ENHANCED WITHDRAWAL RESPONSES TO HEAT AND MECHANICAL STIMULI FOLLOWING INTRAPLANTAR INJECTION OF CAPSAICIN IN RATS

Withdrawal responses to heat and mechanical stimuli applied to the pla ntar surface of the rat hindpaw were measured before and after an intr aplantar injection of capsaicin. In separate groups of rats, capsaicin doses of 1, 10 and 30 mu g, and the vehicle were given into the cente r of the plantar surface in a volume of 10 mu l. Withdrawal latency ev oked by radiant heat and the frequency of withdrawal evoked by mechani cal stimuli (von Frey monofilaments) were obtained from both hindpaws before and after injection. Hyperalgesia to heat was defined as a decr ease in withdrawal latency and mechanical hyperalgesia was indicated b y an increase in withdrawal response frequency. Intraplantar injection of capsaicin evoked nocifensive behavior characterized by lifting and guarding the injected paw which typically lasted up to 3 min followin g injection. Capsaicin produced a decrease in withdrawal latency to he at and increased the frequency of withdrawal to mechanical stimuli in a dose-dependent manner. These effects were observed on the injected p aw only. The duration of hyperalgesia produced by capsaicin was also d ose-dependent. Withdrawal latencies to heat were decreased up to 45 mi n following capsaicin while withdrawal responses to mechanical stimuli remained elevated up to 4 h. The area of mechanical hyperalgesia incl uded most of the plantar surface and extended approximately 9 mm proxi mal and distal to the injection. Injection of the vehicle did not sign ificantly alter withdrawal responses to heat or mechanical stimuli. Th ese studies demonstrate that intraplantar injection of capsaicin in ra ts produces hyperalgesia to heat and mechanical stimuli. This model sh ould be useful for correlative behavioral, physiological and pharmacol ogical studies of underlying mechanisms of capsaicin-evoked hyperalges ia.