The language of serotonin

Or, “What are they saying?

When we mention the word Serotonin (5-hydroxytryptamine, 5-HT), we immediately think of the brain and the Central Nervous System (CNS). People tend to associate serotonin to depression, or mood, or feelings of well-being1

Although that is correct, truth be told, the majority of the serotonin in the human body is actually produced in the gut. In fact, 95% of total serotonin is manufactured by the Enterochromaffin cells (or, Kulchitsky cells) in the gastro-intestinal tract (GI)2,3. These cells live next to the gut epithelium, that covers the cavity of the GI tract, playing a crucial role in the regulation of bowel movements and secretions. If you think that the gut is almost 9 meters (or 30 feet) long, then that’s a lot of cells producing serotonin. 

When in the 50’s, Betty M. Twarog and Irvine H. Page discovered that the brain produced its own serotonin4; then, the gut-made serotonin got reduced to its “Aschenputtel” origins, and relinquished to the favela quarters of the body. As such, brain-derived serotonin always got more attention than its gut-derived counterpart – like a rich vs. poor-cousin type of reputation.


Platelets, also called thrombocytes, are small un-nucleated fragment of cells that, when activated, form blood clots (thrombus) and prevent bleeding. 

Electron microscopy images of circulating platelets, extracted from Zilla et al, 19875

Platelets do not make serotonin, butcan take it up as they circulate through the gut, and carry it along the blood stream6,7. As such, the serotonin produced in the intestine can be carried all over the body. As the chemical messenger serotonin is, it can influence any other cell, in whatever other location, as long as it has a serotonin receptor on it. As such, peripheral serotonin has now discovered its path back into the limelight, and recent research has strengthened the influence that gut-made serotonin has in other parts of the body, functioning as an intestinal-derived hormone. 

Once again, the “Aschenputtel” story comes into mind, but this time through its “Cinderella” version. Let’s take a look…

For example, gut-derived serotonin can directly regulate the liver and mediate liver regeneration8. In Non-Alcoholic Fatty Liver Disease (NAFLD), a group of conditions that are characterized by excessive fat accumulation in the liver and closely track the global public health problem of obesity, researchers showed that inhibiting gut-derived serotonin synthesis could resolve hepatic fat accumulation8,9.

Peripheral serotonin can also be a negative regulator of bone density, by specifically inhibiting osteoblast formation and leading to osteoporosis10 – a common feature in patients with inflammatory bowel disease (IBD). This happens through the action of a common receptor: the low-density Lipoprotein Receptor-related Protein 5(LRP5), which is expressed in both osteoblasts and enterochromaffin cells11. LRP5 inhibits the expression of an important ingredient for serotonin production (Tryptophan hydroxylase-1, Tph1); as such, when LRP5 is deficient or inactivated due to inflammation or disease, blood levels of serotonin are elevated decreasing osteoblast formation; and, consequently, reducing bone mass1,11.

Epidemiologic data suggests a role of serotonin, or Selective Serotonin-Reuptake Inhibitors (typically used as antidepressants, SSRIs) in the development of venous thrombosis12. In fact, patients with depression were reported to have higher incidences of venous thromboembolism in general13; and, the use of SSRIs is associated with an increased venous thromboembolism risk14. No wonder, serotonin and platelets are “brothers in arms”, ready to block any blood vessel along their way…. 

Serotonin and its receptors are also present in the immune system, where evidence suggests it contributes to both innate and adaptive responses. There is now clear evidence of a straight communication between the immune system, the gut and the brain via serotonin15,16.

On top of all and because we are not alone, our gut microbiota plays a critical role in regulating our colonic serotonin. Indigenous spore-forming bacteria (Sp) promote serotonin biosynthesis in our enterochromaffin cells, and with that they can significantly modulate GI movements and platelet function – together with many aspects of our physiology17,18. We now know that the microbiota colonizes the GI tract after birth, with a continuous maturation during the first years of life19. Researchers have now showed in animal models that this developing gut microbiota regulates the development of the adult enteric nervous system via intestinal serotonin networks20. What this actually means, is that the actions of our intestinal bugs during the beginning of our life are determinant for the development of our “gut brain”, our second brain. How about that?…

If we ruminate about it, when we “think” with our gut, we are actually listening to our bugs. By directly signalling our cells to produce serotonin and develop a network of neurons as soon as we are born, our gut-bugs are actually finding a way to communicate with us – the host – in the serotonin language. 

Now, we just need to understand what are they telling us… 

Beethoven’s hearing aids, Beethoven House Museum, Bonn.


1          Gershon, M. D. 5-Hydroxytryptamine (serotonin) in the gastrointestinal tract. Curr Opin Endocrinol Diabetes Obes 20, 14-21, doi:10.1097/MED.0b013e32835bc703 (2013).

2          Bellono, N. W. et al. Enterochromaffin Cells Are Gut Chemosensors that Couple to Sensory Neural Pathways. Cell 170, 185-198.e116, doi:10.1016/j.cell.2017.05.034 (2017).

3          Yaghoubfar, R. et al. Modulation of serotonin signaling/metabolism by Akkermansia muciniphila and its extracellular vesicles through the gut-brain axis in mice. Scientific Reports 10, 22119, doi:10.1038/s41598-020-79171-8 (2020).

4          Twarog, B. M. & Page, I. H. Serotonin Content of Some Mammalian Tissues and Urine and a Method for Its Determination. American Journal of Physiology-Legacy Content 175, 157-161, doi:10.1152/ajplegacy.1953.175.1.157 (1953).

5          Zilla, P. et al. Scanning electron microscopy of circulating platelets reveals new aspects of platelet alteration during cardiopulmonary bypass operations. Tex Heart Inst J 14, 13-21 (1987).

6          Morrissey, J. J., Walker, M. N. & Lovenberg, W. The absence of tryptophan hydroxylase activity in blood platelets. Proc Soc Exp Biol Med 154, 496-499, doi:10.3181/00379727-154-39702 (1977).

7          Hughes, F. B. & Brodie, B. B. The mechanism of serotonin and catecholamine uptake by platelets. J Pharmacol Exp Ther 127, 96-102 (1959).

8          Wang, L. et al. Gut-Derived Serotonin Contributes to the Progression of Non-Alcoholic Steatohepatitis via the Liver HTR2A/PPARγ2 Pathway. Frontiers in Pharmacology 11, doi:10.3389/fphar.2020.00553 (2020).

9          Choi, W. et al. Serotonin signals through a gut-liver axis to regulate hepatic steatosis. Nature Communications 9, 4824, doi:10.1038/s41467-018-07287-7 (2018).

10        Lavoie, B. et al. Gut-derived serotonin contributes to bone deficits in colitis. Pharmacol Res 140, 75-84, doi:10.1016/j.phrs.2018.07.018 (2019).

11        Yadav, V. K. et al. Lrp5 controls bone formation by inhibiting serotonin synthesis in the duodenum. Cell 135, 825-837, doi:10.1016/j.cell.2008.09.059 (2008).

12        Rieder, M., Gauchel, N., Bode, C. & Duerschmied, D. Serotonin: a platelet hormone modulating cardiovascular disease. J Thromb Thrombolysis 52, 42-47, doi:10.1007/s11239-020-02331-0 (2021).

13        Takeshima, M. et al. Prevalence of Asymptomatic Venous Thromboembolism in Depressive Inpatients. Neuropsychiatr Dis Treat16, 579-587, doi:10.2147/NDT.S243308 (2020).

14        Parkin, L. et al. Antidepressants, Depression, and Venous Thromboembolism Risk: Large Prospective Study of UK Women. J Am Heart Assoc 6, doi:10.1161/jaha.116.005316 (2017).

15        Baganz, N. L. & Blakely, R. D. A dialogue between the immune system and brain, spoken in the language of serotonin. ACS Chem Neurosci 4, 48-63, doi:10.1021/cn300186b (2013).

16        Jacobson, A., Yang, D., Vella, M. & Chiu, I. M. The intestinal neuro-immune axis: crosstalk between neurons, immune cells, and microbes. Mucosal Immunology 14, 555-565, doi:10.1038/s41385-020-00368-1 (2021).

17        Yano, J. M. et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell 161, 264-276, doi:10.1016/j.cell.2015.02.047 (2015).

18        Reigstad, C. S. et al. Gut microbes promote colonic serotonin production through an effect of short-chain fatty acids on enterochromaffin cells. Faseb j 29, 1395-1403, doi:10.1096/fj.14-259598 (2015).

19        Bäckhed, F. et al. Dynamics and Stabilization of the Human Gut Microbiome during the First Year of Life. Cell Host Microbe 17, 690-703, doi:10.1016/j.chom.2015.04.004 (2015).

20        De Vadder, F. et al. Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks. Proc Natl Acad Sci U S A 115, 6458-6463, doi:10.1073/pnas.1720017115 (2018).

Clogged Synapses: better call the mailman (or have a shut eye)

Sleep is fundamental for our brain.

Our ability to learn and memorize new things profits from sleep; and, sleep loss leads to cognitive impairment that can only be reversed by closing our eyes and sleeping1. The more time we spend awake, and the further we engage with learning activities, the more our brains will demand for sleep. 

The synapse is the structure that allows the neuron (or nerve cell) to pass an electrical or chemical signal to another neuron, or to the end effector cell that produces the action demanded from our brain. Synapses are the foundation of neuronal plasticity, and, in the adult brain, synapses can change their strength and size within minutes or hours in response to a new experience and learning1. Recent research has shown that the need for sleep and synaptic function are strongly linked together.

Sarah B. Noya and her colleagues2 from the Institute of Pharmacology and Toxicology of the University of Zürich (Switzerland) have recently shown that 70% of the synaptic transcripts change during our 24h circadian cycles. The transcripts and proteins related to synaptic signaling, accumulate before the active phase of the bodies and get further cleared out during the day.  In the meantime, proteins that are associated with the body metabolism and translation, accumulate in the synapses just before the resting phase or sleeping time. As such, just before we go to bed, the synapses get congested with protein information from our bodies daily function, that needs to be compartmentalized and processed.

We can imagine the synapses as a clerk’s room filling up with boxes and parcels that need to be deliver to the proper address.

But what is interesting, is the result that comes from another study published at the same time, from Franziska Brüning and her team3 at the Institute of Medical Psychology of the Ludwig Maximilian University of Munich (Germany). This research study shows that sleep deprivation abolishes nearly all of the compartmentalization of these accumulating proteins at the synapses (98%); which means that, without a proper shut eye, the synapses get completely clogged with accumulating “protein-parcels” that don’t get removed. When a chemical or electrical information wants to get through the synapses the next day, it can’t because there’s accumulating protein transcripts that haven’t been properly processed, or phosphorylated. The information gets stalled, due to the congestion at the synapses.

So, next time your brain feels fizzle in the morning, and throughout the day; promise yourself (and your brain) to go to bed early, and have a proper night’s rest!



1.         Cirelli C and Tononi G. Linking the need to sleep with synaptic function. Science. 2019;366:189-190.

2.         Noya SB, Colameo D, Bruning F, Spinnler A, Mircsof D, Opitz L, Mann M, Tyagarajan SK, Robles MS and Brown SA. The forebrain synaptic transcriptome is organized by clocks but its proteome is driven by sleep. Science. 2019;366.

3.         Bruning F, Noya SB, Bange T, Koutsouli S, Rudolph JD, Tyagarajan SK, Cox J, Mann M, Brown SA and Robles MS. Sleep-wake cycles drive daily dynamics of synaptic phosphorylation. Science. 2019;366.

Habenula, the Master of the Brain

The Habenula, is an area of our brains close to the pineal gland, that is involved in pain processing, reproductive behaviour, nutrition, sleep-wake cycles and stress responses, among other things1. A professor I used to know always said the Habenula was the Master of the Brain… and, indeed, recent research has provided evidence that this tiny bundle of nerves is able to produce Dimethyltryptamine (DMT), a psychedelic drug, “cousin” to the famous LySergic acid Diethylamide (LSD). 

DMT is internally bio-synthesized by the enzymes Aromatic-L-Amino acid DeCarboxylase (AADC) and Indolethylamine-N-Methyltransferase (INMT). Dean and colleagues2 were able to identify INMT messenger RNA in human tissues, by using a RNAscope in situ assay system; a highly sensitive technique, which proved for the first time a clear-cut identification of DMT and its enzymes in human brain. 

Outstanding, was the discovery that there was a significant increase of DMT levels in the rat brain after stimulation of experimental cardiac arrest; showing for the first time, that the brain is capable of synthesizing and releasing DMT under stress. 

This ultimately raises the possibility that this phenomenon may also occur in human brains, when we experience situations of extreme stress. The researchers attest that the cardiac arrest-induced increase of DMT may be related to “near-death experiences”, as reported by Timmermann and collegues3. This group recently reported that human subjects given exogenous DMT, experienced “near-death”-like mental states, including the subjective feeling of transcending one’s body and entering an alternative realm, perceiving and communicating with ‘entities’, and themes related to death and dying.

It’s unbelievable that the more we know about how our body and brain functions, the more I realize that our mind is a construction of our organic biological nature.

What we sometimes perceive as a mystical experience is probably just rooted in an organic mechanism that is tricking our minds into a “trip”.

Into the light


1.         Namboodiri VM, Rodriguez-Romaguera J and Stuber GD. The habenula. Curr Biol. 2016;26:R873-R877.

2.         Jon G. Dean TL, Sean Huff, Ben Sheler, Steven A. Barker, Rick J. Strassman, Michael M. Wang & Jimo Borjigin Biosynthesis and Extracellular Concentrations of N,N-dimethyltryptamine (DMT) in Mammalian Brain. Scientific Reports. 2019;9.

3.         Timmermann C, Roseman L, Williams L, Erritzoe D, Martial C, Cassol H, Laureys S, Nutt D and Carhart-Harris R. DMT Models the Near-Death Experience. Front Psychol. 2018;9:1424.