Delayed Secretion of ACTH in Rats with Hypothalamic Lesions1

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Abstract
Studies have been carried out in rats in an attempt to evaluate the impairment in ACTH secretion which follows placement of hypothalamic lesions. The effects of small and large lesions in various regions of the hypothalamus were investigated with respect to the rapidity of stress-induced ACTH release. Changes in both plasma corticosterone and adrenal ascorbic acid were determined and compared following unilateral adrenalectomy. One of the consequences of a hypothalamic lesion was to delay, rather than to prevent entirely, the ACTH secretory response to stress. The duration of the delay was proportional to lesion size: small lesions resulted in a 1-hr delay, whereas larger lesions delayed for 2 hr the increased secretion of ACTH following a stimulus. These results indicate that, although hypothalamic lesions prevented the rapid stress-induced release of ACTH, they did not prevent a substantial delayed response in which the plasma corticosterone concentration attained was almost 90% of that found in control rats. A correlation was observed between plasma corticosterone changes and adrenal ascorbic depletion. Comparison between these indices of ACTH secretion revealed that a steady rate of secretion of corticosterone was compatible with a stable adrenal ascorbic acid concentration. However, when plasma corticosterone concentration -was increased, a concomitant depletion of adrenal ascorbic acid was observed. The results suggest, but do not prove conclusively, that adrenal ascorbic acid depletion occurs when ACTH secretion increases, whereas during stabilized ACTH secretion adrenal ascorbic acid concentration remains constant. (Endocrinology74: 28, 1964)