Estrogen receptors α and β in the female reproductive tract of the rat during the estrous cycle

The action of steroid hormones is primarily mediated via a process that involves hormone binding to specific receptors in target cells, which leads to transcriptional activation of steroid-responsive genes and, subsequently, to a modification of cellular responses. The aim of the present study was to obtain information about the dynamics of the two types of estrogen receptors (ERs), alpha and beta, by comparing their concentration and distribution in the reproductive tract of the rat during the estrous cycle. Twenty-four 55- to 60-day-old female Sprague-Dawley rats were used. The stage of estrous cycle was determined by vaginal smear. ERalpha was the dominating subtype in uterus, oviduct, and cervix/vagina, with the distribution varying in stroma and epithelium during the estrous cycle. A low level of ERalpha mRNA was observed in ovarian stromal cells, with some scattered positive cells found among granulosa cells. ERbeta expression was observed in the different compartments of uterus and cervix/vagina, but cyclic variation during the estrous cycle was less evident than that of ERalpha. Only a few scattered cells that contained ERbeta mRNA were observed in oviduct. ERbeta mRNA was highly expressed in granulosa cells of developing follicles, with a weaker hybridization signal in new corpora lutea. Immunohistochemistry showed that protein levels of ERalpha and ERbeta have distinct specificity for tissues and cell types, similar to their respective levels of mRNA, as assessed by in situ hybridization. The precise physiological function and importance of ERbeta is still unclear. The relative physiological and pathological function of each ER subtype in the female reproductive tract remains to be further evaluated.