THE CONTRIBUTIONS OF ASSOCIATIVE AND NONASSOCIATIVE MECHANISMS IN THE US PREEXPOSURE EFFECT IN TASTE-AVERSION LEARNING

In conditioned taste aversion learning, rats avoid the consumption of a taste solution that has previously been paired with a drug. When the drug is administered prior to the taste-drug pairing, however, subsequent taste-aversion learning is attenuated. This phenomenon has been called the US preexposure effect. Although this phenomenon is clear, its underlying mechanism is not. For example, depending upon the specific drug examined, either an associative or nonassociative mechanism has been presented to account for the effects of drug preexposure. To test whether the mechanism underlying the US preexposure effect is drug specific or simply a result of procedural differences in the examination of different drugs, rats in Experiment 1 were given five preexposures in a distinctive environment and conditioned either in the distinctive environment or the home cage to either LiCl (Experiment 1A) or morphine (Experiment 1B). As demonstrated, the drug-specific nature of the US preexposure was still evident with identical test procedures. It was suggested that the degree of pharmacological adaptation to the drug during preexposure might be responsible for this drug-specific nature of the preexposure effect. This view was supported in Experiment 2, in which a compound was given during preexposure to which pharmacological adaptation is slow to develop, i.e., methadone hydrochloride. It was predicted that with such a drug, both associative and nonassociative mechanisms may contribute to the preexposure effect simultaneously. As reported, both mechanisms contributed to the effects of methadone preexposure. If the degree of pharmacological change during preexposure determines the mechanism underlying the preexposure effect, it was suggested that the degree of pharmacological adaptation and the subsequent mechanism underlying the preexposure effect should be affected by the specific parameters used during preexposure. This possibility was tested and supported in Experiment 3, in which 10 methadone preexposures were administered. Under this condition, the mechanism underlying the methadone preexposure was totally nonassociative in nature. Based on these results, it was concluded that by knowing the degree of pharmacological adaptation and environmental conditioning during preexposure, the outcome of US preexposure within a taste aversion paradigm may be accounted for and predicted.