Neuroendocrine Control of Thyrotropin Secretion in the Rat

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Abstract
The participation of 2 control mechanisms in the regulation of pituitary thyrotropic function by the hypothalamus was evaluated. Blood flow measurement by Sapirstein's Cardiac Output Distribution method in rats showed that extensive anterior hypothalamic lesions do not limit adenohypophysial blood flow and do not prevent the increase in thyroidal blood flow observed after goitrogen feeding, even when they block gland hypertrophy. Thus, the hypothalamic influence over pituitary thyrotropic function does not appear to be mediated through the control of pituitary blood flow. The participation of a neurohumor of hypothalamic origin was established in experiments in which the pituitary was separated from the hypothalamus without disrupting the vascular supply of either tissue. By joining in parabiotic union a rat whose anterior hypothalamus had been destroyed by electrocoagulation with a hypophysectomized partner, it was shown that the intact hypothalamus of the hypophysectomized partner, acting across the parabiotic union, could reverse the “goiter block” induced in the partner with the pituitary by the hypothalamic lesion. TSH did not cross the parabiotic union efficiently. The neurohumor stimulating TSH secretion across the union appeared to have a half-life in the circulation of the donor animal of the order of several hours, and exerted its effect across the union even in the presence of pituitary remnants in the donor.