IN-SITU ANALYSIS OF ESTROGEN-RECEPTOR MESSENGER-RNA EXPRESSION IN THEBRAIN OF FEMALE RATS DURING PREGNANCY

Estrogen's action in specific brain regions, particularly the medial p reoptic 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 in crease is part of the mechanism by which the brain is readied to suppo rt estrogen-dependent maternal behavior. This experiment determines wh ether an alteration in the levels of estrogen receptor mRNA precedes t he increase in estrogen binding to its receptor. Using in situ hybridi zation, estrogen receptor (ER) mRNA levels were measured in specific b rain regions in females on day 8, 16 or 22 of pregnancy or on postpart um 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-p regnant females or with females on day 16. In the ventromedial nucleus , which is important for estrogen's role in postpartum sexual receptiv ity, there was an increase in ER mRNA levels on day 22 of pregnancy co mpared with day 16 of pregnancy. These results suggest that ER levels may increase in specific, behaviorally relevant brain regions at criti cal times during pregnancy through regulation of ER mRNA levels.