From Scientific American:
Like many people, rats are happy to gorge themselves on tasty, high-fat treats. Bacon, sausage, chocolate and even cheesecake quickly became favorites of laboratory rats that recently were given access to these human indulgences—so much so that the animals came to depend on high quantities to feel good, like drug users who need to up their intake to get high. A new study, published online March 28 in Nature Neuroscience, describes these rats' indulgent tribulations, adding to research literature on the how excess food intake can trigger changes in the brain, alterations that seem to create a neurochemical dependency in the eater—or user. (Scientific American is part of Nature Publishing Group.) Preliminary findings from the work were presented at the Society for Neuroscience meeting in October 2009.
Like many pleasurable behaviors—including sex and drug use—eating can trigger the release of dopamine, a feel-good neurotransmitter in the brain. This internal chemical reward, in turn, increases the likelihood that the associated action will eventually become habitual through positive reinforcement conditioning. If activated by overeating, these neurochemical patterns can make the behavior tough to shake—a result seen in many human cases, notes Paul Kenny, an associate professor in the Department of Molecular Therapeutics at The Scripps Research Institute in Jupiter, Fla., and co-author of the new study. “Most people who are overweight would say, 'I would like to control my weight and my eating,' but they find it very hard to control their feeding behavior,” he says. Despite a growing body of research, it has been unclear whether extreme overeating was initiated by a chemical irregularity in the brain or if the behavior itself was changing the brain's biochemical makeup. The new research by Kenny and his colleague Paul Johnson, a graduate student, shows that both conditions are possible.