Capsaicin reduces substance P immunoreactivity in the lateral nucleus of the solitary tract and nodose ganglion.

The effects of the neonatal administration of capsaicin on substance P (SP) immunoreactivity of the vagal sensory ganglion (nodose ganglion) and the CNS site of termination of vagal afferents (nucleus of the solitary tract-NTS) were evaluated with immunocytochemistry and radioimmunoassay. The SP content of the nodose ganglion was reduced by 58%, and a loss of immunoreactive cell bodies was found in capsaicin treated rats. Regional differences were found in the effects of capsaicin treatment on the SP immunoreactivity in the NTS. Whereas the SP in the medial portions of the nucleus was not altered, a reduction of SP immunoreactivity in the lateral portions of the nucleus was shown with immunocytochemistry and with radioimmunoassay. The data confirm that at least a portion of the vagal afferent cell bodies of the nodose ganglion and their projections to the brainstem are capsaicin sensitive. The finding that the SP in the lateral NTS is depleted by capsaicin administration may be of importance in the use of capsaicin in functional studies, particularly of the role of SP containing vagal afferents in respiration.