Gamma‐aminobutyric acidergic transmission underlies interictal epileptogenicity in pediatric focal cortical dysplasia

Abstract

Objective: Dysregulation of γ-aminobutyric acidergic (GABAergic) transmission has been reported in lesional acquired epilepsies (gliomas, hippocampal sclerosis). We investigated its involvement in a developmental disorder, human focal cortical dysplasia (FCD), focusing on chloride regulation driving GABAergic signals. Methods: In vitro recordings of 47 human cortical acute slices from 11 pediatric patients who received operations for FCD were performed on multielectrode arrays. GABAergic receptors and chloride regulators were pharmacologically modulated. Immunostaining for chloride cotransporter KCC2 and interneurons were performed on recorded slices to correlate electrophysiology and expression patterns. Results: FCD slices retain intrinsic epileptogenicity. Thirty-six of 47 slices displayed spontaneous interictal discharges, along with a pattern specific to the histological subtypes. Ictal discharges were induced in proepileptic conditions in 6 of 8 slices in the areas generating spontaneous interictal discharges, with a transition to seizure involving the emergence of preictal discharges. Interictal discharges were sustained by GABAergic signaling, as a GABA A receptor blocker stopped them in 2 of 3 slices. Blockade of NKCC1 Cl − cotransporters further controlled interictal discharges in 9 of 12 cases, revealing a Cl − dysregulation affecting actions of GABA. Immunohistochemistry highlighted decreased expression and changes in KCC2 subcellular localization and a decrease in the number of GAD67-positive interneurons in regions generating interictal discharges. Interpretation: Altered chloride cotransporter expression and changes in interneuron density in FCD may lead to paradoxical depolarization of pyramidal cells. Spontaneous interictal discharges are consequently mediated by GABAergic signals, and targeting chloride regulation in neurons may be considered for the development of new antiepileptic drugs. ANN NEUROL 2019;85:204-217 M alformations of cortical development are an emblematic etiology of drug-resistant lesional epilepsy in children. Since the pioneer description of focal cortical dysplasia (FCD) by Taylor et al, 1 the classification has evolved into 3 main types of FCD according to the pattern of dyslamination, the presence of abnormal cells (cytomegalic neurons and balloon cells), and the association with another brain lesion. 2,3 This histological classification is crucial to categorize the various features of FCDs, but it does not describe the mechanisms of the View this article online at wileyonlinelibrary.com.

Paroxysmal Discharges in Tissue Slices From Pediatric Epilepsy Surgery Patients: Critical Role of GABAB Receptors in the Generation of Ictal Activity

“…Further, while ictal activity can be generated in areas with histopathologic CD type IIa, areas with CD type IIb do not show seizure activity, suggesting a possible protective role of balloon cells (Boonyapisit et al, 2003). While multitude studies have emphasized that GABA interneuron synchrony jump starts focal seizures (Avoli et al, 1999(Avoli et al, , 2003D'Antuono et al, 2004;Shiri et al, 2015;de Curtis and Avoli, 2016;Blauwblomme et al, 2018;Elahian et al, 2018), the respective role of GABA A and GABA B receptors has not been elucidated. Based on our observations, we can propose that relaxation of GABA Inter (n = 5) 0 2 1 2 inhibition mediated by GABA B receptors is responsible for the transition to ictal activity.…”

Role of NKCC1 and KCC2 in Epilepsy: From Expression to Function

“…In a recent study on pediatric FCD patients, interictal discharges sustained by GABAergic signaling were suppressed by blocking NKCC1. Meanwhile, perturbed Cl − homeostasis related to decreased expression and changes in KCC2 have been observed in the same patients (7). Sharing similar pathological characteristics with FCD, dysplastic neurons in tuberous sclerosis complex (TSC) exhibit depolarizing GABAa Rs mediated spontaneous post-synaptic currents (PSCs), which are controlled by bumetanide (11).…”

“…Although the exact mechanism of epileptogenesis is not yet fully understood, epilepsy is widely believed to be associated with an excitatory-inhibitory imbalance (4,5). There is increasing evidence in recent literature to suggest that epileptogenesis results primarily from a deficit of GABA inhibition (6,7).…”

“…Seventy to eighty percent of neocortical neurons are excitatory glutamatergic pyramidal neurons while the remaining are mostly GABAergic inhibitory interneurons with diverse characteristics (8,9). Based on experimental studies involving seizure models and epilepsy patients, we know that the down-regulation of GABA receptor subunits, loss of GABAergic interneurons, and decreased frequency and amplitude of spontaneous synaptic GABA currents from pyramidal neurons contribute to decreased GABAergic inhibition (7,(10)(11)(12). In neurons, NKCC1 and KCC2 serve as two main regulators of GABA receptors' function by maintaining [Cl − ] i homeostasis (13).…”

GABAA Receptor-Mediated Epileptogenicity in Focal Cortical Dysplasia (FCD) Depends on Age at Epilepsy Onset

“…Immature neuronal networks in FCD lead to altered GABA A receptor function thereby causing abnormal synaptic transmission (Cherubini et al, 1991;Cepeda et al, 2005). Brain specimens obtained from patients with FCD type I and II retain their epileptogenicity, which is dependent on enhanced spontaneous interictal discharges mediated by GABA A receptors (Blauwblomme et al, 2019). To our knowledge, so far, no study has focussed on age at epilepsy onset and GABA A receptormediated epileptogenicity in patients with FCD.…”