Involvement of capsaicin-sensitive neurones in hyperalgesia and enhanced opioid antinociception in inflammation.

The effects of capsaicin pretreatment of adult rats was investigated on consequences of unilateral paw inflammation induced by inoculation with Freund's adjuvant. Decrease in mechanical nociceptive threshold in the inflamed paw, as measured by the paw pressure test, was dose-dependently inhibited by capsaicin (20-150 mg/kg s.c.). In control rats, the antinociceptive action of morphine (0.8-1.9 mg/kg s.c.) was greater in the inflamed than in the non-inflamed paw; this difference was absent in capsaicin-treated animals. Increased volume or skin temperature of the inflamed paw was not influenced by capsaicin. It is concluded that capsaicin-sensitive, presumably C-fibre neurones, but not an alteration of the inflammation itself by capsaicin, mediate hyperalgesia and increased morphine antinociception in the rat paw with adjuvant-induced inflammation.