Neurotoxic action of capsaicin on spinal substance P neurons

The effect of capsaicin injections into neenatal rats on the ultrastructure of the neonatal dorsal horn and on some biochemical and behavioral parameters in the adult were examined. Electron microscopic observations revealed degeneration and glial engulfment of boutons and umyelinated axons in the dorsal horn 2 and 6 h after neonatal subcuntaneous capsaicin injections. Capsaicin treatment had no effect on the activities of glutamic acid decar☐ylase and choline acetytransferase in the dorsal horn. Results of substance P measurements in the CNS showed no effect of capsicin administration on straital, hypothalamic or nigral substance P content, whereas substance P levels in the dorsal horn of teh spinal cord were reduced by half. The density of [3Hnaloxone binding sites in the dorsal horn was significantly reduced, while the affinity was not affected. Capsaicin-treated animals showed significantly increased latencies to respond to a noxious thermal stimulus in both tail-flick and hot- plate tests. The results are discussed in relation to the current concepts of the involvement of substance P and opiate systems in nociception and the potential use of neonatal capsaicin as a selective neurotoxin for the education of the spinal mechanisms of pain.