In situ analysis of estrogen receptor mRNA expression in the brain of female rats during pregnancy.

Estrogen's action in specific brain regions, particularly the medial preoptic nucleus (MPN), is necessary for the onset of maternal behavior in the pregnant female rat. There is an increase in estrogen binding in the MPN during pregnancy, and it has been hypothesized that this increase is part of the mechanism by which the brain is readied to support estrogen-dependent maternal behavior. This experiment determines whether an alteration in the levels of estrogen receptor mRNA precedes the increase in estrogen binding to its receptor. Using in situ hybridization, estrogen receptor (ER) mRNA levels were measured in specific brain regions in females on day 8, 16 or 22 of pregnancy or on postpartum day 1 or in non-pregnant females. ER mRNA levels are significantly higher in the MPN in females on day 8 of pregnancy compared with non-pregnant females or with females on day 16. In the ventromedial nucleus, which is important for estrogen's role in postpartum sexual receptivity, there was an increase in ER mRNA levels on day 22 of pregnancy compared with day 16 of pregnancy. These results suggest that ER levels may increase in specific, behaviorally relevant brain regions at critical times during pregnancy through regulation of ER mRNA levels.