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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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SUMMARY
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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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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ABSTRACT
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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ABSTRACT
Ewes chronically treated with gonadotrophin-releasing hormone (GnRH) agonist were used to investigate the importance of the peripheral concentration of LH in FSH-stimulated follicular development. Twenty-four Welsh Mountain ewes were treated with two agonist implants containing 3·3 mg buserelin. During week 6 of treatment all the ewes were given a 72-h continuous infusion of ovine FSH alone (3 μg/h) or FSH with large (7·5 μg)- or small (2·5 μg) amplitude pulses of ovine LH delivered at 4-hourly intervals. The importance of baseline LH throughout the FSH infusion was evaluated in six animals which were treated with a specific antiserum against bovine LH (LH-AS) 15–20 h before the start of FSH treatment.
In the absence of LH-AS, infusion of FSH alone or with large or small pulses of LH stimulated the development of a normal number of small follicles (≤ 2·5 mm in diameter) and large follicles (> 2·5 mm in diameter). These follicles had normal diameter and steroid secretion compared with control ewes on day 8 of the luteal phase. In contrast, the animals pretreated with LH-AS developed no follicles > 2·0 mm in diameter but the number of small follicles per ewe was significantly (P < 0·05) increased.
These results support the hypothesis that FSH in the absence of pulsatile LH release stimulates preovulatory follicular development in ewes treated with GnRH agonist. The follicular response to LH pulses of different amplitude is dependent on both the stage of development of the follicle and the peripheral concentration of FSH. The endogenous basal level of LH present throughout the FSH infusion is essential for FSH to induce follicle growth beyond > 2·5 mm in diameter.
Journal of Endocrinology (1991) 128, 449–456
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ABSTRACT
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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ABSTRACT
Continuous infusion of a gonadotrophin-releasing hormone (GnRH) agonist (buserelin) by osmotic minipump from day 1 of the luteal phase in five Welsh ewes resulted in a sustained suppression of plasma concentrations of FSH which increased three- to eightfold within 2 days after the end of infusion 29 days later. Plasma concentrations of LH increased three- to eightfold over the first 5 days of infusion and then became basal and non-pulsatile until 1 day after the end of infusion. Duration of the luteal phase and plasma concentrations of progesterone were not significantly different in control and treated ewes. Pulses of LH in control ewes were followed by increases in concentrations of progesterone in samples collected at 10-min intervals for 7 h on days 10 and 14 of the luteal phase. However, progesterone was also released in a pulsatile manner in the absence of LH pulses in both control and GnRH agonist-treated ewes.
After natural luteolysis, no ovulation or corpus luteum function occurred in treated ewes up to 15 days after the end of treatment on day 29, even though oestrus, indicating follicular development and oestrogen secretion, had occurred 8–11 days after treatment ended.
After 30 days of infusion the ovaries of GnRH agonist-treated ewes contained no follicles > 2·5 mm in diameter. In follicles of 1–2 mm in diameter the basal and LH-stimulated production of oestradiol and testosterone in vitro were similar in both control and GnRH agonist-treated ewes, and a similar proportion of these follicles was oestrogenic (> 370 mol oestradiol per follicle) in GnRH agonist-treated and control ewes.
These results show (1) that progesterone secretion by the corpus luteum of the ewe can be sustained in the presence of basal concentrations but absence of pulsatile secretion of LH, and progesterone is released in a pulsatile manner whether or not LH pulses are present, (2) that follicular development beyond 2·5 mm in diameter in the ewe is dependent upon adequate stimulation by both LH and FSH and (3) that the continuous infusion of GnRH agonist is a simple method for providing reproducible suppression of LH and FSH and follicular development in the ewe to allow the study of gonadotrophin action on the ovary in vivo.
J. Endocr. (1987) 115, 273–282
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SUMMARY
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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Department of Physiology, Faculty of Medicine, University of Manitoba, 770 Bannatyne Avenue, Winnipeg, Canada, R3E 0W3
(Received 7 April 1977)
Binding of prolactin to the rat mammary gland is low during pregnancy and only increases at parturition (Holcomb, Costlow, Bushchow & McGuire, 1976). Holcomb et al. (1976) have suggested that failure to demonstrate binding of prolactin may be related to occupancy of the prolactin receptor by placental lactogen. However, the rabbit apparently does not produce a placental lactogen (Kelly, Tsushima, Shiu & Friesen, 1976; McNeilly & Friesen, 1977), and it was therefore of considerable interest to determine whether the results of binding of prolactin to the rabbit mammary gland during pregnancy showed significant differences from those obtained in the rat.
Mammary glands were taken from pregnant New Zealand White rabbits killed by an i.v. overdose of sodium pentobarbitone on each of days 10, 15, 20, 25, 28, 29, 30 and
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SUMMARY
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.
