Serotonin, a well-known neurotransmitter which has a significant role to play in mood disorders, migraine disorders, irritable bowel syndrome and pulmonary and systemic hypertension(1), is commonly associated with the brain, where it does most of its work.
But new research indicates that specific bacteria in the gut are responsible for producing an estimated 90 percent of the body’s serotonin(2).
Increasing numbers of studies using mice have demonstrated that changes in the microbes living in the gut can result in altered behaviours. Researchers are especially interested in the ways in which gut microbes communicate with the nervous system.
The Importance Of Serotonin
Researchers focus on serotonin for a reason – in the brain, this chemical plays a significant role in the pathophysiology of depression, one of the most common mental illnesses plaguing many people worldwide(3).
Much research has been done on how to manage serotonin levels in the human brain, with multiple classes of drugs available to do just that.
However, with so much of the chemical being manufactured in the gut, it’s only natural that serotonin would be a solid starting point for researchers looking to explore the ways our guts and brains are interconnected.
The Mouse Model
Researchers on the new study measured peripheral serotonin levels in mice with small populations of gut bacteria, while simultaneously measuring and comparing peripheral serotonin levels in “germ-free” mice.
The germ-free mice, they found, produced significantly less serotonin than the mice with more “normal” levels of gut bacteria.
Not stopping there, researchers then recolonized the germ-free mice with normal gut microbes, which resulted in the mice’s serotonin levels rising again – showing that serotonin deficits may be reversed with specific types of gut bacteria.
“Our work demonstrates that microbes normally present in the gut stimulate host intestinal cells to produce serotonin,” says Jessica Yano, first author on the paper and a research technician working with Hsiao.
Exploring The Connections Between Gut And Brain
The idea that the bacteria that live in our guts affects the complex chemistry of our brains is a relatively new one, having entered the scene several years ago. Most of the research on this gut-brain connection to date has been done on mice and other small organisms, not humans.
“While the connections between the microbiome and the immune and metabolic systems are well appreciated, research into the role gut microbes play in shaping the nervous system is an exciting frontier in the biological sciences,” says Sarkis K. Mazmanian, a professor of microbiology and a coauthor on the study.
Many feel that this may lead to discovering new therapies for mood and anxiety disorders, as well as for gastrointestinal diseases.
The connection between the brain and the gut appears to be a two-way street, though; according to the American Psychological Association,
“just as the gut bacteria affect the brain, the brain can also exert profound influences on the gut microbiome – with feedback effects on behaviour”(4).
While finding therapeutic uses for this research in humans is a way off yet, many scientists and doctors are excited about what these findings represent – perhaps a future method of treating neurological disorders by adjusting the levels of bacteria in the gut? Only time – and more research – will tell.