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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, 5α‐NET and 3β5α‐NET, has been demonstrated in several species. However, the focus has been mainly on their effects in the uterus. We previously reported that 5α‐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 5α‐NET reduced the percentage of spermatozoa that had undergone progesterone‐induced AR. Both 17β‐estradiol and 3β5α‐NET induced the AR in a considerably lower percentage of spermatozoa than progesterone. In addition, when 5α‐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.