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

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 -520 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 tau, 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 tau 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 tau 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.(ABSTRACT TRUNCATED AT 250 WORDS)