Excessive exercise disrupts dopamine regulation in the brain, study finds

Excessive physical exercise affects more than just the muscles; it can also alter brain chemistry. A new study published in the journal Neuroscience revealed that exhaustive training without adequate rest alters the regulation of dopamine in the hippocampus, a brain region closely linked to memory and the regulation of emotions.

When observing mice subjected to an intense exercise routine, Brazilian researchers noted that dopamine, a neurotransmitter essential for alertness and motivation, remained elevated at a time when the animals should have been resting. If this same mechanism is confirmed in humans, the discovery could help explain why athletes who train excessively often suffer from anxiety, insomnia, and chronic fatigue. This would highlight the biological importance of rest for physical performance and mental health.

"Our findings show that excessive physical exercise affects not only skeletal muscle but also has a pronounced impact on the brain," said Alisson Luiz da Rocha, the first author of the study. He conducted the research as part of his doctoral studies at the Ribeirão Preto School of Physical Education and Sports at the University of São Paulo (EEFERP-USP) in Brazil with a scholarship from FAPESP. According to da Rocha, the data reinforce the view that physical activity has a direct and profound effect on the central nervous system, going far beyond the mere building of muscle mass.

Overtraining simulation

To understand these dynamics, the scientists divided the mice into two main groups. The first group underwent an acute exercise test, running on a treadmill until exhaustion. Meanwhile, the second group simulated the grueling routine of high-performance athletes suffering from overtraining syndrome. This syndrome is characterized by a decline in performance and a loss of appetite, chronic fatigue, muscle pain, an increase in injuries, and changes in the immune system and metabolism (read more at agencia.fapesp.br/55205). 

The rodents in the second group trained for eight weeks with progressively heavier loads, reaching two daily sessions separated by just four hours of recovery in the final week. The researchers then analyzed the animals' hippocampi, measuring levels of dopamine, serotonin, and proteins involved in producing these neurotransmitters, such as tyrosine hydroxylase (TyrH) and vesicular monoamine transporter 2 (VMAT2).

The results showed that dopamine levels rose shortly after the run ended in the acute exercise group. However, the most significant finding occurred in the chronic training group. Levels of dopamine and TyrH, the enzyme responsible for its synthesis, remained elevated for 48 hours after the performance test during the mice's rest phase.

According to the study's coordinator, Adelino Sanchez Ramos da Silva, a professor at EEFERP-USP, similar changes in humans could help shed light on possible links between excessive training, brain regulation, and sleep disorders.

However, it is worth noting that the study did not measure sleep, alertness, or anxiety. Future research may explore the relationship between the findings in mice and these symptoms in humans.

The role of dopamine

The authors emphasize that dopamine is not a biological villain because it is essential for pleasure and keeping us awake at the right times. The real problem, as Rocha points out, is the imbalance created when dopamine levels rise at bedtime, as well as the resulting imbalance relative to other neurotransmitters. Interestingly, the research found no significant differences in the animals' serotonin levels. This contradicts the long-standing hypothesis that serotonin alone is the primary cause of excessive fatigue.

While the direct relationship with sleep and anxiety in humans still needs to be measured in future tests, the main message is clear. To avoid a decline in performance, chronic pain, and immune system dysfunction, it is non-negotiable to allow the body time to recover. For professional and amateur athletes alike, the benefits of exercise can be lost if training is not accompanied by adequate rest and expert guidance.

Rocha conducted part of the research during his postdoctoral fellowship at the School of Applied Sciences (FCA) of the State University of Campinas (UNICAMP) in Limeira, including an internship at the University of Michigan in the United States, where he is currently a researcher. The group also received funding through a Thematic Project coordinated by Silva, as well as a postdoctoral fellowship for Ana Paula Pinto and a direct doctorate scholarship for João Pedro Floriano, both at EEFERP-USP.

The group is now working on a new international project to investigate which biological systems show the first signs of deviation from the expected beneficial adaptations to physical exercise.