Scientists
have found that intestinal bacteria play an important role in inducing
anxiety and depression.
Published
in Nature Communications, the new research is the first of which studies the role
of intestinal microbiota in the altered behaviour that is a consequence of
early life stress. Previously there has been little research on this
subject. All scientists knew was that intestinal bacterium can affect
behaviour, but this was all based upon research conducted on healthy mice.
[Definition: Microbiota
- noun – the microorganisms of a particular site, habitat, or geological period.]
"We have shown for the first time in an established
mouse model of anxiety and depression that bacteria play a crucial role in
inducing this abnormal behaviour," said Premysl Bercik, senior author of
the paper and an associate professor of medicine with McMaster's Michael G.
DeGroote School of Medicine. "But it's not only bacteria, it's the altered
bi-directional communication between the stressed host -- mice subjected to
early life stress -- and its microbiota, that leads to anxiety and depression."
Within this study, the researchers subjected mice to early
life stress with a procedure of maternal separation, meaning that from day
three to 21, newborn mice were separated for three hours each day from their
mothers and then put back with them.
Initially, Bercik and his team confirmed that
conventional mice with complex microbiota. These mice also showed gut
dysfunction based on the release of a major neurotransmitter, acetylcholine.
After that, they repeated the same experiment in germ-free
conditions and found that in the absence of bacteria mice which were maternally
separated still have altered stress hormone levels and gut dysfunction, but
they behaved similar to the control mice, not showing any signs of anxiety or
depression.
Subsequently, they found that when the maternally separated
germ-free mice are colonized with bacteria from control mice, the bacterial
composition and metabolic activity changed within several weeks, and the mice
started exhibiting anxiety and depression.
"However, if we transfer the bacteria from
stressed mice into non stressed germ-free mice, no abnormalities are observed.
This suggests that in this model, both host and microbial factors are required
for the development of anxiety and depression-like behaviour. Neonatal stress
leads to increased stress reactivity and gut dysfunction that changes the gut
microbiota which, in turn, alters brain function," said Bercik.
He said that with this new research, "We are starting to
explain the complex mechanisms of interaction and dynamics between the gut
microbiota and its host. Our data show that relatively minor changes in
microbiota profiles or its metabolic activity induced by neonatal stress can
have profound effects on host behaviour in adulthood."
Bercik said this is another step in understanding how microbiota can
shape host behaviour, and that it may extend the original observations into the
field of psychiatric disorders.
"It would be important to determine whether this also applies
to humans. For instance, whether we can detect abnormal microbiota profiles or
different microbial metabolic activity in patients with primary psychiatric
disorders, like anxiety and depression," said Bercik.
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