Benoît Malpaux

Importance of Pituitary and Neural Actions of Estradiol in Induction of the Luteinizing Hormone Surge in the Ewe

Two experiments were performed to test the importance of both pituitary and neural sites of action of estradiol in inducing the surge of luteinizing hormone (LH) in the ewe. Both experiments were conducted using an animal model in which pulsatile secretion of gonadotropin-releasing hormone (GnRH) and endogenous secretion of ovarian steroids were eliminated by ovariectomy during seasonal anestrus and treatment with Silastic implants which maintained a luteal-phase level of serum progesterone. The hormonal requirements for the surge were then evaluated by systematic application of GnRH and estradiol signals using pulsatile infusion pumps (for GnRH) and Silastic implants (for estradiol). In experiment 1, the circulating level of estradiol and frequency of GnRH pulses were increased either abruptly or progressively (i.e. mimicking the changes in the estrous cycle between luteolysis and just before the LH surge). Abrupt increments led to an LH surge in all ewes; progressive rises to the same absolute levels did not. However, sudden application of a further large increase in GnRH upon the progressive rise elicited an LH surge in every instance. In experiment 2, a GnRH pulse pattern known to be effective in inducing the LH surge was applied under conditions of differing estradiol concentration: no estradiol, basal estradiol, basal rising to peak estradiol. The GnRH signal elicited high-amplitude surges of LH only in the presence of a peak estradiol concentration. Our findings are consistent with the conclusion that two actions are required for a rise in estradiol to elicit a full-amplitude surge of LH in the ewe: an action on the brain to evoke a sudden increase in GnRH release and an action on the pituitary to maximize its response to GnRH.

Photoperiodic requirements for timing onset and duration of the breeding season of the ewe: Synchronization of an endogenous rhythm of reproduction

SummaryA study was conducted to test the hypothesis that different portions of the annual photoperiodic cycle play different roles in timing the breeding season of the ewe, Ovis aries, an animal in which an endogenous rhythm generates the seasonal reproductive transitions. Adult female sheep were pinealectomized to disrupt transduction of photoperiodic cues at 4 times of the year (summer and winter solstices, vernal and autumnal equinoxes), and the effects on seasonal reproductive neuroendocrine activity were evaluated. Time of pinealectomy greatly influenced the subsequent seasonal reproductive cycle such that the following inferences are possible. Lengthening days between the winter and summer solstices synchronize reproductive onset to the appropriate time of year. The relatively long days around the summer solstice act to suppress reproductive activity and forestall the start of the breeding season until late summer/early autumn. The shortening days between the summer solstice and autumnal equinox maintain a normal intensity and duration of reproductive neuroendocrine induction during the impending breeding season. However, the shortening days between the autumnal equinox and winter solstice (i.e., after breeding season onset) do not appear to play a critical role in maintaining the breeding season of that year, but may provide important cues for timing the breeding season of the following year.

Reproductive Refractoriness of the Ewe to Inhibitory Photoperiod Is Not Caused by Alteration of the Circadian Secretion of Melatonin

The 24-hour pattern of melatonin secretion was determined in 5 Suffolk ewes during extended exposure to a long day length to assess whether the eventual loss of response (photorefractoriness) to inhibitory long days resulted from an alteration in the circadian secretion of melatonin. Determinations of the secretory profile of melatonin were made once in short days (8 h light/day) before the switch to long days, and 11 times throughout the 250-day period of exposure to long days (16 h light/day). Samples were obtained hourly for 24 or 48 h. Reproductive state was assessed by response to estradiol-negative feedback, monitored as serum LH in ovariectomized ewes bearing estradiol implants. The characteristic secretory pattern of melatonin (low during the day, high at night), the duration of the melatonin elevation, and its phase relative to the light/dark cycle did not change as ewes became refractory to the inhibitory effects of long days. These results are consonant with the hypothesis that refractoriness of the ewe to inhibitory day length does not result from an alteration of the circadian rhythm of melatonin secretion.