Juvenile circadian core temperature rhythm in Wistar and lean (Fa/-) Zucker rat pups

Abstract

We compared the daily rhythms of cold defense in Wistar and lean (Fa/-) Zucker rat pups isolated from maternal care. Pups, born in a colony with lights-on from 7:00 to 19:00, were artificially reared from 10 to 13 days of age in continuous light at moderate cold loads, while we recorded core temperature (Tc) and metabolic rate (MR). Pups of both strains showed daily rhythms distinctly different from those of adult rats: on the first day of artificial rearing Tc steeply decreased shortly before the time of lights-on in the breeding colony and reached a daily minimum of 31.8 +/- 0.4 degree C at 7:17 +/- 17 min in Zucker pups (+/- SE, N = 24) and of 33.4 +/- 0.2 degree C at 8:59 +/- 9 min (N = 25) in Wistar pups. On each day, the average daily minimum Tc was significantly higher in Wistar than in Zucker rats, whereas the daily maximum Tc were close to 36 degrees C in both strains. The daily rhythms of MR resembled those of Tc. We conclude that circadian differences of cold defense, which must be expected to influence a variety of behavioral and physiological parameters, are not a peculiarity of Zucker rat pups but also occur, though with a smaller amplitude, in 10-day-old Wistar pups.