Effects of single doses of capsaicin on nociceptive thresholds in the rodent

Abstract

The aim of the present investigation was to determine the acute effects of single doses of capsaicin, administered either peripherally or centrally, against responses induced by chemical, heat and pressure nociceptive stimuli in the rat and mouse. In the mouse, subcutaneous capsaicin treatment produced a significant decrease in the number of abdominal constriction responses produced by intraperitoneal injections of either acetylcholine or phenylquinone. Hot plate studies in the same mice showed that capsaicin was ineffective against thermal nociceptive stimuli at doses 20 times greater than those effective against chemical nociceptive stimuli. In the rat, subcutaneous administration of capsaicin caused a dose-related antinociceptive effect against a chemical nociceptive stimulus, using the formalin test. Nociceptive pressure thresholds determined for both hind paws were significantly higher in rats receiving capsaicin either subcutaneously, intracerebroventricularly or intrathecally. In marked contrast, nociceptive heat thresholds were unchanged in rats treated with capsaicin either peripherally or centrally. No gross behavioral changes were observed in the capsaicin treated animals. In conclusion, these results show that single doses of capsaicin administered either peripherally or centrally, caused short-lasting increases in pressure and chemically-induced nociception, whilst nociceptive heat thresholds were unaffected.