The Gut-Brain connection:
This week we’ll discuss the gut-brain connection as it involves mental health and the nervous system. We will also dive into a bit about inflammation (my favorite topic).
Your gut is wired to your brain through your spine and through your vagus nerve, but the vagus is the primary two-way communication channel for mood, so it’s the one we pay the most attention to when discussing psychobiotics. The bacteria in your gut both secrete and respond to neurotransmitters including dopamine, serotonin, and GABA, all which can have antidepressant properties in the brain. It’s likely that this is one of the primary ways gut bacteria influence your mood. Why do bacteria churn out so many neurotransmitters? One theory is that these bacteria want to control your cravings. But the cells lining the gut can also produce cytokines—which means that virtually every cell of the GI tract can trigger an immune response. Immune factors like fatty acids and cytokines activate microglia, the immune cells in the brain, which will fight pathogens but may also lead to anxiety or depression. Pathogenic microbes can encourage inflammation. The good news is that psychobiotic microbes will lower inflammation, either directly or indirectly, by helping to heal the gut lining that became inflamed. Acetylcholine plays a role in this healing. Stimulating the vagus nerve causes acetylcholine to be produced, which, along with cortisol, suppresses the immune system. Many bacteria, including Lacto species, can also produce acetylcholine, and they may thereby gain access to the powerful levers of the immune machine. They effectively inject themselves into your homeostatic system, and that is a major aspect of their psychobiotic effect. Your immune system has been using antibodies as if they were “Wanted” posters identifying foreign microbes. But under stress, the antibody traffic slows down—one of your immune system’s ways to conserve energy for the most essential tasks. That lowers the level of scrutiny, allowing pathogens to sneak through and gain a foothold. The hypothalamus is part of your limbic system—the ancient neurological circuitry shared by birds and crocodiles all the way up to mammals. It is central to your mood, motivation, hunger, and sleep. The hypothalamus is a cross between a brain and a gland, and it bridges the nervous system and the endocrine system. It can reach out past the blood brain barrier (BBB) with specialized nerve cells that sample your blood to detect signs of inflammation. These three organs—the hypothalamus, pituitary gland, and the adrenal gland—constitute the HPA axis, a primary network for communication between the gut and the brain. When inflammation is detected, the HPA axis releases cortisol, among other hormones, to respond to the stress. This response is instrumental in mood regulation, and many people with depression or anxiety have a problematic HPA axis. Fixing problems with this axis, we now understand, can be a way to relieve depression and anxiety. The biological imperative to stabilize the internal environment is called homeostasis, and it’s fundamental to all living creatures. Homeostasis is life pushing back, but it’s a moment-by-moment adjustment not designed for long-term engagements. The ultimate goal is to return to an optimal set point, which is done with negative feedback: When a signal is detected, the feedback quickly dampens it. Thus, chronic inflammation produces stress hormones indefinitely, leading to a surprising number of mental issues, including bipolar disorder, post-traumatic stress disorder, attention deficit disorder, depression, and anxiety. First the nervous system is alerted and your gut signals your brain through the vagus nerve. Then your immune system kicks in, battling the pathogens, but not without collateral damage. Finally your endocrine system is called upon to moderate your raging immune system. The circulation of lymph is entirely driven by body motion, so exercise is essential to lymphatic health. Sometimes your gut bacteria get into the action and set up defenses against pathogens. Lactobacillus reuteri, for example, can directly excite sensory neurons in the gut and influence peristalsis, typically to slow it down. L. reuteri can also dampen pain, with signals to the brain through the vagus that things are okay in the gut. Butyrate is another SCFA with a lot of benefits. It’s the preferred energy source for the cells lining your gut. It can also enter the brain, where it encourages brain growth factors and acts as an antidepressant. It’s all about balancing your microbiota. The nervous system uses neurotransmitters to communicate while the endocrine system uses hormones. Norepinephrine is a clear favorite of many. These molecules are stress hormones, called upon as part of the fight-or-flight response. Their job is to make you anxious and alert. Fight-or-flight reaction takes precedence over all other bodily functions. It subdues the gut and the immune system, which lowers your guard throughout your entire GI tract, potentially allowing pathogens to thrive. Stress can cause other physical and chemical changes in your gut. It can alter the levels of acids, mucus, and other intestinal secretions, disturbing the cozy environment your microbes have grown accustomed to. A different kind of stress is depression. When you get down to it, what is depression? Stripped to its essentials, it is almost indistinguishable from sickness behavior. Depression behavior likely has a long primal history of survival value by keeping you at home when you are sick. It may also conserve energy so that you can fight infection. In the modern world, however, where inflammatory triggers may be more prevalent—coming from stress, high-fat and high-sugar diets, and sedentary lifestyles—depression may no longer be a useful adaptation. Remember, probiotics are not a cure, but an ongoing therapy and gut issues can affect the brain, but the brain can also affect the gut.
