Ultradian Biorhythms of C57BL/6 Mice Body Temperature under Constant Illumination or during Natural Day-Night Cycle

Reduced glucose concentration enhances ultradian rhythms in Pdcd5 promoter activity in vitro

Intrinsically driven ultradian rhythms in the hourly range are often co-expressed with circadian rhythms in various physiological processes including metabolic processes such as feeding behaviour, gene expression and cellular metabolism. Several behavioural observations show that reduced energy intake or increased energy expenditure leads to a re-balancing of ultradian and circadian timing, favouring ultradian feeding and activity patterns when energy availability is limited. This suggests a close link between ultradian rhythmicity and metabolic homeostasis, but we currently lack models to test this hypothesis at a cellular level. We therefore transduced 3T3-L1 pre-adipocyte cells with a reporter construct that drives a destabilised luciferase via the Pdcd5 promotor, a gene we previously showed to exhibit robust ultradian rhythms in vitro. Ultradian rhythmicity in Pdcd5 promotor driven bioluminescence was observed in >80% of all cultures that were synchronised with dexamethasone, whereas significantly lower numbers exhibited ultradian rhythmicity in non-synchronised cultures (∼11%). Cosine fits to ultradian bioluminescence rhythms in cells cultured and measured in low glucose concentrations (2 mM and 5 mM), exhibited significantly higher amplitudes than all other cultures, and a shorter period (6.9 h vs. 8.2 h, N = 12). Our findings show substantial ultradian rhythmicity in Pdcd5 promotor activity in cells in which the circadian clocks have been synchronised in vitro, which is in line with observations of circadian synchronisation of behavioural ultradian rhythms. Critically, we show that the amplitude of ultradian rhythms is enhanced in low glucose conditions, suggesting that low energy availability enhances ultradian rhythmicity at the cellular level in vitro.

Effect of Heliogeophysical and Atmospheric Factors on the Degree of Synchronization of Ultradian Rhythms of Body Temperature in Mice

In a long-term (8 months) study, we examined the degree of synchronization of ultradian body temperature oscillations of two isolated groups of mice kept under constant dim illumination. In most cases, the periods of increased activity accompanied by rapid elevation of body temperature coincided in these groups of mice, but in some days, no significant synchronization between the examined parameters was observed. Analysis of the effects of environmental factors on the degree of synchronization of ultradian rhythms in mice revealed association of this parameter with the dynamics of atmospheric pressure (AtmP) and to a lesser extent with the vertical component of interplanetary magnetic field Bz. The loss in synchronicity of ultradian rhythms of mouse activity occurred after a rapid drop of AtmP or during pronounced negative Bz. Therefore, these factors can be viewed as desynchronizers of the biological ultradian rhythms.

Phase Analysis of Ultradian Rhythms of Body Temperature in Laboratory Mice Maintained under Constant Illumination at Different Longitudinal Locations

The study examined the rhythmic oscillations of body temperature with the period ranging 100-400 min in three groups of laboratory mice maintained under persistent artificial illumination in Moscow and Ulyanovsk. The difference in the moments of sunrise or sunset in these towns is about 1 h. The greatest rhythmic oscillations of body temperature in examined mice had the periods of 100-400 min. The phase analysis of 100-200-min rhythms revealed their synchronicity with local but not universal time despite the mice had no photic signs indicating alternation of day and night. Of them, the most pronounced were the rhythms with the periods of 121, 143, 151, and 186 min. The present data suggest existence of an external environmental synchronizer of body temperature ultradian rhythms related to local solar time.