Belly bacteria boss the brain

Friendly intestinal bacteria not only keep the gut happy, they may help keep their host happy, too, a new study in mice finds.

Mice fed broth fortified with a type of friendly intestinal bacteria called Lactobacillus rhamnosus behaved less anxiously than mice fed broth without bacteria. Those behavior changes were accompanied by differences in levels of a brain-chemical sensor and stress hormones.

The bacteria telegraph these brain-chemical and behavior-changing messages via the vagus nerve, which connects the brain stem to various internal organs, researchers report online August 29 in the Proceedings of the National Academy of Sciences.

Some studies have suggested that changing the mix of bacteria in the intestines could influence behavior (SN: 6/18/11, p. 26). The new research goes a step further to investigate how those changes come about, says Paul Patterson, a neuroimmunologist at Caltech in Pasadena, Calif. “Most people haven’t gone that far to look at what’s happening in the brain,” he says.

The research team — led by John Bienenstock of McMaster University in Hamilton, Canada, and John Cryan of the University College Cork in Ireland — looked at the mice’s brains to examine levels of the GABA receptor, a protein that senses and responds to an important brain chemical messenger called GABA. Alterations in the way GABA and other brain chemical systems work influence behavior. Mice fed bacteria-containing broth had higher levels of the receptor protein in some parts of the brain and lower levels in other parts than did mice fed sterile broth.

Mice usually stay close to walls, but ones that consumed the bacterium spent more time in open spaces in a special maze — a measure that tells scientists the mice are less anxious than usual. The researchers also gave the mice a stress test by forcing the animals to swim in a water tank. Stressed mice that had eaten Lactobacillus rhamnosus had lower levels of stress hormones than did mice that ate broth alone.

When the scientists severed the vagus nerve, the mice no longer had altered levels of the GABA receptor and didn’t exhibit behavior differences, indicating that nerve is probably the major route gut bacteria use to transmit information to the brain.

The vagus nerve “is the obvious route, but that’s not to say it’s the only route,” says Bienenstock. Messages may also be transmitted through other nerves or through chemicals in the blood. The researchers still don’t know what sort of message the bacteria send to the brain or whether bacterial supplements can make a difference in regulating people’s behavior.

Some researchers have proposed, based on experiments in mice, that gut bacteria could play a role in a wide variety of brain and psychiatric disorders, such as depression, anxiety, autism and schizophrenia.

But, “one has to be cautious. This is exciting science in rodents, but you can’t just extrapolate to humans,” says Emeran Mayer, a gastroenterologist and neuroscientist at UCLA’s Center for Neurobiology of Stress who was not involved in the new study.

Drug and food companies that make probiotics — beneficial bacteria taken in a pill or eaten in food such as yogurt — hope the products can help relieve depression, improve weight loss and cure other conditions, but there is little evidence in people that probiotics can accomplish those goals, Mayer says.

“It’s almost like science fiction; you can imagine the most amazing things because so little is known about it,” he says. But, “So far there’s really no evidence that probiotics affect emotions in humans.”