Abstract
Recurrent seizures intensely activate GABAA receptors (GABAA-Rs), which induces transient neuronal chloride ([Cl–]i) elevations and depolarizing GABA responses that contribute to the failure of inhibition that engenders further seizures and anticonvulsant resistance. The K+-Cl– cotransporter KCC2 is responsible for Cl– extrusion and restoration of [Cl–]i equilibrium (ECl) after synaptic activity, but at the cost of increased extracellular potassium which may retard K+-Cl– extrusion, depolarize neurons, and potentiate seizures. Thus, KCC2 may either diminish or facilitate seizure activity, and both proconvulsant and anticonvulsant effects of KCC2 inhibition have been reported. It is now necessary to identify the loci of these divergent responses by assaying both the electrographic effects and the ionic effects of KCC2 manipulation. We therefore determined the net effects of KCC2 transport activity on cytoplasmic chloride elevation and Cl– extrusion rates during spontaneous recurrent ictal-like epileptiform discharges (ILDs) in organotypic hippocampal slices in vitro, as well as the correlation between ionic and electrographic effects. We found that the KCC2 antagonist VU0463271 reduced Cl– extrusion rates, increased ictal [Cl–]i elevation, increased ILD duration, and induced status epilepticus (SE). In contrast, the putative KCC2 upregulator CLP257 improved chloride homeostasis and reduced the duration and frequency of ILDs in a concentration-dependent manner. Our results demonstrate that measuring both the ionic and electrographic effects of KCC2 transport clarify the impact of KCC2 modulation in specific models of epileptiform activity. Anticonvulsant effects predominate when KCC2-mediated chloride transport rather than potassium buffering is the rate-limiting step in restoring ECl and the efficacy of GABAergic inhibition during recurrent ILDs.
Significance Statement In an in vitro preparation that generates spontaneous ictal-like epileptiform discharges (ILDs), we measured the effects of acute KCC2 modulation on both neuronal chloride and ILD activity. We demonstrate that inhibiting KCC2 enhances ictal elevations in [Cl–]i, reduces extrusion and prolongs ILDs. Enhancing KCC2 activity reduces ictal [Cl–]i elevations and ILD duration. These findings regarding the role of KCC2 on baseline chloride, chloride elevations during ILD activity, Cl– extrusion rates and ILD activity resolve conflicting reports in the literature, provide a coherent understanding of the role KCC2 activity in chloride homeostasis during ILDs, and support the feasibility of developing KCC2 modulators into sorely-needed anticonvulsant medications.
Footnotes
The authors declare no competing conflict of interests.
This study was supported by the United States of America National Institutes of Health and National Institute of Neurological Disorders and Stroke grant R01 NS040109.
This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.
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