Influence of Time-of-Day on Maximal Exercise Capacity Is Related to Daily Thermal Balance but Not to Induced Neuronal Activity in Rats

Abstract

In the present study, we investigated whether the daily fluctuations of internal body temperature (Tb) and spontaneous locomotor activity (SLA) interact with the thermal and neuronal adjustments induced by high-intensity aerobic exercise until fatigue. The body temperature and SLA of adult Wistar rats (n = 23) were continuously recorded by telemetry for 48 h. Then, the rats were subjected to a protocol of graded exercise until fatigue or rest on the treadmill during light and dark-phases. Tb, tail skin temperature and ambient temperature during each experimental session were recorded. At the end of the last experimental session, the animals were anaesthetized; the brains were perfused and removed for immunohistochemical analysis of c-fos neuronal activation. The daily rhythms of SLA and Tb were strongly correlated (r = 0.88 and p < 0.001), and this was followed by a daily oscillation in both the ratio and the correlation index between these variables (p < 0.001). Exercise capacity was associated with a lower resting Tb (p < 0.01) and was higher in the light-phase (p < 0.001), resulting in an increased capacity to accumulate heat during exercise (p < 0.01). Independent of time-of-day, high intensity exercise strongly activated the hypothalamic paraventricular nucleus (PVN), the supra-optic nucleus (SON) and the locus coeruleus (LC) (p < 0.001) but not the suprachiasmatic nucleus (SCN). Taken together, our results points toward a role of the circadian system in a basal activity control of the thermoregulatory system as an important component for the onset of physical activities. In fact, rather than directly limiting the adjustments induced by exercise the present study brings new evidence that the effect of time-of-day on exercise performance occurs at the threshold level for each thermoregulatory system effector activity. This assumption is based on the observed resilience of the central clock to high-intensity exercise and the similarities in exercise-induced neuronal activation in the PVN, SON, and LC.

Time-of-Day Effects on Metabolic and Clock-Related Adjustments to Cold

“…As cold exposure is maintained, heat production is increased and heat loss and production reach a steady state in which T core can be successfully preserved. Therefore, as previously observed with other stressors ( 40 , 44 – 47 ), it seems that the time-of-day-dependent effects of cold exposure on body temperature reflect the transitory disturbance in heat loss and heat production mechanisms elicited by mixed signals from the internal circadian time and the thermoregulatory pathways that participate in the homeostatic responses.…”

“…Interestingly, Tokizawa and colleagues ( 29 ) observed that the threshold for increased heat production was elevated during the dark phase, while behavioral curling was elicited earlier during the light phase. In rats, a similar effect of time-of-day on thermoregulatory thresholds was also observed during exercise ( 47 ). To our knowledge, this is the first time that a time-of-day effect on the cold defensive response is reported for the rat model.…”

Independent effects of rapid eye movement sleep deprivation and exposure to environmental heat stress on aerobic performance and thermoregulatory responses in exercising rats

“…This afferent (sensory) feedback includes high T CORE levels, a limiting factor for aerobic performance [26,27,58,59]. In particular, higher T CORE values at exercise initiation have been associated with a lower aerobic performance in rats [26,60,61], although this is not a universal finding [56]. In the present study, despite the fact that RSD rats presented greater T CORE at exercise initiation and a higher heat loss threshold than control rats, no differences in T CORE were observed at fatigue.…”

A Handful of Details to Ensure the Experimental Reproducibility on the FORCED Running Wheel in Rodents: A Systematic Review

“…Because almost all experimental research is developed in nocturnal rodents, it is a priority to point out if the experiments were developed during their active (night) or passive phase (day) (150). Exercise may also re-entrain circadian rhythms, producing a misalignment that can lead to low cognitive performance, deterioration of alertness, weight variations and sleep disruption (33,(151)(152)(153)(154)(155)(156)(157). Furthermore, forced activity during the inactive phase may disrupt gene expression patterns and hormonal regulations (e.g.…”