| Have you ever been so anxious that your stomach started hurting? In addition to its well-known impacts on mental health, stress can also make people feel physically ill—and a new study could help explain why.
Although it’s fairly common knowledge that the gut “talks” to the brain by releasing hormones into the bloodstream, recent studies have shown that the so-called “gut-brain connection” plays a far more pivotal role in health than previously thought. Signals originating in a stressed-out brain can make their way to intestinal nerve cells, causing inflammation and aggravating diseases like ulcerative colitis. Certain microbes in the gut, meanwhile, release chemicals that can alter mood and behavior. Now, scientists have discovered a direct relationship between the brain, gut bacteria, and the immune system—demonstrating this month in Cell that you can, in fact, worry yourself sick. The team zeroed in on small, neuron-filled organs called Brunner’s glands, which line the walls of the small intestine and secrete mucus. Removing these glands in mice triggered inflammation, the researchers report, and made the rodents more susceptible to infection. Humans who’d had tumors removed from the part of the gut that contains Brunner’s glands also showed elevated white blood cells, further solidifying the link between these overlooked organs and the immune system. It turns out that removing Brunner’s glands causes bacteria in the Lactobacillus genus to vanish from the small intestine—with devastating consequences. These microbes normally help reinforce connections between cells in the lining of the gut, so losing them can cause molecules to “leak” out, activating an immune response. As lead researcher Ivan de Araujo tells Nature, “Things that shouldn’t cross into the blood do so.” When the researchers took a closer look at the neurons within Brunner’s glands, they discovered a connection to the vagus nerve—a twisting, 100,000-fiber network that meanders from the base of the brain to the internal organs, influencing digestion, blood pressure, and even memory and mood. Fibers that connect to Brunner’s gland nerves, the team found, lead directly to a region of the brain known as the amygdala, which plays a key role in emotion and the stress response. Sure enough, subjecting mice to chronic stress caused their Brunner’s glands to shut down—causing Lactobacillus to dwindle and intensifying inflammation. Neuroscientist John Cryan, who reviewed the study, calls it a “technical tour de force.” Scientists already knew that bacteria could influence the brain, he tells Nature, so seeing that relationship flow in the opposite direction is “a really cool part of the puzzle.” From AAAS Science Adviser |
