They say if an idea or a theory is repeated long enough, people will start to believe it and accept it as fact.
This seems to be especially true of the hypothesis from the 1960’s that depression is caused by a chemical imbalance in the brain, due to low levels of serotonin. Doctors at the time used mood enhancing medications, which they believed worked by increasing serotonin levels in the brain. This idea stuck as a simple explanation for depression.
This consequently resulted in the widespread use of antidepressants, boosted in the 1990’s with the development of the selective serotonin reuptake inhibitor (SSRI) type of antidepressants, such as Prozac, Lexapro, and Zoloft. Prescriptions for antidepressants have risen dramatically since the 1990’s, with one in six adults in the UK and one in eight adults in the USA being prescribed antidepressants. (These figures are pre-COVID-19 and may currently be conservative.)
Surveys show that more than 80% of the public believe that depression was caused by a chemical imbalance caused by low concentrations of serotonin, which influence their decisions whether to take or continue taking antidepressant medication, while discouraging people from discontinuing treatment. This belief shapes how people understand their moods.
This serotonin hypothesis, reigning supreme as scientific fact, was turned on its head in 2022.
A major research project analyzed serotonin studies:
In the UK, a systematic umbrella review of all relevant studies was done by the Critical and Social Psychiatry Department at University College London (UCL). The study gathered evidence from 361 scientific studies and analyzed antidepressant clinical trial data to look for scientific evidence that low serotonin levels in the brain are responsible for symptoms of depression.
Findings:
According to the researchers at UCL, their comprehensive review of the major strands of research on serotonin shows “there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity”. Most studies found no evidence of reduced serotonin activity in people with depression, compared to people without depression. Serotonin studies have not produced convincing evidence of a biochemical basis to depression.
“We suggest it is time to acknowledge that the serotonin theory of depression is not empirically substantiated”, concluded the researchers at UCL.
Some scientists are of the opinion that the primary effect of the selective serotonin reuptake inhibitor (SSRI) type of antidepressant is to superimpose an abnormal drug state over the symptoms and the small benefits seen in some trials are due to the emotional numbing caused by the medication.
Studies also indicate the possibility that low concentrations of serotonin is associated with long-term use of antidepressants, which shows the converse effect of the serotonin theory.
The importance of serotonin in the body:
Serotonin is a chemical which is one of dozens of neurotransmitters in the brain, each playing a specific role. Communication in the body is important to your health and neurotransmitters are key communicators. In the brain, for example, they relay instructions from one brain cell to the next and they play an active role in transferring information throughout the brain and the body.
As a neurotransmitter, serotonin plays an important role in modulating virtually all the behavioral and neuropsychological processes in the brain, such as mood, emotion, attention, consciousness, perception, reward, anger, aggression, appetite, memory, sexuality, and cognition. Serotonin is also involved in the functioning of all major organ systems in the body, such as the cardiovascular, pulmonary, gastrointestinal, urogenital, and the central nervous system.
Serotonin also does duty as a hormone, which is involved with numerous physiological processes such as sleep, the movement of food through the gastrointestinal tract, eating behavior, blood clotting, and sexual function.
Although serotonin plays a crucial role in the brain, at least 90% of the serotonin in the body is made and used by cells in the digestive tract. Even gut bacteria are important in the production of serotonin, as particular gut bacteria stimulate intestinal cells to produce serotonin. Serotonin is released from the cells lining the gastrointestinal tract into the blood circulation and absorbed by platelets. Only about 10% is produced in the brain.
One of the most common side effects of long-term use of these drugs that change the brain chemistry, is that people get dependent on these drugs. People find it very difficult to stop taking these drugs, with adverse effects linked to the withdrawal process.
Conclusions:
While there is a lack of convincing evidence that depression relates to low levels of serotonin – turning the serotonin theory on its head – the research team at UCL say that further research is needed to clarify the effects of different drugs on neurochemical systems, including the serotonin system, during the use of the drugs as well as over long-term use. The physical and psychological consequences of such effects should also be determined.
Another scientist at UCL, Dr Michael Bloomfield, warns that depression has different symptoms and many scientists do not think that all causes of depression are caused by a simple chemical imbalance in serotonin. The problem he sees with the UCL study on the link between serotonin and depression, is that it has lumped together depression as if it is a single disorder, which does not make sense from a biological perspective.
One of the established causes of depression is having a genetic predisposition, where distressing life events can trigger the onset of depression. Negative life events impact depression, and stress is a known trigger.
The latest theories have moved on from explanations of single neurotransmitters such as serotonin and are looking at the key roles played by regions in the brain such as the amygdala. One such study has shown that people with major depressive disorder displayed abnormally decreased intrinsic connectivity of the amygdala, as well as exaggerated reactivity to negative emotional stimuli.
Experts say that depression is a complex condition with multiple causes.
References:
The serotonin theory of depression: a systematic umbrella review of the evidence. Published 20 July 2022. Molecular Psychiatry. (www.nature.com)
Meet Dr. Joanna Moncrief – the psychiatrist who capsized the depression, serotonin hypothesis. Published 29 July 2022. BizNews. (www.biznews.com)
Further evidence debunking the serotonin depression hypothesis. Published 8 November 2022. Biznews. (www.biznews.com)
Little evidence that chemical imbalance causes depression, UCL scientists find. Published 20 July 2022. The Guardian. (www.theguardian.com)
Study: Low serotonin not cause of depression. Published 15 August 2022. Deutche Welle. (Germany’s international broadcaster.) (www.dw.com)
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