Dopamine is a member of the catecholamine family of neurotransmitters in the brain and is a precursor to epinephrine (adrenaline) and norepinephrine (noradrenaline). Dopamine is synthesized in the body (mainly by nervous tissue and adrenal glands) first by the hydration of the amino acid tyrosine to DOPA by tyrosine hydroxylase and then by the decarboxylation of DOPA by aromatic-L-amino-acid decarboxylase. Dopamine is a major transmitter in the extrapyramidal system of the brain, and important in regulating movement. A family of receptors (dopamine receptors) mediates its action, which plays a major role in reward-motivated behaviour. Dopamine has many other functions outside the brain. In blood vessels, dopamine inhibits norepinephrine release and acts as a vasodilator (at normal concentrations); in the kidneys, it increases sodium excretion and urine output; in the pancreas, it reduces insulin production; in the digestive system, it reduces gastrointestinal motility and protects intestinal mucosa; and in the immune system, it reduces the activity of lymphocytes. Parkinson's disease, a degenerative condition causing tremor and motor impairment, is caused by a loss of dopamine-secreting neurons in an area of the midbrain called the substantia nigra. There is evidence that schizophrenia involves altered levels of dopamine activity, and most antipsychotic drugs used to treat this are dopamine antagonists, which reduce dopamine activity. Attention deficit hyperactivity disorder, bipolar disorder, and addiction are also characterized by defects in dopamine production or metabolism. It has been suggested that animals derived their dopamine-synthesizing machinery from bacteria via horizontal gene transfer that may have occurred relatively late in evolutionary time. This is perhaps a result of the symbiotic incorporation of bacteria into eukaryotic cells that gave rise to mitochondria. Dopamine is elevated in the urine of people who consume bananas. When present in sufficiently high levels, dopamine can be a neurotoxin and a metabotoxin. A neurotoxin is a compound that disrupts or attacks neural tissue. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of dopamine are associated with neuroblastoma, Costello syndrome, leukemia, phaeochromocytoma, aromatic L-amino acid decarboxylase deficiency, and Menkes disease (MNK). High levels of dopamine can lead to hyperactivity, insomnia, agitation and anxiety, depression, delusions, excessive salivation, nausea, and digestive problems. A study has shown that urinary dopamine is produced by Bacillus and Serratia