KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity

Recurrent seizures intensely activate GABA_A receptors (GABA_A-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 (E_Cl) 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 E_Cl and the efficacy of GABAergic inhibition during recurrent ILDs.