Chronic Constriction Injury of the Rat's Infraorbital Nerve (IoN-CCI) to Study Trigeminal Neuropathic Pain

Summary

This manuscript describes a rodent model to study trigeminal neuropathic pain. The methods described include surgical procedures to perform a chronic constriction injury of the rat’s infraorbital nerve and the post-surgical behavioral tests to measure the changes in spontaneous and evoked behavior that are indicative of persistent pain and allodynia.

Abstract

Animal models are important tools to study the pathophysiology and pharmacology of neuropathic pain. This manuscript describes the surgical and behavioral procedures to study trigeminal neuropathic pain in rats. To meet the specificity of trigeminal neuropathic pain syndromes, the infraorbital nerve (IoN) is subjected to a chronic constriction injury (CCI) by loosely ligating the nerve. An intra-orbital approach is presented here to expose and ligate the IoN in the orbital cavity. After IoN ligation, rats exhibit changes in spontaneous behavior and in response to von Frey hair stimulation that are indicative of persistent pain and mechanical allodynia. Two phases can be defined in the development of the behavioral changes. During the first week following IoN-CCI (phase 1), rats show an increased and asymmetric face grooming activity, i.e., with face wash strokes primarily directed to the nerve-injured IoN territory. A distinction is made between face grooming behavior that is part of a more general body grooming behavior, which remains largely unaffected by IoN-CCI, and face grooming that is neither preceded nor followed by body grooming, which is significantly increased after IoN-CCI. During this period, responsiveness to mechanical stimulation of the IoN territory is reduced. This hyporesponsiveness is abruptly replaced by an extreme hyperresponsiveness whereby even very weak stimulus intensities provoke nocifensive behavior (phase 2). The phenomenological similarities between these behavioral alterations and reported signs of facial pain (i.e., responses to noxious stimulation of the face) suggest the presence of dysesthesia/paresthesia and mechanical allodynia in the ligated IoN territory.

Introduction

Damage to somatosensory nerves most often leads to loss of sensation. It is not very well understood why and how, in some cases, peripheral nerve damage leads to chronic pain. As a result, painful neuropathies remain a challenging condition to treat¹ and result in substantial reductions in patients’ quality of life^2-3. Animal models are an important tool to examine the efficacy of existing and newly developed drugs and to identify the pathophysiological mechanisms involved in the development and maintenance of neuropathic pain. Among the key challenges for translational pain research is identifying and measuring spontaneous pain in animals ....

Protocol

Ethics statement: Animals are treated and cared for according to the guidelines of the Committee for Research and Ethical Issues of IASP¹¹. The protocol is approved by the institutional Ethical Committee.

1. Subjects

1. Use male Sprague-Dawley rats (250-300 g at arrival).
2. House rats in solid-bottom cages in a colony room with a humidity of 40-60% and a RT of 21 ± 1 °C.
3. Make water and food available at libitum.
4. Keep rats under a reversed 12:12 hr dark/light cycle (lights on at 20 hr).

2. Surgery

1. Per rat, prepare two pieces of chromic gu....

Results

IoN-CCI rats show a strong post-operative increase in the amount of time spent on isolated face grooming (Figure 1). The increase peaks at the end of the first post-operative week and then declines during the following weeks. Most recent studies have found face grooming to be significantly increased for up to three weeks. Face grooming behavior in sham operated rats is more or less unaffected.

IoN-CCI rats also show an almost complete lack of responsiveness to ipsilateral mech.......

Discussion

The most interesting aspect of the IoN-CCI model is that it provides a unique measure of spontaneous, non-evoked (neuropathic) pain. Most animal studies of neuropathic pain, using spinal nerve models, exclusively address nocifensive behaviors (e.g., vocalization and withdrawal responses), reflecting hypersensitivity to mechanical or thermal stimulations¹². However, the chief clinical complaint in patients is ongoing pain, not hyperalgesia or allodynia¹³.

The pres.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chromic catgut	Dynek	CG504
Nembutal  (pentobarbital)	Pharmacy
Atropine	Pharmacy
Clippers for small animals - Favorita II	Aesculap	GT104
Stereotaxic instrument	Stoelting	51600
Ophthalmic ointment (Liposic)	Pharmacy
Homeothermic monitor system	Harvard Apparatus	PY8 50-7221-F
Cotton applicator	Pharmacy
Precision cotton swab	Qosina	10225
Operation microscope	Kaps	SOM 62
Dumont #5 forceps	Fine Science Tools	11251-10
Dumont forceps - Micro-blunted tips (#5/45)	Fine Science Tools	11253-25
Scissors - blunt tips	Fine Science Tools	14574-09
Polyester sutures - Mersilene (4-0)	Ethicon	R871H
Digital video camera	Sony	HDR-CX330E
Semmes-Weinstein Von Frey Aesthesiometer kit	Stoelting	58011