ACTIVITY OF THE HYPOTHALAMIC-PITUITARY-ADRENAL AXIS AND SYMPATHOADRENAL SYSTEM DURING FOOD AND WATER-DEPRIVATION IN THE RAT

It has been previously shown that chronic water deprivation or hyperto nic saline intake, osmotic stress models with concomitant decrease in food intake, decrease hypothalamic CRH mRNA levels and ACTH responses to acute stimulation, To determine the contribution of food restrictio n to the effects of osmotic stimulation, the function of the hypothala mic pituitary adrenal axis was analyzed in rats subjected to food depr ivation, water deprivation or their combination for 60 h. In all three groups, basal levels of plasma corticosterone were increased, while A CTH and catecholamines were unchanged. Basal plasma vasopressin levels were normal in food deprived rats, but significantly increased in wat er deprived and simultaneously food and water deprived rats. In contra st to the 25% reduction of plasma ACTH responses to 30 min immobilizat ion by water deprivation, food deprivation had no inhibitory effect an d prevented the decreased ACTH responsiveness caused by water deprivat ion. In control rats, plasma corticosterone levels increased 22.5-fold 30 min after immobilization, and this response was significantly pote ntiated in the water deprived, food deprived and combined food and wat er deprived groups. The elevation in plasma catecholamines in response to acute immobilization was also enhanced in both water deprived and food deprived rats. In situ hybridization studies showed a 35% increas e in VP mRNA levels in the PVN after water deprivation, whereas food d eprivation caused a slight decrease and prevented the stimulatory effe ct of water deprivation. CRH mRNA in the PVN was reduced by 27% after food deprivation and by 67% after water deprivation, but simultaneous food and water deprivation caused a significantly smaller reduction si milar to that in food deprivation alone. Anterior pituitary POMC mRNA levels were unchanged after water deprivation alone, but significantly increased after food deprivation and simultaneous food and water depr ivation. The increased pituitary POMC mRNA and pituitary responsivenes s despite enhanced corticosterone responses indicate an altered glucoc orticoid feedback in food deprived rats. The data show that the effect s of food deprivation on the PIPA axis function differ from those of w ater deprivation and that reduced food intake is unlikely to contribut e to the inhibitory effect of osmotic stress on the HPA axis.