A new study by a team of researchers suggests that the gut microbiome could influence how the brains of the mice work, specifically on the neural networks involved in social behavior.
They observed that, like dogs, mice also sniff each other's whiskers and clamber over each other when greeting. However, the germ-free mice without the gut microbiome avoid other mice and remain aloof from others.
Researchers noted that the social impairment of germ-free mice is not unusual. Rather, they want to find out what drives this behavior and whether the gut microbiome has something to do with it.
Gut Microbiome Could Make Mice Avoid Others
In their study, entitled "Microbiota regulate social behavior via stress response neurons in the brain" published in the journal Nature, the team compared the brain activity of rodents that are divided into two groups: one group was raised in a sterile environment, while the other is treated with a combination of antibiotics that depleted their gut microbiome.
They placed the mice in cages with unknown mice to observe their social behavior, according to Live Science. As expected, both the germ-free and antibiotic-treated mice avoided interactions with the unknown mice.
To understand their brain activity during the experiment, the researchers examined the brains for the c-Fos gene whether there is heightened activation or not in the brain regions responsible for stress responses, like the hypothalamus, amygdala, and hippocampus.
The spike of brain activity coincided with the spike of the stress hormone called corticosterone in the germ-free and antibiotic-treated mice, indicating that the two groups have higher stress hormones.
Corticosterone is essentially produced by the hypothalamic-pituitary-adrenal (HPA) axis. The team wondered whether messing the HPA axis could bring the spiked levels of the hormone back down and correct the behavior of rodents.
They found that removing the adrenal gland boost the social behavior of mice without gut microbiome as they meet the unknown mice. Also, blocking the production of corticosterone with drugs increased the sociability of rodents, as well as blocking the receptors that bind the hormone in the brain, known as glucocorticoid.
Their findings showed that hypothalamus cells might be overactive in germ-free mice and that the gut microbiome helps tune them down in the unknown mice to modulate the activity of the HPA axis.
Potential Treatment for Neurological Disorders
Beyond the experiment, the researchers believe that this line of study might someday help scientists find treatment for some neuropsychiatric disorders, such as anxiety and an autism spectrum disorder.
Of course, that is only possible when observations in animals can also be generalized in the human population. Studies published in the journals General Psychiatry and JAMA suggest that anxiety and autism often coincide with gastrointestinal disorders and disruptions in the gut microbiome.
Likewise, the study entitled "Dissecting the contribution of host genetics and the microbiome in complex behaviors" published in the journal Cell reveals that the gut microbiome contributes to certain symptoms associated with complex neurological disorders. These findings suggest that a microbe-inspired therapy may one day help treat these disorders.
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