Single nerve capsaicin: effects on pain and morphine analgesia in the formalin and foot-flick tests

Application of capsaicin to the sciatic nerve reduces responsiveness to pain in the foot-flick test which examines brief, threshold-level pain. The purpose of the present study was to determine if a similar reduction occurs in the formalin test which examines suprathreshold, deep pain that persists for several hours. The sciatic nerve on one side in the rat was exposed and soaked for 15 min in a solution of capsaicin and the saphenous nerve was ligated and cut. The operated foot was tested for sensitivity to pain in the formalin and foot-flick tests 2 days to 12 weeks later both with and without morphine. The capsaicin treatment produced a substantial reduction in sensitivity to foot-flick heat pain at all times after surgery. In the formalin test, the effects were small and tended to suggest that the rats felt more rather than less pain. The capsaicin treatment markedly reduced the sensitivity of formalin test pain to morphine. This effect appeared about one week after surgery and persisted for 12 weeks. The results suggest that capsaicin-sensitive unmyelinated afferents play a role in the threshold-level, non-damaging heat pain, but are not involved in pain resulting from tissue damage. However, these afferents appear to be important for the spinal action of morphine on this type of pain.