Evidence that naloxone attenuates the consumption of food by domestic pigeons through a central influence

Three experiments were performed on domestic pigeons deprived of food for 24 hr to determine whether the anorexic influence of naloxone, which is observed in this species, results from a central effect of this drug. Injections were given 15 min before the delivery of food, and repeated measures of the consumption of food were obtained during the next 20 min. The systemic administration of either 2 or 10 μ mol of naloxone, but not ofequimolar amounts of quaternary naloxone, that does not traverse the blood-brain barrier, reduced the consumption of food of the pigeons. At a dose of 20 μmol, quaternary naloxone also slightly attenuated the ingestion of food, possibly because this drug can penetrate the blood-brain barrier when given in large doses. In another experiment, the intraventricular administration of 12.5, 25, 50 or 100 μg of naloxone attenuated the food intake in a dose-related fashion. Injection of 25 μg of the antagonist was more efficient in this respect when administered intraventricularly rather than systemically. From these experiments, it is concluded that in pigeons, the anorexic influence of naloxone is, at least partly, centrally mediated.