PRONOUNCED JUVENILE CIRCADIAN CORE TEMPERATURE RHYTHMS EXIST IN SEVERAL STRAINS OF RATS BUT NOT IN RABBITS

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.