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Effect of hypothalamic deafferentation on the secretion of thyrotropin during thyroid blockade and exposure to cold in the rat.

Complete hypothalamic deafferentation, either limited to the median eminence, arcuate nucleus, and basal parts of ventromedial nuclei, or comprising ventromedial nuclei and dorsomedial hypothalamus as well reduced considerably the rise of immunoassayable serum TSH in adult male rats rendered hypothyroid by administration of methimazole. Complete hypothalamic deafferentation extended rostrally to include also the caudal half of the anterior hypothalamus did not affect the TSH response to thyroid blockade. Administration of triiodothyronine (T3) to the goitrogen-treated rats reduced serum TSH rapidly and with equal speed in controls and in deafferented animals, which indicates a direct effect of T3 on the pituitary. Animals which, in addition to the hypothalamic deafferentation, had electrolytic lesions in the median eminence, responded both to thyroid blockade and to T3, but the responses were greatly reduced. The cold-stimulated increase of serum TSH was reduced, but not abolished, in animals deafferented several weeks beforehand, but it was absent in rats deafferented 4 days prior to cold exposure. These results suggest that most, if not all of the thyrotropin-releasing factor (TRF) necessary for normal TSH secretion in resting conditions or during the hypothyroid state, is produced and released in the median eminence-arcuate nucleus area, the adequate activity of which, in turn, depends on intact connections with the anterior hypothalamus. Other hypothalamic and extra-hypothalamic areas containing large amounts of TRF do not seem to be required under these conditions.