Effects of phenytoin on normal activity and on penicillin-induced bursting in the guinea pig hippocampal slice

We used both intracellular and field potential recordings to study the effects of phenytoin (20 to 40 μg per milliliter) on CA3 and CA1 neurons of guinea pig hippocampal slices, with or without exposure to convulsant sodium penicillin G. Phenytoin did not change resting membrane potential, input resistance, action potential amplitude or duration, or the threshold intensity for activating action potentials or bursts with intracellular current pulses. Suppression of penicillin-induced bursting was associated with an increased threshold for orthodromically activating neurons. The intrinsic bursting mechanisms of the cells remained intact. Phenytoin seemed to suppress penicillin-induced epileptiform activity by decreasing synaptic transmission.