The role of testosterone in the ventromedial nucleus of the hypothalamus in the regulation of male rat sociosexual behaviors

This work addressed the involvement of the ventromedial nucleus (VMN) of the hypothalamus in the regulation of androgen-dependent behaviors in adult male rats. Four separate studies were designed to examine the VMN and sociosexual behaviors. First, it was shown that bilateral administration of testosterone to the VMN of castrated animals was sufficient to restore partner preference, a measure of sexual motivation. The second study revealed that androgen receptor blockade in the VMN produced impairments in copulation, sexual motivation, and ultrasonic vocalizations. This was in contrast to blockade in the medial preoptic area (MPOA), which only affected copulation.; In addition to androgen receptors, the VMN contains estrogen receptors and aromatase, an enzyme that converts testosterone to estradiol. To assess the relative importance of estradiol at the level of the VMN, an aromatase inhibitor (fadrozole) was selectively administered to animals in the third study. No effects were observed on any of the sociosexual behaviors, suggesting that androgen effects in this area of the hypothalamus are primarily mediated by androgen receptors. In the fourth and final study, microlesions of cells within the dorsomedial portion of the VMN impaired copulatory efficiency, ultrasonic vocalizations and scent marking behaviors. Taken together, these studies suggest that androgen receptor containing neurons in the VMN are critically important for sociosexual behaviors. A model is proposed to integrate the results of these studies and to explain how the VMN fits into neural circuits to influence behavior.