Aquaporin-4 (AQP4) is the main water channel in the brain and primarily localized to astrocytes where the channels are thought to contribute to water and K+ homeostasis. The close apposition of AQP4 and inward rectifier K+ channels (Kir4.1) led to the hypothesis of direct functional interactions between both channels. We investigated the impact of AQP4 on stimulus-induced alterations of the extracellular K+ concentration ([K+]o) in murine hippocampal slices. Recordings with K+-selective microelectrodes combined with field potential analyses were compared in wild type (wt) and AQP4 knockout (AQP4−/−) mice. Astrocyte gap junction coupling was assessed with tracer filling during patch clamp recording. Antidromic fiber stimulation in the alveus evoked smaller increases and slower recovery of [K+]o in the stratum pyramidale of AQP4−/− mice indicating reduced glial swelling and a larger extracellular space when compared with control tissue. Moreover, the data hintat an impairment of the glial Na+/K+ ATPase in AQP4-deficient astrocytes. In a next step, we investigated the laminar profile of [K+]o by moving the recording electrode from the stratum pyramidale toward the hippocampal fissure. At distances beyond 300 μm from the pyramidal layer, the stimulation-induced, normalized increases of [K+]o in AQP4−/− mice exceeded the corresponding values of wt mice, indicating facilitated spatial buffering. Astrocytes in AQP4−/− mice also displayed enhanced tracer coupling, which might underlie the improved spatial re- distribution of [K+]o in the hippocampus. These findings highlight the role of AQP4 channels in the regulation of K+ homeostasis. © 2011 Wiley-Liss, Inc.