Cellular mechanisms underlying adenosine actions on cholinergic transmission in enteric neurons

Whole cell recordings were used to investigate the effects of adenosine and several of its analogues on voltage-activated calcium currents (VACC) of myenteric and submucosal neurons. Electrophysiological and pharmacological properties of the soma VACC recorded in myenteric neurons indicate that they are carried through N-type calcium channels, similar to those of the submucosal neurons and to those of the calcium conductance that mediates acetylcholine release at the submucosal ganglia. Adenosinergic compounds inhibited, in a concentration-response and in a voltage-dependent manner, VACC in neurons from both enteric plexuses. The pharmacological profile of the receptors that mediate this effect was similar to that of the receptors involved in presynaptic inhibition in enteric neurons and likely of the A1 subtype. The effects of 2-chloroadenosine (CADO) on VACC were prevented by pretreatment with pertussis toxin (PTX), became irreversible with guanosine 5'-O-(3-thiotriphosphate) (inside the pipette), and were abolished with N-ethylmaleimide (NEM; known to uncouple receptors from G protein complexes). Intracellular recordings were used to further evaluate presynaptic effects of adenosine at the submucosal plexus. Adenosinergic compounds reduced the amplitude of fast excitatory postsynaptic potentials (EPSPs) by acting at nerve terminals. This effect was insensitive to PTX and staurosporine (a protein kinase inhibitor) but was abolished by NEM. CADO effects on EPSPs were not reversed by increasing the extracellular calcium concentration. In conclusion, activation of A1 adenosine receptors inhibits VACC via PTX-sensitive G proteins in myenteric and submucosal neurons. Reduction of cholinergic transmission also involves A1 adenosine receptors and appears to involve the activation of PTX-insensitive G proteins.