Glutamic acid can be synthesized from several different amino acids, including glutamine. That it can be reconverted back into glutamine when combined with ammonia can create confusion over which amino acid does what. Glutamic acid (sometimes called glutamate) is a major excitatory neurotransmitter in the brain and spinal cord, and is the precursor to glutathione and Gamma-Aminobutyric Acid (GABA), which is a calming agent. Glutamic acid is also a component of folic acid. Glutamic acid does not pass the blood-brain barrier, although it might infiltrate the brain fluids in small amounts. On the other hand, glutamine has no problem passing the barrier, where it is readily converted into glutamic acid in the brain.
After glutamic acid is formed in the brain from glutamine, it then has two key roles. The body uses glutamic acid to fuel the brain and to inhibit neural excitation in the central nervous system. Besides glucose, it is the only compound used for fuel by the brain. Because the brain stores small quantities of glucose, it is dependent upon glutamic acid. The second function is detoxifying ammonia in the brain and removing it. It then reconverts to its original form of glutamine.
This amino acid has been used to treat personality disorders. As a major component of nutritional therapy, it has been successfully used in treating children with behavioral problems and autism. A derivative of glutamic acid (or glutamate), monosodium glutamate contributes to the headache associated with "Chinese Restaurant Syndrome," the name given to the allergic reaction to MSG.