Abstracts

Rationale: Phenytoin (PHT) is a commonly used anti-epilepetic medication, and, together with several other common AED's, most likely exerts its main pharmacological action by modulating voltage gated sodium (Nav)channels to reduce the amplitude of the depolarising sodium currents. There remains some uncertainty as to whether the action of PHT on Nav channels is caused by the fast inactivation process or via other inactivation mechanisms. To clarify this, PHT effects on both fast and slow inactivation processes have been studied. Methods: Single pyramidal cells (n=36) from random sex 3-6 week old Wistar rats were isolated from the CA1 region using an enzymatic dissociation method. Cells were voltage-clamped using the whole-cell patch configuration, and PHT (50 uM)applied by a rapid perfusion system. Equilibrium states were measured with steady-state inactivation protocols using variable interval conditioning pulses, and transitions between closed/closed-inactivated states and open/inactivated states, as well as persistent current ("INap") amplitude were examined. Additionally, in some experiments, fast inactivation was removed with intracellular protease treatment. Effects of PHT in all theses conditions were observed. Results: PHT shifted steady-state activation curves to the left (hyperpolarised), more so with longer pulses (500ms). Open-inactivated transitions were unaffected by PHT, and INap did not appear selectively reduced by PHT. Transitions between closed and closed inactivated states at potentials negative to threshold ~-60 mV) were strongly affected by PHT, but predominantly influenced components with longer time constants, rather than faster interconversions most likely related to the fast inactivation process. Entry into slow inactivated states became faster with PHT whereas exit was slower. PHT exerted proportionally the same reduction in amplitude of INa after enzymatic removal of fast inactivation. Recovery from fast inactivation was also largely unaffected by PHT. Conclusions: PHT appears to modulate sodium channels mainly through slow or intermediate duration inactivation processes. This is surprising,as the effects of PHT have been generally ascribed to the fast inactivation process. It is likely that several other commonly used AED's have a similar mode of action.