The researchers drew their conclusions after performing a series of experiments with obesity-prone and obesity-resistant rats. The obesity-prone strain was developed for obesity research by breeding obese rats with other obese rats. The obesity-resistant rats were developed by breeding lean rats with lean rats, Kotz noted. The study also employed a control group of normal laboratory rats.
Each rat consumed the same number of calories each day. The researchers took baseline measurements of each rat's activity using sensors to measure even minor movements, such as grooming and standing.
They found that the lean group moved significantly more during this baseline period than the obese group, Kotz said. This was true even though the rats were young and both groups weighed the same -- eliminating the obesity itself as the cause of the decreased movement. After the baseline data gathering, the researchers moved to the experimental part of the study.
Lean rats have elevated expression of orexin receptors
"We knew from previous studies that orexin stimulated physical activity, and so we wanted to find out whether it enhances activity more in lean rats than in obese rats, Kotz explained. The researchers injected orexin into the lateral hypothalamus area of the brains of both groups and found that the lean rats became even more active, while the obese rats didn't respond much at all. "Not only do the lean rats have a higher base activity rate but they respond more to orexin," she said.
Orexin must bind to receptors in the brain to produce increased activity, so the researchers reasoned that the lean rats must have more orexin receptors. When they did a blind a
Contact: Christine Guilfoy
American Physiological Society