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C. H. G. Irvine and S. L. Alexander

To study the mechanisms responsible for the unusually slow decline of the ovulatory LH surge in mares, secretion patterns of GnRH, LH and FSH were monitored in pituitary venous blood collected every 2 or 5 min for 10.5–18.0 h from five mares on the third (n = 4) or fifth day after ovulation (first sampling period). To determine the effectiveness of progesterone negative feedback, mares were then given a luteolytic dose of a prostaglandin analogue (PGF) and pituitary venous sampling (every 2 or 5 min for 16 h) recommenced 20–22 h later (second sampling period). During the declining arm of the LH surge, large peaks (detected by the Cluster algorithm) of concurrent LH and FSH secretion occurred infrequently, with four peaks being detected in a combined sampling period of 75 h. Outside the peaks, LH or FSH secretion continued (as assessed by a pituitary to jugular–venous concentration ratio ≥ 1.25) during 46% ± 13 or 40% ± 10, respectively, of the sampling period. GnRH immunoactivity was detected during each spontaneous gonadotrophin peak, but at other times was generally at assay sensitivity. After PGF, plasma progesterone fell (ng ml−1, mean ± SEM; first sampling period: 8.6 ± 0.8; second: 2.0 ± 0.3; P = 0.001) and the frequency of LH (P < 0.05) and FSH (P < 0.02) peaks rose, with 28 peaks detected for each hormone in a total of 80 h sampling. Peaks in LH were smaller during the second period, with decreases observed in maximum (P= 0.027) and mean (P= 0.025) secretion rates. Maximum GnRH secretion rate during peaks also declined (P= 0.010); however, the decrement (−30 ± 6%) was less than that in maximum LH secretion rate (−82 ± 5%; P = 0.040), suggesting that other factors contribute to the reduced LH peak amplitude. In summary, gonadotrophin peak frequency during the downswing of the surge in mares is slow, as in the midluteal phase, and the slow rate of decline in peripheral gonadotrophin concentrations is due, at least in part, to continued secretion between pulses. Moreover, progesterone negative feedback is highly effective in early dioestrus, in that lessening it without complete removal markedly accelerates gonadotrophin pulse frequency.

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J. P. Foster, Margaret J. Evans, and C. H. G. Irvine

Summary. Mares at different stages of the oestrous cycle were given a single intravenous injection of 0·5 mg synthetic Gn-RH.

The mean area of the induced LH peak was significantly less at mid-cycle (Day 10–11) than at any other time. The mean height of the LH peak above preinjection concentration was greater at late oestrus and early cycle (Day 5–6) than at mid-cycle and early oestrus. There were no significant differences in mean FSH responses. The LH:FSH ratio for both height and area of induced peaks was significantly less at mid-cycle than at other times of injection. These results suggest that one releasing hormone could cause the release of both FSH and LH in the normal cyclic mare.

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C. H. G. Irvine, S. L. Alexander, and J. E. Turner

Summary. The possibility of seasonal variation in the feedback effect of testosterone or oestradiol was investigated by giving replacement treatment to geldings for 2–3 weeks during breeding and non-breeding seasons. In the non-breeding season, testosterone suppressed LH values (mean ± s.e.m., ng/ml) in all geldings (before treatment, 7·5 ± 2·3; final treatment week, 1·8 ± 0·2; P <0·05), whereas early in the breeding season, testosterone caused a prolonged rise in LH (before, 6·8 ± 2·3; final week, 18·9 ± 6·4; P <0·05). In all testosterone experiments, LH returned to pretreatment levels within 2 weeks after treatment. Oestradiol treatment caused a prolonged increase (P <0·05) in LH concentrations (mean ± s.e.m., ng/ml) in both seasons (breeding: before 5·2 ± 1·1; final week, 16·2 ± 4·8; non-breeding: before, 10·9 ± 1·9; final week, 20·1 ± 5·2). We conclude that in geldings the feedback effect of testosterone varies with season and, further, that testosterone replacement may be able to restore to geldings the stallion's seasonal pattern of LH secretion. The results suggest that, in male horses, testosterone and possibly oestradiol, are important components in the neuroendocrine pathway controlling seasonal breeding and, moreover, are essential for the generation of a positive signal for LH secretion in the breeding season.

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C. H. G. Irvine, J. E. Turner, S. L. Alexander, N. Shand, and S. van Noordt

In mares, dioestrous FSH profiles based on once-a-day sampling are variable; however, the pulsatility of plasma FSH, which has been suggested by limited windows of intensive sampling, may contribute to this variability. Jugular blood from six mares was sampled at 4 h intervals throughout an ovulatory cycle to determine cyclic FSH and LH patterns more accurately and to measure gonadotrophin pulse frequency during dioestrus. Synchronous pulses of FSH and LH occurred regularly in all mares between day 4 and day 12 (ovulation = day 0) with a mean (± sem) frequency of 1.9 ± 0.1 (FSH) or 1.6 ± 0.1 (LH) pulses day−1. LH pulse amplitude declined (P < 0.0001) between day 4 and day 10, but FSH pulse amplitude remained large and stable, dipping slightly but not significantly on day 6. Daily mean FSH concentrations exceeded (P < 0.0001) early oestrous values between day 4 and day 5, and between day 7 and day 10. However, significantly different patterns were obtained when once-a-day sampling was simulated by selecting samples collected at 08:00 h or noon. LH was higher during the periovulatory surge than during dioestrus (P < 0.0001) and profiles were similar whether daily means or selected samples were used. It is concluded that: (1) the marked pulsatility of plasma FSH during dioestrus makes once-a-day sampling misleading for determining FSH profiles; (2) the dioestrous pattern of large, slow FSH pulses was consistent among mares, unlike that of the daily mean FSH profiles; and (3) no discrete FSH 'surges' were observed during dioestrus, although FSH pulse amplitude tended to undergo alternate increases and decreases. A period of higher amplitude FSH pulses preceded ovulation by 10.2 ± 0.7 days, which corrresponds to the approximate time the ovulatory follicle emerges. Therefore, it is possible that the signal for follicular recruitment in mares is intermittent excursions of plasma FSH above a threshold value.