Patterns of circadian body temperature rhythms in aged rats

1. Old rats of the same chronological age are not homogeneous with respect to their circadian temperature rhythm (CTR). Some old rats maintain a robust CTR, comparable with that of young rats, while others have a weakened and unstable CTR. Often, in the latter group, the CTR becomes flat and disappears. In almost all old rats there is a clear decrease in the amplitude of the CTR. 2. The present paper describes several methods that are useful for distinguishing between old rats in different categories. Three patterns can be distinguished: (i) good (there is a clear difference between the daily body temperature (Tb) means in the light and dark, relatively high density (power) at a period of 24 h, low hour-to-hour fluctuation in Tb within a given day (low intradaily variability; IV) and high day-to-day stability in Tb (high interdaily stability; IS)); (ii) unstable (there is a variable difference between daily Tb means in the light and dark, very low density at the 24h period, moderate IV and low IS); and (iii) flat (there is little or no difference or variation in daily Tb means in the light and dark, low or insignificant density at 24 h, highest IV and lowest IS). 3. The importance of these distinctions in old rats is two-fold. First, differences at a behavioural level may reflect fundamental differences in the brain. Second, the degree of deficits in one system, for example in the CTR, may be correlated with deficits in other systems, such as sleep-waking. The methods described in the present paper are useful for obtaining qualitative information on the progressive deterioration of the CTR as animals age.