Serotonin

Serotonin is a neurotransmitter. Its chemical name is 5-Hydroxytryptamine or 5-HT. It is derived from tryptophan. Serotonin is found in all vertebrates, mainly in the gastrointestinal tract, blood platelets and central nervous system. Its name is derived from its effect on blood pressure: serotonin is a part of the "serum" which regulates the "tonus" of blood vessels.
Serotonin does several jobs within the human body. Unlike dopamine, serotonin causes happiness, while dopamine causes pleasure. It is believed to regulate mood, intestinal activity and appetite, memory, and sleep. Many antidepressant medications are thought to work by increasing the levels of serotonin in the body. Some non-medical treatments for depression have also been shown to raise serotonin levels.
Serotonin is also found in insect venom, fungi and plants. Its presence in the seeds of many fruits helps speed those seeds through the digestive tract of animals that consume them. In insect venom, it causes pain, and sometimes death, through its effect on smooth muscle contraction.
Serotonin is involved in social rank. A lobster injected with serotonin behaves like an alpha male.