Modification of the punishing effects of psychoactive drugs in rats by previous drug experience.

In Experiment 1, it was shown that chronic prior exposure to amphetamine attenuated the conditioned avoidance of saccharin that was produced by both amphetamine and morphine during gustatory conditioning trials; the relationship between morphine and amphetamine was somewhat anomalous because of their pharmacological dissimilarity. The relationship was also asymmetrical, since in Experiment 2, chronic prior exposure to morphine failed to mitigate avoidance conditioning by amphetamine but was effective in attenuating conditioning by morphine itself. In a third experiment, it was found that prior treatment with chlordiazepoxide attenuated saccharin avoidance conditioned by chlordiazepoxide but not by amphetamine or morphine. The findings were related to Parker, Failor, and Weidman's hypothesis, based on findings with morphine, concerning the development of conditioned preferences for substances associated with the repletion of artificially induced biological needs. It was suggested that the findings were best interpreted as a reflection of drug tolerance rather than conditioned preference.