Molecular properties of brain sodium channels: an important target for anticonvulsant drugs.

The voltage-gated sodium channels that are responsible for action potential generation in central neurons are important targets for the actions of antiepileptic drugs. These channels consist of a complex of three glycoprotein subunits: a pore-forming alpha subunit of 260 kd associated noncovalently with a beta 1 subunit of 36 kd and disulfide-linked to a beta 2 subunit of 33 kd. The alpha subunit forms a functional voltage-gated sodium channel by itself, whereas the beta 1 and beta 2 subunits modulate channel gating. The beta 1 and beta 2 subunits also have immunoglobulin-like folds in their extracellular domains that are predicted to interact with extracellular proteins. The alpha subunit is comprised of four homologous domains containing six transmembrane alpha helices (S1 through S6) and additional membrane-associated segments (SS1/SS2). The S4 segments in each domain function as voltage sensors for voltage-dependent activation of the sodium channel. The S5 and S6 segments in each domain and the short SS1/SS2 segments between them form the pore of the channel. The intracellular loop between domains III and IV forms the inactivation gate, which folds into the pore and occludes it within 1 msec of channel opening. The activity of brain sodium channels in modulated by protein phosphorylation G proteins. Activation of muscarinic acetylcholine receptors in hippocampal neurons slows the inactivation of sodium channels and reduces peak sodium currents through activation of protein kinase C (PKC) phosphorylation of sites in the inactivation gate and the intracellular loop between domains I and II of the alpha subunit. Other neurotransmitters that activate the PKC pathway are likely to have similar effects. Activation of D1-like dopamine receptors in hippocampal neurons reduces peak sodium currents through activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase phosphorylation of sites in the intracellular loop between domains I and II. Modulation by PKC and cAMP-dependent protein kinase is convergent--phosphorylation of the inactivation gate by PKC is required before phosphorylation of sites in the intracellular loop between domains I and II can reduce peak sodium currents. Brain sodium channels are also modulated by G proteins. Activation of endogenous G protein-coupled receptors causes negative shifts in the voltage dependence of sodium channel activation and inactivation. Overexpression of G protein beta gamma subunits induces persistent sodium currents. Regulation of sodium channel function by these multiple pathways can produce a flexible tuning of electrical excitability of central neurons in response to neurotransmitters, hormones, and second messengers. The antiepileptic drugs phenytoin, carbamazepine, and lamotrigine inhibit brain sodium channels substantially at clinically relevant concentrations. Their inhibition of sodium channels is increased by depolarization because they bind preferentially to the inactivated state of the channels. This effect increases the inhibition of sodium channels in depolarized tissue at the center of an epileptic focus. Local anesthetics also inhibit sodium channels by preferential binding to the inactivated state. Site-directed mutagenesis experiments show that antiepileptic drugs and local anesthetics bind to a common receptor site in the pore of the channel that is formed in part by three critical amino acid residues in transmembrane segment S6 in domain IV. Mutations in these amino acid residues prevents preferential binding to the inactivated state and thereby greatly reduces the affinity for inhibition of sodium channels by these drugs. Knowledge of the structure-function relationships for drug binding at this receptor site may open the way to development of novel classes of antiepileptic drugs.