REDUCTION OF LEVEL OF L‐GLUTAMIC ACID DECARBOXYLASE BY γ‐AMINOBUTYRIC ACID IN MOUSE BRAIN1

Abstract— The effects of accumulated endogenous GABA on the activity of L‐glutamic acid decarboxylase (GAD) were studied in mouse brain. When the content of GABA in the brain was increased after administration in vivo of aminooxyacetic acid (AOAA), there was a reduction of GAD activity which could not be reversed by the addition of pyridoxal‐5′‐phosphate (PLP). Since inhibition of GAD activity by AOAA could be readily reversed by PLP, the reduction of GAD activity measured in the presence of added PLP indicated a decrease in the level of GAD apoenzyme. Similarly, increase of GABA content by hydrazine was also accompanied by a reduction in the level of GAD. Thiosemicarbazide and hydroxylamine did not affect the content of GABA appreciably, and in both cases levels of GAD remained unchanged when measured in the presence of added PLP. The correlation of the reduction in the levels of GAD with the increases in content of GABA suggests that GABA may regulate its own synthesizing enzyme by feedback repression.