Our Gut Microbes Influence How We Respond To Fear

The reasons why gut microbes have an effect on the brain are still unclear. Joe Techapanupreeda/Shutterstock

Fear, for the most part, is controlled by your brain’s amygdalae, the two almond-shaped segments of your neurological cartography that sound quite a lot like a certain Star Wars princess. Whether or not you feel afraid of or anxious about something is largely dependent on the pondering that goes on between them – but, as a new study points out, they are influenced by something rather unexpected.

Bringing a whole new spin on the phrase “gut reaction”, a team of researchers at University College Cork have discovered that your gut microbiome, that collection of bacteria that lives within your digestive system, appears to also have an effect on your response to and processing of fear – if you’re a mouse, that is.

This was discovered through a truly bizarre series of experiments.

The microbiomes within our tummies don’t just appear at birth; they develop over time from exposure to the world. Wondering what would happen if they cultured mice in a microbe-free environment, the team found that these overly clean rodents appeared to lack any distinguishing fear responses.

When stimulated by things that would normally make these mice run for the hills, the researchers found that their responses were somewhat less pronounced – they appeared to “forget” what made them scared over time.

Weirdly, after they had been exposed to an ecosystem of microbes, their conventionally acute responses to fearful events returned, and they remembered in the long-term what frightened them and what didn’t.

Having a look at the molecular biochemistry of these overtly brave, germ-free mice’s amygdalae, they found that they appeared to be engaging in an altogether different alchemy than conventional mice’s fear centers. Genes that were once quiet were suddenly abnormally active, and overall, the entire section of the brain showed greatly increased neuronal activity.

Afraid of zombies? Clear out your gut microbiome and you probably won't be! Tithi Luadthong/Shutterstock

Writing in Nature: Molecular Psychiatry, the team explain that these amgydalae are in a “hyperactive state” as standard. If this is the baseline level of activity, then this would explain why things that normally scare these mice now failed to do so – their brains cannot process fear at much higher levels.

As for why the presence of a biodiverse gut microbiome would have any effect on this whatsoever, it’s safe to say the jury is still out on that one. Previous research has suggested that there are links between microbes and fear responses – among many other neurological processes – but a definitive cause-and-effect link has so far proved elusive.

Humans and mice both have microbiomes, both have digestive systems, and both have amgydalae. This means that what applies to mice likely applies to humans too. A future featuring drugs that tamper with our microbiome, and thus our ability to handle fear, may not be that far away.

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