Rat uterine melatonin receptors: variation during estrous cycle, pregnancy and effects of gonadal steroid treatment

pp. 53-75 of this journal issue entitled: Receptor and Non-Receptor Mediated Actions of Melatonin - International Symposium, Hong Kong, China, November 6-8, 1999Melatonin receptors are presented in the rat uterine stromal cells. They are of the mt1 receptor subtype [1]. This study was undertaken to examine changes of the rat uterine melatonin receptors during estrous cycle and pregnancy. The level of melatonin binding in the whole rat uterus fluctuated during the estrous cycle, being lower during the metestrus phase. Autoradiography using a melatonin ligand, 2-[125I]iodomelatonin, showed that melatonin receptors were localized in the rat uterine antimesometrial stroma during estrous cycle and pregnancy. Around the time of implantation (day 6), melatonin binding was clearly shown to be confined to the nondecidualized, outer stroma. Throughout pregnancy, 2-[125I]iodomelatonin binding was not seen in the decidual transformation of the stromal cells at the mesometrial region. The role of ovarian hormones in the regulation of uterine melatonin binding was studied by ovariectomy (OVX) and sex steroid administration. Ovariectomy caused an almost 70% reduction of melatonin binding in the rat uterus. Treatment of ovariectomized rats with injections of estradiol (E2) (50 Ìg/kg BW) or progesterone (P4) (90 mg/kg BW) alone or in combination for 11 days induced a marked increase in melatonin binding compared to OVX group. Our results indicate the presence of melatonin receptors in the rat uterine antimesometrial undifferentiated stromal cells. Variation of melatonin binding during estrous cycle, pregnancy and regulation of binding by gonadal steroids, support a role of E2 and/or P4 in the regulation of this receptors in vivo. Reference: 1 Zhao H, Poon AMS, Pang SF: Pharmacological characterization, molecular subtyping, and autoradiographic localization of putative melatonin receptors in uterine endometrial stroma of estrous rats. Life Sci (in press)