Nerve Terminal GABA_A Receptors Activate Ca²⁺/Calmodulin-dependent Signaling to Inhibit Voltage-gated Ca²⁺ Influx and Glutamate Release^*

Abstract

γ-Aminobutyric acid type A (GABA_A) receptors, a family of Cl^--permeable ion channels, mediate fast synaptic inhibition as postsynaptically enriched receptors for γ-aminobutyric acid at GABAergic synapses. Here we describe an alternative type of inhibition mediated by GABA_A receptors present on neocortical glutamatergic nerve terminals and examine the underlying signaling mechanism(s). By monitoring the activity of the presynaptic CaM kinase II/synapsin I signaling pathway in isolated nerve terminals, we demonstrate that GABA_A receptor activation correlated with an increase in basal intraterminal [Ca²⁺]_i. Interestingly, this activation of GABA_A receptors resulted in a reduction of subsequent depolarization-evoked Ca²⁺ influx, which thereby led to an inhibition of glutamate release. To investigate how the observed GABA_A receptor-mediated modulation operates, we determined the sensitivity of this process to the Na-K-2Cl cotransporter 1 antagonist bumetanide, as well as substitution of Ca²⁺ with Ba²⁺, or Ca²⁺/calmodulin inhibition by W7. All of these treatments abolished the modulation by GABA_A receptors. Application of selective antagonists of voltage-gated Ca²⁺ channels (VGCCs) revealed that the GABA_A receptor-mediated modulation of glutamate release required the specific activity of L- and R-type VGCCs. Crucially, the inhibition of release by these receptors was abolished in terminals isolated from R-type VGCC knock-out mice. Together, our results indicate that a functional coupling between nerve terminal GABA_A receptors and L- or R-type VGCCs is mediated by Ca²⁺/calmodulin-dependent signaling. This mechanism provides a GABA-mediated control of glutamatergic synaptic activity by a direct inhibition of glutamate release.