Localization and characterization of 2[125I]iodomelatonin binding sites in female rat uterus

Abstract

Melatonin plays an important role on the animal reproductive system, the hypothalamic-pituitary axis is considered as the site of melatonin action. However, recent studies have suggested a direct action of melatonin on gonads. Ih our laboratory, possible direct action of melatonin on the uterus was also investigated. Autoradiography was used to identify 2[125I]iodomelatonin binding sites in the ovary, oviduct and uterus of adult Sprague-Dawley rats. Positive binding was demonstrated in the uterus. Uterine binding mainly existed in the endometrium, no binding was demonstrated in the overy. Biochemical binding assay was carried out at 4°C, 21°C, 30°C and 37°C on the uterine membrance prepared from the whole uterine homogenates. The optimal condition for the assay was 30°C with maximum binding reached in 1.5 hours. A representative Kinetic study showed a dissociation constant (Kd) of 9.16 pmol/L. Scatchard analyses from uterus at difference stages of the estrous cycle indicated a Kd of 14.63 ± 2.84 pmol/L (n=l 1) and a maximum number of binding sites (Bmax) of 0.45 ± 0.08 fmol/mg protein (n=11). The number of uterine melatonin binding sites varied during the estrous cycle. Results suggest the presence of physiological relevant melatonin receptor in the rat uterine endometrium.