An epilepsy mutation in the β1 subunit of the voltage-gated sodium channel results in reduced channel sensitivity to phenytoin

The antiepileptic drug phenytoin inhibits voltage-gated sodium channels. Phenytoin block is enhanced at depolarized membrane potentials and during high frequency channel activation. These properties, which are important for the clinical efficacy of the drug, depend on voltage-dependent channel gating. In this study, we examined the action of phenytoin on sodium channels, comprising a mutant auxiliary β1 subunit (mutation C121Wβ1), which causes the inherited epilepsy syndrome, generalized epilepsy with febrile seizures plus (GEFS+). Whole cell sodium currents in Chinese hamster ovary (CHO) cells coexpressing human Nav1.3 sodium channels and C121Wβ1 exhibited altered gating properties, compared to currents in cells coexpressing Nav1.3 and wild type β1. In addition mutant channels were less sensitive to inhibition by phenytoin, showing reduced tonic block at −70 mV (EC50 = 26 μM for C121Wβ1 versus 11 μM for wild type β1) and less frequency-dependent inhibition in response to a 20 Hz pulse train (∼40% inhibition for C121Wβ1 versus ∼70% inhibition for wild type β1, with 50 μM phenytoin). Mutant and wild type channels did not differ in inactivated state affinity for phenytoin, suggesting that their pharmacological differences were secondary to their differences in voltage-dependent gating, rather than being caused by direct effects of the mutation on the drug receptor. Together, these data show that a sodium channel mutation responsible for epilepsy can also alter channel response to antiepileptic drugs.