Characterization of the in vitro antiepileptic activity of new and old anticonvulsant drugs

CNS Adverse Effects: From Functional Observation Battery/Irwin Tests to Electrophysiology

This chapter describes various approaches for the preclinical assessment of drug-induced central nervous system (CNS) adverse effects. Traditionally, methods to evaluate CNS effects have consisted of observing and scoring behavioral responses of animals after drug is administered. Among several behavioral testing paradigms, the Irwin and the functional observational battery (FOB) are the most commonly used assays for the assessment of CNS effects. The Irwin and FOB are considered good first-tier assays to satisfy the ICH S7A guidance for the preclinical evaluation of new chemical entities (NCE) intended for humans. However, experts have expressed concern about the subjectivity and lack of quantitation that is derived from behavioral testing. More importantly, it is difficult to gain insight into potential mechanisms of toxicity by assessing behavioral outcomes. As a complement to behavioral testing, we propose using electrophysiology-based assays, both in vivo and in vitro, such as electroencephalograms and brain slice field-potential recordings. To better illustrate these approaches, we discuss the implementation of electrophysiology-based techniques in drug-induced assessment of seizure risk, sleep disruption, and cognitive impairment.

Diphenytoin, riluzole and lidocaine: three sodium channel blockers, with different mechanisms of action, decrease hippocampal epileptiform activity

Epilepsy is a condition affecting 1-2% of the population, characterized by the presence of spontaneous, recurrent seizures. The most common type of acquired epilepsy is temporal lobe epilepsy (TLE). Up to 30% of patients with TLE are refractory to currently available compounds, and there is an urgent need to identify novel targets for therapy. Here, we utilized the in-vitro CA3 burst preparation to examine alterations in network excitability, characterized by changes in interburst interval. Specifically, we show that bath application of three different sodium channel blockers-diphenytoin, riluzole, and lidocaine-slow spontaneous CA3 bursts. This in turn, decreased the epileptiform activity. These compounds work at different sites on voltage-gated sodium channels, but produce a similar network phenotype of decreased excitability. In the case of diphenytoin and riluzole, the change in network activity (i.e., increased interburst intervals) was persistent following drug washout. Lidocaine application, however, only increased the CA3 interburst interval when it was in the bath solution. Thus, its action was not permanent and resulted in returning CA3 bursting to baseline levels. These data demonstrate that the CA3 burst preparation provides a relatively easy and quick platform for identifying compounds that can decrease network excitability, providing the initial screen for further and more complex in-vivo, freely-behaving animal studies.

Cannabidivarin is anticonvulsant in mouse and rat

BACKGROUND AND PURPOSE

Phytocannabinoids in Cannabis sativa have diverse pharmacological targets extending beyond cannabinoid receptors and several exert notable anticonvulsant effects. For the first time, we investigated the anticonvulsant profile of the phytocannabinoid cannabidivarin (CBDV) in vitro and in in vivo seizure models.

EXPERIMENTAL APPROACH

The effect of CBDV (1-100 μM) on epileptiform local field potentials (LFPs) induced in rat hippocampal brain slices by 4-aminopyridine (4-AP) application or Mg(2+) -free conditions was assessed by in vitro multi-electrode array recordings. Additionally, the anticonvulsant profile of CBDV (50-200 mg·kg(-1) ) in vivo was investigated in four rodent seizure models: maximal electroshock (mES) and audiogenic seizures in mice, and pentylenetetrazole (PTZ) and pilocarpine-induced seizures in rats. The effects of CBDV in combination with commonly used antiepileptic drugs on rat seizures were investigated. Finally, the motor side effect profile of CBDV was investigated using static beam and grip strength assays.

KEY RESULTS

CBDV significantly attenuated status epilepticus-like epileptiform LFPs induced by 4-AP and Mg(2+) -free conditions. CBDV had significant anticonvulsant effects on the mES (≥100 mg·kg(-1) ), audiogenic (≥50 mg·kg(-1) ) and PTZ-induced seizures (≥100 mg·kg(-1) ). CBDV (200 mg·kg(-1) ) alone had no effect against pilocarpine-induced seizures, but significantly attenuated these seizures when administered with valproate or phenobarbital at this dose. CBDV had no effect on motor function.

CONCLUSIONS AND IMPLICATIONS

These results indicate that CBDV is an effective anticonvulsant in a broad range of seizure models. Also it did not significantly affect normal motor function and, therefore, merits further investigation as a novel anti-epileptic in chronic epilepsy models.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Phenylethylidenehydrazine, a novel GABA-transaminase inhibitor, reduces epileptiform activity in rat hippocampal slices

Phenylethylidenehydrazine (PEH), an analog of the monoamine oxidase inhibitor, beta-phenylethylhydrazine (phenelzine), inhibits the gamma-aminobutyric acid (GABA) catabolic enzyme GABA-transaminase and increases brain levels of GABA. GABA is the predominant fast inhibitory transmitter counteracting glutamatergic excitation, and increased neural GABA could influence a wide range of synaptic and circuit properties under both physiologic and pathophysiologic conditions. To examine the scope of these effects, we applied PEH (or vehicle) to rat hippocampal slices and measured basal glutamatergic transmission, synaptic plasticity, and epileptiform activity using extracellular field and whole cell patch clamp recordings. In vitro pre-treatment with PEH (100 microM) increased the GABA content of hippocampal slices by approximately 60% over vehicle-treated controls, but it had no effect on basal field excitatory postsynaptic potentials, tonic GABA currents, paired-pulse facilitation, or long-term potentiation. In contrast, pre-incubation with PEH caused a dose- and time-dependent reduction in epileptiform burst frequency induced by superfusion with Mg2+-free or high-K+ artificial cerebrospinal fluid. Thus, the inhibitory effects of PEH are state-dependent: hyper-excitation during epileptiform bursting was reduced, whereas synaptic transmission and plasticity were unaffected.

Antiepileptogenic agents: how close are we?

Epilepsy is a common neurological condition, affecting about 4% of individuals over their lifetime. Epilepsy can be idiopathic, secondary to an underlying genetic abnormality or unknown causes, or acquired. Known potential causes account for about one third of epilepsy. Control of epilepsy has primarily focused on suppressing seizure activity after epilepsy has developed. An intriguing possibility is to control acquired epilepsy by preventing epileptogenesis, the process by which the brain becomes epileptic. Many laboratory models simulate human epilepsy as well as provide a system for studying epileptogenesis. The kindling model involves repeated application of subconvulsive electrical stimulation to the brain, leading to spontaneous seizures. Other models include the cortical or systemic injection of various chemicals. These models suggest that many antiepileptic drugs, from phenobarbital and valproate (valproic acid) to levetiracetam and tiagabine, have antiepileptogenic potential. Some promising other possibilities include N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists as well as the neurotrophins and their receptors. Phenobarbital, phenytoin, valproate, carbamazepine and, to a very limited extent, diazepam have been evaluated in clinical trials to test whether they actually prevent epileptogenesis in humans. Results have been very disappointing. Meta-analyses of 12 different drug-condition combinations show none with significantly lower unprovoked seizure rates among those receiving the active drug. In 4 of the 12, the observed rate was actually slightly higher among treated individuals. None of the newer drugs have been evaluated in antiepileptogenesis trials. Until some drugs demonstrate a clear antiepileptogenic effect in clinical trials, the best course to reduce the incidence of epilepsy is primary prevention of the risk-increasing events--for example, wearing helmets, using seat belts, or decreasing the risk of stroke by reducing smoking.

Effects of antiepileptic drugs on afterdischarge generation in rat hippocampal slices

We have recently reported that ictal-like afterdischarges (ADs) analogous to those in in vivo kindling models are induced by high-frequency stimulation (100 Hz, 1s) to the stratum radiatum of the CA1 region of rat hippocampal slices. To test whether this model can also serve as an in vitro seizure model for temporal lobe epilepsy, we examined the effects of antiepileptic drugs on this model and compared them with their effects on seizures in patients with temporal lobe epilepsy. ADs were progressively enhanced following repetitive high-frequency stimulations to slices treated with 4-aminopyridine, a proconvulsive A-type potassium channel blocker. Bath application of phenytoin (1-100 microM) and carbamazepine (1-100 microM) suppressed AD generation in a concentration-dependent manner. At a clinically relevant concentration of 10 microM, phenytoin reduced the number of spikes in an AD to 50.6% and carbamazepine to 39.7% of the control values. On the other hand, ethosuximide had no effect on AD generation at a concentration of 1 mM, which is clinically effective against absence seizures, but enhanced it at a toxic concentration of 10 mM. These findings indicate that the pharmacological profiles of antiepileptic drugs applied to our model correspond to those applied to seizures in patients with temporal lobe epilepsy. We therefore conclude that this model can be a useful in vitro model for the ictal manifestation of temporal lobe epilepsy.

Inhibition of neuronal hypersynchrony in vitro differentiates levetiracetam from classical antiepileptic drugs

Field potentials were recorded from rat hippocampal slices in order to compare the electrophysiological action of the new antiepileptic drug (AED), levetiracetam (LEV), with that of the classical AEDs, valproate, clonazepam and carbamazepine, on epileptiform responses induced by a 'high K(+)-low Ca(2+)' perfusion fluid. Increasing [K(+)] from 3 to 7.5 mM and decreasing [Ca(2+)] from 2.4 to 0.5 mM, in the bathing fluid, produced population spikes (PSs) of increasing amplitudes in the CA3 area of the slices, repetitive PSs evoked by single stimuli, and spontaneous bursts. Clinically relevant concentrations of LEV, 32 and 100 micro M, consistently reduced the second (PS(2)) and third (PS(3)) population spikes, and the number (N) of repetitive PSs per evoked response. Levetiracetam 32 micro M also opposed the increase in amplitude of the first PS (Delta PS(1)). Neither valproate 1 mM, nor clonazepam 1 micro M, nor carbamazepine 50 micro M, produced any decrease in Delta PS(1)and in PS(2), but all decreased N. These results show that LEV contrasts to reference AEDs by its ability to antagonize neuronal (hyper)synchronization, in the highly seizure-prone CA3 area of rat hippocampal slices.