Dual effects of adenosine on acetylcholine release from myenteric motoneurons are mediated by junctional facilitatory A(2A) and extrajunctional inhibitory A(1) 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 microm) decreased electrically evoked [(3)H]acetylcholine ([(3)H]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 microm), but the A(2A) agonist CGS 21680C (0.003 microm) produced a contrasting facilitatory effect. 3. Increasing endogenous adenosine levels, by the addition of (1) the adenosine precursor AMP (30-100 microm), (2) the adenosine kinase inhibitor 5'-iodotubercidin (10 microm) or (3) inhibitors of adenosine uptake (dipyridamole, 0.5 microm) and of deamination (erythro-9(2-hydroxy-3-nonyl)adenine, 50 microm), enhanced electrically evoked [(3)H]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 [(3)H]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 microm) or by concanavalin A (0.1 mg ml(-1)) attenuated endogenous adenosine formation from AMP, analysed by HPLC, the corresponding reduction in [(3)H]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.
Laboratório de Farmacologia/UMIB, Instituto de Ciências Biomédicas de Abel Salazar (ICBAS), Universidade do Porto, L. Prof. Abel Salazar, 2, 4099-003 Porto, Portugal.
Margarida
M
Duarte-Araújo
Carlos
C
Nascimento
M
M
Alexandrina Timóteo
Teresa
T
Magalhães-Cardoso
Paulo
P
Correia-de-Sá
D015720:5'-Nucleotidase / Q000378:metabolism; D000109:Acetylcholine / Q000378:metabolism*; D000241:Adenosine / Q000096:biosynthesis / Q000494:pharmacology / Q000502:physiology*; D058916:Adenosine A1 Receptor Antagonists; D058917:Adenosine A2 Receptor Antagonists; D000243:Adenosine Deaminase / Q000378:metabolism; D000818:Animals; D005260:Female; D007082:Ileum / Q000294:innervation; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009197:Myenteric Plexus / Q000187:drug effects / Q000378:metabolism*; D009451:Neuroeffector Junction / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D043682:Receptor, Adenosine A1 / Q000502:physiology; D043705:Receptor, Adenosine A2A / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects
D058916:Adenosine A1 Receptor Antagonists; D058917:Adenosine A2 Receptor Antagonists; D043682:Receptor, Adenosine A1; D043705:Receptor, Adenosine A2A; D015720:5'-Nucleotidase; D000243:Adenosine Deaminase; D000241:Adenosine; D000109:Acetylcholine