Swallowing food triggers ‘feel-good hormones’

Scientists have recently discovered an important control circuit involved in swallowing food. The study has shown that fly larvae have special receptors in their esophagus which are triggered as the insect swallows something. 

This leads to the release of serotonin, a messenger substance that is frequently referred to as the “feel-good hormone.” The release of serotonin ensures that the larva continues to eat. 

Although the study focused on flies, the experts argue that humans may also have a similar control circuit. 

Swallowing food makes you want more 

Imagine you’re sitting in a restaurant, feeling hungry, with a delicious pizza in front of you. Its irresistible aroma fills the air. 

You take a bite, savor the flavor as you chew, and the moment you swallow, a wave of satisfaction hits you: Wow, that was amazing! Without hesitation, you reach for another slice and eagerly take another bite.

While the pizza’s smell and taste motivate you to begin eating, it is the pleasurable feeling you have after swallowing which is responsible for you continuing to eat. 

“But how exactly does this process work? Which neural circuits are responsible? Our study has provided an answer to these questions,” said Michael Pankratz, an expert in life and medical sciences at the University of Bonn.

Neurons communicate with the digestive system 

The study was focused on the larvae of the fruit fly Drosophila, which have from 10,000 to 15,000 nerve cells – a manageable number compared to the 100 billion nerve cells in the human brain. 

However, these 15,000 nerve cells form a highly complex network, with every neuron having branching projections through which it contacts dozens or even hundreds of other neurons.

“We wanted to gain a detailed understanding of how the digestive system communicates with the brain when consuming food,” Pankratz said. 

“In order to do this, we had to understand which neurons are involved in this flow of information and how they are triggered.” 

An activated network of neurons

The scientists investigated not just the paths of all of the nerve fibers in the fruit fly larvae but also the connections between neurons. 

To do this, they cut a larva into thousands of very thin slices and photographed them under an electron microscope.

“We used a high-performance computer to create three-dimensional images from these photographs,” Pankratz explained. 

Next, project assistants Andreas Schoofs and Anton Miroschnikow analyzed how all these nerve cells are connected to one another.

Detecting food quality while swallowing 

The investigation helped the researchers identify what they called a “stretch receptor” in the esophagus that is wired to a cluster of six neurons in the larva’s brain which can produce serotonin, the “feel-good hormone” ensuring that we feel rewarded for certain actions.

The serotonin-releasing neurons receive new information about what the fly has just swallowed. 

“They can detect whether it is food or not and also evaluate its quality,” Schoofs explained. “They only produce serotonin if good quality food is detected, which in turn ensures that the larva continues to eat.”

Broader implications of the study 

The scientists believe that this fundamental mechanism most likely also exists in humans, and, when defective, it could lead to eating disorders such as anorexia or binge eating. 

Thus, the results of this study might also have implications for the treatment of such disorders. 

“But we don’t know enough at this stage about how the control circuit in humans actually works.There is still years of research required in this area,” Pankratz cautioned.

Food swallowing in mammals

The study authors noted that serotonin signaling has been implicated in motor function in the mammalian esophagus as well.

“The human esophagus also has distinct regions, with a proximal striated muscle region that is mainly controlled by motor neurons in the brainstem and a distal smooth muscle region that is controlled by central neurons in the medulla oblongata and peripheral neurons of the myenteric plexus.” 

The researchers concluded that, despite the differences in the number of cell types as compared with the fly, it would be interesting to see if serotonin also monitors the completion of a biologically meaningful action such as swallowing or other vital activities in mammals.

The study is published in the journal Current Biology.

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