Binge eating can be a major problem when you're trying to control your weight - the compulsion to keep gorging can be so strong that it feels impossible to stop.
New research has identified a brain pathway that controls impulsive eating in fruit flies, an important discovery that may help explain binge eating in humans.
The research team, led by Ping Shen, a University of Georgia associate professor of cellular biology who also is a member of the Biomedical and Health Sciences Institute, wanted to understand how smell and taste stimulate impulsive eating behavior.
The findings, published in the journal Cell Press on February 28, show how fly larvae can be used to study the evolution of reward-driven impulses based on smell and taste. They suggest that impulsive eating for pleasure is an ancient behavior, and may shed light on the flaws in the system that lead to overeating and binge eating in humans.
Impulsive behaviors like binge eating are often misunderstood because the human brain is so complicated. The nervous systems of fly larvae, however, are much simpler. Their basic structure is very similar to the brains of adult flies and mammals, but with many fewer neural cells and connections.
"A particular function in the brain of mammals may require a large cluster of neurons," said Shen. "In flies, it may be only one or four. They are simpler in number but not principle."
This made fly larvae an ideal subject for observing a possible impulsive binge eating mechanism in the brain.
"We know when insects are hungry, they eat more, become aggressive and are willing to do more work to get the food," Shen said in a statement to UGA. "Little is known about the other half-the reward-driven feeding behavior-when the animal is not so hungry but they still get excited about food when they smell something great.
"The fact that a relatively lower animal, a fly larva, actually does this impulsive feeding based on a rewarding cue was a surprise."
To test reward-driven eating behavior in flies, Shen delivered enticing odors to groups of well-fed larvae. In every case, the already full larvae impulsively ate about 30 percent more sugar-rich food when immersed in the attractive odors. When the insects were offered an unfulfilling meal with less sugar, however, they refused to eat it.
"They have expectations," said Shen. "If we reduce the concentration of sugar below a threshold, they do not respond anymore. Similar to what you see in humans, if you approach a beautiful piece of cake and you taste it and determine it is old and horrible, you are no longer interested."
Shen's team tried to further define the connection between getting excited about food and going on an eating binge. He found when the larvae were briefly presented with an odor, they were not willing to act on the overeating impulse for very long.
"After 15 minutes, they revert back to normal. You get excited, but you can't stay excited forever, so there is a mechanism to shut it down," he said.
Shen's research implies that the brain chemicals that trigger impulsive overeating binges are similar, if not the same, between flies and humans. The brain chemicals, called neuropeptides, are used by neurons in the brain to receive and convert environmental cues into signals that tell animals to act.
Neuropeptide F (NPF) is the brain chemical responsible for causing odor-induced eating in fruit flies, and the scientists found that deficiencies in NPF signaling in flies blocked overeating caused by enticing odors. NPF production is related to dopamine, a major neurotransmitter chemical responsible for the feeling of reward that comes from stimulating behaviors.
The scientists also identified dopamine-producing neurons in flies that became active when presented with appetizing odors, and that projected to a major olfactory center of the brain that can cause binge eating when overstimulated.
The similarities between the basic neural functions of flies, humans, and all animals suggest that the molecular details of these functions are evolutionarily linked. In human brains, the counterpart to the flies' NPF is neuropeptide Y (NPY) - which suggests that NPY may be related to overeating binges in humans.
"There are hyper-rewarding cues that humans and flies have evolved to perceive, and they connect this perception with behavior performance," Shen said. "As long as this is activated, the animal will eat food. In this way, the brain is stupid: It does not know how it gets activated. In this case, the fly says 'I smell something, I want to do this.' This kind of connection has been established very early on, probably before the divergence of fly and human. That is why we both have it."
In the fly's brain model, four neurons control the transferring of signals from the olfactory, or scent-related, neural center to actions like eating. Every odor and receptor translates the response in a slightly different way, and more enticing odors can cause overeating binges.
As you can imagine, human triggers for binge eating are much more diverse and complex that those of flies, but Shen believes the causal brain mechanism for stimulated overeating is the same. After this study, Shen is working with Tianming Liu, assistant professor of computer science at UGA, on a computer model to determine how these odors are interpreted as stimuli.
"Dieting is difficult, especially in the environment of these beautiful foods," said Shen. "It is very hard to control this impulsive urge. "
If research can figure out how compulsive eating is triggered in the human brain, scientists may be able to devise a behavioral method of preventing binge eating.
Eventually, it may even be possible to use to control binge eating with medications, or simply to learn enough to allow ourselves to control our own appetites without too much pain.
Published by Medicaldaily.com