PRIOR EXPOSURE TO PALATABLE SOLUTIONS ENHANCES THE EFFECTS OF NALTREXONE ON FOOD-INTAKE IN RATS

Previous research has suggested that chronic intake of palatable foods and fluids enhances the activity of the endogenous opioid system. To examine this suggestion, the effect of naltrexone on food intake was e xamined in male Long-Evans rats with or without prior exposure to pala table solutions. In Experiment 1, rats were fed laboratory chow alone or laboratory chow and a 32% sucrose solution, and in Experiment 2, we re fed chow alone, chow and a 32% Polycose solution, or chow and a 0.1 5% saccharin solution for three weeks. The sucrose, Polycose, and sacc harin solutions were removed 18 h prior to drug administration. Rats t hen received injections of naltrexone hydrochloride (0.0, 0.3 or 3.0 m g/kg, sc) and chow intakes were measured during the subsequent 1, 2, 4 , 6 and 24 h. Naltrexone injections had minimal effects on intakes of animals which previously had consumed only chow. In contrast, naltrexo ne led to significant dose-related decreases in chow intakes in rats w hich had previously consumed the sucrose, Polycose, or saccharin solut ions. These results provide confirmation for the suggestion that chron ic intake of palatable solutions alters the activity of the endogenous opioid system. (C) 1997 Elsevier Science Inc.