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Summary. The pattern of change in plasma progesterone and LH concentrations was monitored in Clun Forest ewes at a natural oestrus and compared to that observed after removal of progesterone implants. The rate of decline in plasma progesterone concentrations after implant withdrawal (1·8 ± 0·2 ng/ml h⁻¹) was significantly greater (P < 0·001) than that observed at natural luteolysis (0·2 ± 0·1 ng/ml h⁻¹), and this resulted in an abormal pattern of change in tonic LH secretion up to the time of the preovulatory LH surge. This more rapid rate of progesterone removal was also associated with a shortening of the intervals from the time that progesterone concentrations attained basal values to the onset of oestrus (P < 0·05) and the onset of the preovulatory LH surge (P < 0·01). However, there were no significant differences in the duration of the LH peak, preovulatory peak LH concentration, ovulation rate or the pattern of progesterone concentrations in the subsequent cycle. It is suggested that the abnormal patterns of change in progesterone and tonic LH concentrations may be one factor involved in the impairment of sperm transport and abnormal patterns of oestradiol secretion known to occur at a synchronized oestrus.

Department of Physiology and Environmental Studies, University of Nottingham School of Agriculture, Sutton Bonington, Loughborough, Leicestershire LE12 5RD

(Received 2nd December 1974)

Synthetic LH-releasing hormone (LH-RH) has been shown to induce release of both LH and FSH from the ovine pituitary in vivo (Jonas & co-authors, 1973; Symons, Cunningham & Saba, 1974). When administered to seasonally anoestrous Clun Forest ewes as a single intravenous injection of either 150 μg or 300 μg, the induced gonadotrophin release was sufficient to cause ovulation in 23/27 ewes (Haresign, Foster, Haynes, Crighton & Lamming, 1975). Ovulation occurred within 48 hr of injection (W. Haresign, unpublished data). However, peripheral plasma progesterone levels did not rise above basal pre-injection values in most animals after the induction of ovulation. In view of this finding the present experiment was designed to investigate whether the structures visible on the surface of the ovary after injection of LH-RH were representative

Summary. Nine ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were ovariectomized after insertion of subcutaneous implants containing oestradiol-17β. A further 9 ewes of each breed were left entire. All of the ewes were placed in an artificial photoperiod of 8L: 16D on 12 December 1980. After 5 weeks half of the ewes of each breed and physiological state were abruptly changed into a long-day photoperiod (16L:8D) while the rest remained in short days. The time of onset of seasonal anoestrus in entire animals was significantly advanced in ewes changed to long days, with the end of the breeding season coming at the normal time of year in ewes maintained in short days. These differences in oestrous cycle activity were reflected by differences in the time at which LH concentrations in ovariectomized, oestrogen-treated ewes on the two light treatments fell to basal values. Prolactin concentrations showed an immediate and sustained rise in ewes changed to long days, but remained low in ewes maintained in short days. Since the onset of seasonal anoestrus occurred in the absence of high levels of prolactin (in short-day ewes), it is concluded that prolactin is not the major vehicle by which seasonal changes in hypothalamic responsiveness to the negative feedback effects of oestradiol are produced. The results suggest that anoestrus may be due to photorefractoriness.

Summary. Six groups of 4 ewes were given a single i.m. injection of 100 μg ICI 80,996 on Days 3, 5, 8, 11, 14 or 16 of the oestrous cycle. On Day 3 treatment was only partly effective in causing luteal regression, with only 2 of the 4 treated ewes returning to oestrus. Injection had no apparent effect when given on Day 16, at a time when luteal regression had already started naturally. However, treatment between Days 5 and 14 inclusive of the oestrous cycle evoked a rapid fall in progesterone concentrations to basal values of <0·5 ng/ml within 24 h with a high degree of synchrony in the return of oestrus (37·7 ± 1·6 h) and the timing of the LH peak (45·3 ± 3·2 h) after injection.

We conclude that the corpus luteum of the ewe is responsive to ICI 80,996 only between Davs 5 and 14 of the oestrous cycle.

Summary. 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 μg 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.

Summary. Long-term ovariectomized Clun Forest ewes were challenged with a range of doses (12·5–50·0 μg/injection) of oestradiol benzoate every 2 months from March to November. All treatments induced a biphasic pattern of change in LH concentrations, consisting of an initial depression in concentrations followed by an LH peak, similar to a preovulatory LH surge. The positive feedback response to 12·5 μg oestradiol was significantly lower than that after the two higher dose levels, but the magnitude of the response showed no significant seasonal variation. It is concluded that a seasonal change in responsiveness to positive feedback is unlikely to contribute to the absence of ovulation during seasonal anoestrus.

Summary. The current experiment was designed to investigate whether changes in prolactin concentrations might be involved in the seasonal change in responsiveness of the hypothalamic–pituitary axis to the negative feedback effects of oestradiol in the ewe. Twelve Dorset Horn ewes (long breeding season) and 12 Welsh Mountain ewes (short breeding season) were ovariectomized after insertion of empty subcutaneous implants (N = 2 per breed), or ones containing oestradiol-17β (N = 10 per breed). Intact ewes, 10 of each breed, were used to monitor breeding activity. Mean weekly plasma LH concentrations in oestrogen-treated ovariectomized ewes of both breeds were elevated over the period corresponding to their respective breeding seasons, but were basal during anoestrus. The changes in prolactin concentrations, however, followed the same temporal pattern in both breeds and were approximately parallel to changes in daylength. These data support the suggestion of a seasonal change in negative feedback responses to oestradiol which corresponds to changes in breeding activity, but any involvement of prolactin in this response remains questionable.

Summary. Pretreatment of seasonally anoestrous Clun Forest ewes with 750 i.u. PMSG or 50 μg oestradiol benzoate 24 or 7 h respectively before a single injection of 150 μg synthetic LH-RH significantly increased the release of LH compared to that after injection of 150 μg LH-RH alone. Total LH release in the two 'combined' treatments was approximately 70% of that found at a natural oestrus, compared to 25% for LH-RH alone. All but one of the treated ewes ovulated, but only those pretreated with PMSG consistently produced corpora lutea capable of elevating peripheral plasma progesterone concentrations although these were lower than those at natural mid-cycle. These progesterone concentrations were, however, comparable to those during the natural cycle when corrected for the higher metabolic clearance rate found during anoestrus.

Summary. Twenty seasonally anoestrous ewes were pretreated with progesterone for 4 days and divided into four equal groups. Ewes in Group 1 received no GnRH treatment and were slaughtered immediately after progesterone removal. Ewes in Groups 2, 3 and 4 received i.v. injections of 250 ng GnRH every 2 h for 36 h starting at the time of progesterone removal. Ewes in Group 2 were slaughtered immediately after the 36 h GnRH pulsing, while ewes in Groups 3 and 4 were given a bolus injection of 125 μg GnRH at this time and were slaughtered 2 and 10 h after the bolus injection, respectively. Blood samples were collected every 30 min from ewes in Group 4 only, from 4 h before the start of GnRH treatment until 10 h after the bolus injection. Pulsing with GnRH resulted in episodic release of LH, and the bolus injection of GnRH was immediately followed by a preovulatory type LH surge in those ewes in which an endogenous surge had not already begun. The pituitary GnRH receptor numbers were significantly higher for the ewes in Group 2 than for any of the other treatment groups, while there was no significant difference in the receptor numbers between Groups 1, 3 and 4. The results suggest an up-regulation of GnRH receptors resulting from pulsatile GnRH therapy.

Summary. Treatment of 18 cyclic Clun Forest ewes with two i.m. injections of ICI 80,996, given 9 days apart and without reference to stage of the oestrous cycle, synchronized ovulation in all ewes at a mean time interval of 73·1 ±1·6 (s.e.m.) h from the second injection. The interval from the LH peak to ovulation was 22·6 ± 0·7 h and this is comparable to previously reported figures for a natural oestrus.