γ-Aminobutyric acid opens Cl-channels in cultured astrocytes

Cultured astrocytes from cerebral hemispheres of early postnatal rats responded to γ-aminobutyric acid (GABA) with membrane depolarization. This depolarization was affected by changes in extracellular [Cl^-] and depended on the membrane potential. The reversal potential of the GABA-induced depolarization was determined by double electrode voltage clamp or depolarization by elevated [K⁺]₀ and ranged between -38 and -53 mV. Cell input resistance decreased after addition of GABA with the same time course as the membrane depolarization. GABA responses were temperature dependent yielding a peak at about 14°C. At higher temperatures a decrease in the GABA-induced depolarization was seen indicating that the depolarization may not be mediated by an enzyme- coupled carrier system. Addition of ouabain at different temperatures did not change the size of the GABA depolarization. This excludes the possibility that an electrogenic component of the temperature-sensitive Na⁺, K⁺-ATPase activity causes the decrease in GABA-dependent depolarization at higher temperatures. Intracellular [Cl^- was measured with Cl^--sensitive microelectrodes and found to be higher than the value calculated for a free distribution according to the Nernst equation (-40 mV). Addition of furosemide did not alter the reversal potential, but reduced the size of the GABA-induced membrane depolarization. From these observations and previous experiments on the pharmacological properties of the membrane response¹⁸ we conclude that the ionic mechanism underlying the GABA-dependent membrane depolarization of astrocytes results from a transient increase in Cl^- -conductance similar to that of the neuronal GABA_A-receptor.