Obese mice are much more likely than lean mice to overeat in the presence of environmental cues, a behavior that could be related to changes in the brain, finds a new study by a Michigan State University neuroscientist.
The findings offer clues in Alexander Johnson's quest to unpack the interconnected mechanisms of overeating and obesity. Obesity is an epidemic domestically - more than a third of Americans are considered to be obese - and a growing health problem in other parts of the world.
"In today's society we are bombarded with signals to eat, from fast-food commercials to the smell of barbecue and burgers, and this likely drives overeating behaviors," said Johnson, MSU assistant professor of psychology. "Our study suggests both a psychological and neurobiological account for why obese individuals may be particularly vulnerable to these signals."
The study involved two groups of mice - one group that was fed a high-calorie diet until they became obese and a second group that was fed a regular lab chow diet so they stayed lean. Johnson then trained the mice with different auditory cues. Whenever they heard one cue, such as a tone, the mice received a reward of sugar solution; with a second cue, such as a white noise, they received no reward.
All of the mice were then given access to their assigned maintenance diet for three days so they were satiated (i.e., not hungry) for the test phase of the study.
In that test, the sugar solution was available to the mice at all times to see what would trigger them to start eating. When no cue was given, and when the white-noise cue was given (which previously offered no reward), the lean mice and obese mice ate roughly the same amount. When the rewarding tone cue was given, however, the obese mice ate significantly more of the sugar solution than the lean mice.
"From a psychological perspective, this tells us that the obese mice are more vulnerable to the effects of food cues on evoking overeating behavior," Johnson said. "Looking at it through a human lens, this suggests that obese individuals may be more sensitive to, say, the McDonald's Golden Arches."
But why? The final part of the study may offer an explanation.
Johnson also examined the mice's lateral hypothalamus, which is known as a key brain area in appetite and feeding behavior. Using a procedure called immunofluorescence to label neurons in this area of the brain, he found that neurons releasing a certain hormone - melanin-concentrating hormone, or MCH - were more abundant in obese mice. But importantly, these MCH-releasing neurons were more active when the obese mice encountered the environmental reminders of sugar.
"In other words, if you become obese, this may lead to increases in MCH expression, and this may make you more sensitive to this form of overeating," Johnson said.
The novel findings, he added, start to paint a picture in the relationship between brain-behavior mechanisms that may underlie learned overeating in obese individuals.
"This could be one of perhaps many reasons why obese people have the urge to eat more when presented with food cues."
The study, funded by the Michigan Diabetes Research Center and the National Institutes of Health, will be presented this week at the annual meeting of the Society for the Study of Ingestive Behavior in Porto, Portugal. Johnson's co-researchers were MSU students Lauren Raycraft and Ryan Gifford.