Fatigue is mediated by cholinoceptors within the ventromedial hypothalamus independent of changes in core temperature

Abstract

We investigated brain mechanisms modulating fatigue during prolonged physical exercise in cold environments. In a first set of studies, each rat was subjected to three running trials in different ambient temperatures (T(a)). At 8 °C and 15 °C, core body temperature (T(core)) decreased and increased, respectively, whereas at 12 °C, the T(core) did not change throughout the exercise. In another set of experiments, rats were randomly assigned to receive bilateral 0.2 μL injections of 2.5 × 10(-2) M methylatropine or 0.15 M NaCl solution into the ventromedial hypothalamic nuclei (VMH). Immediately after the injections, treadmill exercise was started. Each animal was subjected to two experimental trials at one of the following T(a) : 5 °C, 12 °C or 15 °C. Muscarinic blockade of the VMH reduced the time to fatigue (TF) in cold environments by 35-37%. In all T(a) studied, methylatropine-treated rats did not present alterations in T(core) and tail skin temperature compared with controls. These results indicate that, below the zone of thermoneutrality, muscarinic blockade of the VMH decreases the TF, independent of changes in T(core). In conclusion, our data suggest that VMH muscarinic transmission modulates physical performance, even when the effects of thermoregulatory adjustments on fatigue are minimal.

Association between the increase in brain temperature and physical performance at different exercise intensities and protocols in a temperate environment

“…Each daily session consisted of running at a constant speed of 18 m/min and a 5% inclination for 5 min (17). During the familiarization sessions, the animals were encouraged to run by light electrical stimulation (0.5 mA).…”

“…During the familiarization sessions, the animals were encouraged to run by light electrical stimulation (0.5 mA). The purpose of this familiarization procedure was to show the animals which direction to run, thereby minimizing their exposure to the electrical stimulation during the experimental trials (17). Prior to each familiarization session, the rats were extensively handled by the experimenter, who also briefly restrained the animals and then simulated the insertion of a thermistor.…”

“…The rats were then placed on a treadmill with the inclination always set to 5% and subjected to a running exercise session until volitional fatigue at one of the following treadmill speeds: 18 m/min (S18), 21 m/min (S21), or 24 m/min (S24). Volitional fatigue was defined as the point at which the animals were no longer able to maintain pace with the treadmill, even when being stimulated by light electrical shocks (0.5 mA) for 10 s (17). …”

Role of moderate exercising on Achilles tendon collagen crimping patterns and proteoglycans

“…The program was progressive so that the running time increased from 5 min per session in the first week to 20 min (16 m/min) per session in the third week. In the following 9 weeks, the running session was 30 min per day at a speed of 16 m/min, according to similar literature protocols, where moderate exercise is reported to range from 10 to 20 m/min (48)(49)(50). At the end of the experimental study, all animals were anaesthetized and then euthanized with an i.v.…”

The dynamics of physical exercise-induced increases in thalamic and abdominal temperatures are modified by central cholinergic stimulation

“…After resting for 5 min on the treadmill belt, the rats were required to run for 5 min at a constant speed of 18 m/min and an inclination of 5% [7]. This familiarization protocol was conducted across 5 consecutive days.…”