The gut-brain axis has been the subject of research for several decades, but it is only in the last 15 years that the microrganisms in the enteric tract have been regarded as important in signaling from the intestine to the brain and vice versa.
When we look at the intestinal microbiota we start to see a very complex biochemistry, intertwined with the functions of the brain and other organs. The organism, in fact, depends on intestinal microbes for the production of molecules fundamental to the functioning of the whole organism.
Furthermore, when we analyze the ways in which microbes communicate with the brain, we find two fundamental ways of communication: the vagus nerve and the short chain fatty acids.
Short chain fatty acids, such as butyrate, propionate and acetate, are bacterial metabolic products that can enter the bloodstream and travel to the brain.
The debate about their functions on the brain is still open, but there is evidence that they can act as epigenetic modulators and can change the way genes in neurons actually work. But they also act through classical receptors called “g-protein coupled receptors”.
Another very important role of microbes is the production of tryptophan, a precursor necessary for serotonin, synthesized in particular by bifidobacteria.
Naturally, this means that there is an impact of the intestinal microbiome on the regulation of mood, sleep and appetite.
Ted Dinan of University College Cork, Ireland, gives us an insight into the intestine-brain axis.