Experimental exploration of estrogenic effects of norethindrone and 17α-ethinylestradiol on zebrafish (Danio rerio) gonads.

Synthetic progestins and xenoestrogens found in aquatic habitats are currently gaining attention on global scale. The current study aimed to investigate the time-and dose-dependent effects of synthetic progestin Norethindrone (NET; 100, 500 and 1000 ng/L) and estrogen 17α-ethinylestradiol (EE2; 100 ng/L) individually as well as in binary mixture (1000 ng/L NET + 100 ng/L EE2) on reproductive histology and transcriptional expression profile of genes in adult zebrafish. For this, 20 female (3.15 ± 0.18 cm & 0.33 ± 0.06 g) and 20 male zebrafish in each group (2.93 ± 0.13 cm & 0.29 ± 0.04 g) were exposed to drugs dissolved in water for 30 days in 12 L rectangular tanks. We found that both NET and EE2 exposure reduced the gonadosomatic index in females, while only EE2 exposure caused significant reduction in males (p ≤ 0.05). Interestingly, NET delayed oocyte maturation in females and accelerated spermatogenesis in males, while EE2 consistently suppressed sperm maturation throughout the experiment. Further, qRT-PCR results revealed differential expression pattern of the study genes (er-α, er-β1, er-β2, pgr, vegfaa and p53) in male and female zebrafish. Co-exposure indicated potential inconsistencies in steroidal function in mixtures rather than single exposures. Our findings imply that changes in gonadal histology after NET and EE2 exposure may result from unique regulation of steroid hormone receptors. Additionally, significantly reduced p53 levels (p ≤ 0.05) following co-exposure in both sexes may suggest an elevated risk of neoplastic transformations. Further research with mammalian models will help to explore the mechanisms behind differing effects of alone and co-exposures.