Sympathetic outflow to interscapular brown adipose tissue in cold acclimated mice.

C57BL/6J male mice were subjected to a cold acclimation procedure which consisted of three consecutive cold stress tests: 3-h partial restraint at 6 degrees C at 2-wk intervals. During the week following the last cold stress test, each animal previously subjected to the cold acclimation procedure, and an additional group of naive mice (animals that never had been exposed to an environment below room temperature) were anesthetized with urethan, paralyzed with vecuronium bromide, artificially ventilated, and subjected to cold stimulation for approximately 16 min. Electrical impulse activity from one of the fine sympathetic nerves entering the interscapular brown adipose tissue was recorded before and during cold stimulation, until body temperature dropped 8 degrees C below control level. Sympathetic outflow increased significantly during cold stimulation in all mice. Animals that did not achieve cold acclimation in three repeated cold stress tests (they demonstrated less cold tolerance in the last test) had lower sympathetic nervous outflow to brown adipose tissue at room temperature and during cold stimulation than mice that had achieved cold acclimation. In fact, sympathetic nervous outflow to brown adipose tissue in mice that had failed to show cold acclimation was similar to that of naive mice. These findings indicate that the sympathetic nervous system plays a primary role in cold acclimation.