Abrupt decrease in daylength and short-term changes in the plasma concentrations of FSH, LH and prolactin in...

Area of Science:

• Reproductive Endocrinology
• Animal Physiology
• Chronobiology

Background:

• Seasonal anestrus in ewes is influenced by photoperiod.
• Hormonal regulation of the estrous cycle is critical for reproductive success.

Purpose of the Study:

• To investigate the effects of natural and artificially shortened day lengths on plasma concentrations of FSH, LH, and prolactin during anestrus in ewes.
• To determine if short-day photoperiod advances the onset of the breeding season by altering reproductive hormone profiles.

Main Methods:

• Plasma hormone levels (FSH, LH, prolactin) were measured in ewes over 24-hour periods during anestrus.
• Ewes were exposed to either natural day length or constant short days (8L:16D) to advance the breeding season.
• Hormone concentrations were analyzed in relation to time of day, day length, and proximity to ovulation.

Main Results:

• FSH concentrations did not vary diurnally or with short-day exposure.
• Pulsatile LH release occurred 1-3 times daily, with one pulse potentially synchronized with dawn; short days did not alter LH pulse frequency.
• An increased LH pulse frequency (12-14 days pre-ovulation) was observed, accompanied by decreased pulse magnitude.
• Prolactin levels were higher during darkness and early morning in anestrus; short-day exposure abolished these diurnal changes and reduced prolactin to undetectable levels.

Conclusions:

• Short-day photoperiod advances the breeding season in ewes, primarily by suppressing diurnal prolactin variations and potentially influencing LH pulse dynamics.
• FSH is not a primary mediator of photoperiod-induced reproductive changes in anestrous ewes.
• The observed changes in LH and prolactin secretion in response to short days suggest a mechanism for photoperiodic control of reproduction in sheep.