Introduction

Recent studies have shown that properly cut hippocampal slices contain sufficient circuitry and cellular elements to sustain electrographic seizure activity with a duration of a minute or more. The ‘high-K’ model of hypersynchronous, epileptiform activity is produced by elevation of the extracellular potassium ion concentration ([K+]o) bathing the hippocampal slice. The resulting activity has several unique features. At least two distinct types of epileptiform event arise when [K+]o is raised from 3.5 to 7–8.5 mM, with the concentration of [Ca2+]o and [Mg2+]o maintained nominally at 1.5 mM. So-called interictal bursts, denned as a rapid and brief depolarization of the neuronal membrane coincident with a burst of action potentials, arise predominantly in the CA3b or CA3c subfield and propagate synaptically to CA1 pyramidal cells. In addition, electrographic seizures (often referred to as ictal events) with both tonic– and clonic-like components, appear in, and are restricted to, the CA1 subfield. Thus, the elevation of [K+]o permits the study of both spontaneous interictal activity and electrographic seizures proper in a relatively intact system without the requirement of electrical stimulation of afferent pathways.