An inhibitory neurotransmitter in the central nervous system (principally the brain) that is capable of increasing the permeability of postsynaptic membranes to chloride ions (see inhibitory postsynaptic potential). GABA is synthesized by decarboxylation of the amino acid glutamate. There are three classes of GABA receptors: GABA_A and GABA_C receptors are ionotropic receptors whereas GABA_B receptors are metabotropic receptors. In crustaceans and annelids, GABA is an inhibitory transmitter at synapses influencing muscle activity. Benzodiazepine drugs, such as valium, exert their anti-anxiety effect by allosterically binding to GABA_A receptors and enhancing the action of GABA.