Pre-exposure to morphine and the attenuation of conditioned taste aversion in rats.

Three experiments were done using male Wistar rats to determine whether the mechanisms underlying the attenuation of a conditioned taste aversion to morphine by pre-exposure to the drug were similar to those involved in the development of tolerance to the analgesic response to morphine. This was tested by determining whether the effect of pre-exposure on conditioned taste aversion was situation-specific. In Experiment 1 it was found that having different environments for the pre-exposure injections and for the conditioning injections of morphine had no effect on the attenuation of the taste aversion. This finding was replicated in Experiment 2 in which it was also found that the attenuation of the analgesic effect, tested for in the same animals, was specific to the environment in which repeated injections were given. It was concluded that the attenuation of conditioned taste aversion involved processes different from those responsible for the attenuation of the analgesic effect of morphine. Experiment 3 showed that pairing the pre-exposure injections of morphine with one distinctive taste stimulus prevented the attenuation of the conditioned taste aversion to a second taste stimulus. These results suggest that different associative processes are responsible for the two types of attenuation.