Satellite oligodendrocytes (s-OLs) are closely apposed to the soma of neocortical layer 5 pyramidal neurons but their properties and functional roles remain unresolved. Here we show that s-OLs form compact myelin and action potentials of the host neuron evoke precisely timed Ba 2+-sensitive K + inward rectifying (Kir) currents in the s-OL. Unexpectedly, the glial K + inward current does not require oligodendrocytic Kir4.1. Action potential-evoked Kir currents are in part mediated by gap–junction coupling with neighbouring OLs and astrocytes that form a syncytium around the pyramidal cell body. Computational modelling predicts that glial Kir constrains the perisomatic [K +] o increase most importantly during high-frequency action potentials. Consistent with these predictions neurons with s-OLs showed a reduced probability for action potential burst firing during [K +] o elevations. These data suggest that s-OLs are integrated into a glial syncytium for the millisecond rapid K + uptake limiting activity-dependent [K +] o increase in the perisomatic neuron domain.
Satellite oligodendrocytes (s-OLs) are characterised by their close proximity to neocortical pyramidal cells. Here, the authors find that s-OLs myelinate axons and activity of host neurons evokes inward K + currents in s-OLs which may work to modulate action potential burst firing by buffering extracellular K + levels.