Nonshivering thermogenesis in adult and aged C57BL/6J mice housed at 22°C and at 29°C

Twelve and 28-month-old C57BL/6J male mice were housed either at room temperature of 22°C or at thermoneutrality (29°C) during the two months prior to experiments. Acute experiments were conducted under anesthesia, myorelaxation, and artificial ventilation. We recorded efferent electrical impulse activity in one of the sympathetic nerves innervating the interscapular brown adipose tissue in response to acute cold stimulation, when body temperature was lowered 7.5°C below control level. In separate experiments we measured O₂ consumption and CO₂ production and calculated the nonshivering thermogenesis. We also measured the concentration of uncoupling protein in interscapular brown adipose tissue before and after three-hour cold stress. In aged mice, both sympathetic nervous activity and nonshivering thermogenesis were lower in animals housed at thermoneutrality (29°C) than in mice housed at 22°. Among mice maintained at 22°C, but not at thermoneutrality, aged animals had greater nonshivering thermogenesis and greater cold induced concentration of uncoupling protein in the brown adipose tissue than adults. Sympathetic nervous outflow to brown adipose tissue was always greater in aged mice, regardless of the temperature of acclimation. We concluded that aged mice, housed at 22°C, showed the changes in nonshivering thermogenesis associated with cold acclimation. However, an increased sympathetic outflow to brown adipose tissue in aged animals reflects an age- related elevation of the tone and responsiveness of the sympathetic nervous system.