Right now, nerve cells in your body are passing a certain neurotransmitter—serotonin—back and forth like a chemical basketball. Well-known for its mood-modulating capacity, this chemical is made both in the digestive tract and the brain. And many researchers believe we still haven’t unlocked all its secrets.

Elaine Hsiao, a researcher at University of California, Los Angeles (USA), studies serotonin in the context of the gut-brain axis—in particular, how molecule and cell activities link the gut microbiota to the brain. GMFH editors caught up with Hsiao at the Harvard Probiotics Symposium to get the latest on serotonin, the brain, and the microbes in your gut.

What’s the difference between serotonin made in the gut and serotonin made in the brain?

The serotonin made in the gut is the same structurally as the serotonin in the brain—they’re the same molecules that are made, but they’re just localized in different places and made by different cells.

In the gut, they’re made primarily by endocrine cells called enterochromaffin cells; some of the gut neurons can also make it, and some of the immune cells can potentially make it, too. Whereas in the brain, there’s a specific subtype of neurons, raphe neurons, that are known to make serotonin and supply the adult brain with it. So they’re the same molecule but made by different cell types, and as such, can elicit different localized functions.

“Our work has shown that particular species from the gut microbiome stimulate gut endocrine cells to produce serotonin,” said Hsiao

How do microbes help make gut serotonin?

Our work has shown that particular species from the gut microbiome stimulate gut endocrine cells to produce serotonin. So it’s teamwork between microbes and host cells.

As a result of stimulating gut serotonin, we observe more serotonin in the colon, and also more serotonin that is picked up by blood platelets and circulated systemically.


Is more serotonin always good?

No. As with most things, everything in moderation. Too much serotonin has been linked with particular diseases or infections and inflammation. But there are some cases where there are deficiencies in serotonin that are detrimental as well.


How does the gut-based serotonin affect brain development and behaviour?

This is a question we are actively working on.

We started this work because the majority of pathways that are theorized to link gut microbes to the brain and behaviour seem to require, at some point, something that influences neuron activity, and neurotransmitters seemed like likely candidates for that. That’s why we decided to study the neurotransmitter serotonin.


More broadly in the gut-brain axis, what are some of the ways the gut microbiota influence the brain?

Most of the evidence right now supports one pathway which is indirect, where gut microbes modulate the immune system, and there’s a neuro-immune connection.

Other proposed pathways involve microbial modulation of neuroactive molecules that enter the brain itself, or microbial modulation of peripheral neurons that extend into the brain.


Researchers know the gut and the brain are in constant communication, and the role of the gut microbiota in this communication is a hot topic of scientific study. Further work by Elaine Hsiao and others around the globe, focusing on mechanisms of gut-brain interaction, could lead to new insights about how your gut microbes influence your brain and behaviour.