Neuro-biogenic amines are produced from essential aromatic amino acids and secreted from pre-synaptic neurons into the synapse between nerve cells to stimulate receptors on postsynaptic neurons. While circulating levels of neurobiogenic amines and metabolites may have a variety of sources, urinary levels primarily reflect activity of the peripheral and enteric nervous systems. Up to 20 percent of urinary neurotransmitters are estimated to originate in the central nervous system.