We all know a fitness buff who can’t seem to get enough
exercise, but keeping fit may actually be addicting, finds a study
published in the December issue of
Behavioral Neuroscience. When a mouse’s
running wheel is taken away, its brain shows a jump in neurological
activity much like the symptoms seen during withdrawal from drug
addiction.
Justin Rhodes, a postdoctoral fellow at Oregon Health and Science
University, examined the neurological activity of two sets of mice. One
was a group of ordinary typical lab mice; the other had been bred for 29
generations to have a predisposition for running.
In the study, conducted at the University of Wisconsin-Madison,
each mouse was allowed to run as much as or as little as it wanted for
six days. As expected, the runner mice tended to go longer distances,
covering six miles a day compared to just two for the normal mice. On the
seventh day, researchers took the running wheels away from half of both
mice groups.
Five hours later, at the time of the day when mice usually hit
their wheel-running peak, researchers measured the brain activity of
every mouse by examining levels of Fos, a gene expressed in response to
neurological excitement. The exercise-denied mice had higher brain
activity in 16 out of 25 brain regions. The more a mouse had run during
the previous days, the more brain activity it had.
“These were the same brain regions that become activated when
you prevent rats from getting their daily fix of cocaine, morphine
alcohol or nicotine,” explains Rhodes. In withdrawal, stronger
neurological activity creates the desire to relapse, or return to an
addictive habit.
The researchers note that human research will also be needed before
they can conclude that people also become addicted to exercise.
