Sex and regional differences in decrease of estrogen receptor α-immunoreactive cells by estrogen in rat hypothalamus and midbrain

Sensitivity of neurons to estrogen in down-regulation of estrogen receptor alpha (ER alpha) can be thought to make a sex difference in regulatory system of reproductive activities. In this study, to investigate the sex difference of expression of ER alpha in the hypothalamus and midbrain, the number of ER alpha immunoreactive (-ir) cells was counted in orchidectomized (OCX) and ovariectomized (OVX) rats with or without treatment with estrogen. A week after the gonadectomy, 5 rats in each female and male were injected with 1 mg estradiol benzoate (EB). The remaining 5 rats in both sexes did not receive EB. The brain was fixed 24 h after EB-injection and 50 mu m-serial frozen sections were made. After immunohistochemical staining for ER alpha, the number of ER alpha-ir cells was counted in a 0.2-mm(2) frame in the anteroventral periventricular nucleus (AVPvN), the ventrolateral part of the ventromedial hypothalamic nucleus (vIVMN), the arcuate nucleus (ARCN), and the lateral mesencephalic central gray (IMCG) in 2 or 3 sections. The total number of ER alpha-ir cells was changed to a density value (number per 1 mm(3)). As the results, in EB-treated rats, the density of ER alpha-ir cells in all regions, except the male AVPvN and male IMCG, were lower than those in untreated rats of both sexes. In the vIVMN, the density of ER alpha-ir cells in OVX rats was higher than in OCX rats. These results suggest that there are sex and regional differences in the mechanisms of down-regulation of ER alpha by estrogen in the rat brain. (C) 2009 Elsevier Ireland Ltd. All rights reserved.