Mechanisms of reproductive inhibition in zebrafish (Danio rerio) exposed to ethinylestradiol, nitrate and ammonia

Abstract

The objective of this thesis was to investigate the reproductive effects and mechanisms of action of three common MWWE constituents, 17α-ethinylestradiol (EE2), nitrate and ammonia, using the zebrafish (Danio rerio) as the model. Fecundity assays indicated that ammonia and EE2, but not nitrate, inhibited spawning. Further mechanistic investigations into reproductive processes such as steroidogenesis, maturation, and ovulation indicate that ammonia and EE2 affect reproduction through different mechanisms. EE2 acts by inhibiting steroidogenesis as shown by the reduction of sex steroid levels and expression of aromatase. Ammonia blocked the stimulatory effects of the gonadotropin analog human chorionic gonadotropin (hCG) on ovulation and spawning. Molecular analysis from these experiments indicated a potential role of prostaglandins in this response through the actions of the enzyme COX-2, but no effects on prostaglandin levels or receptor expression were found. Though the mechanism through which ammonia inhibits reproduction remains unknown, these findings help narrow the likely sites of action.