Performance during prolonged exercise capacity diminishes with increasing temperatures. The onset of fatigue under these conditions is not adequately explained by peripheral mechanisms. Recently, drugs which inhibit the reuptake of dopamine and noradrenaline in the brain have been found to improve exercise performance in warm conditions. The aim of this thesis was to further explore and characterise the role of these neurotransmitters during prolonged exercise in warm conditions by manipulating their reuptake or synthesis.
The first series of experiments were designed to further investigate the effects of bupropion, a dopamine and noradrenaline reuptake inhibitor, which has been found to improve performance in warm conditions. To explore gender differences in response to acute bupropion administration, the effects of bupropion on prolonged exercise performance in warm conditions in women was investigated in Chapter 3. The results of this study suggest that during the follicular phase of the menstrual cycle, acute administration of bupropion improves exercise performance. To determine whether there are any dose-dependent effects of bupropion, the experiment in Chapter 4 was designed to test three different doses of bupropion. Exercise performance was only improved for the maximal dose, suggesting a threshold for the performance effects of bupropion.
Catecholamine precursors do not appear to improve exercise performance as consistently as reuptake inhibitors. In agreement with previous studies, the dopamine precursor L-DOPA did not affect exercise performance in warm conditions in Chapter 5. In Chapter 6 the effect of the atypical antidepressant nutritional supplement S-adenosylmethionine was investigated for its role in the synthesis of dopamine and noradrenaline. S-adenosylmethionine appeared to negatively influence cognitive function, increased skin temperature and circulating prolactin concentrations, but no effects on exercise performance were observed.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.