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γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays a role in regulating neuronal excitability throughout the nervous system. In humans, it is also directly responsible for the regulation of muscle tone. In insect species it acts only on excitatory nerve receptors. It is an amino acid, but it is rarely referred to as such in the scientific or medical communities, because the term "amino acid," used without a qualifier, refers to the alpha amino acids, which γ-Aminobutyric acid is not, nor is it incorporated into proteins.