Dual effects of adenosine on acetylcholine release from myenteric motoneurons are mediated by junctional facilitatory A 2A and extrajunctional inhibitory A 1 receptors

Abstract

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 μM) decreased electrically evoked [ 3H]acetylcholine ([ 3H]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 μM), but the A 2A agonist CGS 21680C (0.003 μM) produced a contrasting facilitatory effect. 3. Increasing endogenous adenosine levels, by the addition of (1) the adenosine precursor AMP (30-100 μM), (2) the adenosine kinase inhibitor 5′-iodotubercidin (10 μM) or (3) inhibitors of adenosine uptake (dipyridamole, 0.5 μM) and of deamination (erythro-9(2-hydroxy-3-nonyl)adenine, 50 μM), enhanced electrically evoked [ 3H]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 [ 3H]ACh release by 31 ± 6% (n = 7) and 37 ± 10% (n = 6), respectively. 4. Although inhibition of ecto-5′-nucleotidase by α,β-methylene ADP (200 μM) or by concanavalin A (0.1 mg ml -1) attenuated endogenous adenosine formation from AMP, analysed by HPLC, the corresponding reduction in [ 3H]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.