What makes us indulge in candies, fatty foods, drugs and alcohol to excess? In a new study that involved chocolate M&M candies and rats, scientists reveal why too much of a good thing is never enough.
Scientists from the University of Michigan experimented offered rats an unlimited supply of M&M while also stimulating their neostriatum, the part of the brain associated with movement that researchers suspected may also regulate reward responses, and found that the rats ate more than twice as many candies as they would have otherwise.
The latest findings, published today in the journal Current Biology, suggest that the urge to overeat may be closely related to the neostriatum and fluctuating levels of the naturally occurring drug-like chemical encephalin, the drug the researchers used to stimulate the rats' brains.
"This means that the brain has more extensive systems to make individuals want to over-consume rewards than previously thought," said the study's lead author, Alexandra DiFeliceantonio of the University of Michigan. "It may be one reason why over-consumption is a problem today."
Researchers had studied the neostriatum, the brain portion traditionally thought of as a center of movement coordination, after recent studies that implicated it might also be responsible in producing the "reward" signal that we receive when engaging in pleasurable tasks.
In the first part of the study, researchers measured the activity of the neostriatum while offering the rats M&M's, and monitored the levels of the chemical encephalin in the neostriatum with probes that have been previously implanted in the rats' brains.
Encephalin, like endorphin, is a natural opioid, which is a drug-like chemical that the brain produces, that attaches to the same receptors as many anesthetic or psychoactive drugs.
When researchers give the M&M candies to the rats, the rats expectedly observed "avid consumption" with the average rat eating 10 candies in 20 minutes. Researchers also found that the levels of enkaphalin in the neostriatum portion of their brains spiked, and rats that are the most M&Ms had the quickest and highest peak levels of enkaphalin in that portion of the brain.
Researchers then injected the rats with a dose of synthetic enkaphalin called DAMGO directly into the neostriatum and once again let the rats eat as many M&M's as they wanted.
Alarmingly, the rats injected with the drug ate twice as much candy as before at more than 17 grams, which was roughly 5 percent of their body weight. The equivalent would be a 150 pound person eating 7.5 pounds of M&M's in one sitting.
Researchers said that not only did the rats eat more, they ate faster, suggesting that enkephalin signals the brain to "eat faster" as well as "eat more."
Scientists compared the facial reactions of the experimental and control rats as they ate to determine whether the injection of the synthetic drug made the M&M's taste better, or whether it made the rats want more of the same thing.
Past research found that, for rats, sweeter food trigger more behaviors like tongue protrusion and lip licking, but rather that the brain chemicals increase their desire and impulse to eat them.
They found that for both groups, the facial expression on the face looked the same, leading scientists to suggest that it's not that enkephalins made rats like the M&Ms more, it only made them want to eat more of them at a faster pace.
Researchers believe that the latest findings could apply to people.
"The same brain area we tested here is active when obese people see foods and when drug addicts see drug scenes," DiFeliceantonio said. "It seems likely that our enkephalin findings in rats mean that this neurotransmitter may drive some forms of overconsumption and addiction in people."