A new study conducted by researchers from Johns Hopkins Medicine revealed that human food preference might be determined by a tiny set of brain cells, known as dopamine wedge neurons.
The researchers omitted protein from the diet of fruit flies and those brain cells released dopamine, a chemical signal that seemed to control their craving for yeast, their main protein source, overruling their natural habit to seek out sugar.
The findings can enhance research to find similar processes in humans and other mammals, which could lead to the better understanding of cravings and hunger and eventually obesity and weight gain.
Even though previous studies have identified hormones and sensors in mammals and fruit flies that control the caloric intake, the Johns Hopkins researchers believe this is the first time a study has identified a protein-specific hunger mechanism in any animal.
To find the specific neurons that are responsible for controlling eating behavior, the scientists utilized newly mated female fruit flies that are apt to feed on food sources with high protein content to fill up nutrients in their eggs. The researchers looked at different fruit flies lines, which had been engineered utilized a genetic tool that halted specific groups of neurons from firing. They searched each line to find females that no longer favored consuming high-protein yeast after mating. The researchers added dye to the female fruit flies’ yeast food source and then crushed up the insects. Utilizing an instrument capable of detecting the amount of dye they were to determine the quantity of high-protein yeast the flies consumed.
The researchers were able to identify a set of dopamine neurons that are responsible for controlling protein preference. After further examination of the neurons, they were able to assign food preference signals to only two neurons, which are located on each side of the fruit flies’ brains in a region known as the wedge. This is how the food preference cells were dubbed dopamine wedge neurons.
Utilizing small electrodes, the researchers were able to measure signaling or electrical firing behavior of those neurons in flies that had been deprived of protein-rich yeast. After eight days of being deprived protein in their diets, the researchers measured the insects’ electrical signaling behavior. When compared to fruit flies that were fed a normal protein diet, the protein deprived fruit flies dopamine-wedge neurons fired four times faster.
In nature, fruit flies that have been deprived protein will hunt for food sources of high-protein yeast instead of fruit sugar, which is typically preferred over protein for a quick energy boost. This made the researchers wonder if the dopamine wedge neurons were also responsible for suppressing sugar cravings.
This time the researcher choose male fruit flies that had been genetically engineers with silenced dopamine wedge neurons. The insects were deprived of high-protein yeast, before the researchers measured how much yeast and sugar they consumed. The findings revealed that those flies consumed on average nearly twice as much sugar for each fly, when compared to those fruit flies with silenced dopamine wedge neurons.
Associate professor of neurology at the Johns Hopkins University School of Medicine, Mark Wu, M.D., Ph.D., said, “We found that each of these food preference neurons has two branches, one that controls protein feeding and the other sugar feeding.”
“Typically flies need sugar as a quick source of calories to fly around, so their neurons bypass the protein circuit,” Wu says. “Once you relieve pressure on the protein side by feeding them yeast, the flies can eat sugar again, but they still have a strong craving for protein, because it takes time for the fly to replenish its protein stores and for its neurons branches to go back to their original state,” he adds.
The study was published in the journal Science on May 5, 2017.