In vivo imaging in the rabbit as a model for the study of ovulation-inducing factors

The study of factors responsible for eliciting ovulation in rabbits has been hampered by the lack of a suitable method of monitoring the ovaries in vivo. Ovarian imaging by ultrasound biomicroscopy was used in two experiments designed to determine the effects of seminal plasma on the ovulatory response in rabbits. In Experiment 1, female rabbits were group-housed and treated intramuscularly with saline, gonadotropin releasing hormone (GnRH), or seminal plasma of llamas or rabbits (n = 4 to 6 per group). Rabbits were euthanized eight days later to evaluate the ovarian response by ultrasound biomicroscopy ex situ. No differences among groups were detected in the proportion of rabbits that ovulated or in the number and size of corpora lutea. The high incidence of ovulation in the negative control group was unexpected, and confounded determination of an ovulation-inducing effect of seminal plasma. In Experiment 2, female rabbits were caged individually, and treated as in Experiment 1 (n = 5 to 7 per group). The ovarian response was evaluated in vivo by transcutaneous ultrasound biomicroscopy. Ovulation and formation of corpora lutea were detected only in rabbits given GnRH. A preovulatory surge in plasma luteinizing hormone concentration and a post-ovulatory rise in plasma progesterone concentration were detected only in rabbits treated with GnRH. Surgical translocation of the ovaries to a subcutaneous position enabled longitudinal assessment of the ovulatory response by ultrasound biomicroscopy. Results clearly documented the effect of physical/social interaction on ovulation in rabbits, and did not support the hypothesis that seminal plasma elicits ovulation in rabbits.