A CENTRAL NEGATIVE FEEDBACK ACTION OF THYROID-HORMONES ON THYROTROPIN-RELEASING-HORMONE SECRETION

Two experiments were conducted to test the hypothesis that thyroid hor mones exert central negative feedback effects on the secretion of TRH from the hypothalamus in the ewe. In the first experiment, we examined the effects of thyroidectomy on the secretion of TRH and TSH. Thyroid ectomy was followed by an unambiguous increase in TRH in pituitary por tal plasma and TSH in the peripheral circulation. In the second experi ment, we tested the effects of T-4 replacement to thyroidectomized ewe s. T-4 replacement reversed the effects of thyroidectomy on TRH and TS H release. Of interest, TRH secretion in thyroidectomized ewes was con tinuously elevated during the collection, raising the possibility that TRH is secreted continuously, rather than exclusively in a strictly p ulsatile manner indicative of phasic discharges synchronized among TRH neurosecretory elements. Collectively, these results suggest that thy roid hormones can act centrally to inhibit TRH (and thus TSH) release in the ewe, and they support the concept that at least part of the neg ative feedback action of thyroid hormones is exerted at the hypothalam ic level.