glutamate

Glutamate is an amino acid responsible for the fifth primary taste grouping, savory. (the other four are bitter, salty, sour and sweet). The savory taste, also known as umami* (Japanese for delicious or savory), is the taste associated with things like tomatoes, seaweed, and beef.

Glutamate, or glutamic acid, was first identified in 1908 by Kikaune Ikeda. Glutamates are tremendous taste enhancers, hence the general (but certainly not universal) human predisposition to enjoy things like meat. Glutamate is also responsible for the tremendous power of MSG, which, of course, combines a glutamate with sodium to pack a double-whammy of savory and salt.

According to the glutamic acid writeup, it has a similar molecular structure to aspartic acid, one of the components of aspartame, or NutraSweet.

Sources:
http://chemistry.about.com/science/chemistry/library/weekly/aa062600a.htm
http://ificinfo.health.org/review/ir-msg.htm

* cool wu by sensei

Glutamate is also a neurotransmitter with an excitatory (rather than inhibitory) effect on the Central Nervous System. This, rather than the placebo effect as some claim, is probably what accounts for Chinese Restaurant Syndrome after consuming high levels of Monosodium Glutamate. (Incidentally, high levels of the neurotransmitter dopamine are known to cause psychosis and symptoms of schizophrenia).

Glutamate is the neurotransmitter released by the hair cells of the inner ear responsible for detection of sound. Caffeine acts as a stimulant by suppressing levels of adenosine in the brain, which in turn increases the levels of glutamate.