Variation in Pineal Melatonin Content during the Estrous Cycle of the Rat

Abstract

Pineal 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 such as the hamster and sheep. Additionally, the results suggest that hydroxyindole-0methyltransferase, and not serotonin N-acetyltransferase, is the regulatory enzyme responsible for these estrous stage differences in pineal melatonin content. 416

Plasma melatonin in ewes after ovariectomy

“…Several reports indicate that, in addition to photoperiod, gonadal steroids influence pineal synthesis or secretion of melatonin (Weiss & Crayton, 1970;Cardinali, Nagle & Rosner, 1974;Ozaki, Wurtman, Alonso & Lynch, 1978;Wilkinson & Arendt, 1978;Johnson, Vaughan, Richardson, Petterborg & Reiter, 1982), although the oestrous cycle does not appear to modify plasma melatonin values in the ewe (Rollag et al, 1978;Arendt étal., 1981). In an investigation into factors controlling pineal melatonin secretion in the ewe we have studied the effect of gonadectomy on plasma melatonin levels during 24 h in different (natural) …”

Castration Reduces the Nocturnal Rise of Pineal Melatonin Levels in the Male Rat by Impairing its Noradrenergic Input

“…In the male rat, castration has been shown to reduce both pineal levels of CAMP (19) and the nocturnal elevation of pineal NAT activity (20). In female rats, both pineal melatonin production and release are influenced by the hormonal changes occurring during the rat oestrous cycle (21,22).…”

Melatonin in the Control of the Estrous Cycle of the Indian Desert Gerbil <i>(Meriones hurrianae: </i>Jerdon)

“…Whereas pinealectomy or ad ministration of melatonin affects the estrous cycle [15], the specific role of melatonin in the regulation of vaginal cyclicity is not clear. In the albino rat, pineal weight shows a maxi mum at metaestrus [16], and it is reported that melatonin synthesis is elevated during the nights of metaestrus and diestrus [17,18]. The mechanism whereby melatonin alters the duration of metaestrus and influences the in terval between successive stages of the cycle remains unknown.…”