GABA (gamma aminobutyric acid) is a non-essential amino acid found mainly in the human brain and eyes. It is considered an inhibitory neurotransmitter, which means it regulates brain and nerve cell activity by inhibiting the number of neurons firing in the brain. GABA is referred to as the "brain's natural calming agent". By inhibiting over-stimulation of the brain, GABA may help promote relaxation and ease nervous tension.

Articles on GABA:

http://en.wikipedia.org/wiki/GABA