B. Nuessleinhildesheim et al., PRONOUNCED JUVENILE CIRCADIAN CORE TEMPERATURE RHYTHMS EXIST IN SEVERAL STRAINS OF RATS BUT NOT IN RABBITS, Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology, 165(1), 1995, pp. 13-17
Torpor-like circadian variations of core temperature are well document
ed for suckling-age Zucker rat pups. To determine (1) whether this juv
enile circadian rhythm is as strongly expressed in other rat strains,
and (2) whether a similar rhythm is expressed in rabbit pups, we recor
ded core temperature and metabolic rate of artificially reared pups. W
istar, Brown Norway, and Long Evans pups were studied for 30 h under m
oderate cold loads (ambient temperature = 28 degrees C) when 9-11 days
old, i.e. at the age and ambient temperature for which the rhythm has
been most thoroughly characterized in Zucker rats. Chinchilla bastard
rabbit pups were studied under similar conditions when they were 3-8
days old, the youngest age at which the rhythm can be easily detected
in rats. Rat pups of each strain showed clear circadian rhythms with s
harp decreases of core temperature and metabolic rate in subjective mo
rning. Core temperature amplitudes were in the order Wistar < Brown No
rway < Zucker < Long Evans strain. In contrast, the rabbit pups mainta
ined stable high levels of core temperature and metabolic rate through
out the day. A torpor-like decrease of core temperature in the morning
is thus not a pecularity of the Zucker rat strain but also occurs in
other pigmented rat strains, whereas rabbit pups at a similar developm
ental stage do not show a circadian core temperature rhythm.