Psychology › Brain & biology › Neurotransmitters, the brain's chemical messengers
Serotonin
Isolated 1948· Concept
Well replicatedStrong, repeated evidence.
A neurotransmitter involved in mood, sleep, appetite and digestion; about 90% of the body's serotonin is in the gut.
See it on the Psychology map →What it says
Maurice Rapport, Arda Green and Irvine Page isolated it from blood at the Cleveland Clinic in 1948, naming it for its effect on blood vessels (serum + tonic). In 1953 Betty Twarog and Page found it in the brain. There, a few hundred thousand neurons in the brainstem's raphe nuclei send serotonin fibers almost everywhere, influencing mood, sleep, appetite, impulse control and nausea. SSRI antidepressants such as Prozac (US approval 1987) and psychedelics such as psilocybin act on this system.
Why it matters
It became the target of the best-selling antidepressants and the center of the 'chemical imbalance' story of depression, a story researchers now consider far too simple.
The evidence
Status: well replicated as brain chemistry. A 2022 umbrella review led by Joanna Moncrieff found no consistent evidence that low serotonin causes depression; critics contested parts of it. SSRIs still help many people in trials.
Open questions
Why SSRIs raise serotonin within hours but lift mood only after weeks; leading ideas involve slower changes in brain plasticity and in how people process emotional information.
Connections (4)
▸Serotonin is one of the Neurotransmitters, the brain's chemical messengersWell replicated
The connection
Serotonin is another monoamine neurotransmitter. A few hundred thousand brainstem neurons make it for the brain, but their branches reach nearly every region. At least 14 receptor types respond to it in different ways, which is why drugs as different as Prozac, LSD and anti-nausea pills all work through serotonin.
Who showed it and when
Isolated at the Cleveland Clinic in 1948; found in mammal brains by Betty Twarog and Irvine Page in 1953, which suggested it acted as a brain messenger.
The evidence
Brain tissue studies, receptor mapping and drug effects. In the 1950s, LSD's chemical resemblance to serotonin helped convince scientists that serotonin acts in the brain.
Open questions
What serotonin's core job is: theories include patience, coping with stress and adapting to setbacks, but no single function has won consensus.
▸SSRI antidepressants (e.g., Prozac) boost signaling of SerotoninWell replicated
The connection
After a nerve cell releases serotonin, a transporter pump normally pulls it back in. SSRIs block that pump, so serotonin lingers between cells and its signal strengthens. Over weeks the brain adjusts its receptors in response, which may be what actually lifts mood.
Who showed it and when
Designed by drug companies in the 1970s; at Eli Lilly, Bryan Molloy, David Wong and Ray Fuller created fluoxetine in 1972.
The evidence
Lab studies confirm SSRIs bind the serotonin transporter, and PET scans show about 80 percent of transporters blocked at standard doses.
Open questions
A 2022 review led by Joanna Moncrieff found no consistent evidence that low serotonin causes depression, so SSRIs may help by other routes, such as boosting brain plasticity.
▸The 'chemical imbalance' theory of depression blamed low levels of SerotoninWeak or failed to replicate
The connection
Early antidepressants raised serotonin and norepinephrine, so researchers reasoned backward that depression must involve too little of them. SSRIs, which block serotonin's recycling, then made serotonin the star of the theory and of drug advertising in the 1990s.
Who showed it and when
Alec Coppen in Britain (1967) and others in the 1960s–70s; it reached the public with Prozac's US launch in 1988 and TV drug ads after 1997.
How solid is it?
The link was the theory's own claim; tests of it, summarized in the 2022 Moncrieff review, found no consistent evidence of low serotonin in depression.
Open questions
Serotonin may still play some role in mood and in how drugs work; the debate is whether a deficit causes depression.
▸Psychedelic-assisted therapy (psilocybin, MDMA) acts on receptors for SerotoninWell replicated
The connection
The body converts psilocybin to psilocin, which activates serotonin 5-HT2A receptors, especially in the cortex, producing altered perception and loosened habits of thought. MDMA works differently: it floods synapses with serotonin and raises oxytocin, which may make painful memories easier to talk about.
Who showed it and when
Albert Hofmann at Sandoz in Basel made LSD (1938) and isolated psilocybin (1958); Franz Vollenweider's Zurich team confirmed the 5-HT2A link in humans in 1998.
The evidence
Blocking 5-HT2A receptors with the drug ketanserin largely blocks the psilocybin experience in volunteers, and PET scans show receptor occupancy tracks how intense the trip feels.
Open questions
Whether the therapeutic benefit needs the trip itself; companies are testing non-hallucinogenic drugs that hit the same receptors.