Demonstration of functional coupling between γ-aminobutyric acid (GABA) synthesis and vesicular GABA transport into synaptic vesicles

L-Glutamic acid decarboxylase (GAD) exists as both membrane-associated and soluble forms in the mammalian brain. Here, we propose that there is a functional and structural coupling between the synthesis of gamma-aminobutyric acid (GABA) by membrane-associated GAD and its packaging into synaptic vesicles (SVs) by vesicular GABA transporter (VGAT): This notion is supported by the following observations. First, newly synthesized [H-3]GABA from [H-3]L-glutamate by membrane-associated GAD is taken up preferentially over preexisting GABA by using immunoaffinity-purified GABAergic SVs. Second, the activity of SV-associated GAD and VGAT seems to be coupled because inhibition of GAD also decreases VGAT activity. Third, VGAT and SV-associated Ca2+/ calmodulin-dependent kinase II have been found to form a protein complex with GAD. A model is also proposed to link the neuronal stimulation to enhanced synthesis and packaging of GABA into SVs.