A nonlinear interrelationship between period length and the amount of activity - Age-dependent changes

The circadian activity rhythm undergoes changes in the course of postnatal development. Experiments without external time cues were performed to characterize the endogenous component and to investigate any age-dependent changes. Female laboratory mice were used. At the beginning of the experiment they were 3 (juvenile), 23 (adult) or 72 (senile) weeks old. Animals were kept in climatic chambers (constant darkness, food and water ad libitum, temperature: 22+/-2 degrees C, rel. humidity: 55+/-5%). Locomotor activity was recorded continuously using infrared detectors. The data were stored and analysed by means of the ''Chronobiology Kit'' (Stanford University). The mean period lengths were not statistically different between age groups. The stability of the spontaneous activity rhythms was highest in adult mice, however. The mean activity/day decreased from juvenile to senile mice. A nonlinear interrelationship between period length and amount of activity was obtained. At lower activity levels the period length became shorter with increasing activity; at higher levels it became longer again. The general shape of the curve was similar in all age groups. With respect to the nonlinear curve, one could not establish a general age dependency of period length. At similar ranges of activity the period length would be shortest in senile animals. Taking into account, however, the decline with age of the amount of activity the period of old mice could be shorter than, equal to or longer than that of adult mice. The results show that the endogenous component of the circadian activity rhythm, including feedback loops, matures and stabilizes from the juvenile to the adult. An expected loss of stability in senile mice was not demonstrated, probably due to a high variance of the animals' biological age. These age-dependent changes contribute to the changes of circadian activity rhythms obtained under entrained conditions.