Physical exercise-induced fatigue: the role of serotonergic and dopaminergic systems

Brain serotonin and dopamine are neurotransmitters related to fatigue, a feeling that leads to reduced intensity or interruption of physical exercises, thereby regulating performance. The present review aims to present advances on the understanding of fatigue, which has recently been proposed as a defense mechanism instead of a “physiological failure” in the context of prolonged (aerobic) exercises. We also present recent advances on the association between serotonin, dopamine and fatigue. Experiments with rodents, which allow direct manipulation of brain serotonin and dopamine during exercise, clearly indicate that increased serotoninergic activity reduces performance, while increased dopaminergic activity is associated with increased performance. Nevertheless, experiments with humans, particularly those involving nutritional supplementation or pharmacological manipulations, have yielded conflicting results on the relationship between serotonin, dopamine and fatigue. The only clear and reproducible effect observed in humans is increased performance in hot environments after treatment with inhibitors of dopamine reuptake. Because the serotonergic and dopaminergic systems interact with each other, the serotonin-to-dopamine ratio seems to be more relevant for determining fatigue than analyzing or manipulating only one of the two transmitters. Finally, physical training protocols induce neuroplasticity, thus modulating the action of these neurotransmitters in order to improve physical performance.

脑内血清素（serotonin）与多巴胺（dopamine）是与疲劳相关的神经递质；疲劳指运动强度降低或运动中断的状态，可对运动表现产生调控作用。本综述旨在梳理当前对疲劳的认知进展：近期有研究提出，在长时间有氧（aerobic）运动场景中，疲劳并非一种“生理衰竭”，而是一种防御机制。此外，本文还综述了血清素、多巴胺与疲劳之间关联的最新研究进展。通过运动过程中可直接调控脑内血清素与多巴胺水平的啮齿类动物实验，已明确得出结论：血清素能神经活动增强会降低运动表现，而多巴胺能神经活动增强则与运动表现提升相关。然而，针对人类的相关实验——尤其是涉及营养补充或药理学干预的实验——在血清素、多巴胺与疲劳的关联问题上却得出了相互矛盾的结果。目前在人类受试者中唯一明确且可重复的效应是：使用多巴胺再摄取抑制剂后，受试者在高温环境下的运动表现得以提升。由于血清素能与多巴胺能神经系统存在相互作用，相较于单独分析或调控其中一种递质，血清素与多巴胺的比值似乎与疲劳的发生关联更为紧密。最后，运动训练方案可诱导神经可塑性，进而调控上述神经递质的作用以提升运动表现。