Inhibition of the acrosome reaction (AR) and fertilization capacity of mouse spermatozoa by norethisterone A-ring reduced metabolite (5α-NET)

The contra gestational effects of norethisterone and its main metabolites, 5 alpha-NET and 3 beta 5 alpha-NET, has been demonstrated in several species. However, the focus has been mainly on their effects in the uterus. We previously reported that 5 alpha-NET inhibits the progesterone-induced AR in pig spermatozoa and induces severe morphological damage to fertilized mouse oocytes. In the present study, we analysed the effects of these compounds on the fertilization process in vitro. Oocytes and spermatozoa were obtained from Balb/c female and C57BL/6J male mice, respectively. Both, the AR assays and the fertilization experiments were performed under different steroid treatment schemes using progesterone as a control. Results showed that norethisterone induced the AR, while 5 alpha-NET reduced the percentage of spermatozoa that had undergone progesterone-induced AR. Both 17 beta-estradiol and 3 beta 5 alpha-NET induced the AR in a considerably lower percentage of spermatozoa than progesterone. In addition, when 5 alpha-NET was added to the medium simultaneously with progesterone at the moment of fertilization, the percentage of fertilized oocytes (two-cell stage) decreased by as much as 77% as compared with the control progesterone-treated group. All results suggest that these compounds can have important effects on the fertilization process.