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The mechanism of action of tonically applied choline, the agonist of α7 nicotinic acetylcholine re- ceptors (nAChRs), to the spontaneous and evoked release of a neurotransmitter in mouse motor synapses in diaphragm neuromuscular preparations using intracellular microelectrode recordings of miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) was studied. Exogenous choline was shown to exhibit a presynaptic inhibitory effect on the amplitude and quantal content of EPPs for the activity of neuromuscular junction evoked by single and rhythmic stimuli. This effect was inhibited either by antagonists of α7-nAChRs, such as methyllycaconitine and α-cobratoxin, or by blocking SK-type calcium-activated potassium (K Ca ) chan- nels with apamin or blocking intraterminal ryanodine receptors with ryanodine. A hypothesis was put forward that choline in mouse motoneuron nerve terminals can activate presynaptic α7-nAChRs, followed by the release of the stored calcium through ryanodine receptors and activation of SK-type K Ca channels, resulting in sustained decay of the quantal content of the evoked neurotransmitter release. k E y WORDS quantal content, ryanodine receptors, choline, α7-nicotinic acetylcholine receptors, SK channels. ABBREVIATIONS ACh - acetylcholine; MEPP - miniature endplate potential; nAChRs - nicotinic acetylcholine receptors; EPP - endplate potential.