Conditioning of morphine-induced taste aversion and analgesia.

The process of selective associations is evident in the aversive conditioning literature, where it has been shown that external cues are readily associated with peripheral pain, whereas taste cues are more easily associated with effects of drug administration. Within this framework, it is of interest that the failures to obtain a conditioned analgesic response to a morphine-associated CS have used external cues as conditioned stimuli. In Experiment 1, subjects re-exposed to a morphine-associated CS not only expressed the anticipated taste aversion, but also exhibited a decrease in pain sensitivity that was evident 15 or 30 min following CS re-exposure. Experiment 2 suggested that the conditioned analgesic response was opioid mediated, as pre-test administration of naloxone blocked expression of the analgesic CR. In Experiment 3, an increase in opiate receptor sensitivity produced by chronic naltrexone treatment did not affect the strength of the taste aversion, but resulted in an increase in the magnitude of the conditioned analgesic response. Collectively, these data suggest a neuropharmacological dissociation in systems mediating the two responses.