Effects Of Tributyltin On The Circadian Rhythm Of Zebrafish (Danio Rerio) And The Mechanism Involved

Circadian rhythm plays a crucial role in maintaining the normal physiological and biochemical processes of fish,which has high ecological significance.The changes of circadian rhythm patterns will affect fish foraging,enemy avoidance and reproductive opportunities.tributyltin(TBT),as a persistent organic pollutant,is widely used in agriculture,industry and other fields,and eventually enter the aquatic environment,posing potential threats to fish.Recent studies have shown that fish circadian rhythm system may be a new target of TBT,but the specific interference effects and mechanisms are still unclear.The circadian rhythm system mainly includes three parts: the external rhythm information input,the circadian rhythm oscillator and the rhythm information output.The interference of pollutants on any link will lead to the disorder of circadian rhythm.In this study,we selected adult male zebrafish(Danio rerio)as the research object.After 90 days of exposure to TBT at environmental concentration as the target pollutant,we used behavioral methods to detect the changes in circadian activity rhythm from three aspects: external rhythm information input,circadian rhythm oscillator and rhythm information output.To analyze the effects of TBT on the circadian rhythm system of zebrafish and elucidate its mechanism.Results were as follows:1.TBT causes abnormal circadian rhythm activity in zebrafish.The main manifestations are inactivity and somnolence during 06:00-07:00(Arousing period);there is no regular movement "wave" during 07:00-20:00(Active period)and inability to fall asleep quickly and peacefully during 20:00-24:00(Period of latency to sleep).2.TBT interferes with the external rhythmic information input of zebrafish.The main manifestations are structural changes of the zebrafish retina,such as slight retinal swelling and blurring of the hierarchical structure.Compared with the control group,69 and 242 differentially expressed genes are up-regulated in the 1 ng/L and 10 ng/L TBT groups,respectively.93,253 differentially expressed genes are down-regulated.Through KEGG and gene set enrichment analysis(GSEA),it is found that tyrosine metabolism and retinol metabolism pathways are significantly enriched in the pathways related to circadian rhythm.ELISA also shows that TBT significantly inhibited the activities of tyrosinase,dopachromochrome isomerase and retinol dehydrogenase.3.TBT interferes with the rhythmic information output of zebrafish.It is mainly manifested as a significant inhibition of melanin levels in the retina after TBT exposure.At the same time,TBT could significantly increase the level of melatonin at 18:00,which may be one of the reasons for the change of circadian rhythm behavior.4.TBT disturbs the circadian rhythm oscillator of zebrafish.The expression levels of five circadian Clock oscillation genes(Bmal1,Clock,Per1 a,Per2 and Cry1a)in zebrafish eye tissues and brain tissues are different at four time points(00:00,06:00,12:00 and 18:00),this suggests that the circadian rhythm oscillator is disturbed,but the specific mechanism needs to be further studied.In summary,TBT interferes with circadian rhythms in zebrafish.Environmental concentrations(1 and 10 ng/L)of TBT can affect the circadian rhythm of zebrafish by affecting the external rhythm information input,the circadian rhythm oscillator and the rhythm information output,resulting in the disorder of circadian rhythm behavior.Among them,TBT affects the external rhythm information input by changing the retinal structure and tyrosine metabolism and retinol metabolism pathways.Therefore,the fish circadian system is a new target of TBT and should be considered in future studies of aquatic pollutants on fish circadian rhythm.Innovation:1.Studies on the effects of TBT on fish circadian rhythm will be useful for exploring the new toxic effects of pollutants and will provide a new reference for the monitoring of TBT levels in the environment.2.The study could more truly reflect the response of fish to environmental pollutants.These studies can not only examine the effects of TBT on fish circadian rhythm and its action pathways,but also provide data and information for the development of basic theory of fish chronobiology.