Modulation by calcium of Deiters' neuron GABAA receptors in the rabbit

The function of classical GABAA receptors of the rabbit Deiters' neurons has been studied at the single membrane level by a biochemical micromethod involving the study of labelled chloride permeation. In particular, labelled chloride permeation across microdissected fresh single membranes was studied in a microchamber system. The stimulation of 36Cl- out-->in permeation by "extracellular" GABA was determined under different conditions in the respect of Ca++. When the conditions were such that "intracellular" Ca++ was 0.02 microM there appeared to be an optimal effect by GABA on chloride passage. Conditions presumably resulting in an increase of [Ca++]i beyond the level reported above led to a decreased GABA effect, especially at the highest GABA concentrations used (> or = 10(-4)M). However, complete removal of Ca++ by a high (12 mM) intracellular EGTA concentration erased completely the GABA effect. These results indicate that in these neurons an optimal GABAA receptor function requires [Ca++]i levels well below micromolar. The high [EGTA]i effect seems to imply that too low a [Ca++]i is also harmful to the proper function of these GABAA receptors. However, an alternative explanation is possible.