Adrenal function affects morphine-induced feeding during dark period, but not during light period in rats

The present study was undertaken to investigate the relationships between morphine-induced feeding and the adrenal functions. Morphine (5 mg/kg) was intraperitoneally administered at 10:45 (light period) or 18:45 (dark period). The orectic effects of morphine during the light period in normal rats were not influenced by adrenalectomy; however, the anorectic effects during the dark period in normal rats were attenuated by both adrenalectomy and adrenodemedullation. Corticosterone (10 mg/kg) itself had no effects on feeding during the light and dark period. Morphine did not alter blood insulin levels during the light period, but markedly decreased it during the dark period independently of feeding. These results show that morphine has two different effects on feeding by administration time, and they suggest that the adrenal affects morphine-induced feeding only during the dark period (hungry state), presumably through insulin release, but not during the light period (satiated state).