A CRITICAL PERIOD FOR THYROID-HORMONE ACTION ON SEASONAL-CHANGES IN REPRODUCTIVE NEUROENDOCRINE FUNCTION IN THE EWE

Thyroid hormones are obligatory for the annually recurring termination of reproductive activity in a spectrum of seasonal breeders, includin g sheep. Previous studies involving thyroidectomy and T-4 replacement have led to the hypothesis that, in the ewe, thyroid hormones are nece ssary only during a limited interval late in the breeding season for t he neuroendocrine processes that cause the transition to anestrus. The present series of experiments tested this hypothesis by assessing the influence of thyroidectomy, with or without T-4 replacement for speci fic durations and at different times of the year, on the transition to anestrus. Seasonal alterations in reproductive neuroendocrine activit y were monitored by changes in serum LH concentration in ovariectomize d ewes bearing sc SILASTIC brand silicon tubing implants containing es tradiol. Thyroidectomy in mid-December, just before the putative perio d of thyroid hormone action, prevented the development of the neuroend ocrine anestrous season (fall in LH in this animal model). T-4 replace ment for 90 days beginning in late December (i.e., during the postulat ed period of thyroid hormone action) overcame the blockade of anestrus , causing LH to fall in ewes thyroidectomized several months previousl y. The minimal effective duration of exposure to thyroid hormones requ ired for the transition to anestrus was estimated to be 60-90 days. Fu rther, exposure to T-4 for 60-90 days beginning in late December was f ound to be the only time of the year that thyroid hormones were requir ed to maintain seasonal changes in reproductive neuroendocrine activit y. Finally, replacement of T-4 for 90 days at a different time of year (beginning in August) failed to provoke development of neuroendocrine anestrus in thyroidectomized ewes. These results support the hypothes is that thyroid hormones are necessary only during a limited interval late in the breeding season to promote seasonal reproductive suppressi on in the ewe, Further, the reproductive neuroendocrine axis is not eq ually responsive to thyroid hormone at all times of the year. This sug gests there is a critical period of responsiveness during which thyroi d hormones must be present for anestrus to develop.