The effects of conditioning and novelty on the rat's analgesic and pyretic responses to morphine

Six experiments with rat subjects investigated the role of conditioning in morphine tolerance. Concurrent assessments of body temperature and pain sensitivity were conducted. Experience with morphine produced tolerance to its analgesic effects but enhancement of its hyperthermic effects. Environmental novelty enhanced analgesia but not body temperature (Experiments 1 and 4). Under conditions in which a discriminated hyperthermic CR provided clear evidence that morphine-environment learning had developed, discriminated analgesic tolerance was not obtained (Experiments 2 and 3). Similarly, whereas placebo administrations extinguished the hyperthermic CR (Experiments 4 and 6), analgesic tolerance remained unaffected (Experiments 4, 5, and 6). These experiments suggest that the pyretic and analgesic systems are differentially sensitive to conditioning and the effects of novelty.