Mechanisms of fatigue during prolonged exercise in the heat
Increase in body temperature is a major factor limiting endurance performance in the heat and it is shown in this thesis that the effects of raised body temperature on performance, perception and neuroendocrine response to exercise are mediated by an interaction of body temperatures. Prolactin has been used as an indicator of hypothalamic activity and the pathways regulating its release have been investigated using pindolol as a 5-HT\(_ \) antagonist. The prolactin response to a buspirone challenge has been shown to be approximately 50% serotonergic and 50% dopaminergic, but with a wide inter-subject variation. Passive heating is a potent stimulus for prolactin release and it was shown that 5-HT\(_ \) stimulation plays virtually no part in this process, raising the possibility that prolactin release during hyperthermic exercise may also be largely due to withdrawal of dopamine inhibition. A comparison of exercise tolerance in the heat and the sensitivity of central serotonergic and dopaminergic pathways further indicates the importance of dopamine in central fatigue. The action of caffeine in enhancing endurance performance has been shown not to involve the hypothalamus and this draws attention to other pathways that may be involved in central fatigue including the basal ganglia and limbic system.