Circadian activity rhythms of laboratory mice during the last weeks of their life

The aim of the present paper was a detailed analysis of changes of circadian activity rhythms immediately before natural death. Investigations were carried out on individually housed female laboratory mice. Locomotor activity was measured by passive infrared detectors starting with an age of about 75 weeks up to death. At the beginning all animals had pronounced circadian activity rhythms with a main maximum during the dark time and a secondary one just after light-on. As compared to adult mice the amount of activity and the circadian range of oscillation were lower. The main maximum was phase advanced in most of the animals, so that the percentage of activity during dark time accounted for less than 50% of the total 24-h activity. Towards death the amount of activity and the amplitude decreased even more. A circadian rhythm was preserved however as long as the animals were active, although its synchronization with LD-Zeitgeber deteriorated markedly. The phase position of the main maximum became more unstable, lending in some cases to complete uncoupling (free-run with tau < 24 h). The secondary maximum in contrast was more stable in its phase and remained synchronized longer. The results show that in old age the mechanisms of synchronization break down earlier than the circadian rhythms. Therefore it seems possible to stabilize the circadian rhythms, e.g. by strengthening of Zeitgebers, which in turn may improve wellbeing and performance.