Inhibition of spawning in zebrafish (Danio rerio): Adverse outcome pathways of quinacrine and ethinylestradiol

This study determined the effects of the estrogen receptor agonist ethinylestradiol (EE2) and the phospholipase A2 inhibitor quinacrine (QUIN) on the pathways controlling follicular development, steroidogenesis, oocyte maturation, ovulation and spawning success in adult zebrafish. Both EE2 and QUIN inhibited spawning but did so through different mechanisms. EE2 affected follicular development (reduced ovarian size and reduction in the proportion of cortical alveolus, vitellogenic and mature follicle stages), steroidogenesis (reduced expression of aromatase), maturation (reduced luteinizing hormone receptor expression) and ovulation (reduced expression of cytosolic phospholipase A2 and the nuclear progesterone receptor). Although EE2 alters the proportion of follicle stages within the ovary, the downregulation of gene expression as a consequence of EE2 exposure was primarily due to a decline in expression of the genes of interest in vitellogenic and mature ovarian follicles. QUIN targeted ovulation via a reduction of the steroid 17α,20β dihydroxy-4-prenen-3-one (17α,20β-P) and decreased expression of the prostaglandin metabolizing enzyme cyclooxygenase 2. This study demonstrates the usefulness in defining the impacts of toxicants at the molecular and cellular, organ and whole organism level and how connections between these impacts can be used to describe the adverse outcome pathways (AOPs) that mediate toxicant action. Histological analysis and gene expression were effective tools in defining the AOPs of QUIN and EE2 while the measurement of reproductive hormones level did not provide much valuable information regarding the toxicant's mode of action.