Definition of "GABA"

[Valium] was followed by many imitators, all sharing the same pharmacological mechanism, which is to enhance the actions of the key inhibitory brain chemical messenger GABA.

2001, Leslie Iversen, Drugs: A Very Short Introduction, Oxford, page 58

Its action as an inhibitor also distinguishes GABA from the “big three” amines—dopamine, norepinephrine, and serotonin—which generally stimulate brain circuits.

2003, Samuel H. Barondes, chapter 4, in Better than Prozac, Oxford University Press, page 54