Spatial Buffering during Slow and Paroxysmal Sleep Oscillations in Cortical Networks of Glial CellsIn Vivo

Abstract

The ability of neuroglia to buffer local increases of extracellular K⁺ has been known from in vitrostudies. This property may confer on these cells an active role in the modulation and spreading of cortical oscillatory activities. We addressed the question of the spatial buffering in vivoby performing single and double intraglial recordings, together with measures of the extracellular K⁺ and Ca²⁺ concentrations ([K⁺]_out and [Ca²⁺]_out) in the cerebral cortex of cats under ketamine and xylazine anesthesia during patterns of slow sleep oscillations and spike-wave seizures. In addition, we estimated the fluctuations of intraglial K⁺concentrations ([K⁺]_in). Measurements obtained during the slow oscillation indicated that glial cells phasically take up part of the extracellular K⁺ extruded by neurons during the depolarizing phase of the slow oscillation. During this condition, the redistribution of K⁺ appeared to be local. Large steady increases of [K⁺]_out and phasic potassium accumulations were measured during spike-wave seizures. In this condition, [K⁺]_in rose before [K⁺]_out if the glial cells were located at some distance from the epileptic focus, suggesting faster K⁺ diffusion through the interglial syncytium. The simultaneously recorded [Ca²⁺]_outdropped steadily during the seizures to levels incompatible with efficient synaptic transmission, but also displayed periodic oscillations, in phase with the intraseizure spike-wave complexes. In view of this fact, and considering the capability of K⁺ to modulate neuronal excitability both at the presynaptic and postsynaptic levels, we suggest that the K⁺ long-range spatial buffering operated by glia is a parallel synchronizing and/or spreading mechanism during paroxysmal oscillations.