Phenytoin, phenobarbital, and midazolam fail to stop status epilepticus-like activity induced by low magnesium in rat entorhinal slices, but can prevent its development

RNA Sequencing Demonstrates Ex Vivo Neocortical Transcriptomic Changes Induced by Epileptiform Activity in Male and Female Mice

Seizures are generally associated with epilepsy but may also be a symptom of many other neurological conditions. A hallmark of a seizure is the intensity of the local neuronal activation, which can drive large-scale gene transcription changes. Such changes in the transcriptional profile likely alter neuronal function, thereby contributing to the pathological process. Therefore, there is a strong clinical imperative to characterize how gene expression is changed by seizure activity. To this end, we developed a simplified ex vivo technique for studying seizure-induced transcriptional changes. We compared the RNA sequencing profile in mouse neocortical tissue with up to 3 h of epileptiform activity induced by 4-aminopyridine (4AP) relative to control brain slices not exposed to the drug. We identified over 100 genes with significantly altered expression after 4AP treatment, including multiple genes involved in MAPK, TNF, and neuroinflammatory signaling pathways, all of which have been linked to epilepsy previously. Notably, the patterns in male and female brain slices were almost identical. Various immediate early genes were among those showing the largest upregulation. The set of down-regulated genes included ones that might be expected either to increase or to decrease neuronal excitability. In summary, we found the seizure-induced transcriptional profile complex, but the changes aligned well with an analysis of published epilepsy-associated genes. We discuss how simple models may provide new angles for investigating seizure-induced transcriptional changes.

The brain-specific kinase LMTK3 regulates neuronal excitability by decreasing KCC2-dependent neuronal Cl- extrusion

LMTK3 is a brain-specific transmembrane serine/threonine protein kinase that acts as a scaffold for protein phosphatase-1 (PP1). Although LMKT3 has been identified as a risk factor for autism and epilepsy, its physiological significance is unknown. Here, we demonstrate that LMTK3 copurifies and binds to KCC2, a neuron-specific K+/Cl- transporter. KCC2 activity is essential for Cl--mediated hyperpolarizing GABAAR receptor currents, the unitary events that underpin fast synaptic inhibition. LMTK3 acts to promote the association of KCC2 with PP1 to promote the dephosphorylation of S940 within its C-terminal cytoplasmic domain, a process the diminishes KCC2 activity. Accordingly, acute inhibition of LMTK3 increases KCC2 activity dependent upon S940 and increases neuronal Cl- extrusion. Consistent with this, LMTK3 inhibition reduced intrinsic neuronal excitability and the severity of seizure-like events in vitro. Thus, LMTK3 may have profound effects on neuronal excitability as an endogenous modulator of KCC2 activity.

Can in vitro studies aid in the development and use of antiseizure therapies? A report of the ILAE/AES Joint Translational Task Force

In vitro preparations (defined here as cultured cells, brain slices, and isolated whole brains) offer a variety of approaches to modeling various aspects of seizures and epilepsy. Such models are particularly amenable to the application of anti-seizure compounds, and consequently are a valuable tool to screen the mechanisms of epileptiform activity, mode of action of known anti-seizure medications (ASMs), and the potential efficacy of putative new anti-seizure compounds. Despite these applications, all disease models are a simplification of reality and are therefore subject to limitations. In this review, we summarize the main types of in vitro models that can be used in epilepsy research, describing key methodologies as well as notable advantages and disadvantages of each. We argue that a well-designed battery of in vitro models can form an effective and potentially high-throughput screening platform to predict the clinical usefulness of ASMs, and that in vitro models are particularly useful for interrogating mechanisms of ASMs. To conclude, we offer several key recommendations that maximize the potential value of in vitro models in ASM screening. This includes the use of multiple in vitro tests that can complement each other, carefully combined with in vivo studies, the use of tissues from chronically epileptic (rather than naïve wild-type) animals, and the integration of human cell/tissue-derived preparations.

In vitro and in vivo anti-seizure activity of hydromethanolic extract and fractions of Pterolobium stellatum

ETHNOPHARMACOLOGICAL RELEVANCE

In Ethiopia, the whole plant juice of Pterolobium stellatum is used to treat seizures and epilepsy.

AIM OF THE STUDY

To investigate the antiseizure activity of hydromethanolic crude extract and fractions collected from leaves of P. stellatum using both in vitro, and in vivo seizure models in mice.

MATERIALS AND METHODS

Fresh leaves of P. stellatum were collected from Awash Melka, Addis Ababa, Ethiopia. An 80% crude methanol extract was further fractionated to produce petroleum ether, chloroform, butanol, and aqueous fractions. Anti-seizure activity of the crude extract and fractions (petroleum ether, chloroform, butanol, and water) were assessed at a concentration of 0.7 mg/ml using the in vitro 0 Mg²⁺ model of seizures in mouse brain slices prepared from 14- to 21-day-old C57BL/6 mice. The maximal electroshock seizure (MES) model and the pentylenetetrazol (PTZ) seizure model for seizures were performed on male BALB/c mice using 400 mg/kg and 800 mg/kg of crude 80% methanol extract, as well as the four fractions described above. Diazepam and phenytoin were used as positive controls for PTZ and MES test respectively.

RESULTS

Addition of 0.7 mg/ml of crude 80% methanol extract of P. stellatum prevented the onset of SLEs in most brain slices in the 0 Mg²⁺in vitro model of seizures, with similar efficacy to diazepam (3 μM). The same extract at 400 and 800 mg/kg was efficacious in reducing the hindlimb extension time in the MES model and delaying the onset of myoclonic convulsions in the PTZ model, although not to the same extent as phenytoin (10 mg/kg) or diazepam (5 mg/kg). The chloroform and water fractions of the crude extract also showed significant anti-seizure activity across all three models whilst the non-polar petroleum ether and butanol fractions did not. The UPLC-MS analysis indicated the presence of gallic acid, ellagic acid, kaempferol, myricitrin, isoquercitrin and quercitirin in the crude extract. Gallic acid and ellagic acid were observed in chloroform fraction and in the water fraction ellagic acid, kaempferol, myricitrin and isoquercitrin were detected.

CONCLUSION

The crude hydromethanolic extract of P. stellatum has significant anti-seizure activity. The chloroform and aqueous fractions have antiseizure activity. The extracts have previously identified compounds with anticonvulsant activity which indicates the antiseizure potential of the plant.

Inhibiting with-no-lysine kinases enhances K+/Cl- cotransporter 2 activity and limits status epilepticus

First-in-line benzodiazepine treatment fails to terminate seizures in about 30% of epilepsy patients, highlighting a need for novel antiseizure strategies. Impaired GABAergic inhibition is key to the development of such benzodiazepine-resistant seizures, as well as the pathophysiology of status epilepticus (SE). It is emerging that reduced or impaired neuronal K+/Cl- cotransporter 2 (KCC2) activity contributes to deficits in γ-aminobutyric acid (GABA)-mediated inhibition and increased seizure vulnerability. The with-no-lysine kinase (WNK)-STE20/SPS1-related proline/alanine-rich (SPAK) kinase signaling pathway inhibits neuronal KCC2 via KCC2-T1007 phosphorylation. A selective WNK kinase inhibitor, WNK463, was recently synthesized by Novartis. Exploiting WNK463, we test the hypothesis that pharmacological WNK inhibition will enhance KCC2 activity, increase the efficacy of GABAergic inhibition, and thereby limit seizure activity in animal models. Immunoprecipitation and Western blot analysis were used to examine WNK463's effects on KCC2-T1007 phosphorylation, in vitro and in vivo. A thallium (Tl+) uptake assay was used in human embryonic kidney (HEK-293) cells expressing KCC2 to test WNK463's effects on KCC2-mediated Tl+ transport. Gramicidin-perforated- and whole-cell patch-clamp recordings in cortical rat neurons were used to examine WNK463's effects on KCC2-mediated Cl- transport. In mouse brain slices (entorhinal cortex), field recordings were utilized to examine WNK463's effects on 4-aminopyridine-induced seizure activity. Last, WNK463 was directly deliver to the mouse hippocampus in vivo and tested in a kainic acid model of diazepam-resistant SE. WNK463 significantly reduces KCC2-T1007 phosphorylation in vitro and in vivo (mice). In human embryonic kidney 293 (HEK-293) cells expressing KCC2, WNK463 greatly enhanced the rates Tl+ transport. However, the drug did not enhance Tl+ transport in cells expressing a KCC2-phospho null T1007 mutant. In cultured rat neurons, WNK463 rapidly reduced intracellular Cl- and consequently hyperpolarized the Cl- reversal potential (EGABA). In mature neurons that were artificially loaded with 30 mM Cl-, WNK463 significantly enhanced KCC2-mediated Cl- export and hyperpolarized EGABA. In a 4-aminopyridine model of acute seizures, WNK463 reduced the frequency and number of seizure-like events (SLEs). Finally, in an in vivo kainic acid (KA) model of diazepam-resistant SE, WNK463 slowed the onset and reduced the severity of KA-induced status epilepticus. Last, WNK463 prevented the development of pharmaco-resistance to diazepam in drug-treated mice. Our findings demonstrate that acute WNK463 treatment potentiates KCC2 activity in neurons and limits seizure burden in two well-established models of seizures and epilepsy. Our work suggests that agents which act to increase KCC2 activity may be useful adjunct therapeutics to alleviate diazepam-resistant SE.

Excitatory GABAergic signalling is associated with benzodiazepine resistance in status epilepticus

Status epilepticus is defined as a state of unrelenting seizure activity. Generalized convulsive status epilepticus is associated with a rapidly rising mortality rate, and thus constitutes a medical emergency. Benzodiazepines, which act as positive modulators of chloride (Cl-) permeable GABAA receptors, are indicated as first-line treatment, but this is ineffective in many cases. We found that 48% of children presenting with status epilepticus were unresponsive to benzodiazepine treatment, and critically, that the duration of status epilepticus at the time of treatment is an important predictor of non-responsiveness. We therefore investigated the cellular mechanisms that underlie acquired benzodiazepine resistance, using rodent organotypic and acute brain slices. Removing Mg2+ ions leads to an evolving pattern of epileptiform activity, and eventually to a persistent state of repetitive discharges that strongly resembles clinical EEG recordings of status epilepticus. We found that diazepam loses its antiseizure efficacy and conversely exacerbates epileptiform activity during this stage of status epilepticus-like activity. Interestingly, a low concentration of the barbiturate phenobarbital had a similar exacerbating effect on status epilepticus-like activity, while a high concentration of phenobarbital was effective at reducing or preventing epileptiform discharges. We then show that the persistent status epilepticus-like activity is associated with a reduction in GABAA receptor conductance and Cl- extrusion capability. We explored the effect on intraneuronal Cl- using both gramicidin, perforated-patch clamp recordings and Cl- imaging. This showed that during status epilepticus-like activity, reduced Cl- extrusion capacity was further exacerbated by activity-dependent Cl- loading, resulting in a persistently high intraneuronal Cl-. Consistent with these results, we found that optogenetic stimulation of GABAergic interneurons in the status epilepticus-like state, actually enhanced epileptiform activity in a GABAAR dependent manner. Together our findings describe a novel potential mechanism underlying benzodiazepine-resistant status epilepticus, with relevance to how this life-threatening condition should be managed in the clinic.

Divergent paths to seizure-like events

Much debate exists about how the brain transitions into an epileptic seizure. One source of confusion is that there are likely to be critical differences between experimental seizure models. To address this, we have compared the evolving activity patterns in two widely used in vitro models of epileptic discharges. Brain slices from young adult mice were prepared in the same way and bathed either in 0 Mg2+ or 100 µmol/L 4AP artificial cerebrospinal fluid. We have found that while local field potential recordings of epileptiform discharges in the two models appear broadly similar, patch-clamp analysis reveals an important difference in the relative degree of glutamatergic involvement. 4AP affects parvalbumin-expressing interneurons more than other cortical populations, destabilizing their resting state and inducing spontaneous bursting behavior. Consequently, the most prominent pattern of transient discharge ("interictal event") in this model is almost purely GABAergic, although the transition to seizure-like events (SLEs) involves pyramidal recruitment. In contrast, interictal discharges in 0 Mg2+ are only maintained by a very large glutamatergic component that also involves transient discharges of the interneurons. Seizure-like events in 0 Mg2+ have significantly higher power in the high gamma frequency band (60-120Hz) than these events do in 4AP, and are greatly delayed in onset by diazepam, unlike 4AP events. We, therefore, conclude that the 0 Mg2+ and 4AP models display fundamentally different levels of glutamatergic drive, demonstrating how ostensibly similar pathological discharges can arise from different sources. We contend that similar interpretative issues will also be relevant to clinical practice.

The 4-Aminopyridine Model of Acute Seizures in vitro Elucidates Efficacy of New Antiepileptic Drugs

Up to date, preclinical screening for new antiepileptic substances is performed by a combination of different in vivo models of acute seizures, for which large numbers of animals are necessary. So far, little attention has been paid to in vitro models, which are also able to detect antiepileptic efficacy and in principle could likewise serve for exploratory preclinical screening. One of the established in vitro models of acute seizures is the 4-aminopyridine (4-AP) model. Previous studies have shown that the 4-AP model is capable to recapitulate the antiepileptic efficacy of standard antiepileptic drugs (AEDs) such as valproate or carbamazepine. Here, we employed a dual methodological approach using electrophysiology and optical imaging to systematically test the antiepileptic efficacy of three new-generation AEDs with distinct mechanisms of action (lacosamide, zonisamide, and levetiracetam). We found that frequency of 4-AP induced seizure like events (SLE) was the most sensitive parameter to detect dose-dependent antiepileptic effects in these compounds. Specifically, levetiracetam reduced SLE frequency while lacosamide and zonisamide at higher doses completely blocked SLE incidence. Analysis of the intrinsic optical signal additionally revealed a subiculum-specific reduction of the area involved in the propagation of ictal activity when lacosamide or zonisamide were administered. Taken together, our data adds some evidence that acute seizure models in vitro are in principle capable to detect antiepileptic effects across different mechanisms of action with efficacy similar to acute models in vivo. Further studies with negative controls, e.g., penicillin as a proconvulsant, and other clinically relevant AEDs are needed to determine if this acute in vitro model might be useful as exploratory screening tool. In view of the increasing sensitivity toward animal welfare, an affective in vitro model may help to reduce the number of laboratory animals deployed in burdening in vivo experiments and to preselect substances for subsequent testing in time- and cost-laborious models of chronic epilepsy.

Region-specific differences and areal interactions underlying transitions in epileptiform activity

Key points

Local neocortical and hippocampal territories show different and sterotypical patterns of acutely evolving, epileptiform activity.

Neocortical and entorhinal networks show tonic–clonic‐like events, but the main hippocampal territories do not, unless it is relayed from the other areas.

Transitions in the pattern of locally recorded epileptiform activity can be indicative of a shift in the source of pathological activity, and may spread through both synaptic and non‐synaptic means.

Hippocampal epileptiform activity is promoted by 4‐aminopyridine and inhibited by GABA_B receptor agonists, and appears far more sensitive to these drugs than neocortical activity.

These signature features of local epileptiform activity can provide useful insight into the primary source of ictal activity, aiding both experimental and clinical investigation.

Abstract

Understanding the nature of epileptic state transitions remains a major goal for epilepsy research. Simple in vitro models offer unique experimental opportunities that we exploit to show that such transitions can arise from shifts in the ictal source of the activity. These transitions reflect the fact that cortical territories differ both in the type of epileptiform activity they can sustain and in their susceptibility to drug manipulation. In the zero‐Mg²⁺ model, the earliest epileptiform activity is restricted to neocortical and entorhinal networks. Hippocampal bursting only starts much later, and triggers a marked transition in neo‐/entorhinal cortical activity. Thereafter, the hippocampal activity acts as a pacemaker, entraining the other territories to their discharge pattern. This entrainment persists following transection of the major axonal pathways between hippocampus and cortex, indicating that it can be mediated through a non‐synaptic route. Neuronal discharges are associated with large rises in extracellular [K⁺], but we show that these are very localized, and therefore are not the means of entraining distant cortical areas. We conclude instead that the entrainment occurs through weak field effects distant from the pacemaker, but which are highly effective at recruiting other brain territories that are already hyperexcitable. The hippocampal epileptiform activity appears unusually susceptible to drugs that impact on K⁺ conductances. These findings demonstrate that the local circuitry gives rise to stereotypical epileptic activity patterns, but these are also influenced by both synaptic and non‐synaptic long‐range effects. Our results have important implications for our understanding of epileptic propagation and anti‐epileptic drug action.

Local neocortical and hippocampal territories show different and sterotypical patterns of acutely evolving, epileptiform activity.

Neocortical and entorhinal networks show tonic–clonic‐like events, but the main hippocampal territories do not, unless it is relayed from the other areas.

Transitions in the pattern of locally recorded epileptiform activity can be indicative of a shift in the source of pathological activity, and may spread through both synaptic and non‐synaptic means.

Hippocampal epileptiform activity is promoted by 4‐aminopyridine and inhibited by GABA_B receptor agonists, and appears far more sensitive to these drugs than neocortical activity.

These signature features of local epileptiform activity can provide useful insight into the primary source of ictal activity, aiding both experimental and clinical investigation.

How do we use in vitro models to understand epileptiform and ictal activity? A report of the TASK1-WG4 group of the ILAE/AES Joint Translational Task Force

In vitro brain tissue preparations allow the convenient and affordable study of brain networks and have allowed us to garner molecular, cellular, and electrophysiologic insights into brain function with a detail not achievable in vivo. Preparations from both rodent and human postsurgical tissue have been utilized to generate in vitro electrical activity similar to electrographic activity seen in patients with epilepsy. A great deal of knowledge about how brain networks generate various forms of epileptiform activity has been gained, but due to the multiple in vitro models and manipulations used, there is a need for a standardization across studies. Here, we describe epileptiform patterns generated using in vitro brain preparations, focusing on issues and best practices pertaining to recording, reporting, and interpretation of the electrophysiologic patterns observed. We also discuss criteria for defining in vitro seizure‐like patterns (i.e., ictal) and interictal discharges. Unifying terminologies and definitions are proposed. We suggest a set of best practices for reporting in vitro studies to favor both efficient across‐lab comparisons and translation to in vivo models and human studies.

In vitro and in vivo experimental models employed in the discovery and development of antiepileptic drugs for pharmacoresistant epilepsy

Epilepsy is one of the most common chronic, recurrent and progressive neurological diseases. In spite of the large number of antiepileptic drugs currently available for the suppression of seizures, about one-third of patients develop drug-resistant epilepsy, even when they are administered the most appropriate treatment available. Thus, nonclinical models can be valuable tools for the elucidation of the mechanisms underlying the development of pharmacoresistance and also for the development of new therapeutic agents that may be promising therapeutic approaches for this unmet medical need. Up today, several epilepsy and seizure models have been developed, exhibiting similar physiopathological features of human drug-resistant epilepsy; moreover, pharmacological response to antiepileptic drugs clinically available tends to be similar in animal models and humans. Therefore, they should be more intensively used in the preclinical discovery and development of new candidates to antiepileptic drugs. Although useful, in vitro models cannot completely replicate the complexity of a living being and their potential for a systematic use in antiepileptic drug screening is limited. The whole-animal models are the most commonly employed and they can be classified as per se drug-resistant due to an inherent poor drug response or be based on the selection of subgroups of epileptic animals that respond or not to a specific antiepileptic drug. Although more expensive and time-consuming, the latter are chronic models of epilepsy that better exhibit the disease-associated alterations found in human epilepsy. Several antiepileptic drugs in development or already marketed have been already tested and shown to be effective in these models of drug-resistant epilepsy, constituting a new hope for the treatment of drug-resistant epilepsy. This review will provide epilepsy researchers with detailed information on the in vitro and in vivo nonclinical models of interest in drug-resistant epilepsy, which may enable a refined selection of most relevant models for understanding the mechanisms of the disease and developing novel antiepileptic drugs.

Differential effects of sodium channel blockers on in vitro induced epileptiform activities

Antiepileptic drugs act on voltage gated sodium channels in many different ways: rufinamide is thought to influence the fast inactivation, so its anticonvulsant action could be similar to carbamazepine, whereas lacosamide enhances the slow inactivation; however some antidepressants were also described to act in the same way. Rufinamide, lacosamide, carbamazepine, fluoxetine and imipramine were tested using in vitro models of epileptiform activities. Extracellular local field potentials were recorded using hippocampal slices from immature rats and the pattern of epileptiform activities was analyzed. Seizure-like events (SLE), but not interictal bursts were sensitive to AEDs' action. Rufinamide increased interictal periods by prolonging preictal phase and reducing SLE duration, and was the only tested AED which reduced SLE frequency. Lacosamide's effect resembled that of fluoxetine in the low-Mg²⁺ model: both drugs reduced markedly the SLE duration, but increased their frequency. Imipramine and fluoxetine irreversibly suppressed SLE in all slices. Some proconvulsive type of action on SLEs such as increasing preictal neuronal activity by rufinamide and increasing SLE frequency by lacosamide, fluoxetine and carbamazepine, were also observed. Newer drugs were more efficient than carbamazepine, and the anticonvulsant action of antidepressants on in vitro epileptiform activities may seem somewhat surprising.

Experimental models of status epilepticus and neuronal injury for evaluation of therapeutic interventions

This article describes current experimental models of status epilepticus (SE) and neuronal injury for use in the screening of new therapeutic agents. Epilepsy is a common neurological disorder characterized by recurrent unprovoked seizures. SE is an emergency condition associated with continuous seizures lasting more than 30 min. It causes significant mortality and morbidity. SE can cause devastating damage to the brain leading to cognitive impairment and increased risk of epilepsy. Benzodiazepines are the first-line drugs for the treatment of SE, however, many people exhibit partial or complete resistance due to a breakdown of GABA inhibition. Therefore, new drugs with neuroprotective effects against the SE-induced neuronal injury and degeneration are desirable. Animal models are used to study the pathophysiology of SE and for the discovery of newer anticonvulsants. In SE paradigms, seizures are induced in rodents by chemical agents or by electrical stimulation of brain structures. Electrical stimulation includes perforant path and self-sustaining stimulation models. Pharmacological models include kainic acid, pilocarpine, flurothyl, organophosphates and other convulsants that induce SE in rodents. Neuronal injury occurs within the initial SE episode, and animals exhibit cognitive dysfunction and spontaneous seizures several weeks after this precipitating event. Current SE models have potential applications but have some limitations. In general, the experimental SE model should be analogous to the human seizure state and it should share very similar neuropathological mechanisms. The pilocarpine and diisopropylfluorophosphate models are associated with prolonged, diazepam-insensitive seizures and neurodegeneration and therefore represent paradigms of refractory SE. Novel mechanism-based or clinically relevant models are essential to identify new therapies for SE and neuroprotective interventions.

Possible alterations in GABAA receptor signaling that underlie benzodiazepine-resistant seizures

Benzodiazepines have been used for decades as first-line treatment for status epilepticus (SE). For reasons that are not fully understood, the efficacy of benzodiazepines decreases with increasing duration of seizure activity. This often forces clinicians to resort to more drastic second- and third-line treatments that are not always successful. The antiseizure properties of benzodiazepines are mediated by γ-aminobutyric acid type A (GABA(A) ) receptors. Decades of research have focused on the failure of GABAergic inhibition after seizure onset as the likely cause of the development benzodiazepine resistance during SE. However, the details of the deficits in GABA(A) signaling are still largely unknown. Therefore, it is necessary to improve our understanding of the mechanisms of benzodiazepine resistance so that more effective strategies can be formulated. In this review we discuss evidence supporting the role of altered GABA(A) receptor function as the major underlying cause of benzodiazepine-resistant SE in both humans and animal models. We specifically address the prevailing hypothesis, which is based on changes in the number and subtypes of GABA(A) receptors, as well as the potential influence of perturbed chloride homeostasis in the mature brain.

Age- and region-specific effects of anticonvulsants and bumetanide on 4-aminopyridine-induced seizure-like events in immature rat hippocampal-entorhinal cortex slices

PURPOSE

Seizure-like events (SLEs) induced by 4-aminopyridine in rat organotypic slices cultures, which are prepared early after birth, are resistant to standard antiepileptic drugs. In this study we tested the hypothesis that pharmacoresistance may be an intrinsic property of the immature brain.

METHODS

Frequently recurring SLEs presumably representing status epilepticus were induced by 4-aminopyridine in acute rat hippocampal-entorhinal cortex slices obtained from postnatal day 3-19 (P3-P19), and the effects of carbamazepine, phenytoin, valproic acid, and phenobarbital were examined. In addition, bumetanide was tested, which blocks the Na(+) -K(+) -2Cl(-) (NKCC1) cotransporter, and also acetazolamide, which blocks the carbonic anhydrase and thereby the accumulation of bicarbonate inside neurons.

RESULTS

The efficacy of all antiepileptic drugs in blocking SLEs was dependent on postnatal age, with low efficacy in P3-P5 slices. Antiepileptic drugs suppressed SLEs more readily in the medial entorhinal cortex (ECm) than in the CA3. In P3-P5 slices, valproic acid and phenobarbital increased both tonic and clonic seizure-like activities in the CA3, whereas phenytoin and carbamazepine blocked tonic-like but prolonged clonic-like activity. In P3-P5 slices, bumetanide often blocked SLEs in the CA3, but was not as effective in the ECm. Like with other antiepileptic drugs, the seizure-suppressing effects of acetazolamide increased with postnatal age.

CONCLUSION

We conclude that pharmacoresistance may be inherent to very immature tissue and suggest that expression of the NKCC1 cotransporter might contribute to pharmacoresistance.

Antiepileptic drugs abolish ictal but not interictal epileptiform discharges in vitro

PURPOSE

We established the effects of the antiepileptic drugs (AEDs) carbamazepine (CBZ), topiramate (TPM), and valproic acid (VPA) on the epileptiform activity induced by 4-aminopyridine (4AP) in the rat entorhinal cortex (EC) in an in vitro brain slice preparation.

METHODS

Brain slices were obtained from Sprague-Dawley rats (200-250 g). Field and intracellular recordings were made from the EC during bath application of 4AP (50 microm). AEDs, and in some experiments, picrotoxin were bath applied concomitantly.

RESULTS

Prolonged (>3 s), ictal-like epileptiform events were abolished by CBZ (50 microm), TPM (50 microm), and VPA (1 mm), whereas shorter (<3 s) interictal-like discharges continued to occur, even at concentrations up to 4-fold as high. gamma-Aminobutyric acid (GABA)(A)-receptor antagonism changed the 4AP-induced activity into recurrent interictal-like events that were not affected by CBZ or TPM, even at the highest concentrations. To establish whether these findings reflected the temporal features of the epileptiform discharges, we tested CBZ and TPM on 4AP-induced epileptiform activity driven by stimuli delivered at 100-, 10-, and 5-s intervals; these AEDs reduced ictal-like responses to stimuli at 100-s intervals at nearly therapeutic concentrations, but did not influence shorter interictal-like events elicited by stimuli delivered every 10 or 5 s.

CONCLUSIONS

We conclude that the AED ability to control epileptiform synchronization in vitro depends mainly on activity-dependent characteristics such as discharge duration. Our data are in keeping with clinical evidence indicating that interictal activity is unaffected by AED levels that are effective to stop seizures.

Standard antiepileptic drugs fail to block epileptiform activity in rat organotypic hippocampal slice cultures

BACKGROUND AND PURPOSE

Earlier studies had demonstrated that tonic-clonic seizure-like events (SLEs) resembling electrographic correlates of limbic seizures in animals and humans can be induced in organotypic hippocampal slice cultures (OHSCs). We have explored OHSCs for their suitability to serve as in vitro models of limbic seizures for studying seizure mechanisms and screening new antiepileptic compounds.

EXPERIMENTAL APPROACH

OHSCs were cultivated according to the interface method. Neuronal activity and extracellular potassium concentration were recorded under submerged conditions. SLEs were induced by lowering magnesium concentration or by applying the potassium channel blocker 4-aminopyridine. The effects of standard antiepileptic drugs (AEDs), carbamazepine, phenytoin, valproic acid, clonazepam, diazepam and phenobarbital sodium on SLEs were analysed.

KEY RESULTS

In more than 93% of OHSCs, AEDs did not prevent the induction of SLEs or stop ongoing seizure activity even when toxic concentrations were applied. This pharmacoresistance was independent of the method of seizure provocation, postnatal age at explantation (P2-P10) and cultivation time in vitro (2 months). SLEs were reversibly blocked by glutamate antagonists or the GABA(A)-agonist muscimol.

CONCLUSIONS AND IMPLICATIONS

We present a simple to establish in vitro model of tonic-clonic SLEs that is a priori pharmacoresistant and thus has an advantage over animal models of pharmacoresistant seizures in which responders and non-responders can be sorted out only after an experiment. OHSCs could be suitable for exploring mechanisms of pharmacoresistant seizures and be used for the identification of new anticonvulsive compounds eventually effective in drug refractory epilepsy.

Inhibition of the betaine-GABA transporter (mGAT2/BGT-1) modulates spontaneous electrographic bursting in the medial entorhinal cortex (mEC)

Disruptions in GABAergic neurotransmission have been implicated in numerous CNS disorders, including epilepsy and neuropathic pain. Selective inhibition of neuronal and glial GABA transporter subtypes may offer unique therapeutic options for regaining balance between inhibitory and excitatory systems. The ability of two GABA transport inhibitors to modulate inhibitory tone via inhibition of mGAT1 (tiagabine) or mGAT2/BGT-1 (N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-4-(methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), also known as EF1502) was evaluated using an in vitro model of spontaneous interictal-like bursting (SB). SBs were recorded extracellularly in combined mEC-HC horizontal brain slices (400 microm; 31+/-1 degrees C) obtained from KA-treated rats. Slice recordings demonstrated that EF1502 exhibited a concentration-dependent reduction in SB frequency. EF1502 significantly reduced SB rate to 32% of control at the 30 microM concentration, while reducing the area and duration of SB activity to 60% and 46% of control, respectively, at the 10 microM concentration. In contrast, the GAT1 selective inhibitor tiagabine (3, 10, and 30 microM) was unable to significantly reduce the frequency of SB activity in the mEC, despite significantly reducing both the duration (51% of control) and area (58% of control) of the SB at concentrations as low as 3 microM. The ability of EF1502, but not tiagabine, to inhibit SBs in the mEC suggests that this in vitro model of pharmacoresistant SB activity is useful to differentiate between novel anticonvulsants with similar mechanisms of action and suggests a therapeutic potential for non-GAT1 transport inhibitors.

The source of afterdischarge activity in neocortical tonic-clonic epilepsy

Tonic-clonic seizures represent a common pattern of epileptic discharges, yet the relationship between the various phases of the seizure remains obscure. Here we contrast propagation of the ictal wavefront with the propagation of individual discharges in the clonic phase of the event. In an in vitro model of tonic-clonic epilepsy, the afterdischarges (clonic phase) propagate with relative uniform speed and are independent of the speed of the ictal wavefront (tonic phase). For slowly propagating ictal wavefronts, the source of the afterdischarges, relative to a given recording electrode, switched as the wavefront passed by, indicating that afterdischarges are seeded from wavefront itself. In tissue that has experienced repeated ictal events, the wavefront generalizes rapidly, and the afterdischarges in this case show a different "flip-flop" pattern, with frequent switches in their direction of propagation. This same flip-flop pattern is also seen in subdural EEG recordings in patients suffering intractable focal seizures caused by cortical dysplasias. Thus, in both slowly and rapidly generalizing ictal events, there is not a single source of afterdischarge activity: rather, the source is continuously changing. Our data suggest a complex view of seizures in which the ictal event and its constituent discharges originate from distinct locations.

Phenytoin- and carbamazepine-resistant spontaneous bursting in rat entorhinal cortex is blocked by retigabine in vitro

Hyperexcitability in the medial entorhinal cortex-hippocampal (mEC-HC) circuit in the initial weeks after prolonged seizure activity may contribute to the epileptogenic process in animal models of temporal lobe epilepsy (TLE). The present study examined combined mEC-HC slices (400 microm) using field potential recordings 1-2 weeks following the multiple administration, low-dose kainic acid (KA) model of TLE [Hellier, J.L., Patrylo, P.R., Buckmaster, P.S., Dudek, F.E., 1998. Recurrent spontaneous motor seizures after repeated low-dose systemic treatment with kainate: assessment of a rat model of temporal lobe epilepsy. Epilepsy Res. 31, 73-84]. Field potential recordings in slices from KA-treated rats demonstrated hallmarks of hyperexcitability in the mEC and in the CA1 and CA3 cell body regions of the HC. Spontaneous burst (SB) activity was observed under baseline recording conditions in the mEC of several slices from KA-treated rats, but not in the slices from saline-treated control rats. Elevating ACSF [K(+)](o) (6mM) in the presence of picrotoxin (50 microM) increased SB rates in all slices tested. However, there was a significantly shorter latency to onset of bursting and prolonged evoked response durations in layer II of the mEC of slices from KA-treated rats versus those from controls. Neither carbamazepine (CBZ) nor phenytoin (PHT) abolished SB activity in slices from KA-treated rats; whereas, SB activity in slices from control rats was dose-dependently reduced at 100 microM CBZ. In contrast, the novel anticonvulsant retigabine (RGB) dramatically reduced SB frequency in both control and KA-treated groups. The hyperexcitability observed in combined mEC-HC brain slices from KA-treated rats suggests that the mEC, as well as the HC, may contribute to the epileptogenic process after KA-induced seizure activity. This model may provide an efficient, flexible in vitro paradigm for differentiating novel AEDs in a model of pharmacoresistant bursting.

Feedforward inhibition contributes to the control of epileptiform propagation speed

It is still poorly understood how epileptiform events can recruit cortical circuits. Moreover, the speed of propagation of epileptiform discharges in vivo and in vitro can vary over several orders of magnitude (0.1-100 mm/s), a range difficult to explain by a single mechanism. We previously showed how epileptiform spread in neocortical slices is opposed by a powerful feedforward inhibition ahead of the ictal wave. When this feedforward inhibition is intact, epileptiform spreads very slowly (approximately 100 microm/s). We now investigate whether changes in this inhibitory restraint can also explain much faster propagation velocities. We made use of a very characteristic pattern of evolution of ictal activity in the zero magnesium (0 Mg2+) model of epilepsy. With each successive ictal event, the number of preictal inhibitory barrages dropped, and in parallel with this change, the propagation velocity increased. There was a highly significant correlation (p < 0.001) between the two measures over a 1000-fold range of velocities, indicating that feedforward inhibition was the prime determinant of the speed of epileptiform propagation. We propose that the speed of propagation is set by the extent of the recruitment steps, which in turn is set by how successfully the feedforward inhibitory restraint contains the excitatory drive. Thus, a single mechanism could account for the wide range of propagation velocities of epileptiform events observed in vitro and in vivo.

Adenosine acting via A1 receptors, controls the transition to status epilepticus-like behaviour in an in vitro model of epilepsy

Adenosine has powerful inhibitory effects in the central nervous system. In this study, we aim to understand how adenosine controls the progression of seizure-like events (SLEs) in a seizure-prone region of the brain, the entorhinal cortex. We chose to use a low Mg(2+) model of epilepsy in an in vitro slice preparation where, in the entorhinal cortex, SLEs progress into a type of epileptiform activity called late recurrent discharges (LRDs) that bear resemblance to status epilepticus. Adenosine, acting via its A1 receptor, exerted powerful inhibitory effects to prevent the spontaneous progression to LRDs while the potent A1 receptor antagonist, DPCPX, accelerated the progression in a concentration dependent manner. The spontaneous progression from SLEs to LRDs was associated with a decline in total cellular ATP levels and studies with metabolic inhibitors indicated a key role for the production of endogenous adenosine from ATP. We therefore hypothesise that when ATP becomes rate limiting, extracellular adenosine levels fall, the normal inhibitory brake is removed and the progression from SLEs to LRDs or status epilepticus-like activity can ensue. Moreover, under these conditions, inhibition of the adenine nucleotide salvage pathways reversed the status epilepticus-like activity. Our findings suggest a powerful role for adenosine for the control of the progression to status epilepticus-like activity in an epilepsy model that is refractory to most anti-epileptic drugs. On this basis, manipulation of adenine nucleotide metabolism may represent a potential therapeutic approach for the treatment of status epilepticus.

Trace Elements and Electrolytes Homeostasis and Their Relation to Antioxidant Enzyme Activity in Brain Hyperexcitability of Epileptic Patients

Epileptogenesis is a big challenge. Various experimental and human studies suggested that the homeostasis of trace elements, electrolytes, membrane lipid peroxidation, and antioxidants is crucial for brain function, and they were directly or indirectly implicated as taking part in the pathophysiology of neuronal excitability, neuronal excitotoxicity, and seizure recurrence and its resistance to treatment with antiepileptic drugs (AEDs). In addition, AEDs can also alter the homeostasis of trace elements, electrolytes, and seriously increase membrane lipid peroxidation at the expense of protective antioxidants, leading to an increase in seizure recurrence and an idiosyncratic drug effect. Differential effects were detected among different AEDs treatments in which carbamazepine (CBZ) was found to be better anticonvulsant for the control of free radical related seizures and the level of trace elements were better regulated with CBZ than with valproate (VPA) and phenytoin (PHT) therapies. It is concluded that adequate trace elements and antioxidants supply is important for brain functions and prevention of neurological diseases and further elucidation of the pathological actions of such substances in the brain should result in new therapeutic approaches. Trace elements and antioxidant might have neuroprotective biological targeted benefits when used in epileptic patients.

Effects of antiepileptic drugs on refractory seizures in the intact immature corticohippocampal formation in vitro

PURPOSE

We developed a new in vitro preparation of immature rats, in which intact corticohippocampal formations (CHFs) depleted in magnesium ions become progressively epileptic. The better to characterize this model, we examined the effects of 14 antiepileptic drugs (AEDs) currently used in clinical practice.

METHODS

Recurrent ictal-like seizures (ILEs, four per hour) were generated in intact CHFs of P7-8 rats, and extracellular recordings were performed in the hippocampus and neocortex. AEDs were applied at clinically relevant concentrations (at least two), during 30 min after the third ILE. Their ability to prevent or to delay the next ILE was examined.

RESULTS

Valproic acid and benzodiazepines (clobazam and midazolam) but also phenobarbital and levetiracetam prevent the occurrence of seizures. In contrast, usual concentrations of carbamazepine (CBZ), phenytoin, vigabatrin, tiagabine, gabapentin, lamotrigine (LTG), topiramate, felbamate, and ethosuximide did not suppress ILEs. In addition, LTG and CBZ aggravate seizures in one third of the cases.

CONCLUSIONS

This intact in vitro preparation in immature animals appears to be quite resistant to most AEDs. Blockade of seizures was achieved with drugs acting mainly at the gamma-aminobutyric acid (GABA)A-receptor site but not with those that increase the amount of GABA. Drugs with a broad spectrum of activity are efficient but not those preferentially used in partial seizures or absences. We suggest that this preparation may correspond to a model of epilepsy with generalized convulsive seizures and could be helpful to develop new AEDs for refractory infantile epilepsies.

Cell death and metabolic activity during epileptiform discharges and status epilepticus in the hippocampus

Mechanisms of seizure-induced cell death were studied in organotypic hippocampal slice cultures. These develop after withdrawal of magnesium recurrent seizure-like events (SLE), which lead to intracellular and intramitochondrial calcium accumulation. The intramitochondrial Ca accumulation seems to be involved in causing increased production of NADH, measured as NAD(P)H autofluorescence. During SLEs, depolarization of mitochondria and increased production of free radicals is indicated by fluorescence measurements with appropriate dyes. During recurrent seizures, an increased failure to produce NADH is noted while at the same time free radical production seems to increase. This increase and the decline in NADH production could be involved in transition to late recurrent discharges, a phase in which status epilepticus becomes pharmacoresistant. It also coincides with increased cell death as determined with propidium iodide fluorescence. Interestingly, some of these changes can be prevented by application of alpha-tocopherol, a free radical scavenger, which also has neuroprotective effects under our experimental conditions. The results suggest that free radical-induced mitochondrial impairment is involved in seizure-induced cell death.