Behavioural response to toxic elements, detoxification and organ accumulation are time-of-day-dependent in zebrafish.

Toxic elements, such as mercury (Hg) and arsenic (As), are major pollutants in aquatic environments, posing ecological threats to living organisms due to their toxicity and bioaccumulation. This paper investigated whether zebrafish response to Hg and As displayed day/night differences. Fish were exposed to either 35 µg/L of mercury chloride for 6 h or 65 mg/L of sodium arsenate for 4 h, at two different times of the day: mid-light (day; ML) and mid-darkness (night; MD). Fish were video-recorded to investigate their behavioural response and at the end of each trial, gills and liver samples were collected for gene expression measurement. Gills, liver and brain samples were also obtained to determine Hg and As concentration. A control group (non-exposed) was video-recorded and sampled too. The effect of Hg and As on zebrafish swimming activity and the expression of antioxidant and metallothionein genes was time-of-day-dependent, with a stronger response being observed during the day than at night. However, the neurobehavioural effect of Hg was more affected by the time of exposure than the effect of As. In addition, Hg concentration in the gills was significantly higher in zebrafish exposed at ML than at MD. Altogether, these findings suggest that zebrafish response to Hg and As is time-of-day-dependent and remark the importance of considering toxicity rhythms when using this fish species as a model in toxicological research.

Administration time-dependent effects of poly (I:C) on antioxidant and immune responses along the diurnal time scale in zebrafish.

The circadian clock of vertebrates regulates many biological processes, including the immune system. This paper investigated whether responsiveness to poly (I:C), a synthetic analog of double-stranded RNA used as an immunostimulant, exhibits day/night differences in zebrafish. Fish were intraperitoneally (IP) injected with either phosphate-buffered saline (PBS) or poly (I:C) at two different time points: "Zeitgeber Time" (ZT) 4 (day) and ZT16 (night). Then, 6 h later, fish were euthanized, and tissue samples (skin, liver and kidney) were collected. A control group (intact fish) was also sampled at the same time points. The effect of poly (I:C) on the expression of antioxidant and immune genes was time-of-day-dependent, and the response was stronger following poly (I:C) administration in the day than at night. Time-dependent differences were observed for some genes in the PBS and control groups. However, these differences were tissue-specific. In liver, almost all the genes were affected by time of day. In kidney, poly (I:C) affected the expression of all the gene markers regardless of administration time. These findings highlight the importance of considering the time to administer poly (I:C) when evaluating the fish immune response.