REGULATION OF ACTIVITY IN THE HYPOTHALAMO-PITUITARY-ADRENAL AXIS IS INTEGRAL TO A LARGER HYPOTHALAMIC SYSTEM THAT DETERMINES CALORIC FLOW

We have previously reported that there are diurnal rhythms in the magn itude of ACTH responses to stressors and in the sensitivity of stress- induced ACTH responses to facilitation induced by prior stress and to corticosterone (B) feedback induced by exogenous B. In all cases ACTH was more responsive in the morning than in the evening in nocturnally feeding rats. We have also shown in adrenalectomized rats that an over night fast reduces ACTH responses to restraint in the morning compared with rats fed acl libitum, and we have shown that calorie-containing gavage during the fast increases the amplitude of ACTH responses to re straint in fasted rats. Therefore, this diurnal rhythm is not associat ed with B feedback and is associated with calories. In these studies w e asked whether young, male intact rats that were deprived of food ove rnight had: 1) hypothalamo-pituitary-adrenal (HPA) axis responses duri ng the fasting period; 2) altered basal activity in the HPA axis; 3) a ltered responsivity of ACTH to restraint; and 4) altered sensitivity o f restraint-induced ACTH responses to facilitation or B feedback. Our results show that food deprivation: 1) induces marked-ACTH and B respo nses during the fast that mirrors the pattern of food intake in fed ra ts, with an approximately 3-h lag; 2) results in essentially no change in basal ACTH in the morning; 3) reduces ACTH responsivity to stress in the morning; and 4) reduces ACTH responsivity to prior stress-induc ed facilitation and exogenous B-induced feedback. We conclude that: 1) the HPA axis serves as a default pathway to feeding when food is not available; 2) the diurnal rhyhthms in restraint-induced ACTH secretion are determined by food intake; and 3) the HPA axis is integral to a l arger hypothalamic system that mediates energy flow.