Photoperiodic synchronization of a circannual reproductive rhythm in sheep: identification of season-specific time cues

• Published in: Biology of reproduction Vol. 50; no. 4; pp. 965 - 976
• Main Authors: WOODFILL, C. J. I, WAYNE, N. L, MOENTER, S. M, KARSCH, F. J
• Format: Journal Article
• Language: English
• Published: Madison, WI Society for the Study of Reproduction 01-04-1994
• Subjects: Animals; Biological and medical sciences; Biological Clocks; Circadian Rhythm; Estradiol - administration & dosage; Estradiol - pharmacology; Female; Fundamental and applied biological sciences. Psychology; Luteinizing Hormone - metabolism; Mammalian reproduction. General aspects; Melatonin - administration & dosage; Melatonin - blood; Melatonin - pharmacology; Photoperiod; Pineal Gland - physiology; Pineal Gland - surgery; Reproduction - physiology; Seasons; Sheep; Vertebrates: reproduction; Breeding season; Circannual rhythm; Neuroendocrine regulation; Biological rhythm; Photoperiod; Synchronization; Melatonine; Vertebrata; Mammalia; Sheep; Artiodactyla; Ungulata; Pineal hormone
• DOI: 10.1095/biolreprod50.4.965
• EISSN: 1529-7268
• ISSN: 0006-3363

Seasonal reproduction in the ewe is generated by an endogenous circannual rhythm of reproductive neuroendocrine activity.
Exposure to as few as 70 days of photoperiodic information a year is sufficient to synchronize the rhythm. The present study
was conducted to identify which portions of the photoperiodic cycle are utilized for synchronization. For this purpose, we
used pinealectomized ewes that could not respond reproductively to changes in day length. Selected photoperiodic information
was provided via infusion of melatonin, a hormone that provides the neuroendocrine code for day length in this species. Melatonin
was delivered according to circadian patterns. The infusion patterns were tailored to mimic those of melatonin secretion in
pineal-intact ewes during one of the four seasons: winter, spring, summer, or autumn. The infusions were provided for 90 days
a year during each of the three years following pinealectomy. The ewes were ovariectomized and treated with constant-release
Silastic capsules containing estradiol; reproductive neuroendocrine activity was monitored by measurement of serum concentrations
of LH. In the absence of exogenous melatonin, most (19 of 24) pinealectomized controls exhibited circannual LH cycles that
were not in synchrony, indicating that the rhythm was free-running. Melatonin synchronized the rhythm (such that the period
was 365 days and the stages of the rhythm were both concurrent among animals and in appropriate phase with the geophysical
year), but not all melatonin patterns were equally effective in this regard. The most effective melatonin patterns mimicked
those of secretion during summer. Spring and autumn melatonin patterns were less effective, and winter melatonin patterns
were ineffective. These results support the concept that there is a seasonal specificity with regard to the photoperiodic
cues that synchronize the circannual rhythm of reproductive neuroendocrine activity in the ewe. The rhythm is synchronized
most effectively by long-day photoperiodic cues perceived on or around the summer solstice.