Pineal Melatonin Mediates Photoperiodic Control of Pulsatile Luteinizing Hormone Secretion in the Ewe

Abstract

Seasonal breeding in the ewe is regulated by photoperiod through a pineal-dependent mechanism. Changes in the ability of estradiol to inhibit tonic LH secretion are critical. During anestrus, this ovarian steroid gains the ability to slow the frequency of pulsatile LH secretion through an action on the brain. Exposure of ovariectomized, estradiol-implanted ewes to short photoperiods during summer anestrus revealed that daylength can control LH pulse frequency. After removal of estradiol, LH pulse frequency still differed between long- and short-day ewes, suggesting photoperiodic modulation of LH and presumably GnRH secretion independent of gonadal steroids. Significantly, the effects of daylength expressed both in the presence and the absence of estradiol failed to occur in pinealectomized ewes. Long-term infusions of melatonin, given in physiological patterns to pinealectomized ewes, mimicked the effects of photoperiod on pineal-intact ewes. Specifically, a pattern of melatonin characteristic of that in short days (16-hour night-time rise) led to an increase in LH pulse frequency to a breeding season rate. Conversely, melatonin infusions typifying a long-day pattern (8-hour night-time rise) produced an anestrous pulse pattern. Pituitary sensitivity to GnRH was not reduced in sheep which were reproductively suppressed by photoperiod or melatonin treatments. These observations support the conclusion that day-length acts through pineal melatonin secretion to regulate a neural LH pulse generator which, by changing the frequency of GnRH pulses, determines the ewe’s seasonal reproductive state.

Seasonal and hormonal control of pulsatile LH secretion in the dairy goat (Capra hircus)

“…Daily treatment with melatonin, injected at an hour chosen to reproduce the duration of short-day secretion of this pineal hormone, inhibited this decrease and ultimately led to frequencies which differed significantly between the groups. This effect supports the contention that the duration of melatonin secretion is involved in controlling the hypothalamic system which commands the release of pulses of gonadotrophin-releasing hormone in the goat, as it does in the ewe (Bittman et al, 1985;Karsch et al, 1984;Hansen, 1985). The species do appear to differ, however, in the delay between the initiation of short-day (or melatonin) treatment and the LH or ovulatory response.…”

Residential Magnetic Fields, Light-at-Night, and Nocturnal Urinary 6-Sulfatoxymelatonin Concentration in Women

“…Nevertheless, these results are of considerable interest in the context of mechanisms that could affect development of breast cancer. In its role as a neuroendocrine transducer, the pineal gland provides a hormonal signal that can affect release of gonadotropins (luteinizing hormone and follicle-stimulating hormone) from the pituitary (36)(37)(38)(39). These two hormones are critical in the biosynthesis of steroid hormones in the ovary, including estradiol (40,41).…”

Seasonal changes in ovulatory activity, plasma prolactin, and melatonin concentrations, in Mouflon (Ovis gmelini musimon) and Manchega (Ovis aries) ewes

“…Lincoln et al [24] suggest that the differences between wild and domestic breeds of sheep may be explained by differences in the central neuroendocrine mechanisms relaying the effects of daylength and controlling the secretion of gonadotropic hormones. Photoperiod is the primary cue for seasonal breeding activity, mediated via the pineal gland and melatonin secretion [1,5,16]. Because the pineal gland relays the effects of daylength through the temporal pattern of melatonin secretion and because nocturnal melatonin amplitude was demonstrated to be under a strong genetic of prolactin in rams of various domesticated sheep than in Mouflon rams [23].…”