Gonadal steroids regulate immunoreactive tachykinin in the ventromedial nucleus of the rat hypothalamus.

The ventromedial nucleus of the hypothalamus (VMH) is an important site in the mediation of female receptive behavior (lordosis); however, the sequence of events that eventually facilitate this estrogen-dependent behavior is only partially understood. Here, evidence is presented, suggesting that an interaction between gonadal steroids and tachykinin peptides is involved. Tachykinin-immunoreactive (TACir) neurons were distributed throughout the anterior (VMHa), dorsomedial (VMHdm), central (VMHc), and ventrolateral (VMHvl) subdivisions in the adult male and female rat. Numbers of cells in the VMH of intact males and males that had been castrated for three weeks were similar. Among females, numbers of TACir cells in the VMHvl were significantly increased over numbers seen in ovariectomized females by a two day (1.5 x) and by a two week (1.6 x) exposure to estrogen. In the VMHa, numbers of TACir cells were also significantly higher in females that received replacement estrogen compared to ovariectomized females. Since estrogen concentration by TACir neurons is common in the VMHvl and rare in the VMHa, estrogen may have both direct and indirect effects on tachykinin synthesis. In the caudal pole of the VMH, staining intensity of TACir fibers varied with endocrine state. In intact males, castrated males, and ovariectomized females, fibers were lightly stained. In females exposed to estrogen for two days, fibers were moderately intense, and in females exposed to estrogen for two weeks, they formed a darkly stained plexus. These responses to estrogen suggest that production of tachykinin peptides in the VMH is involved in facilitation of female typical sexual behavior.