Multi-rhythm study on the synchronous ability of mice to the light transitions

Abstract

It is often considered, without clear evidence, that one week is about the time necessary for the locomotion pattern of animals (and human) to return to normal in response to an abrupt change in the day-night cycle, such as during jet lag. In this experiment we wanted to determine accurately the temporal changes in the locomotion pattern following light-transitions using the wavelet transform, a sensitive method in time-frequency analysis. Locomotor activity of mice (Strain C57, aged 8–10 weeks) was studied using a totally non-invasive image-based monitoring system developed in our laboratory. Each animal was individually housed in a top-open cage put in a sound-shielded chamber with ventilation and light control and unrestricted food supply. The displacement of the animal body was detected at an interval of 2 s by summing the pixels in the difference image of two consecutive frames 2 s apart to reflect the extent of locomotion. To study their ability to synchronize with external lighting conditions, we introduced either a light-light (LL) or dark-dark (DD) transition by altering the light schedule once during experiment. Each experimental session lasted 18 days under a controlled dark-light (DL) cycle (12:12 h) in the first four days, and then switching to an experimental reversed light-dark (LD) cycle (12:12 h) thereafter. Specifically that was achieved by changing the light schedule to LL (or DD) on day 5 and then continued into LD (or DL) cycle till day 18. The locomotion signal was analyzed into 7 frequency bands covering specifically the circadian, intermediate, sub-ultradian, ultradian, supra-ultradian to very high frequency rhythms. Our principal findings are: (a) a maximal time shift of 12 hours required 10 days to return to 95% level of the control phase; (b) recovery from the LL transition occurred slightly faster than the DD transition; (c) there is phase difference amongst the high and low rhythmic activities in response to the transition; (d) there is an interesting ‘merge-split’ phenomenon, we called ‘2-1-2’ pattern, observed near the ultradian band. Results showed that (a) 10 days is the time required for the laboratory rat to recover 95% of the normal diurnal cycle following a light-transition, and (b) the time-frequency approach is a very sensitive method of assessing the fine temporal changes in chronobiological rhythms. Supported in part by NSC87-2314-B6-96.