Researchers at Duke University demonstrated that enteroendocrine cells in mice guts form synaptic connections with vagal neurons in the brain.
It has been known for some time that the more than 100 million nerve cells in the human gut communicate with the brain by releasing a variety of hormones into the bloodstream that spurs digestion and suppresses hunger. These messages take about 10 minutes to reach the brain. Diego Bohórquez and colleagues at Duke University have found, however, that there is a second more direct and rapid communication mechanism between the gut and the brain.
The recent results build on an earlier discovery by the scientists that the enteroendocrine cells that line the gut and produce those hormones have footlike protrusions that resemble the synapses neurons use to communicate with one another. They wondered if the enteroendocrine cells also communicate with neurons via electrical signaling. That would require signaling through the vagus nerve, a nerve that connects the gut to the brain.
To find out, the researchers injected the colons of mice with a fluorescent rabies virus known to be transmitted through neuronal synapses. It was indeed found that the enteroendocrine cells formed synaptic connections with vagal neurons. In addition, enteroendocrine cells in a petri dish formed these connections and produced the neurotransmitter glutamate, which plays a role in taste and smell. The vagal neurons picked up the glutamate within 100 milliseconds. The scientists are now looking to find out what information is transmitted through these synaptic connections – such as the nutrients provided by and caloric value of the food being digested.
In a separate study, researchers at the Icahn School of Medicine at Mount Sinai in New York City found that laser stimulation of the sensory neurons that innervate the gut in mice produced rewarding sensations and increased levels of the mood-elevating neurotransmitter dopamine in their brains.
According to the study leader Ivan de Araujo, the results of these two studies help explain why stimulating the vagus nerve with electrical current can treat severe depression and 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,” he says.