(5-hy-DROK-see-TRIP-tuh-meen) A hormone found in the brain, platelets, digestive tract, and pineal gland. It 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 lack of 5-hydroxytryptamine in the brain is thought to be a cause of depression. Also called serotonin.
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