Morphine analgesia: enhancement by shock-associated cues.
Recent research has shown that rats exposed to repeated stress display enhanced morphine analgesia. This study examined the possible contribution of classically conditioned analgesia to this effect. First, drug-naive rats exposed to nine daily sessions of stress, each consisting of a single 45-s exposure to footshock, subsequently displayed enhanced analgesic responsiveness to morphine 1 and 10 days after stress (Experiments 1, 2, and 5). This enhancement was also observed in morphine-experienced rats 1 and 8 days after stress (Experiment 1). Second, the effect of footshock stress on morphine analgesia was found to be specific to the environment in which stress was administered (Experiments 2 and 3). Rats tested in the same distinctive environment in which stress was administered displayed enhanced morphine analgesia; rats shocked elsewhere did not differ from nonshocked controls (Experiment 2). Third, conditioned analgesia was found under the same conditions that yielded enhanced morphine analgesia (Experiments 2 and 4). Lastly, both this conditioned analgesia and the acute analgesia elicited by the footshock stressor were found to be attenuated by naloxone (Experiments 5 and 6). These data are consistent with the hypothesis that the enhanced morphine analgesia observed after repeated footshock stress reflects the contribution of an opioid mediated , conditioned analgesia elicited by cues formerly paired with the stressor.
D000818:Animals; D004597:Electroshock; D008297:Male; D009020:Morphine / Q000627:therapeutic use*; D009270:Naloxone; D010146:Pain / Q000188:drug therapy* / Q000503:physiopathology / Q000523:psychology; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000502:physiology; D013312:Stress, Physiological / Q000503:physiopathology* / Q000523:psychology
D009270:Naloxone; D009020:Morphine