Role of the thyroid gland in seasonal reproduction. III. Thyroidectomy blocks seasonal suppression of gonadotropin-releasing hormone secretion in sheep.

Previous studies demonstrated that the thyroid gland is required for the changes in LH secretion that lead to the end of the breeding season of female sheep, a reproductive transition generated within the ewe by an endogenous annual rhythm. The present study tested the hypothesis that this role of the thyroid is mediated via the GnRH neurosecretory system. The pulsatile secretion of GnRH into hypophyseal portal blood and LH into peripheral blood, as well as the neuroanatomical distribution of GnRH neurons and their light microscopic morphology, were compared among anestrous ewes and thyroidectomized (THX) ewes that failed to make the transition to the anestrous season. The study was conducted under photoperiodic conditions in which the transition to anestrus was generated by the endogenous rhythm. Each animal was ovariectomized and treated sc with a constant release implant of estradiol to normalize gonadal steroid status among thyroid-intact and THX ewes. High frequency pulses of GnRH and LH were evident in THX ewes that failed to make the transition to anestrus, whereas pulsatile secretion of GnRH and LH was generally not observed in thyroid-intact controls that had entered anestrus. This marked effect of thyroidectomy on GnRH secretion was not associated with widespread changes in the total number, distribution, or light microscopic morphology of GnRH neurons in the hypothalamus and preoptic area. Our finding that pulsatile secretion of GnRH is elevated in THX ewes that fail to make the transition to anestrus supports the hypothesis that the thyroid gland is required for the endogenously generated switch in function of the GnRH neurosecretory system that leads to the end of the breeding season of the ewe.