Plasma corticosterone and plasma and pituitary ACTH concentrations were determined during feeding and after application of an acute stress at various times after food and water presentation to male rats maintained on a restricted feeding and watering schedule. Both plasma corticosterone and ACTH concentrations fell after the presentation of food and water, and this fall was accompanied by increased levels of ACTH in the pituitary gland. In addition, a rise in plasma levels of ACTH was inhibited in response to an acute stress applied at 0--5 min after presentation of food and water, but ACTH synthesis was not. This inhibition of ACTH and corticosterone secretion in response to stress was transient and dissipated as a relatively linear function of the interval between food presentation and application of the stress. The results suggest that this feeding-induced, corticosteroid-independent inhibition of pituitary-adrenal activity involves active inhibitory mechanisms operating initially on ACTH secretory processes of the pituitary and later on the synthesis of ACTH or on the secretion of hypothalamic corticotropin-releasing factor.
Rats that were maintained on a 23-h food and water deprivation schedule (food and water available between 09:00 and 10:00 h) for 21 days, and whose blood was sampled at various times following initiation of ingestion showed a decrease in plasma adrenocorticotrophin (ACTH) concurrent with dropping plasma levels of corticosterone (COR). This drop occurred from elevated levels of these hormones, most likely due to altered circadian patterns of pituitary-adrenal activity as a result of the rhythm-entraining properties of the feeding schedule. This inhibition of secretion of ACTH is interpreted as reflecting the inhibitory effect of a central nervous system mechanism that can affect ACTH secretion independently of corticosteroid negative feedback.
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