The role of the central serotonergic system during prolonged exercise in adverse environmental conditions
The central fatigue hypothesis proposes that an increase in central serotonergic (5-HT) activity is detrimental to exercise capacity. The negative effects of high ambient temperatures on exercise capacity are well recognised but are not readily explained in terms of the factors generally associated with the onset of fatigue during prolonged exercise (muscle glycogen depletion, hypohydration and hyperthermia). There is also some evidence to suggest that central 5-HT activity is modified by warm and cold exposure and that the responses of hormones that are considered to be peripheral indices of central 5-HT activity are exacerbated during the later stages of prolonged exercise in the heat but not in the cold. The primary purpose of the studies reported in this thesis was to examine the effects of peripheral or central manipulation of 5-HT activity on the ability to perform prolonged exercise in adverse environmental conditions. The responses of plasma prolactin (Prl) and cortisol were determined in all studies to provide a peripheral index of central 5-HT activity. Chapter 3, examined the effects of pre-cooling and pre-warming by warm and cool water immersion on the responses of Prl and cortisol during exercise in a warm environment. Chapter 4 considered the effect of limiting 5-HT pre-cursor availability by branched chain amino acid (BCAA) ingestion before and during exercise on ability to perform prolonged exercise in a warm environment. Chapter 5 examined the effect of selective 5-HT re-uptake transporter blockade by paroxetine on the time required to reach volitional fatigue in the warm and cool environments. Finally, Chapter 6 considered the effects of 5-HT2C receptor blockade on exercise performance and the thermal response during exercise in a warm environment. None of the studies reported in this thesis provided clear evidence to show that exercise capacity or performance in the heat is influenced by peripheral or central manipulation of central 5-HT activity.