PMID: 9435183Jan 22, 1998Paper

Reduction of cortical pyramidal neuron excitability by the action of phenytoin on persistent Na+ current

The Journal of Pharmacology and Experimental Therapeutics
I LamplW E Crill

Abstract

We examined the effect of the anticonvulsant phenytoin (PT) (20-200 microM) on the persistent Na+ current (INaP), INaP-dependent membrane potential responses and repetitive firing in layer 5 pyramidal neurons in a slice preparation of rat sensorimotor cortex. INaP measured directly with voltage-clamp was reduced in a concentration-dependent manner with an apparent EC50 value of 78 microM. Clear effects on current-evoked membrane potential responses were apparent at 50 microM PT: Subthreshold, depolarizing membrane potential rectification was reduced, rheobase current was increased and the relation between firing rate and injected current was shifted to the right, but action potential amplitude and duration were unaffected. We ascribed these effects of PT largely to the reduction of INaP. A slow decline of firing rate during the injected current pulse also became apparent at moderate PT concentrations. When PT concentration was raised to 150 to 200 microM, this slow adaption was enhanced markedly, and firing ceased during a sufficiently large current pulse. This enhanced slow adaptation and the cessation of firing were associated with a marked decline of spike amplitude and a rise in spike firing level during successive interspi...Continue Reading

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