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B. M. Adams, H. Sakurai and T. E. Adams

The effect of oestradiol on tissue concentrations of GnRH receptor and mRNA encoding GnRH receptor was assessed in orchidectomized sheep (wethers) made deficient in GnRH by passive immunization. Wethers were assigned to one of four groups (n = 6 animals per group). Animals in groups 2 and 4 received ovine anti-GnRH sera (200 ml; i.v.) at passive immunization, while antisera against the carrier protein was administered to wethers in groups 1 and 3. Oestradiol was delivered as a continuous infusion (2 μg h−1) to wethers in groups 1 and 2. Animals in groups 3 and 4 were infused with vehicle alone. Anterior pituitary tissue was collected at the end of the 48 h infusion. Anti-GnRH sera induced a rapid reduction in the serum concentration of LH. Continuous delivery of oestradiol resulted in a twofold increase (P < 0.05) in tissue concentration of GnRH receptor. This oestradiol-induced response was manifest even in wethers in which endogenous GnRH had been neutralized by passive immunization. Conversely, infusion of oestradiol increased (P < 0.05), and intravenous administration of anti-GnRH sera decreased (P <0.05), concentrations of mRNA encoding GnRH receptor in pituitary tissue. When delivered in combination, anti-GnRH sera reduced (P <0.05), but did not eliminate, the oestradiol-induced augmentation of steady-state concentrations of mRNA encoding GnRH receptor. These data demonstrate that the basal concentration of mRNA encoding GnRH receptor is dependent on continued GnRH stimulation. In contrast, the oestradiol-induced increase in steady-state concentration of mRNA encoding GnRH receptor is manifest even in the absence of GnRH.

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H. Sakurai, B. M. Adams and T. E. Adams

The effect of duration of a simulated follicular phase on gonadotrope responsiveness was assessed in orchidectomized sheep (wethers). The oestrogenic and hypothalamic inputs characteristic of the ovine follicular phase were simulated by continuous infusion of oestradiol (5 μg h−1 in 10% (v/v) ethanol-saline) and circhoral delivery of GnRH (200 ng per hourly pulse) for 0, 6, 12, 24, 48 or 96 h (n = 6 wethers per group). Responsiveness increased (P < 0.05) with increasing duration of simulated follicular phase. In a second experiment, responsiveness was assessed 96 h after initiation of infusion of oestradiol in wethers receiving hourly pulses of GnRH or saline. Concurrent administration of GnRH reduced (P < 0.05) the magnitude of the oestradiol-induced increase in gonadotrope responsiveness. In a companion study, anterior pituitary tissue was collected 96 h after the start of infusion of oestradiol and circhoral delivery of GnRH or saline. Pituitary stores of LH and tissue concentrations of GnRH receptor and mRNA encoding the GnRH receptor were increased (P < 0.05) by oestradiol infusion. The magnitude of these oestradiol-induced responses was not affected (P > 0.05) by concurrent GnRH treatment. Tissue concentrations of FSH and mRNA encoding the FSHβ subunit were decreased (P < 0.05) by oestradiol infusion. This suppressive effect of oestradiol was not reversed by GnRH. These results indicate that oestradiol stimulation, but not concurrent delivery of GnRH, is essential for full expression of surge-like secretion of LH. In addition, the oestradiol-induced increase in gonadotrope responsiveness during the simulated follicular phase is sustained throughout the period of oestradiol delivery.

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T. E. Adams, B. M. Adams and J. G. Watson

Summary. Blood samples were collected every 15 min for 6 h during the follicular (1 day before oestrus), and early (Days + 1 to + 3), mid- (Days + 4 to + 8), and full (Days + 9 to +14) luteal phases of the oestrous cycle. Serum concentrations of immunoactive LH were measured by radioimmunoassay. The biological activity of serum LH was determined by an in-vitro bioassay that uses LH-induced testosterone production from mouse interstitial cells as an endpoint. Only ovine and bovine LH and hCG had appreciable activity in this bioassay. The temporal pattern of secretion of bioactive LH paralleled the secretory pattern of immunoactive LH at all stages of the ovine oestrous cycle. However, the secretory pattern itself varied regularly through the oestrous cycle. The frequency of secretory excursions of LH was highest during the follicular phase (6·2 ± 0·9 pulses/6 h) and was progressively reduced through the luteal phase (1·1 ± 0·1 pulses/6 h during full luteal phase). Conversely, amplitude of secretory excursions of immunoactive LH was low during the follicular phase (0·79 ± 0·08 ng/ml) and significantly (P < 0·05) increased during the mid- and full luteal phases (1·49 ± 0·10 and 2·37 ± 0·20 ng/ml, respectively). The biopotency of LH (bioactive LH/immunoactive LH) at the peak of secretory excursions was 1·00 ± 0·03 during the follicular phase, 0·66 ± 0·02 during the mid-luteal phase, and 1·18 ± 0·19 during full luteal activity. The biopotency of LH was markedly reduced during the preovulatory surge of gonadotrophin. During the full luteal phase each pulse of LH secretion was associated with an abrupt increase in secretion of progesterone. These results indicate that both the quantitative and qualitative character of serum LH varies through the oestrous cycle of the sheep.

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K. G. Pirl and T. E. Adams

Summary. Circhoral administration (250 ng/h, i.v.) of GnRH induced a preovulatory-like surge of LH and subsequent luteal function in 4 of 4 ewe lambs 1 month before expected date of puberty. Within 12 h of the start of pulsatile delivery of GnRH, mean concentrations of immunoactive and bioactive LH increased significantly (P < 0·05) and the LH surge occurred by 1·8 ± 0·6 days of treatment. Mean concentrations of serum progesterone were elevated significantly (P < 0·001) 3 days after the surge. The biopotency of LH (bioactive LH/immunoactive LH) before the GnRH-induced surge of LH did not differ from LH biopotency in ewe lambs receiving circhoral delivery of saline (0·41 ± 0·05 and 0·46 ± 0·04, respectively). Biopotency of LH declined markedly at the GnRH-induced LH surge (0·25 ± 0·04), but biopotency of serum LH was significantly augmented (P < 0·05) during the period of luteal activity (0·70 ± 0·07). Regular oestrous cycles were observed in 3 of 4 ewe lambs after the 10-day GnRH treatment period. These results indicate that pulsatile delivery of GnRH is effective in inducing precocious puberty in ewe lambs. Increase in LH biopotency does not appear to be required in the pubertal transition to reproductive cyclicity in this species. Augmented LH biopotency may be important in support of luteal function after first ovulation.

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T. E. Adams, J. F. Quirke, J. P. Hanrahan, B. M. Adams and J. G. Watson

Summary. Rates of ovulation differed significantly (P < 0·01) among ewes of the different genetic lines. However, of the reproductive characteristics studied, only progesterone concentration at the height of luteal function, duration of oestrus, and interval from onset of oestrus to peak of the preovulatory gonadotrophin surge showed significant positive association with rate of ovulation. The pattern of secretion of LH during the periovulatory period did not differ in the Galway and Finnish Landrace breeds. The total amount of LH secreted during the preovulatory surge did not differ amongst lines. Similarly, no difference in the plasma concentration of LH at the height of the preovulatory surge was noted among Galway and reference Finnish Landrace lines. However, the concentration of LH at the height of the surge was significantly (P < 0·05) reduced in the selected Finnish Landrace line. Plasma concentrations of FSH during the preovulatory period were significantly (P < 0·05) elevated in the breed (Galway) with the lowest prolificacy. When contrasted with either of the Finnish Landrace lines, the magnitudes of the preovulatory surge of FSH and the secondary surge of FSH were significantly greater (P < 0·05) in Galway ewes. These results suggest that genetic difference in rate of ovulation among sheep breeds is not tightly coupled to quantitative differences in plasma concentration of gonadotrophic hormones during the periovulatory period.

Keywords: gonadotrophins; periovulatory period; ovulation rate; prolific sheep

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D. R. Mann, S. R. Adams, K. G. Gould, T. E. Orr and D. C. Collins

Summary. In Exp. 1, the effect of treatment with a GnRH agonist on basal concentrations of serum testosterone and peak values of serum testosterone after administration of hCG was determined. One group of adult male monkeys was treated with a low dose (5–10 μg/day) and a second group with a high dose (25 μg/day) of a GnRH agonist for 44 weeks. Basal and peak testosterone concentrations were both significantly reduced by GnRH agonist treatment in all groups compared to untreated control animals, but the % rise in serum testosterone above basal values in response to hCG administration was unchanged by agonist treatment.

In Exp. 2, the GnRH agonist (100 or 400 ng) or a GnRH antagonist (4 μg) was infused into the testicular arteries of adult monkeys. The agonist did not alter testosterone concentrations in the testicular vein or testosterone and LH values in the femoral vein.

In Exp. 3, testicular interstitial cells from monkeys were incubated with three concentrations (10−9, 10 −7 and 10−5 m) of the GnRH agonist or a GnRH antagonist with and without hCG. After 24 h, neither basal nor hCG-stimulated testosterone production was affected by the presence of the GnRH agonist or antagonist.

The results from all 3 experiments clearly suggest that GnRH agonist treatment does not directly alter steroid production by the monkey testis.

Keywords: GnRH; testes; LH; testosterone; rhesus monkey

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C. A. Daley, M. S. Macfarlane, H. Sakurai and T. E. Adams

Stress-like concentrations of cortisol increase the negative feedback potency of oestradiol in castrated male sheep. A similar cortisol-dependent response in female sheep might be expected to suppress gonadotrophin secretion and impair follicular development and ovulation. The oestrous activity of 21 female sheep was synchronized using progestogen-treated vaginal pessaries to test this hypothesis. Stress-like concentrations of cortisol (60–70 ng ml−1) were established by continuous infusion of cortisol (80 μg kg−1 h−1; n = 13) beginning 5 days before, and continuing for 5 days after, pessary removal. Control animals (n = 8) received a comparable volume of vehicle (50% ethanol–saline) over the 10 day infusion period. Serum concentrations of oestradiol increased progressively in control sheep during the 48 h immediately after pessary removal. This increase in serum oestradiol was blocked or significantly attenuated in sheep receiving stress-like concentrations of cortisol. Preovulatory surge-like secretion of LH was apparent in control animals 58.5 ± 2.1 h after pessary removal. In contrast, surge-like secretion of LH was not observed during the 5 days after pessary removal in 54% (7 of 13) of sheep receiving cortisol. Moreover, the onset of the surge was significantly delayed in the cortisol-treated ewes that showed surge-like secretion of LH during the infusion period. The ability of episodic pulses of exogenous GnRH to override the anti-gonadal effect of cortisol was examined in a second study. Oestrous activity of 12 ewes was synchronized using progestogen-containing pessaries as described above. Ewes were randomly assigned to one of three treatment groups (n = 4 ewes per group). Animals received cortisol (100 μg kg−1 h−1; groups 1 and 2) or a comparable volume of vehicle (group 3) beginning 5 days before, and continuing for 2 days after, pessary removal. Pulses of GnRH (4 ng kg−1 h−1, i.v.; group 1) or saline (groups 2 and 3) at 1 h intervals were initiated at pessary removal and continued for 48 h. Serum concentrations of oestradiol were not significantly increased after pessary removal in sheep receiving cortisol alone. Conversely, serum concentrations of oestradiol increased progressively during the 48 h after pessary removal in control ewes and in ewes receiving cortisol and GnRH. At the end of infusion, serum concentrations of oestradiol did not differ (P > 0.05) between control (7.7 ± 0.8 μg ml−1) ewes and ewes receiving cortisol and episodic GnRH (6.4 ± 1.3 μg ml−1). Moreover, these values were significantly greater (P < 0.05) than the serum concentrations of oestradiol in animals receiving cortisol (1.0 ± 0.4 μg ml−1) alone. Collectively, these data indicate stress-like concentrations of cortisol block or delay follicular development and the preovulatory surge of LH in sheep. In addition, episodic GnRH overrides cortisol-induced delay in follicular maturation.