Morphine-elicited feeding: diurnal rhythm, circulating corticosterone and macronutrient selection.

The present study examined the feeding response elicited by morphine, injected either intraperitoneally (IP), or into the paraventricular nucleus (PVN), as a function of diurnal cycle and also in adrenalectomized rats with or without peripheral corticosterone replacement. In animals maintained on a single diet of chow, milk and sugar, a diurnal rhythm in both the peripheral and central morphine-induced feeding responses was observed, with a stronger eating effect occurring in the early dark hours compared with the responses obtained in the early light period. Adrenalectomy significantly reduced the feeding induced by morphine injected IP or into the PVN, and acute corticosterone replacement restored the response. Rats maintained on a self-selection feeding paradigm, with carbohydrate, protein and fat simultaneously available, exhibited a significant increase in total caloric intake after morphine injected IP, along with a preferential increase in the consumption of protein and fat. Adrenalectomy nearly abolished this stimulatory effect of morphine on total intake and altered the diet preference pattern. These findings underscore the importance of corticosterone in the feeding response of morphine injected peripherally or specifically into the PVN. The present findings suggest that corticosterone plays an important role in determining the diurnal rhythm of opiate-induced feeding and the function of endogenous opioids in the regulation of energy balance.