The failure of prolactin to initiate lactogenesis in the ewe.

Infusion of exogenous prolactin (NIH-P-S11) at 1 mg/h for 10 h into ewes pretreated for 30 days with oestradiol benzoate and progesterone was unable to initiate milk secretion. Similarly primed ewes injected with 10 microgram thyrotrophin releasing hormone also failed to lactate but all ewes injected with dexamethasone (10 mg daily for 5 days) after oestrogen plus progesterone treatment secreted copious quantities of milk. The results suggest that prolactin, itself, is not capable of initiating lactogenesis in the ewe.     

Prolactin as a factor in the uterine response to progesterone in rabbits

The direct effect of prolactin on uteroglobin production and on uterine endometrial oestrogen and progesterone receptor concentrations was tested by using ovariectomized rabbits (at least 12 weeks) treated with prolactin; prolactin + progesterone; prolactin + oestradiol + progesterone; oestradiol + progesterone; or progesterone alone. Prolactin treatment produced a significant (P less than 0.05) increase in the concentration of cytosolic oestrogen and progesterone receptors, restoring the concentrations to values found at oestrus. However, the concentration of nuclear receptors remained low. In the remaining treatment categories there was no significant (P greater than 0.05) increase in the concentration of oestrogen and progesterone receptors compared with those in ovariectomized controls. However, the sequential treatment of ovariectomized animals with prolactin + progesterone stimulated uteroglobin production to a concentration equal to that found in intact rabbits on the 5th day of pregnancy. This was not achieved by prolactin or progesterone alone or with oestradiol. These results suggest that prolactin acts as an essential factor in the rabbit uterine response to progesterone, perhaps by the modulation of progesterone receptor activity.     

Effect of ovine trophoblast protein-1, oestrogen and progesterone on oxytocin-induced phosphatidylinositol turnover in endometrium of sheep

In Exp. 1, endometrium was collected from Day-15 cyclic ewes and effects of oTP-1, oxytocin and oTP-1 + oxytocin, in various temporal relationships, on phosphatidylinositol (PI) turnover were determined. Co-treatment of endometrium with oTP-1 and oxytocin inhibited stimulatory effects of oxytocin, while treatment with oTP-1 before and during oxytocin administration had no effect. Turnover of PI was unaffected by oTP-1 alone. In Exp. 2, ovariectomized ewes were treated with progesterone (50 mg/day) for 10 days and then oestrogen (100 micrograms/day) for 2 days and endometrium was collected. Oxytocin stimulated PI turnover in endometrium, but oTP-1 had no effect alone or in combination with oxytocin. In Exp. 3, ovariectomized ewes were treated with corn oil (1 ml/day), oestrogen (50 micrograms/day), progesterone (50 mg/day) or progesterone + oestrogen for 10 days and endometrium was collected. Oxytocin stimulated PI turnover only in ewes that received progesterone. oTP-1 alone had no effect on PI turnover, while co-treatment of endometrium with oxytocin and oTP-1 stimulated PI turnover in ewes treated with progesterone, but not progesterone and oestrogen. Pretreatment of endometrium with oTP-1 stimulated PI turnover when ewes were treated with progesterone or progesterone + oestrogen. Pretreatment of endometrium with oxytocin and then treatment with oTP-1 inhibited PI turnover compared to treatment with oxytocin alone. In Exp. 4, ovariectomized ewes were treated as in Exp. 2. Catheters were placed into the uterine horns and ewes received oTP-1 into one horn and serum into the other twice daily on Days 10-12 of steroid treatment. Endometrium collected on Day 13 was used to measure PI turnover and received either no treatment or oxytocin. Oxytocin stimulated PI turnover in endometrium of these ewes and in-vivo treatment of the ewes with oTP-1 had no effect on PI turnover. These results indicate that antiluteolytic effects of oTP-1 are not mediated by inhibiting effects of oxytocin on phosphatidylinositol turnover if oxytocin receptors are present and that uterine responsiveness to oxytocin is progesterone dependent.     

Lactogenesis induced by ovariectomy in pregnant rats and its regulation by oestrogen and progesterone

Ovariectomy on day 19 of pregnancy augmented galactosyl transferase activity 24 h after surgery preceding by 6 h the significant alpha-lactalbumin accumulation. Progesterone, injected immediately after ovariectomy showed a clear inhibitory effect on both galactosyl transferase and alpha-lactalbumin concentration, measured 30 h after ovariectomy. However, once the synthesis of lactose has been induced, progesterone is no longer inhibitory. Oestrogen induced a significant increase in lactose synthetase activity but no effect was obtained on galactosyl transferase activity. Progesterone, in a time and dose dependent relationship, was capable of preventing the effect of estrogen on lactogenesis. The lactogenic action of oestrogen in ovariectomized pregnant rats might be due to a direct effect at the mammary gland level facilitating the action of prolactin or through an indirect effect mediated via an increase on prolactin release.     

Differences in the rabbit uterine response to progesterone as influenced by growth hormone or prolactin

The direct effect of growth hormone (GH) on the uterine response to progesterone was tested by using ovariectomized rabbits (at least 12 weeks) treated with GH; GH + progesterone; or progesterone alone. These results were compared with the effect of prolactin or prolactin + progesterone on the uterus. Prolactin treatment produced an increase (P less than 0.01) in the endometrial surface area and restored cytosolic oestrogen and progesterone receptor concentrations to oestrous control values. The sequential treatment of does with prolactin + progesterone stimulated uteroglobin production to a concentration equal to that found in intact rabbits on Day 5 of pregnancy. In contrast, GH treatment had no effect on endometrial surface area, produced an increase in the concentration of cytosolic oestrogen receptor but did not produce an increase in the concentration of progesterone receptor. The sequential treatment of does with GH + progesterone failed to stimulate uteroglobin secretion above control (progesterone alone) values. It is concluded that the action of prolactin in the rabbit uterus is no generally somatogenic; rather, prolactin increases the concentration of progesterone receptor and thereby enhances the uterine response to progesterone.     

Oestrogen and progesterone interactions in the control of gonadotrophin and prolactin secretion

Oestrogen and progesterone have marked effects on the secretion of the gonadotrophins and prolactin. During most of the oestrous or menstrual cycle the secretion of gonadotrophin is maintained at a relatively low level by the negative feedback of oestrogen and progesterone on the hypothalamic-pituitary system. The spontaneous ovulatory surge of gonadotrophin is produced by a positive feedback cascade. The cascade is initiated by an increase in the plasma concentration of oestradiol-17 beta which triggers a surge of luteinizing hormone releasing hormone (LHRH) and an increase in pituitary responsiveness to LHRH. The facilitatory action of oestrogen on pituitary responsiveness is reinforced by progesterone and the priming effect of LHRH. How oestrogen and progesterone exert their effects is not clear but the facilitatory effects of oestrogen take about 24 h, and the stimulation of LHRH release is produced by an indirect effect of oestradiol on neurons which are possibly opioid, dopaminergic or noradrenergic and which modulate the activity of LHRH neurons. In the rat, a spontaneous prolactin surge occurs at the same time as the spontaneous ovulatory gonadotrophin surge. The prolactin surge also appears to involve a positive feedback between the brain-pituitary system and the ovary. However, the mechanism of the prolactin surge is poorly understood mainly because the neural control of prolactin release appears to be mediated by prolactin inhibiting as well as releasing factors, and the precise role of these factors has not been established. The control of prolactin release is further complicated by the fact that oestradiol stimulates prolactin synthesis and release by a direct action on the prolactotrophes. Prolactin and gonadotrophin surges also occur simultaneously in several experimental steroid models. A theoretical model is proposed which could explain how oestrogen and progesterone trigger the simultaneous surge of LH and prolactin.     

Control of endometrial oxytocin receptor and uterine response to oxytocin by progesterone and oestradiol in the ewe

The effects of administration of progesterone and oestradiol on ovine endometrial oxytocin receptor concentrations and plasma concentrations of 13,14-dihydro-15-keto prostaglandin F-2 alpha (PGFM) after oxytocin treatment were determined in ovariectomized ewes. Ewes received progestagen pre-treatment, progesterone and/or oestradiol in 11 different treatment schedules. Progestagen pre-treatment decreased oxytocin receptor concentrations in endometrium from ewes treated subsequently with either progesterone for 5 days or progesterone for 5 days plus oestradiol on Days 4 and 5 of progesterone treatment. Oestradiol increased endometrial oxytocin receptor concentrations when administered on Days 4 and 5 of 5 days progesterone treatment. Progestagen pre-treatment followed by progesterone treatment for 12 days caused a large increase in oxytocin receptors and no further increase occurred when ewes were given oestradiol on Days 11 and 12, or when progesterone was withdrawn on Days 11 and 12, or these two treatments were combined. Oxytocin administration caused an increase in plasma PGFM concentrations in ewes which did not receive progestagen pre-treatment, and subsequently received progesterone treatment for 5 days and oestradiol treatment on Days 4 and 5 of progesterone treatment. Similarly treated ewes which received progestagen pre-treatment did not respond to oxytocin. Oxytocin administration also increased plasma PGFM concentrations in ewes which received progestagen pre-treatment followed by progesterone treatment for 12 days, progesterone treatment for 12 days plus oestradiol on Day 11 and 12 of progesterone treatment, progesterone withdrawal on Day 11 and 12, or progesterone withdrawal and oestradiol treatment combined. The results indicate that (1) progesterone pre-treatment affects oxytocin receptor concentrations in the endometrium and uterine responsiveness to oxytocin and (2) progesterone treatment alone for 12 days after a treatment which mimics a previous luteal phase and oestrus is sufficient to induce oxytocin receptors and increase oxytocin-induced PGF release. These results emphasize the importance of progesterone and provide information which can be used to form an hypothesis for control of luteolysis and oestrous cycle length in the ewe.     

Artificial induction of lactation in ewes: the role of prolactin.

The mammary glands of 30 non-pregnant, intact ewes were developed by subcutaneously injecting oestrogen plus progesterone at intervals of 3 days from day 0 to day 27. Two days later (day 29), 15 ewes were injected subcutaneously with 18 mg ergocryptine, to inhibit specifically secretion of prolactin. Then groups of ewes, each comprising five ergocryptiine-treated and five untreated ewes, were injected from days 30 to 34 with either four intravenous injections each day of 1 i.u. syntocinon, one subcutaneous injection each day of 10 mg dexamethasone trimethylacetate, or two subcutaneous injections each day of 2-5 mg oestradiol benzoate plus 6-25 mg progesterone. All ewes were milked by hand on days 30-50. Within 24 h of injecting ergocryptine, levels of prolactin in serum were reduced to negligible values (less than 2 ng/ml). Comparison of results for ewes not receiving ergocryptine showed that syntocinon, dexamethasone and oestradiol benzoate plus progesterone, at the doses used, were equally effective in initiating milk secretion. Peak yields of 0-23-0-27 kg/day were achieved. On the other hand, ewes treated with ergocryptine before syntocinon or dexamethasone produced peak yields of only 0-12-0-13 kg/day and ewes treated with ergocryptine before oestradiol benzoate plus progesterone produced negligible amounts of secretion. The results suggest that syntocinon and dexamethasone were either lactogenic per se or effected the release of hormones of the lactogenic complex other than prolactin. However, oestradiol benzoate plus progesterone appeared to be lactogenic by virtue of the influence of oestrogen on the secretion of prolactin.     

Demonstration of mRNAs for oxytocin and prolactin in porcine granulosa and luteal cells. Effects of these hormones on progesterone secretion in vitro

The relative levels of mRNAs for relaxin, prolactin, inhibin and oxytocin have been measured in porcine granulosa as well as luteal cells by hybridisation to single-stranded synthetic DNA. The likelihood of a paracrine function of oxytocin and prolactin in the porcine ovary was inferred from the in vitro effects of both hormones on progesterone secretion of ovarian cells. Both hormones were found to inhibit progesterone secretion of luteal cells. In contrast, only prolactin but not oxytocin stimulated progesterone secretion in granulosa cells.     

Effects of progesterone and oestradiol on RNA and protein metabolism in the genital tract and on survival of embryos in the ovariectomized ewe.

The hormonal regulation of metabolism in the genital tract and the development of embryos during early pregnancy in the ewe have been examined. Ovariectomized ewes received injections of maintenance progesterone, oestrous oestradiol and priming progesterone according to schedules designed to simulate endogenous ovarian secretion during early pregnancy, around the time of oestrus and during the luteal phase of the oestrous cycle immediately preceding oestrus. The survival and development of embryos was dependent upon the dose of maintenaince progesterone and the duration of treatment at the time of transfer, but changes in progesterone dose did not change endometrial protein or RNA metabolism on particular days. Both priming progesterone and oestrous oestradiol were required for normal embryo development. Priming progesterone and oestrous oestradiol each increased endometrial RNA/DNA ratios during early pregnancy. There were no interactions between priming progesterone and oestrous oestradiol, their effects being simply additive. Neither maintenance nor priming progesterone had any effect on protein and RNA metabolism in the oviduct. It is suggested that in the intact ewe oestrogen secreted at oestrus and progesterone secreted prior to oestrus play important roles in the establishment of a uterine environment suitable for the subsequent normal development of embryos.     

Milking- and suckling-induced secretion of oxytocin and prolactin in parturient dairy cows

Serum concentrations of prolactin were unaffected by either suckling or milking on Day 2 or 3 postpartum in cows housed with their calves following parturition. In contrast, among cows housed without their calves milking elicited a four- to sixfold increase in serum prolactin concentrations. Serum oxytocin levels increased in response to both suckling and milking among cows housed with their calves with suckling being a more potent stimulus (257 ± 32 vs 189 ± 23 pg/ml at peak). However, the greatest increase in oxytocin levels accompanied milking in cows housed without their calves (375 ± 36 pg/ml at peak). These results suggest that stimuli associated with the presence or the absence of the calf can alter maternal secretion of oxytocin and prolactin. Greater understanding of factors which regulate secretion of these hormones may result in techniques to modify milk synthesis and milk ejection in dairy cows.     

Increases in the oxytocin-induced prostaglandin F2 alpha response and reduction in the concentrations of endometrial oxytocin receptors in ewes in response to progesterone.

Ovariectomized ewes were given progesterone and oestrogen priming as steroid pretreatment and subsequently treated with progesterone, prostaglandin F2 alpha (PGF2 alpha), or both. In Expt 1, plasma concentrations of the metabolite 13,14-dihydro-15-keto-PGF2 alpha (PGFM) were measured after an i.v. injection of oxytocin. There was little PGFM response in the untreated control ewes or in the pretreated ewes. Treatment with PGF2 alpha alone had no effect (P greater than 0.05), whereas treatment with progesterone either alone or with PGF2 alpha significantly (P less than 0.05) increased the uterine PGFM response to oxytocin. In Expt 2, chronically ovariectomized ewes had high concentrations of endometrial oxytocin receptors. Treatment with PGF2 alpha alone did not alter the concentrations of the receptors. Treatment with progesterone either alone or with PGF2 alpha significantly (P less than 0.05) reduced the concentrations of the receptors. It is concluded that progesterone promotes the PGFM response to oxytocin, but simultaneously suppresses the concentrations of endometrial oxytocin receptors.     

Role of prolactin and glucocorticoids in the expression of casein genes in rabbit mammary gland organ culture. Quantification of casein mRNA

Milk synthesis is initiated solely by prolactin in the pseudopregnant rabbit and glucocorticoids potentiate this action of prolactin. In organ culture, prolactin, in the presence or in the absence of insulin, enhances casein synthesis and cortisol (inactive alone) amplifies this action. Measurements of casein mRNA concentration in total cellular RNA, by hybridization with DNA complementary to casein mRNA, revealed that the stimulation of casein synthesis by the glucocorticoid is accompanied by an increase in the amount of casein mRNA. A systematic comparison of variations of these two parameters indicated that the major effect of glucocorticoids on lactogenesis in the rabbit at this stage of mammary gland development is mediated through an increase in the quantity of casein mRNA available for translation. No simultaneous control of casein mRNA translation by cortisol was observed.     

Inhibition of ovarian function in subordinate female marmoset monkeys (Callithrix jacchus jacchus)

Plasma concentrations of progesterone, cortisol, LH and prolactin were measured in dominant and subordinate female marmosets in 10 well-established peer groups. Subordinate females never ovulated, had a reduced LH response to LH-RH and showed no positive feedback LH surge after oestrogen administration. There was no evidence of elevated plasma cortisol levels or hyperprolactinaemia in subordinates and all showed a similar prolactin response to TRH in comparison with dominants. However, subordinates showed a reduced prolactin response to metoclopramide. These results clearly indicate that high circulating levels of cortisol or prolactin are not responsible for the inhibition of ovulation in female marmosets.     

Clinical, biological and hormonal study of mid-pregnancy termination in cats with aglepristone

In order to evaluate the efficacy, the safety and the variation in plasma concentrations of estrogens, progesterone, PGFM, oxytocin, cortisol and prolactin after mid-pregnancy termination induced by aglepristone, 61 pregnant queens (33.3 + 4.2 days), were injected subcutaneously with 15 [corrected] mg/kg aglepristone, (Alizine) [corrected] repeated once 24 h later. Five queens served as control and received a placebo. The efficacy of aglepristone was 88.5% and termination of pregnancy was achieved in 50% of the queens within 3 days. Brief periods of depression and anorexia were noted in 9.3% of the queens before fetal expulsion (these symptoms were attributed to the phenomenon of fetal expulsions). Not one of the queens that aborted developed uterine disease. There were no changes in plasma concentrations of estrogen, prostaglandin, prolactin or oxytocin following aglepristone administration. However, there were significant increases in plasma concentrations of progesterone and cortisol 60 and 30 h, respectively, after aglepristone administration. Termination of pregnancy occurred with high plasma progesterone concentrations. Fetal expulsion was characterised by an increase in estrogen, PGFM and oxytocin concentrations, whereas prolactin and cortisol levels remained at a basal level.     

Prolactin-induced accumulation of casein mRNA in mouse mammary explants: A selective role of glucocorticoid

The role of glucocorticoid in the prolactin-induced accumulation of casein mRNA in mammary explants from midpregnant mice has been studied after an initial 4-day incubation to allow the level of messenger to decline to undetectable levels. Subsequent culture for 3 days: 1) with insulin and glucocorticoid did not result in detectable accumulation of messenger; 2) with insulin and prolactin resulted in a very small accumulation; 3) with insulin, glucocorticoid and prolactin elicited a 20-fold greater accumulation of casein mRNA than the system with only insulin and prolactin. Therefore, although glucocorticoids are not an absolute requirement for casein gene expression in mouse mammary tissue, they are necessary for massive accumulation of casein mRNA induced by prolactin. It appears that this dependence is not a result of either mRNA stabilization or alteration in prolactin receptors. By contrast, stimulation of total epithelial RNA synthesis by prolactin does not have this glucocorticoid dependency.     

Role of progesterone and oestradiol in the regulation of uterine oxytocin receptors in ewes.

The interaction between oestrogen and progesterone in the regulation of the uterine oxytocin receptor in sheep was evaluated by measuring the binding of oxytocin to membrane preparations of caruncular and intercaruncular endometrium and myometrium. Ovariectomized ewes were assigned in groups of five to each cell of a 4 x 2 factorial design. The four treatments were (a) vehicle (maize oil) for 12 days, (b) progesterone (10 mg day-1) for 9 days, (c) progesterone for 9 days followed by maize oil until day 12 and (d) progesterone for 12 days. The two oestradiol treatments consisted of the administration of implants in the presence or absence of oestradiol. The ewes were killed on day 10 (group b) or day 13 (groups a, c and d) for collection of uterine tissues. The response of the caruncular and intercaruncular endometrium to the treatments was similar. In the absence of oestradiol, treatment with progesterone continuously for either 9 or 12 days reduced the concentration of the oxytocin receptor in comparison with both the control and the progesterone withdrawal group (in which values were similar). The presence of oestradiol reduced the receptor concentrations in control and both 9- and 12-day continuous progesterone treatment groups, but enhanced the concentration in the progesterone withdrawal group. The myometrial oxytocin receptors responded in a similar way to those in the endometrium to progesterone treatment alone, but the addition of oestradiol produced no further effect. In conclusion, progesterone and oestradiol caused downregulation of the endometrial oxytocin receptor. On the other hand, progesterone withdrawal, similar to that which occurs during luteolysis, increased receptor density in the presence of oestradiol. Progesterone may influence the response of the myometrium to oxytocin by causing a reduction in receptor density.     

Effect of progesterone on the activation of neurones of the supraoptic nucleus during parturition

Parturition is driven by a pulsatile pattern of oxytocin secretion, resulting from burst firing activity of supraoptic oxytocin neurones and reflected by induction of Fos expression. Rats were injected with progesterone on day 20 of pregnancy to investigate the role of the decreasing progesterone:ratio oestrogen ratio, which precedes delivery, in the activation of supraoptic neurones. Progesterone delayed the onset of birth by 28 h compared with vehicle (control) and prolonged the duration of delivery, which was overcome by pulsatile injections of oxytocin, indicating that the slow delivery may reflect impaired oxytocin secretion. Parturient rats pretreated with progesterone had fewer Fos immunoreactive nuclei in the supraoptic nucleus than did parturient rats pretreated with vehicle. The number of Fos immunoreactive nuclei was not restored after oxytocin injection, indicating that appropriate activation of oxytocin neurones is impaired by progesterone and also that there is a lack of stimulatory afferent drive. Fos expression increased in the nucleus of the tractus solitarius during parturition in rats pretreated with either vehicle or progesterone, but not in rats that had been pretreated with progesterone and induced with oxytocin, indicating that this input was inhibited. Endogenous opioids inhibit oxytocin neurones in late pregnancy and the opioid antagonist, naloxone, increases Fos expression in supraoptic nuclei by preventing inhibition. However, progesterone attenuated naloxone-induced Fos expression in the supraoptic nucleus in late pregnancy and naloxone administered during parturition did not accelerate the duration of births delayed by progesterone administration, indicating that progesterone does not act by hyperactivation of endogenous opioid tone. RU486, a progesterone receptor antagonist, enhanced supraoptic neurone Fos expression in late pregnancy, indicating progesterone receptor-mediated actions. Thus, progesterone withdrawal is necessary for appropriate activation of supraoptic and tractus solitarius neurones during parturition.     

The effect of oestrogen on selective transfer of IgG1 into mammary secretion of ewes

Two separate experiments were conducted to examine the effects of exogenous oestrogen on selectivetransfer of IgG1 into mammary secretion of ewes. In one experiment, non-pregnant ewes were induced to lactate artificially by first developing mammary glands with injections of progesterone plus low doses of oestrogen then triggering milk secretion with either glucocorticoid or high doses of oestrogen. In the other experiment, lactating ewes were injected with oestrogen each day for 6 days. The results of the experiments suggest that oestrogen affects selective transfer of IgG1 into mammary secretion of the ewes. Moreover, the results show that, in the absence of high levels of oestrogen in blood, the magnitude of the selective transfer of IgG1 into mammary secretion is related inversely to the synthetic activity of the glandular epithelium.     

Evidence for a systemic role for ovarian oxytocin in luteal regression in sheep

Jugular venous concentrations of oxytocin and progesterone changed in parallel during the oestrous cycle in the ewe, falling at luteal regression and rising with formation of the new corpus luteum. These fluctuations in the circulating concentration of oxytocin were not caused by changes in its metabolic clearance rate. On Days 6-9 of the cycle circulating oxytocin concentrations exhibited a diurnal rhythm, peaking at 09:00 h; this rhythm was absent on Days 11-14. Although there was no evidence for increased production of oxytocin at or preceding luteal regression in samples taken daily, more frequent sampling revealed that two thirds of detected surges of uterine secretion of prostaglandin (PG) F-2 alpha were accompanied by raised levels of oxytocin. This oxytocin was not of pituitary origin. Luteal regression induced with cloprostenol on Day 8 after oestrus caused a decrease in circulating progesterone level followed after 24 h by a fall in oxytocin. Measurements of oxytocin in the ovary and other organs before and after treatment with cloprostenol identified the corpora lutea as a major potential source of oxytocin, and suggested that 98% of luteal oxytocin was available for secretion in response to prostaglandin stimulation. The data are consistent with a role for ovarian secretion of oxytocin in response to uterine release of PGF-2 alpha in the control of luteal regression.