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"Short" Loop Feedback Regulation of Hypothalamic and Brain Thyrotropin-Releasing Hormone Content in the Rat and Dwarf Mouse^*

Division of Endocrinology and Metabolism, University of Massachusetts Medical School Worcester, Massachusetts 01605

Address requests for reprints to: Dr. Apostolos G. Vagenakis, Division of Endocrinology and Metabolism, University of Massachusetts Medical Center, 55 Lake Avenue North, Worcester, Massachusetts 01605.

Abstract

In order to determine whether T₄) TSH, or both affect hypothalamic TRH content, primary or secondary hypothyroidism was induced in the rat by thyroidectomy (Tx) or hypophysectomy (Hx), respectively. Two weeks later, rats were treated with T₄, TSH, or both for 14–16 days. Tx or Hx significantly decreased hypothalamic TRH content, and T₄ treatment restored hypothalamic TRH to normal in the Tx but not in the Hx rats. When TSH was administered simultaneously with T₄ to Hx rats, hypothalamic TRH content was restored to normal. Whole brain TRH content was not affected by Tx, Hx, or by the various treatment regimens.

Similar experiments were performed in Snell dwarf (dw/dw) mice. Hypothalamic TRH content was significantly decreased when compared to normal litter mates (dw/+). As in Hx rats, T₄ administration to dw/dw mice for 16 days did not restore hypothalamic TRH content to normal. Brain TRH content was similar in both groups and was not altered by T₄ treatment.

It is concluded that T₄ affects hypothalamic TRH content, but that TSH is required for this T₄ effect. Whole brain TRH, on the other hand, is unaffected by any of these alterations in thyroid-pituitary function.

Footnotes

^* This work was supported by Grant AM-18919-02 from NIAMDD. It was presented in part at the 59th Annual Meeting of The Endocrine Society, June 8-10, 1977, Chicago, IL.

Received November 19, 1977.

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