Inhibition of Na ⁺ current by imipramine and related compounds: Different binding kinetics as an inactivation stabilizer and as an open channel blocker

Use-dependent block of Na ⁺ channels plays an important role in the action of many medications, including the anticonvulsants phenytoin, carbamazepine, and lamotrigine. These anticonvulsants all slowly yet selectively bind to a common receptor site in inactivated but not resting Na ⁺ channels, constituting the molecular basis of the use-dependent block. However, it remains unclear what channel gating process "makes" the receptor, where the receptor is located, and how the slow drug binding rate (to the inactivated channels) is contrived. Imipramine has a diphenyl structural motif almost identical to that in carbamazepine (a dibenzazepine tricyclic compound), as well as a tertiary amine chain similar to that in many prototypical local anesthetics, and has also been reported to inhibit Na ⁺ channels in a use-dependent fashion. We found that imipramine selectively binds to the inactivated (dissociation constant ∼1.3 μM) rather than the resting Na ⁺ channels (dissociation constant > 130 μM). Moreover, imipramine rapidly blocks open Na ⁺ channels, with a binding rate ∼70-fold faster than its binding to the inactivated channels. Similarly, carbamazepine and diphenhydramine are open Na ⁺ channel blockers with faster binding rates to the open than to the inactivated channels. These findings indicate that the anticonvulsant receptor responsible for the use-dependent block of Na ⁺ channels is located in or near the pore (most likely in the pore mouth) and is made suitable for drug binding during channel activation. The receptor, however, continually changes its conformation in the subsequent gating process, causing the slower drug binding rates to the inactivated Na ⁺ channels.