DEVELOPMENTAL PROFILE AND REGULATION OF ESTROGEN-RECEPTOR (ER) MESSENGER-RNA EXPRESSION IN THE PREOPTIC AREA OF PRENATAL RATS

Estrogen, derived from circulating testosterone; masculinizes the deve loping preoptic area. Expression of estrogen receptors (ERs) within th e preoptic area is one requirement for a possible direct action of est rogen in the process of sexual differentiation of this brain region. U sing a S-35-labeled riboprobe and in situ hybridization to detect ER m RNA 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), coincid ent with the reported onset of the critical period for testosterone-de pendent masculinization of this region. ER mRNA increased significantl y between ED 18 and 19 in both sexes, and continued to increase throug h 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 u ntil PND 0. The lack of a sex difference in ER mRNA prenatally, howeve r, appears to be due to an effect of intrauterine neighbors. ER mRNA l evels in ED 20 embryos were relatively high in females with female-onl y neighbors, whereas ER mRNA levels were relatively low, and comparabl e to males, when the in utero neighbors included one or more males. Tr eatment of pregnant dams with diethylstilbestrol or with tamoxifen did not significantly alter ER mRNA levels in the preoptic area of the em bryos. Although these results suggest that ER mRNA expression is subje ct to hormonal regulation prenatally, the relevant hormone was not ide ntified.