Thyrotropin-releasing hormone immunoreactivity in rat adrenal tissue is localized in mast cells

Pro-thyrotropin-releasing hormone (pro-TRH) has been shown to be present throughout the central nervous system and in several peripheral tissues. In adrenals, TRH immunoreactivity has been reported but not characterized. We show here that two rat pro-TRH-derived peptides, TRH and prepro-TRH[160-169] (Ps4), were detected in extracts of rat adrenal glands by enzyme immunoassay. Endogenous TRH and Ps4 were purified by gel exclusion chromatography and reverse-phase HPLC. Structural identification of each peptide was achieved by chromatographic comparison with synthetic standards. By using the indirect immunofluorescence technique, TRH-immunoreactive cell bodies were found rather widely scattered outside the adrenal, in the brown adipose tissue in which the gland is embedded. These immunofluorescent cells have the typical appearance of mast cells and are metachromatic after histological staining with acidic Toluidine Blue. Our findings suggest that pro-TRH-derived peptides exist in rat mast cells.