Depletion of primary afferent substance P by capsaicin and dihydrocapsaicin without altered thermal sensitivity in rats.

Systemic administration of capsaicin and dihydrocapsaicin to adult rats of two different strains and of capsaicin to neonatal rats, depleted substance P levels in dorsal roots plus ganglia and in dorsal spinal cord. In no case was this depletion accompanied by substantially altered tail-flick latencies. The results are not consistent with a role of the neuropeptide in nociceptive thermal sensitivity in the rat.