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References: Chemical Messengers and the Serotonin Myth

  1. Chapter 3: Synapses and Neurotransmitters: From Neuron to Bedside (book #1 in this series). A free refresher on how synapses work and on neurotransmitters such as serotonin, dopamine, GABA, and acetylcholine. Read it if Section 5.3 moved too fast.

  2. Does serotonin in the intestines make you happy?: Bektaş A, et al. Turkish Journal of Gastroenterology, 2020. A short, readable editorial explaining why gut serotonin can't reach the brain and why the "banana makes you happy" idea fails. The single best source for this chapter's main point.

  3. Serotonin signalling in the gut: functions, dysfunctions and therapeutic targets: Mawe GM, Hoffman JM. Nature Reviews Gastroenterology & Hepatology, 2013. Free full-text review of what gut serotonin does, how SERT clears it, how platelets store it, how SSRIs affect the gut, and how drugs targeting gut serotonin are being developed.

  4. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis: Yano JM, et al. Cell, 2015. The mouse study analyzed in this chapter's "How to read a gut-brain claim" box, showing that spore-forming bacteria raise the gut's own serotonin production. Free full text with funding details.

  5. Enterochromaffin cells are gut chemosensors that couple to sensory neural pathways: Bellono NW, et al. Cell, 2017. Shows that EC cells, less than 1% of gut lining cells, sense chemicals, are electrically excitable, and connect to sensory nerves. The abstract is a good summary of the "gut as sensor" idea.

  6. Influence of tryptophan and serotonin on mood and cognition with a possible role of the gut-brain axis: Jenkins TA, et al. Nutrients, 2016. Open-access review explaining tryptophan as an essential amino acid and how it competes with other amino acids to cross the blood-brain barrier.

  7. Fluoxetine: MedlinePlus, US National Library of Medicine. Plain-language drug information for a common SSRI, including how it works, its important warning about suicidal thoughts in young people, common side effects such as nausea and diarrhea, and why it shouldn't be stopped without a doctor.

  8. The serotonin theory of depression: a systematic umbrella review of the evidence: Moncrieff J, et al. Molecular Psychiatry, 2023 (online 2022). The much-debated review behind Section 5.10. The PubMed page also lists the published comments and critiques, a good way to see a scientific debate in action.

  9. GABA-modulating bacteria of the human gut microbiota: Strandwitz P, et al. Nature Microbiology, 2019. Lab and stool-sample evidence that common gut bacteria such as Bacteroides make GABA. Useful for separating "bacteria make it" from "it reaches your brain."

  10. The neuroactive potential of the human gut microbiota in quality of life and depression: Valles-Colomer M, et al. Nature Microbiology, 2019. A large human study (1,054 people plus validation data sets) linking certain gut bacteria to quality of life and depression, with a clear warning about confounding factors.

  11. The blood-brain barrier: Daneman R, Prat A. Cold Spring Harbor Perspectives in Biology, 2015. Review describing how the brain's blood vessel cells control what passes from blood into the brain. Background for Section 5.7 and Chapter 6.

  12. A leaky umbrella has little value: evidence clearly indicates the serotonin system is implicated in depression: Jauhar S, et al. Molecular Psychiatry, 2023. Free full-text reply to the 2022 umbrella review, written by 35 psychiatrists and brain scientists. Read it next to reference 8 to see both sides of the debate in Section 5.10.