Naloxone blocks that portion of feeding driven by sweet taste in food-restricted rats

We evaluated the potency of naloxone on intake of normal and sweet chow in food-deprived and schedule-fed rats. We found that naloxone's anorectic potency was dependent on the type of chow presented to the rats and the deprivation schedule utilized to stimulate food intake. In 24-h and 48-h deprived rats, naloxone decreased intake of normal rat chow at doses ranging from 0.3 to 3 mg/kg. In chronically deprived rats (80% of normal body wt), these doses of naloxone failed to decrease intake of normal chow. Rats eating sweet chow ate more when energy deprived and were more sensitive than rats eating normal chow to naloxone-induced limitations in food intake, both in acute and chronic food-deprived groups. Thus naloxone decreased intake of sweet chow much more effectively than normal chow even when rats were chronically food deprived. We also found that an extremely low dose of naloxone (0.03 mg/kg) decreased intake of sweet chow by almost 50% in satiated rats.