Potassium-chloride cotransporter 2 activity dampens induced ictal-like activity in neocortical slices containing the seizure propagation zone of temporal lobe epilepsy patients.

Abstract

Objective: The K⁺/Cl^- cotransporter (KCC2), which acts as the main Cl^- extruder in the adult brain, coregulates the driving force and therewith indirectly the amount and polarity of γ-aminobutyric acidergic (GABAergic) currents. Whether the net effect of active KCC2 is inhibitory via such Cl^- extrusion or excitatory due to the concomitant increase of K⁺ in the extracellular space is context-dependent and difficult to predict. Consecutively, in rodent models, antiseizure- as well as seizure-facilitating effects of KCC2 block have been reported. Here, we attempted to gain more insight into KCC2's role in the seizure propagation zone in human temporal neocortex.

Methods: We induced network activity in postoperative acute neocortical brain slices from humans with temporal lobe epilepsy under low Mg²⁺ conditions, with and without elevated K⁺, and recorded it using microelectrode arrays. We analyzed ictal-like events and interictal-like discharges with a developed source-separating approach. Finally, we complemented these network-related studies by patch-clamp recordings of individual pyramidal neurons under regular ionic conditions to assess the inherent functionality of KCC2 and alternative transmembrane Cl^- routes.

Results: Modulation of KCC2 activity altered the induced network activity; KCC2 block reversibly led to substantially increased activity, preferentially in and propagating through supragranular layers. Correspondingly, enhancing KCC2 activity reduced network activity there. Almost all individual supragranular pyramidal neurons tested had functional KCC2, that is, certain Cl^- extrusion capacity that was limited when loaded with higher Cl^- and presented variable predominantly positive values of GABA_A receptor driving force. In addition, we found tonic inhibition that increases after prolonged KCC2 block and may either contribute to Cl^- load or support Cl^- extrusion in supragranular pyramidal neurons, depending on their intracellular Cl^- concentration.

Significance: Our data show that KCC2 mitigates ictal-like activity in the seizure propagation zone of human neocortex, thereby further promoting KCC2 as a therapeutic target.