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_irchannels and then distributed among neighboring astrocytes via gap junctions. Previous reports examining the role of K_irchannels and gap junctions have shown both hyperexcitability and depression when each mechanism is blocked. Here, we tested the effect of blocking K_irchannels 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_irchannels 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_irchannels 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_irchannels 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_irand gap junctions, likely mediated by astrocytes, together play a critical role in maintaining neuronal excitability, particularly during sustained activity.