Differential metabolic capacity of mice selected for magnitude of swim stress-induced analgesia

Abstract

Maximum oxygen consumption (V˙o 2) elicited by swimming in 20°C water or by exposure to −2.5°C in helium-oxygen (Helox) atmosphere is higher in mice selected for low (LA) than for high (HA) stress-induced analgesia (SIA) produced by swimming. However, this line difference is greater with respect to swim- than to cold-elicited V˙o 2. To study the relationship between the analgesic and thermogenic mechanisms, we acclimated HA and LA mice to 5°C or to daily swimming at 20 or 32°C. Next, the acclimated mice were exposed to a Helox test at −2.5°C and to a swim test at 20°C to compareV˙o 2 and hypothermia (ΔT). Cold acclimation raised V˙o 2 and decreased ΔT. These effects were similar in both lines in the Helox test but were smaller in the HA than in the LA line in the swim test. HA and LA mice acclimated to 20 or 32°C swims increasedV˙o 2 and decreased ΔT elicited by swimming, but only HA mice acclimated to 20°C swims increasedV˙o 2 and decreased ΔT in the Helox test. We conclude that the between-line difference in swimV˙o 2 results from a stronger modulation of thermogenic capacities of HA mice by a swim stress-related mechanism, resulting in SIA. We suggest that the predisposition to SIA observed in laboratory as well as wild animals may significantly affect both the results of laboratory measurements ofV˙o 2 and the interpretation of its intra- and interspecific variation.