, Volume 391, Issue 1, pp 35-40

The effect of reduced calcium on quantal unit current and release at the crayfish neuromuscular junction

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Excitatory postsynaptic currents (EPSCs) were recorded extracellularly from large muscle fibers by means of ‘patch clamp’ electrodes. Compared to usual extracellular recordings, better signal/noise ratio and temporal stability were achieved. In the range of extracellular calcium concentrations [Ca]0 between 2.7 and 13.5 mmol/l (normal), the average amplitude of the EPSC increased more than proportional to [Ca]0. The unit quantum current,C 1, and the average release rate,m, were determined from EPSCs and also from spontaneous sEPSCs, using both Poisson and binomial statistics. The main effect of [Ca]0 was onm: at different synaptic sitesm depended on the second to fourth power of [Ca]0. In terms of binomial parameters, the release probabilityp is the [Ca]0-dependent one. In addition, reduction of [Ca]0 from 13.5 to 2.7 mmol/l decreased the unit quantumC 1 consistently to 60%; simultaneously the rise and decay of EPSCs and sEPSCs were shortened by 10–20%. [Ca]0 thus has strong presynaptic effects on the release probability, but in addition smaller ones on teh postsynaptic channel characteristics.

This investigation was supported by the Deutsche Forschungsgemeinschaft