Type 2 dopamine receptors (D2DR)—brain receptors that play a key role in addiction--also play a key role in rats' heightened response to food, according to Scripps Insititute Neuroscientists.
The findings suggest that the same brain mechanisms that fuel drug addiction in humans may also accompany the emergence of compulsive eating behaviors and the development of obesity.
The study was published today in Nature Neuroscience.
When investigators gave rats access to varying levels of high-fat foods, they found unrestricted availability alone can trigger addiction-like responses in the brain, leading to compulsive eating behaviors and the onset of obesity.
Both obesity and drug addiction have been linked to a dysfunction in the brain's reward system. In both cases overconsumption can trigger a gradual increase in the reward threshold—requiring more and more palatable high fat food or reinforcing drug to satisfy the craving over time.
Researchers conducted this study in three groups of male rats over a 40-day period. Each day, the three groups had unlimited access to standard lab food. In addition, two of the groups also had access to high-fat, cafeteria style foods for short (one-hour) or long (18-23 hours) periods.
After 40 days, all groups were denied access to the high-fat foods. Throughout the study, researchers observed the feeding behaviors of each group, noting caloric intake, weight gain, and brain response.
As the rats became obese, the levels of D2DR in the brain's reward circuit decreased. This drop in D2DR is similar to that previously seen in humans addicted to drugs like cocaine or heroin.
"The results of this study could provide insight into a mechanism for obesity," said Paul J. Kenny, one of the study's co-authors and an associate professor at the Scripps Jupiter, Fla., research facility. "It's possible that drugs developed to treat addiction may also benefit people who are habitual overeaters."
Study results also suggest that environmental factors, such as increased or unlimited access to high-fat food options, can contribute to the problem of obesity.
Citation: Paul M Johnson, Paul J Kenny, 'Dopamine D2 receptors in addiction-like reward dysfunction and compulsive eating in obese rats', Nature Neuroscience, March 2010; doi:10.1038/nn.2519