Mechanisms of antiepileptic drug action

Late sodium current: incomplete inactivation triggers seizures, myotonias, arrhythmias, and pain syndromes

Acquired and inherited dysfunction in voltage-gated sodium channels underlies a wide range of diseases. "In addition to the defects in trafficking and expression, sodium channelopathies are also caused by dysfunction in one or several gating properties, for instance activation or inactivation. Disruption of the channel inactivation leads to the increased late sodium current, which is a common defect in seizure disorders, cardiac arrhythmias skeletal muscle myotonia and pain. An increase in late sodium current leads to repetitive action potential in neurons and skeletal muscles, and prolonged action potential duration in the heart. In this topical review, we compare the effects of late sodium current in brain, heart, skeletal muscle, and peripheral nerves. Abstract figure legend Shows cartoon illustration of general Nav channel transitions between (1) resting, (2) open, and (3) fast inactivated states. Disruption of the inactivation process exacerbates (4) late sodium currents. This article is protected by copyright. All rights reserved.

Mechanisms of human motor cortex facilitation induced by subthreshold 5-Hz repetitive transcranial magnetic stimulation

Our knowledge about the mechanisms of human motor cortex facilitation induced by repetitive transcranial magnetic stimulation (rTMS) is still incomplete. Here we used pharmacological conditioning with carbamazepine, dextrometorphan, lorazepam, and placebo to elucidate the type of plasticity underlying this facilitation, and to probe if mechanisms reminiscent of long-term potentiation are involved. Over the primary motor cortex of 10 healthy subjects, we applied biphasic rTMS pulses of effective posterior current direction in the brain. We used six blocks of 200 pulses at 5-Hz frequency and 90% active motor threshold intensity and controlled for corticospinal excitability changes using motor-evoked potential (MEP) amplitudes and latencies elicited by suprathreshold pulses before, in between, and after rTMS. Target muscle was the dominant abductor digiti minimi muscle; we coregistered the dominant extensor carpi radialis muscle. We found a lasting facilitation induced by this type of rTMS. The GABAergic medication lorazepam and to a lesser extent the ion channel blocker carbamazepine reduced the MEP facilitation after biphasic effective posteriorly oriented rTMS, whereas the N-methyl-d-aspartate receptor-antagonist dextrometorphan had no effect. Our main conclusion is that the mechanism of the facilitation induced by biphasic effective posterior rTMS is more likely posttetanic potentiation than long-term potentiation. Additional findings were prolonged MEP latency under carbamazepine, consistent with sodium channel blockade, and larger MEP amplitudes from extensor carpi radialis under lorazepam, suggesting GABAergic involvement in the center-surround balance of excitability.

The effects of anticonvulsant drugs on NMDA-EPSP, AMPA-EPSP, and GABA-IPSP in the rat hippocampus

The effects of phenobarbital, phenytoin, and valproic acid on pharmacologically isolated NMDA-EPSP, AMPA-EPSP, and GABA-IPSPs were examined in rat hippocampal slices. Phenobarbital (0.05 mg/ml) had no effect on the NMDA-EPSP, but decreased the slope of the AMPA-EPSP by 13.4% and facilitated the GABA-IPSP slope by 77.12%. Phenytoin (0.02 mg/ml) had no effects on the NMDA-EPSP, AMPA-EPSP, or GABA-IPSP. Valproic acid (0.1 mg/ml) decreased the NMDA-EPSP slope by 14.3%, increased the GABA-IPSP slope by 54.34%, and had no effect on the AMPA-EPSP. These data suggest that the mechanisms of action of these anticonvulsant drugs may be via their actions on different neurotransmitter systems or ion channels.

The effects of anticonvulsant drugs on long-term potentiation (LTP) in the rat hippocampus

In hippocampal CA1 area, there are at least two forms of long-term potentiation (LTP): one is N-methyl-D-aspartate (NMDA) receptor-dependent LTP (NMDA LTP), which is induced with a 25 Hz tetanus and blocked by 50 microM 2-amino-5-phosphonovaleric acid (APV); the other is NMDA receptor-independent LTP (VDCC LTP), which is induced by 200 Hz tetanus stimulation in the presence of APV and blocked by nifedipine, a voltage-dependent Ca++ channel (VDCC) blocker, or by the intracellular injection of 1,2-bis(2-Aminophenoxoy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). The effects of anticonvulsant drugs phenobarbital, phenytoin, and valproic acid on both NMDA LTP and VDCC LTP were investigated in rat hippocampal slices. The results showed that 0.1 mg/ml valproic acid significantly altered baseline population spike amplitude by 34.6%, but the other drugs had no significant effect on the baseline population spike amplitude. Phenobarbital (0.025 mg/ml) potently blocked NMDA LTP and inhibited VDCC LTP. Phenytoin (0.02 mg/ml) had no effect on NMDA LTP but reduced VDCC LTP. Valproic acid did not inhibit VDCC LTP, but it abolished the expression of NMDA LTP in a similar manner as H-7, a nonspecific protein kinase C inhibitor. These data suggest that the anti-convulsant effects of these three drugs may be via different cellular mechanisms.

Drug interactions in epilepsy

The abolition of seizures using a single antiepileptic agent can be expected in more than 80% of patients, although not necessarily with the first drug tried. The remainder often receive polypharmacy, and current evidence suggests that perhaps only around 10% of these benefit significantly in terms of improved seizure control. Many more experience complicated drug interactions. Carbamazepine, phenytoin, phenobarbital, and primidone (metabolized in part to phenobarbital) all induce the synthesis of hepatic monooxygenase and conjugating enzymes. This will result in an acceleration in the metabolism of other lipid-soluble drugs with likely attenuation of their pharmacological effects. Valproate, on the other hand, is a minor enzyme inhibitor. Pharmacokinetic interactions are almost invariable when more than one antiepileptic drug is coprescribed. The extent and direction of interactions with combinations of these drugs are varied and unpredictable. Discontinuation of an enzyme inducer or inhibitor will influence the concentrations of the remaining drug(s). Pharmacodynamic interactions also cause problems in epileptic patients. A number of commonly prescribed psychoactive drugs, such as tricyclic antidepressants and neuroleptics, can worsen seizure control by reducing the convulsion threshold. In addition, there seems little doubt that ethanol abuse and withdrawal can precipitate seizures in susceptible patients. Antiepileptic polypharmacy is more likely to impair cognitive function than the same drugs used singly. In addition, the more antiepileptic drugs received by a patient in the first trimester of pregnancy, the higher the risk of teratogenesis in the exposed infant. Drug interactions prolong and complicate the process of new drug assessment, particularly when introduced in treated patients with refractory epilepsy. The candidate antiepileptic drug may alter the concentration of concomitant therapy, or its own breakdown may be influenced by coprescribed enzyme inducers or inhibitors. Even if the new drug is excreted unchanged by the kidney, unexpected interactions can be uncovered. Pharmacodynamic interactions need not always be detrimental. Currently, there is no rational approach to the treatment of intractable epilepsy. As more new drugs with single mechanisms of action become available, the potential exists for combining these synergistically. This approach may revolutionize the pharmacological management of the epileptic patient in the 21st century.