Abstract
BACKGROUND:We have shown that circadian rhythm of body temperature (Tb) is finely regulated. However, feeding condition is a factor affecting largely the Tb rhythm. During fasting, Tb gradually decreases in the light (inactive) phase, whereas Tb in the dark (active) phase is maintained at the free‐feeding level. The purpose of the present study was to clarify the mechanism involved in the altered Tb rhythm during fasting.METHODS:Male ICR mice (2–3 mo old), used for the present study, were housed at an ambient temperature (Ta) of 27ºC in a 12:12‐h light‐dark cycle. After 48‐h fasting, Ta was decreased to 20ºC at ZT1‐4 (early light phase) or ZT13‐16 (early dark phase). Tb (biotelemetry) and oxygen consumption (VO2, indirect calorimetry) were measured.RESULTS:During a cold exposure with fasting, Tb decreased by 5.5 ± 2.0ºC at ZT1‐4, and VO2 remained unchanged. However, Tb decreased by only 2.0 ± 0.5ºC at ZT13‐16, which was greater than that in the light phase (p<0.05), and VO2 increased (p<0.05).CONCLUSION:These results indicate that fasting attenuates thermoregulatory response to the cold, and the response differs among time of the day. Factors of feeding condition and time would have influences on the mechanism regulating the Tb rhythm. The possible reason will be presented, based on histological study using c‐Fos immunostaining and molecular analysis of UCP1 mRNA expression in the brown adipose tissue.
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