GABA (gamma-aminobutyric acid) helps to modulate the action of dopamine, norepinephrine, and serotonin. Unlike the other neurotransmitters, it occurs almost exclusively in the CNS and is present in a large percentage of neurons of the brain. Several studies have suggested that some individuals with depression have reduced production of GABA, and that it may play a role in the patient's experience of depression and other mental disorders. GABA is considered a primary neurotransmitter because its effects are very specific.