Urocortin 3 (Ucn 3) is a corticotrophin-releasing factor related neuropeptide with potent anorectic effects in the brain. Ucn 3 nerve fibers heavily innervate the hypothalamic ventromedial nucleus (VMH) and injection of Ucn 3 into the VMH suppresses feeding. Currently, the origin of the Ucn 3 afferent input into the VMH is unknown. In the present study, anatomical tracing shows the major Ucn 3 afferent input to the VMH resides in the anterior parvicellular part of the paraventricular nucleus of the hypothalamus (PVHap) and the adjacent posterior part of the bed nucleus of stria terminalis (pBNST). VMH also receives moderate Ucn 3 input from the medial amygdala. Interestingly, Ucn 3 neurons located immediately caudal to the PVHap/pBNST in the rostral perifornical hypothalamic area (rPFH) provide only minimal input. The paucity of rPFH-VMH Ucn 3 projection is consistent with the finding that only Ucn 3 neurons in the rPFH co-expressed Enkephalin (Enk) and Ucn 3/Enk double-labeled nerve fibers and terminals were observed predominately in the lateral septum (LS) while only a few double-labeled fibers were found in other brain areas including the VMH. Furthermore, retrograde tracing demonstrates that Ucn 3 neurons in the rPFH project to the LS. In conclusion, the present study determines the major Ucn 3 afferent into the VMH originates from the PVHap/pBNST. In addition, anatomical heterogeneity is observed in the major hypothalamic Ucn 3 neuron population as the rostral part (PVHap/pBNST) of the cell group projects to the VMH and the caudal part (rPFH) colocalizes with Enk and provides major afferent input to the LS.
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