IS THE CIRCADIAN CORE TEMPERATURE RHYTHM OF JUVENILE RATS DUE TO A PERIODIC BLOCKADE OF THERMOREGULATORY THERMOGENESIS

Previous studies have demonstrated an endogenous circadian rhythm of c ore temperature (T-c) in suckling-age rat pups. Our aim was to establi sh whether the low and irregular T-c at the beginning of the light pha se is caused by a temporary blockade of thermoregulatory thermogenesis . We therefore isolated and artificially fed 10-day-old pups for 30 h at five ambient temperatures (T-a), ranging from thermoneutrality to s evere cold loading, while continuously recording T-c and metabolic rat e (MR). During the maximum phase of the T-c rhythm MR increased linear ily with decreasing T-a - to as much as 260% of the daily mean MR at t hermoneutrality (TNMR) - so that T-c decreased less than 1 degrees C w ith increasing cold load. During the minimum phase, the MR at all but the lowest T-a fell to, or even below, the TNMR and the amplitude of t he T-c rhythm increased from 2 to 5 degrees C with increasing cold loa d. Under the most severe cold load, however, a further decrease of the minimum T-c was prevented by an increase of MR to 135% of the TNMR. S upplementing the continuously fed synthetic milk with noradrenaline (1 .2 mg kg(-1) h(-1)) during the minimum phase of the T-c rhythm increas ed MR upto 290% of the TNMR. The loose regulation of T-c during the mi nimum phase of the juvenile circadian T-c rhythm is thus not caused by a peripheral impairment of thermo,genic capacitiy at the beginning of the light phase.