High Ambient Temperature Affects Exercise Performance, Thermoregulation And Hypothalamic Neurotransmitters During Exercise In The Rat

Abstract

The attainment of a critically high brain and core temperature has been proposed as an important factor limiting endurance performance in the heat, and several neurotransmitters are thought to be involved in thermoregulation. The preoptic area and anterior hypothalamus (PO/AH), the center for thermoregulation are thought to responsible in controlling thermal and neurochemical homeostasis. However, until now no study has examined the effects of critical brain and core temperature on neurotransmitters during exercise. PURPOSE To investigate the relationship between exercise performance, thermal responses and brain neurotransmitters during exercise in the heat. METHODS Male Wistar rats were exercised for 2.5 wks on a treadmill. A telemetry device was implanted into the peritoneal cavity and two intracerebral guides were implanted under anesthesia. One guide was inserted in the left PO/AH for the microdialysis probe, and one in the right frontal cortex for thermocouple. On the day of the experiments, the microdialysis probe was connected to a microinjection pump and was perfused with a Ringer's solution at a flow rate of 1 μl/min. After 2 h of baseline collections, the animals were exercised at a speed of 26 m/min until exhaustion. One group ran in cool environment (18°C) another in warm environment (30°C). Time to exhaustion, noradrenaline (NA), dopamine (DA), and serotonin (5-HT) in the PO/AH, thermoregulatory responses (brain temperature: Tbrain, core temperature: Tcore, and Tail skin temperature: Ttail) were recorded prior to, during, and post exercise. RESULTS Exercise performance was significantly influenced by temperature (Cool: 147±17min, Warm: 73±5.7min). Thermal responses were not different between two conditions at the start of exercise, but the final temperatures of exercise in Warm were significantly higher than the Cool (Tbrain, Cool: 39.1±0.6°C, Warm: 40.7±0.6°C, Tcore, Cool: 39.9±0.4°C, Warm: 41.7±0.7°C, Ttail, Cool: 32.7±1.5°C, Warm: 35.7±1.1°C). Both NA and DA increased during exercise in both conditions, being lower in Warm than in Cool. No differences were observed between groups for 5-HT. CONCLUSIONS These results suggest that high ambient temperature influenced exercise performance not only via an increase in brain and core temperatures, but also because of a limitation in the heat loss mechanisms. DA and NA in the PO/AH might be responsible in the decrease in exercise performance observed in the heat. Supported by the Japan Society for the Promotion and Sciences and research council of the Vrije Universiteit Brussel (OZR990).