MULTIPLE ACTIONS OF PHENYTOIN ON MOUSE SPINAL-CORD NEURONS IN CELL-CULTURE

Abstract

Concentration-dependence of multiple actions of phenytoin (PT) on mouse spinal cord neurons in primary dissociated cell culture was studied using intracellular microelectrode recording techniques. At concentrations of 2-50 .mu.g/ml, PT did not alter resting membrane potential or input resistance. At 1-2 .mu.g/ml, equivalent to therapeutic CSF concentrations, PT limited the ability to sustain high-frequency repetitive firing of action potentials during long (500-2000 ms) depolarizing current pulses. There was a progressive reduction of maximal rate of rise (.ovrhdot.Vmax) of action potentials during the train until firing failed. Recovery of .ovrhdot.Vmax of single action potentials after repetitive firing was also prolonged. PT did not reduce .ovrhdot.Vmax of a single action potential at 1-2 .mu.g/ml, but did so at 3-40 .mu.g/ml in a voltage-dependent manner. Hyperpolarization partially reversed this reduction of .ovrhdot.Vmax. PT may slow recovery of Na channels from inactivation. At concentrations above 3 .mu.g/ml, PT reduced spontaneous neuronal firing with progressive increase in the number of quiescent neurons, reduced Ca-dependent action potential duration and amplitude, eradicated convulsant-induced paroxysmal bursting and augmented postsynaptic responses to iontophoretically applied .gamma.-GABA. Glutamic acid responses were unaffected at PT concentrations at 10 .mu.g/ml or less. These actions occurred at concentrations equivalent to toxic CSF levels in patients and may be related to PT-induced toxicity. Limitation of sustained high-frequency repetitive firing may account, at least in part, for the anticonvulsant efficacy of PT.