Circulating gonadal steroid hormones regulate estrogen receptor mRNA in the male rat forebrain

In male rats, the conversion of testosterone to estrogen via aromatization is a critical step in a number of androgen-mediated functions, especially reproductive behavior. Within the central nervous system (CNS), locally formed estrogen binds to its cognate estrogen receptor protein. Little is known about what factors regulate the expression of estrogen receptors in the male rat CNS. This study examined whether circulating male gonadal steroid hormones have a role in the regulation of estrogen receptor mRNA in brain regions critical for the expression of male reproductive behavior. Male rats were gonadectomized or sham operated, and 3 days later were sacrificed. Their brains were fixed by perfusion, frozen, and sectioned. Tissue sections were hybridized to an 35S-labeled 850 base cDNA probe, complementary primarily to the steroid binding domain of the estrogen receptor mRNA. Following post-hybridization washes, slides were dipped in photographic emulsion and exposed for 2 weeks. Estrogen receptor mRNA-containing neurons were observed in all brain regions previously shown by steroid hormone autoradiography to concentrate estrogen. Gonadectomy did not alter the number of estrogen receptor mRNA-producing neurons, but did produce a two-fold increase in the relative amount of estrogen receptor mRNA per cell in the medial preoptic nucleus, periventricular preoptic area, and bed nucleus of the stria terminalis. This study shows that circulating gonadal steroids down-regulate steady state levels of estrogen receptor mRNA within specific brain regions, and thereby have the potential to regulate the sensitivity of particular target regions in the CNS to estrogen.