The effects of naloxone on flavor-calorie preference learning indicate involvement of opioid reward systems

Four experiments investigated the effects of the opioid antagonist naloxone hydrochloride on learning and expression of preferences for flavors paired with calories from dextrose or ethanol. Experiments 1 through 3 showed that naloxone eliminated a dextrose-paired flavor preference in two-bottle and one-bottle tests. When naloxone was paired with flavored dextrose in conditioning trials, no preference for the naloxonepaired flavor over the saccharin-paired flavor emerged. Naloxone also eliminated the preference for an ethanol-paired flavor over a saccharin-paired flavor in postconditioning tests. In Experiment 4 shock-induced freezing was used as a probe for conditioned opioidergic activity brought on by tasting a flavor that had been paired with dextrose. Rats froze less after tasting the dextrose-paired flavor than after tasting a saccharin-paired flavor. These results are discussed as evidence for involvement of central opioid reward systems in flavor-calorie learning.