The gut is sometimes referred to as a second brain, since it harbors its own nervous system, which is known as the enteric nervous system. The gut is not only directly linked to the brain through the nervous system. The community of microbes in the gut, called the gut microbiome, can also release and modulate chemicals that can affect the brain. For example, gut microbes can generate some neurotransmitters, and the gut and brain are directly connected by the vagus nerve.
Scientists have now discovered a new kind of sense, which they have called the neurobiotic sense, which enables the brain to quickly react to signals from gut microbes. This research, which was reported in Nature, has focused on a type of cell in the lining or epithelium of the colon, called neuropods. These specialized cells are able to detect to a protein found on bacteria, and they send signals to the brain that regulate hunger.
In this work, the investigators zeroed in on a protein called flagellin, which is commonly found on bacteria and helps them move around. This study showed that some gut microbes release flagellin when food is consumed. The neuropod cells hase a receptor known as TLR5 that detects flagellin, and then the cells release signals to the brain that are transmitted by the vagus nerve.
The researchers tested a hypothesis that gut bacteria can cause neuropods to signal to the brain to reduce appetite; in other words, that gut microbes can directly influence behavior. To do so, a mouse model was not given any food overnight, and then a small amount of flagellin was directly applied to their colons. These mice then ate less food than control mice.
When this process was repeated, but in mice that had no TLR5 receptor, the mice continued to eat and started gaining weight. The researchers suggested that TLR5 is closely connected to the feeling of satiety, in which we feel full and stop eating. Flagellin helps stimulate that receptor, and without it, the satiety feeling is disrupted.
The researchers suggested that these findings may only be the beginning of our understanding of the neurobiotic sense, which could play a role in a wide variety of processes including hunger and satiety, mood, and how the brain may shape the gut microbiome.
"Looking ahead, I think this work will be especially helpful for the broader scientific community to explain how our behavior is influenced by microbes," noted senior study author Diego Bohórquez, PhD, a neuroscientist at Duke University School of Medicine. "One clear next step is to investigate how specific diets change the microbial landscape in the gut. That could be a key piece of the puzzle in conditions like obesity or psychiatric disorders."
Sources: Duke University, Nature