TLDR: Serotonin has no association with depression whatsoever. Instead, some cases are thought to be caused by reduced neuroplasticity during chronic periods of stress (mediated by the HPA axis and cortisol), resulting in a reduction in BDNF (brain-derived neurotrophic factor). SSRIs might work not due to serotonin, but due to significantly increasing BDNF expression, which is likely why psychedelics can also help in some cases.
Few molecules in the history of biology have travelled as strange a road as serotonin.
It was discovered twice, on two continents, by researchers chasing two completely different problems; one studying why blood serum constricts vessels, the other studying a substance in the gut that makes smooth muscle contract.
Within a decade of its chemical identification it leapt from a curiosity of vascular physiology into the centre of psychiatry, becoming the basis of a “chemical imbalance” myth that has shaped how hundreds of millions of people think about depression.
Today, that simple story has collapsed under the weight of evidence, even as serotonin has turned out to be far more biologically important - and far more widely distributed through the body - than its early champions ever imagined.
In this episode, we trace the rise and fall of the “serotonin hypothesis” of depression, dig in to its surprisingly broad function that modern research has revealed, and take a peek at the current leading theories for what makes clinical depression happen (oh, and why SSRIs work at all!)
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In the late 1930s, the Italian pharmacologist Vittorio Erspamer, working in Pavia, was studying a substance concentrated in the enterochromaffin cells of the gastrointestinal mucosa which caused smooth muscle to contract.
He named it enteramine.
Over years of painstaking work he and his colleagues characterised its biological actions, and Erspamer correctly suspected it was an “indole”-derivative (an unpleasantly-oderous organic compound found in coal tar and poo.)
Interestingly, the Italians also played a major role in the West’s discovery of Dopamine, but that’s a whole other story.
Independently, at the Cleveland Clinic in the United States, a team interested in the vasoconstrictor activity of blood serum was trying to isolate the agent responsible for raising vascular tone.
Some 10 years later, in 1948, Maurice Rapport, Arda Green, and Irvine Page succeeded in isolating and crystallising the target agent from beef serum. Because it came from serom and affected vascular tone, they coined the name serotonin.
Rapport then went on to determine its chemical structure, publishing the proposed vasoconstrictor principle the following year. Shortly afterward, serotonin was chemically synthesised, and it became clear that Erspamer’s enteramine and Rapport’s serotonin were the very same molecule: 5-hydroxytryptamine.
This double discovery is why the molecule carries two naming traditions to this day: pharmacologists and neuroscientists usually write 5-HT, while the popular and clinical name remains serotonin.
Then, in 1953, serotonin suddenly became a rather important subject of study to those who investigated diseases of the brain when Betty Twarog and Irvine Page demonstrated that serotonin was present in mammalian brain tissue.
This was the moment serotonin became a candidate neurotransmitter, and the timing couldn’t have been more extraordinary.
It coincided almost exactly with the discovery that the powerful hallucinogen lysergic acid diethylamide (LSD) was structurally related to serotonin, and could even antagonise (promote) its actions on smooth muscle (as serotonin does.)
The inference - that a serotonin-like compound could so profoundly alter perception and mood, and make you hallucinate like the devil - electrified the young field of biological psychiatry and planted the seed of an idea that serotonin governed states of mind.
Even before receptors could be cloned, classical pharmacology hinted at serotonin’s complexity.
In 1957, Gaddum and Picarelli, studying guinea-pig ileum, proposed that serotonin acted on at least two distinct receptor types, which they called the “M” (morphine-blocked) and “D” (dibenzyline-blocked) receptors.
This early two-receptor scheme was the ancestor of what is now recognised as one of the most complicated receptor families in all of pharmacology, encompassing at least 14 distinct receptor subtypes today.
The serotonin theory of depression did not emerge from a direct observation that depressed people lacked serotonin. It emerged, somewhat backwards, from pharmacology.
In the 1950s and 1960s, clinicians noticed that certain drugs altered mood, and researchers reasoned backward from the drugs’ known effects on brain chemistry to a presumed cause of the illness.
Two foundational papers framed the debate:
Joseph Schildkraut proposed the catecholamine hypothesis of affective disorders in 1965, arguing that depression might be associated with a deficiency of noradrenailne at functionally important brain sites.
Two years later, in 1967, the British psychiatrist Alec Coppen advanced the case that 5-HT, rather than (or in addition to) the catecholamines, was central to the biochemistry of affective disorders.
Together these papers crystallised what became the monoamine hypothesis of depression; the idea that mood disorders stem from a deficit of monoamine neurotransmitters in the brain, such as dopamine, noradrenaline, and serotonin.
The hypothesis gained enormous traction because drugs that increase synaptic serotonin can apparently relieve depressive symptoms.
The decisive commercial and cultural moment came with fluoxetine (Prozac), developed at Eli Lilly and described by David Wong and colleagues as the first selective serotonin reuptake inhibitor (SSRI) to reach the market. Wong’s retrospective review traces the deliberate, two-decade evolutionary process by which fluoxetine was engineered specifically to block serotonin reuptake while sparing other systems.
SSRIs supposedly work by inhibiting the serotonin transporter (SERT), thereby raising the concentration of serotonin in the synaptic cleft.
The marketing logic was seductive and simple: if a drug that raises serotonin treats depression, then depression must be caused by too little serotonin - a “chemical imbalance” that the medication corrects.
This framing was widely communicated to the public through advertising and clinical encounters, and it became one of the most successful pieces of medical folk-knowledge of the late twentieth century.
And, as Psychopharmacologist Stephen M. Stahl noted in his 1998 paper, Prozac and similar agents are “among the most frequently prescribed therapeutic agents in all of medicine.”
It turns out, however, this simple pharmacokinetic story was built on far shakier foundations than anyone believed at the time.
Crucially, the inference is a logical error of the form of “the drug raises X, therefore the disease is a deficiency of X.”
Aspirin relieves headaches, but headaches are not caused by an aspirin deficiency.
Even at the time, serious problems were visible, such as the therapeutic delay, and a heterogenous pharmacology of anti-depressants.
SSRIs raise synaptic serotonin within hours, yet clinical antidepressant effects typically take weeks to appear. This temporal mismatch suggested that the relevant therapeutic mechanism is not the acute rise in serotonin itself, but slower, downstream adaptations.
For example, one such adaptation is the desensitisation of somatodendritic 5-HT1A autoreceptors (these are receptors that detect the molecule in the “extra-cellular space”, i.e outside or overflowing the synaptic cleft) in the raphe nuclei.
Meanwhile, drugs with very different effects on serotonin can all have antidepressant activity, which is hard to reconcile with a single, simple serotonin-deficiency model.
Something was clearly not right with any of this. The drugs definitely work; we just couldn’t quite figure out how.
The empirical case against the simple serotonin-deficiency model was assembled most comprehensively in a 2022 systematic umbrella review led by Joanna Moncrieff and colleagues, published in Molecular Psychiatry.
Together, they synthesised the principal bodies of evidence:
* serotonin and 5-HIAA (its main metabolite) concentrations in bodily fluids;
* 5-HT1A receptor binding;
* serotonin transporter (SERT) levels by imaging and post-mortem;
* tryptophan-depletion experiments;
* and SERT gene associations and gene–environment interactions.
Their conclusions were revealing.
First, meta-analysis of the serotonin metabolite - 5-HIAA - showed no association with depression.
A meta-analysis of serotonin in blood plasma showed no relationship with depression, either.
Shockingly, it actually found that lowered serotonin was associated with the use of antidepressants - suggesting these medications might be doing the exact opposite of what we all believed.
Then, further analysis of the largest and highest-quality genetic observations of Serotonin Reuptake Transporter - SERT - showed no relationship whatsoever with depression, and no gene-by-stress interaction.
Not only had we been wrong about how these medications worked, we had been ass-backwards wrong.
Upside-down looney-tunes wrong.
We had been living in opposite land.
The bottom line was that the main areas of serotonin research provide no consistent evidence that depression is caused by lowered serotonin activity or concentration.
The authors wrote that the areas surveyed “provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations”; more provocatively still, “some evidence was consistent with the possibility that long-term antidepressant use reduces serotonin concentration.” (emphasis mine)
The paper - as you might expect - generated intense debate; the journal published numerous commentaries both supporting and criticising its framing and methods, but it crystallised a scientific consensus that had in fact been building for years:
The simple “low-serotonin-causes-depression” story is not supported by the evidence.
It is important to be precise about what collapsed.
The umbrella review undermined the claim that depression is caused by a serotonin deficiency. It does not prove that SSRIs are ineffective, nor that serotonin is irrelevant to mood.
The efficacy of antidepressants is a separate empirical question, addressed by large network meta-analyses such as Cipriani and colleagues’ 2018 study of 21 antidepressants, which found that all examined antidepressants were more effective than placebo for acute major depression, albeit with modest effect sizes and varying acceptability.
The most defensible modern position is that SSRIs can be clinically useful, but that their benefit does not validate a serotonin-deficiency theory of causation.
Stahl puts it bluntly: “the immediate actions of SSRIs are mostly side effects,” and locates the cure elsewhere: “The explanation for therapeutic effects characteristic of SSRIs may be found in delayed neurochemical adaptations,” of which “a leading hypothesis… is desensitization of somatodendritic serotonin 1A autoreceptors in the midbrain raphe.”
If the psychiatric story narrowed and then partially collapsed, the broader biology of serotonin expanded enormously.
The single most important re-framing of the past two decades is captured in the title of a landmark 2009 review by Berger, Gray, and Roth: “The expanded biology of serotonin”.
What we did in fact discover is that most serotonin is not located in the brain.
Although serotonin is famous as a brain neurotransmitter, the overwhelming majority of the body’s serotonin is found outside the central nervous system, predominantly in the gastrointestinal tract, where it is produced by enterochromaffin cells and stored in circulating platelets.
Within the brain, serotonin’s character is one of a neuromodulator via volume transmission, a characteristic it shares mainly with various kinds of peptide: it does not so much carry discrete point-to-point messages as set the gain and tone of large brain networks.
Its functions are remarkably broad; it is involved in the regulation of mood, sleep, appetite, food intake, aggression, impulsivity, and many other processes, and it is the precursor for melatonin synthesis in the pineal gland.
The diversity of serotonin receptor subtypes - each with its own distribution and signalling - is what allows a single molecule to influence so many distinct functions, and it explains both the therapeutic breadth and the side-effect profile of serotonergic drugs.
So, if not serotonin deficiency, then what is depression?
There’s a word - “stress” - that often comes up in the literature these days. I’ve often found myself wondering what exactly kind of “stress” is being referred to here. When we talk about stress, we’re often talking about psychological and psychosocial challenges, such as losing a job, concern over finances, marital instability, and so on.
However, stress in biology is usually something different; infection, illness, disease, anything that results in systemic inflammation, in which large quantities of leukocytes (white blood cells) and cytokines flood the body, with wide-ranging biochemical impacts across all systems.
Something that has emerged recently is an understanding that psychological stress doesn't stay psychological. It is converted into biological signals by the hypothalamic–pituitary–adrenal (HPA) axis, which releases cortisol.
Sustained high cortisol is itself neurotoxic to the relevant circuits; it is one of the proximate drivers of synaptic atrophy, and a reduction in signalling of the brain’s growth hormone, “brain-derived neurotrophic factor” (BDNF.)
It effectively slows your brain’s ability to adapt, significantly reducing its neuroplasticity, as well as weakening existing connections.
Presently, this is hypothesised to be one of the leading causes of clinical depression: the inability for the brain to adapt away from negative thought patterns, due to the massive reduction in neuroplasticity caused by chronically high cortisol.
Interestingly, this also seems to be where SSRIs have their anti-depressant effects; one idea posited that the chain of events that causes the desensitisation of serotonin autoreceptors in the brain may also result in the increase of BDNF expression, restoring levels of neuroplasticity that resemble the “critical period” of brain development.
A related and increasingly influential idea is that serotonergic drugs (and psychedelics) may enhance a window of plasticity that allows the brain to “relearn” healthier patterns, with environment and psychotherapy determining the outcome. In this model, the SSRI unlocks the gate, allowing the patient to pass through.
This would explain why combined SSRI and psychotherapy treatment regimens are typically much more successful than either one on its own: SSRIs seem to give the brain the opportunity to break out of negative thought pathways.
These mechanisms are areas of intense ongoing investigation and are not yet settled.
Although the leading hypothesis of depression has crumbled beneath our feet, the effect on research has been to blast wide-open many new and exciting areas that were once thought to be settled.
The failure to find a robust SERT-gene association with depression is but one, showing depression is almost certainly not one disease but a heterogeneous collection of conditions with overlapping symptoms and many contributing causes, including genetic, developmental, inflammatory, social, and environmental.
Identifying biologically meaningful subtypes, matching them to mechanisms (serotonergic or otherwise), and translating that into clinical treatments, is among the very frontiers of biological psychiatry today.
There’s also more being found on the psychological front, such as how antidepressant treatments “decrease the negative bias in the processing of emotionally salient information early in the course of antidepressant treatment, which leads to the clinically significant mood improvement later in treatment”, as described by Godlewska and Harmer.
This model reflects “a change in the view of psychological and biological processes, from seeing them as separate to complementing one another.”
What we are still discovering may be the most exciting part: how antidepressants and psychedelics actually reshape the brain, how the gut and its microbes talk to the mind, and how a single small indole built from a dietary amino acid came to touch so many corners of human physiology.
Serotonin’s story is not finished.
If anything, after seventy-five years, it is becoming more important than ever.
References
* The discovery of serotonin and its role in neuroscienceWhitaker-Azmitia PM | Neuropsychopharmacology | 1999
* The serotonin theory of depression: a systematic umbrella review of the evidenceMoncrieff J, Cooper RE, Stockmann T, Amendola S, Hengartner MP, Horowitz MA | Molecular Psychiatry | 2023
* Serum vasoconstrictor (serotonin): the presence of creatinine in the complex; a proposed structure of the vasoconstrictor principleRapport MM | Journal of Biological Chemistry | 1949
* Two kinds of tryptamine receptorGaddum JH, Picarelli ZP | British Journal of Pharmacology and Chemotherapy | 1957
* International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-Hydroxytryptamine; Pharmacology and FunctionBarnes NM, Ahern GP, Becamel C, Bockaert J, et al. | Pharmacological Reviews | 2021
* The catecholamine hypothesis of affective disorders: a review of supporting evidenceSchildkraut JJ | American Journal of Psychiatry | 1965
* The biochemistry of affective disordersCoppen A | British Journal of Psychiatry | 1967
* Prozac (fluoxetine, Lilly 110140), the first selective serotonin uptake inhibitor and an antidepressant drug: twenty years since its first publicationWong DT, Bymaster FP, Engleman EA | Life Sciences | 1995
* Mechanism of action of serotonin selective reuptake inhibitors. Serotonin receptors and pathways mediate therapeutic effects and side effectsStahl SM | Journal of Affective Disorders | 1998
* Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder: a systematic review and network meta-analysisCipriani A, Furukawa TA, Salanti G, et al. | Lancet | 2018
* The expanded biology of serotoninBerger M, Gray JA, Roth BL | Annual Review of Medicine | 2009
* A unique central tryptophan hydroxylase isoformWalther DJ, Bader M | Biochemical Pharmacology | 2003
* Patients with high-bone-mass phenotype owing to Lrp5-T253I mutation have low plasma levels of serotoninFrost M, Andersen TE, Yadav V, Brixen K, Karsenty G, Kassem M | Journal of Bone and Mineral Research | 2010
* Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesisYano JM, Yu K, Donaldson GP, Shastri GG, Ann P, Ma L, Nagler CR, Ismagilov RF, Mazmanian SK, Hsiao EY | Cell | 2015
* The role of glutamate underlying treatment-resistant depressionKim J, Kim TE, Lee SH, Koo JW | Clinical Psychopharmacology and Neuroscience | 2023
* Psychedelic Psychiatry’s Brave New WorldNutt D, Erritzoe D, Carhart-Harris R | Cell | 2020
* Cognitive neuropsychological theory of antidepressant action: a modern-day approach to depression and its treatmentBR Godlewska, CJ Harmer | Psychopharmacology | 2020
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