Abstract
Environmental cycles on Earth display different periodicities, including daily, tidal or annual time scales. Virtually all living organisms have developed temporal mechanisms to adapt to such changes in environmental conditions. These biological timing structures—ranging from microsecond to seasonal timing—may have intrinsic properties and even different clock machinery. However, interaction among these temporal systems may present evolutionary advantages, for example, when species are exposed to changing climatic conditions or different geographic locations. Here, we present and discuss a model that accounts for the circadian regulation of both ultradian (less than 24-h) and infradian (more than 24-h) cycles and for the interaction among the three time scales. We show two clear examples of such interaction: (i) between the circadian clock and the seasonal regulation of the Hypothalamic-Pituitary-Thyroid (HPT) axis; and (ii) between the circadian clock and the hypothalamic-nigrostriatal (HNS) ultradian modulation. This remarkable interplay among the otherwise considered isolated rhythms has been demonstrated to exist in diverse organisms, suggesting an adaptive advantage of multiple scales of biological timing.
Highlights
In order to analyze the interplay among the different time scales described above, we propose a coarse-grained conceptual model of some of the various infradian, circadian and ultradian rhythm generators and their interactions (Figure 1B)
We propose a putative neural network that may participate in the interconnection of different scales of biological timing in order to finely regulate behavioral and physiological rhythms
When we turn the clock upside down, we usually isolate different oscillatory mechanisms in order to understand them in detail, not necessarily considering the essential interaction between the convergent sizes and functions of biological gears (Figure 1A, right panel)
Summary
Biological mechanisms that account for temporal information are ubiquitous and essential for physiology and behavior. Biological timing comprises distinct time-related processes that span several orders of magnitude, from microsecond to seasonal events (Buhusi and Meck, 2005; Golombek et al, 2014) Among these temporal orders, almost all living organisms are subjected to the influence of the Earth’s rotational cycle of 24 h. It is puzzling that in most cases, different biological time scales have been researched and considered independently and as isolated compartments, with the notable exception of the circadian reading needed for photoperiodic regulation (e.g., Pittendrigh and Daan, 1976) In this perspective we propose a general framework for the interaction of multiple time scales using well-known examples of different periodicities. Several of the feedback handles among clocks and temporal systems are not well understood, we do urge to consider multiple temporalities and their interactions when analyzing a certain physiological or biochemical variable
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