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This version published online on April 3, 2003
Endocrinology, doi:10.1210/en.2003-0322
A more recent version of this article appeared on July 1, 2003
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Submitted on March 13, 2003
Accepted on March 28, 2003

Evidence that thyroid hormones act in the ventromedial proptic area and the premammillary region of the brain to allow the termination of the breeding season in the ewe

Greg M. Anderson1, Steven L. Hardy1, Miroslav Valent1, Heather J. Billings1, John M. Connors1, and Robert L. Goodman1*

1 Department of Physiology and Pharmacology, West Virginia University (G.M.A., S.L.H., M.V., J.M.C., R.L.G.) Morgantown, West Virginia 26506; Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand (G.M.A.); and Reproductive Sciences Program and Department of Physiology, University of Michigan (H.J.B.), Ann Arbor, Michigan 48109

* To whom correspondence should be addressed. E-mail: bgoodman{at}hsc.wvu.edu.

Thyroid hormones are permissive for various species to enter seasonal anestrus. In the ewe they act centrally to permit the onset of potent estradiol (E2) negative feedback responsible for anestrus, but the specific sites of action are unknown. Therefore, we tested if thyroxine (T4) replacement via chronic microimplants in any of five brain areas could reverse the reproductive effects of thyroidectomy. Diffusion of 125I- T4 from the microimplant was largely (> 98%) limited to a 1.2 mm radius. A marked decline in LH concentration in ovariectomized, E2-treated ewes was used as an index for anestrus. In Exp 1, all thyroidectomized (THX) ewes with microimplants in the medial preoptic area, A15 area and medial basal hypothalamus failed to enter anestrus; instead, LH levels remained elevated similar to those in untreated THX controls. In ventromedial preoptic area (vmPOA)-microimplanted ewes, only the two animals with the most caudal microimplants entered anestrus, as did thyroid-intact controls and THX ewes receiving icv or sc T4 replacement. In Exp 2, all vmPOA-treated ewes with similar placements to those effective in Exp 1 entered neuroendocrine anestrus, along with all ewes microimplanted in the premammillary region (PMR). Thus the PMR and vmPOA are brain sites where thyroid hormones act to permit the onset of seasonal anestrus.
Key words: neuroendocrinology • hypothalamus • luteinizing hormone • seasonal reproduction • thyroid hormones




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