Inhibition of 5α-reductase activity in prepubertal female rats: effect on the timing of first ovulation

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Abstract
The activity of 5α-reductase in the ovary of prepubertal rats decreases prior to first ovulation. To determine whether this decrease causes the onset of first ovulation, prepubertal female were treated with a specific inhibitor of 5α-reductase activity and the effect on the time of first ovulation was studied. The steroid (6-methylene-progesterone, MEP) gives irreversible inhibition of ovarian 5α-reductase activity in vitro with testosterone as well as progesterone as substrate. A MEP-containing silicone tube, releasing 245 nmol per 24 h in vitro, was implanted sc in the back of 25-day-old female rats. This resulted a day 30 in a significantly lower 5α-reductase activity in the ovary than in control rats (42.0 ± 3.8 pmol/min per mg ovary in MEP-treated rats vs 98.8 ± 2.4 pmol/min per mg ovary in control rats). Nearly the same extent of inhibiton was found at day 35 27.4 ± 2.1 in MEP-treated rats vs 56.2 ± 5.9 in control rats). However, implantation of a MEP-containing silicone tube under the skin or inserting a pellet containing MEP under the ovarian capsule did not result in advancement of the first ovulation. These data indicate that in the female rat a decrease of 5α-reductase activity alone is not sufficient to induce the first ovulation.

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