serotonin
Meaning of serotonin: The brain and gut supplies are separate, because the molecule cannot cross from blood into brain tissue, and each pool works on its own.
Definition of serotonin
Serotonin, also written 5-HT, is a signaling chemical built from the amino acid tryptophan. Most of it is made by specialized cells in the lining of the gut, where it regulates movement and secretion. A separate and far smaller supply is made by neurons in the brainstem and acts on mood, sleep and appetite. Platelets carry serotonin but do not manufacture it.
Why are the brain and gut supplies separate?
Serotonin does not pass the blood brain barrier, so the large amount circulating in the body cannot reach the brain and the brain's own supply cannot leak out. The brain therefore manufactures its own from tryptophan arriving in the blood. This is the single most misunderstood point about the molecule: statements about gut serotonin affecting mood directly do not follow, because the two pools never mix. Platelets are a third compartment again. They take serotonin up from plasma, store it, and release it where a vessel is injured, where it narrows the vessel and helps other platelets aggregate.
What does it do?
| Where | Effect |
|---|---|
| Brainstem neurons | Contributes to mood, sleep and waking, appetite and the modulation of pain |
| Gut lining | Drives the wave of muscle contraction that moves contents along, and triggers nausea through nerve signals to the brainstem |
| Platelets and vessels | Narrows blood vessels at a site of injury |
Because it reaches so many tissues, a drug that alters serotonin signaling rarely affects only one of them, and effects on digestion are among the most frequent.
How do serotonin drugs work?
The largest group blocks reuptake, leaving more of the transmitter available where nerve cells meet, and these are widely used in depression and anxiety disorders. Others act on one receptor subtype: migraine drugs constrict specific vessels through it, and several anti-nausea drugs block a different subtype in the gut and brainstem. It is worth being precise about what this establishes. The drugs help many people, but the popular account of depression as a simple serotonin deficiency has not been borne out, and the mechanism by which these medicines work is not fully settled. Combining several serotonin-raising drugs can produce serotonin syndrome, a state of agitation, tremor, muscle rigidity, fever and rapid heartbeat.
Also known as
Used in a sentence
Laboratory testing measured a serotonin breakdown product in the urine as part of the evaluation for a suspected carcinoid tumor.
Good to know
Most of the body's serotonin is in the digestive tract rather than the brain, and because the molecule does not cross the blood brain barrier, the two pools act independently of one another.
Word origin
Where “serotonin” comes from
Coined from sero-, referring to blood serum, and tonin, from tone, reflecting its early identification as a substance affecting blood vessel tone.