Brain Temperature in Spontaneously Hypertensive Rats during Physical Exercise in Temperate and Warm Environments

Abstract

This study aimed to evaluate brain temperature (Tbrain) changes in spontaneously hypertensive rats (SHRs) subjected to two different physical exercise protocols in temperate or warm environments. We also investigated whether hypertension affects the kinetics of exercise-induced increases in Tbrain relative to the kinetics of abdominal temperature (Tabd) increases. Male 16-week-old normotensive Wistar rats (NWRs) and SHRs were implanted with an abdominal temperature sensor and a guide cannula in the frontal cortex to enable the insertion of a thermistor to measure Tbrain. Next, the animals were subjected to incremental-speed (initial speed of 10 m/min; speed was increased by 1 m/min every 3 min) or constant-speed (60% of the maximum speed) treadmill running until they were fatigued in a temperate (25°C) or warm (32°C) environment. Tbrain, Tabd and tail skin temperature were measured every min throughout the exercise trials. During incremental and constant exercise at 25°C and 32°C, the SHR group exhibited greater increases in Tbrain and Tabd relative to the NWR group. Irrespective of the environment, the heat loss threshold was attained at higher temperatures (either Tbrain or Tabd) in the SHRs. Moreover, the brain-abdominal temperature differential was lower at 32°C in the SHRs than in the NWRs during treadmill running. Overall, we conclude that SHRs exhibit enhanced brain hyperthermia during exercise and that hypertension influences the kinetics of the Tbrain relative to the Tabd increases, particularly during exercise in a warm environment.

Fig 1. Thermistor tip positions in the brain.

(A) Photomicrograph of a brain tissue section (50 μm) that was stained with cresyl violet. (B-G) Schematic drawings taken from Paxinos and Watson’s atlas (2007) showing the thermistor tip locations in the rat brain used in this study. Legend: The black arrow indicates the lesioned region. AID, agranular insular cortex, dorsal part; AIV, agranular insular cortex, ventral part; DI, dysgranular insular cortex; fmi, forceps minor of the corpus callosum; Fr3, frontal cortex, area 3; LO, lateral orbital cortex; M1, primary motor cortex; and S1J, primary somatosensory cortex, jaw region.

Fig 2. Thermoregulatory responses during incremental exercise.

Brain (panels A and B), abdominal (panels C and D) and tail skin (panels D and E) temperatures of SHRs and NWRs subjected to incremental exercise in temperate (panels A, C and E) or warm (panels B, D and F) environments. Data are expressed as the mean ± S.E.M. * p < 0.05 compared with the NWR group in the same environment.

Fig 3. Thermoregulatory responses during constant exercise.

Brain (panels A and B), abdominal (panels C and D) and tail skin (panels D and E) temperature in SHRs or NWRs subjected to constant exercise in temperate (panels A, C and E) or warm (panels B, D and F) environments. Data are expressed as the mean ± S.E.M. * p < 0.05 compared with the NWR group in the same environment.

Fig 4. Thermoeffector activity during exercise.

Thermoeffector activity during incremental (panels A and C) or constant (panels B and D) exercise in temperate (panels A and B) or warm (panels C and D) environments. Data are expressed as the mean ± S.E.M.

Fig 5. Brain-abdominal temperature differentials during exercise.

Brain-abdominal temperature differentials during incremental (panels A and C) or constant (panels B and D) exercise in temperate (A and B) or warm (C and D) environments. Data are expressed as the mean ± S.E.M. * p < 0.05 compared with the NWR group in the same environment.

Fig 6. Exercise-induced thermoregulatory responses of rats from the two groups matched for body mass.

(Body mass: NWR, 334 ± 10 g; and SHR, 346 ± 14 g; p = 0.528). Brain (panel A), abdominal (panel B) and tail skin (panel C) temperatures of the SHRs and NWRs subjected to incremental exercise in a temperate environment. Data are expressed as the mean ± S.E.M. * p < 0.05 compared with the NWR rats.