Serial plasma oxytocin levels during pregnancy and labor

The physiology and pharmacology of oxytocin in labor and in the peripartum period

Oxytocin is a reproductive hormone implicated in the process of parturition and widely used during labor. Oxytocin is produced within the supraoptic nucleus and paraventricular nucleus of the hypothalamus and released from the posterior pituitary lobe into the circulation. Oxytocin is released in pulses with increasing frequency and amplitude in the first and second stages of labor, with a few pulses released in the third stage of labor. During labor, the fetus exerts pressure on the cervix of the uterus, which activates a feedforward reflex-the Ferguson reflex-which releases oxytocin. When myometrial contractions activate sympathetic nerves, it decreases oxytocin release. When oxytocin binds to specific myometrial oxytocin receptors, it induces myometrial contractions. High levels of circulating estrogen at term make the receptors more sensitive. In addition, oxytocin stimulates prostaglandin synthesis and release in the decidua and chorioamniotic membranes by activating a specific type of oxytocin receptor. Prostaglandins contribute to cervical ripening and uterine contractility in labor. The oxytocin system in the brain has been implicated in decreasing maternal levels of fear, pain, and stress, and oxytocin release and function during labor are stimulated by a social support. Moreover, studies suggest, but have not yet proven, that labor may be associated with long-term, behavioral and physiological adaptations in the mother and infant, possibly involving epigenetic modulation of oxytocin production and release and the oxytocin receptor. In addition, infusions of synthetic oxytocin are used to induce and augment labor. Oxytocin may be administered according to different dose regimens at increasing rates from 1 to 3 mIU/min to a maximal rate of 36 mIU/min at 15- to 40-minute intervals. The total amount of synthetic oxytocin given during labor can be 5 to 10 IU, but lower and higher amounts of oxytocin may also be given. High-dose infusions of oxytocin may shorten the duration of labor by up to 2 hours compared with no infusion of oxytocin; however, it does not lower the frequency of cesarean delivery. When synthetic oxytocin is administered, the plasma concentration of oxytocin increases in a dose-dependent way: at infusion rates of 20 to 30 mIU/min, plasma oxytocin concentration increases approximately 2- to 3-fold above the basal level. Synthetic oxytocin administered at recommended dose levels is not likely to cross the placenta or maternal blood-brain barrier. Synthetic oxytocin should be administered with caution as high levels may induce tachystole and uterine overstimulation, with potentially negative consequences for the fetus and possibly the mother. Of note, 5 to 10 IU of synthetic oxytocin is often routinely given as an intravenous or intramuscular bolus administration after delivery to induce uterine contractility, which, in turn, induces uterine separation of the placenta and prevents postpartum hemorrhage. Furthermore, it promotes the expulsion of the placenta.

Oxytocin as an adolescent treatment for methamphetamine addiction after early life stress in male and female rats

Early life stress (ELS) is associated with perturbed neural development and augmented vulnerability to mental health disorders, including addiction. How ELS changes the brain to increase addiction risk is poorly understood, and there are no therapies which target this ELS-induced vulnerability. ELS disrupts the oxytocin system, which can modulate addiction susceptibility, suggesting that targeting the oxytocin system may be therapeutic in this ELS-addiction comorbidity. Therefore, we determined whether adolescent oxytocin treatment after ELS could: (1) reduce vulnerability to anxiety, social deficits, and methamphetamine-taking and reinstatement; and (2) restore hypothalamic oxytocin and corticotropin-releasing factor expressing neurons and peripheral oxytocin and corticosterone levels. Long Evans pups underwent maternal separation (MS) for either 15 min or 360 min on postnatal days (PND) 1-21. During adolescence (PNDs 28-42), rats received a daily injection of either oxytocin or saline. In Experiment 1, adult rats were assessed using the elevated plus-maze, social interaction procedure, and methamphetamine self-administration procedure, including extinction, and cue-, methamphetamine- and yohimbine-induced reinstatement. In Experiment 2, plasma for enzyme immunoassays and brain tissue for immunofluorescence were collected from adult rats after acute stress exposure. Adolescent oxytocin treatment ameliorated ELS-induced anxiety and reduced methamphetamine- and yohimbine-induced reinstatement in both sexes, and suppressed methamphetamine intake and facilitated extinction in males only. Additionally, adolescent oxytocin treatment after ELS restored oxytocin-immunoreactive cells and stress-induced oxytocin levels in males, and attenuated stress-induced corticosterone levels in both sexes. Adolescent oxytocin treatment reverses some of the ELS effects on later-life psychopathology and vulnerability to addiction.

Labor induction with oxytocin in pregnant rats is not associated with oxidative stress in the fetal brain

Despite the widespread use of oxytocin for induction of labor, mechanistic insights into fetal/neonatal wellbeing are lacking because of the absence of an animal model that recapitulates modern obstetric practice. Here, we create and validate a hi-fidelity pregnant rat model that mirrors labor induction with oxytocin in laboring women. The model consists of an implantable preprogrammed microprocessor-controlled infusion pump that delivers a gradually escalating dose of intravenous oxytocin to induce birth at term gestation. We validated the model with molecular biological experiments on the uterine myometrium and telemetry-supported assessment of changes in intrauterine pressure. Finally, we applied this model to test the hypothesis that labor induction with oxytocin would be associated with oxidative stress in the newborn brain. Analysis of biomarkers of oxidative stress and changes in the expression of associated genes were no different between oxytocin-exposed and saline-treated pups, suggesting that oxytocin-induced labor was not associated with oxidative stress in the developing brain. Collectively, we provide a viable and realistic animal model for labor induction and augmentation with oxytocin that would enable new lines of investigation related to the impact of perinatal oxytocin exposure on the mother-infant dyad.

In vivo imaging of the GnRH pulse generator reveals a temporal order of neuronal activation and synchronization during each pulse

Hypothalamic oscillators that generate pulsatile patterns of hormone secretion represent a fundamental physiological feature regulating homeostatic systems. How individual cells within these neural ensembles generate and coordinate episodic activity and resultant pulse secretion is unknown. Recently, arcuate KNDy (kisspeptin/neurokinin B/dynorphin) cells were identified as a critical component of the gonadotrophin-releasing hormone (GnRH) pulse generator required for reproduction. Using in vivo calcium imaging of KNDy neurons in freely moving mice, we reveal that, prior to each GnRH pulse, individual KNDy cells demonstrate synchronized activity with striking temporal order, with subsets of cells behaving as “leaders” or “followers.” Future work to distinguish these novel subpopulations and define mechanisms underlying the temporal ordering of cellular synchronization may provide avenues to regulate pulse secretion.

A hypothalamic pulse generator located in the arcuate nucleus controls episodic release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and is essential for reproduction. Recent evidence suggests this generator is composed of arcuate “KNDy” cells, the abbreviation based on coexpression of kisspeptin, neurokinin B, and dynorphin. However, direct visual evidence of KNDy neuron activity at a single-cell level during a pulse is lacking. Here, we use in vivo calcium imaging in freely moving female mice to show that individual KNDy neurons are synchronously activated in an episodic manner, and these synchronized episodes always precede LH pulses. Furthermore, synchronization among KNDy cells occurs in a temporal order, with some subsets of KNDy cells serving as “leaders” and others as “followers” during each synchronized episode. These results reveal an unsuspected temporal organization of activation and synchronization within the GnRH pulse generator, suggesting that different subsets of KNDy neurons are activated at pulse onset than afterward during maintenance and eventual termination of each pulse. Further studies to distinguish KNDy “leader” from “follower” cells is likely to have important clinical significance, since regulation of pulsatile GnRH secretion is essential for normal reproduction and disrupted in pathological conditions such as polycystic ovary syndrome and hypothalamic amenorrhea.

Visualising oxytocin neurone activity in vivo: The key to unlocking central regulation of parturition and lactation

During parturition and lactation, oxytocin neurones in the supraoptic and paraventricular nuclei fire high-frequency bursts of action potentials that are coordinated across the entire population. Each burst generates a large pulse of oxytocin release into the circulation to induce uterine contraction for parturition and mammary duct contraction for milk ejection. Bursts are stimulated by cervical stretch during parturition and by suckling during lactation. However, the mechanisms by which these stimuli are translated into episodic bursts are poorly understood, as are the mechanisms that coordinate bursts across the oxytocin neurone population. An elegant series of experiments conducted in the 1980s and 1990s used serial paired recordings to show that oxytocin neurones do not act as a syncytium during bursts; rather, they start each burst within a few hundred milliseconds of each other but with no distinct "leaders" or "followers". In addition to afferent noradrenergic inputs that relay the systemic stimuli to oxytocin neurones, bursts depend on somato-dendritic oxytocin release within the hypothalamus. Hence, bursts are considered to be an emergent property of oxytocin neurones that is bootstrapped by appropriate afferent stimulation. Although much progress was made using traditional electrophysiological recordings in head-fixed anaesthetised animals, research has effectively stalled in the last few decades. However, the emergence of new technologies to monitor neuronal activity in freely-behaving animals has reinvigorated efforts to understand the biology underpinning burst firing in oxytocin neurones. Here, we report the use of fibre photometry to monitor the dynamics of milk ejection bursts in the oxytocin neurone population of freely-behaving mice. This approach will shed light on the neural mechanisms that control the oxytocin bursts underpinning parturition and lactation.

The Hormonal Milieu by Different Labor Induction Methods in Women with Previous Cesarean Section: a Prospective Randomized Controlled Trial

The physiological pattern of hormonal and signaling molecules associated with labor induction is not fully clear. We conducted a preliminary study in order to investigate hormonal changes during labor induction in women with previous cesarean section. Eighty-seven women at term, with previous cesarean section, were randomized to undergo induction of labor by breast stimulation or intracervical balloon and compared with spontaneous labor (controls). Maternal serum levels of oxytocin, prostaglandin F2α, prostaglandin E2, prolactin, estradiol, and cortisol were analyzed at 0, 3, and 6 h post-induction initiation. Fetal umbilical cord hormones were measured. No significant difference was found in the induction-to-delivery time or mode of delivery between the induction groups. Maternal serum oxytocin levels decreased to a lesser extent in the breast stimulation group vs. the control group (p=0.003, p<0.001). In the breast stimulation and control groups, prostaglandin E₂ levels increased as labor progressed (p=0.005, 0.002, respectively). Prostaglandin F2α levels decreased over time in the balloon group (p=0.039), but increased in the control group (p=0.037). Both induction methods had similar outcomes. The hormonal studies ascertained the hypothesized mechanisms, with oxytocin level higher during breast stimulation and lower in balloon induction. These observations could help clinicians determine the appropriate method for cervical ripening in women with previous cesarean section. Larger future studies are needed to examine the effect of these hormonal trends on the rate of successful labor induction and complications, such as uterine rupture, in women with previous uterine scars. ClinicalTrials.gov Identifier NCT04244747.

Neurosteroid involvement in threatened preterm labour

Introduction

The neurosteroid allopregnanolone modulates oxytocin expression in the brain, and its effects arise from its action on the GABAA receptor. Whether neurosteroid levels and the function of the GABAA receptor are involved in the risk of preterm labour in pregnant women is unknown.

Methods

Pregnant women with (n = 16) or without (n = 20) threatened preterm labour (TPL) in gestational week 33 + 6 days to 37 + 0 days were studied prospectively with procedures including foetal heart rate monitoring, vaginal examination, ultrasound examination and blood tests to determine allopregnanolone, progesterone and oxytocin levels. The GABAA receptor function in both groups was measured with a saccadic eye velocity test (SEVT).

Results

Plasma oxytocin levels were higher in the TPL group than in the control group (41.5 vs. 37.0 pmol/L, respectively, p = .021). Although the allopregnanolone and progesterone levels in both groups did not differ, there was a negative association between blood oxytocin and allopregnanolone (as predictor) levels in the TPL group (B: -3.2, 95% confidence interval (CI): -5.5 to -0.9, p = .012). As a predictor of TPL, progesterone was associated with cervix maturity (odds ratio: 1.02, 95% CI: 1.00-1.04, p = .038). SEVT showed that the women in both groups had similar GABAA receptor functions. In both groups, body mass index correlated with peak saccadic eye velocity (r = .34, p = .044) and negatively with allopregnanolone (r = -.41, p = .013).

Conclusions

Neurosteroid levels were unchanged in the peripheral blood of women with TPL, despite the increase in available oxytocin. Although the function of the GABAA receptor was unchanged in women with TPL, to ensure reliable results, saccadic eye velocity should be investigated during a challenge test with a GABAA receptor agonist.

Neurophysiological and cognitive changes in pregnancy

The hormonal fluctuations in pregnancy drive a wide range of adaptive changes in the maternal brain. These range from specific neurophysiological changes in the patterns of activity of individual neuronal populations, through to complete modification of circuit characteristics leading to fundamental changes in behavior. From a neurologic perspective, the key hormone changes are those of the sex steroids, estradiol and progesterone, secreted first from the ovary and then from the placenta, the adrenal glucocorticoid cortisol, as well as the anterior pituitary peptide hormone prolactin and its pregnancy-specific homolog placental lactogen. All of these hormones are markedly elevated during pregnancy and cross the blood-brain barrier to exert actions on neuronal populations through receptors expressed in specific regions. Many of the hormone-induced changes are in autonomic or homeostatic systems. For example, patterns of oxytocin and prolactin secretion are dramatically altered to support novel physiological functions. Appetite is increased and feedback responses to metabolic hormones such as leptin and insulin are suppressed to promote a positive energy balance. Fundamental physiological systems such as glucose homeostasis and thermoregulation are modified to optimize conditions for fetal development. In addition to these largely autonomic changes, there are also changes in mood, behavior, and higher processes such as cognition. This chapter summarizes the hormonal changes associated with pregnancy and reviews how these changes impact on brain function, drawing on examples from animal research, as well as available information about human pregnancy.

Maternal plasma levels of oxytocin during physiological childbirth - a systematic review with implications for uterine contractions and central actions of oxytocin

Background

Oxytocin is a key hormone in childbirth, and synthetic oxytocin is widely administered to induce or speed labour. Due to lack of synthetized knowledge, we conducted a systematic review of maternal plasma levels of oxytocin during physiological childbirth, and in response to infusions of synthetic oxytocin, if reported in the included studies.

Methods

An a priori protocol was designed and a systematic search was conducted in PubMed, CINAHL, and PsycINFO in October 2015. Search hits were screened on title and abstract after duplicates were removed (n = 4039), 69 articles were examined in full-text and 20 papers met inclusion criteria. As the articles differed in design and methodology used for analysis of oxytocin levels, a narrative synthesis was created and the material was categorised according to effects.

Results

Basal levels of oxytocin increased 3–4-fold during pregnancy. Pulses of oxytocin occurred with increasing frequency, duration, and amplitude, from late pregnancy through labour, reaching a maximum of 3 pulses/10 min towards the end of labour. There was a maximal 3- to 4-fold rise in oxytocin at birth. Oxytocin pulses also occurred in the third stage of labour associated with placental expulsion. Oxytocin peaks during labour did not correlate in time with individual uterine contractions, suggesting additional mechanisms in the control of contractions. Oxytocin levels were also raised in the cerebrospinal fluid during labour, indicating that oxytocin is released into the brain, as well as into the circulation. Oxytocin released into the brain induces beneficial adaptive effects during birth and postpartum.

Oxytocin levels following infusion of synthetic oxytocin up to 10 mU/min were similar to oxytocin levels in physiological labour. Oxytocin levels doubled in response to doubling of the rate of infusion of synthetic oxytocin.

Conclusions

Plasma oxytocin levels increase gradually during pregnancy, and during the first and second stages of labour, with increasing size and frequency of pulses of oxytocin. A large pulse of oxytocin occurs with birth. Oxytocin in the circulation stimulates uterine contractions and oxytocin released within the brain influences maternal physiology and behaviour during birth. Oxytocin given as an infusion does not cross into the mother’s brain because of the blood brain barrier and does not influence brain function in the same way as oxytocin during normal labour does.

Electronic supplementary material

The online version of this article (10.1186/s12884-019-2365-9) contains supplementary material, which is available to authorized users.

Integrative neuro-humoral regulation of oxytocin neuron activity in pregnancy and lactation

Oxytocin is required for normal birth and lactation. Oxytocin is synthesised by hypothalamic supraoptic and paraventricular nuclei neurons and is released into the circulation from the posterior pituitary gland. Under basal conditions, circulating oxytocin levels are relatively constant but during birth and lactation, pulsatile oxytocin release triggers rhythmic contraction of the uterus during birth and milk ejection during suckling. Oxytocin levels are principally determined by the pattern of action potential firing that is, in turn, determined by the interplay between the intrinsic properties of the oxytocin neurons, regulation of their excitability by surrounding glia as well as by synaptic drive from their afferent inputs. During birth and suckling, oxytocin neurons fire high-frequency bursts of action potentials that are coordinated across the population of neurons and these bursts underpin the pulsatile secretion of oxytocin required for normal birth and lactation. Neuroglial regulation of oxytocin neurons changes during pregnancy to favour burst firing. However, these changes still require afferent input activity to drive activity. While it has long been known that noradrenergic inputs to oxytocin neurons are activated during birth and lactation, the involvement of other afferent inputs is less clear. Here, we provide a brief overview of the current understanding of the mechanisms that regulate oxytocin neuron activity during pregnancy and lactation, and focus on recent evidence from our laboratory identifying an input that increases kisspeptin production to excite oxytocin neurons in late pregnancy. This article is protected by copyright. All rights reserved.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Alterations in physiology and anatomy during pregnancy

Pregnant women undergo profound anatomical and physiological changes so that they can cope with the increased physical and metabolic demands of their pregnancies. The cardiovascular, respiratory, haematological, renal, gastrointestinal and endocrine systems all undergo important physiological alterations and adaptations needed to allow development of the fetus and to allow the mother and fetus to survive the demands of childbirth. Such alterations in anatomy and physiology may cause difficulties in interpreting signs, symptoms, and biochemical investigations, making the clinical assessment of a pregnant woman inevitably confusing but challenging. Understanding these changes is important for every practicing obstetrician, as the pathological deviations from the normal physiological alterations may not be clear-cut until an adverse outcome has resulted. Only with a sound knowledge of the physiology and anatomy changes can the care of an obstetric parturient be safely optimized for a better maternal and fetal outcome.

Beyond the GnRH axis: kisspeptin regulation of the oxytocin system in pregnancy and lactation

Circulating oxytocin is critical for normal birth and lactation. Oxytocin is synthesised by hypothalamic supraoptic and paraventricular neurons and is released from the posterior pituitary gland into the circulation. Oxytocin secretion depends on action potentials initiated at the cell body, and we have shown that intravenous (IV) administration of kisspeptin-10 transiently increases the firing rate of supraoptic nucleus oxytocin neurons in anaesthetised, non-pregnant, pregnant and lactating rats. This peripheral effect is likely via vagal afferent input, because disruption of vagal afferents prevented the excitation. In our initial studies, intracerebroventricular (icv) administration of kisspeptin-10 did not alter the firing rate of oxytocin neurons in non-pregnant rats. Remarkably, we have now gathered unpublished observations showing that icv kisspeptin-10 transiently excites oxytocin neurons in late pregnancy and during lactation, suggesting that a central kisspeptin excitation of oxytocin neurons emerges at the end of pregnancy, when increased oxytocin secretion is required for delivery of the fetus and for milk let-down after delivery.

Kisspeptin activation of supraoptic nucleus neurons in vivo

Oxytocin and vasopressin are synthesized by magnocellular neurosecretory cells in the hypothalamic supraoptic and paraventricular nuclei and are released from the posterior pituitary gland into the circulation. Intravenous administration of the ligand for the G protein-coupled receptor 54 receptor, kisspeptin-10, increases plasma oxytocin levels and intracerebroventricular kisspeptin-10 increases vasopressin levels, indicating that kisspeptin might play a role in various physiological functions via stimulation of oxytocin and vasopressin secretion. Because posterior pituitary hormone secretion is dependent on action potential (spike) discharge, we used in vivo extracellular single unit recording to determine the effects of kisspeptin-10 on supraoptic nucleus neurons in urethane-anaesthetized female rats. Intravenous kisspeptin-10 (100 μg) increased the firing rate of oxytocin neurons from 3.7 ± 0.8 to 4.7 ± 0.8 spikes/sec (P = 0.0004), but only a quarter of vasopressin neurons responded to iv kisspeptin-10, showing a short (<3 sec) high-frequency (>15 spikes/sec) burst of firing. By contrast, intracerebroventricular kisspeptin-10 (2 and 40 μg) did not alter oxytocin or vasopressin neuron firing rate. To investigate the pathway involved in the peripheral action of kisspeptin-10, we used i.p. capsaicin to desensitize vagal afferents, which prevented the i.v. kisspeptin-10-induced increase of oxytocin neuron firing rate. This is the first report to show that peripheral, but not central, kisspeptin-10 increases the activity of oxytocin neurons and a proportion of vasopressin neurons and that endogenous kisspeptin regulation of supraoptic nucleus neurons is likely via vagal afferent input, with kisspeptin acting as a hormone rather than as a neuropeptide in this system.

The effects of hydrotherapy on anxiety, pain, neuroendocrine responses, and contraction dynamics during labor

BACKGROUND

Hydrotherapy (immersion or bathing) is used worldwide to promote relaxation and decrease parturient anxiety and pain in labor, but the psychophysiological effects of this intervention remain obscure.

DESIGN

A pretest-posttest design with repeated measures was used to examine the effects of hydrotherapy on maternal anxiety and pain, neuroendocrine responses, plasma volume shift (PVS), and uterine contractions (CXs) during labor. Correlations among variables were examined at three time points (preimmersion and twice during hydrotherapy).

METHODS

Eleven term women (mean age 24.5 years) in spontaneous labor were immersed to the xiphoid in 37 degrees C water for 1 hr. Blood samples and measures of anxiety and pain were obtained under dry baseline conditions and repeated at 15 and 45 min of hydrotherapy. Uterine contractions were monitored telemetrically.

RESULTS

Hydrotherapy was associated with decreases in anxiety, vasopressin (V), and oxytocin (O) levels at 15 and 45 min (all ps < .05). There were no significant differences between preimmersion and immersion pain or cortisol (C) levels. Pain decreased more for women with high baseline pain than for women with low baseline levels at 15 and 45 min. Cortisol levels decreased twice as much at 15 min of hydrotherapy for women with high baseline pain as for those with low baseline pain. beta-endorphin (betaE) levels increased at 15 min but did not differ between baseline and 45 min. During immersion, CX frequency decreased. A positive PVS at 15 min was correlated with contraction duration.

CONCLUSIONS

Hydrotherapy during labor affects neuroendocrine responses that modify psychophysiological processes.

Effects of oxytocin in normal man during low and high sodium diets

AIM

We tested the hypothesis that oxytocin in normal man causes natriuresis by means of nitric oxide and/or atrial natriuretic peptide.

METHODS

Normal male subjects were investigated after 4 days of sodium controlled diets (30 mmol sodium chloride day(-1), n = 8 or 230 mmol sodium chloride day(-1), n = 6). Oxytocin was infused intravenously (1 pmol kg(-1) min(-1) for 240 min).

RESULTS

Mean arterial blood pressure, heart rate and glomerular filtration rate by clearance of chromium-labelled ethylenediaminetetraacetate remained stable. Plasma oxytocin increased from 2 to 3 pg mL(-1) to around 50 pg mL(-1). Oxytocin decreased urine flow (4.2 +/- 0.2--0.75 +/- 0.11 and 4.6 +/- 1.3-1.4 +/- 0.6 mL min(-1), low- and high-salt diet, respectively). During low-salt conditions, oxytocin reduced sodium and potassium excretion (11 +/- 2--4 +/- 2 and 93 +/- 19--42 +/- 3 micromol min(-1), respectively). Plasma renin, angiotensin II, aldosterone and renal excretion of metabolites of nitric oxide (nitrate and nitrite) all decreased. Plasma atrial natriuretic peptide and cyclic guanosine monophosphate were unchanged. A similar pattern was obtained during high-salt conditions but in this case the antinatriuresis was not different from that occurring during the corresponding time control series.

CONCLUSIONS

The data reject the hypothesis. In contrast, we found significant antinatriuretic, antikaliuretic and antidiuretic effects, which were not mediated by the renin-angiotensin-aldosterone system, atrial natriuretic peptide, systemic haemodynamics, or processes increasing urinary excretion of metabolites of nitric oxide. The natriuretic effect of oxytocin found in laboratory animals is species-specific.

Role of nitric oxide on oxytocin-evoked contractions and prostaglandin synthesis in isolated pregnant rat uterus

Uterine contractions elicited by oxytocin (OT), possibly linked with uterus prostaglandin (PG) release, are involved in the final pathway of labor. It is known that nitric oxide (NO) may contribute to the maintenance of uterine contractile quiescence during gestation. Therefore in this study the effect of the inhibition of NO synthase (NOS), with N-monomethyl L-arginine (L-NMMA), on the ability of OT to stimulate uterine contractions and PG synthesis was investigated in isolated rat uterus at days 13 and 21 of pregnancy. L-NMMA did not modify the frequency and the force of contractions elicited by OT at day 13. On day 21 the frequency of contractions evoked by OT were better sustained in the presence of L-NMMA. PGs were not affected by OT on day 13. OT stimulated PGF2alpha on day 21 when NOS had been inhibited with L-NMMA, but not in the absence of L-NMMA. NOS activity was stimulated by OT at day 21 of gestation. In summary these findings indicate that near term NO can regulate OT PGF2alpha induced contractions and PG synthesis in isolated pregnant rat uterus.

Plasma oxytocin and nocturnal uterine activity: maternal but not fetal concentrations increase progressively during late pregnancy and delivery in rhesus monkeys

OBJECTIVE

The purpose of the study was to determine whether rising maternal or fetal plasma oxytocin concentrations could be responsible for the increasing levels of nocturnal uterine activity on the nights preceding delivery.

STUDY DESIGN

Chronically catheterized pregnant rhesus monkeys were exposed to a 16-hour light, 8-hour dark photoperiod (dark 11 PM to 7 AM). Uterine activity and maternal arterial plasma oxytocin concentrations were measured concurrently at weekly intervals in late gestation, on the night preceding term delivery (158 to 167 days, n = 4), and during delivery (149 to 170 days, n = 6). Fetal carotid arterial plasma oxytocin levels were measured during episodes of nocturnal uterine activity in six animals. The effect of oxytocin infusions into the fetus (30 to 480 ng/kg/hr) on uterine activity and on maternal and fetal plasma oxytocin levels was also determined (n = 3).

RESULTS

Maximal nocturnal oxytocin concentrations in the maternal plasma rose progressively during late gestation from 9.9 +/- 3.5 pg/ml at 130 to 139 days to 28.7 +/- 9.8 pg/ml on the night preceding term delivery (p < 0.005); a significant increase in nocturnal uterine activity accompanied this rise (p < 0.001). Maternal oxytocin concentrations were elevated during labor and increased further at delivery (62.5 +/- 5.5 pg/ml, p < 0.05). There was no increase in fetal plasma oxytocin during nocturnal uterine activity (3.1 +/- 0.2 pg/ml) or during labor. Fetal oxytocin infusions raised fetal plasma oxytocin concentrations sixtyfold but had no effect on maternal plasma oxytocin concentrations or on uterine activity.

CONCLUSIONS

Elevated maternal plasma oxytocin concentrations are responsible, at least in part, for the increasing magnitude of nocturnal uterine activity episodes as term approaches and for the elevated uterine activity before delivery at night. Fetal plasma oxytocin does not contribute to nocturnal uterine activity or to maternal plasma oxytocin concentrations.

The stability and short-term fluctuation in serum oxytocinase activity in pregnancy

Maternal serum samples obtained from 123 normal pregnant women between 28 and 40 weeks gestation were analysed for oxytocinase (EC 3.4.11.3) activity using s-benzyl-1-cysteine-4-nitroanilide as substrate. Oxytocinase activity was found to be stable at varying temperatures, with a mean co-efficient of variation of 5.0%. The short-term fluctuation of oxytocinase activity had a mean coefficient of variation of 3.6%. There is no suggestion of a "pulsatile" release of oxytocinae from its placental origin.

Serial oxytocin levels in amniotic fluid and maternal plasma during normal and induced labour

Oxytocin was measured by a specific and sensitive radioimmunoassay in plasma and amniotic fluid after extraction with Sep-Pak cartridges in patients undergoing elective caesarean sections, normal labour and labour induced with oxytocin infusion or prostaglandins. In maternal plasma, levels of oxytocin correlated with the period of gestation; concentrations at term were significantly higher than in the first two trimesters. Maternal plasma levels of oxytocin before the onset of contractions were not significantly different from those at the onset of spontaneous labour or at full cervical dilatation. Levels of oxytocin in patients induced with oxytocin were not statistically different from levels observed in spontaneous labour. Amniotic fluid oxytocin levels were not significantly different between the groups either at the onset of labour or immediately before delivery. Umbilical arterio-venous differences in oxytocin were present in all groups except the prostaglandin-induced group. Increased prostaglandins associated with the onset of normal labour are probably not a consequence of changes in oxytocin concentrations.

Plasma oxytocin and oxytocin neurone activity during delivery in rabbits

The extracellular electrical activity of magnocellular neurones was recorded from unanaesthetized, unrestrained rabbits in birth and the post partum suckling period. The activity of oxytocin neurones was differentiated from that of vasopressin cells on the basis of their stereotyped activity in suckling. Oxytocin neurones showed five to fourteen discrete bursts of accelerated discharge in parturition. Each burst lasted 2-22 s and represented a 3-100-fold increase in the rate of firing, compared with pre-partum values, and was followed 10-34 s later by delivery. After parturition, the spontaneous activity of these neurones returned to pre-partum rates of firing. Vasopressin neurones did not show any bursts of discharge in delivery. There was a significant fall in the discharge frequency compared with pre-partum levels (P less than 0.05: Student's t test) and a significant (P less than 0.01) lengthening of the modal interspike interval. Serial blood samples were obtained during parturition in ten rabbits. Simultaneous recordings of magnocellular neurones activity and plasma oxytocin measurements were made in four of these experiments. Plasma oxytocin profiles were related to the observed events of parturition. Frequent blood samples (0.2-0.3 ml every 10-15 s) were taken throughout delivery and plasma oxytocin measured by sensitive radioimmunoassay in unextracted plasma (lower limit sensitivity of assay 5 pg ml-1). Before birth, plasma oxytocin was 53 +/- 12 pg ml-1 (mean +/- S.E. of mean) and rose to 2846 +/- 326 pg ml-1 within 40-120 s of the onset of the expulsive phase of delivery. Peak concentrations of oxytocin were coincident with delivery of the first or second fetus. No sign of pulsatile release of oxytocin was demonstrated in the profiles but significantly (P less than 0.001) greater variance in oxytocin titres was found during birth compared with pre-partum values which is suggestive of pulsatile release. There was a straight line relationship between peak oxytocin concentrations in the plasma and the speed of delivery, implying that oxytocin facilitates as well as maintains labour in the rabbit.

Plasma levels of oxytocin increase in response to suckling and feeding in dogs and sows

The objective of the present study was to investigate whether oxytocin is released in response to feeding in analogy to the response induced by suckling. Therefore, repeated plasma samples were drawn from dogs and pigs during feeding and suckling and oxytocin levels were determined by radioimmunoassay. As expected suckling gave rise to immediate and short-lasting increases of oxytocin levels in both species. More surprisingly, feeding in female and male dogs as well as in lactating sows was accompanied by a similar-sized rise of oxytocin levels. The oxytocin peak sometimes occurred before the actual period of suckling or feeding, suggesting that the output of oxytocin had been conditioned to visual, olfactory or auditory stimuli associated with both types of situations. It is well known that oxytocin is released in lactating animals in response to touching of the teats. It is possible that also the presence of food in the gastro-intestinal tract activates neurogenic mechanisms which stimulates the release of oxytocin. Since oxytocin causes a release of insulin and VIP (vasoactive intestinal polypeptide), peptides which appear in the circulation following both suckling and feeding, it is suggested that oxytocin may be involved in the control of the suckling- and feeding-related output of these peptides.

Plasma oxytocin in human pregnancy and parturition

Oxytocin concentrations were determined in serial peripheral plasma samples collected from clinically normal women during pregnancy and labor. Measurable concentrations of this hormone were detected in all maternal plasma samples during pregnancy, but there were wide differences in values between patients. Serial samples from individual patients revealed a pattern of gradual rise of oxytocin levels with advancing gestation and the increase in concentration was statistically significant. There were no significant differences in oxytocin levels at any stage of labor, with or without epidural analgesia. Oxytocin levels at the onset of the second stage did not differ statistically from those at crowning. Comparison of cross-sectional data showed no significant difference between the mean oxytocin concentration in early labor and in late pregnancy. Oxytocin surges occurred, but not in a regular pattern. Plasma oxytocin concentration did not increase after pelvic examination, sweeping of the membranes, low amniotomy or after cervical vibration. After spontaneous vaginal delivery, umbilical arterial plasma levels of oxytocin were consistently higher than plasma concentrations from the umbilical vein. The fetal arterio-venous difference was less pronounced at elective cesarean section. At spontaneous vaginal delivery, with and without epidural anesthesia, plasma levels from the umbilical artery were significantly higher than the maternal levels. After vaginal delivery, oxytocin levels in cord plasma were significantly higher than at elective abdominal delivery. Some methodological aspects with regard to blood sampling and to plasma oxytocin radioimmunoassay procedures are discussed. From the results presented it is concluded that the human fetus can be an important source of oxytocin and that neurohumoral birth reflexes described in animals do not occur systematically in man.