Progesterone receptors and the sexual differentiation of the medial preoptic nucleus.

The central component of the medial preoptic nucleus (MPNc) of the rat has served as an excellent model of sexual differentiation. The MPNc is larger in adult males than in females, and its development is regulated by perinatal gonadal hormones. Although testosterone (T) and its metabolite estradiol (E) sexually differentiate this region, the exact mechanism by which they act during development is not known. There is a dramatic sex difference in the expression of progesterone receptors (PR) in the MPN during development; perinatal males express higher levels of PR than females. Additionally, PR expression during this time is dependent on exposure to T. Thus, PR induction may be one mechanism by which T sexually differentiates the MPN. The present study investigated the potential role of PR in the sexual differentiation of the MPNc. Anatomical examination of PR distribution within the MPN of neonatal males revealed the presence of PR immunoreactive cells within the MPNc, suggesting a direct route of action for PR in the development of the MPNc. Additionally, we measured the effects of neonatal RU486 treatment, a progesterone and glucocorticoid receptor antagonist, on subsequent MPNc volume in neonatally T-treated females and neonatally castrated males, given T. RU486 treatment reduced the MPNc volume of T-treated females while it increased the volume in T-treated, neonatally castrated males. These results, taken together with the expression of PR in the MPNc, suggest that PR may influence the sexual differentiation of the MPNc volume.