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J. R. McNeilly, M. Fordyce, R. B. Land, G. B. Martin, A. J. Springbett and R. Webb

Summary. The dynamics of FSH and LH secretion were studied in sheep genetically selected for High (H) and Low (L) rates of testis growth. Gonadotrophin secretion had previously been shown to be affected in the ram lamb with H-line lambs more sensitive to steroid feedback than L. While there were significant differences in mean LH concentrations during the luteal and follicular phases of the oestrous cycle, mean LH values were essentially similar in the two lines in response to ovariectomy, the effect of oestradiol implants on the response to ovariectomy and the response to LHRH. However, the frequency of LH pulses in the H line was similar during both phases of the oestrous cycle, showing a surprising insensitivity to steroid feedback. By contrast, LH pulse frequency was markedly lower in the L-line ewes in the luteal than the follicular phase (0·6 vs 1·1 pulses/h) as expected from the literature. Mean FSH concentrations were significantly higher in the L-line ewes during the follicular phase of the oestrous cycle and after ovariectomy but no significant differences were detected at the other sampling periods. There were no differences in ovulation rate between the lines. It was concluded that selection for testis size had affected the feedback control of gonadotrophin release in the ewe, as in the ram, and hence the expression of the genes controlling this is not sex limited.

Keywords: ewes; gonadotrophins; selection; testis diameter

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J. R. McNeilly, M. Fordyce, R. B. Land, G. J. Lee and R. Webb

Summary. Testis diameter and body weight were recorded from 6 to 76 weeks of age in ram lambs from two established lines selected for high (H) and low (L) testis size. While testis growth was greater in the H line up to 14 weeks of age (P <0·001), body weight was significantly lower, with the L line rams being 10 kg heavier by 76 weeks. There were no differences in plasma LH up to 20 weeks of age, but FSH concentrations were significantly lower at 14 and 20 weeks in the H line. Testosterone concentrations were not significantly higher in the H line from 6 to 20 weeks. In lambs castrated at birth, significantly higher FSH values were recorded from 6 to 20 weeks of age in the H line (P <0·001) whereas there was no difference in LH concentration at 6 and 10 weeks of age between the lines. At 14 and 20 weeks, however, the concentrations of LH were greater in the H than L line lambs (P <0·05). After hemicastration at 6 weeks of age, the rate of growth of the remaining testis in the L line lambs was significantly faster than in entire lambs of that line from 10 to 20 weeks (P <0·05 at 10 weeks to P <0·001 at 20 weeks). There was no difference in the rate of testis growth between the entire and hemicastrated lambs from the H line from 6 to 12 weeks of age.

It can be concluded that there is an underlying genetic difference in pituitary gland and/or hypothalamic activity in ram lambs from the two selected lines.

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J. R. McNeilly, N. P. Evans, T. A. Bramley, P. Brown, A. J. Clark and R. Webb

Selection of the luteinizing hormone (LH) response to exogenous gonadotrophin-releasing hormone (GnRH) in sheep has resulted in the establishment of two lines (High and Low) with a fivefold difference in pituitary sensitivity to GnRH. The effect of selection on gonadotrophin gene expression in the presence or absence of an exogenous gonadotrophin-releasing hormone (GnRH) challenge in twenty-week-old ram lambs from both lines was examined. Before treatment with either GnRH or saline, LH and follicle-stimulating hormone (FSH) concentrations were significantly higher in the High line than in the Low line animals (LH and FSH: P < 0.01). One hour after either GnRH or saline, all animals were slaughtered. In the absence of a GnRH challenge, there were significantly higher concentrations of all three gonadotrophin subunit mRNAs in the High line compared with the Low line, corresponding to the higher basal concentrations of LH and FSH. When comparing treatments between the lines, following a GnRH challenge, LHβ subunit mRNA was significantly (P < 0.001) higher in both lines than before the GnRH, whereas there was no significant change in either α or FSHβ subunit mRNA. These results indicate that the differences in basal gonadotrophin secretion are related to differences in gonadotrophin subunit mRNAs with the High line animals having an inherently greater amount of all three gonadotrophin subunit mRNAs. Selection has not altered the differential amounts of gonadotrophin subunit mRNAs, since there is an overall increase in all three gonadotrophin subunits. GnRH appears to preferentially control LHβ mRNA in both High and Low line animals.

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N. P. Evans, J. R. McNeilly, A. J. Springbett and R. Webb

Summary. Divergent selection in 10-week-old Finn-Dorset ram lambs was based on the luteinizing hormone (LH) response to a pharmacological dose of GnRH (5μg). After eight generations of selection, the LH responses of the two lines (low and high) to GnRH differed by a factor of five. This study investigates the pituitary sensitivity of the two lines to exogenous GnRH. Initially, two pilot studies were performed: one to determine the range of doses of GnRH which would stimulate LH pulses of similar amplitude to those seen endogenously, and the other to confirm that sodium pentobarbitone prevents pulsatile LH secretion in prepubertal ram lambs. The results indicated that barbiturate anaesthesia suppressed pulsatile LH secretion in castrated and intact ram lambs. A model system was therefore constructed in 18 10-week-old intact ram lambs (high n = 7, low n = 11), whereby endogenous pulsatile LH secretion was prevented by sodium pentobarbitone anaesthesia and the amplitudes of LH pulses produced in response to different doses of exogenous GnRH could be measured. The GnRH dose–response curves demonstrated that there was a five-fold difference in the sensitivity of the pituitary glands of the two lines to stimulation with GnRH. The projected minimum concentration of GnRH required to produce a measurable pulse of LH was 4·75 ng for the high-line animals and 26·6 ng for the low-line animals. The results indicated that the low-line animals required five times more GnRH than the high-line lambs to stimulate LH pulses of similar amplitude (high line 43·67 ng; low line 206·55 ng).

These results demonstrate that selection has produced two lines of sheep which differ in the control of LH secretion at the level of the hypothalamus–pituitary gland.

Keywords: pituitary sensitivity; GnRH; sodium pentobarbitone; ram lambs

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N. P. Evans, R. B. Land, J. R. McNeilly and R. Webb

Summary. Divergent selection has resulted in two lines of lambs (high and low) that have a 5-fold difference in their ability to release luteinizing hormone (LH) in response to 5 μg of gonadotrophin-releasing hormone (GnRH).

Baseline gonadotrophin concentrations, the gonadotrophin responses to a GnRH challenge and the concentrations of testosterone and oestradiol were compared in lambs which were castrated at birth and intact lambs from both selection lines at 2, 6, 10 and 20 weeks of age. The pattern of LH and follicle-stimulating hormone (FSH) secretion was similar in the two lines, but differed between the intact and the castrated lambs. Basal LH and FSH secretion were significantly higher in the castrates than in the intact lambs from both selection lines. The high-line lambs had significantly higher basal FSH concentrations at all ages tested and significantly higher basal LH concentrations during the early postnatal period.

The magnitude of the gonadotrophin responses to GnRH differed significantly between the intact and the castrated lambs within each line, the amount of gonadotrophins secreted by the castrated lambs being significantly greater. The removal of gonadal negative feedback by castration did not alter the between-line difference in either LH or the FSH response to the GnRH challenge. Throughout the experimental period, the concentration of testosterone in the intact lambs was significantly greater than in the castrated lambs in both selection lines, but no significant difference was seen in the concentrations of oestradiol. No significant between-line differences were found in the peripheral concentrations of testosterone or oestradiol in the intact lambs from the two selection lines.

Therefore, despite similar amounts of gonadal negative feedback in the selection lines, there were significant between-line differences in basal gonadotrophin concentrations, at 2 and 6 weeks of age, and in the LH and FSH responses to an exogenous GnRH challenge, at all ages tested. Removal of gonadal negative feedback did not affect the magnitude of the between-line difference in the response of the lines to GnRH stimulation. The results indicate that the effects of selection on gonadotrophin secretion are primarily at the level of the hypothalamo–pituitary complex.

Keywords: GnRH; ram lambs; castration; gonadal negative feedback

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J. J. Robinson, R. P. Aitken, T. Atkinson, J. M. Wallace and A. S. McNeilly

Twelve anoestrous ewes maintained under natural photoperiod at 57°N received an oral dose of 3 mg melatonin daily at 15:00 h from 1 May. Starting 41 days later and extending from 11 June until 5 September, six of the ewes were also infused continuously with 0.8 mg thyrotrophin-releasing hormone (TRH) day−1 via subcutaneous osmotic minipumps. The remaining six ewes acted as controls. Behavioural oestrus, ovulation rate and luteal function were determined by exposure to a vasectomized ram, laparoscopy and the measurement of progesterone in peripheral plasma, respectively. TRH infusion stimulated a sustained increase (P < 0.001) in plasma concentrations of thyroxine, tri-iodothyronine and prolactin (thyroxine: 158 ± 9.3 and 65 ± 7.7 nmol l−1 for TRH-infused and control ewes, respectively; tri-iodothyronine, 2.6 ± 0.12 and 1.1± 0.19 nmol−1 and prolactin, 57±12 and 11 ± 2 μg l−1). No ewes were in oestrus before the TRH infusion and the mean number of behavioural oestrous cycles per ewe during the infusion period was 1.3 ± 0.33 and 2.5 ± 0.34 for TRH-infused and control ewes, respectively (P < 0.05). Corresponding mean intervals from 1 May to the onset of the first luteal phase (progesterone > 1 ng ml−1) were 88 ± 8.9 and 79 ± 3.5 days (not significant). TRH infusion had no effect on the mean numbers of corpora lutea (1.7 ± 0.14 and 1.6 ± 0.20 for TRH-infused and control ewes, respectively), but was associated with a lower mean incidence of normal luteal phases (1.5 ± 0.43 versus 2.7 ± 0.21, P= 0.052). Abnormalities in luteal function included delayed initial expression, extended ovarian cycles, suprabasal periovulatory progesterone concentrations and protracted periods of low progesterone secretion between successive ovarian cycles. Thus continuous TRH infusion suppressed plasma prolactin, doubled the circulating concentrations of thyroxine and tri-iodothyronine, and was associated with a wide range of abnormalities in ovarian function and endocrine status, the nature of which varied between ewes.

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J. J. Robinson, J. M. Wallace, R. P. Aitken and A. S. McNeilly

Thirty-two Scottish Blackface ewes that lambed outdoors in March and were weaned at the end of April and individually penned indoors under the natural photoperiod at 57°N were used to determine whether the ovine ovary that was deprived of gonadotrophic support was capable of early activation by melatonin. From 5 May (day 0), 16 of the ewes received an oral dose of 3 mg melatonin in a 4:1 (v:v) mixture of water and ethanol daily at 15:00 h. The remaining 16 ewes received the vehicle alone. Within each of these groups, eight were implanted s.c. on day 0 with an osmotic minipump which infused 50 μg of the gonadotrophin releasing hormone agonist (GnRHa), buserelin day−1. On day 25, a second minipump was inserted to ensure continued infusion of the agonist and on day 50 (24 June) both minipumps were removed. Ovarian activity was assessed by laparoscopy at intervals of 3 weeks from day 29 until the experiment was terminated on day 200 (21 November). Blood samples taken by jugular venepuncture three times a week for the first 50 days, daily from days 51–78 and thereafter three times a week were analysed for progesterone, prolactin and LH. Samples taken at intervals of 15 min for 10 h on days −1, 14, 28, 49, 56, 70 and 91 were assayed for LH. Treatment with GnRHa reduced LH concentrations and abolished pulsatile LH secretion. The onset of ovarian activity (progesterone >3.8 nmol l−1) was not affected by the 50-day GnRHa treatment and occurred for the melatonin-treated ewes at mean (± sem) intervals from 5 May of 66 ± 2.9 (range 51–75) and 71 ± 0.9 (range 68–75) days for non-GnRHa and GnRHa ewes, respectively. For the ewes not receiving melatonin the corresponding intervals were 113 ± 11.6 and 125 ± 9.2 days, respectively. The mean numbers of corpora lutea at first oestrus were not affected by GnRHa treatment and were 1.4 ± 0.13 and 1.5 ± 0.13 for control and melatonin-treated ewes, respectively. First ovulation following GnRHa treatment resulted in luteal concentrations of progesterone of normal duration and magnitude but, within the melatonin-treated ewes, those that received GnRHa returned to anoestrus 21 days earlier (P < 0.03) than did their non-GnRHa counterparts. After their initial suppression by melatonin, prolactin concentrations began to increase after 80 days of melatonin treatment in both non-GnRHa and GnRHa ewes and approximately three months before the ewes returned to anoestrus. The results demonstrate that a period of isolation of the ovaries of the ewe from the pituitary gonadotrophins does not alter their ability to respond to the melatonin-induced activation of the GnRH pulse generator.

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S. M. Rhind, P. J. Goddard, S. R. McMillen and A. S. McNeilly

Ovarian follicle development in response to FSH infusion was investigated in Scottish Blackface ewes with high and low body condition scores in which endogenous gonadotrophin secretion and follicle development to ≥ 2.5 mm diameter was suppressed using subcutaneous implants containing a GnRH agonist. In two experiments conducted during the normal breeding season, groups of 20 (Expt 1) and 15 (Expt 2) ewes were fed to achieve body condition scores ≥2·75 (high; H) or ≤1·75 (low; L). In both experiments GnRH agonist implants were inserted four weeks before FSH was infused for 72 h at 7 μg h−1 to group H animals or at 5 μg h−1 to group L animals; the infusion rates were designed to ensure similar circulating FSH concentrations in animals of both groups. In Expt 2, additional subcutaneous implants containing oestradiol were inserted 21 days after insertion of GnRH agonist implants and 7 days before the FSH infusion began. In both experiments, FSH infusion was associated with an increase in circulatory concentrations of LH (P < 0.01) and FSH (P < 0·001), but there was no difference with body condition in mean circulating gonadotrophin concentrations, the numbers of ovarian follicles ≥ 2.5 mm diameter, the proportion of these follicles that were oestrogenic or the mean rate of oestradiol secretion in vitro. It is concluded that differences in body condition of ewes do not affect the responsiveness of the ovary to FSH, in the presence or absence of oestradiol, as measured by the number, size and steroidogenic capacity of ovarian follicles present following FSH infusion.

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M. Mondain-Monval, O. M. MØller, A. J. Smith, A. S. McNeilly and R. Scholler

Summary. A heterologous radioimmunoassay system developed for the rabbit and suitable for a wide range of mammalian species has been shown to measure prolactin in the plasma of the blue fox. Evaluation of prolactin levels throughout the year showed that concentrations displayed a circannual rhythm with the highest values occurring in May and June. Prolactin concentrations remained low (∼2·5 ng/ml plasma) from July until April with no consistent changes found around oestrus (March—April). In 8 pregnant females, the prolactin increase in late April and May coincided with the last part of gestation and lactation: concentrations (mean ± s.e.m.) increased to 6·3 ± 0·6 ng/ml at mid-gestation, 9·7 ± 2·1 ng/ml at the end of gestation and 26·7 ± 5·0 ng/ml during lactation. In 10 non-pregnant animals, the mean ± s.e.m. values were 7·2 ± 1·2 ng/ml in April, 8·8 ± 2·2 ng/ml in May and 9·8 ± 1·3 ng/ml in June. The prolactin profile in 4 ovariectomized females was similar to that observed in non-pregnant animals, but the plasma values tended to be lower during the reproductive season (April—June). In intact females, the only large LH peak (average 28 ng/ml) was observed around oestrus. During pro-oestrus, baseline LH levels were interrupted by elevations of 3·1–10·4 ng/ml. During the rest of the year, basal levels were < 3 ng/ml. In ovariectomized females, LH concentrations increased within 2 days of ovariectomy and remained high (35–55 ng/ml) at all times of year.

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M. G. Hunter, C. Biggs, G. R. Foxcroft, A. S. McNeilly and J. E. Tilton

Attainment of puberty, cycle lengths, ovulation rate and endocrinology during the periovulatory period were studied in Meishan (MS) and European Large-White hybrid (LW) gilts. The mean age at onset of puberty of 115 days in MS (n = 20) gilts was younger (P < 0.001) than the 235 days in LW (n = 23). In the MS population studied, ovulation rate was not different (P > 0.1) during the third and fourth oestrous cycles, nor were there differences (P > 0.1) in the mean cycle length over the first three cycles. Overall changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and oestradiol did not differ significantly (P > 0.1) between the breeds (MS, n = 6; LW, n = 5) during the periovulatory period, but plasma inhibin concentrations were significantly (P < 0.05) higher in the MS. The time intervals from the oestradiol peak concentration and the onset of the LH surge until the onset of behavioural oestrus were significantly different (P < 0.005) between the breeds, with oestrus occurring earlier in the MS. However, no difference (P > 0.1) was found between the groups when the intervals from the peak oestradiol concentration to the onset of the LH surge were compared. These results indicate differences between the breeds, particularly in terms of the age of attainment of puberty and the timing of the onset of behavioural oestrus relative to the oestradiol and LH surges.