MECHANISMS OF ACTION OF CURRENTLY PRESCRIBED AND NEWLY DEVELOPED ANTIEPILEPTIC DRUGS

Clinically available antiepileptic drugs (AEDs) decrease membrane excitability by interacting with neurotransmitter receptors or ion channels. AEDs developed before 1980 appear to act on sodium (Na) channels, gamma-aminobutyric acid(A) (GABA(A)) receptors, or calcium (Ca) channels. Benzodiazepines and barbiturates enhance GABA(A)-receptor-mediated inhibition. Phenytoin, carbamazepine and, possibly, valproate (VPA) decrease high-frequency repetitive firing of action potentials by enhancing Na channel inactivation. Ethosuximide and VPA reduce a low threshold (T-type) Ca-channel current. The mechanisms of action of recently developed AEDs are less clear. Lamotrigine may decrease sustained high-frequency repetitive firing of voltage-dependent Na action potentials, and gabapentin (GBP) appears to bind to a specific binding site in the CNS with a restricted regional distribution. However, the identity of the binding site and the mechanism of action of GBP remain uncertain. The antiepileptic effect of felbamate may involve interaction at the strychnine-insensitive glycine site of the N-methyl-D-aspartate receptor, but the mechanism of action is not yet proven.