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Temporal Requirements of Thyroid Hormones for Seasonal Changes in LH Secretion

Reproductive Sciences Program (H.J.B., C.V., F.J.K.) and Department of Physiology (H.J.B., F.J.K.), University of Michigan, Ann Arbor, Michigan 48109; and Department of Physiology, West Virginia University (G.M.A., R.L.G., J.M.C.), Morgantown, West Virginia 26506

Address all correspondence and requests for reprints to: Dr. Fred J. Karsch, Reproductive Sciences Program and Department of Physiology, 300 North Ingalls Building, 11th Floor, Ann Arbor, Michigan 48109-0404. E-mail: . fjkarsch{at}umich.edu

The transition between breeding and anestrous seasons in ewes is driven by an endogenous rhythm in responsiveness to estradiol negative feedback. One stage of this rhythm, the transition to anestrus, requires the presence of thyroid hormone during a window of responsiveness that opens in the late breeding season. The primary goal of this study was to assess when ewes lose responsiveness to thyroid hormone (i.e. when the window closes). In addition, we investigated whether thyroid hormone influences aspects of seasonality other than the transition to anestrus. Ovariectomized ewes maintained in a simulated natural photoperiod were implanted with estradiol, thyroidectomized, and treated with T₄ for 100 d beginning at progressively later dates during the anestrous season. Onset of neuroendocrine anestrus (decrease in LH), latency to anestrus, and time of onset of the subsequent neuroendocrine breeding season (rise in LH) were determined. Ewes gradually lost responsiveness to T₄ during the latter half of the anestrous season, as judged by increasing latency to the decrease in LH and, eventually, failure to exhibit a decrease in LH. Progressively later T₄ replacements also caused progressive delays in the subsequent breeding season. In contrast, the annual PRL cycle was not significantly affected by thyroidectomy or T₄ replacement. These findings indicate that 1) responsiveness to T₄ is lost gradually during the mid to late anestrous season; 2) thyroid hormones can influence the timing of the breeding season and thus may be required for the maintenance or entrainment of the endogenous reproductive rhythm; 3) thyroid hormones are not required for all seasonal neuroendocrine cycles.

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