Abstract
The chronobiological landscape has been greatly dominated by circadian rhythms. The existence of shorter-than 24-hour rhythms, called ultradian rhythms, has also emerged. Ultradian rhythms cover a broad spectrum of periodicities ranging from milliseconds to multiple hours. Ultradian rhythms have been associated with a wide spectrum of processes at behavioural, transcriptional, cellular and metabolic levels, including gene expression, hormonal release and behavioural activity. Perhaps the most prominent example of ultradian behaviour in mammals is the common vole (Microtus arvalis), a rodent exhibiting fluctuation in its behavioural activity in the hourly range.
Here, it was attempted to confirm the existence of ultradian rhythms as well as provide new evidence of the role of ultradian rhythms. We hypothesised that ultradian rhythms are observed in the behaviour, metabolite levels and gene expression, using either the common vole or cell lines as investigation models.
In the common vole, our findings show either circadian or ultradian rhythmicities or both in 76 metabolites in males and 85 in females from a total of 184 investigated metabolites. Regarding, gene expression our findings were not conclusive but the existence of an ultradian rhythm cannot be ruled out in Pdcd5, Homer1a, Per2 and Bmal1. This series of experiments also led to the generation of the first ever gene expression and metabolomic profiles in the common vole using an ultradian timescale for sampling.
Gene expression was also interrogated for ultradian rhythmicity in vitro using murine fibroblasts and human kidney cells. Pdcd5 was shown to be expressed in an ultradian fashion, exhibiting periods of expression of about 7 hours, superimposed to a circadian rhythm, in both cellular models.
Taken together, all the data further highlights the existence of ultradian rhythms in systems. It provides additional insight in the general function of ultradian rhythms with a link to metabolic processes being particularly strong.