PURINERGIC MODULATION OF THE EVOKED RELEASE OF [H-3]ACETYLCHOLINE FROM THE HIPPOCAMPUS AND CEREBRAL-CORTEX OF THE RAT - ROTE OF THE ECTONUCLEOTIDASES

Modulation by exogenous and endogenous adenine nucleotides and adenosine of [H-3]acetylcholine release evoked by veratridine (10 mu M) was compared in synaptosomal fractions from the hippocampus and the cerebral cortex of the rat. In both brain areas, exogenously added ATP or adenosine (10-100 mu M) inhibited the evoked tritium release. In the hippocampus, ATP gamma S, an ATP analogue more resistant to catabolism than ATP, was virtually devoid of effect on tritium release, and the effect of ATP was prevented by the ecto-5'-nucleotidase inhibitor alpha,beta-methylene ADP (100 mu M), by adenosine deaminase (2 U/ml) and by the A(1) adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 20 nM). In contrast, in the cerebral cortex, the effect of ATP on tritium release was not prevented by either alpha beta-methylene ADP (100 mu M) or adenosine deaminase (2 U/ml), and several ATP analogues (30 mu M) inhibited release, The order of intensity of the inhibitory effects of the ATP analogues was: ATP gamma S > ATP > beta,gamma-imido ATP > beta,gamma-methylene ATP much greater than 2-methyl-S-ATP, alpha,beta-methylene ATP. The effect of ATP gamma S in the cerebral cortex was prevented by DPCPX (20 nM) and was not affected by the P-2 purinoceptor antagonist suramin (100 mu M). in the hippocampus, alpha,beta-methytene ADP (100 mu M) increased the evoked release of tritium, and adenosine deaminase (2 U/ml) produced an even greater increase; when adenosine deaminase was added in the presence of alpha,beta-methylene ADP, adenosine deaminase still increased the evoked release of tritium. in the cerebral cortex, DPCPX (20 nM) and adenosine deaminase (2 U/ml) increased the evoked tritium release by a similar magnitude, but the effect of adenosine deaminase was smaller than in the hippocampus. It is concluded that in the cerebral cortex ATP as such presynaptically inhibits acety[choline release, whereas in the hippocampus the role of adenine nucleotides is as a source of endogenous extracellular adenosine that tonically inhibits acetylcholine release. The results also show that besides formation of adenosine from adenine nucleotides, released adenosine as such contributes in nearly equal amounts to the pool of endogenous adenosine that presynaptically inhibits acetylcholine release in the hippocampus.