Origin of thyrotropin-releasing hormone neurons that innervate the tuberomamillary nuclei

Abstract Hypophysiotropic TRH neurons function as metabolic sensors that regulate the thyroid axis and energy homeostasis. Less is known about the role of other hypothalamic TRH neurons. As central administration of TRH decreases food intake and increases histamine in the tuberomammillary nuclei (TMN), and TMN histamine neurons are densely innervated by TRH fibers from an unknown origin, we mapped the location of TRH neurons that project to the TMN. The retrograde tracer, cholera toxin B subunit (CTB), was injected into the TMN E1-E2, E4-E5 subdivisions of adult Sprague Dawley male rats. TMN projecting neurons were observed in the septum, preoptic area, BNST, perifornical area, anterior PVN, peduncular and tuberal lateral hypothalamus (TuLH), suprachiasmatic nucleus and medial amygdala. However, CTB/pro-TRH178-199 double-labeled cells were only found in the TuLH. The specificity of the retrograde tract-tracing result was confirmed by administering the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHAL) into the TuLH. Double labeled PHAL-pro-TRH boutons were identified in all subdivisions of the TMN. TMN neurons double labeled for histidine decarboxylase ( Hdc )/PHAL, Hdc / Trhr , and Hdc / Trh . Further confirmation of a TuLH TRH neuronal projection to the TMN was established in a transgenic mouse that expresses Cre-recombinase in TRH-producing cells following microinjection of a Cre-recombinase–dependent AAV that expresses mCherry into the TuLH. We conclude that in rodents, the TRH innervation of TMN originates in part from TRH neurons in the TuLH and that this TRH population may contribute to regulate energy homeostasis through histamine Trhr- positive neurons of the TMN.