Lead a mouse to an exercise wheel and nothing short of unabashed sprinting will surely ensue.
A recent UW-Madison study suggests that for mice, exercise can be as addictive as cocaine in humans. There is reason to believe if mice can exhibit such a craving for physical activity, then so can other animals, possibly even humans.
For years, UW-Madison scientists have selectively bred mice to display high levels of voluntary wheel running behavior. On average, these mice run about three times the distance of normal laboratory mice.
\We decided to look at the brains of these mice with the idea that their brains might have changed to make them more motivated to run,"" said Justin Rhodes, lead author of the study, now a postdoctoral fellow at Oregon Health & Science University.
The study, which is featured in the December issue of Behavioral Neuroscience, compared the brain activity in a control population of normal mice with that of the highly active mice. For six days, the researchers allowed all of the mice free access to exercise wheels. On the seventh day, they allowed only some of the mice to run while restricting the others.
Upon measuring and comparing the neuron stimulation in the brain of each mouse, the researchers were surprised by what they discovered.
""We thought most of the brain activity would be occurring during the running itself,"" Rhodes said. ""Not so, the big changes happened when we prevented the mice from running.""
Interestingly, much of the brain activity occurred in the same regions that are involved with cravings for food, sex and drug abuse in humans, Rhodes said.
""We [saw] the same brain regions activated as when a drug addict is shown an image of drugs, but cannot obtain them,"" added Stephen Gammie, a co-author of the paper and assistant professor of zoology at UW-Madison.
If a creature has exercise addiction it will display signs of withdrawal, such as frustration or stress, when the reward of exercise is withheld.
""With this definition, all the mice [normal and high-runners] were addicted to exercise because they all showed huge amounts of brain activity when they were denied running,"" Rhodes added.
While it is unknown whether similar results could be achieved in animals other than mice, Gammie has his inclinations.
""I think the results could apply to other animals. If mice can show activity in brain regions involved in reward and addiction in association with wanting to run, then it seems likely other animals could link exercise to reward centers in the brain,"" Gammie said.
