Serotonin and depression

Leo Sher, M.D.

A review article, “The serotonin theory of depression: a systematic umbrella review of the evidence” was published in July 2022, in Molecular Psychiatry online ahead of print (1). The abstract of this review article is below:

“The serotonin hypothesis of depression is still influential. We aimed to synthesise and evaluate evidence on whether depression is associated with lowered serotonin concentration or activity in a systematic umbrella review of the principal relevant areas of research. PubMed, EMBASE and PsycINFO were searched using terms appropriate to each area of research, from their inception until December 2020. Systematic reviews, meta-analyses and large data-set analyses in the following areas were identified: serotonin and serotonin metabolite, 5-HIAA, concentrations in body fluids; serotonin 5-HT1A receptor binding; serotonin transporter (SERT) levels measured by imaging or at post-mortem; tryptophan depletion studies; SERT gene associations and SERT gene-environment interactions. Studies of depression associated with physical conditions and specific subtypes of depression (e.g. bipolar depression) were excluded. Two independent reviewers extracted the data and assessed the quality of included studies using the AMSTAR-2, an adapted AMSTAR-2, or the STREGA for a large genetic study. The certainty of study results was assessed using a modified version of the GRADE. We did not synthesise results of individual meta-analyses because they included overlapping studies. The review was registered with PROSPERO (CRD42020207203). 17 studies were included: 12 systematic reviews and meta-analyses, 1 collaborative meta-analysis, 1 meta-analysis of large cohort studies, 1 systematic review and narrative synthesis, 1 genetic association study and 1 umbrella review. Quality of reviews was variable with some genetic studies of high quality. Two meta-analyses of overlapping studies examining the serotonin metabolite, 5-HIAA, showed no association with depression (largest n = 1002). One meta-analysis of cohort studies of plasma serotonin showed no relationship with depression, and evidence that lowered serotonin concentration was associated with antidepressant use (n = 1869). Two meta-analyses of overlapping studies examining the 5-HT1A receptor (largest n = 561), and three meta-analyses of overlapping studies examining SERT binding (largest n = 1845) showed weak and inconsistent evidence of reduced binding in some areas, which would be consistent with increased synaptic availability of serotonin in people with depression, if this was the original, causal abnormaly. However, effects of prior antidepressant use were not reliably excluded. One meta-analysis of tryptophan depletion studies found no effect in most healthy volunteers (n = 566), but weak evidence of an effect in those with a family history of depression (n = 75). Another systematic review (n = 342) and a sample of ten subsequent studies (n = 407) found no effect in volunteers. No systematic review of tryptophan depletion studies has been performed since 2007. The two largest and highest quality studies of the SERT gene, one genetic association study (n = 115,257) and one collaborative meta-analysis (n = 43,165), revealed no evidence of an association with depression, or of an interaction between genotype, stress and depression. The main areas of serotonin research 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. Some evidence was consistent with the possibility that long-term antidepressant use reduces serotonin concentration.”

In response to the Molecular Psychiatry review (1), Prof. Zoltan Rihmer and associates published a paper, “Serotonin and depression – a riposte to Moncrieff et al. (2022)” (2). The abstract of this debate paper is below:

“In their recently published systematic “umbrella” review, Moncrieff and colleagues conclude that there is no consistent evidence that depression is caused by decreased serotonin activity in the central nervous system (CNS). However, this paper – which was extensively publicized and received a lot of attention on the social media – can cause misunderstandings, since the serotonin hypothesis of depression in its original form (i.e. reduced serotonin activity in the CNS = depression) formulated more than 50 years ago has been considered outdated for several decades. It has long been known that depression is a heterogeneous disorder not only genetically, clinically and biologically but also from a pharmacotherapeutic perspective. The decreased activity of serotonin, which undoubtedly plays an essential role in the pathogenesis of depression, is characteristic of only a subgroup of depressed subjects whose clinical picture is mostly dominated by intensified negative emotions, agitation, anxiety, insomnia, decreased appetite, self-blame and suicidality and these individuals are primarily responsive to SSRIs. By contrast, depression cases with reduced positive affects (characterized by anhedonia, anergia, inhibition and reduced cognitive functions) are mainly caused by a disturbance in the metabolism of dopamine and/or noradrenaline. These patients are primarily responsive to dual-action (e.g. SNRI) antidepressants. Results of serotonin and catecholamine (dopamine, noradrenaline) depletion studies also suggest that that the dysregulation of serotonin and dopamine/noradrenaline in the CNS is characteristic of different subgroups of depressed patients. In addition to the serotonergic, dopaminergic and noradrenergic systems, many other neurotransmitter systems (e.g. cholinergic, glutamatergic, GABAergic) and other mechanisms (e.g. neuroinfl ammation) have also been proven to play a role in the development of the disorder. Knowledge of the data presented in our publication is important since the simplistic interpretation by Moncrieffetal. of the role of serotonin in the pathogenesis of depression may undermine confidence in SSRIs in many patients.”


  1. Moncrieff J, Cooper RE, Stockmann T, Amendola S, Hengartner MP, Horowitz MA. The serotonin theory of depression: a systematic umbrella review of the evidence. Mol Psychiatry. 2022 Jul 20. doi: 10.1038/s41380-022-01661-0. Epub ahead of print.
  2. Rihmer Z, Döme P, Katona C. Serotonin and depression – a riposte to Moncrieff et al. (2022). Neuropsychopharmacol Hung. 2022 Sep 1;24(3):120-125.