Abstract
Circadian rhythms control the life of virtually all organisms. They regulate numerous aspects ranging from cellular processes to reproduction and behavior. Besides the light-dark cycle, there are additional environmental factors that regulate the circadian rhythms in animals as well as humans. Here, we outline the circadian rhythm system and considers zebrafish (Danio rerio) as a representative vertebrate organism. We characterize multiple physiological processes, which are affected by circadian rhythm disrupting compounds (circadian disrupters). We focus on and summarize 40 natural and anthropogenic environmental circadian disrupters in fish. They can be divided into six major categories: steroid hormones, metals, pesticides and biocides, polychlorinated biphenyls, neuroactive drugs and other compounds such as cyanobacterial toxins and bisphenol A. Steroid hormones as well as metals are most studied. Especially for progestins and glucocorticoids, circadian dysregulation was demonstrated in zebrafish on the molecular and physiological level, which comprise mainly behavioral alterations. Our review summarizes the current state of knowledge on circadian disrupters, highlights their risks to fish and identifies knowledge gaps in animals and humans. While most studies focus on transcriptional and behavioral alterations, additional effects and consequences are underexplored. Forthcoming studies should explore, which additional environmental circadian disrupters exist. They should clarify the underlying molecular mechanisms and aim to better understand the consequences for physiological processes.
Highlights
Circadian rhythms control the life of virtually all organisms
Little attention has so far been paid to circadian rhythms, they play a crucial role in the development and reproductive performance of aquatic organisms (Bayarri et al, 2009; Blanco-Vives and Sanchez-Vazquez, 2009; Yúfera et al, 2017)
Dysregulation of circadian rhythms can impact numerous aspects ranging from cellular processes to reproduction and behavior
Summary
An endogenous circadian clock exists in organisms ranging from prokaryotes to humans, of which, mechanisms have been described comprehensively in mammals (Rutter et al, 2002; Mohawk et al, 2012). A considerable proportion of non-clock genes are expressed in a circadian fashion. This has been shown from cyanobacteria up to mammals (Dvornyk et al, 2003; Gachon et al, 2004; Li et al, 2013). In promoters of many non-clock genes, which are regulated by circadian rhythms, E-boxes occur (Ripperger and Schibler, 2006). Via such target genes and their effects, central and peripheral circadian clocks directly regulate many aspects of physiological processes (Panda et al, 2002; Takahashi et al, 2008). In principle, be dysregulated by environmental chemicals (Matsuo et al, 2003; Fu et al, 2002; Huang et al, 2016)
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