Quantification of the Motivation to Behaviorally Thermoregulate during Passive Heat Exposure in Humans
Authors: Slyer Julia R. , Vargas Nicole T. , Chapman Christopher L. , Johnson Blair D. , Temple Jennifer L. , Mietlicki‐Baase Elizabeth G. , Schlader Zachary L. 
Published: June 2021

Purpose

Motivation plays a key role in the decision to initiate a change in behavior. Animal models employing operant techniques indicate that the motivation to behaviorally thermoregulate is dependent on the magnitude of changes in body temperature. Similarly, behavioral thermoregulation in humans is driven by the extent of changes in skin and/or core body temperatures. However, motivation to behaviorally thermoregulate has not been quantified in humans. Therefore, we tested the hypothesis that the motivation to behaviorally thermoregulate in humans is dependent on the magnitude of changes in body temperature.

Methods

Following 10 min of seated rest in a 23.6 ± 1.7°C environment, seven healthy subjects (22 ± 2 y, 3 females) underwent 60 min of seated rest in 32.2 ± 0.6°C (T32) or 42.3 ± 0.7°C (T42) environment (20% relative humidity). Trials were completed in in a counterbalanced order separated by one week. The motivation to behaviorally thermoregulate was measured using operant responding on a fixed ratio schedule, in which subjects received thermal reinforcement after clicking a button 100 times. The reinforcer was 30 s of cooling on the dorsal aspect of the neck, an area highly sensitive to cooling during heat exposure. Cooling was enabled via a custom‐made device in which tubing was perfused with −20°C fluid. The motivation to behave was defined as the cumulative number of button clicks over time and behavioral thermoregulation was defined as the change in neck skin temperature. Neck skin temperature, mean skin (10 site), and intestinal temperatures were measured continually. Data are presented as mean ± SD.

Results

There were no differences in mean skin temperature (T32: 31.6 ± 1.0°C, T42:31.4 ± 0.8°C, P=0.47), intestinal temperature (T32: 37.3 ± 0.2°C, T42: 37.3 ± 0.3°C, P>0.99) or neck skin temperature (T32: 34.3 ± 0.6°C, T42: 34.0 ± 0.4°C, P=0.95) before heat exposure. The increase in mean skin temperature was greater in T42 (at 60 min: +4.9 ± 0.8°C) vs. T32 (at 60 min: +2.9 ± 0.8°C, P<0.01). Intestinal temperature during heat exposure decreased from pre‐heat exposure (P≤0.04), but did not differ between conditions (at 60 min ‐ T32: 37.1 ± 0.2°C, T42: 37.2 ± 0.2°C, P=0.16). Neck skin temperature was lower throughout heat exposure in T42 (at 60 min: 33.5 ± 1.9°C) compared to T32 (at 60 min: 34.8 ± 0.2°C, P<0.01). Cumulative responding for the thermal reinforcer increased at 20 min in T42 (P≤0.04), but did not change over time in T32 (P≥0.07). Thus, cumulative responding was greater in T42 (229 ± 160 clicks) compared to T32 (0 ± 0 clicks, P=0.01) at 20 min, which persisted throughout heat exposure (at 60 min ‐ T42: 986 ± 481 clicks, T32: 214 ± 168 clicks, P<0.01).

Conclusions

These data indicate that the motivation to receive thermal reinforcement during heat exposure is dependent on the magnitude of changes in mean skin temperature. Thus, in support of previous animal data, the motivation to behaviorally thermoregulate in humans is dependent on the magnitude of changes in body temperature.

This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.