EFFECTS OF GnRH ON LUTEINIZING HORMONE RELEASE AND ONSET OF CYCLIC OVARIAN ACTIVITY POSTPARTUM IN PELIBUEY EWES

Abstract

Synthetic gonadotropin releasing hormone (GnRH) was administered to Pelibuey ewes to determine its effect on luteinizing hormone (LH) release and on cyclic ovarian activity during the first 3 – 4 wk postpartum. Two GnRH doses were given (10 and 100 μg) on days 4 and 5, 7 and 8, 14 and 15, and 21 and 22, after parturition, respectively. In three trials, 11, 8 and 7 ewes were used. For each trial 5, 3 and 2 ewes were used as controls, respectively; in trial 3, only the 100-μg dose was given once. Results from the three trials indicated that LH release always occurred after GnRH treatment, relative to the saline-treated controls. In trials 1 and 2, the 10-μg dose induced a higher (P < 0.001) LH release than the 100-μg dose; the LH release in response to the 100-μg dose in the three trials was of similar magnitude. Release of LH in response to both doses was lower (P < 0.05) during days 4 – 5 postpartum, than during the following 3 wk. Two types of luteal activity or steroidogenesis were observed in the GnRH-treated ewes, relative to the saline-treated ewes; (1) within 2 – 4 d after each GnRH injection small peaks of progesterone appeared, and (2) within 5 – 15 d after the last GnRH injection there were elevations of progesterone in peripheral plasma to concentrations and of durations characteristic of the luteal phase of the ovine estrous cycle. These results indicate that the pituitary of postpartum Pelibuey ewes regains the ability for maximal release of LH in response to GnRH challenge within 7 d postpartum. Further, treatment with GnRH induces earlier return to cyclic ovarian activity. Key words: Hair sheep, postpartum, GnRH, luteal activity