Zhou Y.X., Han X., Bao Y.Y., Zhu Z.H., Huang J.L., Yang C.Y., He C.Y. and Zuo Z.H,. 2021. Aquatic Toxicology, 238:105917.

Diuron, a commonly used herbicide and antifouling biocide, has been frequently detected in seawater. The effects of diuron on fish behaviour are currently poorly understood. Herein, the marine medaka (Oryzias melastigma) was continuously exposed to environmentally realistic levels of diuron from the fertilised egg stage to the adult stage. Behavioural evaluation of adult marine medaka indicated that exposure to diuron increased anxiety in the light-dark test and increased predator avoidance. In addition, diumn exposure significantly reduced aggression, social interaction, shoaling, and learning and memory ability. However, only negligible variations in foraging behaviour and in behaviour in the novel tank test were observed. Marine medaka chronically exposed to diuron also showed decreased levels of dopamine in the brain, and changes were observed in the transcription of genes related to dopamine synthesis, degradation and receptors. Exposure to 5000 ng/L diumn caused significant downregulation of the expression of the genes of tyrosine hydroxylase and monoamine oxidase and significantly upregulated the expression of the genes of the D₅ dopaminergic receptor. The relative expression of the D₄ dopaminergic receptor was significantly upregulated in the 50, 500 and 5000 ng/L diuron-treated groups. These findings highlight the significant neumtoxic effects of diumn and the extent to which this may involve the dopaminergic system of the brain. More broadly, this study reveals the ecological risk associated with environmentally realistic levels of diuron in marine animals.

Figure 1. Dopamine level in the brains and relative expression of the genes related to dopamine signalling in marine medakas exposed to diuron. (A) Dopamine level in the brains. The data are presented as the mean ± SEM (n = 6). (B) Relative expression of the genes related to dopamine synthesis and degradation of th, ddc, mao, and comt in the brains. (C) Relative expression of the genes related to dopaminergic receptors and transporter of drd1, drd2, drd3, drd4, drd5, and dat in the brains. The data of relative expression of the genes are presented as the mean ± SEM (n = 6). The data were analysed by nested ANOVA followed by the Duncan’s test. The means of treatments not labelled with a common letter are significantly different at P < 0.05. (D) Overview of dopamine signalling in synapsis. Especially, the ovals represent the expression of tyrosine hydroxylase (th), levodopa decarboxylase (ddc), monoamine oxidase (mao), and catechol-O-methyltransferase (comt). Green arrows represent the decreasing of relative mRNA expression of enzymes related to dopamine synthesis and degradation.