Study On Embryonic Toxicity And Reproductive Toxicity Of Mercuric Chloride In Zebrafish

Mercury?Hg?is a global heavy metal pollutant that is widely distributed in the aquatic environment,which seriously threatens the life of fish and endangers human health through the food chain.In nature,Hg mainly exists in three forms as element,inorganic compounds and organic compounds,and organic Hg especially methylmercury is the most toxic form.At present,although inorganic Hg is the most important form of Hg emissions from industrial wastewater,much attention has been paid to organic Hg toxic effects to fish,whereas inorganic Hg-induced toxicity has been somewhat neglected.In this study,zebrafish embryos were exposed to mercuric chloride?HgCl₂?,and developmental parameters,total mercury content,oxidative stress,and immune response were analyzed to investigate the embryonic toxicity of HgCl₂.And then,zebrafish adults were exposed to HgCl₂,and histological structure,sex hormone levels,antioxidant status,and hypothalamic-pituitary-gonadal?HPG?axis-related gene expression levels were examined to expolor the reproductive toxicity of HgCl₂.1.Embryonic toxicity of zebrafish exposed to HgCl₂In order to study the embryotoxicity of HgCl₂ and its mechanism on zebrafish,zebrafish embryos were exposed to different concentrations of Hg²⁺?control,1,4 and16?g/L?from 2 h post-fertilization?hpf?to 168 hpf.The results showed that exposure to 16?g/L Hg²⁺caused developmental damage?includes increased mortality and malformation,decreased body length,and delayed hatching period?.Total Hg in fish increased with the Hg²⁺concentration and exposure time.Meanwhile,Hg²⁺exposure affecting antioxidant enzyme?CAT,GST and GPX?activities,endogenous GSH and MDA contents,as well as the mRNA levels of genes?cat1,sod1,gstr,gpx1a,nrf2,keap1,hsp70 and mt?encoding antioxidant proteins.In addition,the transcription levels of several representative genes?il-1?,il-8,il-10,tnf?2,lyz and c3?related to innate immunity were up-regulated by Hg²⁺exposure.Therefore,the present study provides evidence that HgCl₂ exposure can induce developmental impairment,oxidative stress and immunotoxicity in zebrafish embryos-larvae.2.Reproductive toxicity of HgCl₂ exposure on adult zebrafishIn order to investigate the reproductive toxicity and its mechanism of HgCl₂ on zebrafish,adult zebrafish were exposed to 0?control?,15 and 30?g/L Hg²⁺for 30 days.The results showed that Hg²⁺exposure caused pathological lesions,changed the activities and mRNA levels of antioxidant enzymes?CAT,SOD and GPX?as well as the content of GSH and MDA in zebrafish gonads.In females,although ovarian E2 content remained relatively stable,significant down-regulation of lh?,gnrh2,gnrh3 in brain and lhr,er?in ovary were observed.In males,T level in the testis significantly decreased after Hg²⁺exposure,accompanied by down-regulated expression of gnrh2,gnrh3,fsh?and lh?in the brain as well as fshr,lhr,ar,cyp17 and cyp11b in the testis.Therefore,the present study showed that HgCl₂ exposure induced oxidative stress in the gonads of zebrafish,thereby impairing histological structure,at the same time,HgCl₂ exposure altered sex hormone levels by disrupting the transcription of related HPG-axis genes.Furthermore,different response of the antioxidant defense system,sex hormone and HPG-axis genes between females and males indicated the gender-specific regulatory effect by Hg²⁺exposure.