gamma-Aminobutyric acid-induced ion movements in the guinea pig hippocampal slice.

gamma-Aminobutyric acid (GABA)-induced regional changes of extracellular Cl, K and Na concentration ([Cl]o, [K]o, [Na]o), as well as of the extracellular space were measured with ion-sensitive microelectrodes in guinea pig hippocampal slices. Microdrop application of GABA to the pyramidal cell layer of CA3 or CA1 induced a decrease of [Cl]o, while application to the dendritic layer of CA3 or CA1 induced an increase of [Cl]o in addition. All changes of [Cl]o persisted in the presence of TTX and were blocked by bath-applied bicuculline. The GABA-induced decrease of [Cl]o was reduced by bicuculline application to the pyramidal cell layer. The increase of [Cl]o was blocked by bicuculline application to the dendritic layer. Additionally, GABA induced an increase of [K]o and decreases/increases of [Na]o. Changes of [Cl]o, [K]o and [Na]o together were approximately electroneutral. [Cl]o increases were exaggerated and [Cl]o decreases partly masked by shrinkage of the extracellular space after GABA application. Changing [K] in the superfusate transiently changed GABA-induced [Cl]o movements in a way predicted from a change in driving force due to the effect of [K] on membrane potential. Then a partial recovery followed towards the original [Cl]o change. We conclude that inward and outward Cl transports maintain [Cl]i below equilibrium in CA3 and CA1 pyramidal somata and above equilibrium in CA3 and CA1 dendrites. The significance of this Cl-distribution for hippocampal inhibition is discussed.