POSTNATAL-DEVELOPMENT AND SEX DIFFERENCE IN NEURONS CONTAINING ESTROGEN RECEPTOR-ALPHA IMMUNOREACTIVITY IN THE PREOPTIC BRAIN, THE DIENCEPHALON, AND THE AMYGDALA IN THE RAT

Estrogen has been considered as a key substance that induces sexual di fferentiation of the brain during fetal and neonatal life in the rat. Thus, to define the brain regions involved in the brain sexual differe ntiation, we examined the regions where the estrogen receptor (ER) is located in the developing rat brain. We examined immunohistochemical d istribution of the cells containing estrogen receptor-alpha (ER-alpha) in the preoptic region, the diencephalon, and the amygdala in male an d female rats on postnatal days 1-35 (PD1-PD35). The antibody used rec ognizes ER-alpha equally well for both occupied and unoccupied forms. ER-alpha immunostaining was restricted to the cell nuclei of specific cell groups. In PD1 rats, ER-alpha-immunoreactive (ER-IR) signals were detected in the lateral septum, the organum vasculosum lamina termina lis, the medial preoptic nucleus (MPN), the median preoptic nucleus, t he bed nucleus of the stria terminalis, the hypothalamic periventricul ar nucleus, the lateral habenula, the posterodorsal part of the medial amygdala nucleus, the posterior part of the cortical amygdala nucleus , the hypothalamic ventromedial nucleus (VMH), the hypothalamic arcuat e nucleus, and the posterior hypothalamic periventricular nucleus. The distribution pattern of ER-IR cells in the newborn rat was much the s ame as that in the adult in the preoptic-hypothalamic and amygdala reg ions. Moreover, the signals in the MPN and the VMH were stronger in th e female than in the male, perhaps reflecting the ability of estrogen generated by aromatization of testosterone in the male to down-regulat e the ER signal. Thus, the brain regions showing sex differences may b e sites of sexual differentiation of the brain by aromatizable androge n during the neonatal period. (C) 1997 Wiley-Liss, Inc.