Endometrial Oxytocin Receptor Research Articles

Up-regulation of endometrial oxytocin receptor is associated with the timing of luteolysis in heifers with two and three follicular waves†.

Initiation of luteolysis in ruminants is variable due to ill-defined mechanisms. Cycles of two follicular waves are shorter and have earlier luteolysis than three-wave cycles. This study validated a cytobrush technique for evaluating dynamics of endometrial gene expression and associated changes in mRNA with timing of luteolysis, based on circulating progesterone and ultrasound-determined changes in blood flow and volume of corpus luteum (CL). On day 8 (ovulation=day 0), Holstein heifers were randomized into two groups: cytobrush group (n=9) had an endometrial sample collected every 48h from day 8 until end of luteolysis (CL blood flow ≤ 20%) and control group was sampled only once either before (day 12; n=4) or at the end of luteolysis (n=5). Concentrations of progesterone, CL blood flow, CL volume, and the frequency of two and three-wave cycles were similar between groups. Endometrial mRNA for progesterone receptors and estradiol receptors 1 and 2 was greater on day 8 and decreased thereafter similarly in two and three-wave cycles. Oxytocin receptor mRNA increased earlier in two vs three-wave cycles (day 14 vs 18), and the increase was associated with the onset of luteolysis. In conclusion, the cytobrush technique allowed in vivo collection of multiple endometrial samples during the estrous cycle. Endometrial mRNA expression of steroid receptors did not explain the variability in timing of onset of luteolysis in heifers while the later onset of luteolysis in three-wave cycles was associated with later up-regulation of oxytocin receptor mRNA.

Characteristics and causes of the inadequate corpus luteum

An inadequate luteal phase occurs in domestic ruminants in several physiological situations (e.g. puberty, post partum), and also following the induction of ovulation in anoestrous ewes with GnRH treatment. The induced corpora lutea (CL) initially developed, but then regressed rapidly after Day 4, unless the animals had been primed with progesterone before GnRH therapy or hysterectomized. Significant increases in prostaglandin F-2 alpha metabolite (PGFM) secretion and coincident peaks of oxytocin and PGFM occurred around the time of premature regression. Endometrial oxytocin receptors were also detectable at this time in ewes which had abnormal luteal phases, but not in ewes which had been progesterone primed and thus had normal luteal phases. This suggests that the presence or otherwise of the oxytocin receptor during the early luteal phase may be crucial in determining whether the CL has a short or normal lifespan. These results show that an inadequate luteal phase is often caused by the premature induction of luteolysis and the presence of the endometrial oxytocin receptor in necessary for this occurrence. This receptor in turn is controlled by the steroid environment to which the uterus has previously been exposed.

The oxytocin receptor, luteolysis and the maintenance of pregnancy

During luteal regression episodic pulses of oxytocin secretion become coupled to the release of prostaglandin F2 alpha (PGF2 alpha) following synthesis of endometrial oxytocin receptors, but in early pregnancy the inhibition of oxytocin receptor formation by the conceptus prevents the development of the pulsatile pattern of PGF2 alpha release needed to achieve luteolysis. Oxytocin receptors are present on the luminal epithelium in ovariectomized and anoestrous ewes, in pregnant animals throughout most of gestation (day 21 to term) and in explants of endometrial tissue cultured in vitro. These receptors can be downregulated for a brief period by progesterone (10-12 days in sheep, 12-14 days in cattle). This period of inhibition can be extended by infusion of interferon tau (IFN-tau) (which probably inhibits oxytocin receptor gene transcription) or of oxytocin into the systemic circulation (which may act further downstream, possibly at the level of translation). Oxytocin receptors also develop on the caruncular stroma and deep glands at oestrus, but these need positive upregulation and appear dependent on an oestrogenic environment. Only epithelial receptors are needed to achieve a maximal PGF2 alpha response to an oxytocin challenge, but the presence of oxytocin receptors does not necessarily confer responsiveness as other factors may influence intracellular coupling mechanisms and precursor availability. The duration of the luteal phase is regulated by the time of the initial post-ovulatory rise in progesterone and the duration of exposure to progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular biology of trophoblast interferons and studies of their effects in vivo

Southern blotting of bovine genomic DNA indicated the presence of at least 3 bovine tIFN genes. The full DNA sequence of one of these genes, thought to be expressed in trophoblast, has been determined, including 193 bp of 5' non-coding region. The inferred amino acid sequence of bovine tIFN is more similar to ovine tIFN (80%) than to bovine IFN-alpha II (70%). The 5' flanking sequence has some similarity with bovine IFN-alpha II, and may contain a viral response element. A recombinant bovine alpha I interferon (Ciba Geigy; brIFN), resembling tIFN, extended oestrous cycle length in sheep when administered by intrauterine infusion over the period, Days 12-15 after oestrus, when maternal recognition of pregnancy occurs. Intramuscular injection was only effective at the doses used if given over a longer period (i.e. Days 9-15). Our experiments indicate that both tIFN and brIFN inhibit luteolysis by preventing a rise in endometrial oxytocin receptor concentrations, and suggest that tIFN achieves this by extending the time for which progesterone suppresses oxytocin receptor development. Further studies are required to confirm this hypothesis and to elucidate the interaction of the effects of progesterone and tIFN in endometrial cells.

Maintenance or regression of the corpus luteum during multiple decisive periods of bovinepregnancy

In ruminants, there are specific times during the estrous cycle or pregnancy when the corpus luteum (CL) may undergo regression. This review has attempted to summarize the physiological and cellular mechanisms involved in CL regression or maintenance during four distinct periods. The first period is near day 7 when animals that are ovulating after a period of low circulating progesterone (P4), such as first pubertal ovulation or first postpartum ovulation, are at risk of having a premature increase in Prostaglandin F2α (PGF) secreted from the uterus resulting in early CL regression and a short estrous cycle. The second period is when normal luteolysis occurs at day 18-25 of the cycle or when the CL is rescued by interferon-tau secreted by the elongating embryo. The uterine mechanisms that determine the timing of this luteolysis or the prevention of luteolysis have been generally defined. Induction and activation of endometrial E2 receptors result in induction of endometrial oxytocin receptors that can now be activated by normal pulses of oxytocin. Of particular importance is the observation that the primary mechanisms are only activated through local (ipsilateral) and not a systemic route due to transfer of PGF from the uterine vein to the ovarian artery. In addition at the CL level, studies are providing definition to the cellular and molecular mechanisms that are activated in response to uterine PGF pulses or pregnancy. The third period that is discussed occurs in the second month of pregnancy (day 28-60) when undefined mechanisms result in CL maintenance of an ipsilateral CL but regression of a contralateral (opposite side from pregnancy) CL. The final period that is discussed is regression of the CL just prior to parturition. Although, cortisol from the fetus appears to be the primary initiator of luteolysis, PGF seems to be the final signal that causes regression of the CL. Thus, in all four periods, regression of the CL is likely to be caused by the direct actions of PGF that is secreted from the uterus. The uterine mechanisms that result in secretion of PGF seem to be normally inhibited during the early luteal phase, making short luteal phases not a normal event, and are altered during early pregnancy (day 18-25) resulting in prevention of luteolysis. During much of pregnancy, the mechanisms that cause PGF secretion from the uterus in response to oxytocin are intact but luteolysis does not normally occur, perhaps due to lack of efficient utero-ovarian transfer of PGF.

Effect of pregnancy on endometrial expression of luteolytic pathway components in the mare

Endometrial oxytocin receptors (OXTR) and prostaglandin-endoperoxide synthase 2 (PTGS2) are central components of the luteolytic pathway in cyclic mares, and their suppression is thought to be critical to luteal maintenance during early pregnancy. We examined the effect of pregnancy on endometrial expression of potential regulators of prostaglandin (PG) F2α secretion in mares. Expression of the nuclear progesterone receptor and oestrogen receptor ERα was high during oestrus, and depressed when progesterone was elevated; the opposite applied to the membrane progesterone receptor. PTGS2 was upregulated on Day 14 of dioestrus, but not pregnancy. Although OXTR mRNA expression was not elevated on Day 14 of dioestrus, protein abundance was; this increase in OXTR protein was absent on Day 14 of pregnancy. Intriguingly, gene and protein expression for PTGS2 and OXTR increased markedly between Days 14 and 21 of pregnancy suggesting that, although initial avoidance of luteolysis during pregnancy involves their suppression, this is a transient measure that delays rather than abolishes luteolytic pathway generation. The only oxytocin-PGF2α feedback loop component downregulated on both Days 14 and 21 of pregnancy was the PGF2α receptor we propose that downregulation of the PGF2α receptor uncouples the oxytocin-PGF2α feedback loop, thereby preventing generation of the large PGF2α pulses required for luteolysis.

The Buserelin Enigma; How Does Treatment with this GnRH Analogue Decrease Embryo Mortality?

The use of the GnRH analogue buserelin administered in mid dioestrus has been shown to increase early pregnancy rate, reduce embryo loss and increase litter size in several species. The mechanism by which this works is unclear and may differ between species. In cattle it is believed that buserelin, by reducing oestrogen concentrations, causes the development of endometrial oxytocin receptors to be delayed. Luteal oxytocin binds with these receptors to promote the formation and secretion of prostaglandin. Any delay in the release of PGF2α would allow an underdeveloped embryo extra time to produce and release the maternal recognition of pregnancy signal, which might otherwise be lost following luteolysis. This may also be the mechanism in sheep and pigs although an increase in progesterone concentrations could also be important. However in the mare, following buserelin administration at 9 or 10 days after the detection of ovulation, pregnancy rates are already increased by 12-13 days compared with untreated controls, i.e. before the time of luteolysis, without any change in progesterone concentrations. In addition to an increased pregnancy rate, the twin embryo rate is also increased and subsequent embryo loss rate reduced. The possible mechanisms by which this may result in the mare are discussed.

The progesterone receptor antagonist, onapristone has differential effects on the timing and control of the luteolytic mechanism depending on timing of administration in sheep

Cyclic ewes were treated with control vehicle or progesterone receptor antagonist (onapristone; 100mg i.m. twice daily) during either early (day 3–5) or late (day 12–14) luteal phase and plasma samples collected for hormone analysis and to determine endogenous and oxytocin induced PGF2α release. On day 14 and 17, ewes were euthanised and reproductive tracts collected for ovarian morphology and endometrium for oxytoxin and steroid hormone receptor analysis. Early treatment increased LH, but not progesterone or oestradiol, while late treatment elevated all three hormones. Early treatment delayed the up-regulation of endometrial oxytocin receptors and responsiveness to oxytocin challenge, delaying luteolysis. Late treatment advanced development of oxytocin receptors and responsiveness to oxytocin though not timing of luteolysis. Patterns of hormone receptor mRNA were differentially disrupted by treatments. Results provide mechanistic insight into hormonal control of the oestrous cycle and identify the ability of the luteolytic mechanism to dissociate from functional luteolysis.

Effect of Administration of Oxytocin During Diestrus on Corpus Luteum Function and Endometrial Oxytocin Receptor Concentration in Cycling Mares

The objectives of this study were to (1) compare the effect of twice versus once daily administration of oxytocin on days 7-14 after ovulation on the duration of corpus luteum (CL) function and (2) determine the effect of oxytocin treatment on endometrial oxytocin receptor concentration in mares. In experiment 1, mares were randomly assigned to three groups on day 7: (1) untreated control group (n = 7), (2) twice daily oxytocin treatment group (n = 7), and (3) once daily oxytocin treatment group (n = 8). Oxytocin-treated mares received 60 U of oxytocin intramuscularly (IM) the respective number of times each day on days 7 through 14. One of seven control mares (14%), five of seven (71%) twice daily oxytocin-treated mares, and five of eight (63%) once daily oxytocin-treated mares had prolonged CL function. There was no significant difference in the proportion of mares with prolonged CL function between the two oxytocin-treated groups, and collectively, oxytocin treatment increased (P < .05) the proportion of mares with prolonged CL function compared with no treatment. In experiment 2, mares were randomly assigned to two groups (n = 5/group): (1) saline-treated control mares, and (2) oxytocin-treated mares. Beginning on day 7, control mares received 3 mL of sterile saline IM twice daily, and oxytocin-treated mares received 60 U of oxytocin IM twice daily through day 14. On day 15, endometrial oxytocin-binding capacity was determined (as a measure of oxytocin receptor concentration), and there was no difference (P > .1) between control and oxytocin-treated mares (1,465.7 ± 108 and 1,382.8 ± 108 fmol/mg protein [mean ± standard error of mean], respectively).

Timing of prostaglandin F 2α release episodes and oxytocin receptor development during luteolysis in the cow

The timing of PGF 2α release and the timing and extent of the rise in endometrial oxytocin receptors was determined in relation to the timing of the progesterone fall during luteolysis in cycling cows. In cows undergoing luteolysis ( n = 6), measurement of PGF 2α metabolite in hourly plasma samples collected during daily 10 h sampling periods identified a total of 2.2 ± 0.5 PGF 2α release episodes per animal, each of 4.0 ± 0.4 h duration. In cows in which luteolysis was not observed ( n = 4) no PGF 2α release episodes were identified. In a further three cows in which additional repeated uterine biopsies were collected on days 15, 17, 19, 21 and 23, endometrial oxytocin receptors were initially undetectable (<15 fmol/mg protein) but had increased to 120 ± 19 fmol/mg protein prior to the initiation of PGF 2α release episodes. Receptor concentrations then continued to increase reaching peak concentrations of 651 ± 142 after luteolysis had been completed.

Endometrial Oxytocin Receptor Concentration and Activity in Prepubertal Ewe Lambs

We have measured endometrial oxytocin receptor concentrations during prepuberty in ewe lambs, and have investigated the effect of progesterone on the activity of these receptors. In the first study, oxytocin receptor concentrations were undetectable in 2-week prenatal lambs but had increased immediately following birth and were then maintained throughout prepubertal life. Despite the presence of oxytocin receptors animals showed no prostaglandin F(2alpha) (PGF(2alpha)) release in response to exogenous oxytocin challenge at either 3 or 5 months of age. In a second study in 4-month-old ewe lambs treatment with exogenous progesterone resulted in the appearance of PGF(2alpha) release in response to oxytocin after 10 days of treatment. Thus, during the prepubertal life, ewe lambs possess the prerequisites of a luteolytic mechanism in that they have a dormant population of oxytocin receptors in which progesterone can induce oxytocin-stimulated PGF(2alpha) release.

Oxytocin (OT) action in uterine tissues of cyclic and early pregnant gilts: OT receptors concentration, prostaglandin F 2α secretion, and phosphoinositide hydrolysis

Oxytocin (OT) is involved in the regulation of luteolysis in pigs. However, it is still not clear if OT is responsible for initiation of luteal regression in this species. The objectives of the study were: (1) to compare OT receptors (OTr) concentrations in endometrium and myometrium of cyclic and early pregnant pigs, (2) to examine the effect of OT on plasma PGF 2α secretion during the progressive luteal regression, (3) to ascertain the effect of OT on inositol phosphates (IPs) accumulation in endometrial and myometrial cells of cyclic and early pregnant pigs. Concentrations of OTr on the endometrium and myometrium of cyclic ( n = 33) (days 2–4; 11–13; 14–16; 18–20; day 21) and early pregnant ( n = 4) (days 14–16) gilts were determined and they ranged from 7 ± 3 (days 11–13) to 377 ± 113 fmol/mg protein (day 21) in the endometrium and from 33 ± 11 (days 2–4) to 167 ± 28 fmol/mg protein (days 18–20) in the myometrium. In both tissues, concentrations of OTr were low during the luteal phase and increased ( P < 0.01) during the follicular phase. In contrast to myometrial OTr, endometrial OTr during pregnancy were undetectable. In next experiment, mature gilts ( n = 12) were injected with OT (20 IU; i.v.) for three consecutive days starting on days 14 and 15 of the oestrous cycle and plasma PGF 2α metabolite—13,14-dihydro-16-keto PGF 2α (PGFM) concentration was determined. On days 15–16 and 16–17, OT increased plasma PGFM level. This effect was not observed on days 14–15 of the estrous cycle. A negative correlation was noticed between plasma concentrations of PGFM and progesterone ( r = −0.3; P < 0.05). In last experiment, OT (100 nM) augmented ( P < 0.01) an accumulation of inositol phosphates (IPs) in isolated myometrial cells on days 14–16 ( n = 4) and 18–20 ( n = 3) of the estrous cycle and on days 14–16 ( n = 4) of pregnancy. Oxytocin-stimulated accumulation of IPs was not observed in endometrial cells. In summary: (1) concentrations of OTr on both the endometrium and myometrium were the highest during perioestrus-period in pigs, (2) myometrium of early pregnant sows possessed functional OTr, (3) oxytocin increased plasma PGFM concentration after initiation of luteolysis; and (4) OT-stimulated accumulation of IPs in myometrial, but not in endometrial cells. In conclusion, OT appears to not be involved in the initiation of luteal regression in sows and functional OTr are still present in the myometrium during early pregnancy (days 14–16).

Hormonal regulation of oxytocin-induced prostaglandin F 2α secretion by the bovine and ovine uterus in vivo

In long-term ovariectomized ewes and cows, endometrial oxytocin receptors rest at relatively high levels but oxytocin is unable to induce prostaglandin F 2α release. A series of studies were carried out to investigate the roles of physiological levels of progesterone and estradiol in “activating” these receptors in terms of permitting oxytocin-induced prostaglandin F 2α release. In long-term ovariectomized cows, treatment with progesterone, but not estradiol, resulted in the induction of responsiveness to oxytocin. This responsiveness appeared within 2 d of progesterone treatment, reached a maximum by 6 d and was maintained to Day 18. In ovariectomized ewes, while estradiol treatment did induce temporary responsiveness to oxytocin after 3 d of treatment, treatment with progesterone was required to induce sustained responsiveness that appeared by Day 9 of treatment and was maintained to Day 12. Measurement of endometrial receptors for oxytocin revealed a significant decline in oxytocin receptors by Day 6 of progesterone treatment when responsiveness to oxytocin was maximal, demonstrating that receptor concentrations were not a limiting factor. The most likely mechanism by which progesterone treatment induces responsiveness to oxytocin may be through the up regulation of post receptor signaling pathways and/or enzymes involved in prostaglandin synthesis.

Luteal Deficiency and Embryo Mortality in the Mare

Four separate components combine to produce the progesterone and biologically active 5 alpha-reduced pregnanes needed to maintain pregnancy in the mare. The primary corpus luteum (CL) is prolonged beyond its cyclical lifespan by the down-regulation of endometrial oxytocin receptors to prevent activation of the luteolytic pathway and its waning progesterone production is supplemented from day 40 of gestation by the formation of a series of accessory CL which develop in the maternal ovaries as a result of the gonadotrophic actions of pituitary FSH and the equine chorionic gonadotrophin (eCG). From around day 100 the allantochorion secretes progesterone and progestagens directly to the endometrium and underlying myometrium and, in the last month of gestation, the enlarging foetal adrenal gland secretes appreciable quantities of pregnenelone which is also utilized by the placenta to synthesize progestagens. Between 10 and 15% of mares undergo foetal death and abortion at some time in gestation and the majority of these losses occur during the first 40 days of gestation when the primary CL is the sole source of progesterone. Yet, all the available evidence suggests that untoward luteolysis is not common in this period and the losses that do occur have other underlying causes. Beyond day 40 the secondary CL receive powerful luteotrophic support from eCG and from day 80-100 until term the supply organ (placenta) and target tissues (endometrium and myometrium) are in direct contact with each other over their entire surface. In the face of this interlocking and failsafe system for progestagen production throughout pregnancy, and despite a paucity of evidence that a deficiency of progesterone production is a cause of pregnancy loss in the mare, it is surprising, and worrying, that annually many thousands of pregnant mares throughout the world are given exogenous progestagen therapy during part or all of their gestation as a form of preventative insurance against the possibility of pregnancy failure. Basic investigative research is required urgently to validate or debunk the practice.

The effects of lipopolysaccharide and interleukins-1alpha, -2 and -6 on oxytocin receptor expression and prostaglandin production in bovine endometrium.

Up-regulation of endometrial oxytocin receptor (OTR) expression followed by an increase in pulsatile endometrial prostaglandin (PG) F(2alpha) secretion causes luteolysis in cattle. Inhibition of luteolysis is essential for the maternal recognition of pregnancy but also occurs in association with endometritis. The factors regulating OTR expression at this time are unclear. The OTR gene promoter region contains binding elements for acute phase proteins but their function has not been established. This study investigated the effects of various cytokines on OTR expression and on PGF(2alpha) and PGE(2) production in explant cultures of bovine endometrium. Endometrium was collected in the late luteal phase (mean day of cycle 15.4+/-0.50) or early luteolysis (mean day of cycle 16.4+/-0.24) as determined by the initial concentration of endometrial OTR. Explants were treated for 48 h with: (i) lipopolysaccharide (LPS) and/or dexamethasone (DEX), (ii) ovine interferon-tau (oIFN-tau), or (iii) human recombinant interleukin (IL)-1alpha, -2 or -6. OTR mRNA was then measured in the explants by in situ hybridisation and the medium was collected for measurement of PGF(2alpha) and PGE(2) by RIA. LPS treatment stimulated production of PGF(2alpha), whereas DEX either alone or in combination with LPS was inhibitory to both PGF(2alpha) and PGE(2). Neither of these treatments altered OTR mRNA expression. oIFN-tau reduced OTR mRNA expression but stimulated production of both PGF(2alpha) and PGE(2). In endometrial samples collected in the late luteal phase, IL-1alpha, -2 and -6 all inhibited OTR mRNA expression, but IL-1alpha and -2 both stimulated PGF(2alpha) production. In contrast, when endometrium was collected in early luteolysis, none of the interleukins altered OTR expression or caused a significant stimulation of PGF(2alpha) production but IL-2 increased PGE(2). Neither IL-1alpha nor -2 altered OTR promoter activity in Chinese hamster ovary cells transfected with a bovine OTR promoter/chloramphenicol acetyl transferase reporter gene construct. In conclusion, the action of interleukins on both OTR mRNA expression and endometrial prostaglandin production alters around luteolysis. Pro-inflammatory interleukins suppress OTR expression in the late luteal phase, while LPS stimulates PGF(2alpha) without altering OTR mRNA expression. IL-I and -2 and LPS are therefore unlikely to initiate luteolysis but may cause raised production of PGF(2alpha) during uterine infection.

Regulation of Interferon-.TAU. Gene Expression and the Maternal Recognition of Pregnancy.

In order for pregnancy to be established, the maternal system must recognize the presence of a conceptus(es) within the uterus. This physiological phenomenon, which subsequently causes the extension of the corpus luteum (CL) life-span, is known as maternal recognition of pregnancy. In ruminant ungulates, interferon-tau (IFNτ), produced by the conceptus, binds to the receptor located in the maternal endometrium in a paracrine manner. Its binding then affects levels of endometrial oxytocin receptor and estrogen receptor, which reduces pulsatile secretion of a luteolysin, prostaglandin F2α, resulting in the extension of CL function. Production of IFNτ seems to parallel the degree of trophoblast elongation and as the conceptus begins its attachment to the endometrium, IFNτ levels decline rapidly. Involvement of IFNτ in the process of implantation has long been suspected, but a role for IFNτ other than CL extension or transcriptional regulation of IFNτ genes has not been elucidated. In fact, several transcription factors that regulate the expression of IFNτ have been found, but none could explain its unique regulation. Recent findings related to maternal recognition of pregnancy are presented herein and a new insight into the potential regulation of IFNτ is discussed.

Oestradiol regulation of oxytocin receptor expression in cyclic bovine endometrium

Oestradiol treatment can increase uterine oxytocin receptor expression in vivo. The actions of oestrogen are usually mediated via its receptor, but it also has direct non-genomic effects in some cells. This study investigated the effect of oestradiol and the role of the oestradiol receptor in regulating endometrial oxytocin receptor expression in the bovine uterus. Explant cultures (in triplicate) from late luteal phase non-pregnant endometrium received the following treatments: control (serum-free medium), oestradiol (0. 1 and 0.01 micromol l(-1)), oestradiol (0.1 micromol l(-1)) with the oestradiol receptor antagonist ICI 182780 (0.5 micromol l(-1)), and ICI 182780 (0.5 micromol l(-1)) alone. Explants were collected 12, 24 and 48 h after the start of culture. Oxytocin receptor mRNA expression in the explants was measured by in situ hybridization and oxytocin protein concentrations were measured by autoradiography with the iodinated oxytocin receptor antagonist d(CH(2))(5) [Tyr (Me)(2) Thr(4) Tyr NH(2)(9)]-vasotocin ((125)I-labelled oxytocin receptor antagonist). Oxytocin receptor mRNA and protein expression were initially low but spontaneous upregulation occurred in the luminal epithelium between 24 and 48 h (P < 0.01). Oestradiol increased oxytocin receptor mRNA upregulation in the first 24 h (P < 0.05) but the effect on 125 I-labelled oxytocin receptor antagonist binding was not significant. ICI 182780 inhibited the oestrogenic effect but had no significant effect on oxytocin receptor mRNA expression when given alone. In conclusion, the results showed that oestradiol exerts its effect via the oestradiol receptor. Oestradiol facilitates oxytocin receptor gene transcription by increasing it more rapidly, but spontaneous upregulation of endometrial oxytocin receptor still occurs in the absence of oestradiol.

Regulation of the Oxytocin Receptor in Bovine Reproductive Tissues and the Role of Steroids

ContentsThe oxytocin receptor (OTR) plays a central role in the functioning of the reproductive tract in the female ruminant. During the oestrous cycle endometrial OTR is suppressed through most of the cycle, probably in a progesterone‐dependent manner, to be massively up‐regulated during the few days before and after oestrus. At this time, luteal OT triggers a positive feedback loop, the endometrial OTR mediating the release of luteolytic PGF2α which feeds back to the corpus luteum to cause the secretion of more OT and to induce luteolysis. This feedback is only interrupted if a newly formed blastocyst is present. Ruminant blastocysts secrete the cytokine interferon‐τ, which interacts directly with the endometrial epithelial cells, preventing the up‐regulation of OTR in late luteal phase. During early pregnancy the levels of OTR in endometrium remain very low whereas myometrial OTR are less affected. OTR in endometrium are up‐regulated around day 50 and then progressively increase to reach a maximum at the onset of labour; those in the myometrium are upregulated somewhat later in gestation, around day 100. The OTR in the cervix are differently regulated: their concentrations are very low through most of the cycle and gestation and do not increase until pro‐oestrus and at parturition when they abruptly rise to high values. Ligand binding to the mucosal and endometrial OTR causes specific release of prostaglandins, which in turn activate paracrine pathways in the surrounding tissues to induce softening of the connective tissue of the uterine wall during pregnancy, and of the cervix at term. Additionally, local uterine contractions induced by OT and potentiated by prostaglandins may serve a physiological function during pregnancy by improving uterine blood flow. Due to the appearance of gap junctions in myometrium at term the OT‐induced contractions become synchronized and well propagated and because of concomitant cervical softening the contractions become expulsive. Although OTR‐dependent physiology correlates well with the levels of circulating gonadal steroids, there is no evidence for any direct effect of steroids either on the OTR gene or OTR protein. Instead, progesterone appears to exert an indirect inhibitory effect on the OTR gene, probably mediated via other cell types. This is in contrast to the inhibition by interferon‐τ which appears to activate a pathway which directly suppresses the OTR gene expression. The important role of OTR in female uterine physiology, and its value as an indicator molecule, make it imperative to understand the complex molecular mechanisms underlying its regulation. These are likely to be of fundamental importance for many aspects of uterine function both in ruminants and in other species, including the human.

Suppression of oxytocin-induced prostaglandin F 2α release after intra-uterine nordihydroguariaretic acid administration in ewes

Intrauterine administration of the 5-lipoxygenase inhibitor nordihydroguariaretic acid (NDGA; 5 mg, bid) on Days 9–14 of the ovine estrous cycle (estrus = Day 0) delayed luteolysis and extended the duration of the estrous cycle (20 ± 1, SD, vs. 16 ± 1 days; P < 0.01). In control ewes, plasma concentrations of 13,14,dihydro-15-keto prostaglandin F 2α increased significantly ( P < 0.001) following i.v. administration of oxytocin (10 i.u.) on Day 14; in the nordihydroguariaretic acid-treated ewes, however, there was no such increase. In addition, concentrations of endometrial oxytocin receptors were significantly less ( P < 0.01) in the nordihydroguariaretic acid-treated ewes (218 ± 60 vs. 579 ± 66 fmol/mg tissue). These results suggest that 5-lipoxygenase products of arachidonate metabolism may be involved in the control of ovine luteal function.

Luteolysis: a neuroendocrine-mediated event.

In many nonprimate mammalian species, cyclical regression of the corpus luteum (luteolysis) is caused by the episodic pulsatile secretion of uterine PGF2alpha, which acts either locally on the corpus luteum by a countercurrent mechanism or, in some species, via the systemic circulation. Hysterectomy in these nonprimate species causes maintenance of the corpora lutea, whereas in primates, removal of the uterus does not influence the cyclical regression of the corpus luteum. In several nonprimate species, the episodic pattern of uterine PGF2alpha secretion appears to be controlled indirectly by the ovarian steroid hormones estradiol-17beta and progesterone. It is proposed that, toward the end of the luteal phase, loss of progesterone action occurs both centrally in the hypothalamus and in the uterus due to the catalytic reduction (downregulation) of progesterone receptors by progesterone. Loss of progesterone action may permit the return of estrogen action, both centrally in the hypothalamus and peripherally in the uterus. Return of central estrogen action appears to cause the hypothalamic oxytocin pulse generator to alter its frequency and produce a series of intermittent episodes of oxytocin secretion. In the uterus, returning estrogen action concomitantly upregulates endometrial oxytocin receptors. The interaction of neurohypophysial oxytocin with oxytocin receptors in the endometrium evokes the secretion of luteolytic pulses of uterine PGF2alpha. Thus the uterus can be regarded as a transducer that converts intermittent neural signals from the hypothalamus, in the form of episodic oxytocin secretion, into luteolytic pulses of uterine PGF2alpha. In ruminants, portions of a finite store of luteal oxytocin are released synchronously by uterine PGF2alpha pulses. Luteal oxytocin in ruminants may thus serve to amplify neural oxytocin signals that are transduced by the uterus into pulses of PGF2alpha. Whether such amplification of episodic PGF2alpha pulses by luteal oxytocin is a necessary requirement for luteolysis in ruminants remains to be determined. Recently, oxytocin has been reported to be produced by the endometrium and myometrium of the sow, mare, and rat. It is possible that uterine production of oxytocin may act as a supplemental source of oxytocin during luteolysis in these species. In primates, oxytocin and its receptor and PGF2alpha and its receptor have been identified in the corpus luteum and/or ovary. Therefore, it is possible that oxytocin signals of ovarian and/or neural origin may be transduced locally at the ovarian level, thus explaining why luteolysis and ovarian cyclicity can proceed in the absence of the uterus in primates. However, it remains to be established whether the intraovarian process of luteolysis is mediated by arachidonic acid and/or its metabolite PGF2alpha and whether the central oxytocin pulse generator identified in nonprimate species plays a mediatory role during luteolysis in primates. Regardless of the mechanism, intraovarian luteolysis in primates (progesterone withdrawal) appears to be the primary stimulus for the subsequent production of endometrial prostaglandins associated with menstruation. In contrast, luteolysis in nonprimate species appears to depend on the prior production of endometrial prostaglandins. In primates, uterine prostaglandin production may reflect a vestigial mechanism that has been retained during evolution from an earlier dependence on uterine prostaglandin production for luteolysis.