gamma-Aminobutyric acid-13C4 is the 13C-labeled gamma-Aminobutyric acid. gamma-Aminobutyric acid (4-Aminobutyric acid) is a major inhibitory neurotransmitter in the adult mammalian brain[1][2], binding to the ionotropic GABA receptors (GABAA receptors) and metabotropic receptors (GABAB receptors)[2].