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.