Antihyperalgesic and antiallodynic activity of melatonin in diabetic rats

Abstract

The aim of this study was to examine the effects of melatonin on hyperalgesia (2 weeks) and allodynia (6–8 weeks) in diabetic rats. Experimental diabetes was induced by a single‐dose of streptozotocin (50 mg/kg, ip). Formalin (0.5%) test was used to assess hyperalgesia, whereas that tactile allodynia was determined by the application of von Frey filaments to the paw. Streptozotocin significantly increased blood glucose levels in diabetic rats compared to saline‐treated rats. Streptozotocin treated animals also showed an increase in food and water intake. Melatonin (75–300 mg/kg, po) reduced formalin‐induced nociception and tactile allodynia in a dose‐dependent manner in diabetic rats. The antihiperalgesic effect of melatonin was completely prevented by K‐185 (2 mg/kg, sc), a selective MT2 receptor antagonist, and partially prevented by naltrexone (1 mg/kg, sc), a non‐selective opioid receptor antagonist. Results indicate that melatonin is able to reduce hyperalgesia and tactile allodynia in diabetic rats. Data also suggest that the antihyperalgesic activity of melatonin is mediated by MT2 and opioid receptors. Thus, our data suggest that melatonin could be a valuable tool to treat pain in diabetic patients. This work is supported in part by a grant from CONACYT (199029 and CIC/UMSNH 2006)