‘Food circuit’ identified that could control appetite, study shows
In a surprise discovery the brain cells were connected to taste neurons in the pharynx, or throat, rather than in the tongue, which meant food was being evaluated while it was being eaten.
Researchers believe these findings can be transferred to the human brain, where control of appetite and food choices could be of benefit to those trying to manage weight.
Researchers at Rockefeller University began working with Drosophila flies - a species which exhibits similar mechanisms to humans in controlling taste and food intake.
To isolate the neurons that play a role in regulating eating behaviour, the researchers inhibited different groups of neurons.
IN1 cells, when inhibited, caused the flies to stop eating even if they were still hungry.
When observed under normal situations, even the tiniest amount of sweet food would activate the flies’ IN1 cells. These cells remained active for minutes after the food has been swallowed.
“The neurons are helping the brain evaluate what the animal's eating while it's eating it," said Nilay Yapici, a postdoctoral fellow involved in the study. “Silencing the activity of these neurons appears to suppress food intake."
The team’s thoughts were confirmed as the satiated flies ate as if they were starving when these neurons were turned back on.
Drosophila flies are known to detect and evaluate food using taste cells located in the periphery.
Upon ingestion, food comes in contact with taste neurons located in the pharynx. The function of these taste neurons is poorly understood, but a subset has been shown to regulate sugar ingestion.
This subset - IN1 - has been studied previously and is known to receive selective input from sweet taste neurons in the pharynx.
In trying to explain the results of this study, the researchers thought that the activity of the IN1 cells drove these animals to ingest food.
Subsequently, when the flies weren't hungry and encountered sweet food, the neurons were still active, but quietened down relatively quickly.
When the flies exhibited hungry behaviour and were forced to eat less tasty food, the neurons still showed activity but were quick to revert back to a pre-stimulated state.
The same action was observed when satiated flies were given tasty food. In each case, the activity of IN1 cells mirrored the eating behaviour of the fly.
The team thought that the activity seen in IN1 neurons may be down to intrinsic or extrinsic factors. They pinpointed an area where IN1 cells received input from dopaminergic, serotonergic, and peptidergic sensory neurons.
These neuromodulatory circuits may be sensitive to satiety state helping to prolong IN1 activity aided by sucrose ingestion in fasted animals.
“It is plausible that, once the fly reaches satiety, neuromodulatory activity is tempered, and IN1 neurons return to their basal state,” the study noted.
“Alternatively, IN1 neurons may have unique intrinsic biophysical features that sustain their activity after sucrose stimulation.”
The researchers concluded these findings may have implications for diseases related to food intake such as obesity. "The goal of studying food intake behaviour is to understand the biological signals that make us eat," commented Yapici.
By working with flies, which have relatively small brains compared to mammals, the researchers can more easily identify and manipulate specific circuits that regulate food intake, then see if similar pathways are at play in animals with more complex neurocircuitry, such as mice and other mammals.
"If you find a neural mechanism in the fly, you can look for similar principles in a mouse model -- since you know what you are looking for, it may be easier to find."
Published online ahead of print, doi.org/10.1016/j.cell.2016.02.061
“A Taste Circuit that Regulates Ingestion by Integrating Food and Hunger Signals.”
Authors: Nilay Yapici, Raphael Cohn, Christian Schusterreiter, Vanessa Ruta, Leslie B. Vosshall