The effect of morphine preexposure on the acquisition of morphine-induced taste aversions : A nonassociative effect

In Experiment 1, the amount of time rats spent engaged in a range of behaviors was recorded immediately prior to and following the intraperitoneal administration of morphine sulfate (6 mglkg) or distilled water. No behavioral differences were observed between these groups. In Experiment 2, preexposure to this low dose of morphine attenuated the subsequent acquisition of a morphine-induced taste aversion independent of the similarity of the preexposure and con- ditioning environments. These results with a dose of morphine that does not produce any be- havioral effects, which in turn could potentially mask associative conditioning during preex- posure, confirm that the attenuating effects of morphine preexposure on taste aversion learning are nonassociative.