Contribution of intrinsic neuronal factors in the generation of cortically driven electrographic seizures

Some electrographic seizures are generated intracortically. The cellular and ionic bases of cortically generated spontaneous seizures are not fully understood. Here we investigated spontaneously occurring seizures consisting of spike-wave complexes intermingled with fast runs in ketamine-xylazine anesthetized cats, using dual intracellular recordings in which one pipette contained a control solution and another pipette contained blockers of K(+), Na(+), or Ca(2+) currents. We show that closely located neocortical neurons display virtually identical fluctuations of the membrane potential during electrographic seizures, thus directly demonstrating a high degree of focal synchrony during paroxysmal activity. In addition to synaptic drives, the persistent Na(+) current [I(Na(p))] and probably the high-threshold Ca(2+) current contributed to the generation of paroxysmal depolarizing shifts (PDSs) during cortically driven seizures. Ca(2+)-activated K(+) current [I(K(Ca))] took also part in the control of the amplitude and duration of PDSs. The hyperpolarizing components of seizures largely depended on Cs(+)-sensitive K(+) currents. I(K(Ca)) played a significant, while not exclusive, role in the mediation of hyperpolarizing potentials related to EEG "waves" during spike-wave seizures. We conclude that intrinsic cellular factors have significant role in the generation of depolarizing and hyperpolarizing components of seizures.