Metabolic heat production during repeated testing at 24°C and 6°C in adult and aged male C57BL/6J mice: The effect of physical restraint before cold stress

Abstract

Adult (9–14 month) and aged (29–31 month) male C57BL/6J mice were subjected to 3 baseline tests (BASE), 3 cold tests (COLD), or 3 baseline immediately followed by cold tests (BASE/COLD). All tests consisted of partial restraint, and baseline tests were at 24°C for 1 h while cold tests were at 6°C for 3 h. All tests were started at 0900 and were repeated every 2 weeks. Mice were weighed before each test and colonic temperature, O 2 consumption, and CO 2 production were measured every 4 min for the duration of the test. Mean metabolic heat production during baseline and/or cold and slopes of colonic temperature over time during cold were calculated for each animal. Metabolic heat production at 24°C in both BASE and BASE/COLD was the same in aged mice as adults, however, at 6°C BASE/COLD adult mice increased metabolic heat production compared to 24°C, while aged mice produced a similar amount of heat at both 6°C and 24°C. When comparing metabolic heat production at 6°C between COLD and BASE/COLD mice, adult COLD mice demonstrate an habituation to repeated cold exposure accompanied by increasing heat production, while BASE/COLD adults produce higher heat in all 3 cold exposures. The authors suggest that this is due to a priming of heat production in adults by restraint before the cold. In aged mice, neither COLD nor BASE/COLD groups demonstrate habituation, but BASE/COLD mice produce more heat than COLD during cold exposure, again indicating baseline priming of heat production. The data imply that aged mice have an impairment in specific cold-induced thermogenesis, while their abilities to produce heat in response to restraint-induced sympathetic activation remains intact.