TSH secretion following acute heat exposure, in intact and in hypothalamic deafferentated male rats

Male rats, intact (N) or with complete, anterior, or posterior hypothalamic deafferentations (CHD, AHD, or PHD, respectively), were acutely exposed to an environmental temperature of 36 degrees C, and serum TSH concentrations were determined by RIA. In intact animals, 30 min of exposure to elevated environmental temperature resulted in decreased serum TSH levels. In both CHD and AHD animals, basal serum TSH concentrations were reduced, and heat exposure produced further decreases. In the PHD group, no significant difference in resting serum TSH concentrations was observed, as compared to intacts. Exposure of these animals to 36 degrees C was followed by a reduction in this parameter, similar to that in intact rats. Exposure of intact animals to 3 nonthermal stress modalities produced no changes in serum TSH concentrations. These experiments demonstrate (1) that the hypothalamo-hypophyseal-thyroid axis responds specifically to heat exposure; (2) that extrahypothalamic centers which innervate the medial basal hypothalamus from the rostral direction maintain basal activity of the axis; and (3) that humoral factors, possibly extrahypothalamic thyrotrophin-releasing hormone carried to the median eminence via the CSF, may also mediate both basal and heat-induced secretory activity of the thyrotrophs.