Amino acid neurotransmitter release (exocytosis) is dependent upon calcium Ca2+ and is a presynaptic response. There are inhibitory amino acids (IAA) or excitatory amino acids (EAA). Some EAA are L-Glutamate, L-Aspartate, L-Cysteine, and L-Homocysteine. These neurotransmitter systems will activate post-synaptic cells. Some IAA include GABA, Glycine, β-Alanine, and Taurine. The IAA depress the activity of post-synaptic cells.