Brain, do you hear?
The brain senses intestinal bacteria directly
Kirill Stasevich, "Science and Life"
There is a lot of evidence in favor of the fact that the intestinal microflora and the brain interact with each other, and this interaction can affect the brain itself in different ways. On the one hand, bacteria suppress the symptoms of autism and generally help the brain to think better, on the other hand, depression is accompanied by characteristic changes in the composition of the microflora, and the microflora in some cases accelerates Parkinson's disease.
However, so far little is known about exactly how bacteria communicate with the brain. Most likely, they send some molecules from themselves that get to the brain and interact there with certain receptors on certain cells. But what are these molecules, what are these cells, and what are these receptors? Staff The Pasteur Institute found out that the so-called muropeptides get into the brain from bacteria, and the hypothalamus neurons feel them there.
Article by Gabanyi et al. Bacterial sensing via neuronal Nod2 regulates appetite and body temperature is published in the journal Science.
Muropeptides are fragments of the bacterial cell wall that surrounds their cell membrane, adding strength to bacteria and protecting them from various external adversities. The cell wall consists of peptidoglycan, or murein, which is a special modified carbohydrates, cross-linked with short amino acid chains, that is, peptides.
The murein cell wall is peculiar only to bacteria, and it is not surprising that our immune cells are able to sense muropeptides – for this they have special NOD2 receptors (the abbreviation NOD2 hides some features of their structure and functioning). But it is also known that the genes encoding NOD2 receptors have something to do with neuropsychiatric disorders and diseases of the digestive tract. And now researchers have analyzed in detail where NOD2 receptors work in the brain, and it turned out that they are especially active in the hypothalamus - a small part of the brain that, however, plays a huge role in regulating metabolism, hunger, appetite, thirst, controls sexual behavior, affects memory and emotions, etc.
And so the neurons of the hypothalamus, as we have just said, use NOD2 receptors, and when they felt muropeptides, their electrical activity dropped - the fragments of bacteria seemed to calm the hypothalamic nerve cells. When there are a lot of muropeptides, this usually indicates that the bacteria are multiplying intensively; in other words, the neurons of the hypothalamus become less active when the microflora is growing. The researchers especially emphasize that neurons sense bacterial molecules themselves, without intermediaries in the form of immune cells or anyone else.
What happens if NOD2 receptors are turned off? Such experiments were performed on mice, and it turned out that mice with non-functioning NOD2 receptors in the hypothalamus begin to eat more, their thermoregulation is disrupted, excess weight appears and the likelihood of type 2 diabetes increases, especially in elderly females.
By controlling the microflora, you can act on the NOD2 receptors in the hypothalamus in such a way as to weaken your appetite and generally normalize your metabolism. Maybe bacterial muropeptides can also affect other brain functions, up to higher cognitive ones – let's not forget that the hypothalamus does not affect anything. However, there is still a need to check how this communication channel between the microflora and the brain works in humans.
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