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Biology of Reproduction, Vol 23, 1061-1068, Copyright © 1980 by Society for the Study of Reproduction
1 Reproductive Endocrinology Program,
Departments of Physiology and Pathology,
The University of Michigan,
Ann Arbor, Michigan 48109 In ewes, a pivotal neuroendocrine event regulating seasonal breeding is a change in response of
the hypothalamo-pituitary axis to the negative feedback action of estradiol. Further, the major
environmental parameter regulating seasonal breeding is photoperiod. This study was performed to
examine whether changes in photoperiod regulate breeding seasons by modulating response to
steroid feedback. Six intact ewes, five estradiol-treated ovariectomized ewes, and one vasectomized
ram were housed under artificial short-day (8L:16D) and long-day (16L:8D) photoperiods which
were alternated every 90 days. Under these conditions, the intact ewes underwent two breeding
and anestrous seasons in 1 year. Further, the transitions between breeding seasons coincided with
striking fluctuations in serum LH (between <0.3 and 10 ng/ml) and FSH (50-170 ng/ml) in the
estradiol-treated ovariectomized ewes, long days causing anestrus and an increased response to
estradiol (low LH and FSH), short days resulting in breeding season and a decreased response to
estradiol (high LH and FSH). Similar results were obtained in identical groups of ewes, which were
subjected to 120-day alternations between long and short days, except that there were only three
seasonal transitions and FSH was not measured. These results provide strong support for the
hypothesis that in ewes the mechanism whereby photoperiod regulates seasonal breeding includes
modulation of response to the negative feedback action of estradiol on gonadotropin secretion.
Note:
ACKNOWLEDGMENTS
We are indebted to Dr. Douglas L. Foster for his
assistance in designing this study. We would like to
thank Ms. Marjorie Hepburn and Ms. Barbara Glover
for their expertise in conducting the radioimmunoassays, and Drs. G. D. Niswender and L E. Reichert,
Jr., for providing reagents used in the radioimmunoassays. We also thank Mr. Douglas Doop for designing,
constructing, and maintaining the photoperiod-controlled rooms, and for his invaluable assistance in all
aspects of the animal experimentation.
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L. A. Thrun, G. E. Dahl, N. P. Evans, and F. J. Karsch A Critical Period for Thyroid Hormone Action on Seasonal Changes in Reproductive Neuroendocrine Function in the Ewe Endocrinology, August 1, 1997; 138(8): 3402 - 3409. [Abstract] [Full Text] [PDF]
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C. Viguie, J. Thibault, J.-C. Thiery, Y. Tillet, and B. Malpaux Characterization of the Short Day-Induced Decrease in Median Eminence Tyrosine Hydroxylase Activity in the Ewe: Temporal Relationship to the Changes in Luteinizing Hormone and Prolactin Secretion and Short Day-Like Effect of Melatonin Endocrinology, January 1, 1997; 138(1): 499 - 506. [Abstract] [Full Text] [PDF]
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