Pineal function in the sheep: evidence for a possible mechanism mediating seasonal reproductive activity

Effect of photoperiod on LH, FSH, prolactin and melatonin patterns in ovariectomized prepubertal heifers

“…We have no explanation why the present results show a smaller difference in melatonin concentrations between daytime and nighttime values (2-3-fold rather than 6-10-fold). Arendt et al (1981) have reported significant variation in the magnitude of nighttime melatonin amongst individual ewes, and it may be that the individual heifers used in this experiment were, on average, low responders. Alternatively, the dim red light used to aid in the collection of nighttime samples may have 'dampened' the nocturnal melatonin rise, although the use of red light is common to many studies with sheep (e.g.…”

Plasma melatonin profiles of Romney Marsh sheep in natural photoperiod and in acutely extended darkness

“…Just how melatonin performs this function is unclear. In sheep and some other species the secretion of melatonin has generally been reported to follow the duration of the dark period (Rollag & Niswender, 1976;Kennaway et al, 1977;Lincoln & Short, 1980;Arendt et al, 1981) and this mechanism is believed to underlie the seasonal repro¬ ductive response given the longer period of melatonin secretion in winter than summer (Maywood et al, 1990). However, another major hypothesis is centred around the possibility that there is a phase of the circadian cycle sensitive to the presence of melatonin (Tamarkin et al, 1976;Rollag et al, 1978) …”

Amplitude of the plasma melatonin nycthemeral rhythms is not associated with the dates of onset and offset of the seasonal ovulatory activity in the Ile-de-France ewe

“…This seasonal breeding cycle of the ewe is not directly driven by the photoperiod; rather it appears to be controlled by an endogenous circannual rhythm that is entrained by the photoperiod [1]. The reproductive response to photoperiod is mediated by the pineal gland via changes in the daily secretion of its main secretory product, melatonin [1][2][3][4][5][6]. In sheep, a sudden increase in melatonin release shortly after sunset and a decrease before sunrise, after a continuous nighttime secretion are observed, thus providing a permanently high plasma melatonin during the dark phase [7].…”