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Chronic pain is a significant healthcare issue across the world and is one of the costliest health problems in the United States. Chronic pain is better managed when both pharmacological and non-pharmacological modalities are utilized in a multidisciplinary fashion1. Management of chronic pain is challenging and, in some cases, does not adequately treat the pain2. Therefore, new and complementary methods are needed to improve chronic pain management, and animal models are crucial to investigate innovative therapies.
Chronic neuropathic pain results from lesions or diseases in the somatosensory system, including diabetes, infections, nerve compressions, or autoimmune diseases3. Neuropathic pain relies both on peripheral and central sensitization mechanisms and originates from a lesion of the nerves. This pain can be characterized by both touch- and thermal-evoked hyperalgesia and allodynia, ongoing pain, and changes in the temperature of the affected limb4. To better understand the mechanisms and advance new treatments, several models have been developed in rodents to mimic the symptoms and causes of neuropathic pain5. For example, neuropathic pain can be induced with chemotherapeutic agent injections, spinal nerve ligation (SNL), chronic constriction injury (CCI) of the sciatic nerve, pSNL, spared nerve injury, sciatic nerve transection, and sciatic nerve trisection6. Notably, ligation of the sciatic nerve reproduces multiple features of neuropathic pain observed in humans, such as mechanical and thermal hypersensitivity, or changes in temperature of the affected limb, characteristic of complex regional pain syndrome (CRPS)7. Thus, this model is well-suited for the study of CRPS or any other nerve injury affections that induce chronic neuropathic pain. The model was first developed by Seltzer in 19908, and is widely used in pain studies to investigate novel analgesic compounds or evaluate the cognitive effects of chronic pain9,10,11,12,13. The model presents high reproducibility, and the partial ligation preserves behavioral responses to peripheral stimuli6.
Many of the currently used models have shortcomings not observed in pSNL. The CCI model has a much higher variability of injury between each animal depending on the snugness of the constrictor, and autotomy alters the hind paw digits rendering the model unsuitable for behavioral analysis6. The SNL model is a far more complicated and longer surgery that not only requires advanced technical skills but also carries a high risk of severe motor deficits3. These shortcomings are not seen in the pSNL model. The ease of reproducibility, short duration of the surgery, and the reduced risk of motor deficits seen postoperatively make this model valuable for studying peripheral neuropathic pain8,14. Nevertheless, the partial ligation procedure itself can have variability between experimenters, resulting in less consistency in the number of ligated nerve fibers. Thus, presenting the details of the surgery is crucial to increase reproducibility among studies.
To induce chronic neuropathy, a 9-0 non-absorbable nylon suture is used to ligate a third of the width of the sciatic nerve. Following surgery, responses to thermal and mechanical stimuli are exaggerated, starting at day 1 postoperatively and lasting more than 50 days8. Here, both thermal and mechanical sensitivities were evaluated over 28 days using Hargreaves', hot plate, and von Frey filament tests. All of the behavioral assays demonstrated the consistency of the long-lasting hypersensitivity. This model has been shown to have dose-dependent effects of both morphine and ibuprofen, confirming it is well-suited for preclinical pain studies. Notably, this article describes the instructions for a unique handmade glass tool, referred to as "nerve glass hook." This tool is used in place of forceps to manipulate the nerve and prevent unintended additional nerve injury during surgery.