Anti-allodynic Effect of Nefopam and Morphine in a Rat Model of Neuropathic Pain

Background: Neuropathic pain is a chronic pain due to a disorder in the peripheral or central nervous system with different pathophysiological mechanisms. Current treatments are not effective. Here we compared the analgesic effect of nefopam, and morphine in chronic constriction injury (CCI) model of neuropathic pain. Methods: Male wistar rat (150-200g, n=8) were divided into 3 different groups: 1Saline-treated CCI group, 2Salinetreated sham group, and 3Drug-treated CCI groups. In CCI model of neuropathic pain, the left sciatic nerve was exposed and 4 loose chromic gut ligatures were placed around the nerve proximal to the trifurcation. Ketamine 60mg/kg and xylazine 10 mg/kg were used for anesthesia. Nefopam (10, 20, 30mg/kg), and morphine (1, 3, 5mg/kg) were injected 30 minutes before surgery and continued daily to day 14 post-ligation. Von Frey filaments for mechanical allodynia and acetone test for cold allodynia were respectively used as pain behavioral tests. Experiments were performed on day 0 (before surgery) and days 1, 3, 5,7,10 and 14 post injury. Behavioral studies were performed in a quiet room between 9:00 to 11:00 AM. All experiments followed the IASP guidelines on ethical standards for investigation of experimental pain in animals. Results: Nefopam (20 and 30mg/kg) blocked mechanical and cold allodynia during the experimental period, but the analgesic effects of morphine (5mg/kg) lasted for 7 days. Conclusions: It seems that nefopam could effectively reduce pain behavior compared to morphine with reduced adverse effects.