Influence of heat acclimation on spontaneous wheel‐run exercise in mice

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Introduction We previously found that, in mice, running distance on a running wheel went down in heat in mice, during which body core temperature became lower than that in no‐heat control. These results may suggest that mice determine exercise intensity depending on thermal input from the body surface. In the present study, we hypothesized that heat acclimation disinhibit such suppression of spontaneous running in heat.MethodsTwenty two male C57BL/6J3 mice (age, 7 w) were used, which were divided to two groups: the heat acclimation and control groups (HA group and CON groups, respectively). They had surgery of intra‐abdominal placement of the IC‐chip sensor device (nanotag, Kissei, Nagano) for the measurement of abdominal temperature (Tabd) and counts of movement under inhalation anesthesia with sevoflurane and were allowed to recover for 10 d at least. They were individually housed at an ambient temperature (Ta) of 33°C and 25°C in the HA and CON groups, respectively for 2 w. Before and after the acclimation period, each mouse had two different trials on two‐separated days: exposures at 33°C and 25°C at 19:00–1:00 in a dark phase. In the trials, each mouse was moved to a semi‐closed chamber with a running wheel inside, where Tabd, running distance on the wheel, and metabolism by indirect calorimetry. After the experiments, mice were killed by overdose injection of pentobarbital, and the dorsal root ganglia (DRG, Th 5‐12) was obtained to assess protein expression of TRPV1 and V4.ResultsBefore the acclimation period, the running distance was smaller in the in the 33°C trial than in the 25°C trial in both groups (P<0.05; e.g. 8.98 ± 0.86 and 13.68 ± 1.18 km, respectively, in the HA group). However, after the acclimation period, the distance in the HA group at 33°C trial became greater than the value before the acclimation period (P<0.05; 14.73 ± 1.32 km; post‐25°C, 14.50 ± 0.66 km).VO2 in the HA groups was higher in pre‐33°C trial and lower in post‐33°C trial than any other trial. Before the acclimation period, Tabd was lower in the 33°C trial than in the 25°C trial. After the acclimation period, Tabd was greater in the 33°C trial than in the 25°C trial. (pre‐33°C, 37.52 ± 0.28; pre‐25°C, 37.99 ± 0.30°C; post‐33°C, 37.93 ± 0.23; post‐25°C, 37.82 ± 0.30 °C). The expression TRPV1 on DRG tended to be lower in the HA group compared with the CON group.ConclusionsWe found that, in mice, heat acclimation increases running distance on the wheel and decrease metabolic heat production. Based on physiological and biochemical analyses, decrease in thermal input from the skin and improve metabolic efficiency may be involved in the mechanism.