Glutamic acid is defined as a crystalline amino acid that is widely distributed in plant and animal proteins and that acts throughout the central nervous system especially in the form of a salt or ester as a neurotransmitter which excites postsynaptic neurons. An excitatory amino acid neurotransmitter that is an agonist at all subtypes of glutamate receptors (metabotropic, kainate, NMDA, and AMPA).