γ-Aminobutyric acid (/ˈɡaemə ˈɡæmə əˈmiːnoʊbjuːˈtɪrɨk/ ; ˈaesɨd ˈæSɨD /or/) gaba ˈɡaebə ˈɡæbə is the chief inhibitory neurotransmitter in the mammalian. Central nervous system it plays the principal role in reducing neuronal excitability throughout. the nervous systemabaergic neurons which produce GABA. Exogenous GABA does not penetrate the blood–brain barrier; it is synthesized in the brain. It is synthesized from glutamate using the enzyme L-glutamic acid decarboxylase (GAD) and pyridoxal phosphate (which is the active form of vitamin B6) as a cofactor.