Regulation of γ‐Aminobutyric Acid Synthesis in the Brain

Abstract

Abstract: γ‐Aminobutyric acid (GABA) is synthesized in brain in at least two compartments, commonly called the transmitter and metabolic compartments, and because reglatory processes must serve the physiologic function of each compartment, the regulation of GABA synthesis presents a complex problem. Brain contains at least two molecular forms of glutamate decarboxylase (GAD), the principal synthetic enzyme for GABA. Two forms, termed GAD 65 and GAD 67, are the products of two genes and differ in sequence, molecular weight, interaction with the cofactor, pyridoxal 5′‐phosphate (pyridoxal‐P), and level of expression among brain regions. GAD 65 appears to be localized in nerve terminals to a greater degree than GAD 67, which appears to be more uniformly distributed throughout the cell. The interaction of GAD with pyridoxal‐P is a major factor in the short‐term regulation of GAD activity. At least 50% of GAD is present in brain as apoenzyme (GAD without bound cofactor; apoGAD), which serves as a reservoir of inactive GAD that can be drawn on when additional GABA synthesis is needed. A substantial majority of apoGAD in brain is accounted for by GAD 65, but GAD 67 also contributes to the pool of apoGAD. The apparent localization of GAD 65 in nerve terminals and the large reserve of apo‐GAD 65 suggest that GAD 65 is specialized to respond to short‐term changes in demand for transmitter GABA. The levels of apoGAD and the holoenzyme of GAD (holoGAD) are controlled by a cycle of reactions that is regulated by physiologically relevant concentrations of ATP and other polyanions and by inorganic phosphate, and it appears possible that GAD activity is linked to neuronal activity through energy metabolism. GAD is not saturated by glutamate in synaptosomes or cortical slices, but there is no evidence that GABA synthesis in vivo is regulated physiologically by the availability of glutamate. GABA competitively inhibits GAD and converts holo‐ to apoGAD, but it is not clear if intracellular GABA levels are high enough to regulate GAD. There is no evidence of short‐term regulation by second messengers. The syntheses of GAD 65 and GAD 67 proteins are regulated separately. GAD 67 regulation is complex; it not only is present as apoGAD 67, but the expression of GAD 67 protein is regulated by two mechanisms: (a) by control of mRNA levels and (b) at the level of translation or protein stability. The latter mechanism appears to be mediated by intracellular GABA levels.