Rather enjoy a strawberry milkshake or go for a run…? "Most of us have probably decided at least once to give up sports in favor of one of the many alternative temptations of everyday life," postulate the researchers of ETH Zurich (Switzerland). Denis Burdakov's team wondered what makes us choose between sport and food. They subjected mice to this choice and revealed the crucial role of a hormone: orexin. Their results were published in the journal Nature Neuroscience.
Dopamine is not enough
According to the World Health Organization (WHO), more than 3 in 10 adults do not do enough sport. Despite these statistics, many people manage to resist constant temptations and get enough exercise.", notes Denis Burdakov, professor of neuroscience. We wanted to know what in our brain helps us make these decisions." Often, it's dopamine that plays a role in evaluating options and motivating us. But exercising and eating a tasty dessert are both activities that release dopamine. So how does our brain decide in this situation?
To find out, the researchers offered mice eight choices, including a running wheel and a strawberry milkshake, which is particularly popular with rodents, as well as humans, because it is rich in sugar and fat. The goal of this experiment? To test the influence of orexin, a hormone with lesser-known effects than dopamine, on the mice's choices.
Orexin, a little-known neurotransmitter
Brain activity is regulated by hundreds of substances, vectors of various messages. Serotonin and dopamine in particular. Less popular, orexin was discovered less than 30 years ago and arouses the curiosity of researchers. Its functions, still poorly understood, are gradually becoming clearer. We now know that it is linked to the regulation of food intake, in particular in the stimulation of appetite. But its role certainly does not stop there, according to scientists.
"Without orexin, mice preferred milkshake"
The mice were divided into two groups. In one group, the orexin system was blocked, either by treatment or by genetic modification.
Then, the mammals evolved in an environment with eight different activity options. The result: mice with an intact orexin system largely preferred the wheel to the milkshake. They spent twice as much time exercising as their orexin-deficient counterparts, and half as much time drinking the milkshake. "Without orexin, the decision was heavily in favor of the milkshake, and the mice abandoned exercise in favor of feeding“, observes Denis Burdakov’s team.
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However, when the choices were restricted to two options, eating or running, the behavior of the two groups was surprisingly similar. The researchers analyzed: "This means that the primary role of the orexin system is not to control the amount of movement or food that mice eat." explains Denis Burdakov. "But according to our results, it is essential to make the decision between one and the other, when both options are available."
Researchers are now trying to understand how orexin-producing neurons interact with the rest of the brain during decision-making. If we elucidate how the brain trades off food intake and physical activity, we can develop more effective strategies to combat the global obesity epidemic and associated metabolic disorders.“, concludes Daria Peleg-Raibstein, co-author of the study.
Their work could lead to new research on humans to confirm these initial results. Indeed, some people have a restricted orexin system. On the one hand, for genetic reasons: narcoleptics who suffer from a sleep disorder.
And on the other hand, patients who suffer from insomnia sometimes receive a treatment that blocks orexin. The researchers hope that this new angle will be studied by other scientists, specialized in clinical research on humans, and continue, for their part, to study orexin from the "fundamental" point of view to better understand its effects.
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