The effect of morphine on purine and acetylcholine release from rat cerebral cortex: evidence for a purinergic component in morphine's action.

Morphine enhances the release of adenosine and its metabolites from the rat cerebral cortex and inhibits the release of acetylcholine. Naloxone antagoinizes the effects of morphine on both purine and acetylcholine release. The adenosine antagonists, caffeine and theophylline, reduce morphine's effects on acetylcholine release, and at the same time increase the spontaneous release of acetylcholine. It is suggested that morphine, acting at a naloxone-sensitive site, enhances the level of extracellular adenosine, which in turn inhibits the release of acetylcholine, and that some of morphine's actions are mediated by a purinergic step.