Serotonin: A hormone in the pineal gland, blood platelets, the digestive tract, and the brain. Serotonin acts both as a chemical messenger that transmits nerve signals between nerve cells and that cause blood vessels to narrow.
- "Serotonin definition - Depression Symptoms, Causes, and Treatments Including Clinical and Manic Depression on MedicineNet.com." Web. 10 Dec. 2009. <http://www.medterms.com/script/main/art.asp?articlekey=5468>.