Putative melatonin receptors in the rat uterus: autoradiography and radioreceptor binding studies

Abstract

Melatonin plays an important role in animal reproduction. Recent studies suggest a direct action of melatonin on the gonads. In our laboratory, possible direct action of melatonin on the uterus was investigated. Biochemical binding assay was carried out at 4° C, 21° C, 30°C and 37°C on the membrane prepared by homogenizing the whole uterus. At 37° C, association was completed in 10 min and remained over next 50 min. At 21° C, association was not stable. At 4 ° C, dissociation was incomplete. At 30 ° C, specific binding increased during the first 0.5 h, reached equilibrium and remained stable over next 2.5 h, dissociation was completed in 3 h. The optimal condition for the assay was 30° C incubating for 1.5 h. A representative kinetic study showed an equilibrium dissociation constant (Kd) of 9.16 pmol/l at 30° C. Saturation studies were performed at 30 °C incubating for 1.5 h. Scatchard analyses yielded a Kd of 14.63 B 2.84 pmol/l (n = 11) and a maximum number of binding sites of 0.45 B 0.08 fmol/mg protein (n = 11). The number of uterine melatonin binding sites varied during the estrous cycle. Autoradiography was used to localize 2-[125I]iodomelatonin binding sites in the ovary, oviduct and uterus of adult Sprague-Dawley rats. Positive binding was demonstrated in the uterus. The uterine binding was mainly localized in the endometrial stroma. The endometrial stroma is not a homogeneous tissue; it consists of the glandular epithelium, vascular endothelium, stromal cells and other cells. At present, it is impossible to conclude whether all, or only some of these cells have melatonin receptors. Nevertheless, our results suggest the presence of physiologically relevant melatonin receptors in the rat uterine endometrial stroma. Melatonin might directly regulate the endometrial vascular permeability and decidualization.