Limited contribution of astroglial gap junction coupling to buffering of extracellular K+ in CA1 stratum radiatum

Astrocytes form large networks, in which individual cells are connected via gap junctions. It is thought that this astroglial gap junction coupling contributes to the buffering of extracellular K+ increases. However, it is largely unknown how the control of extracellular K+ by astroglial gap junction coupling depends on the underlying activity patterns and on the magnitude of extracellular K+ increases. We explored this dependency in acute hippocampal slices (CA1, stratum radiatum) by direct K+-sensitive microelectrode recordings and acute pharmacological inhibition of gap junctions. K+ transients evoked by synaptic and axonal activity were largely unaffected by acute astroglial uncoupling in slices obtained from young and adult rats. Iontophoretic K+-application enabled us to generate K+ gradients with defined spatial properties and magnitude. By varying the K+-iontophoresis position and protocol, we found that acute pharmacological uncoupling increases the amplitude of K+ transients once their initial amplitude exceeded similar to 10 mM. Our experiments demonstrate that the contribution of gap junction coupling to buffering of extracellular K+ gradients is limited to large and localized K+ increases.