Brainstem localization of a thyrotropin-releasing hormone-induced change in respiratory function

When rats received microinjections of 100 ng thyrotropin-releasing hormone (TRH) into the medial portions of the nucleus tractus solitarius and 12th nucleus or raphe obscurus, at the level of the obex, a significant decrease in the inspiratory time was found. Examination of TRH immunocytochemistry revealed a high density of TRH-positive nerve terminals in these regions, especially the more caudal aspects. If serotonin was depleted by neonatal 5,7-dihydroxytryptamine treatment, the respiratory response of the adults to TRH appeared potentiated. Even though the neonatal 5,7-dihydroxytryptamine reduces the occurrence of TRH-positive cell bodies, TRH-positive fibers were not appreciably altered. These results are discussed with regard to a possible role of endogenous TRH in the brainstem on rhythmic respiratory activity.