Pre-exposure effects of morphine, diazepam and delta 9-THC on the formation of conditioned taste aversions.

Prior to taste aversion conditioning with morphine, diazepam or delta 9-tetrahydrocannabinol (delta 9-THC), rats received pre-exposures to the vehicle or one of the three drugs. Morphine pre-exposures blocked the aversion normally induced by morphine, but not by delta 9-THC or diazepam. Diazepam pre-exposures attenuated both the morphine- and diazepam-induced taste aversions to a significantly greater degree than the taste aversion induced by delta 9-THC. As a result of delta 9-THC pre-exposures, the aversions induced by diazepam and delta 9-THC were attenuated as well as the morphine-induced aversion, which was the most greatly attenuated. These results demonstrate that pre-exposure effects are not necessarily bi-directional and, moreover, they are inconsistent with current hypotheses which attempt to account for the attenuating effect of drug pre-exposures on taste-aversion conditioning.