Levetiracetam (LEV) inhibits partial refractory epilepsy in human, and both convulsive and absence-like seizures in the spontaneously epileptic rat (SER). Two-thirds of hippocampal CA3 neurons in SER show a long-lasting depolarization shift, with accompanying repetitive firing upon mossy fiber stimulation. This abnormal excitability is probably attributable to abnormalities in the L-type Ca(2+) channels. We performed electrophysiological studies to elucidate the mechanism underlying the antiepileptic effects of LEV via intracellular recording from the hippocampal CA3 neurons in slice preparations of SER and non-epileptic Wistar rats. LEV (100 μM) inhibited the depolarization shift with repetitive firing by mossy fiber stimulation (MFS), without affecting the first spike in SER CA3 neurons. At a higher dose (1mM), LEV suppressed the first spike in all SER neurons (including the CA3 neurons which showed only a single action potential by MFS), while the single action potential of Wistar rat CA3 neurons remained unaffected. SER CA3 neurons with MFS-induced abnormal firing exhibited a higher number of repetitive spikes when a depolarization pulse was applied in the SER CA3 neurons. LEV (100 μM, 1mM) reduced the repetitive firing induced by a depolarization pulse applied without affecting Ca(2+) spike in SER neurons. LEV is known not to bind glutamate and gamma-aminobutyric acid (GABA) receptors. These findings suggest that the therapeutic concentration of LEV inhibits abnormal firing of the CA3 neurons by modulating abnormal synaptic transmission and abnormal Na(+) channels in SER.
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