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

Astrocytes are connected in a functional syncytium via gap junctions, which contribute to the 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. Here, we tested the effect of blocking gap junctions and K _ir channels, both independently and simultaneously, on field excitability of cortical slices in response to a stimulation train. Independently blocking either gap junctions or K _ir channels increased the amplitude of the first fEPSC (field excitatory post-synaptic current) response, followed by suppression of both fiber volley (pre-synaptic action potentials) and fEPSCs during sustained stimulation. Surprisingly, simultaneous block of both gap junctions and K _ir channels enhanced the suppression of neuronal activity, resulting in a ∼75% decrease in fiber volley amplitude in the first response, followed by a fast and strong suppression of fEPSCs during sustained stimulation. Genetic depletion of astrocyte gap junctions showed a reduction but not complete loss of Cx43, indicating partial syncytial decoupling, and, accordingly, had a weaker but similar effect on neuronal excitability as blocking gap junctions. Pharmacological K _ir block in mice with reduced gap junction coupling suppressed sustained firing of the fiber volley but not fEPSCs. That this effect was milder than K _ir block alone suggests that adaptive mechanisms may be recruited upon genetically induced astrocyte decoupling. We conclude that K⁺ buffering via K _ir and gap junctions in astrocytes together play a critical role in maintaining neuronal excitability, particularly during sustained activity, but that other mechanisms can be recruited to perform this function in their absence.