ИСТИНА

The presynaptic activity of choline in mouse motor nerve terminals was studied. Single and rhythmically evoked endplate potentials (EPPs) and miniature endplate potentials (mEPPs) were recorded from hemidiaphragm muscle fibers. Selective agonist of alpha7 nicotinic acetylcholine receptors (nAChR) - choline (100 microM) - did not change the amplitude of mEPPs but caused a decrease in frequency of mEPPs and quantal content of single-evoked EPPs (63±7 % and 82±7 %, respectively). Choline-induced depression was prevented by preapplication of alpha-cobratoxin (5 nM), a blocker of alpha7-nAChR, which by itself does not change the amplitude and frequency of mEPPs and quantal content of EPPs. Choline (100 microM) was found to enhance the depression of EPPs amplitude during short trains (1 sec, 50 Hz) by 12-15 %. This effect was totally abolished by preapplication of selective antagonists of alpha7-nAChR -metyllicaconitine (20 nM) or alpha-cobratoxin (5 nM), as well as by ryanodine receptors blocker ryanodine (2 microM) and SK-type potassium channels blocker apamin (1 microM). The prolonged repetitive stimulation of synapses (50 Hz, up to 1 min) was used to provoke accumulation of endogenous choline within the synaptic cleft. It caused the rundown of EPPs amplitude (55±4 %). We found that such depression can be prevented by 90 % by preapplication of metyllicaconitine (20 nM) or apamin (1microM). So, the presynaptic effects of endogenous choline seem to be very similar to that of the exogenous one and involve activation of presynaptic alpha7-nAChRs and SK-channels. Thus, a mechanism of autoinhibition of acetylcholine secretion in cholinergic mouse motor synapses was discovered. It includes activation of presynaptic alpha7-nAChRs by choline, followed by the influx of calcium, induction of stored calcium release through RyRs, which in turn is aimed to activate SK-type of calcium-activated potassium channels and leads to down regulation of neurotransmitter secretion.