Why do we sometimes keep eating even when we're full and other times turn down food completely? Why do we crave salty things at certain times, and sweets at other times? The answers, according to new neuroscience research at the University of Delaware, may lie in a tiny brain in an organism you might not expect.
Lisha Shao, assistant professor in the Department of Biological Sciences in the College of Arts and Sciences , has uncovered a neural network in the brains of fruit flies that represents a very early step in how the brain decides — minute by minute — whether a specific food is worth eating. The work was published in the journal Current Biology on Jan. 29.
"Our goal is to understand how the brain assigns value — why sometimes eating something is rewarding and other times it's not," Shao said.
Until this discovery, scientists knew very little about the connection between taste and the systems in the brain that determine which foods we pursue, learn and remember.
Although they are the size of a pinpoint, the brains of fruit flies use many of the same kinds of chemical messengers and basic building blocks that are found in mammals and humans, making the work a useful window into the general rules the human brain uses to begin processing rewards.
As our behavior is determined by our brains, understanding which neurological circuits are involved at the beginning of the reward system allows scientists to map the entire system, and give them clues to where unhealthy behaviors—such as eating disorders—can originate.
"Reward drives almost everything we do," Shao said. "If the brain assigns the wrong value to something — too much or too little — behavior goes wrong. That's at the heart of many neurological and psychiatric disorders."
Tiny brains, big insights
Scientists have long known how the body decides whether a food is sweet, salty, bitter or umami. Neurons in the taste buds detect the flavors and the brain gives them those default meanings.
But taste is only part of the story. What's been harder to explain is how the brain interprets the meaning of a taste – how the same food can feel rewarding one moment and not the next, depending on context. For example, from an evolutionary perspective, sweetness usually means a food is nutritionally important for survival. But we don't always eat things that are sweet, even when they're right in front of us.
"If you just ate breakfast and you're full, you'll say no to a donut," Shao said. "But that doesn't mean donuts aren't rewarding. It means the brain is integrating context, internal state and past experience."
Like humans, fruit flies are sophisticated in how they approach food. They won't eat when they're full. In experiments, Shao's team found that activating a pair of neurons – named Fox because they look like fox ears — in the flies' brains would make them eat dramatically more amounts of food, even after they had just been fed.
The experiments also showed certain flies chose what they ate based on what their bodies needed most. Female flies which were reproducing and needed protein to produce eggs, preferred protein-rich food over sugar. Male flies and non-reproductive females ate more of both foods, maintaining their natural balance.
"Fox neurons are the earliest known place in flies where value computation for taste begins," she said. "Scientists are still debating where that very first step happens in mammals."
From flies to safer treatments for humans
In humans, understanding the neural connections that brains use to assign values to an experience can help scientists understand what happens when the reward processing system doesn't work correctly. The result can be disorders like addiction or anorexia or binge eating.
"Behavior starts with value," she said. "If we understand how value is built, we can better understand why we do what we do, and why sometimes it goes wrong."
In today's fast paced, technology-based world our brains are constantly bombarded with new experiences, making it harder for us to assign the correct value to each.
"Our brains evolved to process natural rewards like food and reproduction," Shao said. "But now we're surrounded by artificial rewards — endless short videos, processed foods — that the brain was never designed to handle."
Many current treatments for psychiatric and neurological disorders focus on brain chemical messengers, often including dopamine and serotonin. Both help our brains regulate our moods and some bodily functions. Imbalances in either can cause mental health issues. Medications aim to restore the balance of the chemicals but act on the brain like a "chemical soup," Shao said.
"If dopamine is thought to be too high, we try to lower it everywhere. If serotonin is thought to be too low, we raise it everywhere."
That approach, she said, helps explain why medications can have significant side effects and inconsistent results.
Shao's research opens doors to understanding the full scope of the reward processing system, and the invention of targeted and safer treatments.
"If we understand how decisions are made at the circuit level," she said, "we're one step closer to understanding why they sometimes go wrong, and how to fix them. You can't fix what you don't understand."
