Astrocyte gap junctions and Kir channels contribute to K⁺ buffering and regulate neuronal excitability

Astrocytes are connected in a functional syncytium via gap junctions, which is thought to contribute to maintenance of extracellular K⁺ homeostasis. The prevailing hypothesis is that K⁺ released during neuronal firing is taken up by astrocytes via K_ir channels and then distributed among neighboring astrocytes via gap junctions. Previous reports examining the role of K_ir channels and gap junctions have shown both hyperexcitability and depression when each mechanism is blocked. Here, we tested the effect of blocking K_ir channels and gap junctions, both independently and simultaneously, on field activity of cortical slices in response to a 3 s, 20 Hz stimulation train. Independently blocking either K_ir channels or gap junctions increased the amplitude of the first fEPSC (field excitatory post-synaptic current) in response to a stimulation train, followed by suppression of fEPSCs during sustained stimulation. Surprisingly, blocking both gap junctions and K_ir channels enhanced the suppression of neuronal activity, resulting in a ~75% decrease in fiber volley (pre-synaptic action potentials) amplitude in the first response, followed by a fast and strong suppression of sustained fEPSCs. Our results demonstrate that blocking K_ir channels and gap junctions can increase the excitability of neurons when firing is sparse, but suppression results when the firing frequency is increased to cortical physiological ranges. This suggest that K⁺ buffering via K_ir and gap junctions, likely mediated by astrocytes, together play a critical role in maintaining neuronal excitability, particularly during sustained activity.