Early Inklings of the Gut-Brain Axis

Actually, what is being discussed here is more accurately called the microbiome-gut-brain axis, because what goes on in the digestive tract is pretty much determined by the bacteria and other organisms that live there.

It has long been known that anxiety, stress, and other negative emotions can mess up the orderly processes carried on in the intestines. Recently, it has become more and more apparent that the gut affects higher cerebral operations and mood and behavior. When people are emotionally distressed and not thinking straight, they make a lot of bad decisions that affect health and especially weight.

How could microscopic creatures, even massed in trillions as they are in our gastrointestinal tracts, possibly impact our mental and emotional health? It strains credulity to imagine that various populations of bugs team up to influence our thinking and our actions, but the evidence keeps piling up.

Neuroscientist Dr. John Cryan found some of the earliest indications that signals from bacteria could penetrate the supposedly well-protected brain. In 2011 he published a paper suggesting that…

[…] micro-organisms in the gut tickle a sensory nerve ending in the fingerlike protrusion lining the intestine and carry that electrical impulse up the vagus nerve and into the deep-brain structures thought to be responsible for elemental emotions like anxiety.

At the Texas Tech University Health Sciences Center, Dr. Mark Lyte has been exploring psychoneuroimmunology for more than 30 years. Today, it is widely accepted that stress can suppress the immune system, but back then it was a crazy idea. By the early 1990s, he was pretty sure that stress affects the bacteria that cause infection, and designed lab experiments to prove it.

But was this a two-way street? Could bacteria also induce stress in people?

As usual, figuring this out involved rodents. Stress-inducing ordeals were designed for them, to which they reacted with more anxiety if infected with Campylobacter jejuni. Other scientists had already shown that antibiotics caused mice to be less cautious, which implied that they experienced less anxiety than their untreated compatriots. Related work went on elsewhere, described here by Peter Andrey Smith:

Sven Pettersson, a microbiologist at the Karolinska Institute in Stockholm, published a landmark paper in Proceedings of the National Academy of Science that showed that mice raised without microbes spent far more time running around outside than healthy mice in a control group; without the microbes, the mice showed less apparent anxiety and were more daring.

When the human body is deficient in the amino acid GABA, anxiety is often experienced. This inhibitory neurotransmitter has a calming effect on nervous activity. Xanax, Valium and other anti-anxiety drugs seem to work by targeting GABA receptors in the brain. In nature, immense quantities of GABA are exuded by the bacterium Lactobacillus rhamnosus.

Dr. Cryan and his team from Ireland’s University College Cork collaborated with other scientists from Ontario’s McMaster University on a classic study of “behavioral despair” that became known as “one of the best-known experiments linking bacteria in the gut to the brain.” In what was called the forced-swim test, the curious people in white coats stressed a bunch of mice by letting them believe they would drown. But if the mice were fed a broth containing Lactobacillus rhamnosus, they were much more relaxed, acting in fact as if they were on Prozac.

This probiotic (good or helpful) bacterium is used to create yogurt. Pregnant mice who are stressed tend to pass less Lactobacillus rhamnosus to their babies. Human infants pick up the bacterium while passing through the mother’s birth canal, a fact connected to observations that babies delivered by Caesarean section are more likely than naturally birthed children to become obese.

Somehow, this all fits together and spells out the message that attention paid to the microbiome will not be wasted, and will in fact turn out to be very useful in the battle against obesity.

Your responses and feedback are welcome!

Source: “Can the Bacteria in Your Gut Explain Your Mood?” NYTimes.com, 06/23/15
Photo credit: IsaacMao via Visualhunt/CC BY

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