Prior hot plate exposure enhances morphine analgesia in tolerant and drug-naive rats.
The associative model of morphine tolerance predicts that established tolerance should be attenuated, i.e., extinguished, by placebo injections in the former morphine injection environment. The present study examined the effect of placebo sessions, with and without accompanying nociceptive stimulation, on the extinction of analgesic tolerance. In Experiment 1, rats rendered tolerant to morphine displayed recovery of morphine's analgesic action only following placebo sessions including exposure to a painful hot plate surface (52.5 degrees C); placebo sessions on a cool plate (23-24 degrees C) failed to attenuate tolerance even though these placebo sessions more closely matched the stimulus conditions of tolerance acquisition. In Experiment 2, repeated hot plate exposures were similarly found to enhance morphine analgesia in drug-naive rats. These results question an extinction account of the effect of hot plate placebo sessions observed in Experiment 1. Instead, they suggest that nociceptive hot plate exposures, per se, are sufficient to enhance subsequent morphine analgesia.
D000818:Animals; D004361:Drug Tolerance; D005108:Extinction, Psychological / Q000187:drug effects; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D012718:Set, Psychology
D009020:Morphine