Serotonin or 5-hydroxytryptamine (5-HT) is a molecule that belongs to the class of compounds known as indoleamines. An indoleamine consists of an indole ring that bears an amino group or an alkyl amino group attached to the indole ring. Serotonin has an aminoethyl at position 2 and a hydroxyl group at position 5 of the indole ring. Serotonin exists in all living organisms, ranging from bacteria to plants to humans. In mammals, serotonin functions as a monoamine neurotransmitter, a biochemical messenger and regulator. It is synthesized from the essential amino acid L-Tryptophan. Approximately 90% of the human body's total serotonin is located in the enterochromaffin cells in the GI tract, where it regulates intestinal movements. About 8% is found in platelets and 1‚Äì2% in the CNS. Serotonin in the nervous system acts as a local transmitter at synapses, and as a paracrine or hormonal modulator of circuits upon diffusion, allowing a wide variety of "state-dependent" behavioral responses to different stimuli. Serotonin is widely distributed in the nervous system of vertebrates and invertebrates and some of its behavioral effects have been preserved along evolution. Such is the case of aggressive behavior and rhythmic motor patterns, including those responsible for feeding.  In vertebrates, which display a wider and much more sophisticated behavioral repertoire, serotonin also modulates sleep, the arousal state, sexual behavior, and others. Deficiencies of the serotonergic system causes disorders such as depression, obsessive-compulsive disorder, phobias, posttraumatic stress disorder, epilepsy, and generalized anxiety disorder. Serotonin has three different modes of action in the nervous system: as transmitter, acting locally at synaptic boutons; upon diffusion at a distance from its release sites, producing paracrine (also called volume) effects, and by circulating in the blood stream, producing hormonal effects. The three modes can affect a single neuronal circuit. (PMID: 16047543). Serotonin is also a microbial metabolite that can be found in the feces and urine of mammals. Urinary serotonin is produced by Candida, Streptococcus, Escherichia, and Enterococcus (PMID: 24621061). In plants, serotonin was first found and reported in a legume called Mucuna pruriens. The greatest concentration of serotonin in plants has been found in walnuts and hickory. In pineapples, banana, kiwi fruit, plums and tomatoes the concentration of serotonin is around 3 to 30 mg/kg.