Ion channels and epilepsy.
The role of voltage-gated and ligand-gated ion channels in epileptogenesis of both genetic and acquired epilepsies, and as targets in the development of new antiepileptic drugs (AEDs) is reviewed. Voltage-gated Na+ channels are essential for action potentials, and their mutations are the substrate for generalised epilepsy with febrile seizures plus and benign familial neonatal infantile seizures; Na+ channel inhibition is the primary mechanism of carbamazepine, phenytoin and lamotrigine, and is a probable mechanism for many other classic and novel AEDs. Voltage-gated K+ channels are essential in the repolarisation and hyperpolarisation that follows paroxysmal depolarisation shifts (PDSs), and their mutations are the substrate for the benign neonatal epilepsy and episodic ataxia type 1; they are new targets for AEDs such as retigabine. Voltage-gated Ca2+ channels are involved in neurotransmitter release, in the sustained depolarisation-phase of PDSs, and in the generation of absence seizures; their mutations are a substrate for juvenile myoclonic epilepsy and the absence-like pattern seen in some mice; the antiabsence effect of ethosuximide is due to the inhibition of thalamic T-type Ca2+ channels. Voltage-gated Cl- channels are implicated in GABA(A) transmission, and mutations in these channels have been described in some families with juvenile myoclonic epilepsies, epilepsy with grand mal seizures on awakening or juvenile absence epilepsy. Hyperpolarisation-activated cation channels have been implicated in spike-wave seizures and in hippocampal epileptiform discharges. The Cl- ionophore of the GABA(A) receptor is responsible for the rapid post-PDS hyperpolarisation, it has been involved in epileptogenesis both in animals and humans, and mutations in these receptors have been found in families with juvenile myoclonic epilepsy or generalised epilepsy with febrile seizures plus; enhancement of GABA(A) inhibitory transmission is the primary mechanism of benzodiazepines and phenobarbital and is a mechanistic approach to the development of novel AEDs such as tiagabine or vigabatrin. Altered GABA(B)-receptor function is implicated in spike-wave seizures. Ionotropic glutamate receptors are implicated in the sustained depolarisation phase of PDS and in epileptogenesis both in animals and humans; felbamate, phenobarbital and topiramate block these receptors, and attenuation of glutamatergic excitatory transmission is another new mechanistic approach. Mutations in the nicotinic acetylcholine receptor are the substrates for the nocturnal frontal lobe epilepsy. The knowledge of the role of the ion channels in the epilepsies is allowing the design of new and more specific therapeutic strategies.