Interleukin-6 in Combination with Its Soluble IL-6 Receptor Sensitises Rat Skin Nociceptors to Heat, in Vivo

Pain. Mar, 2002  |  Pubmed ID: 11932061

Interleukin-6 (IL-6) contributes to increased pain and hyperalgesia in inflamed tissue. We have investigated the effects of IL-6, alone or in combination with its soluble receptor (sIL-6R), on the sensitivity of nociceptors to noxious heat, using dermal microdialysis. Plasmapheresis membranes were inserted into the abdominal skin of adult male Wistar rats (n=46) and perfused with modified Ringer solution. After three control samples (20 min each), the skin area above the membrane was heated to 48 degrees C for 20 min. The stimulation was followed by two washout samples. The calcitonin gene-related peptide (CGRP) content of the dialysate was measured with an enzyme immunoassay. Heat stimulation provoked a significant CGRP increase in the dialysate. Intradermal application of IL-6 (200 ng ml-1) did not significantly alter heat-induced CGRP release. However, a significant sensitisation of the heat-induced CGRP release was observed when sIL-6R (25 ng ml-1) was applied, either alone or in combination with IL-6. Neutralisation of endogenous IL-6 with a sheep anti-rat IL-6 serum did not alter heat-induced CGRP release, but abolished the sIL-6R-mediated sensitising effect. We show that IL-6 in combination with its soluble receptor can sensitise nociceptors to heat and provide evidence for the constitutive expression of the signalling molecule gp130, but not of the IL-6-membrane-bound (specific) receptor, in nociceptors.

Prostaglandin E2 Induces Vasodilation and Pruritus, but No Protein Extravasation in Atopic Dermatitis and Controls

Journal of the American Academy of Dermatology. Jul, 2002  |  Pubmed ID: 12077577

Prostaglandin E(2) (PGE(2)) has well-established vasodilatory effects, whereas its effects on protein extravasation and its sensory effects are less clear.

ATP Responses in Human C Nociceptors

Pain. Jul, 2002  |  Pubmed ID: 12098617

Microelectrode recordings of impulse activity in nociceptive C fibres were performed in cutaneous fascicles of the peroneal nerve at the knee level in healthy human subjects. Mechano-heat responsive C units (CMH), mechano-insensitive but heat-responsive (CH) as well as mechano-insensitive and heat-insensitive C units (CM(i)H(i)) were identified. A subgroup of the mechano-insensitive units was readily activated by histamine. We studied the responsiveness of these nociceptor classes to injection of 20 microl 5 mM adenosintriphosphate (ATP) using saline injections as control. Because of mechanical distension during injection, which typically activates mechano-responsive C fibres, interest was focused on responsiveness to ATP after withdrawal of the injection needle. Post-injection responses were observed in 17/27 (63%) mechano-responsive units and in 14/22 (64%) mechano-insensitive units. Excitation by ATP occurred in 9/11 CH units and in 5/11 CM(i)H(i) units. ATP responsive units were found both within the histamine-responsive and the histamine-insensitive group of mechano-insensitive fibres. ATP responses appeared with a delay of 0-180 s after completion of injection; responses were most pronounced during the first 1-3 min of activation, and irregular ongoing activity was observed for up to 10 or even 20 min. ATP responses were dose-dependent, concentrations lower than 5 mM gave weaker responses. No heat or mechanical sensitisation was observed in any of the major fibre classes. In conclusion, we have shown that ATP injections at high concentrations activate C-nociceptors in healthy human skin, without preference for mechano-responsive or mechano-insensitive units. ATP did not sensitise human C fibres for mechanical or heat stimuli. We discuss how various mechanisms might contribute to the observed responses to ATP.

Sensitivity of Human Scalp Skin to Pruritic Stimuli Investigated by Intradermal Microdialysis in Vivo

Journal of the American Academy of Dermatology. Aug, 2002  |  Pubmed ID: 12140471

Although pruritus is common in scalp skin, the forearm has been the main site for investigation in previous experimental studies.

Neural Signal Processing: the Underestimated Contribution of Peripheral Human C-fibers

The Journal of Neuroscience : the Official Journal of the Society for Neuroscience. Aug, 2002  |  Pubmed ID: 12151549

The microneurography technique was used to analyze use-dependent frequency modulation of action potential (AP) trains in human nociceptive peripheral nerves. Fifty-one single C-afferent units (31 mechano-responsive, 20 mechano-insensitive) were recorded from cutaneous fascicles of the peroneal nerve in awake human subjects. Trains of two and four suprathreshold electrical stimuli at interstimulus intervals of 20 and 50 msec were applied to the receptive fields of single identified nociceptive units at varying repetition rates. The output frequency (interspike interval) recorded at knee level was compared with the input frequency (interstimulus interval) at different levels of accumulated neural accommodation. At low levels of use-dependent accommodation (measured as conduction velocity slowing of the first action potential in a train), intervals between spikes increased during conduction along the nerve. At increasing levels of neural accommodation, intervals decreased because of a relative supernormal period (SNP) and asymptotically approached the minimum "entrainment" interval of the nerve fiber (11 +/- 1.4 msec) corresponding to a maximum instantaneous discharge frequency (up to 190 Hz). For neural coding, this pattern of frequency decrease at low activity levels and frequency increase at high levels serves as a mechanism of peripheral contrast enhancement. The entrainment interval is a good minimum estimate for the duration of the refractory period of human C-fibers. At a given degree of neural accommodation, all afferent C-units exhibit a uniform pattern of aftereffects, independent of fiber class. The receptive class of a fiber only determines its susceptibility to accommodate. Thus, the time course of aftereffects and existence or absence of an SNP is fully explained by the amount of preexisting accommodation.

Pathological C-fibres in Patients with a Chronic Painful Condition

Brain : a Journal of Neurology. Mar, 2003  |  Pubmed ID: 12566278

Little is known about the contribution of C-afferent fibres to chronic painful conditions in humans. We sought to investigate the role of C-fibres in the pathophysiology of pain and hyperalgesia in erythromelalgia as a model disease for chronic pain. Erythromelalgia is a condition characterized by painful, red and hot extremities, and patients often report tenderness on walking. We made microneurographic recordings from single C-fibres in cutaneous fascicles of the peroneal nerve in patients suffering from this disease. All patients had had a pain attack recently and psychophysical signs of allodynia and punctate hyperalgesia were found. We obtained recordings from a total of 103 C-fibres and found significantly lower conduction velocities and increased activity-dependent slowing of the conduction velocity of afferent C-fibres in the patients compared with healthy controls. Furthermore, several units with biophysical properties of mechano-insensitive fibres were pathological, being spontaneously active or sensitized to mechanical stimuli. Since these fibres also mediate the axon reflex flare, their hyperexcitability might account not only for ongoing pain and tenderness but also for redness and warming in this pain syndrome. The changes in conductive properties found in the C-fibres of these patients could be the first signs of a small-fibre neuropathy. This is the first systematic study of single C-fibres in patients and it shows an active contribution of mechano-insensitive fibres to chronic pain.

Itch

Lancet. Feb, 2003  |  Pubmed ID: 12606187

Itch is a common skin sensation, with substantial effects on behaviour. Neurophysiological research has permitted accurate definition of neural pathways of itch, and has confirmed the distinctiveness of itch pathways in comparison with pain. A clinical classification of itch, based on such improved understanding, describes the difference between peripheral (pruritoceptive) and central (neurogenic or neuropathic) itch. New specific and sensitive investigational methods in people and animals enable us to better understand this bothersome symptom, and have important clinical implications. We describe the clinical classification of itch, new findings on neuropathophysiology of itch, methods for assessment, and improved treatments.

The Effect of the Nitric Oxide Synthase Inhibitor N-nitro-L-arginine-methyl Ester on Neuropeptide-induced Vasodilation and Protein Extravasation in Human Skin

Journal of Vascular Research. Mar-Apr, 2003  |  Pubmed ID: 12808346

Endogenous neuropeptides released from nociceptors can induce vasodilation and enhanced protein extravasation (neurogenic inflammation). The role of nitric oxide (NO) in the induction of neurogenic inflammation is controversial. In this study, dermal microdialysis was used in awake humans (n = 39) to deliver substance P (SP; 10(-7) and 10(-6)M) or calcitonin gene-related peptide (CGRP; 5 x 10(-7)M and 2 x 10(-6)M). Neuropeptide-induced local and axon reflex erythema was assessed by laser Doppler imaging. Total protein concentration in the dialysate was measured to quantify local protein extravasation. The responses were assessed in the absence and the presence of the nitric oxide synthase inhibitor, N-nitro-L-arginine-methyl ester (L-NAME) added to the perfusate at concentrations of 5, 10 or 20 mM. L-NAME (5 mM) applied via the dialysis catheters reduced local blood flow by approximately 30%. In addition, L-NAME inhibited SP-induced vasodilation by about 40% for 10(-7)M SP and 30% for 10(-6)M SP (n = 11, p < 0.01). In contrast, CGRP-induced vasodilation was only marginally inhibited by L-NAME. SP, but not CGRP, provoked a dose-dependent increase in protein extravasation. L-NAME (5 mM) inhibited this increase by up to 40% for both SP concentrations used (n = 11, p < 0.01). Higher concentrations of L-NAME did not further reduce SP- or CGRP-induced vasodilation or SP-induced protein extravasation. Exogenously applied SP induces vasodilation and protein extravasation, which is partly NO mediated, whereas CGRP-induced vasodilation appears to be NO independent.

Differential Modulation of Remifentanil-induced Analgesia and Postinfusion Hyperalgesia by S-ketamine and Clonidine in Humans

Anesthesiology. Jul, 2003  |  Pubmed ID: 12826855

Experimental studies and clinical observations suggest a possible role for opioids to induce pain and hyperalgesia on withdrawal. The authors used a new experimental pain model in human skin to determine the time course of analgesic and hyperalgesic effects of the mu-receptor agonist remifentanil alone or in combination with the N-methyl-D-aspartate-receptor antagonist S-ketamine or the alpha(2)-receptor agonist clonidine.

Central Origin of Secondary Mechanical Hyperalgesia

Journal of Neurophysiology. Jul, 2003  |  Pubmed ID: 12843313

The contribution for the development of secondary mechanical hyperalgesia by peripheral mechanisms has not been fully elucidated. We have reevaluated the effects of local anesthetics on electrically evoked flare reaction and mechanical hyperalgesia in human skin. We applied 2% lidocaine via intradermal microdialysis fibers at a length of 10 cm for 110 min to the volar forearm to establish a narrow and stable "anesthetic strip." After 60 min of lidocaine perfusion, transdermal electrical stimulation (1 Hz, 50 mA) was applied at a distance of 1 cm from the microdialysis fibers for 30 min. The areas of allodynia and punctate hyperalgesia were marked at the end of the stimulation period. The flare reaction was assessed by laser Doppler scanner and infrared thermography. Total protein content of the dialysate collected at the stimulating electrode was measured photometrically. We found no increase in protein content during electrical stimulation. Flare area (12.4 +/- 2.3 vs. 3.5 +/- 1.2 cm2) and intensity (426 +/- 24 vs. 257 +/- 21 PU) were significantly reduced beyond the lidocaine strip. The mean temperature increase in the area beyond the lidocaine strip was significantly reduced (1.1 +/- 0.1 vs. 0.2 +/- 0.1 degrees C) and did not differ from control areas. In contrast, allodynia (7.4 +/- 0.7 and 8.6 +/- 0.9 cm) and punctate hyperalgesia (7.6 +/- 0.7 and 8.6 +/- 0.9 cm) developed symmetrically on both sides of the anesthetic strip. Allodynia subsided 4 min after the end of the electrical stimulation. We conclude that the development of allodynia and punctate hyperalgesia in human skin is centrally mediated, whereas the axon reflex vasodilation is of peripheral origin.

Proteinase-activated Receptor-2 Mediates Itch: a Novel Pathway for Pruritus in Human Skin

The Journal of Neuroscience : the Official Journal of the Society for Neuroscience. Jul, 2003  |  Pubmed ID: 12867500

We examined whether neuronal proteinase-activated receptor-2 (PAR-2) may be involved in pruritus of human skin. The endogenous PAR-2 agonist tryptase was increased up to fourfold in atopic dermatitis (AD) patients. PAR-2 was markedly enhanced on primary afferent nerve fibers in skin biopsies of AD patients. Intracutaneous injection of endogenous PAR-2 agonists provoked enhanced and prolonged itch when applied intralesionally. Moreover, itch upon mast cell degranulation was abolished by local antihistamines in controls but prevailed in AD patients. Thus, we identified enhanced PAR-2 signaling as a new link between inflammatory and sensory phenomena in AD patients. PAR-2 therefore represents a promising therapeutic target for the treatment of cutaneous neurogenic inflammation and pruritus.

Substance-P-induced Protein Extravasation is Bilaterally Increased in Complex Regional Pain Syndrome

Experimental Neurology. Sep, 2003  |  Pubmed ID: 12957502

Pain, mechanical hyperalgesia, edema, increased skin temperature, and skin reddening are characteristic symptoms of acute complex regional pain syndrome (CRPS). We have recently demonstrated facilitated neurogenic inflammation on the affected limb. To further elucidate the underlying mechanisms, exogenous substance P (SP) in ascending concentrations (10(-9), 10(-8), 10(-7), 10(-6) M) was intradermally applied to the affected and the unaffected limbs, respectively, in two groups of 11 CRPS patients each using the microdialysis technique. Fourteen healthy volunteers served as controls for SP application, and 9 volunteers and 10 patients served as controls for saline perfusion. Dialysate protein content was measured photometrically to assess plasma protein extravasation. Significant differences in dialysate protein content were found after 10(-9) M SP (affected side, 98.4 +/- 8.4% of baseline value; unaffected side, 104.4 +/- 5.6%; controls, 70.7 +/- 4.1%; P < 0.005) and after 10(-6) M SP (affected, 169.7 +/- 24.2%; unaffected, 189.4 +/- 19.1%; controls, 122.2 +/- 12.0%; P < 0.05). While 10(-9) M SP induced no protein extravasation in controls, it provoked protein extravasation in 6 of 11 patients on the affected and in 5 of 11 patients on the unaffected side (P < 0.01). We conclude that SP-induced plasma protein extravasation is increased in CRPS patients on both the affected and unaffected limbs. The underlying mechanism might be impaired SP inactivation. Thus, our results further support the hypothesis that neurogenic inflammation plays an important role in the initiation of CRPS.

Proinflammatory Role of Proteinase-activated Receptor-2 in Humans and Mice During Cutaneous Inflammation in Vivo

FASEB Journal : Official Publication of the Federation of American Societies for Experimental Biology. Oct, 2003  |  Pubmed ID: 14519665

Proteinase-activated receptor-2 belongs to a new subfamily of G-protein-coupled receptors. Its precise role during inflammation and the underlying mechanisms is still unclear. Our study establishes that PAR-2 plays a direct proinflammatory role during cutaneous inflammation in mice and humans in vivo. In a model of experimentally induced allergic (ACD) and toxic (ICD) contact dermatitis (CD) we show that ear swelling responses, plasma extravasation, and leucocyte adherence were significantly attenuated in PAR-2 null mutant (PAR-2-/-) mice compared with wild-type (PAR-2+/+) mice, especially at early stages. The proinflammatory effects by PAR-2 activation were significantly diminished using nitric oxide-synthase inhibitors, while NF-kappaB and neuropeptides appear to play a minor role in these mechanisms. PAR-2-mediated up-regulation of E-selectin and cell adhesion molecule ICAM-1; enhanced plasma extravasation was observed in humans and mice and of interleukin-6 in mice in vivo. Thus, PAR-2 may be a beneficial therapeutic target for the treatment of inflammatory skin diseases.

Short-term Infusion of the Mu-opioid Agonist Remifentanil in Humans Causes Hyperalgesia During Withdrawal

Pain. Nov, 2003  |  Pubmed ID: 14581110

Numerous animal studies suggest that acute and chronic exposure to opioids can be associated with the development of hyperalgesia, i.e. an increased sensitivity to noxious stimuli. Hyperalgesia has been documented during withdrawal and on occasion while animals were still exposed to opioids. A pivotal role in the genesis of opioid-associated hyperalgesia has been attributed to a pain facilitating system involving the N-methyl-D-aspartate (NMDA)-receptor. In humans little direct evidence documents opioid-associated hyperalgesia, albeit observational data suggest that such hyperalgesia may be relevant in a clinical context. This study used a double blind, randomized, crossover and placebo-controlled design to test in opioid-naïve, healthy human volunteers whether hyperalgesia would develop within 30 min of stopping a 90-min infusion with the mu-opioid agonist remifentanil, and whether co-administration of the NMDA-receptor antagonist S-ketamine would prevent such hyperalgesia. We found that a skin area with pre-existing mechanical hyperalgesia was significantly enlarged after stopping the remifentanil infusion. However, the pain response to heat assessed in regular skin was not different before and after the infusion of remifentanil. Co-administration of the NMDA-receptor antagonist S-ketamine abolished observed enlargement of the hyperalgesic skin area. This study provides direct evidence in humans that short-term administration of an opioid can enhance hyperalgesia as observed during withdrawal and points to a potential role of the NMDA-receptor system in mediating such a hyperalgesic response. This study also points to a differential susceptibility of different pain modalities for the expression of hyperalgesia associated with opioid administration.

Naloxone Provokes Similar Pain Facilitation As Observed After Short-term Infusion of Remifentanil in Humans

Pain. Nov, 2003  |  Pubmed ID: 14581115

In contrast to an expected preventive analgesic effect, clinical observations suggest that intraoperatively applied opioids can induce postoperative hyperalgesia. We tested the development of post-infusion hyperalgesia in a newly developed experimental model of electrically induced pain and secondary mechanical hyperalgesia. In a double-blind, placebo controlled, cross-over study, 13 subjects received either saline placebo, remifentanil (0.05 or 0.1 microg/kg/min) or naloxone (0.01 mg/kg). Remifentanil dose-dependently reduced pain and mechanical hyperalgesia during the infusion, but upon withdrawal, pain and hyperalgesia increased significantly above control level (p<0.01 and p<0.05, respectively). Naloxone infusion similarly resulted in increased pain (anti-analgesia) (p<0.001) and mechanical hyperalgesia (p<0.01). Increased pain ratings following withdrawal of remifentanil significantly correlated to anti-analgesia evoked by the mu-opioid antagonist naloxone (p<0.01) and was of similar magnitude, suggesting inhibition of endogenous opioids as an underlying mechanism. In contrast, hyperalgesia after remifentanil was more pronounced than hyperalgesia after naloxone administration and did not correlate to the observed anti-analgesic effects, suggesting the involvement of additional receptors systems other than the endorphin system.

Neuronal Sensitization for Histamine-induced Itch in Lesional Skin of Patients with Atopic Dermatitis

Archives of Dermatology. Nov, 2003  |  Pubmed ID: 14623705

Lowered threshold of neurons (ie, neuronal sensitization) in atopic dermatitis was investigated by testing sensitivity to histamine.

Neurophysiology of Pruritus: Cutaneous Elicitation of Itch

Archives of Dermatology. Nov, 2003  |  Pubmed ID: 14623706

Itching is defined as an unpleasant cutaneous sensation leading to the desire to scratch. It serves as a physiological self-protective mechanism as do other cutaneous sensations like pain, touch, vibration, cold, and heat to help defend the skin against harmful external agents. Pruritus can be evoked in the skin directly by mechanical and thermal stimuli or indirectly through chemical mediators. It may also be generated in the central nervous system independently of peripheral stimulation. Single-nerve-fiber recordings have shown that histamine-evoked itch is transmitted by selective slow-conducting subpopulations of unmyelinated C-polymodal neurons. Recent experimental studies using improved methods have demonstrated which of the suspected chemical itch mediators such as histamine, neuropeptides, prostaglandins, serotonin, acetylcholine, or bradykinin act pruritogenically on C-fibers. Moreover, investigations have revealed new receptor systems such as vanilloid, opioid, and cannabinoid receptors on cutaneous sensory nerve fibers that may modulate itch and thereby represent targets for antipruritic therapy. This review focuses on the peripheral generation of itch, including neurotransmitters, neuropeptides, and inflammatory mediators.

Neurophysiology of Pruritus: Interaction of Itch and Pain

Archives of Dermatology. Nov, 2003  |  Pubmed ID: 14623707

The discovery of an itch-specific neuronal pathway, which is distinct from the pain-processing pathway, has clarified the neuronal basis for the itch sensation. Albeit being distinct, there are complex interactions between pain and itch. The inhibition of itch by pain is well known and can explain the antipruritic effect of scratching. However, the opposite effect also exists and has major clinical implications: inhibition of pain processing (eg, by spinal opioids) can generate itch. Conversely, blockade of spinal opioid receptors can be used as an antipruritic therapy. Moreover, the spinal processing of pain and itch can be modulated, resulting in a hypersensitivity or hyposensitivity to pain or itch: similar to chronic painful conditions, ongoing activity of pruriceptors can induce a spinal hypersensitivity for itch in patients with chronic pruritus. Therapeutic antipruritic approaches therefore should target both local inflammation and spinal sensitization of itch processing.

Modern Aspects of Cutaneous Neurogenic Inflammation

Archives of Dermatology. Nov, 2003  |  Pubmed ID: 14623709

Recent findings have shed new light on the role of peripheral nerves in the skin and established a modern concept of cutaneous neurobiology. Closely related monodirectional and/or bidirectional pathways exist in which the central and peripheral nervous system, the endocrine and immune system, and almost all skin cells are involved. Information is emerging about the factors involved in these immunomodulatory mechanisms, which are defined as neuropeptides, neurotransmitters, neurotrophins, and neurohormones. The interaction between peripheral nerves and the immune system is mediated by different types of cutaneous nerve fibers that release neuromediators and activate specific receptors on target cells in the skin such as keratinocytes, mast cells, Langerhans cells, microvascular endothelial cells, fibroblasts, and infiltrating immune cells. These interactions influence a variety of physiologic and pathophysiologic functions including cellular development, growth, differentiation, immunity, vasoregulation, leukocyte recruitment, pruritus, and wound healing. A variety of mechanisms lead to the termination of cellular responses to released neuropeptides under physiologic circumstances. Herein, we highlight some of the recent advances of neurocutaneous biology and discuss the role of nerves in mediating cutaneous inflammation. Understanding the mechanisms and the factors controlling neuromediators and their receptors and degrading enzymes will lead to the identification of novel therapeutic targets for the treatment of cutaneous diseases.

Effects of Gender and Level of Surgical Sympathetic Block on Vasoconstrictor Function

Clinical Autonomic Research : Official Journal of the Clinical Autonomic Research Society. Dec, 2003  |  Pubmed ID: 14673680

Interruption of sympathetic outflow by surgical sympathetic block has been used to treat hyperhidrosis for decades. In this study the effect of gender and the level of sympathetic block (T2 vs. T3) on the rewarming kinetics following ice water immersion were assessed in a prospective study on 60 hyperhidrosis patients before, 2 days, and 3 months postoperatively. Rewarming kinetics following endoscopic sympathetic block (ESB) was massively enhanced 2 days post-operatively, but had returned to pre-operative conditions at the 3 month follow-up for ESB at level T3. ESB at level T2 provoked significantly faster rewarming as compared to T3 at the 2 day and 3 month follow-up. Independent of the level of ESB, there was a slower rewarming in women already pre-operatively. This gender difference was clearly reduced at the 2 day follow-up, but had increased again at the 3 month follow-up. There was no correlation between the rewarming kinetics of the fingertips and palmar sweating. We conclude that for the sympathetic vasoconstriction of the fingertips the sympathetic ganglion T2 is crucial. Gender differences have to be taken into account when assessing effect of ESB by cold induced vasoconstriction. It remains to be established whether the quantification of vasoconstriction has some predictive value for the long-term prognosis of sudomotor blockade.

Differential Effects of Surgical Sympathetic Block at the T2 and T4 Level on Vasoconstrictor Function

Clinical Autonomic Research : Official Journal of the Clinical Autonomic Research Society. Dec, 2003  |  Pubmed ID: 14673681

Endoscopic sympathetic block (ESB) is used as a treatment of excessive palmar sweating. In a prospective study we compared the effect of ESB at the level of the second (T2) and fourth thoracic ganglion (T4) on vasoconstriction and sweating of the hands. Sympathetic vasoconstriction was measured by computerassisted infrared thermography following ice water immersion of the hands in 22 hyperhidrosis patients before, two days and 3 months post op. In addition, palmar sweating before and 3 months post op was assessed by sudometry. After ESB the rewarming was accelerated in both T2 and T4 patients, but was significantly slower in the T4 group. Three months postoperatively rewarming had returned to the preoperative pattern in T4 patients but was still significantly faster in the T2 group. These effects were more pronounced in the fingertips than the hand dorsum. Sudomotor function was blocked in all T2 patients but had relapsed in 2 patients in the T4 group. Two T4 patients had not shown an effect on sudomotor function postoperatively. The normalization of rewarming kinetics may be explained by remaining fibers, denervation hypersensitivity or stimulation of catecholamine receptors, or neuronal reorganization. The effect of ESB T4 on sudomotor function has to be proven.

Opioid-induced Mast Cell Activation and Vascular Responses is Not Mediated by Mu-opioid Receptors: an in Vivo Microdialysis Study in Human Skin

Anesthesia and Analgesia. Feb, 2004  |  Pubmed ID: 14742371

Activation of mast cells and the systemic release of histamine is a common side effect of opioids. Nevertheless, fentanyl and its derivatives show only a slight activation of mast cells with a subsequent liberation of histamine and tryptase. In this study, we used intradermal microdialysis to assess whether this stimulatory effect of opioids on mast cells depends on the activation of opioid receptors. This new approach allowed us to measure the dose-dependent release of histamine and tryptase from mast cells and the subsequent vascular and sensory effect without systemic side effects in volunteers. The opiate codeine and the synthetic opioids meperidine, fentanyl, alfentanil, sufentanil, remifentanil, buprenorphine, and the opioid antagonist naloxone were tested. Only codeine and meperidine induced mast cell activation with the release of tryptase and histamine, leading to protein extravasation, flare reactions, and itch sensations. Because naloxone did not attenuate these effects, it is unlikely that mu-opioid receptors are involved in the activation of mast cells. IMPLICATIONS: Opioid effects on mast cells were assessed using intradermal microdialysis. Mast cell activation was seen with codeine and meperidine; no other opioid induced degranulation. Therefore, histamine release seen at large concentrations of potent micro agonists is caused by an unspecific effect rather than an activation of opioid receptors.

Electrically Stimulated Axon Reflexes Are Diminished in Diabetic Small Fiber Neuropathies

Diabetes. Mar, 2004  |  Pubmed ID: 14988263

Axon reflex mediated flare depends on the density and the function of cutaneous C-fibers and may be impaired in diabetic neuropathy. We induced neurogenic axon reflex flare by intracutaneous electrical stimulation and analyzed size and intensity of the flare on the dorsum of the foot and ventral thigh with laser Doppler imaging (LDI). We investigated 12 diabetic subjects with small fiber neuropathies (SFNs), 5 diabetic subjects without neuropathy (NO-Ns), and 14 healthy control subjects. Five of the normal subjects were reassessed after 12 months. In comparing patients with SFN to control subjects, we found that SFN flare size but not the intensity of vasodilation (flux) was reduced on the feet (P < 0.001) and thighs (P < 0.007). Furthermore, electrical thresholds for flare induction were increased (thighs P < 0.001 and feet P < 0.03). In NO-Ns, flare size at the feet (P < 0.02) and flux at both sites (thighs P < 0.001 and feet P < 0.002) were even increased. Test/retest evaluation of our method revealed a good correlation (r = 0.83, P < 0.004). Intracutaneous electrical stimulation of C-fibers and scanning the flare with LDI is a sensitive tool to reliably detect small fiber impairment in diabetic SFN subjects and even increased neuropeptide release in NO-Ns.

Perioperative Intravenous Lidocaine Has Preventive Effects on Postoperative Pain and Morphine Consumption After Major Abdominal Surgery

Anesthesia and Analgesia. Apr, 2004  |  Pubmed ID: 15041597

Sodium channel blockers are approved for IV administration in the treatment of neuropathic pain states. Preclinical studies have suggested antihyperalgesic effects on the peripheral and central nervous system. Our objective in this study was to determine the time course of the analgesic and antihyperalgesic mechanisms of perioperative lidocaine administration. Forty patients undergoing major abdominal surgery participated in this randomized and double-blinded study. Twenty patients received lidocaine 2% (bolus injection of 1.5 mg/kg in 10 min followed by an IV infusion of 1.5 mg. kg(-1). h(-1)), and 20 patients received saline placebo. The infusion started 30 min before skin incision and was stopped 1 h after the end of surgery. Lidocaine blood concentrations were measured. Postoperative pain ratings (numeric rating scale of 0-10) and morphine consumption (patient-controlled analgesia) were assessed up to 72 h after surgery. Mean lidocaine levels during surgery were 1.9 +/- 0.7 microg/mL. Patient-controlled analgesia with morphine produced good postoperative analgesia (numeric rating scale at rest,

High Local Concentrations and Effects on Differentiation Implicate Interleukin-6 As a Paracrine Regulator

Obesity Research. Mar, 2004  |  Pubmed ID: 15044662

To examine the possibility that interleukin-6 (IL-6) can act as a paracrine regulator in adipose tissue by examining effects on adipogenic genes and measuring interstitial IL-6 concentrations in situ.

Facilitated Neurogenic Inflammation in Unaffected Limbs of Patients with Complex Regional Pain Syndrome

Neuroscience Letters. Apr, 2004  |  Pubmed ID: 15050689

Pain, edema, increased skin temperature, reddening and trophic changes characterize complex regional pain syndrome (CRPS). Recently, we have been able to show facilitated neurogenic inflammation on the affected limb. In the current study unaffected limbs were examined after resolution of the CRPS symptoms to assess possible generalized changes predisposing to CRPS. In 12 patients and in 12 healthy volunteers dermal microdialysis in combination with electrical C-fiber stimulation was employed to induce neuropeptide release. Dialysate protein concentration and axon reflex vasodilation were measured. Neither in patients nor in controls did electrical stimulation lead to protein extravasation, while axon reflex vasodilation was significantly enhanced even on the patients' unaffected limbs (P < 0.05). Our results support the hypothesis that facilitated neurogenic inflammation is a predisposing factor for CRPS. The lack of protein extravasation indicates that an initiating trauma is necessary to induce neuropeptide up-regulation in primary afferents.

The Cyclooxygenase Isozyme Inhibitors Parecoxib and Paracetamol Reduce Central Hyperalgesia in Humans

Pain. Mar, 2004  |  Pubmed ID: 15109518

Non-steroidal antiinflammatory drugs (NSAIDs) are known to induce analgesia mainly via inhibition of cyclooxygenase (COX). Although the inhibition of COX in the periphery is commonly accepted as the primary mechanism, experimental and clinical data suggest a potential role for spinal COX-inhibition to produce antinociception and reduce hypersensitivity. We used an experimental model of electrically evoked pain and hyperalgesia in human skin to determine the time course of central analgesic and antihyperalgesic effects of intravenous parecoxib and paracetamol (acetaminophen). Fourteen subjects were enrolled in this randomized, double blind, and placebo controlled cross-over study. In three sessions, separated by 2-week wash-out periods, the subjects received intravenous infusions of 40 mg parecoxib, 1000 mg paracetamol, or placebo. The magnitude of pain and areas of pinprick-hyperalgesia and touch evoked allodynia were repeatedly assessed before, and for 150 min after the infusion. While pain ratings were not affected, parecoxib as well as paracetamol significantly reduced the areas of secondary hyperalgesia to pinprick and touch. In conclusion, our results provide clear experimental evidence for the existence of central antihyperalgesia induced by intravenous infusion of two COX inhibitors, parecoxib and paracetamol. Since the electrical current directly stimulated the axons, peripheral effects of the COX inhibitors on nociceptive nerve endings cannot account for the reduction of hyperalgesia. Thus, besides its well-known effects on inflamed peripheral tissues, inhibition of central COX provides an important mechanism of NSAID-mediated antihyperalgesia in humans.

Mechanically Induced Axon Reflex and Hyperalgesia in Human UV-B Burn Are Reduced by Systemic Lidocaine

European Journal of Pain (London, England). Jun, 2004  |  Pubmed ID: 15109974

The mechanisms for the induction of primary mechanical hyperalgesia are unclear. We analyzed the neurogenic axon reflex erythema (flare) following phasic mechanical stimulation in normal and in UV-B irradiated skin. In a cross-over double blind design (n = 10), low dose of systemic lidocaine suppressed mechanical hyperalgesia in sunburned skin and in the mechanically induced flare. Phasic mechanical stimulation, even at painful intensities, did not evoke a flare reaction in normal skin. However, stimulation within the UV-B burn dose-dependently provoked an immediate flare reaction. Systemic lidocaine suppressed the mechanically induced flare as well as the mechanical hyperalgesia in sunburned skin, while leaving the impact-induced ratings in normal skin unchanged. Systemic lidocaine reduced these effects of sensitization, but did not reduce ratings in normal skin. As mechanically insensitive ("sleeping") nociceptors have been shown to mediate the axon-reflex in human skin, sensitization of this class of nociceptors might contribute also to the UV-B-induced primary mechanical hyperalgesia.

Catecholamine Release in Human Skin--a Microdialysis Study

Experimental Neurology. Jul, 2004  |  Pubmed ID: 15191805

Dermal microdialysis might be a promising tool to investigate properties of sympathetic neurons in the skin as investigation of peripheral noradrenergic neurons in humans usually relies on highly variable vasoconstrictor reflexes or on indirect measurements like skin temperature recordings. To evaluate this technique, 21 experiments were performed in 15 healthy subjects with four intracutaneous microdialysis fibers (diameter, 200 microm; cutoff, 5 kDa) at hands or feet. After 60 min, saline perfusion tyramine at concentrations of 0.195 to 200 microg/ml was applied for 15 min followed by a 15-min saline perfusion again. Catecholamine concentrations were detected through high-performance liquid chromatography with electrochemical detection. Control experiments were performed in human skin homogenates with and without tyramine incubation. In vivo, norepinephrine (NE) concentration increased from 36.3 +/- 10.2 pg/ml to 84.4 +/- 18.4 pg/ml (P < 0.001) during stimulation with tyramine, dialysate dopamine (DA) concentration increased from 105.2 +/- 36.5 pg/ml to 7162.4 +/- 3972.4 pg/ml (P < 0.001). Both tyramine-induced NE and DA release were dose-dependent (NE: r = 0.438, P < 0.05; DA: r = 0.894, P < 0.001). In skin homogenates, tyramine incubation led to a significant increase of DA concentrations (387.0 +/- 34.8 pg/ml, controls: 13.2 +/- 2.4 pg/ml; P < 0.05), while NE and epinephrine levels remained unchanged. In conclusion, our experiments show that dermal microdialysis is capable of locally measuring catecholamines in human skin. This offers the opportunity to investigate the function of the peripheral sympathetic nervous system. Additional to non-enzymatic oxidation, DA increase probably reflects metabolic degradation of tyramine by non-neuronal pathways and therefore does not reflect local sympathetic innervation.

Neural Activation During Experimental Allodynia: a Functional Magnetic Resonance Imaging Study

The European Journal of Neuroscience. Jun, 2004  |  Pubmed ID: 15217377

Abstract Pain induced by gentle stroking, i.e. dynamic-mechanical allodynia, is one of the most distressing symptoms of neuropathic pain. The underlying neuronal pathways are still a matter of debate. Here, we investigated the cortical activations associated with dynamic-mechanical allodynia in an experimental human pain model by functional magnetic resonance imaging (fMRI). Large and stable areas of brush-evoked allodynia were induced in 11 healthy subjects by topical capsaicin (2.5%, 30 min) application following local heating (45 degrees C for 5 min), thus combining both physical and chemical sensitization. During the fMRI experiments, allodynia was rekindled by local heat application (40 degrees C for 5 min) immediately before the allodynia testing. Brushing the untreated forearm (control condition) led to activations of the contralateral primary somatosensory cortex (S1), contralateral parietal association cortex (PA), bilateral secondary somatosensory cortices (S2) and insula (contralateral). Brushing the allodynic skin was painful and the cortical responses were partially overlapping with those induced by the nonpainful brush stimulation. Additionally, the contralateral inferior frontal cortex (IFC) and the ipsilateral insula were activated. Direct comparison between nonpainful brushing and brush-evoked allodynia revealed significant increases in blood oxygenation level-dependent (BOLD) signals in contralateral S1, PA, IFC and bilateral S2/insula during allodynia. This study highlights the importance of a cortical network comprising S1, PA, S2/insula and IFC in the processing of dynamic-mechanical allodynia in the human brain. Furthermore, it demonstrates that the combined heat/capsaicin model can be used successfully in the exploration of brain processes underlying stimulus-evoked pain.

Sensitization to Bradykinin B1 and B2 Receptor Activation in UV-B Irradiated Human Skin

Pain. Jul, 2004  |  Pubmed ID: 15275768

Bradykinin B1 and B2 receptors contribute to nociceptor sensitization under inflammatory conditions. Here, we examined the vascular inflammatory responses and nociceptive effects resulting from activation of B1 and B2 receptors in healthy and UV-B irradiated skin in human volunteers. The B1 receptor agonist des-Arg(10)-Kallidin (10(-6)-10(-3)M) and the B2 receptor agonist bradykinin (10(-9)-10(-4)M) were administered by dermal microdialysis to the ventral thigh. UV-B irradiation was performed 24 h prior to the experiment with the threefold minimum erythemal dose. Pain sensation perceived during the stimulation with the bradykinin receptor agonists was estimated on a numeric scale. Local and axon reflex-induced vasodilations were recorded by laser Doppler imaging. For protein extravasation, total protein content in the dialysate was assessed as a measure of increased endothelial permeability. In normal skin, both B1 and B2 receptor activation dose-dependently evoked pain, vasodilatation and protein extravasation. In UV-B irradiated skin, pain sensation and axon reflex vasodilatation were enhanced by both B1 and B2 agonists, whereas local vasodilatation was increased only following B1 receptor activation. The UV-B irradiation did not enhance B1 and B2 receptor-induced protein extravasation indicating a differential sensitization of the neuronal, but not the vascular response.

Chemically and Electrically Induced Sweating and Flare Reaction

Autonomic Neuroscience : Basic & Clinical. Jul, 2004  |  Pubmed ID: 15331047

Both thin afferent (nociceptors) and efferent (sympathetic sudomotor) nerve fibers can be activated electrically and chemically, resulting in neurogenic erythema and sweating. These reactions have been used before to assess the impairment of sympathetic and nociceptor fibers in humans. In this study, electrically induced sweating and erythema were assessed simultaneously in the foot dorsum and thigh, and were compared to chemically induced activation. Reproducible intensity-response relations (stimulation intensities 0-30 mA, 1 Hz) were obtained from 32 subjects. The steepest increase of the sweat response was induced at lower intensities as compared to that of the erythema (18.3 mA vs. 25.7 mA, p<0.01) and reached a plateau for intensities above 25 mA, suggesting lower electrical thresholds for sudomotor fibers. Maximum flare areas induced electrically with 30 mA were smaller than those evoked chemically (flare size: 4.5 cm2 vs. 10.6 cm2). In contrast, the electrically evoked sweating rate was higher than that evoked chemically (acetylcholine, or ACh; sweating rate 0.31 vs. 0.21 microl/cm2/min, p<0.01), which might be attributed to an increased effectiveness of synchronized discharge in sympathetic fibers upon electrical stimulation.

The Effect of Intravenous Infusion of Adenosine on Electrically Evoked Hyperalgesia in a Healthy Volunteer Model of Central Sensitization

Anesthesia and Analgesia. Sep, 2004  |  Pubmed ID: 15333416

Human pain models invoking central sensitization, one of the key mechanisms of chronic pain, may be useful for characterizing new analgesics. A new model of electrical hyperalgesia can detect the efficacy of several analgesic mechanisms. Because IV adenosine can alleviate neuropathic pain, we investigated its effect on experimental sensitization. This was a double-blinded, randomized, two-period crossover study in 20 healthy volunteers. Current pulses (0.5 ms; 1 Hz) were applied intracutaneously to achieve pain rating of approximately 5 on a 0-10 numeric rating scale. Pain, areas of pinprick hyperalgesia, and tactile allodynia were assessed during the 2.5-h stimulation period. Adenosine (50 microg. kg(-1). min(-1)) and placebo were infused IV over 60 min. Additional testing was performed 24 h after each treatment. Adenosine reduced the area of pinprick hyperalgesia during the infusion compared with placebo; there was no significant effect on tactile allodynia or pain rating. The effect on hyperalgesia developed over 15 min and was significant (P < or = 0.05) for the rest of the infusion period. There was no difference between treatments at 24 h. Thus, in accordance with reports on neuropathic pain, adenosine reduced central sensitization in the human model of electrical hyperalgesia. However, adenosine did not have the long-term effects seen in patients. The model can investigate mechanisms of drugs for the treatment of chronic pain.

No Alpha-adrenoreceptor-induced C-fiber Activation in Healthy Human Skin

Journal of Applied Physiology (Bethesda, Md. : 1985). Apr, 2004  |  Pubmed ID: 14672966

In healthy volunteers, flare responses induced by norepinephrine (NE) iontophoresis have been observed. However, as NE iontophoresis is a combined electrical and chemical stimulus axon, reflexes cannot be directly linked to pharmocological activity of NE. Different concentrations of NE, clonidine (CL), and phenylephrine (PE) (NE: 10(-10)-10(-3) M; CL and PE: 10(-8)-10(-3) M) were applied via intradermal microdialysis fibers into the skin of healthy volunteers. Simultaneously, skin blood flow was visualized by laser-Doppler imaging scans and quantified in a vasoconstriction skin area directly above the membranes to control drug effects and in expected axon reflex vasodilation areas that were 0.75 cm apart. NE, PE, and CL caused dose-dependent vasoconstriction. However, neither in the presumed axon reflex areas (quantitative analysis) nor on laser-Doppler imaging pictures (qualitative analysis) were any vasodilation observed. Even at concentrations causing maximum vasoconstriction (10(-3) M for any drug), no vasodilation was induced. Our results indicate that, in healthy human skin, exogenously supplied alpha-adrenoreceptor agonists alone do not activate nociceptors sufficiently to induce axon reflex flare.

Electrically Evoked Itch in Humans

Pain. Jan, 2005  |  Pubmed ID: 15621375

We compared itch sensations and axon reflex flare induced by transcutaneous electrical (0.08-8 ms, 2-200 Hz) and chemical (histamine iontophoresis; 100 microC) stimulation. Stimuli were applied to non-lesional volar wrist skin in 20 healthy human subjects and 10 patients with atopic dermatitis. Intensity of evoked itch and pain sensations were rated on a numerical rating scale (NRS) of 0 (no sensation) to 10 (the maximum sensation imaginable). The axon reflex erythema was measured by laser Doppler imager and areas of alloknesis (itch evoked by light brushing) and hyperknesis (itch evoked by pricking) were assessed psychophysically. Electrical stimulation was most effective for stimulus durations >or=2 ms and frequencies >or=50 Hz. It evoked pure itch as threshold sensation in 80% of the subjects that was perceived with a delay of approximately 1 s. Itch intensities of up to 7/10 were not accompanied by an axon reflex flare. In contrast, histamine provoked a massive increase of axon reflex erythema and maximum itch ratings of 3.1+/-0.2. The extention of alloknesis areas (2.3+/-0.5 cm) evoked by electrical stimulation clearly exceeded those induced by histamine (0.7+/-0.3 cm). Healthy subjects and patients with atopic dermatitis did not differ significantly in their response to either stimulation. We conclude that C-fiber activation underlies the electrically evoked itch sensation. The low electrical thresholds and the absence of an axon reflex flare suggest that these fibers are not identical with the previously described mechano-insensitive histamine responsive C fibers, but represent a separate peripheral neuronal system for the induction of itch.

Transcutaneous Penetration of Toluene in Rat Skin a Microdialysis Study

Experimental Dermatology. Feb, 2005  |  Pubmed ID: 15679579

Percutaneous absorption of lipophilic substances has major implications for therapeutical use or toxicological effects. We, therefore, using dermal microdialysis, measured local toluene concentrations and assessed the effects of duration of exposure, skin barrier disruption and the use of skin-care products. Three microdialysis membranes (3000 kDa) were inserted intradermally at a length of 2 cm in the abdominal skin of 82 anaesthetized male Wistar rats. They were perfused with albumin solution (5%) at 10 microl/min. A skin area of 1.5 x 0.6 cm above the membranes was exposed to toluene (100%, 200 microl) for 15 or 240 min. Dialysate was sampled at 20-min intervals. Using GC-FPD (gas charomotography flame photometric detector), it was analysed for toluene. In addition, the effects of tape stripping and pretreatment with topical products were assessed. In each of the 12 permutations of exposure time, pretreatments and tape stripping, five to eight animals were investigated. Maximum toluene concentrations were reached at 60 min after exposure (3.07 +/- 0.40 microg/ml, 15 min; 5.38 +/- 0.92 microg/ml, 240 min). In 15-min exposure experiments, dermal toluene concentrations decreased slowly to reach baseline values after 240 min. After 240-min exposure, a plateau of approximately 6 microg/ml was reached after 60 min. Neither tape stripping nor the pretreatment with barrier cream induced a significant change on dermal toluene concentrations. The slow kinetics of toluene penetration results in a steep concentration gradient in the skin with very-high local toluene concentrations and a delayed wash out, which might be relevant not only toxicologically, but also therapeutically.

Altered Central Excitability and Analgesic Treatment in Patients with Restless Legs Syndrome

Brain : a Journal of Neurology. Jun, 2005  |  Pubmed ID: 15900027

Distribution of Cannabinoid Receptor 1 (CB1) and 2 (CB2) on Sensory Nerve Fibers and Adnexal Structures in Human Skin

Journal of Dermatological Science. Jun, 2005  |  Pubmed ID: 15927811

Cannabinoid receptors mediate the psychopharmacological action of marijuana and have been localized in the central and peripheral nervous system as well as on cells of the immune system.

Different Profiles of Buprenorphine-induced Analgesia and Antihyperalgesia in a Human Pain Model

Pain. Nov, 2005  |  Pubmed ID: 16154698

Different mechanisms were proposed for opioid-induced analgesia and antihyperalgesia, which might result in different pharmacodynamics. To address this issue, the time course of analgesic and antihyperalgesic effects of intravenous (i.v.) and sublingual (s.l.) buprenorphine was assessed in an experimental human pain model. Fifteen volunteers were enrolled in this randomized, double-blind, and placebo controlled cross-over study. The magnitude of pain and the area of secondary hyperalgesia following transcutaneous stimulation were repetitively assessed before and up to 150 min after administration of (1) 0.15 mg buprenorphine i.v. and placebo pill s.l., (2) 0.2 mg buprenorphine s.l. and saline 0.9% i.v. or (3) saline 0.9% i.v. and placebo pill s.l. as a control. The sessions were separated by 2 week wash-out periods. For both applications of buprenorphine the antihyperalgesic effects were more pronounced as compared to the analgesic effects (66+/-9 vs. 26+/-5% and 43+/-10 vs. 10+/-6%, for i.v. and s.l. application, respectively). This contrasts the pattern for the intravenous administration of pure mu-receptor agonists in the same model in which the antihyperalgesic effects are weaker. The apparent bioavailability of buprenorphine s.l. as compared to buprenorphine i.v. was 58% with a 15.8 min later onset of antinociceptive effects. The half-life of buprenorphine-induced analgesic and antihyperalgesic effects were 171 and 288 min, respectively. In contrast to pure mu-receptor agonists, buprenorphine exerts a lasting antihyperalgesic effect in our model. It will be of major clinical interest whether this difference will translate into improved treatment of pain states dominated by central sensitization.

Activation of Naloxone-sensitive and -insensitive Inhibitory Systems in a Human Pain Model

The Journal of Pain : Official Journal of the American Pain Society. Nov, 2005  |  Pubmed ID: 16275600

We investigated naloxone effects in a model of electrically induced pain and hyperalgesia. In a double-blind, placebo-controlled, cross-over study, 15 volunteers underwent four 150-minute sessions of high-current-density electrical stimulation of their forearms. After 60 minutes, naloxone or placebo was given intravenously (increasing plasma concentrations of 0.1, 1, and 10 ng/mL; 30 minutes each) in 3 of the 4 sessions. Pain ratings and areas of mechanical hyperalgesia were assessed at regular intervals during all sessions. The low doses of naloxone did not cause any significant change of pain rating of areas of hyperalgesia. In terms of intrasession effects, pain ratings and areas of hyperalgesia significantly decreased during the sessions to 62% (pain rating), 70% (area of punctuate hyperalgesia), and 82% (area of allodynia) of the initial values. Naloxone (10 ng/ml) reversed these decreases. In terms of between-session effects, the time course of pain ratings remained constant from session to session. In contrast, the areas of punctate hyperalgesia successively decreased to 60% of initial value at the fourth repetition. The session effect was not reversed by naloxone. High-current-density electrical stimulation provokes central sensitization, but in addition inhibitory systems are activated that are only partly naloxone-sensitive. PERSPECTIVE: Endogenous inhibitory systems are of major importance for clinical pain conditions, but are not reflected in traditional human pain models. Here we show activation of a naloxone-sensitive short-term and a naloxone-insensitive long-term inhibitory system in a new model of electrically induced pain and hyperalgesia.

Itch and Pain

Dermatologic Therapy. Jul-Aug, 2005  |  Pubmed ID: 16297001

The discovery of a specialized neuronal pathway for itch has markedly improved our understanding of itch processing under physiological conditions. However, the complex interactions of pain and itch are only partly understood. This review focuses on the neurophysiological mechanisms involved in clinical and experimental itch conditions. There is emerging evidence that similar patterns of peripheral and central sensitization occur in chronic pain and chronic itch conditions. It will be of major interest to reveal whether the underlying mechanism for sensitization in the itch and pain pathways are also similar, as this might have major implications for therapy.

Neurogenic Components of Trypsin- and Thrombin-induced Inflammation in Rat Skin, in Vivo

Experimental Dermatology. Jan, 2006  |  Pubmed ID: 16364032

Activation of protease-activated receptors (PAR) can induce vasodilation (VD) and increase of vascular permeability either directly by stimulating endothelial cells or indirectly via activation of nociceptors and subsequent release of neuropeptides (neurogenic inflammation). We aimed to estimate the relative contribution of the two pathways for stimulation with endogenous activators of PAR-2 (trypsin) and of PAR-1, 3 and 4 (thrombin) using in vivo dermal microdialysis in rats. Protein extravasation (PE) was assessed by increase of protein concentration in the dialysate, and VD was quantified by laser Doppler scanning. Both trypsin (10(-8)-10(-4) M) and thrombin (10(-6), 10(-5.5) and 10(-5) M) provoked PE and local VD in a dose-dependent manner. Trypsin (10(-4) M)-induced PE was inhibited by 87.2 +/- 21% due to the substance P (SP) NK1 receptor antagonist SR140333. VD was blocked by 58.15 +/- 10.1% in response to the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP(8-37). By contrast, CGRP(8-37) did not affect thrombin-induced VD, while blockade of SP receptors prevented the PE elicited only by low doses of thrombin (10(-6) M), being ineffective at higher thrombin concentrations. In conclusion, intradermal trypsin elicits a neurogenic inflammation in rat, probably mediated via PAR-2 activation on nociceptors and subsequent SP and CGRP release. Thrombin-induced PE and VD are mediated mainly by a non-neurogenic mechanism.

Angiotensin Converting Enzyme Has an Inhibitory Role in CGRP Metabolism in Human Skin

Peptides. Apr, 2006  |  Pubmed ID: 16406141

The neutral endopeptidase (NEP) is important for calcitonin gene related peptide (CGRP) degradation, while the role of angiotensin converting enzyme (ACE) remains unclear. By using dermal microdialysis we explored the effect of phosphoramidon (NEP blocker), captopril (ACE blocker) and a mixture of both drugs on the intensity of electrically-induced CGRP-mediated neurogenic flare. The results reveal that phosphoramidon elevated flare intensity, but that this was not further increased by adding captopril. In contrast, neurogenic flare was decreased when the drug mixture was applied in compared to NEP only. Electrically released CGRP levels could be measured directly in perfusates containing phosphoramidon and the mixture. Again, CGRP levels were elevated in phosphoramidon treated sites, and significantly reduced upon adding captopril. These findings suggest that NEP and ACE do not have additive effects regarding neuropeptide degradation. In contrast, inhibition of ACE seems to augment CGRP catabolism.

Duration of Effects of Aspirin on Platelet Function in Healthy Volunteers: an Analysis Using the PFA-100

Journal of Clinical Anesthesia. Feb, 2006  |  Pubmed ID: 16517326

The aim of the study was to determine the duration and effects of aspirin on platelet function.

Pathophysiology and Treatment of Pain in Joint Disease

Advanced Drug Delivery Reviews. May, 2006  |  Pubmed ID: 16626837

Deep somatic pain originating in joints and tendons is a major therapeutic challenge. Spontaneous pain and mechanical hypersensitivity can develop as a consequence of sensitization of primary afferents directly involved in the inflammatory process, but also following sensitization of neuronal processing in the spinal cord (central sensitization) or higher centres. Inflammatory pain is linked to sensitization of sensory proteins at the nociceptive endings whereas pain originating from nerve damage (neuropathic pain) has been linked to changes in axonal ion channels producing ectopic discharge in nociceptors as a source of pain. New targets for analgesic therapy include sensory proteins at the nociceptive nerve endings such as the activating TRPV and ASIC channels, but also inhibitory opioid and cannabinoid receptors. Therapeutic targets are also found among the axonal channels that set membrane potential and modulate discharge frequency such as voltage sensitive sodium channels and various potassium channels.

Frontiers in Pruritus Research: Scratching the Brain for More Effective Itch Therapy

The Journal of Clinical Investigation. May, 2006  |  Pubmed ID: 16670758

This Review highlights selected frontiers in pruritus research and focuses on recently attained insights into the neurophysiological, neuroimmunological, and neuroendocrine mechanisms underlying skin-derived itch (pruritogenic pruritus), which may affect future antipruritic strategies. Special attention is paid to newly identified itch-specific neuronal pathways in the spinothalamic tract that are distinct from pain pathways and to CNS regions that process peripheral pruritogenic stimuli. In addition, the relation between itch and pain is discussed, with emphasis on how the intimate contacts between these closely related yet distinct sensory phenomena may be exploited therapeutically. Furthermore, newly identified or unduly neglected intracutaneous itch mediators (e.g., endovanilloids, proteases, cannabinoids, opioids, neurotrophins, and cytokines) and relevant receptors (e.g., vanilloid receptor channels and proteinase-activated, cannabinoid, opioid, cytokine, and new histamine receptors) are discussed. In summarizing promising new avenues for managing itch more effectively, we advocate therapeutic approaches that strive for the combination of peripherally active antiinflammatory agents with drugs that counteract chronic central itch sensitization.

Chronic Itch and Pain--similarities and Differences

European Journal of Pain (London, England). Jul, 2006  |  Pubmed ID: 16678456

Both, pruritus and pain are aversive, but clearly distinct sensations originating in the peripheral and central nervous system. During the last years, many interactions between itch and pain in acute transmission and sensitization processes have been identified. It is common experience that the itch sensation can be reduced by the painful sensations caused by scratching. Vice versa analgesia may reduce this inhibition and thus enhance itch. This phenomenon is particularly relevant to spinally administered mu-opioid receptor agonists, which induce segmental analgesia often combined with segmental pruritus. The peripheral and central sensitization to pain and to itch exhibits striking similarities. Classical inflammatory mediators such as bradykinin have been shown to sensitize nociceptors for both itch and pain. Also regulation of gene expression induced by trophic factors, such as NGF, plays a major role in persistently increased neuronal sensitivity for itch and pain. Finally, itch and pain exhibit corresponding patterns of central sensitization. The knowledge of antagonistic interaction, but also of similar sensitization processes has major implication for antipruritic therapeutic approaches.

Reduction of Pulmonary Edema by Short-acting Local Anesthetics

Regional Anesthesia and Pain Medicine. May-Jun, 2006  |  Pubmed ID: 16701192

Local anesthetics (LAs) possess a variety of effects that cannot be explained by the typical block of neuronal sodium channels. Antithrombotic effects of LAs are well known, but LAs also act as bactericides. Therefore, an investigation of the influence of LAs on the inflammatory response of the isolated rat lung (n = 78) to an N-formyl-l-leucin-methionyl-l-phenylalanine (FMLP) stimulus was performed.

The Neurobiology of Itch

Nature Reviews. Neuroscience. Jul, 2006  |  Pubmed ID: 16791143

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

Delta-9-THC Based Monotherapy in Fibromyalgia Patients on Experimentally Induced Pain, Axon Reflex Flare, and Pain Relief

Current Medical Research and Opinion. Jul, 2006  |  Pubmed ID: 16834825

Fibromyalgia (FM) is a chronic pain syndrome characterized by a distinct mechanical hyperalgesia and chronic pain. Recently, cannabinoids have been demonstrated as providing anti-nociceptive and anti-hyperalgesic effects in animal and human studies. Here, we explored in nine FM patients the efficacy of orally administered delta-9-tetrahydrocannabinol (THC) on electrically induced pain, axon reflex flare, and psychometric variables. RESEARCH DESIGN AND METHods: Patients received a daily dose of 2.5-15 mg of delta-9-THC, with a weekly increase of 2.5 mg, as long as no side effects were reported. Psychometric variables were assessed each week by means of the West Haven-Yale Multidimensional Pain Inventory (MPI), Pittsburgh Sleep Quality Index (PSQI), Medical outcome survey-short form (MOS SF-36), the Pain Disability Index (PDI), and the Fibromyalgia Impact Questionnaire (FIQ). In addition, patients recorded daily, in a diary, their overall pain intensity on a numeric scale. Each week, pain and axon reflex flare was evoked experimentally by administration of high intensity constant current pulses (1 Hz, pulse width 0.2 ms, current increase stepwise from 2.5-12.5 mA every 3 minutes) delivered via small surface electrodes, attached to the volar forearm skin.

Bradykinin is a Potent Pruritogen in Atopic Dermatitis: a Switch from Pain to Itch

Pain. Dec, 2006  |  Pubmed ID: 16842920

Histamine, substance P, serotonin and bradykinin were applied by iontophoresis to lesional and visually non-lesional skin of 14 patients with atopic dermatitis, and normal skin of 15 healthy volunteers. Itch could be evoked by light stroking of skin with a cotton swab (alloknesis) in all lesional skin sites, but not in non-lesional or normal skin. Substances were applied in the same skin area before and 3 h after administration of placebo or antihistamine (olopatadine hydrochloride: H1-receptor-blocker). Intensities of itch and pain sensation and areas of flare and wheal were measured. All the substances induced significantly more intense itch in lesional skin than in non-lesional skin of patients. Even bradykinin, which evoked only weak itch and pain of similar intensities in non-lesional skin of patients and in healthy volunteers, induced intense itch in lesional skin, while the simultaneously increased pain did not suppress the itch sensation, indicating central sensitization. Histamine- and substance P-induced itch was almost completely suppressed by antihistamines, whereas bradykinin- and serotonin-induced itch was not. This suggests that substance P is a histamine-dependent pruritogen also in lesional skin under sensitized conditions but that bradykinin and serotonin are histamine-independent pruritogens in lesional skin. It is concluded that serotonin and bradykinin, classic endogenous algogens, can turn into potent histamine-independent pruritogens in lesional skin of atopic dermatitis.

Neurophysiological, Neuroimmunological, and Neuroendocrine Basis of Pruritus

The Journal of Investigative Dermatology. Aug, 2006  |  Pubmed ID: 16845410

Pruritus (itch) can be defined as an unpleasant cutaneous sensation associated with the immediate desire to scratch. Recent findings have identified potential classes of endogenous "itch mediators" and establish a modern concept for the pathophysiology of pruritus. First, there in no universal peripheral itch mediator, but disease-specific sets of involved mediators. Second, numerous mediators of skin cells can activate and sensitize pruritic nerve endings, and even modulate their growth. Our knowledge of itch processing in the spinal cord and the involved centers in the central nervous system is rapidly growing. This review summarizes the current information about the significance of neuron-skin interactions, ion channels, neuropeptides, proteases, cannabinoids, opioids, kinins, cytokines, biogenic amines, neurotransmitters, and their receptors in the pathobiology of pruritus. A deeper understanding of these circuits is required for the development of novel antipruritic strategies.

Modulation of Remifentanil-induced Analgesia and Postinfusion Hyperalgesia by Parecoxib in Humans

Anesthesiology. Nov, 2006  |  Pubmed ID: 17065897

Numerous experimental and clinical studies suggest that brief opioid exposure can enhance pain sensitivity. It is suggested that spinal cyclooxygenase activity may contribute to the development and expression of opioid tolerance. The aim of the investigation was to determine analgesic and antihyperalgesic properties of the cyclooxygenase-2 inhibitor parecoxib on remifentanil-induced hypersensitivity in humans.

Abnormal Function of C-fibers in Patients with Diabetic Neuropathy

The Journal of Neuroscience : the Official Journal of the Society for Neuroscience. Nov, 2006  |  Pubmed ID: 17079656

The mechanisms underlying the development of painful and nonpainful neuropathy associated with diabetes mellitus are unclear. We have obtained microneurographic recordings from unmyelinated fibers in eight patients with diabetes mellitus, five with painful neuropathy, and three with neuropathy without pain. All eight patients had large-fiber neuropathy, and seven patients had pathological thermal thresholds in their feet, indicating the involvement of small-caliber nerve fibers. A total of 163 C-fibers were recorded at knee level from the common peroneal nerve in the patients (36-67 years old), and these were compared with 77 C-fibers from healthy controls (41-64 years old). The ratio of mechano-responsive to mechano-insensitive nociceptors was approximately 2:1 in the healthy controls, whereas in the patients, it was 1:2. In patients, a fairly large percentage of characterized fibers (12.5% in nonpainful and 18.9% in painful neuropathy) resembled mechano-responsive nociceptors that had lost their mechanical and heat responsiveness. Such fibers were rarely encountered in age-matched controls (3.2%). Afferent fibers with spontaneous activity or mechanical sensitization were found in both patient groups. We conclude that small-fiber neuropathy in diabetes affects receptive properties of nociceptors that leads to an impairment of mechano-responsive nociceptors.

Chapter 18 Itch and Cold Allodynia

Handbook of Clinical Neurology / Edited by P.J. Vinken and G.W. Bruyn. 2006  |  Pubmed ID: 18808840

Chapter 29 Microneurography in the Assessment of Neuropathic Pain

Handbook of Clinical Neurology / Edited by P.J. Vinken and G.W. Bruyn. 2006  |  Pubmed ID: 18808851

Potentiation of Nociceptive Responses to Low PH Injections in Humans by Prostaglandin E2

The Journal of Pain : Official Journal of the American Pain Society. May, 2007  |  Pubmed ID: 17337250

Inflammation and trauma lead to tissue acidification and release of inflammatory mediators, including prostaglandin E2 (PGE2). Protons can evoke pain through acid-sensing ion channels (ASICs) and TRPV1 receptors. In this study, we examined whether PGE2 can potentiate proton-induced nociception in humans on injection into skin and muscle. Psychophysical and vascular responses to microinjections of protons (pH 6.0 and 6.5), PGE2 (10-6 and 10-7 M) and their combinations into forearm skin (30 microL) or anterior tibial muscle (50 microL) were assessed in 16 male subjects. Pain intensity, axon reflex erythema, and heat pain thresholds were recorded after skin challenge; pain intensity and thresholds for pressure-evoked pain were recorded after intramuscular injections. Intradermal or intramuscular injections of PGE2 induced very low levels of pain similar to saline. Administration of low pH caused moderate pain within 5 seconds that declined rapidly over 15 to 20 seconds. In comparison, coinjection of low pH with PGE2 led to a biphasic profile of the pain response. Combined pH + PGE2 stimulation provoked significantly increased pain in the second phase after injections (20 to 100 seconds) both in skin and muscle, whereas the initial injection pain was not enhanced. Heat pain thresholds were reduced after PGE2 and combined pH + PGE2, whereas flare responses were rather attenuated on coadministration of low pH with PGE2. Intriguingly, when compared with skin, muscle pain was significantly lower in the initial phase (0 to 15 seconds) but significantly higher in the second phase (20 to 100 seconds after injection). PERSPECTIVE: PGE2 can potentiate nociceptor activation by protons in human skin and muscle, indicated by increased sustained pain ratings. This can be best explained by TRPV1 sensitization in the presence of PGE2, a mechanism potentially relevant for inflammatory and injury-induced pain.

Monocyte Chemoattractant Protein-1 in Subcutaneous Abdominal Adipose Tissue: Characterization of Interstitial Concentration and Regulation of Gene Expression by Insulin

The Journal of Clinical Endocrinology and Metabolism. Jul, 2007  |  Pubmed ID: 17456576

The chemokine monocyte chemoattractant protein-1 (MCP-1) is implicated in obesity-associated chronic inflammation, insulin resistance, and atherosclerosis.

The Impact of Opioid-induced Hyperalgesia for Postoperative Pain

Best Practice & Research. Clinical Anaesthesiology. Mar, 2007  |  Pubmed ID: 17489220

Clinical evidence suggests that--besides their well known analgesic activity - opioids can increase rather than decrease sensitivity to noxious stimuli. Based on the observation that opioids can activate pain inhibitory and pain facilitatory systems, this pain hypersensitivity has been attributed to a relative predominance of pronociceptive mechanisms. Acute receptor desensitization via uncoupling of the receptor from G-proteins, upregulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA)-receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days in rodents, and also in humans large-doses of intraoperative micro-receptor agonists were found to increase postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients is often associated with a requirement for increasing doses and the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha2-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor/selectivity.

Opioids and the Skin: "itchy" Perspectives Beyond Analgesia and Abuse

The Journal of Investigative Dermatology. Jun, 2007  |  Pubmed ID: 17502859

Opioids are intimately linked to central pain inhibition and their abuse potential. Thus, peripheral opioid receptors in the skin have been studied initially with a focus on their peripheral analgesic properties. Recent results, however, clearly indicate that opioids play a specific role in skin homeostasis by modulating keratinocyte differentiation, wound healing, and inflammatory responses.

Differential Sensitivity of Thick and Thin Fibers to HIV and Therapy-induced Neuropathy

Autonomic Neuroscience : Basic & Clinical. Oct, 2007  |  Pubmed ID: 17561445

The study assessed HIV-related and anti-retroviral therapy-induced neuropathy in myelinated and unmyelinated nerve fibers. One hundred consecutive HIV patients were examined clinically and standard nerve conduction velocities were measured. In addition, electrically induced sympathetic skin response (SSR) was assessed in the palms and soles. The difference in delay of SSR in palms and soles (DeltaSSR) was calculated as an indirect measure of C-fiber conduction velocity. Thick fiber conduction velocities significantly decreased with age and increasing stage of the disease, whereas no effect of stage was found for DeltaSSR (p=0.6). In contrast, medication of at least one of the most known neurotoxic drugs zalcitabine, stavudine, or didanosine did not result in significantly lower conduction velocities in thick fibers (51.29+/-3.4 m/s vs. 50.86+/-3.5 m/s), but was related to an increased DeltaSSR. DeltaSSR allows an indirect measurement of C-fiber conduction velocity. In HIV this measure of unmyelinated sympathetic fibers was most sensitive to anti-viral treatment whereas conduction velocity of myelinated somatic fibers was more sensitive to disease-related neuropathy. The results suggest that HIV neuropathy preferably affects myelinated and anti-retroviral therapy unmyelinated fibers.

Rapid Flare Development Evoked by Current Frequency-dependent Stimulation Analyzed by Full-field Laser Perfusion Imaging

Neuroreport. Jul, 2007  |  Pubmed ID: 17589307

We analyzed, with a new imaging technique, the rapid axon reflex flare responses in human skin upon transcutaneous delivery of electrical stimuli at 1, 5, 10 and 50 Hz in single bursts of five pulses each. Two-dimensional perfusion images covering an area of 8 x 8 cm(2) were captured at 25 Hz and their averages saved at 0.5 Hz. The stimulation caused an axon reflex flare (maximum 3 cm(2), 20 s after stimulation) that gradually resolved within 2 min. Maximum flare responses developed at 5 Hz, whereas pain ratings increased with stimulation frequency. The highest neuropeptide release at 5 Hz correlates to the discharge characteristics of mechanoinsensitive C-fibers, whereas the maximum pain intensity at 50 Hz may be attributed to the activation of A-delta fibers.

Clinical Classification of Itch: a Position Paper of the International Forum for the Study of Itch

Acta Dermato-venereologica. 2007  |  Pubmed ID: 17598029

Chronic itch is a common and distressing symptom that arises from a variety of skin conditions and systemic diseases. Despite this, there is no clinically based classification of pruritic diseases to assist in the diagnosis and cost-effective medical care of patients with pruritus. The proposed classification focuses on clinical signs and distinguishes between diseases with and without primary or secondary skin lesions. Three groups of conditions are proposed: pruritus on diseased (inflamed) skin (group I), pruritus on non-diseased (non-inflamed) skin (group II), and pruritus presenting with severe chronic secondary scratch lesions, such as prurigo nodularis (group III). The next part classifies the underlying diseases according to different categories: dermatological diseases, systemic diseases including diseases of pregnancy and drug-induced pruritus, neurological and psychiatric diseases. In some patients more than one cause may account for pruritus (category "mixed") while in others no underlying disease can be identified (category "others"). This is the first version of a clinical classification worked out by the members of the International Forum for the Study of Itch. It is intended to serve as a diagnostic route for better evaluation of patients with chronic pruritus and aims to improve patients' care.

Catecholamine-induced Excitation of Nociceptors in Sympathetically Maintained Pain

Pain. Feb, 2007  |  Pubmed ID: 16997471

Sympathetically maintained pain could either be mediated by ephaptic interactions between sympathetic efferent and afferent nociceptive fibers or by catecholamine-induced activation of nociceptive nerve endings. We report here single fiber recordings from C nociceptors in a patient with sympathetically maintained pain, in whom sympathetic blockade had repeatedly eliminated the ongoing pain in both legs. We classified eight C-fibers as mechano-responsive and six as mechano-insensitive nociceptors according to their mechanical responsiveness and activity-dependent slowing of conduction velocity (latency increase of 0.5+/-1.1 vs. 7.1+/-2.0 ms for 20 pulses at 0.125 Hz). Two C-fibers were activated with a delay of several seconds following strong endogenous sympathetic bursts; they were also excited for about 3 min following the injection of norepinephrine (10 microl, 0.05%) into their innervation territory. In these two fibers, a prolonged activation by injection of low pH solution (phosphate buffer, pH 6.0, 10 microl) and sensitization of their heat response following prostaglandin E2 injection were recorded, evidencing their afferent nature. Moreover, their activity-dependent slowing was typical for mechano-insensitive nociceptors. We conclude that sensitized mechano-insensitive nociceptors can be activated by endogenously released catecholamines and thereby may contribute to sympathetically maintained pain. No evidence for ephaptic interaction between sympathetic efferent and nociceptive afferent fibers was found.

Nociceptor Sensitization to Mechanical and Thermal Stimuli in Pig Skin in Vivo

European Journal of Pain (London, England). Feb, 2008  |  Pubmed ID: 17611131

Primary hyperalgesia to mechanical and thermal stimuli are major clinical symptoms of inflammatory pain and can be induced experimentally by ultraviolet-B (UV-B) irradiation in humans. We set-up a pig model in order to have more options for pharmacological intervention on primary hyperalgesia. Pig skin was irradiated with a dose one- to threefold higher than the minimum erythema dose (MED) and investigated for mechanical and heat responsiveness 24 and 48 h post UV-B treatment. C-fiber activation upon mechanical and thermal stimulation was assessed indirectly by extent of the axon reflex erythema (flare) measured by laser Doppler imaging. Mechanical stimulation with von Frey filaments (100 mN) induced flare responses in UV-B treated skin at 24 and 48 h, but no effect was measured in normal untreated skin. Increased mechanical stimulation (600 mN) elicited a small flare response in normal skin in an area of 1.8 cm(2) on average that was extending about 2.5 cm(2) in the UV-B irradiated sites. Thermal stimuli provoked in normal pig skin flare areas of approximately 2 cm(2) (45 degrees C, 10 s) and 4.5 cm(2) (47 degrees C, 10 s) which increased to about 3.5 cm(2) (45 degrees C) and 5.5 cm(2) (47 degrees C) following UV-B irradiation at 24 and 48 h. No significant differences of mechanically or thermally induced hypersensitivity were seen between 24 and 48 h after irradiation. We conclude that UV-B induced mechanical and heat sensitization of primary afferent nociceptors can be assessed in pig skin, providing a new human-like model of primary hyperalgesia. Sensitization of primarily mechano-insensitive (silent) nociceptors, which are underlying the flare response in humans, most probably contributes to the observation presented here.

Endothelin 1 Activates and Sensitizes Human C-nociceptors

Pain. Jul, 2008  |  Pubmed ID: 17884295

Microneurography was used to record action potentials from afferent C-fibers in cutaneous fascicles of the peroneal nerve in healthy volunteers. Afferent fibers were classified according to their mechanical responsiveness to von Frey stimulation (75g) into mechano-responsive and mechano-insensitive nociceptors. Various concentrations of Endothelin1 (ET1) and Histamine were injected into the receptive fields of C-fibers. Activation and heat sensitization were monitored. Axon reflex flare and psychophysical ratings were assessed after injection of ET1 and codeine into the forearms after pre-treatment with an H1 blocker or sodium chloride. 65% of mechanosensitive nociceptors were activated by ET1. One-third showed long lasting responses (>15min). In contrast, none of thirteen mechano-insensitive fibers were activated. Sensitization to heat was observed in 62% of mechanosensitive and in 46% of mechano-insensitive fibers. Injection of ET1 produced a widespread axon reflex flare, which was suppressed by pre-treatment with an H1 receptor blocker. In addition, pain sensations were induced more often than itching by ET1 in contrast to codeine. No wheal was observed after injection of ET1. Both itching and pain were decreased after H1 blocker treatment. In summary: (1) In humans ET1 activates mechanosensitive, but not mechano-insensitive, nociceptors. (2) Histamine released from mast cells is not responsible for all effects of ET1 on C-nociceptors. (3) ET1 could have a differential role in pain compared to other chemical algogens which activate additionally or even predominantly mechano-insensitive fibers.

Representation of UV-B-induced Thermal and Mechanical Hyperalgesia in the Human Brain: a Functional MRI Study

Human Brain Mapping. Dec, 2008  |  Pubmed ID: 17948883

Surrogate models of pain and hyperalgesia allow the investigation of underlying mechanisms in healthy volunteers. Here, we investigated brain activation patterns during mechanical and heat hyperalgesia in an inflammatory human pain model using functional magnetic resonance imaging. Heat and mechanical hyperalgesia were induced on the right forearm by UV-B application in 14 healthy subjects. All four conditions (nonsensitized heat and nonsensitized mechanical pain, sensitized heat and sensitized mechanical pain) were perceptually matched. A 2 x 2 factorial analysis was performed. Areas with main effect of sensitization were insula, anterior cingulate cortex (ACC), prefrontal cortices (PFC), parietal association cortices (PA), thalamus, and basal ganglia. A main effect of modality with more activation during heat hyperalgesia was found in primary somatosensory cortex (S1), ACC, PFC, and PA. A main effect of modality with more activation during mechanical hyperalgesia was found in secondary somatosensory cortices, posterior insula, and contralateral inferior frontal cortex (IFC). An interaction of sensitization and modality was found bilaterally in IFC. Areas with similar effects of sensitization in both stimulus modalities were ACC, bilateral anterior insula and bilateral IFC. We conclude that different types of hyperalgesia in a human surrogate model of inflammatory pain produce different brain activation patterns. This is partly due to a differential processing of thermal and mechanical pain and an interaction of sensitization and modality in the caudal portion of the IFC. Finally, the data provide evidence for the existence of a common "sensitization network" consisting of ACC, bilateral anterior insula, and parts of the IFC.

A Subpopulation of Capsaicin-sensitive Porcine Dorsal Root Ganglion Neurons is Lacking Hyperpolarization-activated Cyclic Nucleotide-gated Channels

European Journal of Pain (London, England). Aug, 2008  |  Pubmed ID: 18218331

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels contribute to stabilizing resting membrane potential, thus controlling neuron excitability. Subclasses of nociceptive neurons differ in their excitability, therefore, these channels could be a distinguishing marker. We investigated isolated dorsal root ganglion neurons from a non-rodent species, the pig, Sus scrofa domesticus. Single labeling revealed capsaicin-induced cobalt-uptake in 54.3% and transient receptor potential V1 (TRPV1) immunoreactivity in 55.1% of all neurons. Ruthenium red and capsazepine suppressed capsaicin-induced cobalt-uptake. HCN-1 and HCN-2 channel isoform immunoreactivity was detected in 82.6% and 88.3%, respectively, and binding of IB4 in 29.4% of all neurons. Double labeling revealed that out of the capsaicin-positive neurons, 42.3% were IB4-positive, 80.0% immunoreactive for the HCN-1, and 77.3% for the HCN-2 channel isoform, respectively. Neurons lacking HCN-1 or HCN-2 channel isoforms were mostly capsaicin-positive and IB4-negative. The soma size of neurons lacking HCN-1 and/or HCN-2 channels was small to medium. Western blot analysis showed protein products of sizes similar to those of HCN-1 and HCN-2 channel isoforms. Functionally, in patch-clamp experiments, some neurons were unresponsive to membrane hyperpolarization, thus, probably lacking HCN channels. In conclusion, in porcine dorsal root ganglion neurons there is a subset of capsaicin-positive, IB4-negative neurons lacking HCN-1 and/or HCN-2 channel isoforms.

Neuropeptides, Neurogenic Inflammation and Complex Regional Pain Syndrome (CRPS)

Neuroscience Letters. Jun, 2008  |  Pubmed ID: 18423863

This review explains symptoms and nature of neuropeptide signaling and its importance for clinical symptoms of CRPS. Neurogenic inflammation regularly accompanies excitation of primary afferent nociceptors. It has two major components-plasma extravasation and vasodilatation. The most important mediators are the calcitonin gene-related peptide (CGRP) and substance P (SP). After peripheral trauma immune reaction (e.g. cytokines) and the attempts of the tissue to regenerate (e.g. growth factors) sensitize nociceptors and amplify neurogenic inflammation. This cascade of events has been demonstrated in rat models of CRPS. Clinical findings in these animals strongly resemble clinical findings in CRPS, and can be prevented by anti-cytokine and anti-neuropeptide treatment. In CRPS patients, there is meanwhile also plenty of evidence that neurogenic inflammation contributes to clinical presentation. Increased cytokine production was demonstrated, as well as facilitated neurogenic inflammation. Very recently even "non-inflammatory" signs of CRPS (hyperhidrosis, cold skin) have been linked to neuropeptide signaling. Surprisingly, there was even moderately increased neurogenic inflammation in unaffected body regions. This favors the possibility that CRPS patients share genetic similarities. The future search for genetic commonalities will help us to further unravel the "mystery" CRPS.

Role of TRPM8 and TRPA1 for Cold Allodynia in Patients with Cold Injury

Pain. Sep, 2008  |  Pubmed ID: 18440147

Local cold injury often induces hypersensitivity to cold and cold allodynia. Sensitisation of TRPM8 or TRPA1 could be the underlying mechanisms. This was evaluated by psychophysics and axon-reflex-flare induction following topical menthol and cinnamaldehyde application in cold injury patients and healthy subjects. The patients had no signs of neuropathy except cold allodynia. We applied 20% cinnamaldehyde and 40% menthol solutions in the cold-allodynic area of the patients and in a corresponding area in healthy subjects and obtained sensory ratings during application. Thermotesting and Laser Doppler Imaging were performed before and after exposure to the compounds. Menthol did not induce axon-reflex-erythema in patients or in controls. After menthol cold pain threshold was decreased in healthy subjects; however, no further sensitisation was observed in the patients moreover in some patients an amelioration of their cold allodynia was observed. Cinnamaldehyde-induced pain sensation did not differ between patients and controls. Heat pain thresholds following cinnamaldehyde were lowered to a similar extent in patients and controls (43-39.8 and 44-39 degrees C) and also the axon-reflex-flare responses were comparable. No evidence for sensitisation of responses to TRPM8 or TRPA1-stimulation was found in patients with cold injury-induced cold allodynia. The lack of TRPM8 induced axon-reflex indicates that also de-novo expression of TRPM8 on mechano-insensitive C-nociceptors does not underlie cold allodynia in these patients. We conclude from these data that the mechanisms for the induction of cold allodynia in the patients with cold injury are independent of TRPM8 or TRPA1 and differ therefore from neuropathic pain patients.

Separate Peripheral Pathways for Pruritus in Man

Journal of Neurophysiology. Oct, 2008  |  Pubmed ID: 18562548

Recent findings suggest that itch produced by intradermal insertion of cowhage spicules in human is histamine independent. Neuronal mechanisms underlying nonhistaminergic itch are poorly understood. To investigate which nerve fibers mediate cowhage induced itch in man, action potentials were recorded from cutaneous C-fibers of the peroneal nerve in healthy volunteers using microneurography. Mechano-responsive and -insensitive C-nociceptors were tested for their responsiveness to cowhage spicules, histamine, and capsaicin. Cowhage spicules induced itching and activated all tested mechano-responsive C-units (24/24), but no mechano-insensitive C-fibers (0/17). Histamine also induced itch, but in contrast to cowhage, it caused lasting activation only in mechano-insensitive units (8/12). In mechano-responsive C-units, histamine caused no or only short and weak responses unrelated to the time course of itching. Capsaicin injections activated four of six mechano-responsive fibers and three of four mechano-insensitive C-fibers. Cowhage and histamine activate distinctly different nonoverlapping populations of C-fibers while inducing similar sensations of itch. We hypothesize that cowhage activates a pathway for itch that originates peripherally from superficial mechano-responsive (polymodal) C-fibers and perhaps other afferent units. It is distinct from the pathway for histamine-mediated pruritus and does not involve the histamine-sensitive mechano-insensitive fibers.

Painful and Nonpainful Phantom and Stump Sensations in Acute Traumatic Amputees

The Journal of Trauma. Oct, 2008  |  Pubmed ID: 18849803

The formation, prevalence, intensity, course, and predisposing factors of phantom limb pain were investigated to determine possible mechanisms of the origin of phantom limb pain in traumatic upper limb amputees.

In Situ Profiling of Adipokines in Subcutaneous Microdialysates from Lean and Obese Individuals

American Journal of Physiology. Endocrinology and Metabolism. Nov, 2008  |  Pubmed ID: 18780773

Adipose tissue (AT) had emerged as an endocrine organ and a key regulator of the metabolically triggered inflammation. The aims of this study were 1) to investigate the usefulness of a multiplexed bioassay in characterizing a panel of adipokines in subcutaneous (sc) microdialysate samples and 2) to determine whether lean and obese individuals differ in their interstitial adipokines levels following microdialysis (MD) probe insertion. Ultrafiltrating MD membranes were inserted in opposite sites of the sc abdominal AT of six lean (L) and six obese (OB) males at the beginning (M1) and during the last 120 min (M2) of the study. Interstitial and serum concentrations of adipokines were quantified using the Luminex technique and ELISA at 60-min intervals for 5 h. In comparison with L subjects, OB subjects exhibited elevated interstitial leptin (P < 0.001), IL-8 (P < 0.05), and IL-18 levels (P = 0.05), as well as higher serum concentrations of leptin (P < 0.0001), IL-6 (P < 0.0001), tumor necrosis factor-alpha (P < 0.001), IL-8 (P = 0.01) and interferon-gamma-inducible protein 10 (P < 0.05). In samples from the M1 membranes, leptin decreased and IL-1alpha, IL-18, and RANTES (regulated on activation, normal T-cell expressed and secreted) remained relatively stable, whereas IL-6, IL-8, and monocyte chemoattractant protein-1 significantly increased after the first hour (P < 0.0001 vs. baseline). Notably, either the magnitude of increase from the initial values or the time pattern of all the adipokines in M1 and M2 dialysates were similar between the groups. In conclusion, the current work provides valuable information on the optimal time frame to collect in situ AT microdialysate samples. Further studies are needed, however, to unravel the intricate interplay of cytokines in AT interstitial fluid.

Acute Hyperinsulinemia Differentially Regulates Interstitial and Circulating Adiponectin Oligomeric Pattern in Lean and Insulin-resistant, Obese Individuals

The Journal of Clinical Endocrinology and Metabolism. Nov, 2009  |  Pubmed ID: 19820029

Hyperinsulinemia emerges as a negative modulator of the circulating high-molecular-weight adiponectin multimers.

Neurotrophins in the Cerebrospinal Fluid of Patient Cohorts with Neuropathic Pain, Nociceptive Pain, or Normal Pressure Hydrocephalus

The Clinical Journal of Pain. Oct, 2009  |  Pubmed ID: 19920725

The pathophysiology of neuropathic pain is still poorly understood. Studies in experimental animals showed that neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), and nerve growth factor (NGF) might be involved in the pathophysiology of neuropathic pain. On the basis of these findings it is conceivable that neurotrophic factors also play a role in pain processing in man. Thus far, it remains unknown whether neurotrophic factors are altered in the cerebrospinal fluid (CSF) of patients with different pain syndromes. Here, we analyzed the concentrations of neurotrophic factors in the CSF of patients with chronic neuropathic pain in comparison to controls with nociceptive pain or hydrocephalus.

Pain: Itch Without Pain-a Labeled Line for Itch Sensation?

Nature Reviews. Neurology. Dec, 2009  |  Pubmed ID: 19953111

Differential Endogenous Pain Modulation in Complex-regional Pain Syndrome

Brain : a Journal of Neurology. Mar, 2009  |  Pubmed ID: 19153154

Endogenous pain modulation may provide facilitation or inhibition of nociceptive input by three main mechanisms. Firstly, modification of synaptic strength in the spinal dorsal horn may increase or decrease transmission of nociceptive signals to the brain. Secondly, local dorsal horn interneurons provide both feed-forward and feed-back modulation to spinothalamic and spinobulbar projection neurons. Thirdly, descending systems originating in the brainstem exert top-down modulation of nociceptive input at the spinal level. Not much is known on the activity of these systems in complex regional pain syndrome (CRPS). CRPS is a chronic pain condition characterized by burning pain and abnormalities in the sensory, motor and autonomous nervous system. In the present study, we tested changes in endogenous pain modulation in 27 CRPS patients compared with age-matched healthy controls. We applied repetitive noxious electrical stimuli (stimulation frequency 1 Hz) at the dorsal aspect of affected and unaffected hands in patients and to corresponding hands in controls. As known from previous studies this protocol simultaneously activates inhibitory and facilitatory pain modulating systems. This results in adaptation to the repetitive noxious stimulus, and simultaneously and at the same site, in development of an area of pinprick hyperalgesia. We measured (i) pain adaptation during the course of stimulation and (ii) the provoked area of pinprick hyperalgesia. These parameters were used as activity measures of pain inhibitory and pain facilitatory systems. As both measures result from gross inhibitory and gross facilitatory activity in pain modulatory systems, pain adaptation reflects net pain inhibition and area of pinprick hyperalgesia net pain facilitation. We found (i) decreased adaptation to painful electrical stimuli on both affected and unaffected hands of CRPS patients compared to healthy controls and (ii) increased areas of hyperalgesia on affected hands of CRPS patients compared to unaffected hands of CRPS patients and healthy controls. These findings imply a shift from inhibition towards facilitation of nociceptive input in CRPS patients, based on differential activation of subcomponents of the endogenous pain modulatory system. The differences were not correlated with duration of the disease, pain intensity, autonomic or motor function scores, presence or degree of evoked pain. However, significant correlation was found with the extent of adaptation and hyperalgesia on the unaffected hand. Thus, we hypothesize that differential activity in endogenous pain modulating systems may be not only a result of CRPS, but a potential risk factor for its development.

Neural Fractalkine Expression is Closely Linked to Pain and Pancreatic Neuritis in Human Chronic Pancreatitis

Laboratory Investigation; a Journal of Technical Methods and Pathology. Mar, 2009  |  Pubmed ID: 19153557

The chemokine fractalkine induces migration of inflammatory cells into inflamed tissues, thereby aggravating inflammatory tissue damage and fibrosis. Furthermore, fractalkine increases neuropathic pain through glial activation, which can be diminished by blocking of its receptor, CX3CR1, through neutralizing antibodies. As chronic pancreatitis (CP) is characterized by tissue infiltration of inflammatory cells, fibrosis, pancreatic neuritis and severe pain, the roles of fractalkine and CX3CR1 were investigated in CP (n=61) and normal pancreas (NP, n=21) by QRT-PCR, western blot and immunohistochemistry analyses. Their expression correlated with the severity of pancreatic neuritis, fibrosis, intrapancreatic nerve fiber density and hypertrophy, pain, CP duration and with the amount of inflammatory cell infiltrate immuno-positive for CD45 and CD68. To investigate the influence of fractalkine on pancreatic fibrogenesis, human pancreatic stellate cells (hPSCs) were isolated from patients with CP, incubated with fractalkine and then Collagen-1 and alpha-smooth muscle actin (alpha-SMA) expressions were measured. CX3CR1, but not fractalkine, mRNA was overexpressed in CP. In contrast, the protein levels of both CX3CR1 and fractalkine were upregulated. Neuro-immunoreactivity for fractalkine and CX3CR1 was strongest in patients suffering from severe pain and pancreatic neuritis. Long-term suffering from CP was noticeably related to increased neural immunoreactivity of fractalkine. Furthermore, fractalkine and CX3CR1 mRNA overexpressions were associated with enhanced lymphocyte and macrophage infiltration. Advanced fibrosis was associated with increased fractalkine expression, whereas in vitro fractalkine had no significant impact on collagen-1 and alpha-SMA expressions in hPSCs. Therefore, pancreatic fractalkine expression appears to be linked to visceral pain and to the recruitment of inflammatory cells into the pancreatic tissue and nerve fibers, with subsequent pancreatic neuritis. However, pancreatic fibrogenesis is probably indirectly influenced by fractalkine. Taken together, these novel findings suggest that CX3CR1 represents a potential novel therapeutic target to reduce inflammation and modulate pain in CP.

Translating Nociceptive Processing into Human Pain Models

Experimental Brain Research. Experimentelle Hirnforschung. Expérimentation Cérébrale. Jun, 2009  |  Pubmed ID: 19404625

As volunteers can easily communicate quality and intensity of painful stimuli, human pain models appear to be ideally suited to test analgesic compounds, but also to study pain mechanisms. Acute stimulation of nociceptors under physiologic conditions has proven not to be of particular use as an experimental pain model. In contrast, if the experimental models include sensitization of the peripheral or central pain processing they may indeed mimic certain aspects of chronic pain conditions. Peripheral inflammatory conditions can be induced experimentally with sensitization patterns correlating to clinical inflammatory pain. There are also well-characterized models of central sensitization, which mimic aspects of neuropathic pain patients such as touch evoked allodynia and punctate hyperalgesia. The main complaint of chronic pain patients, however, is spontaneous pain, but currently there is no human model available that would mimic chronic inflammatory or neuropathic pain. Thus, although being helpful for proof of concept studies and dose finding, current human pain models cannot replace patient studies for testing efficacy of analgesic compounds.

Post-junctional Facilitation of Substance P Signaling in a Tibia Fracture Rat Model of Complex Regional Pain Syndrome Type I

Pain. Aug, 2009  |  Pubmed ID: 19464118

Tibia fracture in rats evokes nociceptive, vascular, and bone changes resembling complex regional pain syndrome (CRPS). Substance P (SP) signaling contributes to the hindpaw warmth, increased vascular permeability, and edema observed in this model, suggesting that neurogenic inflammatory responses could be enhanced after fracture. Four weeks after tibia fracture we measured SP and calcitonin gene-related peptide (CGRP) protein levels in the sciatic nerve and serum. Hindpaw skin extravasation responses and SP receptor (NK1), CGRP receptor (calcitonin receptor-like receptor, CRLR) and neutral endopeptidase (NEP) protein levels were also determined. Gene expression levels of these peptides, receptors, and peptidase were examined in the DRG and skin. Spontaneous and intravenous SP-evoked extravasation responses were increased ipsilateral, but not contralateral to the fracture. Fracture increased SP and CGRP gene expression in the ipsilateral L4,L5 DRG and neuropeptide protein levels in the sciatic nerve and in serum, but had no effect on electrically evoked SP and CGRP release. NK1 receptor expression was increased in the ipsilateral hindpaw skin keratinocytes and endothelial cells after injury, but CRLR and NEP expression were unchanged. Fracture also increased epidermal thickness, but had no effect on epidermal skin neurite counts. These results demonstrate that spontaneous and intravenous SP-evoked extravasation responses are enhanced in the ipsilateral hindlimb after fracture and that fracture chronically increases the expression of endothelial and keratinocyte NK1 receptors in the injured limb. We postulate that SP activation of these up-regulated NK1 receptors results in skin warmth, protein leakage, edema, and keratinocyte proliferation in the injured limb.

How Pain Becomes Itch

Pain. Jul, 2009  |  Pubmed ID: 19467787

Predominant CB2 Receptor Expression in Endothelial Cells of Glioblastoma in Humans

Brain Research Bulletin. Jun, 2009  |  Pubmed ID: 19480992

The most abundant malignant brain tumor in human is glioblastoma and patients with this type of tumor have a poor prognosis with high mortality. Glioblastoma are characterized particularly by fast growth and a dependence on blood vessel formation for survival. Cannabinoids (CBs) inhibit tumor growth by inducing apoptosis of tumor cells and impairing tumor angiogenesis. The distribution of CB1 and CB2 receptors in glioblastoma and associated endothelial vessels is still unknown.

Hematopoietic Colony-stimulating Factors Mediate Tumor-nerve Interactions and Bone Cancer Pain

Nature Medicine. Jul, 2009  |  Pubmed ID: 19525966

Pain is one of the most severe and debilitating symptoms associated with several forms of cancer. Various types of carcinomas and sarcomas metastasize to skeletal bones and cause spontaneous bone pain and hyperalgesia, which is accompanied by bone degradation and remodeling of peripheral nerves. Despite recent advances, the molecular mechanisms underlying the development and maintenance of cancer-evoked pain are not well understood. Several types of non-hematopoietic tumors secrete hematopoietic colony-stimulating factors that act on myeloid cells and tumor cells. Here we report that receptors and signaling mediators of granulocyte- and granulocyte-macrophage colony-stimulating factors (G-CSF and GM-CSF) are also functionally expressed on sensory nerves. GM-CSF sensitized nerves to mechanical stimuli in vitro and in vivo, potentiated CGRP release and caused sprouting of sensory nerve endings in the skin. Interruption of G-CSF and GM-CSF signaling in vivo led to reduced tumor growth and nerve remodeling, and abrogated bone cancer pain. The key significance of GM-CSF signaling in sensory neurons was revealed by an attenuation of tumor-evoked pain following a sensory nerve-specific knockdown of GM-CSF receptors. These results show that G-CSF and GM-CSF are important in tumor-nerve interactions and suggest that their receptors on primary afferent nerve fibers constitute potential therapeutic targets in cancer pain.

Long-acting Local Anesthetics Attenuate FMLP-induced Acute Lung Injury in Rats

Anesthesia and Analgesia. Sep, 2009  |  Pubmed ID: 19690261

Endothelin-1 (ET-1) is a mediator of lung diseases and a potent pulmonary vasoconstrictor. In addition to thromboxane A2, it participates in the formation of lung edema. Both lidocaine and mepivacaine attenuate the increase of pulmonary arterial pressure (PAP) and lung edema development. We examined the effects of procaine, bupivacaine, and ropivacaine on experimentally evoked PAP increase and ET-1 release.

Facilitated Neurotrophin Release in Sensitized Human Skin

European Journal of Pain (London, England). Apr, 2009  |  Pubmed ID: 18571954

The neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) play a pivotal role in the generation and maintenance of hyperalgesia. In the present study we analyzed NGF and BDNF levels in human skin of the upper arm and axilla skin sites by dermal microdialysis and multiplexed assay. Skin sensitization and inflammatory responses were evoked experimentally by repetitive shaving of one axilla provoking local erythema and reduced heat pain thresholds. Acute excitation of skin nociceptors was performed by perfusion of the microdialysis catheters with citric acid pH 3. At baseline, neurotrophin concentrations did not differ significantly between the investigated skin sites. On average, NGF concentration was 5.8 fg/microug protein/ml sample volume and BDNF concentration was 87.5 fg/microg/ml. Citric acid perfusion marginally increased NGF levels to 7.3 fg/microg/ml on average in the upper arm and control axilla. Similarly, BDNF values increased at these control skin sites to about 122 fg/microg/ml following proton stimulation. In contrast, perfusion of the inflamed axilla with citric acid significantly enhanced the release of both NGF and BDNF. On average, NGF levels were analyzed at 14.6 fg/microg/ml and BDNF values at 202 fg/microg/ml. These data demonstrate enhanced level of neurotrophin release in inflamed human skin in vivo which might well contribute to peripheral sensitization. Analyses of neurotrophic factors by dermal microdialysis are useful endogenous markers to further explore their role in neuronal sensitization processes in human.

Single-fiber Recordings of Unmyelinated Afferents in Pig

Neuroscience Letters. Feb, 2010  |  Pubmed ID: 19818831

Microneurography has provided valuable data on single-fiber characteristics in healthy volunteers and patients, featuring a unique setting that allows linking discharge of single fibers to percept. This advantage is of particular value, when pain patients are examined. Latest results on specific changes of axonal excitability differentially expressed in various C-fiber classes have incited studies linking axonal excitability changes to the mechanism of chronic pain. These studies are mainly based on investigating patients rather than establishing animal models and have led to specific questions on the molecular mechanisms underlying axonal excitability changes. However, pharmacological interventions are limited in human microneurography and therefore an animal model is required. Ideally, the distribution of fiber classes and their sensory and axonal characteristics should be comparable to human. Here we report on corresponding C-fiber classes between human and pig. Nociceptive and non-nociceptive fiber classes found in pig correlate with human fiber classes, in both distribution and axonal excitability changes. It is suggested that the pig is an attractive model for studying modulation of excitability in nociceptive and non-nociceptive C-fiber classes that correspond to those in man.

NGF Induces Non-inflammatory Localized and Lasting Mechanical and Thermal Hypersensitivity in Human Skin

Pain. Mar, 2010  |  Pubmed ID: 20022698

Nerve growth factor (NGF) modulates sensitivity and sprouting of nociceptors. We explored the spatial and temporal sensitization induced by NGF injection (1 microg) in human skin. Hyperalgesia was investigated in 16 volunteers (36+/-9 years) at day 1, 3, 7, 21, and 49. Areas of mechanical (brush, pin-prick) and heat (43 degrees C) sensitization were mapped and thermal (heat and cold) pain thresholds, mechanical (impact stimulation) and electrically evoked pain, and axon reflex flare were assessed. No spontaneous pain or local inflammation was recorded upon NGF injection and during 49 days. Sensitization to heat was maximum at day 3 and lasted 21 days. Hyperalgesia to cold was recorded at day 7 and 21. Hypersensitivity to mechanical impact stimuli developed delayed, reached maximum at day 21, and persisted throughout 49 days. Fifty percent of all volunteers reported a static allodynia to tonic pressure until day 21. Electrical stimulation at 7.5 mA was more painful at the NGF site at day 21, which correlated significantly to maximum impact pain. Axon reflex flare was unaffected by NGF. Sensitization was limited to the NGF injection site, no touch- or pin-prick evoked secondary hyperalgesia was observed. Spatially restricted hyperalgesia indicates a peripheral rather than central mechanism. The temporal profile of lasting nociceptor sensitization suggests an altered peripheral axonal expression of sensory proteins specifically leading to mechanical and thermal sensitization. Intradermal NGF administration provokes a pattern of sensitization that can be used as experimental model for neuropathic pain.

Nerve Growth Factor-evoked Nociceptor Sensitization in Pig Skin in Vivo

Journal of Neuroscience Research. Jul, 2010  |  Pubmed ID: 20143422

Peripheral sensitization of skin nociceptors by nerve growth factor (NGF) was explored in pig skin in vivo. As an objective output measure, the area of axon-reflex-mediated erythema was assessed upon mechanical, thermal, chemical, and electrical stimuli delivered at 1, 3, and 7 days after i.d. injection of 1 microg NGF into the pig's back skin (n = 8). Pretreatment with NGF provoked a sensitization to mechanical (600 mN), thermal (10 sec 49 degrees C) and chemical (15 microl, pH 3) stimuli that lasted for 7 days. No sensitization, however, was found in response to weak mechanical (100 mN), weak thermal (10 sec 45 degrees C), or electrical stimuli. Irrespective of the skin pretreatment (NGF or PBS vehicle control), the area of electrically induced erythema decreased upon repetition (days 1-7) by 70% (P < 0.05). Sensitization of sensory endings by NGF upon mechanical, heat, and chemical stimuli suggests recruitment of sensory transducer molecules [e.g., TRPV1, acid-sensing ion channels (ASICs)]. In contrast, the gradual decrease in electrically induced erythema over 7 days might be attributable to axonal desensitization and possibly activity-dependent down-regulation of sodium channels. Thus, long-lasting sensitization processes of nociceptor endings or axonal sodium channel desensitization mechanisms can be explored in the pig as a translational experimental animal model.

Thoracoscopic Sympathectomy at the T2 or T3 Level Facilitates Bradykinin-induced Protein Extravasation in Human Forearm Skin

Pain Medicine (Malden, Mass.). May, 2010  |  Pubmed ID: 20202140

The endogenous peptide bradykinin (BK) is an inflammatory mediator that induces nociceptor activation and sensitization as well as protein extravasation and vasodilation.

Itch and Pain

Neuroscience and Biobehavioral Reviews. Feb, 2010  |  Pubmed ID: 19146873

Decades of pain research have succeeded in elucidating complex mechanisms of acute activation and chronic sensitization of nociceptors leading to pain. In contrast, itch conditions have received less attention and even basic mechanisms for the induction of itch are still unclear. In this review we describe itch-specific pathways, but also evidence for a modified pattern theory of pruritus offering independent mechanisms for the itch induction. Traditionally pain and itch have been regarded as antagonistic as painful stimuli such as scratching suppress itch and opioids suppress pain, but generate itch. However, concerning mechanisms of sensitization to itch or pain, surprisingly similar patterns have been observed lately in both inflamed tissue and in the spinal cord. These similarities open up two highly interesting perspectives: the role of well established analgesic therapeutic concepts can be validated in chronic itch conditions and on the other hand investigations of sensitization in easily accessible pruritic skin may help to validate concepts of nociception in humans. These perspectives illustrate that itch and pain research no longer follows separate paths, but can be advantageously interconnected.

Patterns of Activity-dependent Conduction Velocity Changes Differentiate Classes of Unmyelinated Mechano-insensitive Afferents Including Cold Nociceptors, in Pig and in Human

Pain. Jan, 2010  |  Pubmed ID: 19913997

Activity-dependent slowing of conduction velocity (ADS) differs between classes of human nociceptors. These differences likely reflect particular expression and use-dependent slow inactivation of axonal ion channels and other mechanisms governing axonal excitability. In this study, we compared ADS of porcine and human cutaneous C-fibers. Extracellular recordings were performed from peripheral nerves, using teased fiber technique in pigs and microneurography in humans. We assessed electrically-induced conduction changes and responsiveness to natural stimuli. In both species, the group of mechano-insensitive C-fibers showed the largest conduction slowing ( approximately 30%) upon electrical stimulation (2Hz for 3min). In addition, we found mechano-insensitive cold nociceptors in pig that slowed only minimally (<10% at 2Hz), and a similar slowing pattern was found in some human C-fibers. Mechano-sensitive afferents showed an intermediate conduction slowing upon 2Hz stimulation (pig: 14%, human 23%), whereas sympathetic efferent fibers in pig and human slowed only minimally (5% and 9%, respectively). In fiber classes with more pronounced slowing, conduction latencies recovered slower; i.e. mechano-insensitive afferents recovered the slowest, followed by mechano-sensitive afferents whereas cold nociceptors and sympathetic efferents recovered the fastest. We conclude that mechano-insensitive C-fiber nociceptors can be differentiated by their characteristic pattern of ADS which are alike in pig and human. Notably, cold nociceptors with a distinct ADS pattern were first detected in pig. Our results therefore suggest that the pig is a suitable model to study nociceptor class-specific changes of ADS.

Pituitary Adenylate Cyclase Activating Polypeptide: an Important Vascular Regulator in Human Skin in Vivo

The American Journal of Pathology. Nov, 2010  |  Pubmed ID: 20889562

Pituitary adenylate cyclase-activating peptide (PACAP) is an important neuropeptide and immunomodulator in various tissues. Although this peptide and its receptors (ie, VPAC1R, VPAC2R, and PAC1R) are expressed in human skin, their biological roles are unknown. Therefore, we tested whether PACAP regulates vascular responses in human skin in vivo. When injected intravenously, PACAP induced a significant, concentration-dependent vascular response (ie, flush, erythema, edema) and mediated a significant and concentration-dependent increase in intrarectal body temperature that peaked at 2.7°C. Topical application of PACAP induced marked concentration-dependent edema. Immunohistochemistry revealed a close association of PACAP-immunoreactive nerve fibers with mast cells and dermal blood vessels. VPAC1R was expressed by dermal endothelial cells, CD4+ and CD8+ T cells, mast cells, and keratinocytes, whereas VPAC2R was expressed only in keratinocytes. VPAC1R protein and mRNA were also detected in human dermal microvascular endothelial cells. The PACAP-induced change in cAMP production in these cells demonstrated VPAC1R to be functional. PACAP treatment of organ-cultured human skin strongly increased the number of CD31+ vessel cross-sections. Taken together, these results suggest that PACAP directly induces vascular responses that may be associated with neurogenic inflammation, indicating for the first time that PACAP may be a crucial vascular regulator in human skin in vivo. Antagonists to PACAP function may be beneficial for the treatment of inflammatory skin diseases with a neurogenic component.

Electrically Induced Quantitative Sudomotor Axon Reflex Test in Human Volunteers

Autonomic Neuroscience : Basic & Clinical. Jan, 2011  |  Pubmed ID: 20923721

Chemically-induced quantitative sudomotor axon reflex test (QSART) and quantitative sensory testing (QST) are established clinical tools to assess thin fiber function in humans. We investigated stimulus-response functions to transcutaneous electrical stimuli of different current intensity (3.75 to 10mA) and pulse frequency (5 to 100Hz) comparing sweat output (ml/h/m(2)) and pain intensity (numeric rating scale [NRS], 0-10). Efferent sudomotor and afferent nociceptive responses were recorded after a 30s electrical stimulation period of distal (hand and foot) and proximal (forearm and thorax) body sites with 3 repetitive measures per body site. Sweat responses increased intensity dependently and peaked (~100ml/h/m(2)) at highest currents (10mA) that had been administered. Similarly, pain ratings increased with an escalating current intensity. At a constant stimulus intensity of 7.5mA, sudomotor activity was highest (~75ml/h/m(2)) at a stimulus frequency of 20Hz without further increase at 50 or 100Hz. In contrast, pain ratings increased frequency dependently and reached NRS 7 at 100Hz. Sudomotor activity, but not pain ratings, was significantly different between the body sites (p<0.05, ANOVA) with maximum sweat responses obtained at the ventral forearm. Varying response patterns for higher stimulation frequencies between sweating (peak maximum at 20Hz) and pain (maximum at 100Hz) might indicate differential axonal properties of sympathetic efferent and nociceptive afferent fibers. Electrically induced QSART could be a useful explorative and clinical method to indirectly study characteristics of frequency-dependent axonal excitability changes of sudomotor fibers.

Pathogenesis of Pruritus

Journal Der Deutschen Dermatologischen Gesellschaft = Journal of the German Society of Dermatology : JDDG. Jun, 2011  |  Pubmed ID: 21208378

The pathogenesis of acute and chronic (> 6 weeks duration) pruritus is complex and involves in the skin a network of resident cells (e. g., mast cells, keratinocytes, sensory neurons) and transient inflammatory cells (e. g., eosinophils). Though pruritus and pain show overlapping mechanisms, recent studies have provided evidence that pruritus and pain pathogenesis differ in important points. In the skin, the sensory C-nerve fibers have been investigated intensively. Several classes of histamine-sensitive or histamine-insensitve C-fibers have been described. Epidermal and dermal sensory nerve fibres are now assumed to be of major importance in pruritus induction. They interact with keratinocytes, inflammatory cells such as T lymphocytes, eosinophils and basophils which have been shown to release multiple pruritogenic mediators (e.g., nerve growth factor, interleukin-31) which lead to activation, sensitization and sprouting of skin nerves. Specific receptors have been discovered on cutaneous and spinal neurons to be exclusively involved in the processing of pruritic signals. Just recently, the gastrin-releasing peptide receptor (GRPR) was identified on spinal neurons that are crucially involved in pruritus but not pain processing. Chronic pruritus is notoriously difficult to treat. Newer insights into the underlying pathogenesis of pruritus have enabled novel treatment approaches that target the pruritus-specific pathophysiological mechanism. For example, kappa-opioid receptor agonists and neurokinin-1 antagonists have been found to relieve chronic pruritus.

Differential Central Pain Processing Following Repetitive Intramuscular Proton/prostaglandin E₂ Injections in Female Fibromyalgia Patients and Healthy Controls

European Journal of Pain (London, England). Aug, 2011  |  Pubmed ID: 21277243

While the etiology of fibromyalgia syndrome (FMS) remains unclear, it is assumed that both peripheral and central components are involved.

Chimpanzees Know That Others Make Inferences

Proceedings of the National Academy of Sciences of the United States of America. Feb, 2011  |  Pubmed ID: 21282649

If chimpanzees are faced with two opaque boards on a table, in the context of searching for a single piece of food, they do not choose the board lying flat (because if food was under there it would not be lying flat) but, rather, they choose the slanted one- presumably inferring that some unperceived food underneath is causing the slant. Here we demonstrate that chimpanzees know that other chimpanzees in the same situation will make a similar inference. In a back-and-forth foraging game, when their competitor had chosen before them, chimpanzees tended to avoid the slanted board on the assumption that the competitor had already chosen it. Chimpanzees can determine the inferences that a conspecific is likely to make and then adjust their competitive strategies accordingly.

Effects of COX Inhibition on Experimental Pain and Hyperalgesia During and After Remifentanil Infusion in Humans

Pain. Jun, 2011  |  Pubmed ID: 21396775

Opioids may enhance pain sensitivity resulting in opioid-induced hyperalgesia (OIH). Activation of spinal cyclooxygenase may play a role in the development of OIH. The aim of this study was to demonstrate remifentanil-induced postinfusion hyperalgesia in an electrical pain and a cold pain model, and to investigate whether COX-2 (parecoxib) or COX-1 (ketorolac) inhibition could prevent hyperalgesia after remifentanil infusion. Sixteen healthy males were enrolled in this randomized, double-blind, placebo-controlled crossover study. Each subject went through 4 sessions: control, remifentanil, parecoxib+remifentanil, and ketorolac+remifentanil. Transcutaneous electrical stimulation induced acute pain and areas of pinprick hyperalgesia. The areas of pinprick hyperalgesia were assessed before, during, and after a 30-minute infusion of either remifentanil or saline. The cold-pressor test (CPT) was performed before, at the end of, and 1 hour after the infusions. The subjects received a bolus of either saline, 40 mg parecoxib, or 30 mg ketorolac intravenously after the first CPT. The areas of pinprick hyperalgesia and CPT pain after the end of remifentanil infusion increased significantly compared to control (P < 0.001 and P = 0.005, respectively). Pretreatment with parecoxib or ketorolac reduced the postinfusion area of pinprick hyperalgesia (P < 0.001 and P = 0.001, respectively), compared to the remifentanil group. Parecoxib reduced the area significantly more than ketorolac (P = 0.009). In the CPT, pretreatment with parecoxib or ketorolac did not prevent postinfusion hyperalgesia. These results demonstrated OIH in both models, and may suggest that COX-2 inhibition is more important than COX-1 inhibition in reducing hyperalgesia. Remifentanil-induced hyperalgesia was demonstrated for both electrically induced pain and cold-pressor pain. Both parecoxib and ketorolac prevented hyperalgesia in the electrical model, parecoxib to a larger extent.

Time Course of Acetylcholine-induced Activation of Sympathetic Efferents Matches Axon Reflex Sweating in Humans

Journal of the Peripheral Nervous System : JPNS. Mar, 2011  |  Pubmed ID: 21504500

Action potentials from postganglionic C-fibres were recorded in healthy volunteers by microneurography in the peroneal nerve. Their responsiveness to mechanical or heat stimuli or to sympathetic reflex provocation tests was determined by transient slowing of conduction velocity following activation. Twenty units were classified as sympathetic efferent units. Acetylcholine (ACh) iontophoresis (10%, 1 mA, 1 min) inside their innervation territory activated 8 of 20 sympathetic fibres with a mean delay of 61 ± 12 s, peak response at 175 ± 38 s, and a duration of 240 ± 42 s, whereas iontophoresis of saline did not activate any of them. The time course of neuronal activation correlated with the axon reflex sweating measured by an evaporimeter in a separate session (delay 76 ± 9 s, peak at 195 ± 12 s, decline to 50% of peak 312 ± 25 s). No ACh-induced vasoconstriction was observed by laser Doppler scanning (n = 11) even after depletion of neuropeptides by chronic topical capsaicin treatment (n = 8). We conclude that ACh iontophoresis activates about half of the sympathetic fibres in human skin and provokes a corresponding axon reflex sweating. The absence of ACh-induced vasoconstriction even after the depletion of neuropeptides by capsaicin suggests that only sudomotor fibres, but not sympathetic vasoconstrictor fibres are activated by this stimulus.

NGF Enhances Electrically Induced Pain, but Not Axon Reflex Sweating

Pain. Aug, 2011  |  Pubmed ID: 21546161

High-affinity receptors for nerve growth factor (NGF) are found on nociceptors and sympathetic efferents. NGF is known to sensitize nociceptors, increase innervation density, and fire frequency of sympathetic fibers. We explored axonal sensitization of afferent and efferent fibers following intracutaneous injection of NGF in human and pig skin. In humans, frequency-dependent (5, 20, 100 Hz) electrically induced pain was assessed 1, 3, 7, 21, and 49 days post injection. Sweat output was recorded in parallel using the quantitative sudomotor axon reflex test (QSART). Electrically induced pain ratings (7.5 mA for 30 s) significantly increased at the NGF sites for 5 Hz (numeric rating scale [NRS] 6±0.5 vs 3.7±0.4), 20 Hz (NRS 7.2±0.4 vs 5±0.5), and 100 Hz stimulation (NRS 6.9±0.4 vs 5.4±0.3) at day 21, and also for 5 Hz at day 49 (NRS 5.4±0.4 vs 3.8±0.3). Electrically evoked QSART increased frequency dependent, but was not altered by NGF throughout the entire observation period (average QSART at 5 Hz: 3 mL/h/m(2), 20 Hz: 9 mL/h/m(2), 100 Hz: 10 mL/h/m(2)). Similarly, NGF did not change the activity-dependent slowing of conduction of sympathetic efferents (6±2% vs 5.1±1.5%, for 3 minutes, 2 Hz) in pig single-fiber recordings. In parallel to the increased pain ratings recorded in humans, activity-dependent slowing of mechano-insensitive nociceptors was reduced by NGF (18.1±2% vs 29±1.4%). In summary, axonal sensitization of nociceptors by NGF could underlie the hyperalgesia to electrical stimulation. Enhanced responses were limited to nociceptors, as no sensitization was found in sympathetic efferent neurons.

Neuronal Sensitivity of the Skin

European Journal of Dermatology : EJD. May, 2011  |  Pubmed ID: 21628129

The skin is equipped with nerve fibers subserving the senses for touch, temperature, pain and itch. Thickly myelinated Aβ-fibers are linked to low threshold mechano-receptors responsible to detect vibration and slight indentation of the skin. Among the thinly myelinated Aδ-fibers one class is crucial for the cold detection, but there also nociceptive Aδ-fibers for the detection fast rising noxious heat and punctate mechanical stimuli. Unmyelinated C-fibers consist of various classes mediating nociceptive (pain and itch) and non-nociceptive (warmth, pleasant touch) sensations. The unmyelinated C-fibers have close contact to the keratinocytes and the interaction between C-fibers and local skin cells is of particular interest, as efferent neuronal function has both local trophic and immunomodulatory function. In turn, excitability of C-fibers is governed in part by neurotrophins from neighbouring skin cells such as keratinocytes. This mechanism is held responsible to induce and maintain chronic itch and pain conditions. Beyond this interaction recently a direct involvement of keratinocytes in the transduction process has been discussed especially for temperature and noxious stimuli, as keratinocytes express a variety of sensory transduction molecules. Thus, the interplay between neurons and non-neuronal cells is operational not only in basic physiology, but also in chronic itch and pain patients.

Structural and Functional Differences Between Neuropathy with and Without Pain?

Experimental Neurology. Oct, 2011  |  Pubmed ID: 21683699

We aimed to find functional and structural differences in neuropathy between patients with and without chronic pain following nerve injury. We included 30 patients requiring hand surgery after a trauma, with 21 reporting chronic pain for more than one year after the injury, while 9 did not suffer from injury-related chronic pain. We assessed mechanical sensitivity, thermal thresholds, electrically induced pain and axon reflex erythema and cutaneous nerve fiber density in skin biopsies of the injured site and its contralateral control. Epidermal fiber density of the injured site was reduced similarly in both patient groups. Thresholds for cold and heat pain and axon reflex areas were reduced in the injured site, but did not differ between the patient groups. Only warmth thresholds were better preserved in the pain patients (35.2 vs. 38.4°C). Neuronal CGRP staining did not reveal any difference between pain and non-pain patients. Epidermal innervation density correlated best to warmth detection thresholds and deeper dermal innervation density to the area of the axon reflex erythema. No specific pattern of subjective, functional or structural parameters was detected that would separate the neuropathy patients into pain and non-pain patients. Specific staining of additional targets may help to improve our mechanistic understanding of pain development.

Nerve Growth Factor Selectively Decreases Activity-dependent Conduction Slowing in Mechano-insensitive C-nociceptors

Pain. Sep, 2011  |  Pubmed ID: 21763072

Nerve growth factor (NGF) induces acute sensitization of nociceptive sensory endings and long-lasting hyperalgesia. NGF modulation of sodium channel expression might contribute to neurotrophin-induced hyperalgesia. Here, we investigated NGF-evoked changes of the activity-dependent slowing of conduction in porcine C-fibers. Animals received intradermal injections of NGF (2 μg or 8 μg) or saline in both hind limbs. Extracellular recordings from the saphenous nerves were performed 1 week later. Based on sensory thresholds and electrically induced activity-dependent slowing (ADS) of axonal conduction, C-fibers were classified as mechano-sensitive afferents, mechano-insensitive afferents, cold nociceptors, and sympathetic efferents. NGF (2 μg) increased conduction velocity in C-fibers from 1.0±0.05 m/s to 1.2±0.07 m/s. In mechano-insensitive afferents, NGF (8 μg) reduced activity-dependent slowing of conduction, from 5.3±0.2% to 3.2±0.5% (0.125-0.5 Hz stimulation) and from 28.5±1.3% to 20.9±1.9% (2 Hz stimulation), such that ADS no longer differentiated between mechano-sensitive and mechano-insensitive fibers. Accordingly, the number of fibers with pronounced ADS decreased but more units with pronounced ADS were mechano-sensitive. Spontaneously active C-fibers were increased above the control level (1%) by NGF 8 μg (8%). The results demonstrate that NGF changes the functional axonal characteristics of mechano-insensitive C-fibers and enhances spontaneous activity thereby possibly contributing to hyperalgesia.

A Study of Serum Concentrations and Dermal Levels of NGF in Atopic Dermatitis and Healthy Subjects

Neuropeptides. Dec, 2011  |  Pubmed ID: 21893340

Nerve growth factor (NGF) was reported to be increased in the serum and skin of atopic dermatitis (AD) patients, to the extent that serum nerve growth factor levels were proposed to serve as a marker of disease severity. We studied NGF levels in the serum and dermis using skin microdialysis and attempted to correlate them with disease severity. We also examined if potential differences between morning and evening levels of NGF can explain the phenomenon of nocturnal itch. In addition, neurogenic inflammation and itch were induced using histamine iontophoresis in lesional and non-lesional skin and the effect of experimental itch on dermal NGF concentration was examined. We found that systemic (serum) and eczematous skin levels of NGF in AD are significantly lower in comparison to healthy controls. Serum NGF decreases from morning to late afternoon in both groups. Interestingly, serum NGF levels were correlated to disease severity in the morning in AD, although the NGF concentration in AD were significantly lower than in the healthy group. The local itch and neurogenic inflammation induction via experimental histamine reduced local NGF levels in the eczema and non-lesional skin in atopics, but not in the healthy controls, where it was slightly increased. The higher the clinical severity of the eczema, a significantly less pronounced effect of neurogenic inflammation on the local levels of NGF was found. The availability of measurable NGF might be reduced by a higher expression of NGF receptors. The fluctuations of NGF levels during the day suggest a complex modulation of this neurotrophin, potentially linked to stress or to an altered neurophysiological mechanism.

Experimental Thermal Lesions Induce Beta-thromboglobulin Release from Activated Platelets

European Journal of Pain (London, England). Jan, 2011  |  Pubmed ID: 20510637

Time courses of beta-thromboglobulin release, and protein extravasation after thermal inflammation of human skin was compared to neurogenic inflammation induced by histamine iontophoresis. Beta-TG and protein levels were sampled by intradermal microdialysis. Three microdialysis membranes were inserted at each stimulation site. The collected samples were measured photometrically for protein by the Coomassie blue method and for beta-TG by ELISA. Heat stimuli of 40°C and 47°C, and histamine iontophoresis were inflicted directly above the membranes. In vitro recovery rates of beta-TG and bovine albumin were measured using a latex chamber. Recovery rates at a continuous flow rate of 3 μl/min for bovine albumin ranged between 20% and 35%; those for beta-TG ranged between 14% and 17%. Heat stimulation at 40°C showed a slight but insignificant increase of beta-TG during the stimulation period. Temperatures of 47°C produced a significant, long-lasting increase of beta-TG, whereas histamine iontophoresis did not. Protein extravasation corresponded with beta-TG release; a long-lasting significant increase during and after the burn lesion, and only an initial increase of protein extravasation during the 40°C heat stimulus. Long lasting heat stimuli to the skin induced beta-TG release from platelets, whereas histamine iontophoresis although provoking protein extravasation and neurogenic flare, did not. Using microdialysis techniques we detected beta-TG release during an inflammatory response. We conclude that local platelet activation is induced by the heating stimulus. Platelet mediators, such as beta-TG might contribute to the subsequent inflammatory process which is also characterized by mechanical and heat hyperalgesia.

Does Spontaneous Activity in C-nociceptors Provide a Readout to Quantify Neuropathic Pain?

Pain. Jan, 2012  |  Pubmed ID: 22014531

Double Spikes to Single Electrical Stimulation Correlates to Spontaneous Activity of Nociceptors in Painful Neuropathy Patients

Pain. Feb, 2012  |  Pubmed ID: 22154219

Multiple firing of C nociceptors upon a single electrical stimulus has been suggested to be a possible mechanism contributing to neuropathic pain. Because this phenomenon maybe based on a unidirectional conduction block, it might also be related to neuropathic changes without a direct link to pain. We investigated painful neuropathy patients using microneurography and analysed nociceptors for the occurrence of multiple spiking and spontaneous activity. In 11 of 105 nociceptors, double spiking was found, with 1fibre even showing triple spikes on electrical stimulation. The interval between the main action potential and the multiple spikes ranged from 13 to 100ms. There was a significant association between spontaneous activity and multiple spiking in C nociceptors, with spontaneous activity being present in 9 of 11 fibres with multiple spiking, but only in 21 of 94 nociceptors without multiple spiking (P<.005, Fisher exact test). Among the 75 C nociceptors without spontaneous activity, only 2 nociceptors showed multiple spiking. In 8 neuropathy patients without pain, double spiking was found only in 4 of 90 nociceptors. Multiple spiking of nociceptors coincides with spontaneous activity in nociceptors of painful neuropathy patients. We therefore conclude that rather than being a generic sign of neuropathy, multiple spiking is linked to axonal hyperexcitability and spontaneous activity of nociceptors. It is still unclear whether it also is mechanistically related to the clinical pain level.