Effect of phenytoin on sodium conductances in rat hippocampal CA1 pyramidal neurons

The antiepileptic drug phenytoin (PHT) is thought to reduce the excitability of neural tissue by stabilizing sodium channels (Na_V) in inactivated states. It has been suggested the fast-inactivated state (I_F) is the main target, although slow inactivation (I_S) has also been implicated. Other studies on local anesthetics with similar effects on sodium channels have implicated the Na_V voltage sensor interactions. In this study, we reexamined the effect of PHT in both equilibrium and dynamic transitions between fast and slower forms of inactivation in rat hippocampal CA1 pyramidal neurons. The effects of PHT were observed on fast and slow inactivation processes, as well as on another identified "intermediate" inactivation process. The effect of enzymatic removal of I_F was also studied, as well as effects on the residual persistent sodium current (I_NaP). A computational model based on a gating charge interaction was derived that reproduced a range of PHT effects on Na_V equilibrium and state transitions. No effect of PHT on I_F was observed; rather, PHT appeared to facilitate the occupancy of other closed states, either through enhancement of slow inactivation or through formation of analogous drug-bound states. The overall significance of these observations is that our data are inconsistent with the commonly held view that the archetypal Na_V channel inhibitor PHT stabilizes fast inactivation states, and we demonstrate that conventional slow activation "I_S" and the more recently identified intermediate-duration inactivation process "I_I" are the primary functional targets of PHT. In addition, we show that the traditional explanatory frameworks based on the "modulated receptor hypothesis" can be substituted by simple, physiologically plausible interactions with voltage sensors. Additionally, I_NaP was not preferentially inhibited compared with peak I_Na at short latencies (50 ms) by PHT.