Effects of some antiepileptic drugs on the repetitive activity of the node of Ranvier

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Abstract
1 Effects of some antiepileptic drugs on the repetitive activity of the node of Ranvier have been tested on frog myelinated nerve fibres. 2 Nerve fibres were stimulated by supraliminal direct current pulses of long duration. Motor fibres responded with a single action potential; sensory fibres responded with repetitive firing at a frequency of about 300/s. Chlordiazepoxide hydrochloride (0.1 mm), phenobarbitone sodium (0.25 mm) or diphenylhydantoin sodium (0.5 mm) suppressed the repetitive activity. 3 Sensory and motor nerve fibres stimulated by a 10 kHz alternating current of strength twice the threshold responded with repetitive firing at a frequency of 400–500/s. Superfusion of the node with chlordiazepoxide hydrochloride (0.2 mm), phenobarbitone sodium (0.5 mm) or diphenylhydantoin sodium (1.7 mm) reduced the frequency of firing by 50% either in sensory or in motor fibres activated by a.c. stimulation; at the same concentrations, the drugs altered amplitude of the action potential and threshold for electric excitability by less than 10%. 4 Unlike local anaesthetics, chlordiazepoxide, phenobarbitone and diphenylhydantoin are more selective in inhibiting repetitive firing than in reducing the amplitude of the action potential or increasing the threshold for electric excitability. 5 Trimethadione (up to 5 mm) was ineffective on repetitive firing elicited either with direct or with alternating current.