Serotonin speculation of melancholy

The serotonin speculation of affective (temper) problems is over 50 years outdated (Coppen., 1965), and like loads of hypotheses from that period, was constructed on a mix of clinically noticed phenomena and psychopharmacology.

At its primary stage it prompt that lowered ranges of monoamines (dopamine, noradrenaline and/or serotonin (5-hydroxytryptamine, 5-HT)) contributed to the assemble of melancholy, based mostly on the remark that depleting monoamines by reserpine led folks to expertise melancholy signs, and that iproniazid (a remedy for tuberculosis) contributed a few of its therapeutic results by way of actions on (stopping degradation of) serotonin, along with different monoamines (Schildenkraut, 1967), (Jauhar, Browning and Cowen, 2023).

The speculation has undergone revisions over time (and proof has modified, e.g. reserpine findings challenged (Strawbridge et al, 2022)), and latterly integrated each experimental medication and molecular imaging research. Genetic analysis additionally prompt a hyperlink between melancholy and polymorphisms within the gene for serotonin binding protein and life occasions. Using the Selective Serotonin Reuptake Inhibitors (SSRIs) within the Nineties coincided with an exponential enhance in analysis on this space (in addition to the gross sales of those medication), and attainable hyperbole regarding the monoamine speculation of melancholy. Over time this optimism has waned, and although SSRIs are little doubt efficient for a proportion of individuals, they aren’t a panacea for melancholy. This has led some to query the validity of the analysis in regards to the serotonin system in melancholy.

On this context, Moncrieff et al. (2022) performed an umbrella evaluate to look at whether or not the serotonin system is implicated within the aetiology of melancholy.

The serotonin hypothesis on depression was introduced decades ago, yet remains relevant in understanding the aetiology of depression.

The serotonin speculation of melancholy was launched many years in the past, but stays related in understanding the aetiology of melancholy.


The authors performed an umbrella evaluate (a evaluate of systematic evaluations and meta-analyses), through which proof from these sources is collected, appraised and synthesised.

They searched databases together with PubMed, EMBASE and PsycINFO by way of the Healthcare Databases Superior Search device. They recognized 6 areas they wished to look at:

  • Serotonin and serotonin metabolite, 5-HIAA, concentrations in physique fluids;
  • Serotonin receptor binding;
  • Serotonin transporter (SERT) ranges;
  • Tryptophan depletion research;
  • SERT gene polymorphisms; and
  • SERT gene-environment interactions.

The PROSPERO pre-specified protocol said inclusion of

“Any systematic analysis synthesis together with systematic evaluations, meta-analysis, umbrella evaluations and particular person affected person meta-analysis or very giant dataset evaluation based mostly on a pre-specified set of research or dataset.” and for individuals, “Inclusion: Adults with melancholy and wholesome volunteers”.

The authors included research involving folks with depressive problems, or, for experimental research, wholesome volunteers through which temper was a measured final result. The place larger than 5 evaluations or giant analyses existed, solely the 5 most up-to-date research had been included. Animal research and research completely involved with melancholy in bodily circumstances or a particular subtype of melancholy had been excluded.

The authors utilized varied high quality standards together with AMSTAR-2 (A MeaSurement Device to Assess systematic Critiques) and their very own model of GRADE (Grading of Suggestions, Evaluation, Improvement and Evaluations) standards for assessing certainty. The latter concerned two authors taking pre-defined standards and agreeing consensus on whether or not these had been measured or not, then agreeing a ‘stage of certainty’ as to the proof.


361 publications had been recognized, and 17 research fulfilled inclusion standards.

For tryptophan depletion research they state, “In areas through which no systematic evaluate or meta-analysis had been carried out throughout the final 10 years, we additionally chosen the ten most up-to-date research on the time of looking out (December 2020) for illustration of newer findings.”

Serotonin and 5-HIAA concentrations

Of the three research included, the authors said that one meta-analysis demonstrated decrease ranges of plasma 5-HT in post-menopausal girls with melancholy, which didn’t meet statistical significance. The opposite two meta-analyses didn’t discover an affiliation between 5-HIAA concentrations in cerebrospinal fluid (CSF) and melancholy.

In abstract, the research did not show a statistically important affiliation between serotonin and 5-HIAA ranges and melancholy.

Serotonin 5-HT1A receptor binding

The authors restricted their evaluation to research regarding 5HT1A receptors solely. The authors said that the 5HT1A receptor was an ‘auto-receptor’, i.e. a pre-synaptic receptor the place serotonin inhibits its personal additional launch into the synapse. The authors said that: “if melancholy had been to be the results of lowered serotonin exercise brought on by abnormalities within the 5HT1Areceptor, folks with melancholy could be anticipated to point out elevated exercise of 5HT1A receptors, in comparison with these with out”.

Two evaluations happy standards. The authors discovered that almost all of outcomes throughout each evaluations recommend there was both no distinction, or decrease binding, to the 5HT1A receptor in folks with melancholy in comparison with controls.

If utilizing the authors’ scientific rationale above, the findings recommend these with melancholy have both no change or a rise in ranges of serotonin.

Serotonin transporter (SERT) ranges

Three evaluations had been recognized inspecting serotonin transporter (SERT) ranges, and the authors state, “General, the info indicated attainable reductions in SERT binding in some mind areas, though areas through which results had been detected weren’t constant throughout the evaluations.”

Tryptophan depletion

The authors recognized two evaluations in regards to the impact of tryptophan depletion on temper. One of many evaluations discovered that folks with a household historical past of melancholy confirmed a larger discount in temper following tryptophan depletion, in comparison with sham remedy. In each evaluations, folks with melancholy confirmed larger temper discount following tryptophan depletion.

The authors additionally included the ten most up-to-date research investigating the impact of tryptophan depletion on temper. They concluded that tryptophan depletion had no convincing impact on temper.

“Knowledge from research involving volunteers principally confirmed no impact” and “Throughout each evaluations, research involving folks recognized with melancholy confirmed barely larger temper discount following tryptophan depletion than sham remedy total, however most individuals had taken or had been taking antidepressants and participant numbers had been small.”

SERT gene and gene-environment interactions

Two meta-analyses confirmed a statistically important affiliation between the 5-HTTLPR (serotonin-transporter-linked promoter area) and melancholy. Nonetheless, two newer research didn’t discover an affiliation between the SERT gene and melancholy. Equally, though the authors discovered that earlier research prompt a statistically important interplay between the SERT gene and melancholy, some latest research didn’t.

Within the dialogue they state, “there is no such thing as a convincing proof that melancholy is related to, or brought on by, decrease serotonin concentrations or exercise.” and “Most research discovered no proof of lowered serotonin exercise in folks with melancholy in comparison with folks with out, and strategies to scale back serotonin availability utilizing tryptophan depletion don’t constantly decrease temper in volunteers.”

The authors state that areas of analysis that supplied average or excessive certainty, based mostly on their modified GRADE standards, confirmed no affiliation between markers of serotonin exercise and melancholy. They remark that some areas recommend findings per elevated serotonin exercise, which was of low certainty. The authors notice that tryptophan depletion research confirmed very low certainty of proof of lowered serotonin exercise or availability in these with a household historical past of melancholy. No remark is made on trytophan depletion research in folks with melancholy.

They conclude, “This evaluate means that the massive analysis effort based mostly on the serotonin speculation has not produced convincing proof of a biochemical foundation to melancholy.”

The review indicated that those with depression have either no significant change or an increase in levels of synaptic serotonin.

This evaluate (Moncrieff et al, 2022) indicated that these with melancholy have both no important change or a rise in ranges of synaptic serotonin.


From their evaluation of the literature the authors conclude that:

 There is no such thing as a convincing proof that melancholy is related to, or brought on by, decrease serotonin concentrations or exercise.

The authors concluded that no convincing evidence suggests links of association or causation between depression and lower serotonin concentration.

The authors concluded that no convincing proof suggests hyperlinks of affiliation or causation between melancholy and decrease serotonin focus.

Strengths and limitations

A energy of the paper is that the protocol was registered with PROSPERO. Nonetheless, it was up to date to mirror the modification of the standard score system and embrace the authors modified GRADE standards for certainty. Though the measurement of research high quality seems typical, the GRADE standards will not be. The authors state they’re in line with prior literature (Kennis et al., 2020), but there is no such thing as a clear similarity seen and the factors are arbitrarily utilized post-hoc after submission of the unique Prospero protocol. Moreover, GRADE units observational research (i.e. these examined right here) as low certainty. By their very nature, tryptophan depletion and molecular imaging research could have small pattern sizes, and given the factors set by the authors, it’s a self-fulfilling prophecy that this literature will probably be of “very low certainty”. The authors seem to disregard the truth that pattern sizes for these research are dictated by regulatory our bodies, practicalities and funding, and will probably be decrease than these of genetic research.

On examination, the methodology permits for a excessive diploma of bias relating to research choice, appraisal and inference. Firstly, it doesn’t conform to a standard umbrella evaluate, which might usually solely research a single space (i.e. genetics, molecular imaging, experimental research) because the measure of impact won’t be comparable throughout the completely different scientific disciplines. Secondly, it’s obscure why overlapping research imply a separate meta-analysis couldn’t be performed as this has occurred with different evaluations.

Moreover, the inclusion standards within the evaluate seem to vary from these specified within the PROSPERO protocol, i.e. inclusion of latest tryptophan depletion research. Together with particular person research is just not in line with typical umbrella evaluations. It’s puzzling how they will justify this. If evaluations had been too outdated they may have included all research printed because the evaluate, and if that they had carried out so they’d have picked up a research which did present an impact of tryptophan depletion in folks with melancholy (Yatham et al, 2012).

Concerning evaluation of serotonin receptor and SERT research, one of many included “meta-analyses” doesn’t seem like a meta-analysis, merely pooling information collectively (Nikolaus et al., 2016). Due to this fact the outcomes of this evaluate have little worth.

When discussing outcomes of the evaluate of 5HT1A receptors (imaging) the authors make a scientific error; they state that the 5HT1A receptor is a pre-synaptic auto-receptor, and conclude that decrease binding would suggest larger focus or exercise of serotonin. Nonetheless, the 5HT1A receptor is expressed each pre- and post- synaptically within the mind, and due to this fact has differing results in numerous mind areas. Interpretation of decreased binding is difficult, and the evaluate the authors cite states “Decreased 5HT1A binding might act as a compensating issue to enhance postsynaptic serotonin reuptake throughout a depressive episode.” (Wang et al, 2016), i.e. in line with lowered serotonin, versus what Moncrieff et al surmise. It’s puzzling how Moncrieff et al quote the evaluate, although ignore the inferences drawn by the evaluate authors and substitute their very own reasoning, based mostly on a misunderstanding of primary neuroscience.

The interpretation of PET findings is rather more nuanced than acknowledged by Moncrieff et al, they usually additionally omit point out of methodologically extra rigorous replicated research of this technique that recommend alterations in melancholy (for particulars see Jauhar et al, 2023).

Concerning SERT imaging, Moncrieff et al state “areas through which results had been detected weren’t constant throughout the evaluations”. That is at odds with the 2 meta-analyses, the place constant findings are seen in amygdala and midbrain. This could have been addressed by a standard umbrella evaluate or simply quoting findings from the latest meta-analysis.

Interpretation of lowered SERT is once more simplistic, as demonstrated by the authors stating lowered SERT signifies larger synaptic serotonin. Puzzlingly, the quotation Moncrieff et al give for this interpretation provides this as certainly one of 4 attainable methods of understanding present findings, together with decrease 5-HT (Meyer, 2007). By attributing lowered SERT to antidepressant use, the authors seem unaware that nearly half of the sufferers in a single meta-analysis they cite had been drug naïve, and that lowered SERT is seen in research of individuals naïve of antidepressants. Once more, there’s little nuance or acknowledgement of how PET findings ought to be interpreted. The unique evaluations the authors cite acknowledge this, so it’s obscure how Moncrieff et al draw differing conclusions, and selectively cite papers to concur with their views.

The reporting of the tryptophan depletion research is once more obscure. Within the evaluate they cite the impact dimension for the results of tryptophan depletion on temper in depressed folks not taking antidepressants, from 8 samples, was giant (Hedge’s g=-1.9 (95% CIs -3.02 to -0.78). Moncrieff et al report this as “slight”. A few of these had been from the identical group, and confidence intervals admittedly had been giant (Ruhe et al, 2007, Jauhar et al, 2023). It’s complicated that the authors place emphasis on destructive tryptophan research in wholesome volunteers, when the proof suggests it’s these with melancholy who present depressive signs with trytophan depletion.

Certainly one of many tenets of the serotonin speculation was an alteration within the system in these with vulnerability to melancholy, versus these with out melancholy.

On reflection, it appears the authors have chosen their stance, modified their quality criteria to fit this, and selectively reported findings accordingly.

On reflection, it seems the authors have chosen their stance, modified their high quality standards to suit this, and selectively reported findings accordingly.

Implications for observe

It’s tough to type any implications for observe following this evaluate. It doesn’t current any new findings, and given the excessive diploma of bias, doesn’t present the reader with any dependable conclusions.

What’s vital to notice, is that irrespective of the present proof of the monoamine speculation of melancholy, the antidepressant results of SSRIs are a separate matter. No implications for the usage of such medication can reliably be concluded from this research.

What this evaluate does spotlight is the persevering with want for ongoing efforts within the understanding of the biopsychosocial causes within the assemble of such a fancy sickness, melancholy.

Irrespective of the current evidence of the monoamine hypothesis of depression, the antidepressant effects of SSRIs are a separate matter.

Irrespective of the present proof of the monoamine speculation of melancholy, the antidepressant results of SSRIs are a separate matter.


On reflection, it seems the authors might have chosen their stance, modified their standards for certainty to suit this, and selectively reported findings accordingly.

The factors for an umbrella evaluate haven’t been met:

  • research favouring the authors’ arguments have been given extra prominence,
  • research not favouring the authors stance have been neglected, and
  • interpretation of the meta-analyses is, in some instances, in distinction with what the meta-analyses themselves truly report.

That is odd, as Moncrieff et al seem content material to report writer’s findings, with no new analyses, although not interpretations made by the unique authors. The interpretation of advanced neuroscience is oversimplified, once more showing in favour of the authors’ narrative.

Mom Nature is advanced, and the factual errors highlighted above, and inferences drawn make it obscure how the authors may be so sure of their daring conclusions.

The serotonin speculation of melancholy has been round for over fifty years, and as identified by ourselves, rumours (and opinion expressed on this “evaluate”) of it’s demise might be untimely.

The serotonin hypothesis of depression has been around for over 50 years, rumours of it's demise may well be premature.

The serotonin speculation of melancholy has been round for over 50 years, rumours of it’s demise might be untimely.

Assertion of pursuits

Rebecca Wilkinson has no pursuits to declare.

Sameer Jauhar has obtained honoraria for non-promotional instructional talks on antipsychotics from Janssen, Sunovian and Lundbeck, and on causes of schizophrenia for Boehringer-Ingelhim. He has additionally obtained honoraria for consulting on antipsychotics for LB Prescribed drugs. He additionally sits on Council for the British Affiliation for Psychopharmacology and was a latest Panel Member for the Wellcome Belief.


Major Paper

Moncrieff, J., Cooper, R.E., Stockmann, T. et al. The serotonin principle of melancholy: a scientific umbrella evaluate of the proof. Mol Psychiatry (2022).

Different references

Coppen, A. (1967) ‘The Biochemistry of Affective Problems’, The British Journal of Psychiatry, 113(504), pp. 1237–1264. Accessible at:

Schildkraut JJ. The catecholamine speculation of affective problems: a evaluate of supporting proof. Am J Psychiatry. 1965 Nov;122(5):509–22.

Jauhar S, Cowen PJ, Browning M. Fifty years on: Serotonin and melancholy. J Psychopharmacol 2023; 37: 237–41.

Strawbridge, R. et al. (2022) ‘The consequences of reserpine on melancholy: A scientific evaluate’, Journal of Psychopharmacology (Oxford, England), p. 2698811221115762. Accessible at:

Kennis M, Gerritsen L, van Dalen M, Williams A, Cuijpers P, Bockting C. Potential biomarkers of main depressive dysfunction: a scientific evaluate and meta-analysis. Mol Psychiatry. 2020 Feb;25(2):321–38.

Yatham LN, Liddle PF, Sossi V, Erez J, Vafai N, Lam RW, et al. Positron Emission Tomography Examine of the Results of Tryptophan Depletion on Mind Serotonin(2) Receptors in Topics Not too long ago Remitted From Main Despair. Archives of Normal Psychiatry. 2012 Jun 1;69(6):601–9.

Ruhé HG, Mason NS, Schene AH. Temper is not directly associated to serotonin, norepinephrine and dopamine ranges in people: a meta-analysis of monoamine depletion research. Mol Psychiatry. 2007 Apr;12(4):331–59.

Nikolaus S, Müller HW, Hautzel H. Totally different patterns of 5-HT receptor and transporter dysfunction in neuropsychiatric problems–a comparative evaluation of in vivo imaging findings. Rev Neurosci. 2016 Jan;27(1):27–59.

Meyer JH. Imaging the serotonin transporter throughout main depressive dysfunction and antidepressant remedy. J Psychiatry Neurosci. 2007 Mar;32(2):86–102.

Jauhar, S., Arnone, D., Baldwin, D.S. et al. (2023) A leaky umbrella has little worth: proof clearly signifies the serotonin system is implicated in melancholy. Molecular Psychiatry 2023.

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