Rationale:
It is well established that brain injury induces cytotoxic edema as neurons swell due to the accumulation of water and sodium and potassium chloride salts. In vitro data indicate that the chloride accumulation is progressive in both developing and mature neurons and erodes inhibition by changing the driving force for transmembrane Cl- currents, leading to depolarizing GABA responses that foster seizures and anticonvulsant resistance via failure of GABAergic inhibition. Cation-chloride cotransporters (CCCs) facilitate neuronal Cl- accumulation after injury. Under these conditions, inhibition of the Cl--accumulating Na+-K+-2Cl- co-transporter (NKCC1) and/or activation of the Cl--extruding K+-Cl- co-transporter (KCC2) should improve neuronal chloride homeostasis, GABAergic inhibition, and control of seizures. We determined now which is the most effective: NKCC1 inhibition, KCC2 activation or mutual NKCC1 and KCC2 modulation via regulatory WNK-SPAK/OSR1 kinase activity, and whether the best approach may depend on the severity of seizures and the rate of neuronal chloride accumulation.
Methods:
We compared the net effects of the NKCC1 inhibitor bumetanide, the putative KCC2 activator CLP257 and the WNK-SPAK/OSR1 inhibitor WNK463 that reciprocally inhibits NKCC1 and activates KCC2 activity on [Cl-]i elevation and extrusion rates during spontaneous ictal-like discharges, and the correlation between ionic and electrographic effects. Organotypic hippocampal slice cultures from mice expressing the Cl- sensitive fluorescent protein Clomeleon or Super Clomeleon were studied as a model of acute traumatic brain injury and epileptogenesis in vitro.
Results:
After a 1-week latent period, slice cultures developed spontaneous ictal-like epileptiform discharges (ILDs). We found that: (i) inhibition of the NKCC1 by bumetanide reduced [Cl-]i in injured neurons with higher initial [Cl-]i and reduced the frequency and power of early post-traumatic ILDs; (ii) the putative KCC2 activator CLP257 reduced base-line [Cl-]i in all seizing neurons, and reduced the frequency and power of ILDs in a concentration-dependent manner; (iii) CCCs inhibitors did not prevent the effects of CLP257 on base-line [Cl-]i; (iv) WNK463 progressively reduced elevated [Cl-]i in injured neurons, enhanced Cl- extrusion rates during ILDs and abolished recurrent ILDs; (v) WNK463 had no effects in the presence of GABAA-receptor antagonist or CCCs inhibitors.
Conclusions:
In the context of seizure treatment, neither NKCC1 antagonists nor putative KCC2 activators are more “effective” than the other, and the best approach depends on the severity of seizures and the rate of chloride elevation. Our findings also demonstrate that mutual NKCC1 inhibition and KCC2 activation via the regulatory WNK-SPAK/OSR1 pathway exerts powerful anti-ictal effects by facilitating [Cl-]i extrusion and provide a coherent understanding of the role of WNK-SPAK/OSR1 kinase and cation-chloride co-transport activity in neuronal chloride homeostasis, GABA signaling and ictogenesis.
Funding:
This study was supported by the United States of America National Institutes of Health and National Institute of Neurological Disorders and Stroke grants R01NS120973 and R35NS116852