Fast Excitatory Postsynaptic Currents in Voltage-clamped Mammalian Sympathetic Ganglion Neurones

Fast excitatory postsynaptic currents (EPSCs) were recorded in voltage-clamped neurones of isolated superior cervical ganglion of the rabbit. The rise time, decay time and whole duration of EPSC, as well as miniature EPSC, were shorter than those of corresponding postsynaptic potentials. Characteristic impedance for EPSC was 5.5 +/- 1.1 M omega, and was a few times lower than for current evoked by iontophoretic application of ACh. The rise time of EPSC was 2.0 +/- 0.2 msec, the time constant of decay was 3.6 +/- 0.5 msec, and the mean amplitude of EPSC was -5.5 +/- 1.0 nA at the resting potential level (-53.8 +/- 1.4 mV) and at 36 degrees C. Amplitude of EPSC varied with membrane potential almost linearly at negative potentials, non-linearly at positive potentials, and nullified at -8.9 +/- 1.8 mV. The decay of EPSC was exponential over the most of its time course and the rate constant of decay (alpha) varied exponentially with membrane potential according to the relationship alpha(V) = B exp(AV), with A = 0.00716 +/- 0.00101 mV-1 and B = 0.46 +/- 0.07 msec-1. The voltage sensitivity of EPSC decay is interpreted in terms of voltage sensitivity in ionic channel lifetimes.