GABA (Gamma amino butyric acid) is your major inhibitory/calming neurotransmitter in the central nervous system. It is often considered to act as "mother nature's Xanax." Indeed, it is throught the GABA system that most sedatives, prescription sleep aids, and tranquilizers work. It helps the neurons recover after firing and thereby reduces anxiety, worry, and fretfulness. In addition, GABA regulates norepinephrine, adrenaline, dopamine, and serotonin, it is a significant mood modulator.
When neurotransmitter testing reveals unusually high levels of GABA, it often reflects your body's attempt to compensate for or balance out abnormally elevated excitatory neurotransmitter activity (stress for example). The extra GABA may "calm" things down but can lead to unwanted effects such as sluggishness, sleepiness, and brain fog.
Too little GABA, on the other hand, can be associated with high anxiety, impulsivity, inability to handle stress, restlessness, and irritability.