Summary: Researchers explain why the serotonin hypothesis for depression may not be as accurate as previously believed.
Source: The Conversation
For decades, the public has been told that depression stems from a “chemical imbalance” in the brain—most commonly, a deficit of the neurotransmitter serotonin. Our recent umbrella review of the evidence finds that this explanation is not supported by the available scientific research.
The idea that low serotonin causes depression dates back to the 1960s but gained widespread traction in the 1990s alongside aggressive promotion of selective serotonin reuptake inhibitors (SSRIs). Professional bodies, clinicians and media outlets propagated the chemical imbalance story, and many people accepted it as fact. During this period antidepressant prescriptions rose sharply; for example, they are now prescribed to about one in six adults in England.
Some academics and clinicians have long questioned the serotonin hypothesis, arguing that the evidence is weak or inconsistent. Until now, however, there had not been a single comprehensive review that brought together findings from all major strands of serotonin research to assess the hypothesis as a whole.
At first glance, the fact that SSRIs interact with the serotonin system might seem to support the serotonin-deficit idea. SSRIs increase serotonin availability in the short term, but this pharmacological effect does not automatically mean that depression is caused by the opposite state. Antidepressant effects are complex and may arise through multiple mechanisms, and clinical trials show that the benefit of antidepressants over placebo is often small. Some antidepressants also produce a general blunting of emotion, which could influence mood independently of correcting any biochemical deficit.
First comprehensive review
We carried out an umbrella review: a systematic synthesis of previous systematic reviews and meta-analyses across the various areas that study serotonin and depression. This approach allowed us to integrate evidence from multiple lines of inquiry—biochemical measures, receptor and transporter studies, experimental depletion studies, and genetic investigations—rather than considering each domain in isolation.
Studies measuring serotonin or its breakdown products in blood or cerebrospinal fluid generally failed to show consistent differences between people with depression and those without. Research examining serotonin receptors produced mixed results: many studies found no meaningful difference, while some reported increased receptor activity in people with depression—contrary to the idea of reduced serotonin function.
Investigations of the serotonin transporter—the protein targeted by SSRIs—also did not provide clear support for a deficit model. Where differences were reported, they could often be explained by prior or current antidepressant use among study participants, which alters serotonin system measures.
Experimental studies that temporarily lower serotonin levels in healthy volunteers have been used to test whether reduced serotonin can trigger depressive symptoms. Multiple systematic reviews and a range of individual studies show that lowering serotonin does not reliably produce depression in healthy people. There is tentative evidence for a small effect in a very limited subgroup with a family history of depression, but that finding is based on small samples and remains uncertain.
Large-scale genetic studies have examined variation in genes related to the serotonin system, including the gene encoding the serotonin transporter. These well-powered investigations have not found consistent differences in the frequency of serotonin-related gene variants between people with depression and healthy controls. Earlier, smaller studies that claimed interactions between transporter gene variants and life stress have not been confirmed by larger, more comprehensive analyses. Stressful life events themselves, however, remain a robust risk factor for developing depression.
Some studies that included participants who were taking or had previously taken antidepressants found evidence that those medications can alter serotonin measures, sometimes lowering concentrations or activity. This complicates interpretation of many clinical studies, since prior drug exposure can change the very biological markers under investigation.
Not supported by the evidence
Taken together, the breadth of evidence does not substantiate the simple serotonin-deficit explanation for depression. The serotonin theory has been one of the most influential biological models of depression, but our synthesis finds that it is not adequately supported by current scientific data. This raises important questions about how antidepressants are understood and prescribed.
Most commonly used antidepressants are thought to act on serotonin, and some also affect noradrenaline. Evidence for noradrenaline’s role in depression is generally weaker than the claims historically made about serotonin. Because there is no widely accepted alternative pharmacological mechanism that fully explains antidepressant benefits, it remains possible that much of their apparent clinical effect reflects placebo responses, emotion blunting, or other non-specific factors.
Framing depression primarily as a biochemical brain disorder was often promoted as a way to reduce stigma. Yet research indicates this biological framing can increase pessimism about recovery and may not reduce stigma in practice. People who believe their depression is due to a fixed chemical imbalance tend to feel less hopeful about getting better.
It is important to communicate that the chemical imbalance model is a hypothesis not proven by the evidence. We still do not understand precisely what temporary increases in serotonin caused by drugs do to the brain, or whether those changes are central to recovery. Given the uncertainties about mechanisms and benefits, it is not possible on current evidence to state unequivocally that taking SSRI antidepressants is always worthwhile or entirely safe.
If you are taking antidepressants, do not stop them without consulting your prescribing clinician. People should be provided with accurate, balanced information so they can make informed decisions about the potential benefits and risks of these medications.
About this depression research news
Author: Joanna Moncrieff and Mark Horowitz
Source: The Conversation
Contact: Joanna Moncrieff and Mark Horowitz – The Conversation
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