THYROXINE IS PERMISSIVE TO SEASONAL TRANSITIONS IN REPRODUCTIVE NEUROENDOCRINE ACTIVITY IN THE EWE

The observation that circulating thyroxine concentration increases dur ing the breeding season of the ewe, coupled with the finding that thyr oid hormones are required for the transition from the breeding season to anestrus in this species, led us to test the hypothesis that the tr ansition to anestrus is driven by a rise in circulating thyroxine. Suf folk ewes were thyroidectomized (THX) late in the anestrous season. Th yroxine was then either not replaced or provided at doses that produce d nadir, incremental (simulating the seasonal rise), or mildly hyperth yroid concentrations in serum. Additional ewes remained thyroid-intact . To monitor seasonal changes in reproductive neuroendocrine activity, the ewes were ovariectomized and received implants of constant-releas e silastic capsules containing estradiol. Serum concentrations of LH a nd thyroxine were determined in samples collected twice weekly. In all groups, LH increased in mid-September, signifying that manipulation o f thyroid status did not influence onset of the neuroendocrine breedin g season. In thyroid-intact controls, LH decreased to low concentratio ns in mid-January, marking the neuroendocrine transition to anestrus. As expected, LH remained elevated through the end of the study (April) in THX controls not receiving thyroxine, confirming that the neuroend ocrine transition to anestrus is dependent on thyroid hormones. The se asonal decrease in LH was seen in all ewes treated with thyroxine. Thi s decrease in LH was neither advanced in mildly hyperthyroid ewes nor delayed in ewes exposed to low serum concentrations of thyroxine. Thes e results lead to the conclusion that the seasonal increase in circula ting thyroid hormone in the ewe does not drive the transition from the breeding season to anestrus. This supports the view that thyroid horm ones act permissively to allow the neuroendocrine changes that lead to the seasonal cessation of reproductive activity.