Two papers just came out, each showing – in different ways – how the gut is directly connected to the brain. My gut feeling is that this is going to be critically important research in the future.
We already knew that the gut lining contained over 100 million nerve cells, and that the gut talks indirectly to the brain via hormones. This is why, for example, it takes about 10 minutes for your belly to tell your brain that you are full. The feeling of satiety is hormone-driven. (All those of you who have dieted know these rules: eat slowly, to give your brain time to register the food intake; drink a glass of water 10 minutes before you eat, so you feel less hungry, etc.) Hormones work (relatively) slowly.
Remember: over a year ago, in June 2017, I wrote about research in Parkinson’s out of Duke University that showed that the immune protein, alpha-synuclein, which was known to become damaged (folded) in the disorder, seems to originate in the endocrine cells of the gut and then makes its way into the brain via gut nerve cells. (The current belief is that copious amounts of alpha-synuclein are released by the gut in order to fight some kind of gut infection. The nature of that infection is currently unknown.) These scientists were actually able to video the gut endocrine cells moving toward the neuron fibers and establishing a connection. Take a few seconds to watch that video here – it’s seriously cool. “With the new finding of alpha-synuclein in endocrine cells, Liddle and colleagues now have a working explanation of how malformed proteins can spread from the inside of the intestines to the nervous system, using a non-nerve cell that acts like a nerve.”[i]
In this latest experiment, scientists (also from Duke University) injected a fluorescent rabies virus (which is transmitted through nerves) into the colons of mice and “…waited for the enteroendocrine cells and their partners to light up. Those partners turned out to be vagal neurons…”[ii] The endocrine cells released the excitatory neurotransmitter, glutamate, to communicate directly with brain and nervous system.[iii] “By synapsing with the vagus nerve, neuropod cells connect the gut lumen to the brainstem. Neuropod cells transduce sensory stimuli from sugars in milliseconds by using glutamate as a neurotransmitter. The neural circuit they form gives the gut the rapidity to tell the brain of all the occurrences of the day, so that he, too, can make sense of what we eat.”
In a separate study, also just published, scientists used lasers to stimulate nerves in the gut and were not only able to induce a sense of reward (which led to the mice repeating behaviors) but also measured increases in dopamine in the brain, which enhances mood and motivation. In fact, the vagus nerve connection from the gut led to stimulation of, and dopamine-release in, the Substantia nigra, the particular part of the brain affected in Parkinson’s. “Specifically, right, but not left, vagal sensory ganglion activation sustained self-stimulation behavior, conditioned both flavor and place preferences, and induced dopamine release from Substantia nigra.”[iv]
Says Dr. Bohorquez, lead author of the first study, “”We think these findings are going to be the biological basis of a new sense…One that serves as the entry point for how the brain knows when the stomach is full of food and calories. It brings legitimacy to [the] idea of the ‘gut feeling’ as a sixth sense.”[v]
[iii] Kaelberer, MM, et. al. A gut-brain neural circuit for nutrient sensory transduction. Science. 2018. 361(6408). DOI: 10.1126/science.aat5236
[v] Op cit., sciencemag.org/news