Serotonin : a possible central mechanism of fatigue during exercise
It has been well-documented that carbohydrate supplementation during exercise improves cycling and running performance and may alter brain serotonin concentration at rest. In addition it was found that carbohydrate supplementation attenuates the rise in plasma free tryptophan and ratio of tryptophan to the branched chain amino acids during constant pace running at a moderate intensity. This did not, however, occur during prolonged running of varying high, moderate and low intensities. It is possible that carbohydrate supplementation may therefore not only alter metabolic fuel selection during exercise but also the concentration of brain tryptophan. The previous chapters shave shown that manipulation of the plasma tryptophan to branched chain amino acid ratio is possible both before and during exercise. The possible link between this ratio and performance is, however, complicated by peripheral changes in substrate supply brought about by such dietary manipulation. The experimental design of the next chapter used a more specific central agent, a serotonin re-uptake inhibitor, to increase synaptosomal serotonin activity. The reduction of exercise performance following the ingestion of this pharmacological agent suggested that brain serotonin activity is a probable central mechanism of fatigue during prolonged exercise in man. Although brain serotonin turnover has been reported to increase during exercise, these results have been obtained from measurement of the brain tissue levels of serotonin from rats that are forced to exercise. The use of the microdialysis technique allowed continuous measurement of serotonin release during exercise to be performed in the final experiment chapter of this thesis. It was found that exercise may increase brain serotonin release but that this is related to the ability of the animal to exercise.