Hunger affects decisions and risk perception
Hungry people are often rather difficult contemporaries. A rich meal can have an effect not only on our mood, but also on our willingness to take risks. This phenomenon is also known in the animal kingdom for a wide variety of animal species. We have shown in the fruit fly Drosophila that hunger not only alters behavior, but also the processing pathways in the brain.
The influence of hunger on processing pathways in the brain
Whether and how much food is available has a decisive influence on animal behavior. Studies show that the willingness of many animals to take risks increases or decreases depending on whether an animal is full or hungry. For example, more dangerous prey is hunted only when the hunter is on the verge of starvation. In recent years, this behavior has also been documented in humans: In one study, hungry subjects took significantly more financial risks than their sated counterparts.
Even the fruit fly Drosophila changes its behavior depending on its nutritional status. For example, the animals usually perceive even small amounts of carbon dioxide as a danger signal and take flight. However, rotting fruits and plant parts - the flies' main food sources - also emit carbon dioxide. We have discovered how the fly brain deals with this constant conflict decision between hazardous substance and food odor.
Projection neurons trigger escape responses in hungry animals
In various experiments, the scientists presented the flies with environments containing carbon dioxide or a carbon dioxide-food scent mixture. They found that hungry flies overcame their carbon dioxide aversion significantly faster than sated flies - if a food scent was present at the same time. If there is the prospect of food, hungry animals are thus much more willing to take risks than sated flies. But how does the brain manage to choose between these options?
Carbon dioxide avoidance is an innate behavior and should therefore be generated outside the so-called mushroom body in the fly brain: Mushroom body neurons have previously been associated only with learning and behaviors based on learned associations. However, when the scientists temporarily inactivated these neurons in the experiment, hungry flies no longer showed any response to carbon dioxide. Satiated flies, on the other hand, continued to avoid carbon dioxide.
In further studies, the researchers then identified a so-called projection neuron, which carries the carbon dioxide information to the mushroom body. This neuron is crucial for triggering a flight response in hungry, but not in sated, animals. "In sated flies, nerve cells outside the mushroom body are sufficient for flies to flee from carbon dioxide. In hungry animals, on the other hand, the nerve cells in the mushroom body and the projection neuron that brings the carbon dioxide information there are essential for the flight response. Therefore, carbon dioxide no longer disturbs exclusively hungry flies when the mushroom body or the projection neuron are inhibited," explains Ilona Grunwald-Kadow, the study's leader.
The results show that innate escape behavior to carbon dioxide in fruit flies is controlled by two parallel neural circuits, depending on their state of satiety. "When the fly is hungry, it no longer relies on 'direct conduction' but uses brain centers that allow it to weigh internal and external signals and make a balanced decision," Grunwald Kadow explains, adding, "It's fascinating to see the extent to which metabolism and hunger affect processing in the brain."