Aquaporin-4 Regulates the Velocity and Frequency of Cortical Spreading Depression in Mice

The astrocyte water channel aquaporin-4 (AQP4) regulates extracellular space (ECS) K+ concentration ([K+](e)) and volume dynamics following neuronal activation. Here, we investigated how AQP4-mediated changes in [K+](e) and ECS volume affect the velocity, frequency, and amplitude of cortical spreading depression (CSD) depolarizations produced by surface KCl application in wild-type (AQP4(+/+)) and AQP4-deficient (AQP4(-/-)) mice. In contrast to initial expectations, both the velocity and the frequency of CSD were significantly reduced in AQP4(-/-) mice when compared with AQP4(+/+) mice, by 22% and 32%, respectively. Measurement of [K+](e) with K+-selective microelectrodes demonstrated an increase to approximate to 35mM during spreading depolarizations in both AQP4(+/+) and AQP4(-/-) mice, but the rates of [K+](e) increase (3.5 vs. 1.5mM/s) and reuptake (t(1/2) 33 vs. 61s) were significantly reduced in AQP4(-/-) mice. ECS volume fraction measured by tetramethylammonium iontophoresis was greatly reduced during depolarizations from 0.18 to 0.053 in AQP4(+/+) mice, and 0.23 to 0.063 in AQP4(-/-) mice. Analysis of the experimental data using a mathematical model of CSD propagation suggested that the reduced velocity of CSD depolarizations in AQP4(-/-) mice was primarily a consequence of the slowed increase in [K+](e) during neuronal depolarization. These results demonstrate that AQP4 effects on [K+](e) and ECS volume dynamics accelerate CSD propagation.