Temporal relationships between peripheral plasma concentrations of oxytocin, progesterone and 13,14-dihydro-15-keto-prostaglandin F2 alpha during the oestrous cycle and early pregnancy in the ewe

Effect of Neurosteroids on Basal and Stress-Induced Oxytocin Secretion in Luteal-Phase and Pregnant Sheep

Oxytocin (OT) is a neuropeptide synthesized in the hypothalamic nuclei that modulates both behavioral and reproductive functions, associated with the increased neurosteroid synthesis in the brain. Therefore, the present study tested the hypothesis that manipulation of central neurosteroid levels could affect oxytocin synthesis and release in non-pregnant and pregnant sheep under both basal and stressful conditions. In Experiment 1, luteal-phase sheep were subjected to a series of intracerebroventricular (icv.) infusions of allopregnanolone (AL, 4 × 15 μg/60 μL/30 min) for 3 days. In Experiment 2, pregnant animals (4th month) received a series of infusions of the neurosteroid synthesis blocker, finasteride (4 × 25 μg/60 μL/30 min), conducted for 3 days. In non-pregnant sheep AL alone was shown to differentially modulate OT synthesis in basal conditions, and strongly inhibit OT response to stress (p < 0.001). In contrast, in pregnant animals, basal and stress-induced OT secretion was significantly (p < 0.001) increased during finasteride infusion compared to controls. In conclusion, we showed that neurosteroids were involved in the control of OT secretion in sheep, particularly under stress and pregnancy conditions and are part of an adaptive mechanism which is responsible for protecting and maintaining pregnancy in harmful situations.

Menstrual cycle-related fluctuations in oxytocin concentrations: A systematic review and meta-analysis

Oxytocin affects physiological and psychological functions that are often expressed sex-specifically, suggesting interactions between oxytocin and sex hormones. As female sex hormone concentrations change during the menstrual cycle, oxytocin might fluctuate, too. This systematic review and meta-analysis investigated endogenous oxytocin concentrations across menstrual cycle phases in healthy women. Data from 13 studies (120 women) showed a significant increase of oxytocin concentrations from the early follicular phase to ovulation (g = 0.39 [0.25; 0.53], p < .001) and a significant decrease from ovulation to the mid-luteal phase (g = -0.50 [-0.81; -0.18], p < .001). There were no significant differences between the early follicular and mid-luteal phase (g = -0.19 [-0.70; -0.32], p = .471). These findings contribute to a deeper understanding of differences in normal and abnormal psychobiological processes in women. They highlight the necessity to consider the menstrual cycle phase in studies on oxytocin in women.

Nongenomic action of progesterone inhibits oxytocin-induced phosphoinositide hydrolysis and prostaglandin F2alpha secretion in the ovine endometrium

Experiments were conducted to characterize the nongenomic effects of progesterone (P4) on binding of oxytocin (OT) to its receptor and signal transduction in the ovine endometrium. The dose-response relationship of P4 to OT binding was examined. Membranes from endometrial tissue of ovariectomized hormone-treated ewes were preincubated in the presence of P4 for 1 h followed by OT receptor analysis. P4 interfered with the binding of OT in a dose-dependent manner. Endometrium was then recovered from cyclic ewes and divided into explants. Treatment consisted of two dosages of P4 and two dosages of OT. Explants were analyzed for total inositol monophosphate, bisphosphate (IP(2)), and trisphosphate (IP(3)) content. Preincubation with P4 for 10 min significantly interfered with OT stimulation of IP(2) and IP(3) synthesis. Oxytocin increased monophosphate production, but there was no detectable effect of P4. In the next experiment, endometrial explants were cultured in the absence or the presence of arachidonic acid. Explants were then exposed for 1 h to medium containing vehicle or P4. After incubation, explants were challenged with OT and the media were collected and analyzed for 13,14 dihydro-15-keto prostaglandin F(2alpha) by RIA. Treatment of explants with AA increased PGF(2alpha) content compared with that of controls. Brief exposure to P4 significantly decreased OT-induced PGF(2alpha) secretion from explants previously exposed to medium or AA. Collectively, these data are interpreted to indicate that the observed reduction in OT-induced IP(2) and IP(3) production and OT-induced PGF(2alpha) secretion was due to P4 inhibition of OT binding to its receptor.

Oestrogenic effects of ICI 182,780, a putative anti-oestrogen, on the secretion of oxytocin and prostaglandin F2 alpha during oestrous cycle in the intact ewe

The effect of ICI 182,780, oestrogen antagonist, on the concentration of oxytocin and uterine PGF2 alpha was investigated in intact Border Leicester Merino cross ewes during the late oestrous cycle. Twelve cyclic ewes (n = 6 per group) were randomly assigned to receive, at 6 h intervals, intra-muscular injection of either peanut oil or ICI 182,780 (1.5 mg kg-1 day-1) in oil for 2 days, starting at 1900 h on day 13 until 1300 h on day 15 post-oestrus. Hourly blood samples were collected via a jugular catheter from 0800 h on day 14 for 37 h and then daily over days 16, 17 and 18 post-oestrus. Peripheral plasma concentrations of oxytocin, the metabolite of prostaglandin F2 alpha, 15-keto-13,14-dihydro-prostaglandin F2 alpha, (PGFM) and progesterone were measured by radioimmunoassay. All ewes treated with ICI 182,780 exhibited functional luteal regression as indicated by a marked reduction in plasma progesterone concentrations to less than 1000 pg/ml over the period of 18-36 h during sampling period on days 14 and 15 of the oestrous cycle. In five of six vehicle-treated ewes, progesterone concentrations declined between day 16 and day 18 post-oestrus. In the remaining control ewe, progesterone concentrations reach less than 1000 pg/ml within 36 h of the commencement of the sampling period. During the frequent sampling period, the number of oxytocin pulses in the ICI 182,780 treated ewes was significantly higher compared to control ewes (2.7 +/- 0.3 vs. 0.8 +/- 0.3). The mean amplitude of oxytocin pulses observed was also greater (70.4 +/- 19.5 pg/ml) in ewes treated with ICI 182,780, but was not significantly different from the control ewes (33.5 +/- 12.9 pg/ml). Oxytocin pulses may however have occurred following the initial two ICI 182,780 injections but before commencing blood sampling. The oxytocin pulses were detected at a mean of 3.2 +/- 0.2 h following each injection with ICI 182,780 during blood sampling. In the ICI 182,780-treated ewes, the pulsatile pattern of plasma PGFM in jugular blood samples over the 37 h sampling period on days 14 and 15 post-oestrus had a higher amplitude (512.9 +/- 158.9 vs 121.7 +/- 78.7 pg/ml) and pulse area (618.1 +/- 183.3 vs 151.5 +/- 102.9 (ph/ml)tau) compared to the vehicle-treated ewes (P < 0.05) respectively.. The average number of PGFM pulses observed per ewe was 3.0 +/- 0.7 in the ICI 182,780-treated group and was significantly (P < 0.02) higher than the number of pulses (0.5 +/- 0.3) observed in ewes treated with vehicle alone. The PGFM pulses were detected at 4.2 +/- 0.6 h following each injection with ICI 182,780 during blood sampling. The percentage of PGFM pulses that occurred coincidently with significant elevation of oxytocin concentrations was 44.4% in ICI 182,780-treated compared to 66.6% in control ewes. We conclude that administration of oestrogen antagonist ICI 182,780 accelerated development of the luteolytic mechanism by enhancing pulsatile secretion of oxytocin and PGFM which suggests that ICI 182,780 acts as an agonist for oxytocin and prostaglandin f2 alpha release in intact ewes when administered at 1.5 mg/kg/day over day 13 to 15 post-oestrus.

Intramammary pressure and luteal oxytocin after PGF2 alpha administration in cycling and early pregnant ewes

The aim of this work was to investigate whether luteal oxytocin released after non-luteolytic prostaglandin F2 alpha (PGF2 alpha) challenge could explain the intramammary pressure (IMP) rises previously described in pregnant ewes after the normal life span of a corpus luteum. Blood oxytocin levels and IMP after challenge were measured in cycling and pregnant lactating ewes until the response ceased (approximately 55 d post oestrus). Oxytocin release was not significantly different for cycling and pregnant ewes, and fell to its lowest levels on days 16-18 post oestrus in both groups, i.e. with or without luteolysis. However, although IMP response fell with luteolysis in cycling ewes, the response persisted until day 55 in pregnant ewes. This lack of correlation between oxytocin release and IMP response raised interesting questions including: is there an unknown oxytocic compound that can be released after PGF2 alpha stimulation of the corpora lutea?

Paracrine regulation of luteal function

The mechanisms controlling luteal function may involve factors that are produced both within the corpus luteum and outside the ovary. The process of luteal control appears to involve a series of molecular species, proteins, peptides, steroids and prostaglandins. Each of these factors may act independently or in concert modifying the actions of one another. The effect of GnRH on luteal function has not been completely examined and thus its significance is unclear. The neurohypophyseal peptides, oxytocin and arginine vasopressin, in combination with LH, prolactin, oestrogens and prostaglandins may play an important regulatory role on the corpus luteum.

Contribution to the pathogenesis of dysmenorrhea

Menstrual blood was collected from five eumenorrheic and seven dysmenorrheic women aged between 20 and 35 years for a period of three cycles each. The levels of prostaglandin F2 alpha (PGF2 alpha), prostaglandin E2 (PGE2), 6-keto-prostaglandin F1 alpha (6-k-PGF1 alpha)-the stable metabolite of prostacyclin (PGI2)-, oestradiol, oestrone, and progesterone were determined radioimmunologically. Both eumenorrheic and dysmenorrheic women showed identical blood losses. The levels of oestradiol excreted by the dysmenorrheic women were markedly elevated as compared to the non-dysmenorrheic subjects (2 p less than 0.05). Oestrone excretion was in the same order of magnitude in all subjects examined. The concentration of progesterone per menstruation was significantly higher in the eumenorrheic women (2 p less than 0.02) than in the dysmenorrheic patients. Menstrual excretion of PGF2 alpha was 2.5 times higher in the dysmenorrheic women compared to the normal subjects (2 p less than 0.05). The levels of PGE2 was identical in both groups. Excretion of 6-k-PGF1 alpha was significantly lower in the dysmenorrheic women than in the eumenorrheic subjects (2 p less than 0.02). The oestradiol/progesterone ratio showed a distinct predominance of oestradiol in the dysmenorrheic patients. PGF2 alpha dominance in the dysmenorrheic patients is expressed by the PGF2 alpha/6-k-PGF1 alpha and the PGF2 alpha/PGE2 ratios. A shift in the oestradiol/progesterone ratio in favour of oestradiol seems to be the underlying pathogenic principle of dysmenorrhea. The oestradiol dominance is associated with a shift in the PGF2 alpha/PGI2 and the PGF2 alpha/PGE2 proportions. Thus, the PGF2 alpha predominance and a simultaneous reduction of PGI2 in uterine tissue seem to be responsible for dysmenorrheic bleeding.

Secretion of neurophysins by the ovary in sheep

Measurements of venoarterial concentration differences across the ovary in anesthetized sheep have demonstrated that the ovary secretes ovine neurophysin I/II (oNP I/II) and that this process is stimulated by the prostaglandin F2 alpha analogue, cloprostenol. A parallel increase in the secretion of oxytocin (OT) was observed in response to cloprostenol, and the mean molar ratio of oNP I/II to OT secreted was 1.2. There was no detectable ovarian secretion of oNP III. Secretion of oNP I/II and OT was absent after hysterectomy. The data support other evidence indicating that the corpus luteum synthesizes OT, and confirm that the neurophysin associated with OT in the sheep is oNP I/II.

The effect of oxytocin on the corpus luteum of the monkey

The effect of a pharmacologic dose of synthetic oxytocin on corpus luteum function was evaluated in rhesus monkeys during normal menstrual cycles, or during menstrual cycles in which the corpus luteum was concomitantly stimulated by injections of human chorionic gonadotropin (hCG). Oxytocin administered by intramuscular injection at a total dose of 4.5 milligrams (2250 I.U.) on Day +6 of the normal luteal phase (Day 0 is the day of the midcycle LH surge) did not change the concentrations of progesterone in the peripheral serum of monkeys or alter the duration of the luteal phase. The same dose of oxytocin, administered to monkeys on Day 22 of menstrual cycles in which hCG was also given on Days 20-22, caused a small, but statistically significant, reduction in serum progesterone values. The results indicate that oxytocin does not alter luteal life span or markedly change blood progesterone concentrations in primates.

Corpus luteum regression induced by ultra-low pulses of prostaglandin F2 alpha

In view of the pulsatile nature of PGF2 alpha secretion from the ovine uterus at the time of luteolysis, experiments were designed to examine the effect of pulsed infusions of PGF2 alpha on luteal function and to re-examine the minimal effective levels of PGF2 alpha required to induce luteolysis. To mimic physiological conditions, hour-long infusions of PGF2 alpha in increasing concentrations were given either 4 times in 19 h or 5 times in 25 h into the arterial supply of the autotransplanted ovary in conscious sheep on day 12 of an induced cycle. Blood flow and progesterone secretion rate from the ovary were used to monitor directly the luteolytic effect of administered PGF2 alpha. The concentration of LH in peripheral plasma was measured throughout each infusion experiment and the presence of a preovulatory peak of LH was used as an indicator of the permanence of luteal regression. Four pulses of PGF2 alpha in 19 h caused complete corpus luteum regression in only 1 of 4 animals whereas the addition of a fifth pulse (5 pulses in 25 h) caused permanent regression in 4 out of 4 animals. Infusion of 5 hour-long pulses of saline or PGF2 alpha at a rate less than 0.04 microgram/h did not induce permanent suppression of progesterone secretion. The average total effective dose of PGF2 alpha required to induce luteal regression when given as 5 pulses was 1/40th of the amount currently regarded as the minimal effective one when given by constant infusion into the ovarian artery. In another series of experiments the luteolytic effect of a single hour-long pulse of 0.1 microgram/h PGF2 alpha given daily for either 3 or 4 days was investigated. A significant fall (ANOVA, F0.01) in progesterone secretion rate, which reached a nadir at 5.3 +/- 2.2 h (means +/- S.D., n = 15), was followed by a recovery of progesterone secretion rate. Permanent luteal regression did not occur with this protracted regimen, suggesting that a relatively short pulse frequency of PGF2 alpha over a minimal period of 24 h is a necessary condition for physiological regression of the corpus luteum in sheep.

Progesterone withdrawal without parturition

It has been demonstrated that administration of 100 mg of trilostane (an inhibitor of 3 beta-hydroxysteroid dehydrogenase) to late pregnant sheep will rapidly lower circulating levels of progesterone and that delivery ensues. Our intention was to reduce the dose of trilostane in order to separate the latter two sequelae and thereby obtain insight into the relationship between progesterone and prostaglandin biosynthesis. At the dose chosen (10 mg) the treatment did not induce parturition in 4 chronically catheterized sheep during late pregnancy. Circulating progesterone concentrations declined precipitously in all ewes but recovered to near basal values by 24 h after administration of trilostane. Circulating concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha rose slightly but significantly at 4-5 h after administration of trilostane but never reached values normally associated with labor. Plasma estradiol levels were unchanged by treatment. These results are consistent with the view that progesterone withdrawal must be of a critical magnitude and duration for prostaglandin biosynthesis to be sufficiently stimulated to induce labor in sheep during late gestation.

Oxytocin in the fetal and maternal circulations during induced and spontaneous parturition in sheep

Oxytocin has been measured in fetal and maternal plasma samples obtained serially from 6 sheep during late pregnancy, spontaneous labor and labor induced by intrafetal infusion of Synacthen (ACTH1-24). Fetal plasma contained significantly (P less than 0.001) greater concentrations of oxytocin than maternal plasma. Concentrations of oxytocin rose in maternal plasma during the later stages of spontaneous and induced labor, whereas concentrations in fetal plasma did not exhibit a consistent trend.

Peripheral plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha and progesterone around luteolysis and during early pregnancy in the goat

Peripheral plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) and progesterone were determined during both luteolysis in the oestrous cycle and early pregnancy in four goats. Luteal regression, characterised by decreasing progesterone concentrations, began on day 12 or 13. PGFM concentrations showed a pulsatile pattern around this time, with peak concentrations increasing markedly as progesterone levels fell and oestrus approached. During early pregnancy progesterone concentrations did not fall after day 12 and no marked elevation of PGFM above basal values of 50-150 pg/ml was detected.

Embryonic suppression of oxytocin--associated neurophysin release in early pregnant sheep

Pulsatile release of the oxytocin-neurophysin (OT-N) occurs during luteal regression in non-pregnant sheep. The effect on this neurophysin release by the presence of a conceptus was investigated. It was found that the transfer of an embryo, on either Day 12 or 13, to the uterus of a non-pregnant sheep inhibited the pulsatile release of OT-N. This result suggests that in the early pregnant sheep, the embryo may prevent corpus luteum regression by suppressing the pulsatile release of maternal oxytocin which is considered to induce the release of uterine prostaglandin F2 alpha.