Variation in pineal melatonin content during the estrous cycle of the rat.

Abstract

AbstractPineal levels of melatonin exhibit a circadian rhythm in the rat. To determine if this rhythm varies during the estrous cycle, adult rats were sacrificed every 2 hr from 2000 hr (prior to lights off) until 0800 hr (2 hr after lights on) throughout each day of the 4-day estrous cycle. Pineal glands were assayed for melatonin by radioimmunoassay. A significant circadian rhythm in the pineal content of melatonin was evident each day of the cycle with peak levels exhibited during the dark phase. There were significant differences in titers of melatonin present at 0200, 0400, and 0600 hr between various stages of the cycle. In general, highest nighttime levels occurred during the evenings of metestrus and diestrus, with levels at a minimum on the evening of estrus. These data indicate that the circadian pineal melatonin rhythm is a function of estrous cycle stage in the rat, and, thus, important differences exist in the pineal biosynthetic dynamics of this species relative to those of seasonal breeders...