Effects of antiepileptic drugs on thalamocortical excitability.

Comparative effects of anticonvulsant drugs on the thalamocortical system were analyzed quantitatively. Paired stimuli were delivered to the ventrolateral thalamus with evoked responses recorded from the ipsilateral sensorimotor cortex in the cat. Threshold and excitability profiles were developed with an on-line computer. Effects of phenytoin and diazepam were generally similar, with depression of excitability and slight elevation of thresholds. Ethosuximide produced a pronounced pair-interval dependent effect of unchanged or increased excitability and lowered threshold at shorter intervals, with depressed excitability and raised threshold at longer intervals. These data demonstrate a marked difference in effect of the petit mal and grand mal agents tested and suggest a basis for the effectiveness of ethosuximide in controlling 3-per-second repetitive activity.