Our internal states, such as hunger or thirst, influence much more than just our desire to eat or drink. They can alter our decisions, even in situations unrelated to food. In a study published in
Nature Communications, scientists show how, in the fruit fly Drosophila, these states act on specific brain circuits, influencing behaviors in the face of danger.
When hunger changes how we react to danger
It is now well established that our nutritional state does not only determine our appetite, but can also influence other types of decisions, such as risk-taking. For example, being hungry can push one to act more boldly, even in risky contexts. But the mechanisms by which internal signals like hunger or thirst modify our brain and our behaviors remain poorly understood.
To advance this understanding, scientists need to be able to map the connections between neurons in the brain, record their function and interactions, and see how they are modulated by internal signals such as circulating hormones or molecules released in the brain depending on the body's state. And then measure the impact of these variations on behavior. Studying this directly in the mammalian brain is very complex and we lack experimental tools.
Diet influences the behavior of the fruit fly Drosophila
To overcome these obstacles, scientists, in a paper published in the journal
Nature Communications, chose a much simpler model, the Drosophila or vinegar fly, and more specifically its larva, whose compact brain is very accessible, allowing for the detailed study of neural circuits.
Using a map representing all the connections between all the neurons in the Drosophila larva's brain obtained from electron microscopy images, the scientists were able to identify the neurons that transmit information about the internal state (such as hunger) from those that control defensive behaviors, like startle or escape.
Thanks to an imaging technique that allows tracking the activity of neurons in the living larva, they were able to show that this activity is influenced by the feeding state: when the larvae are starved, the activity of neurons that trigger the startle response decreases, while that of neurons that promote escape increases.
This change makes the larvae more inclined to flee than to startle, showing that their internal state guides their strategy in the face of danger.
The scientists also showed that this modulation is partly regulated by neuropeptides, which are small proteins produced by neurons that serve to transmit information from one neuron to another, homologous to neuropeptide Y involved in feeding behavior in mammals, including humans.
By manipulating the activity of these neurons, the scientists were able to highlight clear changes in the larvae's behavior. This also shows that some neurons, organized in reciprocal inhibition circuits (where neurons inhibit each other), are capable of combining information from the environment and the internal state of the organism to arbitrate between different behaviors. This organization allows the brain to flexibly choose the response most suited to the immediate situation.
Overall, this work has made it possible to decipher, at the molecular and cellular scale, how hunger can influence behaviors not directly linked to feeding.