Effect of Continuous Light on the Circadian Rhythms of Running-Wheel Activity and Body Temperature in Spontaneously Hypertensive Rats

Abstract

The circadian rhythms of running-wheel activity and body temperature in a strain of spontaneously hypertensive rats (Aoki) and a strain of normotensive rats (Wistar Kyoto) were analysed. When the rats were exposed to continuous light, the free-running rhythm of activity persisted for more than 30 days in the hypertensive rats, but for only 16–17 days at most in the normotensive ones. The rigidity of the circadian rhythm of activity, judged by comparison with the cycle of light and dark of the initial conditions, was greater in the hypertensive rats and the activity time was longer. Body temperatures were measured rectally at 4-hour intervals in both strains of rats and under both types of lighting conditions. The circadian rhythm of body temperature under continuous illumination was shorter in the hypertensive rats, but its amplitude was larger. Thus the nature of the circadian oscillating system differs in these two strains.