Felbamate is a subunit selective modulator of recombinant gamma-aminobutyric acid type A receptors expressed in Xenopus oocytes

Drug treatment of epilepsy: From serendipitous discovery to evolutionary mechanisms

Epilepsy is a chronic brain disorder caused by abnormal firing of neurons. Up to now, using antiepileptic drugs is the main method of epilepsy treatment. The development of antiepileptic drugs lasted for centuries. In general, most agents entering clinical practice act on the balance mechanisms of brain "excitability-inhibition". More specifically, they target voltage-gated ion channels, GABAergic transmission and glutamatergic transmission. In recent years, some novel drugs representing new mechanisms of action have been discovered. Although there are about 30 available drugs in the market, it is still in urgent need of discovering more effective and safer drugs. The development of new antiepileptic drugs is into a new era: from serendipitous discovery to evolutionary mechanism-based design. This article presents an overview of drug treatment of epilepsy, including a series of traditional and novel drugs.

Mechanisms of action of currently used antiseizure drugs

Antiseizure drugs (ASDs) prevent the occurrence of seizures; there is no evidence that they have disease-modifying properties. In the more than 160 years that orally administered ASDs have been available for epilepsy therapy, most agents entering clinical practice were either discovered serendipitously or with the use of animal seizure models. The ASDs originating from these approaches act on brain excitability mechanisms to interfere with the generation and spread of epileptic hyperexcitability, but they do not address the specific defects that are pathogenic in the epilepsies for which they are prescribed, which in most cases are not well understood. There are four broad classes of such ASD mechanisms: (1) modulation of voltage-gated sodium channels (e.g. phenytoin, carbamazepine, lamotrigine), voltage-gated calcium channels (e.g. ethosuximide), and voltage-gated potassium channels [e.g. retigabine (ezogabine)]; (2) enhancement of GABA-mediated inhibitory neurotransmission (e.g. benzodiazepines, tiagabine, vigabatrin); (3) attenuation of glutamate-mediated excitatory neurotransmission (e.g. perampanel); and (4) modulation of neurotransmitter release via a presynaptic action (e.g. levetiracetam, brivaracetam, gabapentin, pregabalin). In the past two decades there has been great progress in identifying the pathophysiological mechanisms of many genetic epilepsies. Given this new understanding, attempts are being made to engineer specific small molecule, antisense and gene therapies that functionally reverse or structurally correct pathogenic defects in epilepsy syndromes. In the near future, these new therapies will begin a paradigm shift in the treatment of some rare genetic epilepsy syndromes, but targeted therapies will remain elusive for the vast majority of epilepsies until their causes are identified. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

cAMP-dependent protein kinase A activity modulates topiramate potentiation of GABA(A) receptors

Activation of cAMP-dependent protein kinase A (PKA) prevents inhibition of non-NMDA glutamate receptors by the anticonvulsant topiramate. Using two-electrode voltage-clamp techniques, we demonstrate that PKA activity also modulates topiramate potentiation of recombinant GABA(A) receptors expressed in Xenpus laevis oocytes. PKA activators, dibutyryl-cAMP and forskolin, attenuate topiramate potentiation, whereas the PKA inhibitor H-89 increases topiramate potentiation. Thus, endogenous PKA activity and receptor phosphorylation states may contribute to topiramate treatment efficacy.

Diverse antiepileptic drugs increase the ratio of background synaptic inhibition to excitation and decrease neuronal excitability in neurones of the rat entorhinal cortex in vitro

Although most anti-epileptic drugs are considered to have a primary molecular target, it is clear that their actions are unlikely to be limited to effects on a single aspect of inhibitory synaptic transmission, excitatory transmission or voltage-gated ion channels. Systemically administered drugs can obviously simultaneously access all possible targets, so we have attempted to determine the overall effect of diverse agents on the balance between GABAergic inhibition, glutamatergic excitation and cellular excitability in neurones of the rat entorhinal cortex in vitro. We used an approach developed for estimating global background synaptic excitation and inhibition from fluctuations in membrane potential obtained by intracellular recordings. We have previously validated this approach in entorhinal cortical neurones [Greenhill and Jones (2007a) Neuroscience 147:884–892]. Using this approach, we found that, despite their differing pharmacology, the drugs tested (phenytoin, lamotrigine, valproate, gabapentin, felbamate, tiagabine) were unified in their ability to increase the ratio of background GABAergic inhibition to glutamatergic excitation. This could occur as a result of decreased excitation concurrent with increased inhibition (phenytoin, lamotrigine, valproate), a decrease in excitation alone (gabapentin, felbamate), or even with a differential increase in both (tiagabine). Additionally, we found that the effects on global synaptic conductances agreed well with whole cell patch recordings of spontaneous glutamate and GABA release (our previous studies and further data presented here). The consistency with which the synaptic inhibition:excitation ratio was increased by the antiepileptic drugs tested was matched by an ability of all drugs to concurrently reduce intrinsic neuronal excitability. Thus, it seems possible that specific molecular targets among antiepileptic drugs are less important than the ability to increase the inhibition:excitation ratio and reduce overall neuronal and network excitability.

Molecular determinants of the anticonvulsant felbamate binding site in the N-methyl-D-aspartate receptor

The antiepileptic effect of felbamate (FBM) is ascribable to gating modification of NMDA receptors. Using site-directed mutagenesis and electrophysiological studies, we found that single-point mutations of four pairs of homologous residues in the external vestibule of the receptor pore, namely V644(NR1)-L643(NR2B) (the two inner pairs) and T648(NR1)-T647(NR2B) (the two outer pairs), significantly decrease FBM binding. Moreover, double mutations involving either the inner or the outer pair always show cooperative (nonadditive) effects on FBM binding, whereas double mutations involving both inner and outer pairs always show additive (noncooperative) effects. Most interestingly, triple mutations of any three of the four critical residues essentially abolish the effect of FBM. These findings indicate that T648(NR1)/T647(NR2B) and V644(NR1)/L643(NR2B) act cooperatively to contribute directly to the "outer binding region" and "inner binding region" in the FBM binding site, respectively. The outer and inner binding regions, however, seem to contribute independently to FBM binding. We conclude that residues L643 and T647 in NR2B as well as homologous residues V644 and T648 in NR1 are the major, and very likely the exclusive, molecular determinants constituting the FBM binding site in the NMDA receptor.