Acquisition and expression of conditioned hypoalgesia in morphine-naive and morphine-tolerant rats

The present study used a within-subject design to examine acquisition and e xpression of conditioned hypoalgesia in 50 male Wistar rats. Morphine-naive rats preexposed to a heat stressor with saline were hypoalgesic when subse quently tested for latencies to tail flick or paw lick. However, morphine-t olerant rats preexposed to the heat stressor with saline failed to display hypoalgesia when tested for latencies to tail flick, but showed hypoalgesia when tested for latencies to paw lick. Taken together, these findings sugg est that expression of conditioned hypoalgesic responses in morphine-tolera nt rats may depend on the nociceptive test used. Both morphine-naive and mo rphine-tolerant rats preexposed to the heat stressor with morphine failed t o display hypoalgesia on either the tail-flick or the hot-plate test, demon strating that morphine's ability to block acquisition of conditioned hypoal gesia is independent of the test used to assess nociceptive sensitivity. (C ) 1999 Elsevier Science Inc.