Opiate receptors, food intake and obesity.

The present studies tested the effect of acute and chronic administration of naloxone on food intake of lean and genetically obese (ob/ob) mice. Acute administration of naloxone, a drug which blocks opiate receptors, produced a greater reduction of food intake in obese (ob/ob) mice than in the lean littermates. For chronic experiments with naloxone, the daily feeding period was shortened to eight hours and two injections of naloxone were given four hours apart. With this procedure of scheduled-feeding the food intake of both lean and obese mice was depressed during the first hour after injecting naloxone. However, beginning on the second day of treatment, the lean mice began to eat more food than the untreated controls during the eight hour feeding period. Food consumption by lean mice reached values 140 to 200% above the control levels between the fourth and sixth day. In the obese mice the rise in food intake was more gradual and did not reach 200% of the control value until the sixth day. Body weight changes reflected the changes in food intake. In contrast to naloxone, chronic treatment with morphine lowered food intake and blocked the stimulatory effect of naloxone. Our findings suggest that endogenous opioids may play a role in signalling satiety and in regulating long-term energy balance.