Electron microscope immunocytochemical localization of thyrotropin-releasing hormone (TRH) prohormone in the rat hypothalamus.

Using an antiserum to a synthetic peptide corresponding to the sequence Phe-178-Ser-199 of the precursor of thyrotropin-releasing hormone (pro-TRH), we have studied by immunoelectron microscopy the ultrastructural localization of pro-TRH in the rat hypothalamus. In the paraventricular nucleus neuronal cell bodies, dendrites and axons were labelled. The dense core vesicles were strongly immunoreactive. Immunostained cell bodies, dendrites and endings were frequently observed in contact with unidentified neuronal elements. In the median eminence, numerous endings containing positive dense core vesicles were detected. These results suggest that pro-TRH and/or fragment(s) of pro-tRH could play a neuromodulator/neurotransmitter role in the hypothalamus and could also be released into the pituitary portal plexus.