Changes in neuronal conductance during different components of cortically generated spike-wave seizures

Neuronal conductance was studied in anesthetized cats during cortically gen erated spike-wave seizures arising from slow sleep oscillation. Single and dual intracellular recordings from neocortical neurons were used. The chang es were similar whether the seizures occurred spontaneously, or were evoked by electrical stimulation or induced by bicuculline. In all seizures, the conductance increased from the very onset of the seizure and returned to co ntrol values only at the end of the postictal depression. Simultaneous intr acellular recordings from two neurons showed that the neuron leading the ot her neuron displayed the largest increase in membrane conductance. The chan ges in neuronal conductance during the two phases of the slow sleep oscilla tion, i.e. highest during depolarizations and lowest during hyperpolarizati ons, were similar to those occurring during the "spike" and "wave" componen ts of seizures. (1) Maximal conductance was found during the paroxysmal dep olarizing shift corresponding to the electroencephalogram "spike" (median: 252 nS; range: 90 to more than 400 nS). It was highest at the onset of the depolarized plateau and decreased thereafter. (2) During the hyperpolarizat ion corresponding to the electroencephalogram "wave", the conductance was s ignificantly lower (median: 71 nS; range: 41 to 140 nS). (3) The conductanc e was elevated during the fast runs (median: 230 nS; range: 92 to 350 nS) w hich occurred in two-thirds of the seizures. (4) The conductance values dur ing postictal depression were situated between those measured during the se izure hyperpolarizations and during sleep hyperpolarizations. The conductan ce decreased exponentially back to the values of the slow sleep oscillation over the total duration of the postictal depression. The data suggest that the major mechanism underlying the "wave"-related hyp erpolarizing component of spike-wave seizures relies mainly not on active i nhibition, but on a mixture of disfacilitation and potassium currents. (C) 2000 IBRO. Published by Elsevier Science Ltd.