The food preference neurons (green) in a fruit fly head. (Credit: Wu laboratory)

Fruit flies typically depend on yeast as a main protein source, but have a natural tendency to seek out a quick fix of sugar.

New research from scientists at Johns Hopkins University found a small set of brain cells, called dopamine wedge neurons, are responsible for influencing the insects to crave yeast once they’ve been deprived of the nutrient. The researchers were also able to control those cells, affecting what the flies ate.

Little is understood about how the brain regulates nutrient-specific hunger, but the new findings could further investigations into a similar process in mammals and this understanding could help provide clues to addressing weight gain and obesity issues.

For the study, published May 5 in Science, the team, led by Mark Wu, M.D., Ph.D., associate professor of neurology at Johns Hopkins, starved flies of yeast for one week, and found that following deprivation, the insects consumed more yeast and less sugar than flies on a control diet.

Wu’s team, using novel genetic tools, identified a set of dopamine neurons that controlled protein-seeking behavior.

Using small electrodes to measure the signaling behavior of the neurons, the researchers found that when the fruit flies were deprived of protein-rich yeast, the cells in the circuit were active, firing four times faster than those in insects fed a normal protein diet.

Interestingly, when the circuit was activated, the flies consumed more yeast than normal, but when the circuit was turned off they ate less yeast.

The insects’ general hunger and thirst was not affected by turning the circuit on or off.

“We showed that just a few neurons forming a circuit in the fruit fly brain adopts processes used in learning and memory to control persistent, motivated behaviors, food preferences in this case,” Wu said in a statement.

The team also found that the circuit not only promotes protein feeding behavior, but it also serves the dual function of suppressing sugar cravings.

Fruit flies genetically engineered to have the circuit turned off were starved of yeast and observed to measure how much sugar and yeast they ate. Those with the silenced dopamine wedge neurons at about twice as much sugar compared to those whose food-preference circuit had not been turned off.

Structural changes were also observed in one branch of the brain cells in normally fed flies, following high-protein yeast deprivation. Increased size, as well as a greater number of active zones were observed, and this increased size remained for hours even after the flies began eating protein again.

“We found that each of these food preference neurons has two branches, one that controls protein feeding and the other sugar feeding,” Wu said.

“Adult flies usually have a sweet tooth, but when they are starved of protein, the brain makes it a priority to find this nutrient. Once they finally get some protein, the blockade on sugar feeding lifts but the flies still continue to be interested in eating protein,” Wu explained. “In this way, the circuit we identified promotes a single-minded focus on eating protein when the animal is protein-starved, but also allows for more flexible eating patterns with a continued preference for protein, when the need for protein is less.”

Up next Wu and his team will investigate specific molecules responsible for causing the neurons in the hunger circuit to fire and then examine similar protein-specific hunger circuits in mammals.

Contributing Editor/Science Writer