2023-10-27 07:24:30
Study points to serotonin deficiency as a possible reason
by Dr. Bettina Albers
(October 27, 2023) The cause and development of long-term effects of a COVID-19 infection have not yet been clarified. A current study has now demonstrated reduced serotonin levels in the blood when symptoms persist following COVID-19, which leads to disorders of neurovegetative functions and might therefore explain some of the core symptoms of long/post COVID.
The study provides a plausible hypothesis that links various previously suspected pathomechanisms. It might possibly also explain the development of other post-viral syndromes. After a COVID-19 illness, persistent symptoms can occur (long or post COVID, also called PASC/“post-acute sequelae of COVID”). It is a sometimes serious post-viral syndrome, the cause of which is still unclear.
Common clinical symptoms include fatigue, cognitive impairment, headaches, sleep and anxiety disorders. The severity of the acute infection and psychosocial factors obviously play a role in the development of PASC. In addition, four hypotheses for the pathogenesis are discussed: virus persistence, chronic inflammatory processes, hypercoagulability and autonomic dysfunction (ie disorders in the autonomic nervous system).
A new publication in the renowned journal “Cell” has now traced these four hypotheses to a single, newly postulated pathomechanism using clinical and animal experiments. In studies on mice that were infected with SARS-CoV-2 or in which a corresponding inflammatory reaction was chemically triggered, the study group showed that the SARS-CoV-2 infection is associated with a reduced serotonin concentration in the blood. The researchers then analyzed data from a total of 1,540 PASC patients from different cohorts and came to the conclusion that those with PASC can also have a reduced serotonin concentration in the blood.
However, the degree of serotonin reduction varied in the PASC patient cohorts examined and was not detectable at all in some. Overall, however, the author team concluded that acute COVID-19 disease often leads to a reduction in serotonin levels, which persists in severe PASC.
How can the drop in serotonin levels be explained? Apparently, viral infection results in reduced intestinal absorption of the serotonin precursor tryptophan, an essential amino acid from food, as well as platelet overactivation with thrombocytopenia, which reduces endogenous serotonin stores, and also increased MAO enzyme-mediated serotonin turnover. It has also been shown that viral RNA-induced type I interferons increase serotonin deficiency by, among other things, reducing tryptophan absorption in the intestine.
“The resulting peripheral serotonin deficiency might be a possible explanation for some PASC symptoms, because it impairs the signal transmission of the vagus nerve and thus the activity of the autonomic nervous system as well as functions of the hippocampus,” explains Prof. Dr. Peter Berlit, Secretary General and Press Spokesman of the German Society for Neurology (DGN). “However, the pathomechanisms of PACS described in the study must now first be validated in prospective follow-up studies with control groups.”
Both the study group itself and the author of a discussion published in “Science” point to limitations of the study: the biggest weakness is that the reduction in serotonin levels might not be consistently demonstrated in all cohorts. And in animal experiments, the serotonin deficiency was found in the blood, but not in the brains of the mice. The described “link” between PASC and enteral tryptophan uptake and thrombocytopenia following a SARS-CoV-2 infection is ultimately just a hypothesis.
“First of all, the study provides a new possible explanation for long-COVID symptoms that needs to be further researched,” says Prof. Berlit. According to the expert, if the results are confirmed, they might also be significant beyond SARS-COV-2: reduced serotonin levels are not COVID-19-specific, but are also known from other viral diseases that can also trigger post-viral syndromes, such as the dengue fever. “It is therefore important that COVID-19 and PASC continue to be researched, even if the pandemic is now considered to be largely over.” – Idw.
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