Microbes in the Gut Can Affect Human Behavior

Several mechanisms allow the gut to influence the brain.

Posted Feb 10, 2020

This column was written by Eugene Rubin M.D., Ph.D., and Charles Zorumski M.D.

Over the last several years, it has become increasingly apparent that the chemical and neuronal systems connecting the gut and the brain are important modulators of human behavior. Microbes living in the gut can exert a major influence on this gut-brain relationship, and substantial progress is being made in elucidating the ways through which gut microbes can affect behavior. A paper published by Christine Fülling, Timothy Dinan, and John Cryan in the journal Neuron provides a concise review of this exciting area of science.  

Research on this topic has been facilitated by the ability to study germ-free mice, that is, mice that have been born and raised in germ-free conditions. When microbes from the gastrointestinal (GI) systems of people suffering from certain illnesses are transferred into germ-free mice, some of the symptoms of these illnesses develop in the mice. For example, germ-free mice that are inoculated with gut microbes from individuals with Parkinson’s disease develop some of the motor symptoms of Parkinson's disease. Another example of the influence of gut microbes on behavior is the finding that microbes from anxious mice can transfer anxiety traits to non-anxious mice.

How do microbes that live in the gut influence brain and behavior? One way involves chemicals released either by these microbes or by cells lining the GI system that are influenced by these microbes. These chemicals are released into the bloodstream and then cross the blood-brain barrier and influence the growth, function, and connectivity of nerve cells.

Some of these chemicals also influence cells involved in the brain’s immune system. In addition, it has been known for a long time that certain chemicals synthesized in the gut are also made by cells in the brain (examples include somatostatin and cholecystokinin, among others) and that these brain-synthesized gut chemicals powerfully modulate brain function.

Another method that GI microbes use to influence brain structure and function involves the vagus nerve. The vagus nerve is a major parasympathetic nerve that is the conduit for communication between many organs in the body, including the GI system and the brain. This nerve is a two-way highway, meaning that signals are sent from the gut to the brain as well as from the brain back to the gut.

Chemicals released from microbes and cells lining the gut can directly lead to changes in the electrical activity of the vagus nerve and brain regions innervated by the vagus nerve. The activity of the vagus nerve is well known to influence human behavior. In fact, electrical devices implanted to stimulate this nerve (vagus nerve stimulators) can be effective in treating individuals who suffer from severe, treatment-resistant depression and epilepsy.

Over the last decade, it has become increasingly evident that chemicals synthesized in the gut can have substantial effects on how we feel and how we act. It is amazing that the bacteria and other microbes living in the gut can influence humans in these ways. As more is learned about the regulation of gut/brain interactions, it is possible, and even likely, that new therapies for psychiatric and neurologic conditions may be developed.

References

Fulling, C., Dinan, T.G., & Cryan, J.F. (2019 Mar 20). Gut microbe to brain signaling: what happens in vagus... Neuron. 101(6): 998-1002.