Glossary of Medical Terms

serotonin

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 serotonin in the brain is thought to be a cause of depression. Also called 5-hydroxytryptamine.

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