Estrogen receptors alpha and beta in the female reproductive tract of the rat during the estrous cycle

The action of steroid hormones is primarily mediated via a process that inv olves 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 t o obtain information about the dynamics of the two types of estrogen recept ors (ERs), alpha and beta, by comparing their concentration and distributio n in the reproductive tract of the rat during the estrous cycle. Twenty-fou r 55- to 60-day-old female Sprague-Dawley rats were used. The stage of estr ous cycle was determined by vaginal smear, ER alpha was the dominating subt ype in uterus, oviduct, and cervix/vagina, with the distribution varying in stroma and epithelium during the estrous cycle. A low level of ER alpha mR NA was observed in ovarian stromal cells, with some scattered positive cell s found among granulosa cells. ER beta expression was observed in the diffe rent compartments of uterus and cervix/vagina, but cyclic variation during the estrous cycle was less evident than that of ER alpha, Only a few scatte red cells that contained ER beta mRNA were observed in oviduct, ER beta mRN A was highly expressed in granulosa cells of developing follicles, with a w eaker hybridization signal in new corpora lutea, Immunohistochemistry showe d that protein levels of ER alpha and ER beta have distinct specificity for tissues and cell types, similar to their respective levels of mRNA, as ass essed by in situ hybridization, The precise physiological function and impo rtance of ER beta is still unclear, The relative physiological and patholog ical function of each ER subtype in the female reproductive tract remains t o be further evaluated.