Ciliary Neurotrophic Factor (CNTF) Activation of Astrocytes Decreases Spreading Depolarization Susceptibility and Increases Potassium Clearance

Waves of spreading depolarization (SD) have been implicated in the progressive expansion of acute brain injuries. SD can persist over several days, coincident with the time course of astrocyte activation, but little is known about how astrocyte activation may influence SD susceptibility. We examined whether activation of astrocytes modified SD threshold in hippocampal slices. Injection of a lentiviral vector encoding Ciliary neurotrophic factor (CNTF) into the hippocampus in vivo, led to sustained astrocyte activation, verified by up-regulation of glial fibrillary acidic protein (GFAP) at the mRNA and protein levels, as compared to controls injected with vector encoding LacZ. In acute brain slices from LacZ controls, localized 1M KCl microinjections invariably generated SD in CA1 hippocampus, but SD was never induced with this stimulus in CNTF tissues. No significant change in intrinsic excitability was observed in CA1 neurons, but excitatory synaptic transmission was significantly reduced in CNTF samples. mRNA levels of the predominantly astrocytic Na+/K+-ATPase pump 2 subunit were higher in CNTF samples, and the kinetics of extracellular K+ transients during matched synaptic activation were consistent with increased K+ uptake in CNTF tissues. Supporting a role for the Na+/K+-ATPase pump in increased SD threshold, ouabain, an inhibitor of the pump, was able to generate SD in CNTF tissues. These data support the hypothesis that activated astrocytes can limit SD onset via increased K+ clearance and suggest that therapeutic strategies targeting these glial cells could improve the outcome following acute brain injuries associated with SD. GLIA 2015;63:91-103 Main Points We found that constitutive CNTF expression led to long-term activation of astrocytes which significantly elevated spreading depolarization (SD) threshold. This increase in SD threshold may be the result of changes in K+ homeostasis in CNTF tissues.