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. 2018 Mar 13;12:10. doi: 10.3389/fnint.2018.00010

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Copyright © 2018 Laje, Agostino and Golombek.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Schematic representation of the interaction among different scales of biological timing. (A) The left panel depicts one side of a gear clock containing examples of ultradian (red), circadian (blue) and infradian (green) events. Some events, such as cortisol release or the menstrual cycle, are regulated by more than one scale of biological timing. Thus, cortisol release has both ultradian and circadian components (red/blue), whereas the menstrual cycle presents circadian and infradian regulation (blue/green). The right panel shows the other side of this clock, in which each temporal scale may act as a gear, interacting with the others to produce a fine regulation of temporal events. (B) A simplified conceptual model of the various biological rhythms and their interactions. Arrows represent excitation/activation; flat-ended lines represent inhibition/repression. Dashed lines represent known but disregarded interactions. Black stars mark our proposed experimental manipulations: short and long photoperiod (short photoperiod, SP an longphotoperiod, LP, respectively) conveyed by Mel; short Zeitgeber period (SH); fasting, represented by an increased efficacy of the connection ARC→TRH (via higher NPY); slower HNS oscillations due to increased Dopamine (DA). Abbreviations: SCN, suprachiasmatic nucleus; HPA, Hypothalamic-Pituitary-Adrenal axis; HPT, Hypothalamic-Pituitary-Thyroid axis; HNS, Hypothalamic-Nigro-Striatal axis; CRH, corticotropin-releasing hormone; ACTH, adrenocorticotropic hormone; CORT, cortisol/corticosterone; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone; T3, triiodothyronine; Mel, melatonin; EYA3, eyes-absent transcriptional coactivator and phosphatase 3; ARC, arcuate nucleus; SN/VTA, substantia nigra/ventral tegmental area; NAcc, nucleus accumbens.