Serotonin · Grey Matter

Serotonin (5-HT) is a neuromodulator released across the brain from the raphe nuclei of the brainstem, which adjusts mood, patience, sleep, appetite and how strongly circuits respond to stress and punishment.

Serotonin. Serotonin (5-HT) is a neuromodulator released across the brain from the raphe nuclei of the brainstem, which adjusts mood, patience, sleep, appetite and how strongly circuits respond to stress and punishment.

Inside the brain it is made only by raphe neurons, from the amino acid tryptophan, which crosses the blood-brain barrier on a transporter. Their axons spread to the whole forebrain, cerebellum and spinal cord. Serotonin acts on at least fourteen receptor types, all metabotropic except 5-HT3, which is an ion channel; 5-HT1A receptors on the raphe cells themselves act as autoreceptors, and 5-HT2A receptors on cortical pyramidal cells are the main target of psychedelics. It is cleared by the transporter SERT and broken down by MAO.

Most of the body's serotonin is elsewhere. About 90 % is made by enterochromaffin cells of the gut, where it controls motility and is stored in platelets; it does not cross the blood-brain barrier, so the two pools are separate.

It is a state signal, without one single meaning. Raphe neurons fire steadily in waking, slow in deep sleep and fall nearly silent in REM sleep; theories link serotonin to waiting for delayed rewards, coping with aversive events and flexibility.

Drugs that block SERT (SSRIs) raise serotonin within hours but improve depression only over weeks, a delay attributed to slower adaptations such as autoreceptor desensitisation and plasticity.

Serotonin tunes the tone of the brain more than the content of any message.

Its many receptors let one small set of neurons set mood, sleep and patience at once, which is also why its drugs have broad effects.

Questions: What does an SSRI actually block? A selective serotonin reuptake inhibitor binds the serotonin transporter (SERT), the protein on serotonin terminals that pulls released serotonin back into the cell. With SERT blocked, serotonin stays longer in the extracellular space and reaches receptors farther from its release sites. Selective means it spares the dopamine and noradrenaline transporters that older tricyclic antidepressants also hit, which reduces side effects. The serotonin rise is immediate, while the clinical effect on depression takes weeks, so the drug's action begins at the transporter but its benefit depends on slower adaptations. Where does the brain's serotonin come from, and how does it reach so many regions? Nearly all of it comes from the raphe nuclei, clusters of neurons along the midline of the brainstem; the upper ones (the dorsal and median raphe) project to the forebrain and the lower ones to the spinal cord. Like the noradrenaline and dopamine systems, a small population of cells sends highly branched axons that release serotonin over wide territories, often outside classical synapses. Serotonin then acts through more than a dozen receptor types, almost all metabotropic, so the same release can excite some cells and inhibit others. This layout makes serotonin a modulator of state (mood, sleep, appetite, the response to stress) more than a carrier of specific messages. If SSRIs raise serotonin within hours, why do they take weeks to work? Blocking the serotonin transporter raises serotonin around the raphe neurons first, where it acts on their 5-HT1A autoreceptors and slows their firing, so serotonin in the forebrain rises less than expected at the start. Over two to several weeks those autoreceptors desensitise and the neurons fire normally again with the transporter still blocked, which is one classic explanation for the delay. Other proposed explanations involve slow changes downstream (plasticity, new gene expression, changes in how emotional information is processed), and the delay is probably the sum of several of them. The question matters because it shows that a drug's immediate chemical effect and its clinical effect can be separated by weeks of adaptation. If most of the body's serotonin is made in the gut, why does it not reach the brain? About 90 % of the body's serotonin is made by enterochromaffin cells in the lining of the gut, where it regulates motility and is carried in the blood inside platelets. Serotonin is a charged molecule and does not cross the blood-brain barrier, so this peripheral pool and the brain's pool stay separate. The brain makes its own serotonin in the raphe nuclei from tryptophan, an amino acid that does cross the barrier on a transporter. The gut can still influence the brain through the vagus nerve, immune signals and the supply of tryptophan, which are indirect routes.