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

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, C1, 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 on m: at different synaptic sites m depended on the second to fourth power of [Ca]0. In terms of binomial parameters, the release probability p is the [Ca]0-dependent one. In addition, reduction of [Ca]0 from 13.5 to 2.7 mmol/l decreased the unit quantum C1 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 the postsynaptic channel characteristics.