Dear Colleagues,

On planet Earth, living organisms have adapted to the sidereal period of rotation of the Earth on its axis, which determines the alternation of sunlight and darkness and a solar day of 24 hours on average. The tilt of the Earth's axis gives rise to different seasons, which are opposite in each hemisphere. Living organisms on this planet are subject to predictable fluctuations in light and temperature, and cellular and body tissue physiology are profoundly affected by these geophysical cycles. A wide range of species, from cyanobacteria to humans, has evolved endogenous biological clocks that allow for the anticipation of these daily (and seasonal) variations. The biological rhythms in unicellular and multicellular organisms are controlled by a hierarchical network structure that allows for the coordination of molecular oscillation in individual cells, tissues and organ systems. There is a master pacemaker in the suprachiasmatic nucleus of the hypothalamus, which works as a transducer; it receives inputs from the external environment (light, temperature), reprocesses the signal and organizes the oscillating activity of peripheral biological clocks through output pathways. In every single cell, a biological clock goes on ticking and the set of oscillations of the single cells is responsible for the rhythmic tissue function. Harmonized biological rhythms are apparent in numerous facets of our physiology, managed by the internal timing system known as circadian clock circuitry. Circadian desynchrony, i.e., loss of resonance between body rhythmicity and environmental cues cycles, and alterations in the rhythms of each single tissue critically contribute to the mechanisms implicated in metabolic, neoplastic, infectious, immune-inflammatory and neurodegenerative diseases.

In this Special Issue in Biology, we will bring together articles dealing with the numerous and various aspects of up-to-date scientific research on multi-frequency biological rhythms in cells, animal models and humans, in order to promote an increasingly in-depth knowledge of the molecular processes underlying the rhythmicity of cell processes, intracellular signaling and tissue functions as well as their rhythmic regulation through circadian clock circuitry, both in physiological and pathological conditions. This Special Issue welcomes the submission of original research articles and reviews.

Prof. Dr. Gianluigi Ubaldo Mazzoccoli
Dr. Marina Maria Bellet
Guest Editors

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• biological rhythms
• molecular clockworks
• circadian timing system
• circadian desynchrony
• circadian disruption
• chronopathology
• chronopharmacology and chronotherapeutics
• chronomedicine

Title: The ups and downs of insect life: Circadian rhythms of hormones in the physiology of adult insects
Authors: Michael Cardinal-Aucoin
Affiliation: York University, Toronto, Canada
Abstract: The circadian system profoundly influences the behavior and physiology of adult insects, and its disruption has been associated with reduced reproductive output and lifespan. The central role of hormones within the circadian system of adult insects has become increasingly evident over the past two decades. Previous research in this area has tended to focus on larval insects and the hormonal regulation of development while less attention has been paid to hormonal rhythms in adults. Several major hormones including juvenile hormone (JH), ecdysteroids, and neuropeptides such as pigment dispersing factor (PDF) and insulin-like peptides (ILPs), exhibit rhythmic secretion patterns, tightly regulated by the circadian clock. The neuroendocrine and circadian systems of adult insects are intimately connected both morphologically and functionally. Central clocks in the brain convey external timing cues to brain neurosecretory cells while peripheral clocks, including in endocrine tissues, coordinate numerous processes from metabolism to reproduction. The resulting hormone rhythms allow the communication of important temporal information to the cells and tissues of the adult insect, thus acting as vital mediators in the circadian system. The circadian modulation by hormones of diverse processes from metabolism to reproduction ultimately orchestrates the physiology and behavior of adult insects, shaping their adaptation to dynamic environmental conditions.