Fruit flies’ brains may be wired to count calories.
Several genes in the brain appear to help the flies learn to distinguish between normal-calorie and high-calorie foods — and to remember to choose the healthier option later. Feeding the flies a constant diet of high-calorie foods disrupts their ability to make these metabolic memories, researchers report April 7 in Nature Communications.
Preliminary studies suggest that mice make similar metabolic memories, the researchers say. Taken together, the results hint that human brains may also be wired to do the same thing, which could have implications for weight control and health. But constant exposure to high-calorie foods may have damaged humans’ abilities to make metabolic memories, says study coauthor Dongsheng Cai of Albert Einstein College of Medicine in New York. Being able to rebuild humans’ metabolic memory could help control diseases such as obesity and diabetes, he says.
Cai and his colleagues gave fruit flies (Drosophila melanogaster) a choice between yeast mixed with a moderate amount of a sugar called sorbitol and yeast mixed with an extra amount of this sugar. Flies spent a day in a vial with the moderately sugary yeast and then a day in a vial with extra-sugary (and therefore higher-calorie) yeast. After cycling through the vials several times, the flies started to eat more of the moderately sugary yeast. The food choices tasted the same, but each was matched with a particular smell. Flies put in a container with only the smells that matched the two food options (but no food) preferred the smell associated with the moderate-calorie yeast.
When the flies spent consecutive days in vials with the more sugary yeast, however, they lost their ability to distinguish between the moderate-calorie and high-calorie options. This observation suggests that access to high-calorie food may damage the flies’ ability to make metabolic memories, the researchers say. These flies also had higher levels of sugar and fat accumulated in their bodies, conditions that predispose mammals to diabetes.
Additional experiments showed that the flies also had trouble making metabolic memories when they had faulty learning and memory genes in a region of their brain called the pars intercerebralis. This brain region is similar to the hypothalamus, which controls hunger and thirst in mice and humans.
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In a preliminary study of mice, Cai’s team found that when a strain of mice prone to obesity was overfed, some of the rodents’ learning and memory genes didn’t work properly in the hypothalamus. The results, paired with the experiments in flies, suggest that the hypothalamus might play a leading role in animals’ abilities to make metabolic memories, the researchers conclude.
“Knowing the nutritional value of a food is critical for survival,” says MIT neuroscientist Kay Tye, who was not involved in the study. It makes sense that animals would learn to associate foods with some type of fixed value of calories in the brain region that controls hunger, she says.
Tye says more work needs to be done to establish if animals’ metabolic memories are made in the hypothalamus alone. Identifying which brain circuits are involved in metabolic memory is one of the next steps in the research, Cai says.