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Brain neurotransmitters in fatigue and overtraining

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Abstract:

Since the publication of the serotonin hypothesis, numerous theories involving the accumulation or depletion of different substances in the brain have been proposed to explain central fatigue. Although the theoretical rationale for the “serotonin-fatigue hypothesis” is clear, several seemingly well-conducted studies have failed to support a significant role for 5-hydroxytryptamine in the development of fatigue. As brain function appears to be dependent upon the interaction of a number of systems, it is unlikely that a single neurotransmitter system is responsible for central fatigue. Several other mechanisms are involved, with evidence supporting a role for the brain catecholamines. Fatigue is therefore probably an integrated phenomenon, with complex interaction among central and peripheral factors. When prolonged and excessive training happens, concurrent with other stressors and insufficient recovery, performance decrements can result in chronic maladaptations that can lead to the overtraining syndrome (OTS). The mechanism of the OTS could be difficult to examine in detail, perhaps because the stress caused by excessive training load, in combination with other stressors, might trigger different “defence mechanisms” such as the immunological, neuroendocrine, and other physiological systems that all interact and probably therefore cannot be pinpointed as the “sole” cause of the OTS. It might be that, as in other syndromes, the psychoneuroimmunology (study of brain–behavior–immune interrelationships) might shed a light on the possible mechanisms of the OTS, but until there is a definite diagnostic tool, it is of utmost importance to standardize measures that are now thought to provide a good inventory of the training status of the athlete. It is very important to emphasize the need to distinguish the OTS from overreaching and other potential causes of temporary underperformance such as anemia, acute infection, muscle damage, and insufficient carbohydrate intake.

Depuis la publication de l’hypothèse sérotoninergique, de nombreuses théories faisant état de l’accumulation ou de la déplétion de certaines substances dans le cerveau ont été avancées pour expliquer la fatigue centrale. Bien que la justification théorique du rôle de la sérotonine dans la manifestation de la fatigue soit correcte, plusieurs études, selon toute apparence bien menées, n’ont pu confirmer un rôle significatif à la 5-hydroxytryptamine dans l’installation de la fatigue. Comme les fonctions cérébrales semblent liées à l’interaction de nombreux systèmes, il est peu probable qu’un seul neurotransmetteur soit responsable de la fatigue centrale. D’autres mécanismes entrent en jeu ainsi que le confirment d’autres études portant sur le rôle joué par les catécholamines du cerveau. La fatigue est fort probablement le résultat de l’interaction de nombreux facteurs centraux et périphériques. En présence d’un entraînement excessif et prolongé conjugué à d’autres agents stressants et à un manque de récupération, on assiste à des baisses de performance résultant de mésadaptations chroniques qui définissent le syndrome de surentraînement (OTS). Il est vraisemblablement difficile d’analyser le mécanisme de l’OTS, car le stress engendré par la surcharge d’entraînement combiné à d’autres agents stressants peut déclencher différents mécanismes de défense au sein des systèmes immunologiques, neuroendocriniens et autres systèmes physiologiques se confondant l’un dans l’autre et compliquant la tâche d’identifier la «seule» cause de l’OTS. Tout comme pour d’autres syndromes, la psychoneuroimmunologie (l’étude des interactions du système nerveux, du comportement et du système immunologique) peut apporter sa contribution à la compréhension des mécanismes de l’OTS, mais en attendant un outil de diagnostic validé, il est primordial de standardiser les mesures qui constituent présentement l’inventaire du statut de l’entraînement d’un athlète. En outre, il faut aussi mettre les efforts pour distinguer le syndrome de surentraînement des actions de dépassement ou autres causes potentielles de diminution de performance tels l’anémie, l’infection aiguë, les lésions musculaires et l’apport glucidique insuffisant.

Document Type: Research Article

Publication date: 2007-10-01

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  • This bimonthly journal has a 30-year history of publishing, first as the Canadian Journal of Sport Sciences, and later as the Canadian Journal of Applied Physiology. It publishes original research articles, reviews, and commentaries, focussing on the application of physiology, nutrition, and metabolism to the study of human health, physical activity, and fitness. The published research, reviews, and symposia will be of interest to exercise physiologists, physical fitness and exercise rehabilitation specialists, public health and health care professionals, as well as basic and applied physiologists, nutritionists, and biochemists.
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