Modulation of GABAergic receptor binding by activation of calcium and calmodulin-dependent kinase II membrane phosphorylation

gamma-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Because of the important role that GABA plays in the CNS, alteration of GABA(A) receptor function would significantly affect neuronal excitability. Protein phosphorylation is a major mechanism for regulating receptor function in the brain and has been implicated in modulating GABA(A) receptor function. Therefore, this study was initiated to determine the role of calmodulin-dependent kinase II (CaM kinase II) membrane phosphorylation on GABA(A) receptor binding. Synaptosomal membrane fractions were tested for CaM kinase II activity towards endogenous substrates. In addition, muscimol binding was evaluated under equilibrium conditions in synaptosomal membrane fractions subjected to either basal (Mg2+ alone) or maximal GaM kinase II-dependent phosphorylation. Activation of endogenous GaM kinase II-dependent phosphorylation resulted in a significant enhancement of the apparent B-max for muscimol binding without significantly altering the apparent binding affinity. The enhanced muscimol binding could be increased further by the addition of exogenous CaM kinase II to synaptosomal membrane fractions. Co-incubation with inhibitors of kinase activity during the phosphorylation reactions blocked the CaM kinase II-dependent increase in muscimol binding. The data support the hypothesis that activation of GaM kinase II-dependent phosphorylation caused an increased GABA(A) receptor binding and may play an important role in modulating the function of this inhibitory receptor/chloride ion channel complex. (C) 1998 Elsevier Science B.V. All rights reserved.