Bioconcentration,Tissue Distribution And Metabolism Of Neonicotinoid Clothianidin In Zebrafish

As pesticides play an indispensable role in the agricultural production,their environmental fate and toxicity have always been a hot topic in the environmental toxicology research.Neonicotinoids are the most widely used insecticides in the world.The mode of action of neonicotinoids is binding to nicotinic acetylcholine receptors(nAChRs)of insects and affecting their central nervous system,causing paralysis and death.However,it became increasingly evident that neonicotinoids could have various toxic effects on nontarget organisms.Due to the facts that neonicotinoids are extensively detected in the environment(especially in the aquatic environment)with a high concentration level and/or high detection frequency,the bioconcentration potential,tissue distribution and metabolism of neonicotinoids in aquatic organisms become an important scientific tissue.In this study,the model animal zebrafish were exposed to a typical neonicotinoid clothianidin in the laboratory condition,aiming to evaluate the bioconcentration,tissue-specific distribution,metabolism and elimination of clothianidin in zebrafish.The main conclusions have been described as follows:(1)The uptake and elimination of clothianidin in zebrafish was investigated in laboratory.The concentration of clothianidin in the whole zebrafish were detected during the 14-days of uptake period and 7-days of elimination period.Results show that clothianidin can be rapidly accumulated during the uptake period and reach steady-state within 2 days.The elimination process is also fast and clothianidin residue in zebrafish could not be detected after 2 days.The uptake and elimination kinetics of clothianidin in zebrafish can be well described by the first-order toxicokinetic models.Both the kinetic bioconcentration factor(BCF_kinetic)and the steady-state bioconcentration factor(BCF_steady-state)indicate the low bioconcentration potential of clothianidin in the whole zebrafish.(2)In order to characterize the uptake and elimination of clothianidin in different tissues of zebrafish,six tissues(intestine,liver,gill,kidney,brain and muscle)were selected and the clothianidin concentrations in these tissues were detected during the uptake and elimination period.Results show that the uptake and elimination kinetics trends in intestine,gill,brain and muscle are generally very similar to that in the whole zebrafish,i.e.,increasing quickly within the first 2 days and reaching the plateau,and decreasing slightly after day-7 during the 14-days uptake period and decreasing quickly within 2 days during the elimination period.However,different trend was observed in liver,in which the concentration of clothianidin increases continuously during the 14-days uptake period and still can be detected after 3-days of depuration.Based on the tissue-blood partitioning coefficients calculated by the PBTK model,together with the concentrations and BCF in different tissues,clothianidin is prone to accumulate in the intestine and liver of zebrafish.(3)Eight metabolites of clothianidin in zebrafish were identified by the high-resolution mass spectrometer,mainly including desmethyl clothianidin,clothianidin urea,N-denitro clothianidin.According to these metabolites,N-demethylation and nitro-reduction were proposed to be the possible metabolism pathways for clothianidin in zebrafish.Investigation on the uptake and elimination kinetics of two metabolites(desmethyl clothianidin,clothianidin urea)shows that both the intestine and liver are the important tissues for metabolism of clothianidin in zebrafish and desmethyl clothianidin is the major metabolite of clothianidin in zebrafish.The N-demethylation pathway of clothianidin catalyzed by cytochrome P450enzymes is investigated by quantum chemistry calculation and the mechanism of this reaction is H-abstraction-OH-rebound-water-assisted hydroxymethyl decomposition.(4)During the whole uptake and elimination period,clothianidin and its metabolites still can be detected in the muscle.Although the concentration is relatively low,the large amount of muscle in fish imply that it is the most important storage tissue for clothianidin and its metabolites in zebrafish.