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Summary. To characterize the changes in LH pulse frequency during the transition to breeding season, LH pulse patterns and serum progesterone profiles were determined in 8 intact ewes from mid-anoestrus to the early breeding season. Overall, 8 increases in LH pulse frequency were observed and these were restricted to 5 ewes. Of the 8 increases, 7 occurred during the 4 weeks before the first cycle, 5 of them within 1 week after a pulse frequency typical of anoestrus (0–2 per 8 h). Six of them occurred less than 1 week before either a full-length luteal phase (n = 2) or a 1–3-day increment in progesterone (n = 4). Seven of these brief progesterone increases were observed in 6 ewes, 5 of them immediately preceding the first full-length luteal phase. These results are consistent with the hypothesis that the seasonal decrease in response to oestradiol negative feedback at the beginning of the breeding season causes an increase in GnRH, and thereby LH pulse frequency. In addition, they demonstrate that the first increase in tonic LH secretion occurs in < 1 week and, in most ewes, initiates either the first fulllength cycle or a transient increase in progesterone, the latter occurring more often.

Keywords: LH; pulse frequency; progesterone; seasonal transition; short luteal phase; ewes

Summary. To determine whether the first LH surge of the breeding season initiates a transient rise in progesterone in most ewes, serum progesterone (daily) and LH (every 4 h) concentrations were measured in samples collected from 7 ewes between 19 July and first oestrus or 8 September, whichever came first. In 6 of the 7 ewes, the first LH surge of the breeding season was followed within 5 days by a transient, 2-day rise in progesterone. Within less than 5 (N = 4), or 9 (N = 1) or 10 (N = 1) days later, a second LH surge occurred, which was similar in maximum amplitude and duration to the first surge, and which initiated the first full-length luteal phase of the breeding season. In the remaining ewe, the first LH surge of the breeding season induced an abbreviated (9 days) and insufficient (maximum progesterone, 0·94 ng/ml) luteal phase. These results demonstrate that most ewes have more than one LH surge before the first full-length luteal phase, the first surge inducing a transient rise in progesterone. Therefore, although the seasonal decrease in response to oestradiol negative feedback is sufficient for initiation of the first LH surge of the breeding season, additional endocrine mechanisms may be necessary to induce the first full-length luteal phase.

Keywords: seasonal transition; LH surge; serum LH; short luteal phases; progesterone

Summary. Administration of a GnRH agonist (5 μg) every 12 h to long-term ovariectomized ewes for 5 or 10 days during the breeding season suppressed mean LH levels from around 6 to 1 ng/ml on Days 1 and 4 after treatment; on Day 1 after treatment LH pulse frequency and amplitude were lower than pretreatment values. On Day 4 after treatment LH pulse frequency was restored to pretreatment levels (1 per h) whereas LH pulse amplitude had only slightly increased from 0·5 to 1 ng/ml, a value 25% of that before treatment. This increase in amplitude was greater the shorter the duration of treatment. Ovariectomized ewes treated with the agonist for 5 days exhibited both negative and positive feedback actions after implantation of a capsule containing oestradiol; however, compared to control ewes treated with oestradiol only, the positive and negative feedback actions of oestradiol were blunted.

These results suggest that the recovery of tonic LH concentrations after GnRH agonist-induced suppression is limited primarily by changes in LH pulse amplitude. The results also demonstrate that the feedback actions of oestradiol are attenuated, but not blocked, by GnRH agonist treatment.

Summary. Intact female lambs were chronically treated with low levels of oestradiol by Silastic implant from 20 weeks of age. Reproductive cycles were initiated in only 33% of these lambs (3 of 9) compared to 80% of untreated females (11 of 14) by 45 weeks when the study was terminated. Moreover, in the 3 oestradiol-treated lambs which began cycles, the age at first oestrus was delayed 3 weeks (37 ± 1 weeks of age vs 34 ± 1 weeks of age for untreated controls). Retardation of the pubertal process was not due to absence of the pubertal rise in circulating LH. At about 32 weeks of age, chronic oestradiol treatment was no longer able to suppress tonic LH secretion and serum LH increased in intact, oestradiol-treated lambs. These results indicate that a maturational decrease in responsiveness to oestradiol inhibition of tonic LH secretion can be demonstrated in the intact female, as in the ovariectomized female. However, chronic oestradiol suppression of prepubertal LH secretion also delays onset of reproductive cycles. This finding raises the possibility that low tonic LH secretion, presumably in the form of slow pulses, is necessary for development or maintenance of ovarian function before puberty. In the absence of LH during the last part of sexual maturation, the ability of the ovary to respond to the high frequency LH pulses during the pubertal gonadotrophin rise may be delayed.