Luteal stage dependence of pituitary response to gonadotrophin-releasing hormone in cyclic dairy ewes subjected to synchronisation of ovulation

Abstract

Possible hormonal aberrations precluding conception or maintenance of pregnancy in dairy ewes subjected to ovulation synchronisation were investigated in this study. The pituitary response to exogenous gonadotrophin-releasing hormone ( GnRH) was tested at different luteal stages in 36 ewes. Oestruses were synchronised by using progestagen-impregnated sponges and the animals were randomly allotted into one of three treatment groups ( A, B and C; n= 12 for each). Treatments commenced on Days 4, 9 and 14 of the new cycle ( oestrus was defined as Day 0). Ewes were given two GnRH injections, 5 days before and 36 h after a prostaglandin F-2 alpha (PGF(2 alpha)) injection, and the animals were inseminated 12-14 h after the second GnRH injection ( modified OVSYNCH). For luteinising hormone ( LH) determination blood samples were withdrawn from six ewes of each group at the time of GnRH administration, and 30, 90, 180, 270 and 360 min later. Progesterone was assayed in samples taken every other day starting from oestrus and for 17 days after the second GnRH injection, and in an additional sample collected on the day of insemination. After the first GnRH injection, the LH concentration was higher in Group C than in Groups B and A (mean +/- s.d.: 64.8 +/- 10.0 ngmL(-1), 41.3 +/- 3.7 ngmL(-1) and 24.6 +/- 9.0 ngmL(-1), respectively; P < 0.05), whereas after the second GnRH injection a uniform LH release was found in all groups. PGF2 alpha caused a significant decrease in progesterone (P-4) concentration in all groups; however, at artificial insemination ewes that conceived had significantly lower P-4 concentration in comparison with those that failed to conceive. As early as Day 5, pregnant animals had higher P-4 concentrations than non- pregnant animals. Overall, 21 animals conceived ( seven, nine and five ewes from Groups A, B and C, respectively). These results indicate that the proposed protocol is equally effective in inducing a preovulatory LH surge at any stage of the luteal phase, and that elevated P-4 concentration along with a delayed P-4 increase should be considered as a causative factor for inability to conceive.