Prolonged activation of the N-methyl-d-aspartate receptor–Ca(2+) transduction pathway causes spontaneous recurrent epileptiform discharges in hippocampal neurons in culture

The molecular basis for developing symptomatic epilepsy (epileptogenesis) remains ill defined. We show here in a well characterized hippocampal culture model of epilepsy that the induction of epileptogenesis is Ca(2+)-dependent. The concentration of intracellular free Ca(2+) ([Ca(2+)](i)) was monitored during the induction of epileptogenesis by prolonged electrographic seizure activity induced through low-Mg(2+) treatment by confocal laser-scanning fluorescent microscopy to directly correlate changes in [Ca(2+)](i) with alterations in membrane excitability measured by intracellular recording using whole-cell current–clamp techniques. The induction of long-lasting spontaneous recurrent epileptiform discharges, but not the Mg(2+)-induced spike discharges, was prevented in low-Ca(2+) solutions and was dependent on activation of the N-methyl-d-aspartate (NMDA) receptor. The results provide direct evidence that prolonged activation of the NMDA–Ca(2+) transduction pathway causes a long-lasting plasticity change in hippocampal neurons causing increased excitability leading to the occurrence of spontaneous, recurrent epileptiform discharges.