Stimulation of ³⁶Cl⁻ permeation in the in → out direction across the deiters' neuron membrane by GABA on its cytoplasmic side: Effect of different ionic conditions

Abstract

The permeation of labelled Cr ions across single plasma membranes microdissected from rabbit Deiters' neuron was studied in a microchamber system. In particular, we studied ³⁶Cl⁻ in out permeation and its stimulation by 10⁻⁶ M GABA on the cytoplasmic side under different ionic conditions on both sides of the membrane. Three main results were found:

a) The GABA effect turns up at a cytoplasmic side [Cl⁻], 21 mM, in the range of the normal steady state intracellular Cl⁻ concentration. It disappears both at tracer level of Cl⁻ on the cytoplasmic side and when [Cl⁻] is high there (140mM).

b) The increase in ionic strength due to the equinormal substitution of monovalent anions (Cl⁻ and acetate) with the trivalent impermeant anion citrate on both sides of the membrane erases the GABA effect even if Cl⁻ is at the optimal cytoplasmic side concentration, 21 mM.

c) Citrate ions reduce to the limit of significance the GABA effect even when they are only on the membrane extracellular side and [Cl⁻] is at the optimal level on the other side.

These results confirm that GABA stimulated Cl⁻ permeation in the in out direction is via Cl⁻ channels exposing positive charges at their cytoplasmic mouth. In addition, they point out that such GABA activated channels are endowed with a partial electrical positivity at their extracellular exit.

Key Words:

• Deiters' neurone
• ¹⁶Cl⁻ permeation
• cytoplasmic side GABA_A receptors
• ionic conditions