Responses to exercise in the heat related to measures of hypothalamic serotonergic and dopaminergic function

We have studied 12 recreationally active men to measure their responses to exercise in the heat and relate these to measures of hypothalamic function explored with a buspirone [5-hydroxytryptamine 1A (5-HT_1A) agonist, dopaminergic D₂ antagonist] neuroendocrine challenge, with and without pretreatment with pindolol (5-HT_1A antagonist). Pindolol treatment allowed the serotonergic and non-serotonergic components of prolactin release to be distinguished. Subjects exercised at 73 (5)% maximal rate of oxygen uptake (V̇O_2max) until volitional fatigue at 35°C (relative humidity, 30%). On another two occasions they underwent a buspirone challenge [0.5 mg (kg body mass)⁻¹], once with, and once without, pindolol [0.5 mg (kg body mass)⁻¹] pretreatment and the circulating plasma concentrations of prolactin were measured for the next 2.5 h. Rectal temperature increased throughout exercise, whilst mean skin temperature remained constant. There was a wide inter-subject variation in prolactin response to the neuroendocrine challenges. The proportion of the prolactin response to buspirone attributable to a non-serotonergic component (most likely dopaminergic) correlated both with exercise duration (r=0.657, P=0.028), rectal temperature at fatigue (r=0.623, P=0.041) and the rate of temperature rise (r=−0.669, P=0.024). Our results suggest that high activity of the dopaminergic pathways in the hypothalamus is a predictor of exercise tolerance in the heat.