Two articles capture the new connection between the intestines and the brain

The illustration depicts the microbial makeup
of the human body.
Image source: National Human Genome Institute

To learn to socialize, zebrafish need to trust their gut
.
New University of Oregon research on zebrafish suggests that gut microbes prompt specific cells to prune additional connections
in brain circuits that control social behavior.
Pruning is essential
for the development of normal social behavior.

The researchers also found that these "social" neurons in zebrafish and mice were similar
.
This suggests that the findings could be translatable across different species and could point the way
to treating a range of neurodevelopmental diseases.

"It's a big step forward," said Judith Eisen, a neuroscientist at the University of Oregon who co-led the work
with neuroscientist Philip Washbourne.
"It also sheds light on what's happening
to large, hairy animals.
"

The team reports their findings in two new papers, published in PLOS Biology and BMC Genomics
.

While social behavior is a complex phenomenon that involves many parts of the brain, Washbourne's lab previously identified a group of neurons in the zebrafish's brain that are necessary for
a particular social interaction.
Normally, if two zebrafish see each other through a glass partition, they will come close to each other and swim
side by side.
But zebrafish without these neurons would not show interest
.

Here, the team discovered a pathway
that connects the microbes in the gut with these neurons in the brain.
In healthy fish, gut microbes stimulate a type of cell called microglia to prune out extra links
between neurons.

Pruning is a normal part
of healthy brain development.
Like clutter on the counter, extra neural connections can get in the way of really important information, leading to information clutter.

In zebrafish without these gut microbes, pruning does not occur and the fish exhibit social deficits
.

Washbourne said: "We have long known that the microbiome affects a lot of things
during development.
But there's not much specific data
yet on how the microbiome affects the brain.
We have already made a lot of efforts
in this area.
”

In the second paper, the team identified two defining features of this group of social neurons, which may be common to
mice and zebrafish.
One is that these cells can be identified by turning on similar genes — a clue that they may play a similar role
in the brains of both species.
These iconic signals can be used to identify neurons
that play this role in different brains.
Another, Eisen said, is that "neurons with the same genetic characteristics in mice are in roughly the same brain location
as social neurons in zebrafish.
" ”

The discovery reinforces the researchers' belief that their work on zebrafish can be translated into mice or humans
.
Scientists can observe the formation of neural circuits through the zebrafish's transparent body, making it easier to study the specifics
of zebrafish brain development.
Researchers can then take these insights from zebrafish and use them as a starting point
for understanding other species.

Both gut microbiota disruption and poor synaptic pruning have been linked
to a range of neuropsychiatric disorders, such as autism spectrum disorder.

Joseph Bruckner, a postdoc in Eisen and Washbourne's lab and first author of the PLOS Biology paper, said: "If we can combine these, it may lead to better treatments
for a variety of diseases.
" His next step is to figure out what molecules connect bacteria to microglia, mapping out the pathways
between microbes and behavior in more detail.