Episodic Ultradian Events-Ultradian Rhythms

Abstract

In the fast lane of chronobiology, ultradian events are short-term rhythms that have been observed since the beginning of modern biology and were quantified about a century ago. They are ubiquitous in all biological systems and found in all organisms, from unicellular organisms to mammals, and from single cells to complex biological functions in multicellular animals. Since these events are aperiodic and last for a few minutes to a few hours, they are better classified as episodic ultradian events (EUEs). Their origin is unclear. However, they could have a molecular basis and could be controlled by hormonal inputs-in vertebrates, they originate from the activity of the central nervous system. EUEs are receiving increasing attention but their aperiodic nature requires specific sampling and analytic tools. While longer scale rhythms are adaptations to predictable changes in the environment, in theory, EUEs could contribute to adaptation by preparing organisms and biological functions for unpredictability.

Keywords: central nervous system; gene; methodology; short-term rhythms; temperature.

Figure 1

Three-day profiles of temperature in the scrotum of an alpaca ((A): sampling rate = 1 min), the brain of a gemsbok ((B): sampling rate = 5 min), the peritoneal cavity of a non-pregnant female lion ((C): sampling rate = 1 min), a female duck ((D): sampling rate = 5 min), and a pregnant female rabbit ((E): sampling rate = 5 min). NB: the episodic ultradian events are superimposed on the strong circadian rhythms. All the animals were kept outdoors, either in outdoor paddocks with free access to food and water (A,D) or in their natural environment [12] (B,C,E). Lion data courtesy of Andrea Fuller [13].

Figure 2

Twenty-four-hour temperature profiles in the brain, carotid artery, abdomen, retroperitoneal fat, and subcutaneous fat of a castrated sheep. The raw data (blue line) were collected from the brain, carotid artery, and abdomen every 1 min and from the two fat tissues every 5 min. The circadian patterns (red line) were fitted to the data using a cosinor analysis [17]. There is a visible synchrony between most of the episodic ultradian events (EUEs) across the different tissues (vertical lines). The sheep was kept indoors under a 12/12 h dark–light cycle and fed at requirement level at 09:00 [12].

Figure 3

Wavelet power spectrum (left) and time series (right; original data and reconstructed from the wavelet analysis) of a three-day profile of body temperature measured in the abdomen of a mouse sampled at different frequencies based on [24]. Wavelet analysis revealed a loss of EUEs when the sampling interval was longer than 5 min. NB: the number of detected EUEs is indicated in the bottom right corner of the right profiles.

Figure 4

Profiles of activity and body temperature in a mouse kept at an ambient temperature of 20 °C (top two panels) or 30 °C (bottom two panels) to illustrate the synchrony between the EUEs of temperature and the EUEs of activity [79] NB: It has been demonstrated that the EUEs of temperature precede the EUEs of activity [80].

Figure 5

Schematic of the key regions involved in the regulation of EUEs approximated in the rat brain [81]. Major neuronal connections are shown in black arrows and demonstrate a putative network that generates and/or coordinates physiological EUEs. A midbrain dopaminergic oscillator in the VTA/SNc region appears largely responsible for generating EUEs in locomotion [82]. Orexin neurons in the lateral hypothalamus are implicated in generating EUEs of body temperature, arousal and locomotion, but not feeding and heart rate [83]. The pathways connecting the SCN with the ARC and the SPZ may represent sites of crosstalk between the ultradian and circadian systems. The SPZ-PVN region generates EUEs in intracellular calcium levels [84], and EUEs in clock gene expression in the SCN have been observed [34]. The parvocellular and magnocellular neurons in the PVN may regulate endocrine EUEs. However, the functional significance of ultradian clock gene rhythms in the SCN remains unclear.