Substance P immunoreactive neurons following neonatal administration of capsaicin.

Neonatal administration of capsaicin on the days 2, 10 or 20 leads to a long-lasting loss of substance P immunoreactive material in fibers of primary sensory neurons in the spinal cord and medulla oblongata. The degree of depletion examined 6 months after treatment was related to the day of injection. Injections on the second day produced dramatic losses of substance P in fibers of the substantia gelatinosa and the marginal layer of the spinal cord and the spinal nucleus of the trigeminal nerve, although these losses were never complete. The observed depletion of substance P immunoreactive material was homogenous throughout the superficial layers of the dorsal horn and the spinal nucleus of the trigeminal nerve. No changes were observed for the immunoreactivity of Leu-enkephalin in the substantia gelatinosa and the marginal layer of the spinal cord in consecutive sections from the same treated animals. In the medulla oblongata a reduction of substance P immunofluorescent fibers was found in the nucleus tractus solitarii and the spinal nucleus of the trigeminal nerve. Other areas of the central nervous system with a rich innervation of substance P immunoreactive fibers were not affected by capsaicin treatment.