Independent mechanisms of potassium clearance by astrocytes in gliotic tissue

The "glial impairment hypothesis" states that astrocytes which change from normal into the reactive type lose their ability to clear extracellular K+, which in turn leads to hyperexcitability in the gliotic tissue. As this hy pothesis was never proven or disproven, the question of glial efficiency in K+ clearance in gliotic tissue is still controversial, mainly due to the l ack of direct measurements of the intracellular Kf concentration of reactiv e astrocytes. In order to investigate K+ accumulation by glial cells of gli otic tissue, we used hippocampal slices. Adult rats, previously treated wit h kainic acid, exhibited loss of neurons and gliosis in the CA1 layer of th e hippocampus within 3 days, After this time period, double-barrelled micro electrodes were used to inject Lucifer yellow into cells of the stratum rad iatum of the CA1 subfield in 400-mu m-thick hippocampal slices. These cells had electrophysiological and morphological characteristics of astrocytes. Most injected cells (70%) were dye-coupled to other cells and were glial fi brillary acidic protein (GFAP)-positive (80%). We found, however, that GFAP -positive cells were dye-coupled not only to each other, but also to GFAP-n egative cells, In another set of experiments, we investigated the glial mem brane potential during reduction of the extracellular Cl(-)concentration an d the use of the Cl- channel blocker 4,4'-diisothiocyanostilbene-2,2' disul phonic acid (DIDS). The results suggest that reactive astrocytes have a sig nificant resting Cl- conductance. K+-selective microelectrodes were used to analyze the intracellular glial Kf concentration. When the extracellular K + concentration was increased from 3.5 mM to 10 mM, the intracellular K+ co ncentration increased by 23 mM, Experiments in which different ion transpor t systems were blocked with ouabain and DIDS suggest that this increase is dependent on two mechanisms, which can substitute each other: the Na+, K+ - ATPase and passive K+ and anion fluxes. Inhibition of either of the two mec hanisms did not block the K+ uptake. If, however, the Na+, K+-ATPase and Cl - channels were inhibited at the same time, the net accumulation of K+ was blocked, It appears, therefore, that astrocytes in the gliotic stratum radi atum of the hippocampal slice have the capacity to limit increases in extra cellular K+ that are produced by hyperactive surviving hippocampal neurons by passive mechanisms and hence independently of blood and oxygen supply. ( C) 1999 Wiley-Liss,Inc.