Effect of capsaicin on neuropeptides in areas of termination of primary sensory neurones.

Capsaicin stimulates chemosensitive peripheral pain receptors, and neonatal administration produces degeneration of a population of primary afferent fibres. It has been shown previously that the effects of capsaicin are accompanied by the loss of substance P from areas of primary afferent termination and that enkephalin is not depleted from such areas. However, a number of other peptides are thought to be contained in sensory fibre systems and so we have used immunohistochemistry to examine the effect of capsaicin on the distribution of five different peptides in the substantia gelatinosa of the spinal trigeminal nucleus and spinal cord. Neonatal capsaicin treatment produces a depletion of somatostatin and cholecystokinin immunofluorescence in addition to substance P, but enkephalin and neurotensin immunofluorescence are not depleted. The implications of this result for theories of peptide involvement in nociceptive mechanisms are discussed.