The male-induced pregnancy block (the Bruce effect) in mice: re-evaluation of the ability of exogenous progesterone in preventing implantation failure.

The ability of exogenous progesterone administration to prevent implantation failure in newly inseminated females exposed to alien males for 3 days (from 10.00 hr on day 1 to 10.00 hr on day 4 post coitum) was evaluated. Administration of progesterone, 1 mg/female/day, for 5 days beginning at 10.00 hr on day 1 or at 10.00 hr on day 2 prevented the return of oestrus and induced implantation of blastocysts in alien male-exposed females. Administration of progesterone beginning at 22.00 hr on day 2 was only partially effective in preventing return of oestrus in the male-exposed females; however, the majority of the females which exhibited vaginal oestrus showed implanted embryos. Progesterone injection beginning at 10.00 hr on day 3 or later was totally ineffective in preventing return of oestrus in the females. Administration of progesterone beginning at 10.00 hr or at 22.00 hr on day 3 failed to induce implantation in the male-exposed females which returned to oestrus on day 3; however, the treatment induced implantation in the majority of the females which returned to oestrus on day 4. Progesterone administration beginning at 10.00 hr on day 4 was totally ineffective in inducing implantation in the male-exposed females exhibiting the Bruce effect. The results indicate that administration of progesterone beginning at any time prior to the appearance of oestrus facilitates implantation of blastocysts in male-exposed females; however, progesterone administration beginning after the onset of oestrus is totally ineffective in inducing implantation in females. The results also strongly suggest that in females exhibiting the Bruce effect there is a total loss of the blastocysts.