Extracellular potassium concentration,[K+](0), is a major determinant of neuronal excitability. In the healthy brain, [K+](0) levels are tightly controlled. During seizures, [K+](0) increases up to 15 mM and is thought to cause seizures due to its depolarizing effect. Although astrocytes have been suggested to play a key role in the redistribution (or spatial buffering) of excess K+ through Connexin-43 (Cx43)-based Gap Junctions (GJs), the relation between this dynamic regulatory process and seizure generation remains unknown. Here we contrasted the role of astrocytic GIs and hemichannels by studying the effect of GJ and hemichannel blockers on [K+](0), regulation in vivo. [K+](0) was measured by K+-sensitive microelectrodes. Neuronal excitability was estimated by local field potential (LFP) responses to forepaw stimulation and changes in the power of resting state activity. Starting at the baseline [K+](0) level of 1.61 +/- 0.3 mM, cortical microinjection of CBX, a broad spectrum connexin channel blocker, increased [K+](0). to 11 +/- 3 mM, Cx43 GJ/hemichannel blocker Gap27 increased it from 1.9 +/- 0.7 to 9 1 mM. At these [K+](0) levels, no seizures were observed. Cx43 hemichannel blockade with TAT-Gap19 increased [K+](0) by only-1 mM. Microinjection of 4-aminopyridine, a known convulsant, increased [K+]. to-10 mM and induced spontaneously recurring seizures, whereas direct application of K+ did not trigger seizure activity. These findings are the first in vivo demonstration that astrocytic GJs are major determinants for the spatial buffering of [K+](0)-10 and that an increase in [K+](0) alone does not trigger seizures in the neocortex. (C) 2016 Elsevier Inc. All rights reserved.