Biologists have discovered a sensor in the throats of fly larvae that seems to trigger serotonin—a feel-good hormone—that is released in the brain when high-quality food is swallowed.
The same mechanism is likely to exist in humans too, the scientists said, which might have implications for disordered eating behaviors and how they're treated in the future.
"We wanted to gain a detailed understanding of how the digestive system communicates with the brain when consuming food," said professor Michael Pankratz of Germany's University of Bonn in a statement about the study.
"In order to do this, we had to understand which neurons are involved in this flow of information and how they are triggered," he said.
Biologists at the University of Bonn and the U.K.'s University of Cambridge used fruit fly larvae for this study because they have a network of around 10,000 to 15,000 nerve cells—much more manageable than the 100 billion in a human brain. But these nerve cells still form a complex network of neurons and synapses, which the researchers mapped in their entirety.
The scientists cut a larva into thousands of razor-thin slices and photographed them under a microscope. "We used a high-performance computer to create three-dimensional images from these photographs," said Pankratz.
Then, study lead author Andreas Schoofs and co-author Anton Miroschnikow used these photographs to investigate how the nerve cells were connected across the digestive system and brain.
The team identified a receptor in the throat, linked via the vagus nerve (sometimes referred to as the gut-brain axis), to a group of six neurons in the larva's brain that could produce serotonin.
They concluded that the process of swallowing, combined with sensory information about the value of the food, could prompt serotonin to be released in the brain to encourage more eating.
"[Animals] can detect whether it is food or not and also evaluate its quality," said Schoofs in a statement. "They only produce serotonin if good-quality food is detected, which in turn ensured that the larva continues to eat."
The scientists also speculated about the impact that their research might have on human health if a similar mechanism exists in humans.
They suggested that a defective swallowing mechanism—where too much or not enough serotonin was released in the brain in response to food—could cause patterns of disordered eating or even conditions such as anorexia or binge eating disorder. Further research could have implications for the treatment of eating disorders, they said.
"But we don't know enough at this stage about how the control circuit in humans actually works," said Pankratz. "There is still years of research required in this area."
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Reference
Schoofs, A., Miroschnikow, A., Schlegel, P., Zinke, I., Schneider-Mizell, C.M., Cardona, A., & Pankratz, M.J. (2024). Serotonergic modulation of swallowing in a complete fly vagus nerve connectome. Current Biology. https://doi.org/10.1016/j.cub.2024.08.025
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