Conditional hyperalgesia is elicited by environmental signals of morphine

Groups of rats were administered a placebo and tested for sensitivity to “hot plate” stimulation in the presence of an environmental stimulus which had, in the past, either been paired or unpaired with either morphine or saline injections. Paired morphine groups showed significantly more reactivity to the aversive stimulation than the unpaired morphine groups and the saline control groups. Unpaired morphine groups did not differ from saline control groups. This conditional hyperalgesia elicited by a cue paired with morphine cannot be interpreted as the result of differential pretest assessment experience or novelty.