INDUCTION OF OVULATION IN THE ACYCLIC POSTPARTUM EWE FOLLOWING CONTINUOUS, LOW-DOSE SUBCUTANEOUS INFUSION OF GNRH

Pituitary and ovarian responses to subcutaneous infusion of GnRH were investigated in acyclic, lactating Mule ewes during the breeding seaso n. Thirty postpartum ewes were split into 3 equal groups; Group G rece ived GnRH (250 ng/h) for 96 h; Group P+G was primed with progestagen f or 10 d then received GnRH (250 ng/h) for 96 h; and Group P received p rogestagen priming and saline vehicle only. The infusions were deliver ed via osmotic minipumps inserted 26.6+/-0.45 d post partum (Day 0 of the study). Blood samples were collected for LH analysis every 15 min from 12 h before until 8 h after minipump insertion, then every 2 h fo r a further 112 h. Daily blood samples were collected for progesterone analysis on Days 1 to 10 following minipump insertion, then every thi rd day for a further 25 d. Tn addition, the reproductive tract was exa mined by laparoscopy on Day -5 and Day +7, and estrous behavior was mo nitored between Day -4 and Day +7. Progestagen priming suppressed (P<0 .05) plasma LH levels (0.27+/-0.03 vs 0.46+/-0.06 ng/ml) during the pr einfusion period, but the GnRH-induced LH release was similar for Grou p G and Group P+G. The LH surge began significantly (P<0.05) earlier ( 32.0+/-3.0 vs 56.3+/-4.1 h) and was of greater magnitude (32.15+/-3.56 vs 18.84+/-4.13 ng/ml) in the unprimed than the primed ewes. None of the ewes infused with saline produced a preovulatory LH surge. The GnR H infusion induced ovulation in 10/10 unprimed and 7/9 progestagen-pri med ewes, with no significant difference in ovulation rate (1.78+/-0.1 5 and 1.33+/-0.21,respectively). Ovulation was followed by normal lute al function in 4/10 Group-G ewes, while the remaining 6 ewes had short luteal phases. In contrast, each of the 7 Group-P+G ewes that ovulate d secreted progesterone for at least 10 d, although elevated plasma pr ogesterone levels were maintained in 3/7 unmated ewes for > 35 d. Thro ughout the study only 2 ewes (both from Group P+G) displayed estrus. T hese data demonstrate that although a low dose, continuous infusion of GnRH can increase tonic LH concentrations sufficient to promote a pre ovulatory LH surge and induce ovulation, behavioral estrus and normal luteal function do not consistently follow ovulation in the progestage n-primed, postpartum ewe.