Swallowing linked to brain sensations that stimulate food intake

Researchers have discovered how animals understand hunger

In a study of fruit fly larvae, an international team of scientists found that the process of food absorption is associated with the release of serotonin in the brain, a substance responsible for mood and sleep-wake cycles. The discovery could change the understanding of why animals eat, the researchers say.

Scientists took a detailed look at the larvae's nervous system located in their intestines, and the results shed light on swallowing. According to the data, the researchers consider this action "perhaps one of the most significant decisions made by animals."

“We sought to gain a complete understanding of the interaction between the digestive system and the brain during food,” says neuroscientist Michael Pankratz. “To do this, we needed to figure out which neurons are involved in this process and how they are activated.”

The researchers really went all out: they carefully cut one of the larvae into thousands of small pieces and photographed them under powerful microscopes, mapping neural pathways and connections. From these images, a three-dimensional model was created using computer software, leading to the discovery of a biological mechanism known as a stretch receptor, located, for example, in the esophagus, which connects the mouth to the stomach.

This receptor is connected to six specific neurons in the larva's brain, which receive information about the swallowing process and what exactly it is eating. If the larva finds tasty food, the brain emits a reward signal.

“Neurons are able to determine whether it is food and evaluate its quality,” emphasizes neuroscientist Andreas Schoofs. He added that "the electrically excitable cell releases serotonin only when good quality food is detected, which helps the larva continue to live."

Fruit flies have fewer than 200,000 neurons, compared with about 100 billion in the human brain. However, the complexity of these nerve cells allows them to be viewed as a scaled-down model of the human system, making it easier to catalog and understand the underlying mechanism necessary for animal survival.

In the future, researchers intend to find out whether a similar pattern of neuronal arrangement and serotonin secretion exists in other animals, including humans. The fact is that humans have more biological similarities to fruit flies than one might think, but understanding the human circuitry will require a significant expansion of research. The study will help understand whether this factor in eating well is common to everyone.

“At this stage, we still don’t know enough about how the human control system works,” concludes Michael Pankratz. “Many more years of research will be needed in this area.”