Levels of estrogen receptor immunoreactivity are altered in behaviorally-relevant brain regions in female rats during pregnancy

Pregnancy and parturition are accompanied by unique behavioral changes. Only some of the neural mechanisms behind the dramatic changes in behavior are understood. Estrogen's action within the medial preoptic nucleus (MPN) is necessary for the induction of maternal behavior around the time of parturition, and estrogen acts within the ventromedial nucleus (VMN) to trigger postpartum sexual receptivity shortly after parturition. We have hypothesized that the sensitivity of various brain regions to estrogen may be altered by pregnancy to support these unique behavioral patterns. Using immunocytochemistry, this study examined whether the levels of estrogen receptor (ER) protein, within behaviorally relevant brain regions, differ among females on day 8, day 16, and day 22 of pregnancy, or on postpartum day 1. On day 16 and day 22 of pregnancy, the MPN contained a significantly greater number of cells expressing high levels of ER-ir compared to day 8 or postpartum day 1. In the VMN, the mean amount of ER-ir per cell was significantly higher on day 22 of pregnancy than on day 16 or postpartum day 1. In the bed nucleus of the stria terminalis, ER-ir levels were significantly increased on postpartum day 1 compared to day 22 of pregnancy. There were no significant changes in ER-ir in the medial amygdala. These results demonstrate regionally and temporally specific regulation of ER protein in the brain during pregnancy. Alterations in the levels of ER at critical times in regions such as the MPN and VMN may underlie the unique expression of maternal and sexual behavior that occur during pregnancy and at the time of parturition.