OPIATE RECEPTOR SUPERSENSITIVITY PRODUCED BY CHRONIC NALOXONE TREATMENT - DISSOCIATION OF MORPHINE-INDUCED ANTINOCICEPTION AND CONDITIONED TASTE-AVERSION

In 3 separate experiments, rats were used to assess the effects of chronic administration of naloxone on specific binding of 3H-naloxone in various regions of the CNS, and on the efficacy of morphine to produce antinociception and a conditioned taste aversion. Chronic naloxone treatment increased opiate binding in medulla-pons, midbrain, hypothalamus, hippocampus, striatum and prefrontal cortex, but not in the spinal cord or cerebellum. In those CNS regions exhibiting increased opiate binding, the duration of increased binding following termination of the naloxone treatment differed between regions. In conjunction with the increase in opiate binding, the efficacy of morphine to produce antinociception was potentiated, while the efficacy to produce a conditioned taste aversion was unchanged. Administration of naloxone during behavioral testing blocked completely the antinociceptive effect, but not the aversive effect, of morphine. Thus, morphine-induced antinociception and conditioned taste aversion may be dissociated neuropharmacologically.