Ups and Downs in the Nervous System

  • Published28 Oct 2015
  • Reviewed28 Oct 2015
  • Source Wellcome Trust

The vagus nerve brings information from all over the body to the brain – but is it being hacked by the bacteria in our gut?

This story first appeared on Mosaic, which is published by the Wellcome Trust, and is republished here under a Creative Commons license.

Some 80 per cent of the traffic along the vagus nerve is sensory information sent up to the brain by the body, and researchers are beginning to realise this has a significant influence on the mind. There is also now strong evidence from animal studies that the gut’s microbial residents – known as the microbiome – can activate the vagus nerve, with effects on brain and behaviour.

The link between gut flora and disorders such as depression and autism has been recognised for some years. Recent experiments have demonstrated that mice with behavioural disorders (equivalent to certain human mental health problems) have a significantly different microbiome make-up from healthy mice, and that these bacteria may be causing the problems.

One study, which compared mice born via Caesarean section with ones born vaginally, found the first group, in which the newborns were colonised by bacteria from the mother’s skin rather than vaginal bacteria, were more anxious and prone to depression. In another study, mice with autistic symptoms were found to have very low levels of a common gut bacterium (Bacteroides fragilis), and high levels of a bacterial chemical (4EPS) in their blood. When the researchers fed these mice B. fragilis, it reversed their symptoms. And when they injected 4EPS into healthy mice, it caused them to exhibit behavioural problems similar to autism.

The gut’s influence over mental health can begin before we’re even born. One-fifth of all people with schizophrenia were exposed in the womb to viral or microbial infections because their mother became sick while pregnant, scientists estimate. Mouse studies have shown infected infants are more likely to suffer symptoms of mental health disorders from anxiety to schizophrenia. Gut flora can control these pathogens and modify brain development and behaviour, researchers have found.

But how do gut microbes have this mind-altering affect? Bacteria that inhabit the intestines secrete waste products that can cause inflammation. They can also produce neurotransmitters, including serotonin and dopamine. This generates electrochemical signals that travel up the vagus nerve, triggering chemical changes in the brain that affect behaviour. The vagus even appears to differentiate between non-pathogenic and potentially pathogenic bacteria and induce feelings of anxiety or calm depending on the stimulus, scientists believe.

Disorders of mental health, personality and mood are poorly understood and difficult to treat, so this newly found relationship between the mind and gut, mediated by the vagus nerve, offers the tantalising possibility that therapies will one day be possible through the relatively simple intestines. Doctors are already experimenting by introducing different gut bacteria: Lactobacillus has been shown to reduce anxiety in people, for example. Meanwhile, other researchers are looking at ways to engineer the microbiome using genetically modified bacteria designed to improve health. In the future, we may be able to treat conditions such as schizophrenia and autism this way, or tweak our personalities to become more empathic or outgoing.

The possibilities are intriguing, even if we’re still a long way from probiotic cures for mental illness.

By Gaia Vince


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