Serotonin: the Wellbeing Messenger That Lives Mostly in the Gut

The fact that surprises everyone

When people think of serotonin, they picture the brain: neurons, depression, antidepressants. But 95 % of the body's serotonin is in the gastrointestinal tract, mainly in the enterochromaffin cells of the gut lining. The brain holds only the remaining 5 %, and that 5 % cannot be topped up from the rest of the body, because serotonin does not cross the blood-brain barrier.

The brain makes its own serotonin completely independently of the gut. What the gut produces acts locally, regulating motility, secretion and visceral sensation, and affects the brain only indirectly, through the vagus nerve.

Structure: an indolethylamine

Serotonin (5-hydroxytryptamine, 5-HT, C₁₀H₁₂N₂O) is an indolethylamine. Unlike dopamine and adrenaline, catecholamines built on a benzene ring, serotonin has an indole ring (benzene fused to pyrrole) with a hydroxyl group at position 5 and an ethylamine chain at position 3.

This structural difference matters for pharmacology: drugs designed to act selectively on the serotonin system without touching the dopamine system exploit the geometric and electronic differences between the two families of receptors, although the selectivity is never perfect.

Its synthesis starts from the essential amino acid tryptophan:

L-Tryptophan → 5-hydroxytryptophan (5-HTP) [by tryptophan hydroxylase, TPH] 5-HTP → Serotonin (5-HT) [by DOPA decarboxylase, the same enzyme that makes dopamine]

Tryptophan hydroxylase is the rate-limiting step, and its essential cofactor is tetrahydrobiopterin (BH4), the same one tyrosine hydroxylase uses to make dopamine. That is why genetic defects in BH4 synthesis affect serotonin and dopamine at the same time.

The 14 serotonin receptors

Serotonin's functional complexity comes not from its structure but from the diversity of its receptors. There are 7 families of 5-HT receptors (5-HT1 to 5-HT7), with 14 different subtypes in total, all coupled to G proteins except 5-HT3, which is a ligand-gated ion channel.

This diversity explains why serotonin's effects can seem so contradictory:

• 5-HT1A (presynaptic autoreceptor): inhibits the serotonin neuron itself, the opposite of the "activating" role usually associated with serotonin
• 5-HT2A (cortex): involved in perception, cognition and the mechanism of classic hallucinogens (LSD and psilocin, the active metabolite of psilocybin, are partial 5-HT2A agonists)
• 5-HT3 (periphery, gut): mediates nausea and vomiting; 5-HT3 antagonists (ondansetron) are the standard anti-sickness drugs in chemotherapy
• 5-HT4 (gut): stimulates gastrointestinal motility, the target of prokinetic drugs such as prucalopride
• 5-HT6 and 5-HT7: involved in cognition and circadian rhythm

Serotonin and depression: a hypothesis that got complicated

For decades, the "serotonin hypothesis of depression" was the dominant framework: depression results from low serotonin in the brain, and SSRI antidepressants (selective serotonin reuptake inhibitors) work by increasing the serotonin available at synapses.

This hypothesis has serious problems that recent research has made impossible to ignore. The systematic review by Moncrieff et al. (2022) in Molecular Psychiatry, which caused considerable controversy, examined the direct evidence that serotonin levels or metabolism are altered in depression and found it inconsistent, not supporting the simple hypothesis.

What we do know:

SSRIs work clinically. They reduce depressive symptoms in a significant subset of patients with moderate to severe depression. The clinical effect is real, reproducible and statistically significant.

But the mechanism is not what we thought. SSRIs block the serotonin transporter (SERT) from the very first dose, raising synaptic serotonin immediately. Yet the antidepressant effect takes 2–4 weeks to appear. If the mechanism were simply "more serotonin", the effect should be immediate.

What happens over those weeks is more complex. The presynaptic 5-HT1A autoreceptors (which inhibit the serotonin neuron) gradually become desensitised under sustained SSRI exposure, finally letting the neuron fire more often. At the same time, SSRIs promote hippocampal neurogenesis (the formation of new neurons in the hippocampus), a process that also takes weeks and may be part of the real antidepressant mechanism.

Serotonin in the gut: the second brain

The gut's enterochromaffin cells release serotonin in response to mechanical stimuli (stretching of the gut wall) and chemical ones (gut contents, certain foods, toxins). That serotonin acts locally:

• It stimulates peristalsis through neurons of the enteric nervous system (5-HT4)
• It activates afferent fibres of the vagus nerve (5-HT3), sending information to the brain about the state of the gut
• It regulates intestinal fluid secretion

The serotonin-mediated gut-brain axis runs both ways: psychological stress alters gut motility via the enteric nervous system (the physiological basis of irritable bowel syndrome in many patients), and gut disturbances can affect mood via vagal signals.

The gut as a "second brain" is not just a metaphor: the enteric nervous system has more neurons (100–500 million) than the spinal cord and works semi-autonomously. Serotonin is one of its main neurotransmitters, along with acetylcholine.

Dietary tryptophan and the warm-milk myth

It is widely believed that eating tryptophan-rich foods (turkey, milk, bananas) raises brain serotonin and improves mood or sleep. The biochemistry is more nuanced.

Tryptophan is the least abundant amino acid in most food proteins. After a protein meal, tryptophan competes for the large neutral amino acid (LNAA) transporter of the blood-brain barrier with phenylalanine, leucine, isoleucine, valine and tyrosine, all present in larger amounts. As a result, after a mixed protein meal, tryptophan entry into the brain is, paradoxically, lower than when fasting.

Carbohydrates change things: the insulin they trigger makes muscles take up the amino acids that compete with tryptophan, improving tryptophan's relative share in the blood and its entry into the brain. That is why carbohydrate plus tryptophan (not protein alone) raises brain serotonin the most.

If warm milk with honey helps you sleep, it is mainly thanks to the carbohydrates and the relaxing ritual: a glass provides little tryptophan and a negligible amount of melatonin.

The next article explores the third great molecule of stress and emergency: adrenaline, the catecholamine that in fractions of a second prepares the body to run or fight, and whose tiny structure can switch on the heart, lungs, muscles and liver at the same time.