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 preferences conditioned by the postingestive actions of carbohydrates were investigated. Food-restricted rats (Experiment 1) were given one-bottle training with one flavored saccharin solution (CS+) paired with intragastric (IG) infusions of 16% sucrose, and another flavored saccharin solution (CS -) paired with water infusions. In two-bottle tests CS+ was preferred to CS -, and naltrexone (1.0-5.0 mg/kg) reduced total intake but not CS+ preference. In 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 - than did saline-treated control rats. Yet, the NTX and control groups displayed similar CS+ preferences during two-bottle tests when treated with saline or naltrexone (0.1-5.0 mg/kg). In Experiment 3, rats were trained to accept more CS+ than CS - in one-bottle tests. Naltrexone (0.1-2.5 mg/ kg) reduced the one-bottle intakes of both solutions, and the rats continued to consume more CS+ than CS -. We conclude that the opioid system modulates the consumption of flavored solutions, but is not critically involved in the acquisition or expression of flavor preferences conditioned by IG carbohydrate. (C) 2000 Elsevier Science Inc. All rights reserved.