Asymmetrical Cross-Tolerance Between Morphine- and Scopolamine-Induced Antinociception in the Primate.
Technical rept. Sep 73-Aug 74,
EDGEWOOD ARSENAL ABERDEEN PROVING GROUND MD
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Two experiments were designed to explore further the role of the cholinergic system in mediating morphine-induced analgesia in the Rhesus monkey. Experiment 1 tested for cross-tolerance between morphine and scopolamine using the shock titration technique. It was found that tolerance to morphine attenuated the response to scopolamine but that tolerance to scopolamine had no effect on morphine-induced antinociception. In experiment 2, injections of scopolamine and arecoline were made into brain sites which had previously been found to be analgesically responsive to morphine injections. Neither compound was found to be effective in modifying the shock threshold. These findings imply that morphine and scopolamine do not exert their antinociceptive effects through identical neural substrates. The findings from Experiment 1, however, do suggest that there may be some degree of overlap between such substrates.