On the structure of ictal events in vitro

Propose: To analyze the cellular and network mechanisms of sustained seizures, we reviewed the literature and present new data on in vitro epileptiform events, We considered single and recurring synchronized population bursts occurring on a time scale from tens of milliseconds to 1 min. Methods: We used intracellular and field potential recordings, together with computer network simulations, derived from three types of experimental epileptogenesis: gamma-aminobutyric-acid, (GABA(A)) blockade, low extracellular [Mg2+](o), and 4-aminopyridine (4-AP). Results: In all three models, sustained depolarizing synaptic currents developed, either through N-methyl-D-aspartate (NMDA) receptors, depolarizing GABA(A) receptors, or both. Ectopic action potentials (APs), probably originating in axonal structures, occurred in 4-AP and (as shown by other researchers) after tetanic stimulation; ectopic APs, occurring at sufficient frequency, should also depolarize dendrites, by synaptic excitation, enough to trigger bursts. Conclusions: Ictal-like events appear to arise from two basic mechanisms. The first mechanism consists of sustained dendritic depolarization driving a series of dendritic bursts. The second mechanism consists of an increase in axonal and presynaptic terminal excitability driving a series of bursts analogous to interictal spikes.