Physiological impact of caloric restriction on the lower critical temperature and body temperature dynamics in mice (1105.2)

Abstract

The lower critical temperature (TLC) is that ambient temperature (Ta) where metabolic rate (MR) increases linearly with decreasing Ta. We tested the hypothesis that chronic caloric restriction (CR) would elevate TLC. Control male C57Bl/6J mice (n=9) were allowed free access to food and ate 4.2 g/day while CR mice (n=9) were restricted to 2.7 g/day for 8 weeks. Indirect calorimetry demonstrated that CR mice had a higher TLC than control mice (30.3±0.5 vs. 27.7±1.0 °C, respectively), with a lower basal MR (0.45±0.02 vs. 0.66±0.01 mL O2/min), and lower average MR (0.86±0.04 vs. 1.17±0.04 mL O2/min) when compared to controls. In a separate study to examine the impact of CR and Ta on body temperature (Tb), outbred female mice were weaned and separated into one of four groups: 1) housed and given food ad libidum at 20°C, n=37; 2) housed ad lib at 30°C, n=35; 3) CR at 20°C, n=34; and 4) CR at 30°C, n=34. Ad lib feeding and CR lasted ~1 year. CR mice weighed ~55% of their ad lib counterparts for both housing temperatures (20°C and 30°C). A subset (n=7 for each group) of these mice were implanted with Tb telemeters, allowed 2 weeks for recovery, and continued with CR or ad lib. While ad lib mice housed at both 30°C and 20°C had similar minimum Tbs (35.7±0.1 and 35.5±0.2°C, respectively), the CR mice housed at 20°C experienced daily torpor, with a minimum Tb of 24.7±1.1°C, significantly lower than that of CR mice housed at 30°C (32.3±0.3°C). These findings indicate that caloric restriction 1) elevates TLC, and 2) exerts a minimal effect on the Tb of mice housed closed to their TLC, but has a great effect on Tb of mice housed well below their TLC.