OBJECTIVETo measure simultaneously the time course for the circadian rhythm of brown adipose tissue(BAT) thermogenesis and core temperature, and analyzing their temporal relationship.

METHODSThe circadian rhythm of core temperature (Tc), BAT temperature (T(BAT)), axillary temperature (Tax) and motor activity were simultaneously measured by telemetry in adult male Sprague-Dawley rats at an ambient temperature of 22 degrees C during a 12-h light:12-h dark photoperiod (lights on at 06:00 h and lights off at 18:00 h).

RESULTS(1) T(BAT) was 0.67 degrees C lower than Tc group under the light phase, but it was similar to that Tc during the dark phase. The rate of increase in T(BAT) was higher than corresponding increases in Tc at the start of transition from the light to dark phase, and increase in T(BAT) commenced approximately 8 min before Tc increases. Whereas at the start of transition from the dark to light phase, decrease in T(BAT) commenced approximately 4 min before Tc decreases. (2) The amplitude of the circadian Tax rhythm was similar to that of Tc. During either the light phase or dark phase, Tax was lower than simultaneous measurement of Tc. (3) Increases in behavioral activity commenced before increases in T(BAT) and Tc at the start of transition from the light to dark phase.

CONCLUSIONBAT thermogenesis contributes to increase in core temperature during the dark phase, indicating that circadian changes of BAT thermogenesis does indeed play significant role in the overall maintenance of the circadian rhythm of core temperature.