Seasonally anoestrous ewes were injected i.v. with 250, 500 or 1000 ng Gn-RH at 2-h intervals for 8 days (2 sheep/treatment). Each injection of 250 or 500 ng Gn-RH resulted in a transient rise in plasma LH concentrations. Treatment with 1000 ng Gn-RH per injection resulted in a more sustained rise in plasma LH concentrations in 1 of 2 sheep during the early part of the treatment period. A preovulatory-type LH peak occurred 17-48 h after the start of treatment in all ewes, with a second preovulatory-type peak 106-133 h later in those ewes receiving 500 or 1000 ng Gn-RH per injection. Ovulation, with subsequent normal luteal function, occurred in all sheep. However, the rise in plasma progesterone concentrations appeared to be delayed in those ewes treated with 500 or 1000 ng Gn-RH compared to ewes treated with 250 ng Gn-RH. These data suggest that the absence of ovulation during seasonal anoestrus is due to an inadequate pattern of episodic LH secretion.

Two experiments were conducted to explore the effectiveness of synthetic peptide-based vaccines for active and passive autoimmunization of sheep against inhibin. In the first experiment, adult Romney ewes (n = 20) were actively immunized against a synthetically produced peptide that corresponded to the N-terminus of the alpha-subunit of bovine inhibin (bI alpha(1-29)-Tyr30). This peptide was conjugated to tuberculin purified protein derivative (PPD) to increase its antigenic properties. Control groups comprised non-immunized (n = 10) and PPD-immunized (n = 10) ewes. Primary immunization (400 micrograms conjugate/ewe) was followed by two booster immunizations (200 micrograms conjugate/ewe), given 5 and 8 weeks later. Following synchronization of oestrus using progestagen sponges, ovulation rates were assessed by laparoscopy. Weekly blood samples were taken throughout the experiment. All inhibin-immunized ewes produced antibodies which bound 125I-labelled bovine inhibin (Mr 32,000), and ovulation rate in inhibin-immunized ewes (2.15 +/- 0.22; mean +/- S.E.M.) was significantly (P less than 0.01) greater than in both non-immunized (0.90 +/- 0.23) and PPD-immunized (1.20 +/- 0.13) control groups. Immunization against the peptide, but not against PPD alone, resulted in a modest rise in plasma FSH, with mean levels after the second boost being significantly (P less than 0.025) higher (22%) than those before immunization. Moreover, when blood samples were taken (2-h intervals) from randomly selected groups of control (n = 7) and inhibin-immunized (n = 7) ewes for an 84-h period following withdrawal of progestagen sponges, the mean plasma concentration of FSH during the 48 h immediately before the preovulatory LH surge was 37% greater (P less than 0.025) in immunized than in control animals. However, more frequent blood sampling (every 15 min for 12 h) during follicular and mid-luteal phases of the oestrous cycle revealed no significant differences between treatment groups in mean plasma concentrations of FSH. In addition, neither mean concentrations of LH nor the frequency and amplitude of LH episodes differed between immunized and control ewes. However, the mean response of LH to a 2 micrograms bolus of gonadotrophin-releasing hormone, given during the luteal phase, was significantly (P less than 0.05) less in immunized than in control ewes. These findings indicate that active immunization of Romney ewes against a synthetic fragment of inhibin can promote a controlled increase in ovulation rate, but this response cannot be unequivocally related to an increase in plasma levels of FSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Four groups, each of 5 seasonally anoestrous ewes, were treated i.v with small doses (75, 125, 250 or 500 ng) of Gn-RH at 2-h intervals for 48 h. A further 15 ewes received 14 days pretreatment with progesterone and then the 250 ng Gn-RH treatment. Gn-RH injections induced an episodic pattern of LH secretion which differed significantly for the doses of Gn-RH used. A preovulatory LH surge occurred in all but 1 of the ewes during the period of Gn-RH treatment. Ovulation occurred in all 15 ewes pretreated with progesterone and in 19/20 ewes treated with Gn-RH alone. Although normal luteal function occurred in all ewes pretreated with progesterone, it was present in only 5 of the 20 ewes treated with Gn-RH alone. Oestrus, as shown by mating, occurred at a mean time of 34.7 +/- 2.6 h after the start of Gn-RH treatment only in those ewes receiving progesterone pretreatment. These results indicate that progesterone pretreatment has a marked effect on the ability of small doses of Gn-RH to induce ovulation and normal luteal function in seasonally anoestrous ewes.

Chronically ovariectomized prepubertal heifers were used for a comparison of the effects of highly purified bovine inhibin (Mr 32,000) and steroid-free bovine follicular fluid (bFF) on the secretion of FSH and LH. In view of the limited availability of highly purified inhibin, an initial study was undertaken to establish the optimal method for administration of bFF inhibin activity. In comparison with the FSH response to a single large i.v. bolus injection of bFF (50 ml; 3250 mg protein), a far more effective suppression of plasma FSH concentrations was achieved when considerably less bFF (6.3 ml; 410 mg protein) was administered gradually over an extended time-period (2 days) either as a continuous i.v. infusion or as a series of 2-hourly i.v. injections. Following a single i.v. bolus injection of bFF, immunoreactive inhibin was cleared rapidly from the circulation (half-life 51 +/- 8 (S.E.M.) min, n = 5), presumably accounting for its limited ability to suppress FSH secretion when administered in this manner. In a second experiment, treatment of ovariectomized heifers (three per group) with highly purified Mr 32,000 bovine inhibin at a dose rate of 15 micrograms/2 h for 2 days significantly (P less than 0.05) suppressed plasma FSH concentrations, which reached their minimum values (40% suppression) during day 2 of treatment. At a lower dose rate (5 micrograms/2 h), inhibin did not significantly affect plasma FSH levels. Administration of bFF was also associated with a dose-dependent suppression of FSH secretion. For each of three dose rates tested (three heifers per group), plasma FSH concentrations were maximally suppressed during day 2 of treatment (65 mg/2 h, 86% suppression, P less than 0.001; 21.7 mg/2 h, 66% suppression, P less than 0.001; 7.2 mg/2 h, 15% suppression, P greater than 0.05). Neither highly purified inhibin nor bFF significantly affected mean plasma LH concentrations, LH pulse frequency or LH pulse amplitude. Thus we have shown for the first time that highly purified Mr 32,000 bovine inhibin does possess in-vivo biological activity in cattle, promoting a selective suppression of plasma FSH concentrations qualitatively similar to that evoked by steroid-free bFF.(ABSTRACT TRUNCATED AT 250 WORDS)

Oestrus was detected using an oestradiol-treated steer, and ovulation inferred from progesterone profiles. A 'short luteal phase' oestrous cycle preceded the first observed oestrus, and this was followed in all heifers by a normal length luteal phase. However, no increase in mean LH concentrations, basal LH concentrations, LH episode frequency, LH episode amplitude or change in mean FSH concentration could be directly associated with the onset of puberty. It is therefore concluded that the gonadotrophic stimulus for first ovulation must occur abruptly.

Ovulation was induced in seasonally anoestrous ewes by repeated 2-h injections of 250 ng Gn-RH, after 12 days (Group 1, N = 7; Group 2, N = 8), 2 days (Group 3, N = 8) or no (Group 4, N = 7) progesterone pretreatment. A preovulatory LH peak occurred spontaneously at a mean (+/- s.e.m.) time of 43.1 +/- 2.0 h, 38.5 +/- 3.1 h and 26.8 +/- 1.7 h after the start of Gn-RH treatment in Groups 1, 3 and 4 respectively, and was artificially induced in ewes in Group 2, after 24 h of treatment, by a single i.v. injection of 150 micrograms Gn-RH. Normal luteal function occurred in all progesterone-pretreated ewes, but in only 1/7 animals not treated with progesterone. These results demonstrate that, although normal luteal function in progesterone-primed ewes induced to ovulate with repeated injections of low doses of Gn-RH is associated with a delayed preovulatory LH peak, it is not this extended period of follicle development which is responsible for functional competence of the resultant corpus luteum. Since as little as 2 days of exposure to elevated plasma progesterone concentrations is effective, it is suggested that progesterone may act directly on the preovulatory follice.

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 non-pretreated ewes, those recovered from progesterone-pretreated ewes responded to the LH surge by significantly increasing oestradiol secretion (P less than 0.01) and binding of 125I-labelled hCG (P less than 0.05) to granulosa cells. Overall there was also more (P less than 0.05) hCG binding to granulosa and theca cells from progesterone-pretreated animals. Non-ovulatory follicles recovered from progesterone-primed ewes had more (P less than 0.05) binding of 125I-labelled hCG to theca and a higher testosterone concentration in follicular fluid (P less than 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.

Ten ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were given subcutaneous oestradiol-17 beta implants and then ovariectomized. Another 10 ewes of each breed were left intact. On 3 May 1982, all the ewes were housed in an artificial photoperiod of 16L:8D. After 4 weeks, half of the ewes of each breed and physiological state were abruptly exposed to a short-day (8L:16D) photoperiod while the others remained in long days (16L:8D). The time of onset of the breeding season was significantly (P less than 0.05) advanced in ewes switched to short days (12 August +/- 10 days) compared to those maintained in long days (4 September +/- 14 days). Dorset Horn ewes began to cycle (20 July +/- 7 days) significantly (P less than 0.001) earlier than Welsh Mountain ewes (19 September +/- 6 days). Disparities in the time of onset of cyclic activity in ewes of different breeds and daylength groups were echoed in disparities in the time at which plasma LH and FSH concentrations rose in oestrogen-implanted, ovariectomized ewes of the same light treatment group. Prolactin concentrations showed an immediate decrease in ewes switched to short days, but remained elevated in long-day ewes. Since the breeding season started in the presence of high prolactin concentrations in long-day ewes, it seems unlikely that prolactin is an important factor determining the timing of the onset of cyclic activity.