Evidence for chloride ions as intracellular messenger substances in astrocytes

Cultured rat hippocampal astrocytes were used to investigate the mechanism underlying the suppression of Ba2+-sensitive K+ currents by GABAA receptor activation. Muscimol application had two effects on whole cell currents: opening of the well-known Cl- channel of the GABAA receptor and a secondary longer-lasting blockade of outward K+ currents displaying both peak and plateau phases. This blockade was independent of both Na+ (inside and outside) and ATP in the pipette. It also seemed to be independent of muscimol binding to the receptor because picrotoxin application showed no effect on the K+ conductance. The effect is blocked when anion efflux is prevented by replacing Cl- with gluconate (both inside and out) and is enhanced with more permeant anions such as Br- and I-. Moreover, the effect is reproduced in the absence of muscimol by promoting Cl- efflux via lowering of extracellular Cl- levels. These results, along with the requirement for Cl- efflux in muscimol experiments, show a strong dependency of the secondary blockade on Cl- efflux through the Cl- channel of the GABAA receptor. We therefore conclude that changes in the intracellular Cl- concentration alter the outward K+ conductances of astrocytes. Such a Cl--mediated modulation of an astrocytic K+ conductance will have important consequences for the progression of spreading depression through brain tissue and for astrocytic swelling in pathological situations.