A NONLINEAR INTERRELATIONSHIP BETWEEN PERIOD LENGTH AND THE AMOUNT OFACTIVITY - AGE-DEPENDENT CHANGES

The circadian activity rhythm undergoes changes in the course of postn atal development. Experiments without external time cues were performe d 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) we eks old. Animals were kept in climatic chambers (constant darkness, fo od 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 ''Chrono biology Kit'' (Stanford University). The mean period lengths were not statistically different between age groups. The stability of the spont aneous activity rhythms was highest in adult mice, however. The mean a ctivity/day decreased from juvenile to senile mice. A nonlinear interr elationship between period length and amount of activity was obtained. At lower activity levels the period length became shorter with increa sing activity; at higher levels it became longer again. The general sh ape of the curve was similar in all age groups. With respect to the no nlinear curve, one could not establish a general age dependency of per iod length. At similar ranges of activity the period length would be s hortest in senile animals. Taking into account, however, the decline w ith age of the amount of activity the period of old mice could be shor ter than, equal to or longer than that of adult mice. The results show that the endogenous component of the circadian activity rhythm, inclu ding feedback loops, matures and stabilizes from the juvenile to the a dult. An expected loss of stability in senile mice was not demonstrate d, probably due to a high variance of the animals' biological age. The se age-dependent changes contribute to the changes of circadian activi ty rhythms obtained under entrained conditions.