Glucocorticoids (GCs) are steroid hormones mainly acting as key regulators of body homeostasis and stress responses. Their activities are primarily based on the binding to the GC receptor (GR), a member of the nuclear receptor family, that regulates tissue-specific sets of genes. GCs secretion follows a circadian rhythmicity with a peak linked to the animal’s activity phase. In mammals, GCs are also implicated in feeding entrainment mechanisms as internal zeitgeber. Here, we investigated, by means of behavioural and molecular approaches, the circadian clock and its regulation by light and food in wild-type (WT) and null glucocorticoid receptor (gr^−/−) zebrafish larvae, juveniles and adults. In both WT and gr^−/− larvae and adults, behavioural activity and clock gene expression were entrained to the light–dark (LD) cycle and rhythmic in constant conditions. Differences in the pattern of clock genes’ expression indicated a modulatory role of GCs. A significant role of Gr was detected in the feeding entrainment which was absent or markedly dampened in mutants. Furthermore, the expression of two clock-regulated genes involved in glucidic and lipidic metabolism was altered, highlighting the participation of GCs in metabolic processes also in fish. Taken together, our results confirmed the role of GC-mediated Gr signaling in the feeding entrainment in a non-mammalian species, the zebrafish.

Intensive and trait‐selective mortality of fish and wildlife can cause evolutionary changes in a range of life‐history and behavioural traits. These changes might in turn alter the circadian system due to co‐evolutionary mechanisms or correlated selection responses both at behavioural and molecular levels, with knock‐on effects on daily physiological processes and behavioural outputs. We examined the evolutionary impact of size‐selective mortality on group risk‐taking behaviour and the circadian system in a model fish species. We exposed zebrafish Danio rerio to either large or small size‐selective harvesting relative to a control over five generations, followed by eight generations during which harvesting was halted to remove maternal effects. Size‐selective mortality affected fine‐scale timing of behaviours. In particular, small size‐selective mortality, typical of specialized fisheries and gape‐limited predators targeting smaller size classes, increased group risk‐taking behaviuor during feeding and after simulated predator attacks. Moreover, small size‐selective mortality increased early peaks of daily activity as well as extended self‐feeding daily activity to the photophase compared to controls. By contrast large size‐selective mortality, typical of most wild capture fisheries, only showed an almost significant effect of decreasing group risk‐taking behaviour during the habituation phase and no clear changes in fine‐scale timing of daily behavioural rhythms compared to controls. We also found changes in the molecular circadian core clockwork in response to both size‐selective mortality treatments. These changes disappeared in the clock output pathway because both size‐selected lines showed similar transcription profiles. This switch downstream to the molecular circadian core clockwork also resulted in similar overall behavioural rhythms (diurnal swimming and self‐feeding in the last hours of darkness) independent of the underlying molecular clock. To conclude, our experimental harvest left an asymmetrical evolutionary legacy in group risk‐taking behaviour and in fine‐scale daily behavioural rhythms. Yet, the overall timing of activity showed evolutionary resistance probably maintained by a molecular switch. Our experimental findings suggest that size‐selective mortality can have consequences for behaviour and physiological processes.

Circadian rhythms are biological oscillations that occur with an approximately 24 h period and optimize cellular homeostasis and responses to environmental stimuli. A growing collection of data suggests that chronic circadian disruption caused by novel lifestyle risk factors such as shift work, travel across time zones, or irregular sleep-wake cycles has long-term consequences for human health. Among the multiplicity of physiological systems hypothesized to have a role in the onset of pathologies in case of circadian disruption, there are redox-sensitive defensive pathways and inflammatory machinery. Due to its location and barrier physiological role, the skin is a prototypical tissue to study the influence of environmental insults induced OxInflammation disturbance and circadian system alteration. To better investigate the link among outdoor stressors, OxInflammation, and circadian system, we tested the differential responses of keratinocytes clock synchronized or desynchronized, in an in vitro inflammatory model exposed to O₃. Being both NRF2 and NF-κB two key redox-sensitive transcription factors involved in cellular redox homeostasis and inflammation, we analyzed their activation and expression in challenged keratinocytes by O₃. Our results suggest that a synchronized circadian clock not only facilitates the protective role of NRF2 in terms of a faster and more efficient defensive response against environmental insults but also moderates the cellular damage resulting from a condition of chronic inflammation. Our results bring new insights on the role of circadian clock in regulating the redox-inflammatory crosstalk influenced by O₃ and possibly can be extrapolated to other pollutants able to affect the oxinflammatory cellular processes.

Urbanization and human activities are increasing global levels of artificial light at night (ALAN). Several studies have shown that ALAN negatively impacts animals, altering their perception, physiology, and behaviour, eventually leading to reduced fitness. ALAN also disrupts biological rhythms, affecting animals' ability to synchronize with natural cycles. There still is a critical lack of knowledge on ALAN's effects on freshwater ecosystems, which are highly threatened biodiversity hotspots. Our study aimed to understand the molecular effect of prolonged ALAN exposure in the zebrafish, a model species in this field. Using high-throughput RNA sequencing, we investigated the transcriptome-wide gene expression responses in whole brains of ALAN-exposed zebrafish. Samples were collected at four different timepoints along the day, two at daytime and two at nighttime, to identify daily changes in gene expression and metabolic pathways. A one month exposure to ALAN varied the expression of several genes and metabolic pathways, both at night, when zebrafish were directly exposed to artificial light, and during the day, at natural light conditions. The highest number of differentially expressed genes were found at the early hours of the day and at night. Overall, circadian clock genes and those related to immunity, stress response, responses to sensory stimuli, energy production, motor activity, and reproductive processes changed in expression levels. These results provide new insights into the mechanisms through which ALAN affects animal biology, suggesting broader impacts than previously recognised. To safeguard aquatic ecosystems and their biodiversity, it is paramount to mitigate light pollution.

Circadian rhythms are 24-h oscillations driven by a hypothalamic master oscillator that entrains peripheral clocks in almost all cells, tissues and organs. Circadian misalignment, triggered by industrialization and modern lifestyles, has been linked to several pathological conditions, with possible impairment of the quality or even the very existence of life. Living organisms are continuously exposed to air pollutants, and among them, ozone or particulate matters (PMs) are considered to be among the most toxic to human health. In particular, exposure to environmental stressors may result not only in pulmonary and cardiovascular diseases, but, as it has been demonstrated in the last two decades, the skin can also be affected by pollution. In this context, we hypothesize that chronodistruption can exacerbate cell vulnerability to exogenous damaging agents, and we suggest a possible common mechanism of action in deregulation of the homeostasis of the pulmonary, cardiovascular and cutaneous tissues and in its involvement in the development of pathological conditions.

Individual differences in cognition have been historically recognized in humans, and recent evidence suggests that such variation is widespread across the animal kingdom. Despite its importance for individuals' behaviour and fitness, the biological roots of cognitive variation remain poorly understood. We hypothesize that variation in brain gene expression is important in determining individual cognitive differences. To test this, we focused on 6 neural plasticity genes and examined fish, which exhibit the largest cognitive variation reported for vertebrates. Zebrafish (Danio rerio) exposed to visual discrimination tasks showed substantial variation in their performance, with some learning over 7 times faster than others. Expression of two genes positively predicted learning performance. However, expression levels of most genes were related at the individual level, suggesting that multi-gene expression patterns may be more relevant than single gene variation. Principal component analysis identified two axes of multi-gene expression variation: the first loaded by all genes except neurotrophin bdnf, the second mainly loaded by bdnf and neurod1 expression. Only the latter component significantly predicted learning performance in a visual discrimination task, indicating that individual variation in bdnf expression and with lesser extend neurod1 are critical for learning. Our study bridges the gap between cognitive differences and molecular mechanisms underlying brain function, providing foundation for new understanding what makes individual unique.