Effect of melatonin on estrogen and progesterone receptors in relation to uterine contraction in rats.

The present study was designed to investigate the possible modulator effect of melatonin on uterine estrogen and progesterone receptors in rats as well as the uterine response to oxytocin. Non-pregnant rats were pretreated with melatonin in a dose of 0.8 mg kg(-1) per day for 15 consecutive days. Control animals received the vehicle. The uterus was dissected out and uterine contraction in one horne was recorded in vitro for each animal as a response to oxytocin (0.5 x 10(-11) to 2 x 10(-11)M). The other uterine horne was subjected to estrogen and progesterone receptors detection by immunohistochemical and image analysis techniques. The results reveal a significant reduction (59%) in the number of uterine estrogen receptors with concomitant increase in the progesterone receptors (53%) in melatonin-pretreated rats as compared to the control ones. In addition, our data show an inhibitory effect of melatonin on the uterine contraction as a response to oxytocin (0.5 x 10(-11), 1 x 10(-11), and 2 x 10(-11)M) amounting to 48, 77, and 59.5% reduction, respectively, in the amplitude of contraction as well as 62, 19.9, and 47% reduction in the area under the curve (AUC) of uterine contractions, respectively. The data, so far obtained, may indicate a possible relationship of melatonin-induced modulation of the number of estrogen and progesterone receptors and its inhibitory effect on uterine contraction. These findings merit further investigations on the possible beneficial role of melatonin in a plethora of hormone-dependent uterine disorders.