A substance that is found mostly in the digestive tract, central nervous system (brain and spinal cord), and platelets. Serotonin acts both as a neurotransmitter (a substance that nerves use to send messages to one another) and a vasoconstrictor (a substance that causes blood vessels to narrow). A low level of serotonin in the brain is thought to be a cause of depression. Also called 5-hydroxytryptamine.