Hormonal therapeutic strategy on the induction of accessory corpora lutea in relation to follicle size and on the increase of progesterone in sheep

Abstract

We determined the effect of GnRH or hCG treatment on day 4 post-time artificial insemination (FTAI) onthe formation of accessory corpora lutea (acc-CL) and on the concentration of serum progesterone (P4)insheep. Multiparous adult Merino ewes (n¼36) were synchronized for estrus using double injection ofPGF2aagonist (125mg Cloprostenol) with an interval of 14 days. At 53e56 h after the second PGapplication, FTAI was performed. On day 4 post FTAI, ewes were either treated with analogue of GnRH(4mg buserelin; n¼12) or hCG (300 IU, hCG; n¼12) or saline solution (1 ml; Control; n¼12). Twolaparoscopic ovarian examinations were performed on days 4 and 10 post FTAI. In thefirst observation,we determined the number of post ovulation corpora lutea (po-CL) and the site, number and diameter offollicles present in both ovaries. In the second laparoscopy, we observed the number of po-CL and acc-CL.The sizes of the follicles that generated the acc-CL were determined according to the position of thefollicles observed in thefirst laparoscopy. Serum P4concentration was determined on days 4, 7, 10, 13, 17and 21 post FTAI by chemiluminescence. A similar follicular population in size and number was observedin the three experimental groups prior to the beginning of treatments (Follicles 2 mm: 6.4±3.7, 3 mm:3.0±2.3, 4 mm: 1.1±0.5, 5 mm: 1.4±0.8; P˃0.05). The formation of 1.0±0.4 and 1.1±0.3 acc-CL wasobserved in the GnRH and hCG groups, respectively (P˃0.05), but was not observed in the Control group(P<0.05). Follicle sizes from which acc-CL generated were 3, 4 and 5 mm and did not differ betweenhormonal treatments (P˃0.05). The hCG group had higher mean concentrations of P4on days 7, 10, 13and 17 post FTAI compared with the GnRH group and the Control group (P<0.05), while no differenceswere observed between these two latter groups (P>0.05). Mean P4concentrations in ewes treated withhCG showed no differences according to the size of the follicle from which acc-CL were generated(P˃0.05). In conclusion, administration of hCG or GnRH on day 4 post FTAI induced the formation of oneacc-CL from follicles of 3, 4 or 5 mm, indistinctly. However, serum P4concentration increased signifi-cantly only in the hCG group. The serum P4concentrations of acc-CL that originated from different folliclesizes did not differ.