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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.

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I K M Liu, JW Turner Jr, E M G Van Leeuwen, D R Flanagan, J L Hedrick, K Murata, V M Lane, and M P Morales-Levy

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I K M Liu, J W Turner Jr, E M G Van Leeuwen, D R Flanagan, J L Hedrick, K Murata, V M Lane, and M P Morales-Levy

In this study of equids, we investigated the antibody response and the effect on the estrous cycle following a single inoculation of porcine zonae pellucidae (pZP) employing controlled-release methodology. We also investigated the use of two different water-soluble adjuvants as an alternative to oil-based adjuvants. Twenty-seven domestic mares were inoculated with various formulations of pZP and adjuvant. We showed that the anti-pZP antibodies generated as a result of the inoculations persisted for at least 43 weeks (length of the study). Of the various formulations used in the study, pZP and QS-21 water-soluble adjuvant, administered in combination with an emulsified preparation of pZP and Freund’s Complete Adjuvant generated a significantly (P < 0.05) higher titer of anti-pZP antibodies when compared with other formulations employing the water-soluble adjuvant, Carbopol. Hormone analyses for cyclicity indicated a high incidence and extended duration of persistent corpora lutea among the treated mares. The positive control group of mares receiving two standard inoculations of pZP and Freund’s Complete and Incomplete Adjuvants, as well as the placebo group of mares injected with QS-21 only, also exhibited high incidences of persistent corpora lutea. However, all mares eventually returned to normal cyclicity. The basis for the high incidence and extended duration of persistent corpora lutea was unexplained. The results demonstrate for the first time the persistent generation of anti-pZP antibodies following a single inoculation of pZP incorporated into a controlled-released preparation in the horse. This study further suggests that a single inoculation of pZP sequestered in a controlled-release lactide-glycolide polymer may serve as an alternative to traditional two-inoculation protocols for contraception investigations in the equine.