The Level of GAD67 Protein Is Highly Sensitive to Small Increases in Intraneuronal γ‐Aminobutyric Acid Levels

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Abstract
Increases (> 2.5-fold) in GABA levels in rat brain lead to a large decrease in the level of the 67-kDa form of glutamate decarboxylase (GAD67) through a mechanism involving either a change in GAD67 protein stability or a change in GAD67 mRNA translation. In the present study, brain levels of GABA were manipulated by treating rats with various doses of gamma-vinyl-gamma-aminobutyric acid (GVG), and the dependence of total GAD activity and levels of GAD67 and GAD65 protein on the levels of GABA was analyzed. Initial studies showed that both GABA and GAD67 protein levels reached new steady-state levels after two to four daily injections; GABA increased 1.5- (30 mg of GVG/kg) and fourfold (150 mg of GVG/kg), and GAD67 protein content decreased by 30 and 70%. To assess the sensitivity of GAD67 to GABA, rats were injected with eight different doses of GVG (15-150 mg/kg) for 5 days. With increasing doses of GVG, we observed a gradual increase in both whole-tissue and synaptosomal GABA levels and a gradual decrease in GAD67 protein and GAD activity. The levels of GAD65 remained constant at all GVG doses. GAD67 was remarkably sensitive to GABA. The synaptosomal GAD67 level decreased approximately 12% and the whole-neuron GAD67 level decreased approximately 3% for each 1% increase in nerve terminal GABA content when it was close to its physiological level. Our results clearly demonstrate that GAD67 is tightly controlled by intraneuronal GABA, and we suggest that this regulatory mechanism has important implications for the physiological regulation of GABAergic function in the mammalian brain.