Effects of medroxyprogesterone acetate (MAP) on ovarian antral follicle development, gonadotrophin secretion and response to ovulation induction with gonadotrophin-releasing hormone (GnRH) in seasonally anoestrous ewes.

When ovulation is induced with gonadotrophin-releasing hormone (GnRH) in anoestrous ewes, a proportion of animals fail to form normal (full-lifespan) corpora lutea (CL). Progesterone treatment before GnRH prevents luteal inadequacy. It remains uncertain whether a similar effect, achieved with medroxyprogesterone acetate (MAP) from intravaginal sponges, is mediated by influences on growing ovarian follicles and/or secretion of gonadotrophic hormones, before and after GnRH treatment. Two experiments were performed, on 13 and 11 anoestrous Western white-faced ewes, respectively. Seven and six ewes, respectively, received MAP-containing sponges (60 mg) for 14 days; the remaining ewes served as untreated controls. To test the effect of timing of GnRH administration after pre-treatment with MAP-releasing sponges, GnRH injections (250 ng every 2h for 24h followed by a bolus injection of 125 microg of GnRH i.v.) were given either immediately (Experiment 1) or 24h after sponge removal in the treated ewes (Experiment 2). Ovarian follicular dynamics (follicles reaching >or=5mm in size) and development of luteal structures were monitored using transrectal ultrasonography. In Experiment 1, the mean ovulation rate (0.7+/-0.3 and 1.0+/-0.4) and proportion of ovulating ewes (57 and 67%, respectively) did not vary (P>0.05) between MAP-treated and control ewes. Normal (full-lifespan) CL were detected in 29% of treated and 67% of control ewes (P>0.05). In Experiment 2, the mean ovulation rate (2.3+/-0.2 and 1.2+/-0.6; P<0.05) and percentage of ewes with normal (full-lifespan) CL (100 and 40%, respectively; P<0.10) were greater in the treated compared to control ewes. In Experiment 1, the mean peak concentration of the GnRH-induced LH surge was lower (P<0.05) in MAP-treated than in control ewes. There were no significant differences between MAP-treated and control ewes in the characteristics of follicular waves, mean daily serum FSH concentrations, and secretory parameters of LH/FSH, based on intensive blood sampling conducted 1 day before sponging and 1 day before sponge removal. It is concluded that treatment with MAP has no effect on the tonic secretion of LH/FSH or follicular wave development in anoestrous ewes. However, the GnRH-stimulated LH discharge was attenuated in the ewes that received MAP-impregnated sponges for 14 days and were treated with GnRH immediately after sponge withdrawal. Ovulatory response and CL formation were increased when GnRH was administered 24 h after sponge removal.