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

Abstract

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 D(2) 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 (VO(2max)) until volitional fatigue at 35 degrees 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.

Effect of transdermal nicotine administration on exercise endurance in men

“…This observation is similar to observations of the effects of caffeine supplementation (Pasman et al 1995;Bridge et al 2000). Despite the widespread use of nicotine and general acceptance that it promotes many of the characteristics provided by other stimulants, such as caffeine, cocaine and amphetamines, there has been no quantitative research about its benefits on endurance capacity during moderate and prolonged exercise, where the progressive development of central fatigue is believed to be a contributing factor in maintaining motivation.…”

“…The hypothesis of the present study was that nicotine administration would improve endurance capacity and reduce perception of exertion in a similar way to that observed by Bridge et al (2000) after caffeine consumption.…”

“…It is therefore likely that administration of nicotine may enhance motivation during prolonged exercise owing to an increased release of dopamine, possibly involving similar pathways to those suggested for the central actions of caffeine. Bridge et al (2000) demonstrated that caffeine supplementation improved exercise endurance by ∼20% and significantly reduced the perception of exertion whilst cycling at 18 and 35…”

Heat Stress and Cardiovascular, Hormonal, and Heat Shock Proteins in Humans

“…Prolonged exercise capacity is reduced markedly if the ambient temperature is high enough to trigger PRL release. 28 In fact, the heat-induced increase of PRL with exercise or heat stress has been used as an indirect marker of serotonergic activity 12 and central fatigue. 28 Serotonergic and dopaminergic neurotransmitter pathways in the brain have been implicated as contributing to central fatigue during prolonged exercise.…”

“…Prolactin, which is one of these hormones, is an indirect measure of dopaminergic-serotonergic transmitters in the brain. 12 The extent to which passive heat stress triggers a cascade of responses is the basis for this study. Therefore, the primary purpose of our study was to determine whether whole-body passive heat stress triggers cardiovascular (heart rate, blood pressure), hormonal (prolactin, catecholamines), and extracellular protein (HSP72) responses that commonly are reported during exercise.…”