Localization of immunoreactive thyrotropin releasing hormone in the lower brainstem of the rat.

The distribution of thyrotropin releasing hormone (TRH) in individual nuclei of the rat brainstem was examined by radioimmunoassay. TRH was detectable in 36 of 40 brainstem nuclei investigated and the localization of TRH in the brainstem was unlike other known brainstem neuropeptides. By far the highest concentration of TRH in the brainstem (1.2 ng/mg protein) is present in the nucleus of the solitary tract. The concentration of TRH was relatively high in the motor nuclei of the IIIrd, Vth, VIIth, Xth and XIIth cranial nerves, and less high in the area postrema, nucleus gracilis, locus coeruleus, lateral reticular nucleus (A1-catecholamine cell group), dorsal raphe and central gray matter. Cerebellum and pontine nuclei contained very low levels (less than 0.03 ng/mg protein) of TRH.