The intense relationship between the digestive system and the brain is widely documented by science. The human intestine is filled with more than 100 million nerve cells; and communicates with the brain by releasing hormones into the bloodstream that, in the course of about 10 minutes, tell us if we have food or maybe we have eaten too much.
Now, a research by the Lunenfeld-Tanenbaum Research Institute in Toronto (Canada) reveals that the intestine has a much more direct connection to the brain, which is through a neural circuit through which signals can be transmitted in a few seconds.
Although this research proposes more questions than answers, “this is a great new piece of the puzzle,” says Daniel Drucker, a clinical scientist who studies intestinal disorders. The study reveals “the pathways that intestinal cells use to communicate quickly with the brainstem,” he explains. The findings, according to its authors, could lead to new treatments for obesity, eating disorders and even depression and autism, which the scientific literature has already linked to poor bowel function.
In 2010, neuroscientist Diego Bohórquez of Duke University in Durham, North Carolina (USA) discovered that enteroendocrine cells, which line the intestine and produce hormones that stimulate digestion and suppress hunger, have protuberances similar to those of the feet that resemble the synapses that neurons use to communicate with each other.
Bohórquez knew that enteroendocrine cells could send hormonal messages to the central nervous system, but he also wondered if they could “talk” to the brain using electrical signals, as neurons do. If so, they would have to send the signals through the vagus nerve, which travels from the intestine to the brainstem.
To test this, his team injected a fluorescent rabies virus, which is transmitted through the neuronal synapses, into the colon of mice and waited for the enteroendocrine cells and their pairs to light up. These partners turned out to be vagal neurons, the researchers report in Science.
In a Petri dish, the enteroendocrine cells extended to the vagal neurons and formed synaptic connections with each other. The cells even expelled glutamate, a neurotransmitter involved in smell and taste, which vagal neurons recovered in 100 milliseconds, faster than a blink, and much faster than hormones can travel from the intestine to the brain. Through the bloodstream, says Bohorquez: “The slowness of hormones can be responsible for the failure of many appetite suppressants that attack them.” The next step, they say, is to study if this signaling of the brain and the intestine provides the brain with important information about the nutrients and the caloric value of the food we eat.
Benefits of good communication
According to the researchers, this ultra-fast communication involves some obvious advantages, such as the immediate detection of toxins and poison, and there may also be a benefit in detecting the contents of our casings in real time. Additional clues about how intestinal sensory cells benefit us are found in a separate study, published in Cell.
The researchers used lasers to stimulate the sensory neurons that innervate the intestine in mice, which produced gratifying sensations that the rodents tried to repeat. On the other hand, this technique also increased the levels of a neurotransmitter that stimulates the state of mind called dopamine in the brain of rodents.
Combined, the two documents help explain why stimulation of the vagus nerve with electrical current can treat severe depression in people, according to Iván de Araujo, a neuroscientist at the Icahn School of Medicine at Mount Sinai in New York (EE. .U.U.), who directed the study of the cell.
The results can also explain why, at a basic level, eating makes us feel good. “Even though these neurons are outside the brain, they perfectly fit the definition of reward neurons” that drive motivation and increase pleasure, concludes the researcher.