Melatonin-binding sites in brain and caudal arteries of the female rat during the estrous cycle and after estrogen administration.

Melatonin-binding sites were studied by quantitative autoradiography in brain and caudal (tail) arteries and in selected brain areas of cycling female rats, ovariectomized rats, and ovariectomized rats treated with estradiol. In the caudal artery, the number of melatonin-binding sites was significantly lower during proestrus, estrus, and metestrus than during diestrus. In ovariectomized rats, melatonin binding to the caudal artery was similar to that found during proestrus, estrus, and metestrus, and a further decrease in the receptor number was produced by estradiol replacement. Similarly, melatonin binding in anterior cerebral arteries was higher during diestrus than at other stages of the estrous cycle. In ovariectomized rats, estradiol replacement also resulted in decreased melatonin binding in anterior cerebral arteries. Conversely, no changes in the number of melatonin-binding sites were observed in the suprachiasmatic nucleus or the area postrema during the estrous cycle, in ovariectomized rats, or after estradiol replacement. The present results suggest that in the female rat, reproductive hormones modulate the expression of cerebral and caudal arterial melatonin-binding sites.