Effects of morphine and caffeine on adenosine release from rat cerebral cortex: is caffeine a morphine antagonist

Morphine (1 mg/kg) enhances the efflux of labelled adenosine and its derivatives from the rat cerebral cortex. Caffeine (40 mg/kg) does not alter either basal purine efflux rates or the enhanced effux elicited by morphine. The antagonism by caffeine of morphine-evoked inhibition of acetylcholine (ACh) release from the cerebral cortex is unlikely therefore to be the result of an action of caffeine on the opiate receptor but is instead due to an antagonism of the inhibitory effect of released adenosine on ACh release.