Lack of cross-tolerance between morphine and autoanalgesia.

The acquisition of autoanalgesia (behaviorally-induced antinociception) was investigated in morphine-tolerant and non-tolerant rats. Tolerance to morphine did not affect analgesia acutely-elicited by a brief (15 sec) schedule of footshock. Similarly, analgesia elicited by classically conditioned fear wasnot attenuated by morphine tolerance. These data suggest that endorphins may not be the principle mediators of autoanalgesic phenomena.