Glucocorticoid Feedback Inhibition of Adrenocorticotropic Hormone Secretagogue Release

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Abstract
Feedback inhibition of the adrenocortical axis by circulating glucocorticoids occurs at the pituitary and CNS sites. In the CNS, both hypothalamic and suprahypothalamic sites have been implicated as mediators of glucocorticoid feedback activity. In the present experiments, we have attempted to identify specific CNS regions mediating the feedback and to characterize which hypothalamic adrenocorticotropic hormone secretagogues are under glucocorticoid inhibitory control. Adrenalectomized rats were presented with a delayed feedback signal in the form of systemic infusion with corticosterone or dexamethasone. Hypophysial-portal concentrations of corticotropin-releasing factor (CRF), arginine vasopressin (AVP), and oxytocin (OT) were determined before and during a hypotensive stressor in the face of varying levels of feedback. The rats were then killed, and the extent of total, type I, and type II corticosteroid receptor occupancy in hippocampus, hypothalamus, and amygdala was determined. The following observations were made: (1) increased hippocampal corticosteroid receptor occupancy was associated with suppressed adrenocorticotropic hormone secretagogue concentrations; (2) the major, significant predictor of initial (prehypotensive) concentrations of CRF, AVP, and OT was the extent of occupancy of hippocampal type II receptors, often in combination with occupancy of hippocampal type I or hypothalamic receptors; (3) secretion of CRF induced by hypotension was best predicted by hippocampal type I and type II receptor occupancy (stress-induced OT secretion was best predicted by hippocampal type II and hypothalamic receptor occupancy), and (4) the ‘shape’ of the hippocampal type II receptor occupancy versus initial AVP concentration curve suggested a nonlinear, threshold type of relationship, implying tight hippocampal regulation of AVP secretion.