Synaptic triggering of epileptiform discharges in Ca2 pyramidal cells in vitro.

Intra- and extracellular recordings were made in the transverse hippocampal slice in vitro to study the requirements for the triggering of epileptiform discharges of CA1 cells. Spontaneous and induced epileptiform discharges were produced by adding small amounts of sodium benzyl penicillin. Recorded intracellularly, the epileptiform activity consisted of a burst of action potentials superimposed on a depolarizing wave. Extracellular recordings demonstrated a marked synchronization. The epileptiform activity of the CA1 cells appeared without changes in the passive membrane properties or in the spike generating mechanism. Spontaneous epileptiform discharges of the CA2 cells depended upon a synaptic activation from the CA3 region. Stimulation of afferent fibres evoked an early and a late burst response in the CA2 cells. The long latency burst was caused by a re-excitation from the CA3 region. The early burst response seems to be an intrinsic property of the CA1 cells and may be induced by synaptic activation of either apical or basal dendrites. The findings suggest that synaptic depolarization is necessary for the generation of epileptiform discharges of the CA1 cells.