Cesium has been widely used to study the roles of the hyperpolarization-activated (Ih) and inwardly rectifying potassium (KIR) channels in many neuronal and nonneuronal cell types. Recently, extracellular application of cesium has been shown to produce epileptiform activity in brain slices, but the mechanisms for this are not known. It has been proposed that cesium blocks the KIR in glia, resulting in an abnormal accumulation of potassium in the extracellular space and inducing epileptiform activity. This hypothesis has been tested in hippocampal slices and cultured hippocampal neurons using potassium-sensitive microelectrodes. In the present study, application of cesium produced spontaneous epileptiform discharges at physiological extracellular potassium concentration ([K+]o) in the CA1 and CA3 regions of hippocampal slices. This epileptiform activity was not mimicked by increasing the [K+]o. The epileptiform discharges induced by cesium were not blocked by the N-methyl-d- aspartate (NMDA) receptor antagonist AP-5, but were blocked by the non-NMDA receptor antagonist CNQX. In the dentate gyrus, cesium induced the appearance of spontaneous nonsynaptic field bursts in 0 added calcium and 3 mM potassium. Moreover, cesium increased the frequency of field bursts already present. In contrast, ZD-7288, a specific Ihblocker, did not cause spontaneous epileptiform activity in CA1 and CA3, nor did it affect the field bursts in the dentate gyrus, suggesting that cesium induced epileptiform activity is not directly related to blockade of the Ih. When potassium-sensitive microelectrodes were used to measure [K+]o, there was no significant increase in [K+]o in CA1 and CA3 after cesium application. In the dentate gyrus, cesium did not change the baseline level of [K+]o or the rate of [K+]o clearance after the field bursts. In cultured hippocampal neurons, which have a large and relatively unrestricted extracellular space, cesium also produced cellular burst activity without significantly changing the resting membrane potential, which might indicate an increase in [K+]o. Our results suggest that cesium causes epileptiform activity by a mechanism unrelated to an alteration in [K+]o regulation.
NMDA Receptor Inhibition of L-Type Calcium Channels via ER Calcium Depletion and Activation of STIM1 in Cultured Hippocampal Neurons