Membrane responses to changes in the extracellular potassium concentration in isolated hippocampal pyramidal neurons

The responses of freshly isolated hippocampal pyramidal neurons to rapid elevations of the external potassium concentration ([K+](out)) were investigated using the whole-cell variation of a patch-clamp technique. An elevation of [K+](out) induced a two-phase inward current at the membrane potentials more negative than the reversal potential for K ions. This current consisted of a leakage current and a time-dependent current (tau = 40-50 msec at 21 degrees C), the latter designated below as I-Delta K. It displayed first-order activation kinetics that showed neither voltage, nor concentration dependence. The amplitude of this current was determined by the external K+ concentration and increased with hyperpolarization. Voltage dependence of I-Delta K measured within the range from -20 to -120 mV was similar to that for inward rectifier. Activation of I-Delta K was utterly dependent on Na+; substitution of extracellular Na+ with choline chloride almost completely depressed I-Delta K. I-Delta K was absent in the cells freshly dissociated from the nodosal and dorsal root ganglia. This suggests that this earlier unrecognized current is instrumental in preserving densely packed hippocampal pyramidal neurons from sudden increases in [K+](out) and following spontaneous over-excitation. It prevents the neurons from responding to K+-induced depolarizations by slowing down potassium influx.