Effect of Adenosine, Adenosine Derivatives, and Caffeine on Acetylcholine Release from Brain Synaptosomes: Interaction with Muscarinic Autoregulatory Mechanisms

Abstract

Synaptosomes, prepared from rat cerebral cortex and hippocampus, were preincubated with [methyl-3H]choline. The effect of adenosine, cyclohexyladenosine, N-ethylcarboxamide adenosine, 2''-deoxyadenosine, and oxotremorine on K+-evoked 3H efflux was investigated. High-voltage electrophoresis separation showed that in the presence of physostigmine, the K+-evoked 3H efflux from hippocampal synaptosomes was 90% [3H]acetylcholine and 10% [3H]choline. Adenosine (30 .mu.M) and oxotremorine (100 .mu.M) both decreased [3H]acetylcholine release from hippocampal synaptosomes. The effect was inversely proportional to the KCl concentration and disappeared at a KCl concentration of 50 mM. Cyclohexyladenosine was .apprx. 3,000 times more active then adenosine, whereas N-ethylcarboxamide adenosine and 2''-deoxyadenosine were inactive. The indicates that A1 adenosine receptors were involved in the inhibitory effect. Caffeine antagonized the adenosine effect, and at a concentration of 100 .mu.M, it stimulated [3H]acetylcholine efflux. The inhibitory effect of oxotremorine was as great in cortical as in hippocampal synaptosomes. In contrast, adenosine was much less active in cortical than in hippocampal synaptosomes. When inhibitory concentrations of adenosine and oxotremorine were added together into the incubation medium, the effect of adenosine on [3H]acetylcholine release was consistently reduced. An interaction between muscarinic and A1 adenosine presynaptic receptors at a common site modulating acetylcholine release can be assumed.