Study co-author Dr. Gerard Clarke, of the APC Microbiome Institute at University College Cork in the Republic of Ireland, and colleagues recently reported their findings in the journal Microbiome.
This is not the first study to suggest a link between gut bacteria and anxiety. A study reported by Medical News Today in 2015, for example, found that stress in early life may alter gut bacteria in a way that may increase the risk of anxiety in later life.
But what are the mechanisms that underly the possible link between gut bacteria and anxiety? The new study from Dr. Clarke and team helps to shed light.
Gut bacteria and miRNAs
To reach their findings, the researchers analyzed groups of mice that had three different gut bacteria conditions:
- Germ-free mice, which had no gut bacteria due to being bred in an environment free of microorganisms
- Ex germ-free mice, which had been colonized with gut bacteria in later life
- Mice with normal gut bacteria, which had been bred in their usual conditions
The team notes that mice bred in germ-free conditions are more likely to develop symptoms of anxiety, depression, problems with sociability, and cognitive dysfunctions.
As part of the study, the team looked at how the absence of gut bacteria affected miRNAs in the rodents' brains. miRNAs are tiny RNA molecules that regulate gene expression.
Specifically, the researchers sought to pinpoint which miRNAs were present in the amygdala and prefrontal cortex of the mice in each gut bacteria condition.
Compared with the conventional mice, the researchers found that the germ-free mice showed differences in 103 miRNAs in the amygdala - which is the brain region involved in emotional processing - and 31 changes in miRNAs in the prefrontal cortex - which is the brain region involved in behavior, planning, and impulse control, among other functions.
Importantly, when the researchers introduced bacteria to the guts of the germ-free mice in later life, some of the differences in miRNAs within the amygdala and the prefrontal cortex disappeared.
As such, the team speculates that a healthy gut microbiome is required for normal miRNA regulation.
"Gut microbes seem to influence miRNAs in the amygdala and the prefrontal cortex," says Dr. Clarke. "This is important because these miRNAs may affect physiological processes that are fundamental to the functioning of the central nervous system and in brain regions, such as the amygdala and prefrontal cortex, which are heavily implicated in anxiety and depression."
A path to psychobiotics?
Additionally, the researchers assessed the brains of adult rats whose gut bacteria had been weakened with antibiotics.
They found that the miRNAs of these rats were comparable with those of germ-free mice. This indicates that, despite having normal gut bacteria in early life, changes to gut bacteria in adulthood may influence miRNA diversity in a way that fuels anxiety.
Further studies are needed before the team can make any firm conclusions about the link between gut bacteria and anxiety.
That said, the researchers believe that they may have come across a potential treatment for anxiety disorders, which currently affect more than 40 million adults in the United States.
"This is early-stage research but the possibility of achieving the desired impact on miRNAs in specific brain regions by targeting the gut microbiota - for example, by using psychobiotics - is an appealing prospect."
Dr. Gerard Clarke