Most of your serotonin works in your gut, not your brain
Serotonin is famous as a brain chemical, yet about 90 percent of the body's supply is made in the digestive tract, where it keeps the intestines moving. Only 1 to 2 percent sits in the central nervous system. And the popular idea that low serotonin causes depression lacks scientific support.
Chemically, serotonin is 5-hydroxytryptamine, an indoleamine built from the amino acid tryptophan in two steps: a hydroxyl group is added to the ring, then a carboxyl group is removed. Specialised enterochromaffin cells lining the gut produce the lion's share. Smaller amounts come from the raphe nuclei of the brainstem, Merkel cells in skin, cells in the lungs and taste receptor cells on the tongue. Blood platelets soak it up and hold around 8 percent of the total, releasing it during clotting to tighten blood vessels and help platelets clump. Depending on dose and setting, it can either narrow or widen vessels.
Inside the brain, nine raphe nuclei strung along the midline of the brainstem supply most serotonin-releasing neurons, and their fibres reach almost every part of the central nervous system. The molecule is involved in mood, appetite, sleep, learning, memory, pain, temperature control, sexual activity and body rhythms. It acts through 14 known receptor types; all but one are G-protein-coupled receptors that trigger chains of signals inside cells, while the exception, 5-HT3, is an ion channel. Many psychedelic drugs also act on these receptors.
Signalling usually ends when a transporter called SERT pulls serotonin back into the neuron that released it. Several antidepressant families, including SSRIs, block that reabsorption, and cocaine and the cough suppressant dextromethorphan interfere with it too. A 2006 study found a second, lower-affinity transporter, PMAT, handles a large share of clearance as well. Leftover serotonin is broken down mainly in the liver into 5-HIAA and excreted by the kidneys. Since 2003, research has also shown serotonin can bond directly onto proteins, a process called serotonylation that has been linked to several diseases.
Serotonin is far older than humans. Nearly all bilaterally symmetrical animals use it, including insects, spiders and worms, and fungi and plants make it too. Some plants and insect venoms deploy it to cause pain, and its presence in fruits and seeds may speed digestion and help spread them.
Source: Serotonin
