A context-specific detrimental effect of UCS preexposure on place conditioning with morphine

Abstract

The effect of morphine preexposure on place conditioning with morphine was investigated. In the first experiment, five injections of 10 mg/kg morphine were administered to rats prior to place conditioning or taste-aversion training with morphine. Although this number of preexposures retarded taste-aversion learning, there was no effect on place conditioning. In subsequent experiments we investigated the role of context blocking in UCS preexposure in place conditioning. In one experiment, preexposure to five morphine injections prior to place conditioning resulted in a reduced place preference, compared with preexposure and place conditioning in different contexts. However, the overall detrimental effect of morphine preexposure was questionable, because the rats that were preexposed were only marginally different from those that were not preexposed. In a final experiment we examined the effect of a context change from preexposure to place conditioning with 15 preexposures and demonstrated a detrimental effect of preexposure on place conditioning that was context specific. These results support a role of classical conditioning in place-preference conditioning with morphine.