Adenosine activating A sub(2A)-receptors coupled to adenylate cyclase/cyclic AMP pathway downregulates nicotinic autoreceptor function at the rat myenteric nerve terminals

In addition to the somatodendritic region, myenteric motoneuron terminals are endowed with nicotinic autoreceptors. We aimed at investigating the effect of nicotinic receptor (nAChR) activation on [ super(3)H]-acetylcholine ([ super(3)H]- ACh) release from longitudinal muscle-myenteric plexus of the rat ileum and to evaluate whether this could be modulated by adenosine, an endogenous neuromodulator typically operating changes in intracellular cyclic AMP. The nAChR agonist, 1, 1-dimethyl-4-phenylpiperazinium (DMPP, 1-30 mu M, 3 min) increased [ super(3)H]-ACh release in a concentration-dependent manner. DMPP (30 mu M)-induced [ super(3)H]-ACh outflow was attenuated by hexamethonium (0.1-1 mM), tubocurarine (1-5 mu M), or by removing external Ca super(2+) (plus EGTA, 1 mM). In contrast to veratridine (0.2-10 mu M)-induced [ super(3)H]-ACh release, the DMPP (30 mu M)-induced outflow was resistant to tetrodotoxin (1 mu M) and cadmium (0.5 mM). Pretreatment with adenosine deaminase (0.5 U/mL) or with the adenosine A sub(2A)-receptor antagonist, ZM 241385 (50 nM), enhanced nAChR-induced transmitter release. Activation of A sub(2A) receptors with CGS 21680C (3 nM) reduced the DMPP-induced release of [ super(3)H]-ACh. CGS 21680C (3 nM) inhibition was prevented by MDL 12, 330A (10 mu M, an adenylate cyclase inhibitor) and by H-89 (10 mu M, an inhibitor of protein kinase A), but was potentiated by rolipram (300 mu M, a phosphodiesterase inhibitor). DMPP-induced transmitter release was decreased by 8-bromo-cyclic AMP (1 mM, a protein kinase A activator), rolipram (300 mu M), and forskolin (3 mu M, an activator of adenylate cyclase). Both MDL 12, 330A (10 mu M) and H-89 (10 mu M) facilitated DMPP-induced release of [ super(3)H]-ACh. The results indicate that nAChR-induced [ super(3)H]-ACh release is triggered by the influx of Ca super(2+), independent of voltage-sensitive calcium channels, presumably directly through nAChRs located on myenteric axon terminals. It was also shown that endogenous adenosine, activating A sub(2A) receptors coupled to the adenylate cyclase/cyclic AMP transducing system, is tonically downregulating this nAChR-mediated control of [ super(3)H]-ACh release.