After showing that obese rats had clear addiction-like food seeking behaviors, Johnson and Kenny next investigated the underlying molecular mechanisms that may explain these changes. They focused on a particular receptor in the brain known to play an important role in vulnerability to drug addiction and obesity - the dopamine D2 receptor. The D2 receptor responds to dopamine, a neurotransmitter that is released in the brain by pleasurable experiences like food or sex or drugs like cocaine. In cocaine abuse, for example, the drug alter the flow of dopamine by blocking its retrieval, flooding the brain and overstimulating the receptors, something that eventually leads to physical changes in the way the brain responds to the drug.

The new study shows that the same thing happens in junk food addiction.

"These findings confirm what we and many others have suspected," Kenny said, "that overconsumption of highly pleasurable food triggers addiction-like neuroadaptive responses in brain reward circuitries, driving the development of compulsive eating. Common mechanisms may therefore underlie obesity and drug addiction."

Consistent with common mechanisms explaining addiction and obesity, levels of the D2 dopamine receptors were significantly reduced in the brains of the obese animals, similar to previous reports of what happens in human drug addicts, Kenny noted. Remarkably, when the scientists knocked down the receptor using a specialized virus, the development of addiction-like eating was dramatically accelerated.

"This addiction-like behavior happened almost from the moment we knocked down the dopamine receptors," Kenny noted. "The very next day after we provided access to the palatable food, their brains changed into a state that was consistent with an animal that had been overeating for several weeks. The animals also became compulsive in their eating behaviors almost immediately. These data are, as far as we know, the strongest support for the idea that overeating of palatable food can become habitual in the same manner and through the same mechanisms as consumption of drugs of abuse."

Source: Scripps Research Institute