serotonin (sĕr´ətō´nĬn), organic compound that was first recognized as a powerful vasoconstrictor occurring in blood serum. It was partially purified, crystallized, and named in 1948, and its structure was deduced a year later. Independent work indicated that serotonin was widely distributed in nature and occurred in tissues other than blood. It has been shown to be in many representatives of the animal kingdom, in wasp stings and scorpion venom, in various fruits, such as pineapples, bananas, and plums, and in various nuts. It has been estimated that an adult human contains about 5 to 10 mg of serotonin, 90% of which is in the intestine and the rest in blood platelets and the brain. One role of the compound is as a neurotransmitter whose participation is being sought in diverse functions including learning, sleep, and control of mood. The structural similarity of serotonin to several drugs known to cause mental aberrations, such as LSD, has prompted much speculation as to the role of serotonin in naturally occurring mental disorders such as schizophrenia or depression. Serotonin also acts as a hormone. Serotonin produced by the intestines inhibits the production of the bone tissue by osteoblasts, while in the brain it acts indirectly to promote the formation of bone. The placenta has been discovered to produce serotonin that stimulates the development of neural connections in the forebrain of the embryo. The function of serotonin in blood platelets is not entirely clear, but by acting as a vasoconstrictor and possibly in other ways it helps to regulate the clotting process. Its function in stings and venoms might be that of an irritant, since intravenous injections of serotonin in humans produce pain at the site of injection, gasping, coughing, a general tingling and prickling sensation, nausea, cramps, and other unpleasant symptoms.