A key protein in the brain may be the reason why some people cannot be bothered to take up physical exercise, French researchers believe.
They have found that the CB1 cannabinoid receptor, which is associated with motivation and reward systems, controls the amount of time that an individual will undertake voluntary physical exercise.
“The inability to experience pleasure during physical activity, which is often quoted as one explanation why people partially or completely drop out of physical exercise programmes, is a clear sign that the biology of the nervous system is involved,” says Francis Chaouloff, research director at Inserm’s NeuroCentre Magendie, at the Université Bordeaux Ségalen.
The exact role of the endocannabinoid system during physical activity has remained a mystery, despite the discovery ten years ago that physical exercise activated the endocannabinoid system in trained sportsmen.
Three years ago, it was also discovered that mice lacking the CB1 cannabinoid receptor ran for a shorter time and less distance than healthy mice.
Chaouloff and colleagues believe their latest research into mice models, published in the January edition of Biological Psychiatry, can now help scientists to begin understanding how, where and why the lack of CB1 receptor reduces voluntary exercise performance by between 20 and 30 per cent.
The researchers showed that the CB1 receptor controlling running performance is located at the GABAergic nerve endings and went on to show that the receptor is located in the ventral tegmental area of the brain, which is involved in motivational processes relating to reward.
Based on the results of the latest study and earlier work, the team concluded that endocannabinoid stimulation of the CB1 receptor during physical exercise inhibits the release of GABA, an inhibitory neurotransmitter that controls the activity of the dopamine neurons associated with the motivation and reward processes.
Because the stimulation of the CB1 receptor activates the dopaminergic neurons in the ventral tegmental area, it must be stimulated before the exercise can go on for longer and the body must receive the necessary motivation, explained Chaouloff.
“If confirmed, this motivational hypothesis would imply that the role played by the CB1 receptor has more to do with ‘staying power’ in the exercise than with actual physical performance levels,” say the researchers.
“After endorphins, we now need to consider endocannabinoids as another potential mediator of the positive effects that physical exercise has on our mood.”
Biological Psychiatry January 2013
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