Oxytocin plays an important role at parturition due to its involvement in uterine contractions, foetal expulsion and the onset of maternal behaviour. The role of the related neurohypophysial hormone, vasopressin, is less clear; however, there is some evidence that it is also involved in maternal behaviour and its role in osmotic regulation is well established. The aim of this study was to investigate the inhibitory effects of endogenous opioids on these hormones during the expulsive phase of parturition in the pig, and to examine how opioid restraint interacts with environmental restriction. The subjects of this study were 31 Large Whitex Landrace primiparous sows (gilts). An indwelling jugular catheter was implanted under general anaesthesia at 12 days before the expected parturition day (EPD). From 5 days before the EPD 15 of the gilts were individually housed in a restrictive parturition crate without straw and 16 were individually housed in a straw-bedded pen. Blood samples were taken with increasing frequency towards and during parturition through a catheter extension to reduce disturbance. At 7.5 min after the birth of the first piglet half of the gilts in each environment received a dose of the opioid receptor antagonist naloxone (1 mg/kg, i.v.) with the remaining gilts receiving saline as a control. Overall, there was no effect of environment on either circulating oxytocin or vasopressin. However, both oxytocin and vasopressin were inhibited by endogenous opioids during the expulsive phase. The inhibitory effects of opioids on these hormones did not appear to have any adverse effects on the progress of parturition as judged by cumulative piglet birth intervals. The regulation of the opioid inhibition of oxytocin and vasopressin during parturition is discussed in relation to other neurotransmitters and whether opioid inhibition of these neurohypophysial hormones is part of the 'normal' physiological response to parturition or whether it is stress-induced.

The present study was designed to investigate a possible role for ovarian steroids in the regulation of rat uterine oxytocin receptor (OTR) mRNA expression before labour. By using a competitive RT-PCR system, we have previously reported that parturition was associated with high levels of uterine OTR mRNA in all the animals examined. On the other hand, near term, some rats showed high OTR mRNA levels while others did not. We therefore examined the changes in OTR mRNA expression before and during prostaglandin F(2)(alpha) (PGF(2)(alpha))-induced parturition; a paradigm adopted to reduce the variation in the onset of parturition. Injection of PGF(2)(alpha) on day 18 of pregnancy significantly increased OTR mRNA expression in all the rats within 24 h of treatment, suggesting that the variation in OTR mRNA levels during spontaneous parturition may be due to the difference in the timing of the onset of parturition. The increase in OTR mRNA was significantly abolished by injection of the anti-oestrogen compound, tamoxifen. The stimulatory action of oestrogen on OTR mRNA expression was then examined in the presence or absence of ovarian factors. Pregnant rats were ovariectomized (OVX) or sham-operated on day 18 of pregnancy and either oestrogen or vehicle was administered 6 h after the surgical operation. Oestrogen increased OTR mRNA significantly in OVX rats 18 h after administration compared with sham-operated animals. Moreover, ovariectomy alone on day 18 of pregnancy increased OTR mRNA expression to a level which reached statistical significance 24 h after the operation. In addition, oestrogen treatment increased OTR mRNA levels in OVX virgin rats in which progesterone tubes were implanted for 1 week and removed 6 h before oestrogen injection. The stimulatory effect of oestrogen was not observed in rats in which the progesterone tubes were implanted for 1 week and not removed. These results suggest that the decline of progesterone is necessary for the expression of the stimulatory effects of oestrogen on uterine OTR mRNA.

Oxytocin (OT) is present in the mammalian testis and has been shown to play a role in the modulation of seminiferous tubule contractility and steroidogenesis. However, stage-specific effects of the peptide have not been previously investigated. In this study, computer-assisted analysis and time-lapse videomicrography were used to investigate basal contractility and the response to OT of seminiferous tubules at specific stages of the spermatogenic cycle. Adult rat testes were placed in fresh oxygenated DMEM F12 medium, decapsulated, and the tubules gently teased apart. Stages were identified by transillumination and a 10 mm section of tubule at each of stages IV-V, VII-VIII and XIII-I was placed in a microslide chamber and perifused with medium. After a control period of 3 h, OT (2 nM) was given for 1 h, followed by another control period of 1 h. The experiment was repeated using tubules from different rats and data were analysed to give arbitrary units of tubule contractility. Contractility was observed in all the tubules studied and the contractile activity was shown to vary depending on the stage of the spermatogenic cycle. Mean basal contractility at stages VII-VIII, the time when sperm are shed from the epithelium, was significantly lower than that at stages IV-V and XIII-I. The response of the tubules to OT was also stage-dependent, with the peptide producing the largest increases in contractile activity at stages VII-VIII and having no effect at stages IV-V. We postulate that these stage-specific differences in basal and OT-stimulated contractility may be important in co-ordinating the movement of developing germ cells towards the lumen of the seminiferous epithelium and in the process of spermiation.

The aims of this study on porcine ovarian granulosa cells were to examine the effect of GH on oxytocin (OT), IGF-I and IGF-I receptors, IGF-binding protein-3 (IGFBP-3), progesterone and prostaglandin E (PGE), as well as to determine whether IGF-I and/or OT may be mediators of GH action. The cells were cultured either with porcine GH (pGH) (1 ng/ml to 10 microg/ml or 100 ng/ml only), antiserum against IGF-I (0.1%), antiserum against OT (0.1%) or a combination of GH (10 ng/ml) with antiserum against IGF-I or antiserum against OT (0.1%). The secretion of IGF-I, OT, IGFBP-3, progesterone and PGE was determined using RIA/IRMA, whilst the IGF-I binding sites were measured using a radioreceptor assay. It was observed that pGH increased the secretion of IGF-I and the abundance of IGF-I binding sites in granulosa cells. Furthermore, GH inhibited OT release, stimulated progesterone and PGE output, but had no significant effect on IGFBP-3 secretion. Immunoneutralization of IGF-I by antiserum against IGF-I inhibited PGE secretion, but it did not influence progesterone or IGFBP-3 secretion. Binding of OT by antiserum suppressed IGFBP-3, PGE, but not progesterone secretion. Neither immunoneutralization of IGF-I nor OT substantially prevented the effects of GH on progesterone, IGFBP and PGE. These observations demonstrate the involvement of GH, IGF-I and OT in the control of porcine ovarian secretory activity and the ability of GH to regulate IGF-I and OT production and IGF-I reception. Nevertheless, lack of correlation between the effects of GH, antiserum against IGF-I and antiserum against OT, as well as the inability of blockade of IGF-I or OT to prevent the effects of GH, suggests that IGF-I and OT, despite their dependence on GH, do not mediate GH action on ovarian cells.

The expression of oxytocin receptor (OTR) in the uterine endometrium plays an important role in the initiation of luteolysis. During early pregnancy, the conceptus secretes interferon tau (IFN|gt) which inhibits OTR up-regulation and luteolysis. In this study, uterine horn cross sections were collected on day 16 from 15 pregnant cows (PREG), 9 uninseminated controls and 5 inseminated cows with no embryo present. The latter two groups had similar results and were combined to form a single non-pregnant (NP) group. The animals were given an oxytocin challenge shortly before tissue collection to assess prostaglandin F2alpha (PGF2alpha) release through the measurement of the metabolite 13,14-dihydro-15-keto PGF2alpha (PGFM). The mRNAs for OTR, oestrogen receptor (ER) and progesterone receptor (PR) were localised by in situ hybridisation. The results were quantified by optical density (OD) measurements from autoradiographs using image analysis. OTR protein was measured by autoradiography with iodinated oxytocin antagonist and ER and PR protein was detected by immunocytochemistry. The release of PGFM after the oxytocin challenge was significantly higher in the 14 NP cows (187%+/-15%) compared with the PREG group (131%+/-11%) (P<0.01). Low concentrations of OTR mRNA were localised to the luminal epithelium (LE) in 6 out of the 14 NP cows, of which 2 also expressed OTR protein, while OTR mRNA and protein were undetectable in all the pregnant animals. These results indicated that the sampling time coincided with the onset of the luteolytic mechanism in the NP cows. On day 16 ER mRNA was detectable in both the LE and glands of both PREG and NP animals. There were no differences in either ER mRNA or protein between NP and PREG samples. PR mRNA was moderately expressed in the caruncular stroma, with lower levels in the dense caruncular-like stroma and glands. There were no differences between PREG and NP animals. The expression of PR mRNA and protein in the deep glands was variable between animals. These results suggested that, in cows, the presence of an embryo suppressed the expression of OTR, but had no effect on the expression of the transcriptionally regulated ER on day 16.

Oxytocin receptor (OTR) mRNA expression has previously been demonstrated in human myometrium, decidua, chorion and amnion but the effect of gestational age and the onset of labour has not been determined in these individual tissues. Spatial OTR mRNA expression was examined by in situ hybridization and ligand binding was confirmed using autoradiography with the iodinated oxytocin antagonist d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH29]-vasotocin (125I-OTA). Tissue was collected at term (>37 weeks of gestation) or preterm (24-36 weeks of gestation) caesarean section and classified as labour (contractions every 5 min associated with cervical dilatation) or non-labour. OTR mRNA expression was measured as optical density units from autoradiographs. There was a highly significant (P<0.001) effect of tissue type on expression of OTR mRNA with expression greatest in myometrium, low in decidua and chorion and not detected in placenta. Similar results were obtained with the 125I-OTA-binding studies, indicating that the message was translated. Amnion had an apparently high level of both hybridization and 125I-OTA binding in some samples, but a lack of specificity prevented quantification of the signal in this tissue type. Term myometrium (labour and non-labour) had significantly higher (P<0.01) OTR mRNA expression than preterm myometrium, but there was no further increase in mRNA concentration associated with labour onset. In contrast, 125I-OTA binding in myometrium was already high at 33 weeks and did not increase further either later in pregnancy or with labour. In decidua there was no effect of gestational age or labour onset on OTR mRNA expression or 125I-OTA binding. In summary, OTR mRNA expression in the myometrium increased in late pregnancy whereas decidual expression was much lower and did not rise at term.

The hormonal regulation of uterine oxytocin receptors (OTR) during the establishment of pregnancy and at parturition has been studied extensively, but little information is available during mid-pregnancy. This study investigated the localisation of OTR mRNA in the ovine placentome throughout gestation and related this to expression patterns for the putative regulatory agents aromatase, oestradiol receptor, progesterone receptor and oxytocin. Placentomes were collected at regular intervals throughout pregnancy for in situ hybridisation analysis and immunocytochemistry (oestradiol receptor only). Results were quantified by optical density measurements of autoradiographs. Progesterone receptor mRNA was localised to the caruncular tissues on day 30 but became undetectable by day 34. Aromatase mRNA appeared in the fetal villi at days 34-40, with concentrations peaking at days 52-55 and again at days 132-137. Oestradiol receptor mRNA was localised to the caruncular tissues from days 13 to 30 and found in the maternal villi and placentome capsule from days 45 to 70. Oestradiol receptor protein was barely detectable in either tissue. OTR mRNA was localised to the placentome capsule at days 34-40, remaining high at day 45 and declining to basal levels by days 132-137. Oxytocin mRNA was not detected in the placentome. In conclusion: (1) progesterone acting via its receptor may suppress the expression of aromatase and OTR in early pregnancy; (2) the up-regulation of OTR expression in the capsule may not involve the oestradiol receptor; (3) there is a differential regulation between different regions of the uterus as the increase in the placentome capsule occurs at a time when concentrations in the rest of the endometrium and myometrium remain low; (4) oestradiol receptor expression in the placentome may be regulated at the translational level; and (5) there is no local production of oxytocin in the sheep placenta. The role of ORTs in the capsule during mid-pregnancy remains to be determined.

In the pig, nest building occurs in the day preceding parturition (gestation=114--116 days). Nest building behaviour can be induced in pregnant, pseudopregnant and cyclic female pigs following injection of prostaglandin F2alpha. Here we investigated behaviour and endocrine changes after the administration of indomethacin, which inhibits cyclo-oxygenase enzymes and thus prostaglandin synthesis. In experiment 1, pregnant primiparous pigs (gilts) were blood sampled through jugular vein catheters every 20 min from 1000 h on day 113 of pregnancy and behaviour was recorded until birth. Two hours after pre-partum nest building began, animals received 4 mg/kg indomethacin (n=7) or control vehicle (n=8) intramuscularly. Indomethacin-treated animals showed less nest building than controls between 1 and 5 h after injection (P<0.05), during which time they were mostly inactive and lay down for longer than controls. From 5 h before birth until birth there was no significant treatment difference in nest building behaviour. There was a tendency for the start of birth to be delayed in indomethacin-treated animals. Plasma 13,14-dihydro-15-keto-prostaglandin F2 alpha (a major metabolite of prostaglandin F2 alpha) rose during pre-injection nest building and then fell following indomethacin treatment, but was not significantly different between groups when behaviour differed. Plasma oxytocin, cortisol and progesterone were not significantly affected by treatment. In experiment 2, indomethacin-treated non-pregnant gilts (n=7) did not show any changes in activity or posture compared with vehicle-treated controls (n=6) between 90 and 150 min after treatment. These results suggested that indomethacin treatment reversibly and specifically inhibits porcine pre-partum nest building by a mechanism that may involve endogenous prostaglandin F2 alpha synthesis inhibition but is independent of circulating oxytocin, cortisol and progesterone concentrations.

The precise regulatory mechanisms of cyclic oviductal contraction in the cow are unclear. The purpose of this study was to evaluate the effect of luteinizing hormone (LH), steroids, prostaglandins (PGs) and peptides on the oviductal contraction and secretion of PGs and endothelin (ET-1). In addition, the cyclic expression of mRNA for ET-1 and its receptors (ET-R) was evaluated by reverse transcription-polymerase chain reaction (RT-PCR). In the in vitro microdialysis study, an infusion of LH alone or in combination with progesterone (P(4)), estradiol-17beta (E(2)) and/or ET-1 stimulated pronounced release of PGE(2), PGF(2alpha) and ET-1 in the oviducts from cows in the follicular and postovulatory phases. The addition of LH, LH+P(4)+E(2) and/or ET-1 to the medium increased the amplitude of oviductal contraction. However, oxytocin (OT) completely blocked the responses of oviductal secretion and contraction. In contrast, these substances did not show any effect in the oviducts from cows in the mid luteal phase. Similar expression patterns of mRNA encoding for ET-R type A and type B were found, which were highest during the postovulatory phase, lower during the luteal phase, with the lowest expression during the follicular phase. We suggest that the preovulatory LH surge, together with increasing E(2) levels from the Graafian follicle and a basal P(4) from regressing corpora lutea (CL), stimulates maximum oviductal production of PG and ET-1, resulting in oviductal contraction for a rapid transport of gametes. OT released from the newly-formed CL may block these mechanisms, and slow contractions for transport of the embryo to the uterus.

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.