Researchers at Duke have discovered the scientific basis of our gut feelings—literally.
For decades, our understanding of the connection between the digestive system and the brain was that nutrients go inside the intestines, which cause the release of hormones that eventually reach the brain. This process can take hours, even though humans feel satisfaction from food in seconds.
Now, Duke researchers have found sensory neurons lining the walls of our stomach that send signals to our brain in as little as 10 microseconds—faster than the blink of an eye.
“The gut has its own sense, just like the tongue, ears, and nose,” said Diego Bohórquez, senior author of the study and assistant professor in medicine. “It has the ability to recognize components of a meal and pass those signals very fast to the brain, which influences how we respond to foods and other stimuli.”
Bohórquez recognized that there may be a connection between the brain and the digestive system after observing a friend’s change in behavior after gastric-bypass surgery.
“Before the surgery, the sight of eggs made her queasy,” he said. “Afterwards, not only could she stomach those eggs, but she craved them. My immediate question was how altering the stomach can rewire the brain.”
The neurons in our gut are essentially our sixth sense. They also cause the queasiness felt after eating an unpleasant food, the nausea before an important event and the butterflies before something exciting.
The Bohórquez lab made this discovery using an innovative technique involving the rabies virus.
“We used rabies specifically because the virus loves neurons and is really good at finding them,” Bohórquez said. “That’s why when a rabid animal bites someone on their finger, the virus spreads immediately to the brain and alters the person’s behavior.”
The Bohórquez lab injected modified rabies virus into the stomach linings of mice, hypothesizing that if any neurons were in the gut, the virus would find them. Sure enough, the modified virus, which fluoresces green and is easily trackable, found clusters of sensory neurons throughout the stomach lining.
The Bohórquez lab found that the sensory neurons in the gut were directly connected to the vagus nerve—the longest of the cranial nerves.
“Gut neurons are not quite like the ones in our brains,” Bohórquez said. “But they’re able to ‘sniff out’ some very complicated things and pass them on to our brains.”
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The relationship between neurons in the brain and in neurons in the gut is a two-way street.
“Take butterflies in your stomach, for example,” he said. “The gut is usually the one advising the brain, but in this case, the brain tells the gut ‘look, I have a message for you’ and gives you that feeling of inexplicable stimulation. This connection opens up the possibility to study how the brain controls our gut sensitivity in the near future.”
One application of this research is behavior modification in children with autism. About 90 percent of children with autism suffer from gastrointestinal complications that seem to be sensory in nature.
“A normal patient’s gut neurons can fire in 100 milliseconds,” Bohórquez said. “But this might happen in 10 milliseconds or maybe 2 seconds in a patient with autism. This greatly alters their behavior—it might make them hyper-aware or lethargic, which has tremendous implications on body weight, mood and metabolism.”
Bohórquez also plans to explore the complex interactions between gut neurons and gut flora—the millions of bacteria that live inside our stomach. These bacteria may interact with gut neurons, influencing them to send messages to our brains to intake foods and nutrients that are beneficial to their survival.
Humans and these bacteria, Bohórquez suggested, share an innate connection that is more powerful than we may think.
“We know very little about how these bacteria talk to the brain,” he said. “Who knows, maybe we are just their slaves. Maybe they control our desires. Maybe the decisions we make on a daily basis are made to keep these bugs happy.”