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Progesterone pretreatment has a direct effect on GnRHinduced preovulatory follicles to determine their ability to develop into normal corpora lutea in anoestrous ewes

Summary. In two experiments carried out during seasonal anoestrus, Romney Marsh ewes were treated with small-dose (250 ng) multiple injections of GnRH at 2-h intervals with and without progesterone pretreatment. In Exp. 1, 8/8 progesterone-primed ewes ovulated and produced functionally normal corpora lutea compared with 2/9 non-primed ewes.

Follicles were recovered from similarly treated animals 18 or 28 h after the start of GnRH treatment (at least 14 h before the estimated time of the LH peak) and assessed in terms of diameter, granulosa cell number, oestradiol, testosterone and progesterone concentrations in the follicular fluid, oestradiol production in vitro and binding of ¹²⁵I-labelled hCG to granulosa and theca. There were no significant differences in any of these measures in 'ovulatory' follicles recovered from the progesterone-pretreated compared to non-pretreated animals.

In Exp. 2, follicles were removed from similar treatment groups just before and 2 h after the start of the LH surge. Unlike 'ovulatory' follicles recovered from the nonpretreated ewes, those recovered from progesterone-pretreated ewes responded to the LH surge by significantly increasing oestradiol secretion (P < 0·01) and binding of ¹²⁵I-labelled hCG (P < 0·05) to granulosa cells. Overall there was also more (P < 0·05) hCG binding to granulosa and theca cells from progesterone-pretreated animals. Non-ovulatory follicles recovered from progesterone-primed ewes had more (P < 0·05) binding of ¹²⁵I-labelled hCG to theca and a higher testosterone concentration in follicular fluid (P < 0·05) than did those from non-primed ewes.

These results suggest that inadequate luteal function after repeated injections of GnRH may be due to a poor response to the LH surge indicative of a deficiency in the final maturational stages of the follicle.

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