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Thirty-six series of serial jugular blood samples were collected from 12 goats during suckling during the first 3 weeks of lactation and the transient appearance of oxytocin (5–86 μu./ml plasma) was detected in 24 of the series. Blood oxytocin was assayed on the lactating guinea-pig mammary gland. Forty series of serial blood samples were also collected from 12 goats during hand-milking over the first 6 weeks of lactation and the transient appearance of oxytocin (5–160 μu./ml plasma) was observed in 25 of these.

The results indicated that: (1) oxytocin may be released at any time during the suckling or milking process; (2) in only 16% of experiments where oxytocin was released and 5·6% of all experiments investigated could a conditioned release of oxytocin be induced in goats before suckling and in no case before hand-milking despite a vigorous and prolonged conditioning period; (3) there is a large degree of variability in the pattern of oxytocin release between animals and between individual suckling and milking episodes in the same animal; (4) suckling and hand-milking are equally effective in causing the release of oxytocin; (5) stimuli arising during various stages of the suckling and hand-milking routines may be assigned a figure indicative of the relative effectiveness of these stimuli in terms of percentage probability in causing oxytocin release; (6) there is a greater probability of oxytocin release occurring before suckling than before hand-milking; (7) there is a very marked similarity in the pattern of oxytocin release both during and after teat stimulation in response to suckling and milking stimuli; (8) there is a greater probability of oxytocin being released in response to hand-milking during early lactation than during late lactation; (9) the milk-ejection reflex in the goat does not appear to be important for the achievement of normal milk yields during suckling or hand-milking.

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A. S. McNeilly

Prolactin is the hormone principally involved with stimulating milk production. Therefore, at first sight, a role for prolactin in controlling gonadotrophin secretion may seem strange, especially as its involvement at the ovarian level has only been irrevocably established in rodents, particularly the rat (see McNeilly, 1984). However, in almost all species studied so far, the high levels of prolactin induced by suckling and essential for lactation are also associated with a reduction in normal gonadotrophin secretion, principally the pulsatile secretion of luteinizing hormone (LH), and as a consequence, a suppression of ovarian activity. A similar association between increased levels of prolactin and a reduction in both LH and follicle-stimulating hormone (FSH) occurs during seasonal infertility in male and female ungulates and in pathological hyperprolactinaemia in both men and women.

In view of this apparent relationship between increased prolactin and decreased gonadotrophin secretion, a specific role for prolactin itself in suppressing

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Specific radioimmunoassays were used to assess the content of LH, FSH, the gonadotrophin α-subunit and the LH β-subunit in four adult, 19 normal foetal pituitary glands (9·5–32 weeks of gestation) and a pituitary extract from an anencephalic foetus (36 weeks). The hormones and subunits were further identified by column chromatography on Sephadex G-100. All pituitary glands contained free α-subunit and intact LH but the α-subunit:LH ratio was significantly higher in the early foetal pituitaries (9·5–16 weeks) than in the four adult pituitaries. Only small or undetectable amounts of LH β-subunit and 'undetectable' FSH were found in these early foetal pituitaries (9·5–11·5 weeks). The concentration of intact hormones or subunits in the pituitaries showed no significant sex difference in any of the groups. In contrast to these results, only α-subunit was detectable in the pituitary of the anencephalic foetus.

For 14 early foetuses (age of gestation 10–16 weeks) the serum levels of LH–HCG, FSH, and α-subunit in the circulation were significantly higher than in 26 foetuses at term (37–41 weeks). On the basis of these results a theory for the development of the gonadotrophin secretion from the foetal pituitary gland is outlined.

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J Brooks and A S McNeilly


To investigate the regulation of the sheep gonadotrophin-releasing hormone receptor (GnRH-R) gene expression, two different treatment regimes were used. Experiment 1 examined the effects of twice daily injections of ovine follicular fluid (oFF, 15 ml s.c.) as a source of inhibin, and daily GnRH antagonist injections (Nal-Glu.HOAc, 2 mg s.c.) on days 9–12 of the oestrous cycle. Luteolysis was induced on day 12 with prostaglandin (PG) and the ewes killed at two different stages; day 12 (luteal) and 18 h after PG injection. Experiment 2 examined the effect of a single injection of oestradiol benzoate (100 μg i.m.) 18 h before death in luteal phase ewes and ewes chronically implanted with the GnRH agonist, buserelin. In both experiments, pituitaries were removed at death for determination of pituitary GnRH binding, LH content and levels of GnRH-R and LHβ mRNA. In addition in experiment 1, follicles ≥2·5 mm were dissected from the ovaries for determination of oestradiol content.

In experiment 1, oFF treatment during the luteal phase completely inhibited follicle oestradiol production but was without effect on the other parameters measured. After cessation of oFF treatment and induction of luteolysis, a significant (P<0·05) increase in plasma LH occurred but the normal follicular increase in both GnRH-R mRNA levels and GnRH binding seen in control ewes was prevented. GnRH antagonist treatment alone or in combination with oFF also inhibited follicle oestradiol production, prevented the increase in GnRH-R mRNA, completely inhibited GnRH binding and significantly decreased LHβ mRNA levels. Pituitary LH content was unaffected by any treatment. In experiment 2, oestradiol treatment did not affect GnRH-R mRNA levels, GnRH binding, LHβ mRNA or pituitary LH content in luteal phase ewes, whilst chronic GnRH agonist treatment acted to decrease these parameters dramatically. A single injection of oestradiol in the GnRH agonist treated ewes significantly (P<0·05) increased GnRH-R mRNA levels and completely restored GnRH binding to luteal levels, without any effect on LHβ mRNA or pituitary LH content.

These results suggest that the control of GnRH receptor expression in the sheep is directly related to oestradiol and not to the action of GnRH itself.

Journal of Endocrinology (1994) 143, 175–182

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Prolactin was isolated from frozen goat pituitary glands by a simple procedure involving gel filtration and chromatography on DEAE-cellulose. The major product (yield, 2·5 mg/g pituitary tissue) had high pigeon crop sac-stimulating activity (27 i.u./mg) and was free of growth hormone and other pituitary hormones. The molecular weight was similar to that of ovine prolactin.

Caprine prolactin was immunologically indistinguishable from ovine prolactin in radioimmunoassays, in which ovine prolactin antiserum and either ovine or caprine prolactin labelled with 125I were used.

The results indicate that caprine and ovine prolactin are closely related and that radioimmunoassay for ovine prolactin may be used to estimate caprine prolactin in serum.

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A. S. McNEILLY and C. A. FOX

Although certain of the prostaglandins are known to be potent smooth-muscle contracting agents (Horton, 1969), very little is known of their action on the mammary gland. Prostaglandin F2α (PGF2α) had no effect on the mammary epithelium of the mouse or on blood flow through the perfused goat udder (J. L. Linzell, personal communication; cited by Pickles, 1967). PGF1 had no effect per se on the milk-ejection pressure when given by close i.a. or i.v. injection in the lactating rabbit (Türker & Kiran, 1969) but did decrease the milk-ejection activity of oxytocin in the rabbit (Türker & Kiran, 1969) and abolished the milk-ejection activity of oxytocin in the rat (Haldar, Maiweg & Grosvenor, 1970).

We now report results obtained during investigations into the effect of prostaglandins in the lactating guinea-pig assay for oxytocin (Tindal & Yokoyama, 1962). Changes in intramammary pressure were recorded after the close

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G. A. Lincoln and A. S. McNeilly


Changes in the concentration of inhibin, FSH, LH and testosterone were measured in the peripheral blood of adult Soay rams during a reproductive cycle induced by exposure to an artificial lighting regimen (long days with a 16-week period of short days) or treatment with melatonin (long days with a 12-week period when melatonin was administered daily in mid-light phase to simulate the effect of short days). In both experimental situations, changes in the plasma concentrations of inhibin occurred in parallel with the cycle in the diameter of the testes with a four- to fivefold increase in the inhibin concentrations from the nadir to the peak of the testicular cycle. Increases in the plasma concentrations of FSH, LH and testosterone also occurred in association with the reactivation of the reproductive axis. The weekly changes in the plasma concentrations of inhibin were positively correlated with the changes in plasma FSH values during the developing and regressing stages of the testicular cycle but negatively correlated during the active stage.

In a group of castrated rams exposed to the same lighting regimen, the plasma concentrations of inhibin were always below the detection limit of the radioimmunoassay. The testosterone values were also very low in the castrates while the plasma concentrations of FSH and LH were 10-50 fold higher than normal and varied in relation to the light cycle.

The results show for the first time that inhibin is secreted into the peripheral blood in the ram exclusively from the testes. The positive correlation between the changes in plasma concentrations of FSH and inhibin during the developing and regressing phases of the testicular cycle indicate that FSH stimulates the secretion of inhibin. The negative correlation between FSH and inhibin in the active phase of the testicular cycle, is consistent with the role of inhibin in the negative-feedback control of FSH secretion. This is only evident because the testes undergoes reactivation of its full function during the change from the regressed to the active state which is especially obvious in the highly seasonal Soay ram.

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Blood oxytocin was assayed on the lactating guinea-pig mammary gland. Thirty-six series of serial jugular blood samples were collected from 33 oestrous female goats during normal mating and the transient appearance of oxytocin (2 to 190 μu./ml plasma) was detected in 28 of the series. Coitus did not appear to be a major stimulus for the release of oxytocin. However, oxytocin release occurred in many experiments shortly before coitus when the male was present and continued after coitus for varying times up to 16 min after the male had left the mating room. Oxytocin (9–150 μu./ml plasma) was also released during simulated mating in all six experiments carried out in six dioestrous female goats.

In a further 16 oestrous female goats, oxytocin release (7–73 μu./ml plasma) occurred in response to individual exteroceptive stimuli associated with mating and these stimuli could be classified in descending order of effectiveness in causing oxytocin release as follows: presence of another goat > smell of the male > sound of the male > sight of the male.

The results indicate that oxytocin release occurs in oestrous female goats in response to a complex of stimuli occurring during mating and that physical stimuli associated with coitus are only important in causing oxytocin release during simulated mating in dioestrous female goats.

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J. M. Wallace and A. S. McNeilly


Treatment of Damline ewes with twice-daily i.v. injections of bovine follicular fluid during the luteal phase for 10 or 2 days before prostaglandin-induced luteolysis resulted in a delay in the onset of oestrous behaviour and a marginal increase in ovulation rate. During the treatment cycle, blood samples were withdrawn at 15-min intervals for 25 h from 08.00 h on days 1, 6 and 10 (day 0 = oestrus). At all three stages of the luteal phase, plasma FSH concentrations were suppressed relative to controls 3 h after the 09.00 h injection of follicular fluid and remained low until

06.00 h on the following day. In the 10-day treatment group LH pulse amplitude was significantly greater than that of controls on days 6 and 10. Pulse frequency remained high throughout treatment and was significantly higher relative to controls on day 10 despite normal progesterone levels. The results suggest that the higher pulsatile LH secretion during the luteal phase is due to reduced negative feedback effects of oestradiol occurring as a result of the follicular fluid-induced reduction in FSH.

J. Endocr. (1986) 111, 317–327

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A. S. McNeilly and D. T. Baird


Changes in the plasma concentration of inhibin were measured by radioimmunoassay in ovarian venous blood collected at 10-min intervals for 5-h periods between 16 and 21 h and 40 and 45 h after cloprostenol-induced luteal regression in six Finn–Merino sheep. Episodes of inhibin secretion occurred with an interpulse interval of 66 ± 5 min in both stages of the follicular phase. These changes in inhibin were unrelated to pulses of LH or oestradiol. There was no relationship with plasma concentrations of FSH, which did not change in a pulsatile manner. These results suggest that the release of inhibin by the preovulatory follicle(s) occurs in a pulsatile manner and is under local control by unknown factors.

Journal of Endocrinology (1989) 122, 287–292