The gut and brain communicate – typically through hormones. Throughout the day, your gut releases hormones that travel through the bloodstream and, over the course of about 10 minutes, reach the brain. These signals tell you that you are hungry, or that you should stop eating. In a recent study, researchers found that there may be a direct gut-brain link via a neural circuit.
The study reveals “a new set of pathways that use gut cells to rapidly communicate with…the brain stem,” says Dr. Daniel Drucker, a clinician-scientist who studies gut disorders at the Lunenfeld-Tanenbaum Research Institute in Toronto, Canada, who was not involved with the work. Although many questions remain before the clinical implications become clear, he says, “This is a cool new piece of the puzzle.”
In 2010, neuroscientist Dr. Diego Bohórquez of Duke University in Durham, North Carolina, discovered that hormone-producing gut cells, called enteroendocrine cells, have protrusions that resemble synapses, or connections between neurons. Based on this anatomical observation, he wondered whether or not these gut cells send signals to the brain through the vagus nerve, which travels from the gut to the brain stem.
To address this, Bohórquez and his team injected a specialized green fluorescent rabies virus – engineered to label cells that share synaptic connections – into the intestines of mice. They noticed that both the enteroendocrine cells and vagal nerves lit up green, researchers report in the journal Science. This means that the cells lining the gut directly communicate with the vagus nerve and the brain.
Next, researchers grew enteroendocrine cells and vagal neurons in a petri dish and observed that gut cells formed synapses with neurons. Then, they showed that common nutrients (sugar) are sensed by enteroendocrine cells, which transmit this sensation to vagal neurons. But how quick does this signal travel? By stimulating enteroendocrine cells, researchers found that these cells released glutamate, a chemical that excites neurons. Cells that communicate using glutamate do so quickly – on a millisecond time scale.
That’s much faster than hormones can travel from the gut to the brain through the bloodstream, Bohórquez says. Hormones’ sluggishness may be responsible for the failures of many appetite suppressants that target them, he says.
“We think these findings are going to be the biological basis of a new sense,” Bohórquez says in a statement. “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.”