Dual effects of adenosine on acetylcholine release from myenteric motoneurons are mediated by junctional facilitatory A2A and extrajunctional inhibitory A1 receptors

1 The coexistence of both inhibitory A(1) and facilitatory A(2) adenosine receptors in the rat myenteric plexus prompted the question of how adenosine activates each receptor subtype to regulate cholinergic neurotransmission. 2 Exogenously applied adenosine (0.3-300 muM) decreased electrically evoked [H-3]acetylcholine ([H-3]ACh) release. Blocking A(1) receptors with 1,3-dipropyl-8-cyclopentylxanthine (10 nM) transformed the inhibitory action of adenosine into a facilitatory effect. Adenosine-induced inhibition was mimicked by the A(1) receptor agonist R-N-6-phenylisopropyladenosine (0.3 muM), but the A(2A) agonist CGS 21680C (0.003 muM) produced a contrasting facilitatory effect. 3 Increasing endogenous adenosine levels, by the addition of (1) the adenosine precursor AMP (30-100 muM), (2) the adenosine kinase inhibitor 5'-iodotubercidin (10 muM) or (3) inhibitors of adenosine uptake (dipyridamole, 0.5 muM) and of deamination (erythro-9(2-hydroxy-3-nonyl) adenine, 50 muM), enhanced electrically evoked [H-3]ACh release (5 Hz for 40 s). Release facilitation was prevented by adenosine deaminase (ADA, 0.5 U ml(-1)) and by the A(2A) receptor antagonist ZM 241385 (50 nM); these compounds decreased [H-3]ACh release by 31 +/- 6% (n = 7) and 37 +/- 10% (n = 6), respectively. 4 Although inhibition of ecto-5'-nucleotidase by alpha, beta-methylene ADP (200 muM) or by concanavalin A (0.1 mg ml(-1)) attenuated endogenous adenosine formation from AMP, analysed by HPLC, the corresponding reduction in [H-3] ACh release only became evident when stimulation of the myenteric plexus was prolonged to over 250 s. 5 In summary, we found that endogenously generated adenosine plays a predominantly tonic facilitatory effect mediated by prejunctional A(2A) receptors. Extracellular deamination and cellular uptake may restrict endogenous adenosine actions to the neuro-effector region near the release/production sites.