Naltrexone fails to block the acquisition or expression of a flavor preference conditioned by intragastric carbohydrate infusions

The effects of naltrexone on the expression and acquisition of flavor prefe rences conditioned by the postingestive actions of carbohydrates were inves tigated. Food-restricted rats (Experiment 1) were given one-bottle training with one flavored saccharin solution (CS+) paired with intragastric (IG) i nfusions of 16% sucrose, and another flavored saccharin solution (CS -) pai red with water infusions. In two-bottle tests CS+ was preferred to CS -, an d naltrexone (1.0-5.0 mg/kg) reduced total intake but not CS+ preference. I n Experiment 2 food-restricted rats that received naltrexone (0.1 or 1.0 mg /kg; NTX group) throughout one-bottle training consumed less CS+ and CS - t han did saline-treated control rats. Yet, the NTX and control groups displa yed similar CS+ preferences during two-bottle tests when treated with salin e or naltrexone (0.1-5.0 mg/kg). In Experiment 3, rats were trained to acce pt more CS+ than CS - in one-bottle tests. Naltrexone (0.1-2.5 mg/ kg) redu ced the one-bottle intakes of both solutions, and the rats continued to con sume more CS+ than CS -. We conclude that the opioid system modulates the c onsumption of flavored solutions, but is not critically involved in the acq uisition or expression of flavor preferences conditioned by IG carbohydrate . (C) 2000 Elsevier Science Inc. All rights reserved.