Consistent with these behavioral findings, two genes implicated in anxiety -- nerve growth factor-inducible clone A (NGF1-A) and brain-derived neurotrophic factor (BDNF) -- were found to be down-regulated in multiple brain regions in the germ-free animals. These changes in behavior were also accompanied by changes in the levels of several neurotransmitters, chemicals which are responsible for signal transmission between nerve cells. The neurotransmitters dopamine, serotonin and noradrenaline were elevated in a specific region of the brain, the striatum, which is associated with the planning and coordination of movement and which is activated by novel stimuli, while there were there were no such effects on neurotransmitters in other brain regions, such as those involved in memory (the hippocampus) or executive function (the frontal cortex).
When Pettersson’s team performed a comprehensive gene expression analysis of five different brain regions, they found nearly 40 genes that were affected by the presence of gut bacteria. Not only were these primitive microbes able to influence signaling between nerve cells while sequestered far away in the gut, they had the astonishing ability to influence whether brain cells turn on or off specific genes.
How, then, do these single-celled intestinal denizens exert their influence on a complex multicellular organ such as the brain? Although the answer is unclear, there are several possibilities: the Vagus nerve, for example, connects the gut to the brain, and it’s known that infection with the Salmonella bacteria stimulates the expression of certain genes in the brain, which is blocked when the Vagus nerve is severed. This nerve may be stimulated as well by normal gut microbes, and serve as the link between them and the brain. Alternatively, those microbes may modulate the release of chemical signals by the gut into the bloodstream which ultimately reach the brain. These gut microbes, for example, are known to modulate stress hormones which may in turn influence the expression of genes in the brain.
Regardless of how these intestinal “guests” exert their influence, these studies suggest that brain-directed behaviors, which influence the manner in which animals interact with the external world, may be deeply influenced by that animal’s relationship with the microbial organisms living in its gut. And the discovery that gut bacteria exert their influence on the brain within a discrete developmental stage may have important implications for developmental brain disorders.
Are you a scientist? And have you recently read a peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.