Effects of Astrocyte Specific Swelling on Neuronal Excitability in Elevated Potassium
Neurotransmitter and ion influx into astrocytes generates osmotic gradients coupled to water movement into the cell, resulting in transient or prolonged fluctuations in cell volume. Increases in cell volume reduce the size of the extracellular space (ECS) and are associated with elevated brain tissue excitability. However, the precise mechanisms at play in coupling astrocyte volume changes to ion movements remain controversial, as does the effect of acute astrocyte volume fluctuations on neuronal function. Here we set out to determine the effects of raised extracellular potassium concentrations ( [K+]o) on volume responses of CA1 pyramidal neurons and stratum radiatum astrocytes in the hippocampus. First, we found that elevated [K+]o within a physiological range (6.5 and 10.5 mM from a baseline of 2.5 mM) and up to 26 mM produces dose-dependent increases in astrocyte volume, with no effect on neuronal volume. Astrocyte volume increases in elevated [K+]o were not dependent on AQP4, Kir4.1, the sodium-bicarbonate cotransporter NBCe1, or the electroneutral cotransporter NKCC1, but were significantly attenuated in 1 mM BaCl2 and by the Na+/K+ pump inhibitor ouabain, suggesting that astrocyte volume increases are due to K+ influx from nonspecific K+ channels and/or the Na+/K+ ATPase. High [K+]o-induced astrocyte swelling resulted in significant increases in neuronal excitability in the form of NMDA receptor-dependent slow inward currents (SICs) and mixed AMPA/NMDA mEPSCs. Direct depolarizing effects of high [K+]o on neuronal spiking were prevented by application of TTX, and the amount of depolarization was insufficient to activate voltage-gated Ca2+ channels, suggesting that changes in neuronal excitability were not due to elevated [K+]o-related increases in synaptic transmission. Finally, we show that astrocyte-specific swelling in elevated [K+]o and effects on neuronal excitability can be completely negated by addition of mannitol, which we found selectively shrinks astrocytes. Overall, our findings suggest that astrocyte-selective volume increases in elevated [K+]o conditions are due to activity of the Na+/K+ ATPase, which result in astrocyte-specific increases in neuronal excitability independent of direct depolarizing effects of high [K+]o on neurons.