Thyroid hormones act primarily within the brain to promote the seasonal inhibition of luteinizing hormone secretion in the ewe

In the ewe, thyroid hormones are required for the seasonal suppression of GnRH and LH secretion, thereby maintaining an annual rhythm in reproductive activity. The primary site of action of thyroid hormones is unknown; in particular, there is no evidence to distinguish a central from a peripheral ac...

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Bibliographic Details
Published in:Endocrinology (Philadelphia) 1999-03, Vol.140 (3), p.1111-1117
Main Authors: Viguié, C, Battaglia, D F, Krasa, H B, Thrun, L A, Karsch, F J
Format: Article
Language:English
Subjects:
Analysis of Variance
Animals
Brain - physiology
Dose-Response Relationship, Drug
Female
Gonadotropin-Releasing Hormone - secretion
Luteinizing Hormone - secretion
Neurosecretory Systems - physiology
Reproduction - physiology
Seasons
Secretory Rate
Sheep
Thyroid Gland - physiology
Thyroidectomy
Thyroxine - physiology
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Summary:In the ewe, thyroid hormones are required for the seasonal suppression of GnRH and LH secretion, thereby maintaining an annual rhythm in reproductive activity. The primary site of action of thyroid hormones is unknown; in particular, there is no evidence to distinguish a central from a peripheral action. In this study, we test the hypothesis that thyroid hormones can act directly within the brain to promote GnRH/LH seasonal inhibition. Ovariectomized estradiol-treated ewes were thyroidectomized late in the breeding season to prevent seasonal LH inhibition. T4 was then infused for 3 months, either peripherally or centrally. Neuroendocrine reproductive state was monitored by assaying the LH concentration in biweekly blood samples. Central infusion of low dose T4, which restored a physiological concentration of the hormone in cerebrospinal fluid of these thyroidectomized ewes, promoted the neuroendocrine changes that lead to anestrus. The serum LH concentration in these animals fell at the same time as the seasonal LH decline in euthyroid controls. Neither this same T4 dose infused peripherally nor vehicle infused centrally was effective; LH remained elevated, signifying blockade of the mechanism for anestrus. Our results provide strong evidence that thyroid hormones can act directly within the brain to promote seasonal inhibition of neuroendocrine reproductive function in the ewe.
ISSN:0013-7227
DOI:10.1210/en.140.3.1111