A substance P projection from the VMH to the dorsal midbrain central gray: Implication for lordosis

Abstract

Substance P has been implicated in the modulation of lordosis behavior at the level of the dorsal midbrain central gray (dMCG). Bilateral injections of substance P into the dMCG facilitate estrogen-induced lordosis behavior in ovariectomized female rats. Input from the ventromedial nucleus of the hypothalamus (VMH) to the dMCG is a vital link in the central nervous system control that mediates the expression of lordosis behavior. Substance P-containing cells have been localized in the VMH and substance P binding sites are localized in the dMCG; this suggested to us that substance P neurons originating in the VMH may terminate in the dMCG. The present study examined the projection of substance P-immunoreactive neurons (SP-IR) in the VMH to the dMCG. The retrograde tract tracer fluorogold revealed cell bodies throughout the extent of the VMH and sP immunofluorescence labelled a subpopulation of these cells particularly in the ventrolateral part of the VMH. The majority of sP-projection cells was localized in the caudal two-thirds of the VMH. Thirteen percent of the sP-IR cells were observed to project to the dMCG, while approximately 17% of the sP-IR cells of the ventrolateral part of the VMH projected to the dMCG. These results provide morphological evidence for a substance P projection from the VMH to an area where substance P has been demonstrated to facilitate lordosis behavior. We hypothesize that hormonal conditions known to be important for the expression of lordosis behavior in the female rat regulate the dynamic balance of synaptic events between substance P and Met-enkephalin release in the dMCG from neurons originating within the ventrolateral part of the VMH, and that this represents a mechanism by which these two peptides regulate lordosis behavior in the female rat.