The present study was aimed at elucidating the possible mechanisms underlying the anticonvulsant efficacy of phenytoin using intracellular recording techniques in the in vitro amygdalar slice preparation. Synaptic response mediated by the N-methyl-D-aspartate (NMDA) receptor (EPSPnmda) was isolated pharmacologically by application of a solution containing non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 μmol/l) and γ-aminobutyric acidA receptor antagonist bicuculline (20 μmol/l). Phenytoin inhibits the amplitude of EPSPnmda without affecting the postsynaptic depolarization induced by exogenous application of NMDA. In addition, phenytoin increases the magnitude of paired-pulse facilitation which is consistent with a presynaptic mode of action. These results suggest that inhibition of transmitter release due to presynaptic blockade of Na+ and/or Ca2+ channels may account largely for the anticonvulsant efficacy of phenytoin.
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