Development of context-specific tolerance to morphine: support for a dual-process interpretation

Four experiments were concerned with the development in rats of context-specific tolerance to the sedating and analgesic effects of morphine. Experiment 1 was conducted to assess the temporal course of activity changes and analgesia consequent to acute morphine administration. Experiments 2, 3, and 4 were conducted to assess the development of context-specific morphine tolerance in the two measures under different conditions of pairing of morphine with a distinctive environment. In support of a dual-process model (postulating both a general tendency for conditioned diminution of unconditioned responding and a more restricted influence of the development of specific conditioned compensatory responses), tolerance was observed in both measures, but evidence of conditioned compensatory response was found only in the activity measure. The differential evidence of conditioned compensatory response in the two measures was interpreted as consistent with the fact that the activity measure showed a biphasic unconditioned response in Experiment 1 whereas the analgesic measure did not.