Influence of the extracellular ionic environment on ganglionic fast excitatory postsynaptic currents

Abstract

The influence of changing the extracellular sodium or calcium concentration as well as the addition of strontium or magnesium on characteristics of nicotinic fast excitatory postsynaptic currents (EPSCs) has been studied in voltage-clamped sympathetic ganglion B cells of the bullfrog. Lithium substitution for sodium influenced both the EPSC size and decay time course in a concentration-dependent manner. In those cells exposed to either a 50% or 100% lithium-substituted solution, the EPSC decay was faster than that of control EPSCs. With a 50% replacement of lithium for sodium, the EPSC size at −50 mV was similar to control values. However, with a 100% substitution, the EPSC size was significantly reduced below control values although the voltage dependence of the decay τ, the shape of the peak EPSC-voltage relationship, or the EPSC reversal potential was not changed by replacing lithium for sodium. The change in EPSC size and decay τ in the lithium solution was due to the presence of lithium and not simply the consequence of a reduction in the external sodium concentration; as with a 50% substitution of sucrose or mannitol for sodium chloride the EPSC decay was slowed. EPSC size at −50 mV and the voltage dependence of τ was similar to control values when 50% of the sodium was replaced by sucrose. The peak EPSC-voltage relationship was linear in cells exposed to either the control or the 50% sucrose-substituted solution, although the EPSC reversal potential was shifted to a more negative voltage with 50% sucrose substitution. Facilitation of phasic transmitter release, estimated using paired pulses was depressed below control levels in those cells exposed either to a 100% lithium-substituted solution or a 50% sucrose-substituted solution suggesting that phasic transmitter release is enhanced in both experimental conditions. Both EPSC size and decay τ increased as a function of calcium concentration in the range 0.9–5.4 mM without any change in the shape of the peak current-voltage relationship, voltage-dependence of the decay τ, or EPSC reversal potential. The addition of 2 mM strontium prolonged the EPSC time course similar to that of doubling calcium, but reduced the EPSC size. In contrast, the addition of 8 mM magnesium had no influence on the EPSC decay time course but did reduce EPSC size. It is concluded that changes in the extracellular ionic environment can directly modify the kinetics of nicotinc receptor-channel gating in sympathetic postganglionic cells.