Kindling as a model of drug-resistant partial epilepsy: selection of phenytoin-resistant and nonresistant rats.

Complex partial seizures comprise the major uncontrolled seizure type in adult patients with epilepsy. Any improvement of our understanding of the mechanisms through which these seizures are often refractory to antiepileptic drugs is therefore of considerable importance. By examining the effects of the anti-epileptic drug phenytoin in a large group of kindled rats, a widely used model of complex partial seizures, animals with different sensitivity to this drug were selected. Using determination of the focal seizure threshold for evaluation of phenytoin's anticonvulsant effect, most animals (about 60%) showed variable effects in response to phenytoin. However, about 20% of the animals ("phenytoin nonresponders") showed no increase in their focal seizure threshold at repeated test trials with phenytoin, and 20% ("phenytoin responders") exhibited reproducible increases in focal seizure threshold after injection of phenytoin. Phenytoin responders and nonresponders thus selected were used for subsequent experiments. The different response of focal seizures to phenytoin was not related to differences in pharmacokinetics or location of the stimulating electrode. Although phenytoin reproducibly increased the threshold for induction of afterdischarges in responders, it did not alter severity or duration of the elicited seizure response. In contrast to phenytoin, carbamazepine induced increases in focal seizure threshold in all kindled rats. Duration of seizures and afterdischarges were significantly reduced by carbamazepine in phenytoin responders, but not in nonresponders, although plasma levels of carbamazepine were the same in both groups. The difference in response of kindled rats to phenytoin was restricted to kindled seizures, because phenytoin induced the same anticonvulsant effect on the threshold for generalized tonic electroconvulsions (determined via transauricular electrodes) in both groups of kindled rats.(ABSTRACT TRUNCATED AT 250 WORDS)