Research has shown that the communities of bacteria that live in the gut of a mouse are essential for the animals to display normal social behavior with other mice. Mice that have been bred to be germ-free, without a gut microbiome, display significant antisocial behaviors, such as avoiding a strange mouse rather than interacting with it .
But what is the chain of events that occur at the molecular and cellular level, from gut bacteria to the brain and behavioral changes ?
Extrapolation to humans
A new study has identified a specific circuit of neurons that is directly influenced by the gut microbiome and is subsequently responsible for antisocial behaviors in mice that lack a gut microbiome. Transplants of fecal matter from mice with healthy gut microbiomes to these germ-free mice were enough to change the activity of these neurons and thus improve their social behavior. The researchers also identified a specific bacterial species that can increase sociability .
Germ-free mice had already been shown chemically to have significantly higher levels of the hormone corticosterone (the analog of the so-called stress hormone cortisol in humans) than mice with healthy microbiomes.
So what was it about the gut bacteria, or lack thereof, that was causing the corticosterone levels to spike in the first place? To address this, the team performed fecal transplants from wild-type mice with normal gut microbiota to germ-free mice.
Germ-free mice that were colonized with E. faecalis showed improved social behaviors and reduced levels of corticosterone. The mechanisms through which E. faecalis can mediate this improvement will be the subject of future research .
Identifying interactions between gut microbes, neurons, and effects on health throughout the body (such as behavioral changes) may be an important line of research on ways to one day help improve social deficits , like those that concur with depression and autism.
Current practices to treat these types of problems include the prescription of drugs, such as antidepressants and anxiolytics. However, it is difficult to get these drugs to the right brain regions in the correct concentrations, and much of the drug ends up throughout the body. Understanding the gut-brain connections adds to the evidence that neuropsychiatric disorders can be indirectly improved by treating the gut microbiome, which is much easier to access pharmaceutically than the brain .