Analysis of miniature spontaneous inhibitory postsynaptic currents (sIPSCs) from current noise in crayfish opener muscle

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Abstract
Deviating from the normal situation, some crayfish muscle fibres showed spontaneous inhibitory activity: discharge of large inhibitory postsynaptic currents, IPSCs, alternating with long lasting bursts of current noise. Analysis of the bursts of current noise revealed that they are composed of spontaneous miniature unit currents, sIPSCs. In the burst periods the sIPSCs occurred with an average rate of 3.5–10 kHz and had an amplitude of about ã=90 pA at a driving force ΔE=10 mV. The peak conductance γã=ã/ΔE of the sIPSCs was γã=9.2 nS±0.5 (S.D.,n=5) for membrane potentials between E=−60 mV and E=−80 mV. γã seemed to decrease when the membrane was hyperpolarized. The time constants of decay, τ, of the sIPSCs were identical with τ of the IPSCs. Further, τ and its potential dependence agreed with the mean lifetimes of inhibitory postsynaptic channels operated by γ-aminobutyric acid (GABA) [Dudel et al. 1977, 1980]. Synchronized opening of about 750 inhibitory synaptic channels generates a sIPSC. Analysis of this anomalous bursting inhibitory activity thus yields the size of the inhibitory quantum of transmission, which could not be obtained from IPSCs.

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