No abstract

Changes in chloride (Cl Ϫ) homeostasis may be involved in the generation of some epileptic activities. In this study, we asked whether Cl Ϫ homeostasis, and thus GABAergic signaling, is altered in tissue from patients with mesial temporal lobe epilepsy associated with hippocampal sclerosis. Slices prepared from this human tissue generated a spontaneous interictal-like activity that was initiated in the subiculum. Records from a minority of subicular pyramidal cells revealed depolarizing GABA A receptor-mediated postsynaptic events, indicating a perturbed Cl Ϫ homeostasis. We assessed possible contributions of changes in expression of the potassium-chloride cotransporter KCC2. Double in situ hybridization showed that mRNA for KCC2 was absent from ϳ30% of CaMKII␣ (calcium/calmodulindependent protein kinase II␣)-positive subicular pyramidal cells. Combining intracellular recordings with biocytin-filled electrodes and KCC2 immunochemistry, we observed that all cells that were hyperpolarized during interictal events were immunopositive for KCC2, whereas the majority of depolarized cells were immunonegative. Bumetanide, at doses that selectively block the chloride-importing potassium-sodium-chloride cotransporter NKCC1, produced a hyperpolarizing shift in GABA A reversal potentials and suppressed interictal activity. Changes in Cl Ϫ transporter expression thus contribute to human epileptiform activity, and molecules acting on these transporters may be useful antiepileptic drugs.

Major advances in the identification of genes implicated in idiopathic epilepsy have been made. Generalized epilepsy with febrile seizures plus (GEFS+), benign familial neonatal convulsions and nocturnal frontal lobe epilepsy, three autosomal dominant idiopathic epilepsies, result from mutations affecting voltage-gated sodium and potassium channels, and nicotinic acetylcholine receptors, respectively. Disruption of GABAergic neurotransmission mediated by gamma-aminobutyric acid (GABA) has been implicated in epilepsy for many decades. We now report a K289M mutation in the GABA(A) receptor gamma2-subunit gene (GABRG2) that segregates in a family with a phenotype closely related to GEFS+ (ref. 8), an autosomal dominant disorder associating febrile seizures and generalized epilepsy previously linked to mutations in sodium channel genes. The K289M mutation affects a highly conserved residue located in the extracellular loop between transmembrane segments M2 and M3. Analysis of the mutated and wild-type alleles in Xenopus laevis oocytes confirmed the predicted effect of the mutation, a decrease in the amplitude of GABA-activated currents. We thus provide the first genetic evidence that a GABA(A) receptor is directly involved in human idiopathic epilepsy.

Mechanisms involved in the transition to an epileptic seizure remain unclear. We studied this question in tissue slices from human subjects with mesial temporal lobe epilepsies. Ictal-like discharges were induced in the subiculum by increasing excitability together with an alkalinization or low Mg 2+ . During the transition, distinct pre-ictal discharges emerged concurrently with interictal events. Intracranial recordings from the mesial temporal cortex of epileptic subjects revealed similar discharges before seizures were restricted to seizure onset sites. In vitro, pre-ictal events spread faster, have a larger amplitude and a distinct initiation site than interictal discharges. They depend on glutamatergic mechanisms and are preceded by pyramidal cell firing, while interneuron firing precedes interictal events which depend on both glutamatergic and depolarizing GABAergic transmission. Once established, recurrence of these pre-ictal discharges triggers seizures. Thus the subiculum supports seizure generation and the transition to seizure involves a novel, emergent glutamatergic population activity.3

Diffuse low-grade gliomas are highly epileptogenic brain tumours. We aimed to explore the natural course of epileptic seizures, their predictors and the prognostic significance of their occurrence in adult patients harbouring a diffuse low-grade glioma. An observational retrospective multicentre study examined 1509 patients with diffuse low-grade gliomas to identify mutual interactions between tumour characteristics, tumour course and epileptic seizures. At diagnosis, 89.9% of patients had epileptic seizures. Male gender (P = 0.003) and tumour location within functional areas (P = 0.001) were independent predictors of a history of epileptic seizures at diagnosis. Tumour volume, growth velocity, cortical location, histopathological subtype or molecular markers did not significantly affect epileptic seizure occurrence probability. Prolonged history of epileptic seizures (P < 0.001), insular location (P = 0.003) and tumour location close to functional areas (P = 0.038) were independent predictors of uncontrolled epileptic seizures at diagnosis. Occurrence of epileptic seizures (P < 0.001), parietal (P = 0.029) and insular (P = 0.002) locations were independent predictors of uncontrolled epileptic seizures after oncological treatment. Patient age (P < 0.001), subtotal (P = 0.007) and total (P < 0.001) resections were independent predictors of total epileptic seizure control after oncological treatment. History of epileptic seizures at diagnosis and total surgical resection were independently associated with increased malignant progression-free (P < 0.001 and P < 0.001) and overall (P < 0.001 and P = 0.016) survivals. Epileptic seizures are independently associated with diffuse low-grade glioma prognosis. Patients diagnosed with epileptic seizures and those with complete and early surgical resections have better oncological outcomes. Early and maximal surgical resection is thus required for diffuse low-grade gliomas, both for oncological and epileptological purposes.

Rationale-Diffuse brain gliomas induce seizures in a majority of patients. As in most epileptic disorders, excitatory glutamatergic mechanisms are involved in the generation of epileptic activities in the neocortex surrounding gliomas. However, chloride homeostasis is known to be perturbed in glial tumor cells. Thus the contribution of GABAergic mechanisms which depend on intracellular chloride and which are defective or pro-epileptic in other structural epilepsies merits closer study.Objective-We studied in neocortical slices from the peritumoral security margin resected around human brain gliomas, the occurrence, networks, cells and signaling basis of epileptic activities.Results-Postoperative glioma tissue from 69% of patients spontaneously generated interictallike discharges. These events were synchronized, with a high frequency oscillation signature, in superficial layers of neocortex around glioma areas with tumor infiltration. Interictal-like events Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts depended on both glutamatergic transmission and on depolarizing GABAergic signaling. About 65% of pyramidal cells were depolarized by GABA released by interneurons. This effect was related to perturbations in Chloride homeostasis, due to changes in expression of chloride cotransporters: KCC2 was reduced and expression of NKCC1 increased. Ictal-like activities were initiated by convulsant stimuli exclusively in these epileptogenic areas.Conclusions-Epileptic activities are sustained by excitatory effects of GABA in the peritumoral human neocortex, as in temporal lobe epilepsies. Glutamate and GABA signaling are involved in oncogenesis and chloride homeostasis is perturbed. These same factors, induce an imbalance between synaptic excitatory and inhibition underly epileptic discharges in tumor patients.

Hippocampal population discharges such as sharp waves, epileptiform firing, and GDPs recur at long and variable intervals. The mechanisms for their precise timing are not well understood. Here, we show that population bursts in the disinhibited CA3 region are initiated at a threshold level of population firing after recovery from a previous event. Each population discharge follows an active buildup period when synaptic traffic and cell firing increase to threshold levels. Single-cell firing can advance burst onset by increasing population firing to suprathreshold values. Population synchrony is suppressed when threshold frequencies cannot be reached due to reduced cellular excitability or synaptic efficacy. Reducing synaptic strength reveals partially synchronous population bursts that are curtailed by GABA(B)-mediated conductances. Excitatory glutamatergic transmission and delayed GABA(B)-mediated signals have opposing feedback effects on CA3 cell firing and so determine threshold behavior for population synchrony.

Epilepsy often develops in patients with glioma, and the two conditions share common pathogenic mechanisms. Altered expression of glutamate transporters, including the cystine-glutamate transporter (xCT) system, increases concentrations of extracellular glutamate, which contribute to epileptic discharge, tumour proliferation and peripheral excitotoxicity. Furthermore, mutation of the isocitrate dehydrogenase 1 gene in low-grade gliomas causes production of D-2-hydroxyglutarate, a steric analogue of glutamate. Dysregulation of intracellular chloride promotes glioma cell mitosis and migration, and γ-aminobutyric acid (GABA) signalling suppresses proliferation. In neurons, however, chloride accumulation leads to aberrant depolarization on GABA receptor activation, thereby promoting epileptic activity. The molecular target of rapamycin (mTOR) pathway and epigenetic abnormalities are also involved in the development of tumours and seizures. Antitumour therapy can contribute to seizure control, and antiepileptic drugs might have beneficial effects on tumours. Symptomatic treatment with antiepileptic drugs carries risks of adverse effects and drug interactions. In this Review, we discuss the potential for single therapeutic agents, such as the xCT blocker sulfasalazine, the chloride regulator bumetanide, and the histone deacetylase inhibitor valproic acid, to manage both gliomas and associated epilepsy. We also provide guidance on the evidence-based use of antiepileptic drugs in brain tumours. The development of solo therapies to treat both aspects of gliomas promises to yield more-effective treatment with fewer risks of toxicity and drug interactions.