Imagine you are sitting in a restaurant. What would you order if you are very thirsty? Would you make a different decision if you are also hungry?

How are thirst and hunger represented in the brain? How do they control foraging behaviors? To answer these questions, Bhagyashree Senapati, a Ph.D. student in the TIGP-MCB program, from Dr. Suewei Lin’s laboratory at the Institute of Molecular biology trained fruit flies to associate odors with food or water. The flies’ responses to these learned odor cues are controlled by their thirst and hunger states. They only approach water-related odors when thirsty and only seek food-related odors when hungry. Hence, thirst and hunger not only affect flies’ innate responses to water and food but also influence their utilization of learned information.

The research team showed that a neuropeptide called leucokinin functions as a thirst switch in the fly brain. Leucokinin is released when flies are dehydrated, and it drives flies to search for learned water cues by inhibiting two groups of dopamine-releasing neurons (thirst-DANs). Further study suggests that leucokinin is not only a thirst signal. Starvation also triggers its release in the brain. Interestingly, under this condition, leucokinin promotes learned food-seeking behavior via activating another group of dopamine-releasing neurons (hunger-DANs). Therefore, leucokinin is a shared signal for both hunger and thirst, but it acts on distinct groups of neurons to control how a fly responds to food or water cues.

But, how can a fly distinguish hunger and thirst if these two derivational states are represented by the same molecule? Why does not leucokinin inhibit thirst-DANs to evoke water-seeking responses when flies are hungry? The team solved these puzzles, showing that in starved flies, other hunger signals increase the activity of the thirst-DANs and thus neutralize the effect of leucokinin on these neurons. This study, therefore, identifies the first molecule that broadcasts thirst in the fly brain and reveals an elegant neural mechanism for how thirst and hunger signals coordinate to prioritize foraging decisions. This study was published in Nature Neuroscience on December, 2019.

Article website: https://www.nature.com/articles/s41593-019-0515-z

A thirst circuit in the fly brain