A number of amino acids have direct effects in the central nervous system and can be considered as possible treatments for mental disorders. Gamma-aminobutyric acid (GABA) is one such amino acid. However although GABA is an amino acid, it is not an alpha amino acid because it is not incorporated into proteins. It is also not essential as it can be synthesised from glutamic acid in the brain using the enzyme glutamate decarboxylase. In the central nervous system GABA acts as an inhibitory amino acid which inhibits neuronal activity. Here it is released from GABAergic neurones and binds to postsynaptic GABA receptors. Ligand receptor interactions leads to a closing of chloride channels in the postsynaptic neurone. This depolarises the postsynaptic cell membrane and this in turn reduces the likelihood that an action potential will fire on the postsynaptic neurone. In this way GABA has an inhibitory effect on the postsynaptic neurones and decreases neuronal excitation.
The ability of GABA to inhibit neuronal activity explains the relaxing properties of supplemental GABA. The plasma, brain and cerebrospinal fluid of depressed subjects are low in GABA which suggests that GABA may play a role in depression. In particular, GABA may be able to effectively treat depression that is related to anxiety, as the antidepressant and antianxiety effects of benzodiazepines are likely due to their effects on the GABA neuronal systems. Some evidence suggests that depression caused by alcohol or poor dietary habits may be triggered by reductions in the GABA concentrations of the brain. Supplemental GABA is also a useful sleep aid for those without depression because it inhibits neurotransmitter activity in the brain and reduces anxiety, which may increase sleepiness. Taking GABA supplements before bed is therefore the best strategy, as this will negate the drowsiness some experience from taking GABA. Both capsules and powders appear effective at increasing brain levels of GABA.