Hypothalamic dorsomedial nucleus neurons innervate thyrotropin-releasing hormone-synthesizing neurons in the paraventricular nucleus.

To determine whether the hypothalamic dorsomedial nucleus (DMN) may serve as a relay center for the central actions of leptin on thyrotropin-releasing hormone (TRH)-synthesizing neurons in the paraventricular nucleus (PVN), axonal projections from the DMN to TRH-containing neurons in the PVN were studied using the anterogradely transported marker substance, Phaseolus vulgaris-leucoagglutinin (PHA-L). Stereotaxic injections of PHA-L were targeted to the mid-dorsal and mid-ventral portions of the DMN. After 10-14-day survival, the brains were prepared for immunohistochemistry and immunostained with an antibody directed against PHA-L. Focal injections confined to the DMN were identified in 14 animals and gave rise to a fiber bundle that entered the PVN at the caudal pole of the nucleus, densely innervating all parvocellular subdivisions of the PVN. In double-labeled preparations using antisera to PHA-L and preproTRH 178-199, the latter as a marker for TRH-containing neurons in the PVN, proTRH-IR neurons were observed to be enmeshed in a network of PHA-L-containing fibers. When the injection site covered the entire DMN or the mid-dorsal part of the DMN, PHA-L-containing axon varicosities were juxtaposed to approximately 97 and 90% of proTRH neurons, respectively, in all parvocellular subdivisions of the PVN, and by ultrastructural analysis were shown to be synaptic. In contrast, when the injection site was centered primarily in the mid-ventral part of the DMN, only approximately 52% of proTRH-synthesizing neurons appeared to be innervated by PHA-L-containing axons. These data demonstrate that a major projection pathway exists from the DMN, specifically to TRH-producing neurons in the PVN, and suggest that the DMN is anatomically situated to exert a regulatory effect on TRH-synthesizing neurons in the PVN.