The representation in memory of morphine’s unconditioned motivational effects depends on the nature of the conditioned stimulus

Abstract

In conditioning paradigms, morphine has both rewarding and aversive motivational properties. The rewarding motivational property is preferentially associated with places, whereas the aversive property is preferentially associated with tastes. We used this preferential associability of stimuli in combination with a higher order conditioning procedure to ask how the motivational effects of morphine are represented in memory. First-order conditioning involved the pairing of a novel taste or distinct place with an intraperitoneal injection of morphine (15 mg/kg) in two separate groups of rats. As expected, conditioned taste aversions developed in one group and conditioned place preferences developed in the other. Second-order conditioning involved pairing the firstorder taste conditioned stimulus (CS) with a distinct place CS in the absence of morphine in one group and pairing the first-order place CS with a distinct taste CS in the absence of morphine in the other group. If the first-order taste CS, for example, elicits from memory a representation of the entire motivational spectrum of morphine, then the second-order place cues might be expected to reveal conditioned place preferences on the basis of differential associability findings. However, if the first-order taste CS calls up only a representation of morphine’s aversive effects, then second-order conditioned place aversions would be expected. We found that the first-order taste CS produced second-order conditioned place aversions. Similarly, in the other group, the first-order place CS produced second-order taste preferences. In other words, only conditioning within one motivational system was observed. We propose that the first-order stimulus conditioned to morphine represents in memory only that single motivational effect of morphine to which the first-order conditioned stimulus is preferentially associable.