Developmental profile and regulation of estrogen receptor (ER) mRNA expression in the preoptic area of prenatal rats

Estrogen, derived from circulating testosterone; masculinizes the developing preoptic area. Expression of estrogen receptors (ERs) within the preoptic area is one requirement for a possible direct action of estrogen in the process of sexual differentiation of this brain region. Using a S-35-labeled riboprobe and in situ hybridization to detect ER mRNA on both him and emulsion-coated slides, we were able to detect ER mRNA within the rat preoptic area by embryonic day 18 (ED 18), coincident with the reported onset of the critical period for testosterone-dependent masculinization of this region. ER mRNA increased significantly between ED 18 and 19 in both sexes, and continued to increase through postnatal day 0 (PND 0 = day of birth) in females, but not males. ER mRNA levels were nor significantly greater in females than in males until PND 0. The lack of a sex difference in ER mRNA prenatally, however, appears to be due to an effect of intrauterine neighbors. ER mRNA levels in ED 20 embryos were relatively high in females with female-only neighbors, whereas ER mRNA levels were relatively low, and comparable to males, when the in utero neighbors included one or more males. Treatment of pregnant dams with diethylstilbestrol or with tamoxifen did not significantly alter ER mRNA levels in the preoptic area of the embryos. Although these results suggest that ER mRNA expression is subject to hormonal regulation prenatally, the relevant hormone was not identified.