The fact that surprises everyone
95% of the body's serotonin is in the gastrointestinal tract, mainly in enterochromaffin cells of the intestinal mucosa. The brain contains only the remaining 5% — and crucially, it cannot be replenished from the periphery: serotonin does not cross the blood-brain barrier. The brain synthesizes its own serotonin completely autonomously.
Structure: an indolethylamine
Serotonin (5-hydroxytryptamine, 5-HT, C₁₀H₁₂N₂O) is an indolethylamine — unlike dopamine (catecholamine with benzene ring), serotonin has an indole ring (benzene fused with pyrrole) with a hydroxyl at position 5. Synthesized from tryptophan in two steps: tryptophan hydroxylase (rate-limiting, requires BH4 cofactor) → DOPA decarboxylase.
14 serotonin receptors
Seven families (5-HT1–7), 14 subtypes, all G-protein coupled except 5-HT3 (ion channel). Key examples: 5-HT1A (presynaptic autoreceptor): inhibits the serotonergic neuron — opposite to the "activating" effect usually associated with serotonin. 5-HT2A (cortex): involved in perception, cognition, and the mechanism of classic hallucinogens (LSD, psilocybin are partial agonists). 5-HT3 (periphery, gut): mediates nausea — 5-HT3 antagonists (ondansetron) are first-line antiemetics in chemotherapy.
Serotonin and depression: the hypothesis that got complicated
The "serotonin hypothesis of depression" — that depression results from low serotonin levels, and SSRIs work by correcting this — has been seriously challenged. The Moncrieff et al. (2022) systematic review found the evidence for altered serotonin metabolism in depression is inconsistent. SSRIs do work clinically, but the mechanism is more complex: their antidepressant effect takes 2–4 weeks despite immediate SERT blockage, suggesting the real mechanism involves gradual 5-HT1A autoreceptor desensitization and hippocampal neurogenesis — a process that also takes weeks.
The gut-brain axis
Intestinal serotonin from enterochromaffin cells activates vagal afferent fibers (5-HT3) that send information to the brain. Psychological stress alters intestinal motility via the enteric nervous system — the physiological basis of IBS in many patients. The enteric nervous system (100–500 million neurons) operates semi-autonomously with serotonin as its primary neurotransmitter.