Brain and abdominal temperatures at fatigue in rats exercising in the heat

Abstract

We measured brain and abdominal temperatures in eight male Sprague-Dawley rats (350-450 g) exercising voluntarily to a point of fatigue in two hot environments. Rats exercised, at the same time of the day, in three different trials, in random order: rest 23 degrees C, exercise 33 degrees C; rest 23 degrees C, exercise 38 degrees C; and rest 38 degrees C, exercise 38 degrees C. Running time to fatigue was 29.4 +/- 5.9 (SD), 22.1 +/- 3.7, and 14.3 +/- 2.9 min for the three trials, respectively. Abdominal temperatures, measured with intraperitoneal radiotelemeters, at fatigue in the three trials (39.9 +/- 0.3, 39.9 +/- 0.3, and 39.8 +/- 0.3 degrees C, respectively) were not significantly different from each other. Corresponding brain temperatures, measured with thermocouples in the hypothalamic region (40.2 +/- 0.4, 40.2 +/- 0.4, and 40.1 +/- 0.4 degrees C), also did not differ. Our results are consistent with the concept that there is a critical level of body temperature beyond which animals will not continue to exercise voluntarily in the heat. Also, in our study, brain temperature was higher than abdominal temperature throughout exercise; that is, selective brain cooling did not occur when body temperature was below the level limiting exercise.

Differential effects of targeted tongue exercise and treadmill running on aging tongue muscle structure and contractile properties

“…For the ET, rats were given a 1-min warm-up running period at 20 cm/s. Treadmill speed was then increased to 30 cm/s on a 20°incline, and rats ran until exhaustion, which was defined as the inability of the rat to return to and maintain the pace of the treadmill for two continuous minutes (22,40,42,85). Time to exhaustion (s) was recorded.…”

The Involvement of Brain Monoamines in the Onset of Hyperthermic Central Fatigue

“…Brain function may also be altered during hyperthermia. Changes in behavior seen with overheating, such as confusion, a loss of coordination, syncope, and, in extreme hyperthermia, a loss of consciousness or seizures, have been consistently observed (Fuller et al, 1998;Schlader et al, 2010;Walters et al, 2000). These data indirectly suggest a change in central nervous system function with hyperthermia Nybo, 2008).…”

“…It has been shown that hyperthermia reduces physical performance in many mammalian species (Bruck & Olschewski, 1987;Fuller et al, 1998;González-Alonso et al, 1999;Nielsen et al, 1993;Walters et al, 2000), reduces central nervous system drive for exercise performance (Nielsen et al, 1993;Walters et al, 2000) and precipitates feelings of fatigue at a sublethal threshold by establishing a safety level against heat stroke, thus protecting the brain, among other tissues, from thermal damage (Caputa et al, 1986;Marino,2004). The reduction in exercise performance as a consequence of hyperthermic stress has been described in isometric tasks (Nybo & Nielsen, 2001;Thomas et al, 2006;Todd et al, 2005), in dynamic exercises with fixed intensity, i.e., a constant workload (Nybo & Nielsen, 2001), and in self-paced prolonged exercises (Ely et al, 2010;Tatterson, 2000).…”

“…The thermoregulatory-sensed variables that initiate these central inhibitory signals of exercise performance in order to avoid heat stroke are still the subject of debate. Even though it has been stated that central fatigue coincides with the attainment of fixed high body and brain temperatures (Fuller et al, 1998;Gonzalez-Alonso et al, 1999;Nielsen et al, 1993;Walters et al, 2000), there is evidence that dynamic mechanisms are consistent regulators of feelings of fatigue. In support of this hypothesis, the rates of heat storage and body heating that are associated with changes in neurotransmitter content in thermoregulatory centers emerge as important factors in determining fatigue Hasegawa et al, 2008;Lacerda et al, 2005Lacerda et al, , 2006Leite et al, 2006, L. O. Rodrigues et al, 2003A.…”

Peer Review #2 of "Automatic analysis of treadmill running to estimate times to fatigue and exhaustion in rodents (v0.2)"

“…To make it translationally valid, the researchers usually include clearly 92 recognizable and characteristic behavioral pattern that reflects the loss of the optimal physical 93 performance. If this pattern is observed for more than some predefined period of time, it is 94 considered as an objective sign of fatigue (Fuller et al 1998). This behavior is also accompanied 95 by the animal having transient periods when it cannot keep pace with the treadmill so that the 96 animal moves toward the back of the treadmill, may receive a prodding stimulus, and then 97 returns to the front of the treadmill.…”