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'The crudest form of that is "Depression is caused by lack of serotonin", which makes it sound like a vitamin deficiency that can be cured by dietary supplements. A more realistic version is "Depression is a disorder of serotonin-based signaling in particular brain regions", but that still leaves an awful lot of hand-waving to be done.'
https://www.science.org/content/blog-post/serotonin-hypothesis-lives
@cyrilpedia
I'm not sure depression is caused by serotonin dysfunction at all. Tissues that have been subject to chronic inflammation tend to be low in serotonin, but attempting to correct that by targeting serotonin directly isn't going to address the underlying cause. I keep the Serotonin Hypothesis in the same drawer with the Amyloid Hypothesis, Blue Light Hypothesis, and unmatched socks.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9706857/#:~:text=Dysregulated%20complement%20system%20is%20linked,putative%20mechanism%20of%20immune%20activation.