Abstracts

Rationale: Focal seizures correlate with stereotyped electrophysiological patterns that can be reproduced in animal models. The analysis of the cellular and network changes that subtend these patterns contribute to the understanding of ictogenesis. We recently demonstrated that seizure onset in an in vitro model of focal seizures correlates with a complete interruption of neuronal firing in principal neurons, that resumes within 5-10 seconds (Gnatkovsky et al. 2008). We test here the hypothesis that ectopic firing induced by elevations of extracellular potassium is responsible for progression of seizure synchronization. Methods: We analyzed seizure-like discharges generated in the entorhinal cortex of the in vitro isolated guinea pig brain preparation by 3-minute applications of the GABAA receptor antagonist, bicuculline. We focussed our investigation on the features of action potentials recorded in principal neurons and interneurons recorded intracellularly with sharp electrodes. Results: Analysis of phase plots of action potentials (dV/dt versus voltage) was utilized to characterize threshold, amplitude and repolarization features of spikes in the different phases of the seizure-like discharge. During the transition toward the extracellular irregular spiking, action potential firing resumed in principal cells: in layer II-III neurons spikes showed higher threshold, lower peak amplitude and faster repolarization compared to pre-ictal spikes. These action potentials could be interpreted as ectopic spikes. Spike doublettes characterized by a second action potential with features of calcium spike were also observed. Within few seconds, burst firing emerged and become progressively more regular. Changes in action potential features correlate with increases in extracellular potassium concentration. Conclusions: Analysis of action potential features contributes to the understanding of the mechanisms of focal seizure generation and progression. Ectopic firing, possibly due to enhancement of potassuim concentration plays a primary role in focal seizure development.