Michael A. Rogawski

The neurobiology of antiepileptic drugs

Antiepileptic drugs (AEDs) provide satisfactory control of seizures for most patients with epilepsy. The drugs have the remarkable ability to protect against seizures while permitting normal functioning of the nervous system. AEDs act on diverse molecular targets to selectively modify the excitability of neurons so that seizure-related firing is blocked without disturbing non-epileptic activity. This occurs largely through effects on voltage-gated sodium and calcium channels, or by promoting inhibition mediated by GABAA (γ-aminobutyric acid, type A) receptors. The subtle biophysical modifications in channel behaviour that are induced by AEDs are often functionally opposite to defects in channel properties that are caused by mutations associated with epilepsy in humans.

The neurobiology of antiepileptic drugs for the treatment of nonepileptic conditions.

Antiepileptic drugs (AEDs) are commonly prescribed for nonepileptic conditions, including migraine headache, chronic neuropathic pain, mood disorders, schizophrenia and various neuromuscular syndromes. In many of these conditions, as in epilepsy, the drugs act by modifying the excitability of nerve (or muscle) through effects on voltage-gated sodium and calcium channels or by promoting inhibition mediated by γ-aminobutyric acid (GABA) A receptors. In neuropathic pain, chronic nerve injury is associated with the redistribution and altered subunit compositions of sodium and calcium channels that predispose neurons in sensory pathways to fire spontaneously or at inappropriately high frequencies, often from ectopic sites. AEDs may counteract this abnormal activity by selectively affecting pain-specific firing; for example, many AEDs suppress high-frequency action potentials by blocking voltage-activated sodium channels in a use-dependent fashion. Alternatively, AEDs may specifically target pathological channels; for example, gabapentin is a ligand of α2δ voltage-activated calcium channel subunits that are overexpressed in sensory neurons after nerve injury. Emerging evidence suggests that effects on signaling pathways that regulate neuronal plasticity and survival may be a factor in the delayed clinical efficacy of AEDs in some neuropsychiatric conditions, including bipolar affective disorder.

GYKI 52466, a 2,3-benzodiazepine, is a highly selective, noncompetitive antagonist of AMPA/kainate receptor responses

In whole-cell voltage-clamp recordings from cultured rat hippocampal neurons, the 2,3-benzodiazepine GYKI 52466 was a potent antagonist of kainate- and AMPA-activated currents (IC50 values, 7.5 and 11 microM, respectively), but was inactive against N-methyl-D-aspartate (NMDA) or gamma-aminobutyric acid responses. The block produced by GYKI 52466 occurred in a noncompetitive fashion, was voltage independent, and failed to show use dependence, indicating an allosteric blocking mechanism. In kinetic experiments with kainate as the agonist, the GYKI 52466 binding and unbinding rates were 1.6 x 10(5) M-1 s-1 and 3.2 s-1, respectively. GYKI 52466 also suppressed non-NMDA receptor-mediated spontaneous synaptic currents via a postsynaptic action. Non-competitive AMPA/kainate antagonists such as GYKI 52466 could offer advantages over competitive antagonists in the treatment of glutamate-associated neurological disorders, particularly under conditions in which high levels of the amino acid would render the competitive antagonists relatively ineffective. Moreover, the results demonstrate the existence of a novel recognition site for an atypical benzodiazepine on non-NMDA receptors.

The A-current: how ubiquitous a feature of excitable cells is it?

Abstract The membranes of most excitable cells contain a distinct set of potassium channels that rapidly open and close following depolarization, giving rise to a transient outward membrane current ( I A ). This current seems to play an important role in allowing neurons to encode graded depolarization into spike train information; in action potential repolarization, particularly in cardiac muscle cells; and in several aspects of synaptic transmission. As a modifier of the responsiveness of postsynaptic neurons to synaptic potentials, I A is a critical determinant of the integrative behavior of certain neural networks. Recent studies indicate that I A can be physiologically modulated by synaptic influences and during learning. The discovery of Drosophila mutants where I A is defective or absent provides the hope that this may soon be the first potassium channel amenable to structural analysis by molecular genetic techniques.

Molecular Targets for Antiepileptic Drug Development

SummaryThis review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the α subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, α2-δ voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABAA receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABAB and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated currentIh; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABAA receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

Therapeutic potential of excitatory amino acid antagonists: channel blockers and 2,3-benzodiazepines

NMDA and non-NMDA (AMPA/kainate) antagonists have potential in the treatment of a diverse group of neurological disorders associated with excessive activation of excitatory amino acid receptors. Here Michael Rogawski reviews recent progress in the development of therapeutically useful NMDA receptor channel blockers and a new class of selective AMPA/kainate receptor antagonists, the 2,3-benzodiazepines. Research on these novel noncompetitive excitatory amino acid antagonists has opened promising new avenues for the development of drugs to treat epilepsy, ischaemia, neurodegeneration and Parkinsons disease.

Colchicine Myopathy and Neuropathy

Although colchicine has been used for centuries, its neuromuscular toxicity in humans is largely unrecognized. In this report we describe a characteristic syndrome of myopathy and neuropathy and present 12 new cases of the condition. Colchicine myopathy may occur in patients with gout who take customary doses of the drug but who have elevated plasma drug levels because of altered renal function. It usually presents with proximal weakness and always presents with elevation of serum creatine kinase; both features remit within three to four weeks after the drug is discontinued. The accompanying axonal polyneuropathy is mild and resolves slowly. Electromyography of proximal muscles shows a myopathy that is marked by abnormal spontaneous activity. Because of these features, colchicine myoneuropathy is usually misdiagnosed initially, either as probable polymyositis or as uremic neuropathy. The myopathy is vacuolar, marked by accumulation of lysosomes and autophagic vacuoles unrelated to necrosis or to the mild denervation in distal muscles. The morphologic changes in muscle suggest that the pathogenesis involves disruption of a microtubule-dependent cytoskeletal network that interacts with lysosomes. Correct diagnosis may save patients with this disorder from inappropriate therapy.

Diverse mechanisms of antiepileptic drugs in the development pipeline

There is a remarkable array of new chemical entities in the current antiepileptic drug (AED) development pipeline. In some cases, the compounds were synthesized in an attempt improve upon the activity of marketed AEDs. In other cases, the discovery of antiepileptic potential was largely serendipitous. Entry into the pipeline begins with the demonstration of activity in one or more animal screening models. Results from testing in a panel of such models provide a basis to differentiate agents and may offer clues as to the mechanism. Target activity may then be defined through cell-based studies, often years after the initial identification of activity. Some pipeline compounds are believed to act through conventional targets, whereas others are structurally novel and may act by novel mechanisms. Follow-on agents include the levetiracetam analogs brivaracetam and seletracetam that act as SV2A-ligands; the valproate-like agents valrocemide, valnoctamide, propylisopropyl acetamide, and isovaleramide; the felbamate analog flurofelbamate, a dicarbamate, and the unrelated carbamate RWJ-333369; the oxcarbazepine analog licarbazepine, which probably acts as a use-dependent sodium channel blockers, and its prodrug acetate BIA 2-093; various selective partial benzodiazepine receptor agonists, including ELB139, which is a positive allosteric modulator of alpha3-containing GABA(A) receptors. A variety of AEDs that may act through novel targets are also in clinical development: lacosamide, a functionalized amino acid; talampanel, a 2,3-benzodiazepine selective noncompetitive AMPA receptor antagonist; NS1209, a competitive AMPA receptor antagonist; ganaxolone, a neuroactive steroid that acts as a positive modulator of GABA(A) receptors; retigabine, a KCNQ potassium channel opener with activity as a GABA(A) receptor positive modulator; the benzanilide KCNQ potassium channel opener ICA-27243 that is more selective than retigabine; and rufinamide, a triazole of unknown mechanism.

Glia and epilepsy: excitability and inflammation

Epilepsy is characterized by recurrent spontaneous seizures due to hyperexcitability and hypersynchrony of brain neurons. Current theories of pathophysiology stress neuronal dysfunction and damage, and aberrant connections as relevant factors. Most antiepileptic drugs target neuronal mechanisms. However, nearly one-third of patients have seizures that are refractory to available medications; a deeper understanding of mechanisms may be required to conceive more effective therapies. Recent studies point to a significant contribution by non-neuronal cells, the glia--especially astrocytes and microglia--in the pathophysiology of epilepsy. This review critically evaluates the role of glia-induced hyperexcitability and inflammation in epilepsy.

Neuroprotective and disease-modifying effects of the ketogenic diet.

The ketogenic diet has been in clinical use for over 80 years, primarily for the symptomatic treatment of epilepsy. A recent clinical study has raised the possibility that exposure to the ketogenic diet may confer long-lasting therapeutic benefits for patients with epilepsy. Moreover, there is evidence from uncontrolled clinical trials and studies in animal models that the ketogenic diet can provide symptomatic and disease-modifying activity in a broad range of neurodegenerative disorders including Alzheimers disease and Parkinsons disease, and may also be protective in traumatic brain injury and stroke. These observations are supported by studies in animal models and isolated cells that show that ketone bodies, especially &bgr;-hydroxybutyrate, confer neuroprotection against diverse types of cellular injury. This review summarizes the experimental, epidemiological and clinical evidence indicating that the ketogenic diet could have beneficial effects in a broad range of brain disorders characterized by the death of neurons. Although the mechanisms are not yet well defined, it is plausible that neuroprotection results from enhanced neuronal energy reserves, which improve the ability of neurons to resist metabolic challenges, and possibly through other actions including antioxidant and anti-inflammatory effects. As the underlying mechanisms become better understood, it will be possible to develop alternative strategies that produce similar or even improved therapeutic effects without the need for exposure to an unpalatable and unhealthy, high-fat diet.