LIGHT-INDUCED PHASE AND PERIOD RESPONSES OF CIRCADIAN ACTIVITY RHYTHMS IN LABORATORY MICE OF DIFFERENT AGE

Age-dependent changes of circadian activity rhythms concern particular ly the ability to synchronize with external light/dark (LD)-periodicit ies. The aim of the present study, therefore, was to examine whether t he phase and period responses to light pulses (LP) are different in mi ce of various ages. Female laboratory mice of different ages were used (36 juveniles; 3 weeks old, 12 adults; 20 weeks old, 12 preseniles; 6 5 old). They were kept in climatic chambers (constant darkness, food a nd water ad libitum, temperature 22 +/- 2 degrees C, rel, humidity; 55 -65%). Light pulses (15', 150 lux) were applied every two weeks at dif ferent circadian times. Locomotor activity was recorded continuously u sing infrared detectors. PRCs showed a delay region from CT11 through CT20 for all age groups. However, the minima were delayed in juvenile and presenile mice compared to adult animals. Phase advances occured b etween CT7 and CT11 in adult and presenile mice, and between CT21 and CT24 in juveniles. The maximal delay (3.5 h) exceeded the maximal adva nce (1.5 h) and was lowest in adult mice. Period changes after light p ulses were most pronounced in juvenile mice (-35' to +20') and depende d on circadian time (shortening from CT15 through CT1, at the other ti mes lengthening), in older animals the curves were flattened and the p hase delayed. The results show that the response of the circadian cloc k to the LD-Zeitgeber changes depending on age. Even though the differ ences were small, they may be one of the causes of the ontogenetic dif ferences in the ability to synchronize circadian rhythms. But, further factors, e.g. non-photic Zeitgebers, should contribute as well.