ATP but not adenosine inhibits nonquantal acetylcholine release at the mouse neuromuscular junction

The postsynaptic membrane of the neuromuscular synapse treated with antiace tylcholinesterase is depolarized due to nonquantal release of acetylcholine (ACh) from the motor nerve ending. This can be demonstrated by the hyperpo larization produced by the application of curare (H-effect). ATP (1 x 10(-5 ) M) decreased the magnitude of the H-effect from 5 to 1.5 mV. The membrane input resistance and the ACh sensitivity were unchanged, and so changes in these cannot explain the ATP effect. Adenosine alone was without effect on the nonquantal release. On the other hand, both ATP and adenosine depresse d the frequency of spontaneous miniature endplate potentials, to 56% and 43 % respectively. The protein kinase A inhibitor Rp-cAMP or the guanylyl cycl ase inhibitor 1 H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one did not affect t he inhibitory influence of ATP on the H-effect, whereas staurosporine, an i nhibitor of protein kinase C, completely abolished the action of ATP, Suram in, an ATP antagonist, enhanced the H-effect to 8.6 mV and, like staurospor ine, prevented the inhibitory effect of ATP. ATP thus suppresses the nonqua ntal release via a direct action on presynaptic metabotropic P2 receptors c oupled to protein kinase C, whilst adenosine exerts its action mainly by af fecting the mechanisms underlying quantal release. These data, together wit h earlier evidence, show that nonquantal release of ACh can be modulated by several distinct regulatory pathways, in particular by endogenous substanc es which may or may not be present in the synaptic cleft at rest or during activity.