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.

Oxytocin: recent developments

Oxytocin is a neurohypophyseal hormone that is produced centrally by neurons in the paraventricular nucleus and supraoptic nucleus of the hypothalamus. It is released directly into higher brain centres and into the peripheral circulation where it produces a multitude of effects. Classically, oxytocin is known for inducing uterine contractions at parturition and milk ejection during suckling. Oxytocin also acts in a species and gender specific manner as an important neuromodulator. It can affect behaviours associated with stress and anxiety, as well social behaviours including sexual and relationship behaviours, and maternal care. Additionally, oxytocin has been shown to have a variety of physiological roles in peripheral tissues, many of which appear to be modulated largely by locally produced oxytocin, dispelling the notion that oxytocin is a purely neurohypophyseal hormone. Oxytocin levels are altered in several diseases and the use of oxytocin or its antagonists have been identified as a possible clinical intervention in the treatment of mood disorders and pain conditions, some cancers, benign prostatic disease and osteoporosis. Indeed, oxytocin has already been successful in clinical trials to treat autism and schizophrenia. This review will report briefly on the known functions of oxytocin, it will discuss in depth the data from recent clinical trials and highlight future targets for oxytocinergic modulation.

Neuropeptide trefoil factor 3 attenuates naloxone-precipitated withdrawal in morphine-dependent mice

RATIONALE

The persistence of physical dependence and craving in addicts is considered to contribute to relapse. Increasing evidence indicates that neuropeptide systems are associated with several phases of drug addiction, but little is known about whether the neuropeptide trefoil factor affects withdrawal symptoms.

OBJECTIVES

This study aims to investigate the potential effects of the neuropeptide trefoil factor 3 (TFF3) on naloxone-precipitated withdrawal symptoms in morphine-dependent mice.

RESULTS

Mice received increasing doses of morphine over 3 days. On day 4, the mice were injected with TFF3 (1.0 mg/kg, i.p.) 30 min after the last dose of morphine. Thirty minutes after TFF3 treatment, naloxone (1 mg/kg, i.p.) was injected, and body weight, jumping behavior, wet-dog shakes, and locomotor activity were assessed 30 min later. Naloxone caused significant weight loss and increased jumping behavior and wet-dog shakes in morphine-dependent mice. TFF3 (1.0 mg/kg) reversed these behavioral symptoms caused by morphine withdrawal, suggesting that TFF3 might ameliorate physical dependence associated with opiate addiction. Furthermore, TFF3 pretreatment significantly reduced morphine withdrawal-induced increases in plasma corticosterone and adrenocorticotropic hormone levels. The glucocorticoid receptor agonist RU486 blocked the behavioral effects of TFF3 on morphine withdrawal symptoms. Finally, Fos expression in the medial prefrontal cortex which was decreased during morphine withdrawal was increased by TFF3 pretreatment.

CONCLUSION

These findings indicate that TFF3 might be a potential therapeutic candidate for opiate addiction by regulating glucocorticoid secretion and neuronal activation in the prefrontal cortex.

Allopregnanolone in the brain: protecting pregnancy and birth outcomes

A successful pregnancy requires multiple adaptations in the mother's brain that serve to optimise foetal growth and development, protect the foetus from adverse prenatal programming and prevent premature delivery of the young. Pregnancy hormones induce, organise and maintain many of these adaptations. Steroid hormones play a critical role and of particular importance is the progesterone metabolite and neurosteroid, allopregnanolone. Allopregnanolone is produced in increasing amounts during pregnancy both in the periphery and in the maternal and foetal brain. This review critically examines a role for allopregnanolone in both the maternal and foetal brain during pregnancy and development in protecting pregnancy and birth outcomes, with particular emphasis on its role in relation to stress exposure at this time. Late pregnancy is associated with suppressed stress responses. Thus, we begin by considering what is known about the central mechanisms in the maternal brain, induced by allopregnanolone, that protect the foetus(es) from exposure to harmful levels of maternal glucocorticoids as a result of stress during pregnancy. Next we discuss the central mechanisms that prevent premature secretion of oxytocin and consider a role for allopregnanolone in minimising the risk of preterm birth. Allopregnanolone also plays a key role in the foetal brain, where it promotes development and is neuroprotective. Hence we review the evidence about disruption to neurosteroid production in pregnancy, through prenatal stress or other insults, and the immediate and long-term adverse consequences for the offspring. Finally we address whether progesterone or allopregnanolone treatment can rescue some of these deficits in the offspring.

Immune stress in late pregnant rats decreases length of gestation and fecundity, and alters later cognitive and affective behaviour of surviving pre-adolescent offspring

Immune challenge during pregnancy is associated with preterm birth and poor perinatal development. The mechanisms of these effects are not known. 5α-Pregnan-3α-ol-20-one (3α,5α-THP), the neuroactive metabolite of progesterone, is critical for neurodevelopment and stress responses, and can influence cognition and affective behaviours. To develop an immune challenge model of preterm birth, pregnant Long-Evans rat dams were administered lipopolysaccharide [LPS; 30 μg/kg/ml, intraperitoneal (IP)], interleukin-1β (IL-1β; 1 μg/rat, IP) or vehicle (0.9% saline, IP) daily on gestational days 17-21. Compared to control treatment, prenatal LPS or IL-1β reduced gestational length and the number of viable pups born. At 28-30 days of age, male and female offspring of mothers exposed to prenatal IL-1β had reduced cognitive performance in the object recognition task compared to controls. In females, but not males, prenatal IL-1β reduced anxiety-like behaviour, indicated by entries to the centre of an open field. In the hippocampus, progesterone turnover to its 5α-reduced metabolites was lower in prenatally exposed IL-1β female, but not in male offspring. IL-1β-exposed males and females had reduced oestradiol content in hippocampus, medial prefrontal cortex and diencephalon compared to controls. Thus, immune stress during late pregnancy reduced gestational length and negatively impacted birth outcomes, hippocampal function and central neurosteroid formation in the offspring.

Neuroendocrine mechanisms of change in food intake during pregnancy: a potential role for brain oxytocin

During pregnancy body weight, and particularly adiposity, increase, due to hyperphagia rather than decreased energy metabolism. These physiological adaptations provide the growing fetus(es) with nutrition and prepare the mother for the metabolically-demanding lactation period following birth. Mechanisms underlying the hyperphagia are still poorly understood. Although the peripheral signals that drive appetite and satiety centers of the brain are increased in pregnancy, the brain may become insensitive to their effects. For example, leptin secretion increases but hypothalamic resistance to leptin actions develops. However, several adaptations in hypothalamic neuroendocrine systems may converge to increase ingestive behavior. Oxytocin is one of the anorectic hypothalamic neuropeptides. Oxytocin neurons, both centrally-projecting parvocellular oxytocin neurons and central dendritic release of oxytocin from magnocellular neurons, may play a key role in regulating energy intake. During feeding in non-pregnant rats, magnocellular oxytocin neurons, especially those in the supraoptic nucleus, become strongly activated indicating their imminent role in meal termination. However, in mid-pregnancy the excitability of these neurons is reduced, central dendritic oxytocin release is inhibited and patterns of oxytocin receptor binding in the brain alter. Our recent data suggest that lack of central oxytocin action may partly contribute to maternal hyperphagia. However, although opioid inhibition is a major factor in oxytocin neuron restraint during pregnancy and opioids enhance food intake, an increase in opioid orexigenic actions were not observed. While changes in several central input pathways to oxytocin neurons are likely to be involved, the high level of progesterone secretion during pregnancy is probably the ultimate trigger for the adaptations.

Neuroactive steroids attenuate oxytocin stress responses in late pregnancy

In late pregnant rats neuroendocrine stress responses, expressed as increased oxytocin secretion and activation of the hypothalamo-pituitary-adrenal axis, are attenuated. These adaptations preserve the oxytocin store for parturition and prevent pre-term birth, and protect the fetuses from adverse programming by exposure to excess glucocorticoid. Mechanisms of adaptations for oxytocin neurones are reviewed, using challenge with systemic interleukin-1beta, simulating activation of immune signaling by infection, as a stressor of special relevance in pregnancy. In virgin rats, systemic interleukin-1beta stimulates the firing of oxytocin neurones, and hence oxytocin secretion, but interleukin-1beta has no effects in late pregnant rats. This lack of response is reversed by naloxone treatment just before interleukin-1beta administration, indicating endogenous opioid suppression of oxytocin responses in late pregnancy. This opioid presynaptically inhibits noradrenergic terminals impinging on oxytocin neurones. Finasteride pretreatment, inhibiting progesterone conversion to allopregnanolone, a positive GABA(A) receptor allosteric modifier, also restores an oxytocin response to interleukin-1beta. This finasteride effect is reversed by allopregnanolone treatment. In virgin rats allopregnanolone attenuates the oxytocin response to interleukin-1beta, which is exaggerated by naloxone. The effects of naloxone and finasteride in late pregnant rats in restoring an oxytocin response to interleukin-1beta are not additive. Accordingly, allopregnanolone may both enhance GABA inhibition of oxytocin neurone responses to interleukin-1beta, and induce opioid suppression of noradrenaline release onto oxytocin neurones.

Dopaminergic mechanisms involved in prolactin release after mifepristone and naloxone treatment during late pregnancy in the rat

BACKGROUND/AIMS

During late pregnancy, the antiprogesterone mifepristone facilitates prolactin release. This effect is enhanced by administration of the opioid antagonist naloxone, suggesting an inhibitory-neuromodulatory role of the opioid system. Since hypothalamic dopamine (DA) is the main regulator of prolactin release, in this study we explored the role of DA on prolactin release induced by mifepristone and naloxone treatment.

METHODS/RESULTS

Rats on day 19 of pregnancy were used. Naloxone treatment did not modify the 3,4-dihydroxyphenylacetic acid/DA (DOPAC/DA) ratio or serum prolactin concentration in control rats. After mifepristone treatment, DA activity diminished significantly without modifying serum prolactin levels. Naloxone administration to antiprogesterone-treated rats did not change the DOPAC/DA ratio but increased serum prolactin. Tyrosine hydroxylase (TH) expression in medial basal hypothalamus (MBH) protein extracts was lowered by pretreatment with mifepristone, with no additional effect of naloxone. While mifepristone decreased the intensity of TH immunoreactivity in the arcuate and periventricular nuclei and in fibers of the median eminence, naloxone treatment had no further effect.

CONCLUSIONS

(1) A reduction of tuberoinfundibular dopaminergic (TIDA) neuron activity is suggested by the fall of the DOPAC/DA ratio and the low expression of MBH TH; (2) this reduction facilitates prolactin secretion by naloxone, indicating that progesterone stimulates DA neurons to maintain low serum prolactin; (3) naloxone action seems to depend on a previous decrease of DA tone induced by mifepristone, without involve a direct effect on neuronal DA activity, and (4) endogenous opioids may inhibit prolactin secretion through a non-dopaminergic neuronal system that regulates prolactin secretion in which as yet undetermined prolactin-releasing factors may participate.

Prepartum plasma estradiol and postpartum cortisol, but not oxytocin, are associated with interindividual and breed differences in the expression of maternal behaviour in sheep

Consistent, individual differences in the expression of maternal behaviour have been described in several species including the sheep. The neuroendocrine mechanisms underlying the onset of maternal behaviour in the sheep have been described, although the relationship between hormonal events and individual differences in behaviour has yet to be determined. In this study, we examined whether the individual differences in plasma estradiol, progesterone, oxytocin and cortisol concentrations were related to observed individual and breed differences in maternal behaviours in two breeds of sheep (Scottish Blackface and Suffolk) known to differ in maternal behavioural expression. Maternal estradiol concentration increased rapidly before parturition and was higher in Blackface ewes than Suffolk ewes. Plasma progesterone declined before parturition and was higher in Suffolk than Blackface ewes. Prepartum estradiol, but not progesterone, was related to individual differences in maternal grooming of the lamb. Plasma oxytocin did not differ between breeds in late gestation. There was a tendency for oxytocin to be higher in Blackface than Suffolk ewes immediately after birth. However, there were no significant relationships between prepartum or postpartum oxytocin and any maternal behaviours. Plasma cortisol was higher in Blackface than Suffolk ewes in the last days of pregnancy but rose in both breeds over the last 24 h before parturition and did not differ at delivery. Cortisol peripartum was negatively related to individual differences in maternal affiliative behaviours. These data suggest that estradiol, and potentially cortisol, may mediate individual differences in maternal behaviour in sheep.

Oestrogen receptor-beta and neurohypophysial hormones: functional interaction and neuroanatomical localisation

Oestrogens affect fluid balance, influencing both ingestive behaviour and renal excretion. The renal effects are partly due to altered release of vasopressin and oxytocin. This study was designed to explore the role of oestrogen receptor-beta (ERbeta) in neurohypophysial hormonal function. Following dietary administration, soya isoflavones reach the brain in sufficient concentration to activate ERbeta, but not oestrogen receptor-alpha (ERalpha). ERbeta function was therefore manipulated by feeding rat diets differing in soya isoflavone content. Fluid balance and neurohypophysial hormone release were measured in male rats maintained for 14 days on a soya isoflavone-free diet or one containing 150 microg/g genistein+daidzein. Food and water intake, body weight, urine flow, osmolality and sodium concentrations were determined daily. After 14 days, plasma and urine osmolality and sodium, vasopressin and oxytocin concentrations were determined. There was no significant difference in weight gain between the two groups or in their excretion of sodium and water or plasma sodium and plasma oxytocin. However, plasma vasopressin was significantly lower in the iso-free group. Double-label immunocytochemistry was used to assess colocalisation of ERbeta with the neurohypophysial hormones in male rats. Cell nuclei showing ERbeta immunoreactivity were abundant in the posterior magnocellular paraventricular nucleus (PVNpm) and in the supraoptic nucleus (SON). Vasopressin-immunoreactive neurones were similarly distributed, forming the core of the PVNpm and the ventral portion of the SON; majority were positive for ERbeta. Cells with oxytocin immunoreactivity were located mainly at the periphery of the PVNpm and in the dorsal SON; only approximately a quarter of these cells showed ERbeta immunoreactivity. Thus, the difference in the effects of the soya diet on vasopressin and oxytocin release may be related to the ERbeta-activating properties of this diet and to the preponderance of this receptor in vasopressin as opposed to oxytocin cells.

Prolactin and oxytocin interaction in the paraventricular and supraoptic nuclei: effects on oxytocin mRNA and nitric oxide synthase

We investigated the contribution of prolactin and oxytocin to the increase in staining for NADPH-d and oxytocin mRNA in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) observed at the end of pregnancy, or following a steroid-priming regimen that mimics the hormonal profile of late pregnant females. Ovariectomized rats received chronic implants of silastic capsules containing oestrogen and progesterone followed by progesterone removal. In experiment 1, oxytocin antagonist (OTA) was administered to rats to investigate whether intranuclear oxytocin release was necessary for NADPH-d staining. In experiments 2a and b, rats received concurrent treatment with bromocryptine (0.5 mg/day) to suppress endogenous prolactin release, and either systemic prolactin (0.5 mg once daily), or prolactin (2 micro g/ micro l), or vehicle infused twice a day into the third ventricle, or chronic oxytocin infusion (24 ng/day) for 3 days following progesterone removal. Brains were then processed for NADPH-d histochemistry. In experiment 3, the interaction of prolactin and oxytocin on oxytocin mRNA within the SON and PVN was examined. NADPH-d staining in the SON and PVN was reduced by the highest dose of the OTA, and by bromocryptine treatment. Central prolactin and oxytocin replacement completely restored NADPH-d staining in bromocryptine-treated rats. Finally, both bromocryptine and the OTA suppressed oxytocin mRNA expression and prolactin replacement restored expression levels to that of controls. Together, these data suggest that the increased capacity to produce nitric oxide in the SON and PVN during late pregnancy is dependent on prolactin stimulating oxytocin gene mRNA and hence intranuclear oxytocin release.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

Epidural analgesia: effects on labor progress and maternal and neonatal outcome

The intended and unintended effects of epidural labor analgesia are reviewed. Mothers randomized to epidural rather than parenteral opioid analgesia have better pain relief. Fetal oxygenation is not affected by analgesic method; however, neonates whose mothers received intravenous or intramuscular opioids rather than epidural analgesia require more naloxone and have lower Apgar scores. Epidural analgesia does not affect the rates of cesarean delivery, obstetrically indicated instrumented vaginal delivery, neonatal sepsis, or new-onset back pain. Epidural analgesia is associated with longer second labor stages, more frequent oxytocin augmentation, and maternal fever (particularly among women who shiver and women receiving epidural analgesia for > 5 hours) but not with longer first labor stages. Epidural analgesia has no affect but intrapartum opioids decrease lactation success. Epidural use and urinary incontinence are weakly, but probably not causally, associated. Epidural labor analgesia would improve if the mechanisms of these unintended effects could be determined.

Progesterone receptor expression in the pregnant and parturient rat hypothalamus and brainstem

Oxytocin is synthesized by magnocellular neurons in the supraoptic and paraventricular nuclei (SON and PVN) and during pregnancy progesterone prevents premature activation of oxytocin neurons. Progesterone receptors (PR) are not detectable in SON oxytocin neurons of non-pregnant rats, so we sought to determine whether they are expressed during pregnancy and parturition. In addition, we examined PR expression in brainstem and hypothalamic regions that have known direct projections to the SON. Neuronal immunoreactive PR (irPR)-labeled nuclei were counted in sections from proestrous virgin, late pregnant (day 21) and parturient rats (90 min from birth onset). IrPR nuclei were not evident in the SON at any stage but irPR expression in the medial preoptic nucleus (MPA) significantly increased in pregnancy and parturition (159% and 189% of proestrous controls, respectively). Other hypothalamic areas did not exhibit a significant change in irPR expression. In the nucleus tractus solitarius (NTS) in the brainstem, there was no significant change in irPR in late pregnancy, but there was a significant reduction in irPR expression at parturition (22% of proestrous controls). Very few NTS neurons immunoreactive for tyrosine hydroxylase (irTH), and thus putatively noradrenergic, contained irPR. These findings taken with evidence that brainstem irTH neurons projecting to the SON are stimulated at parturition, whereas MPA cells projecting to the SON are not, suggest that any direct actions of progesterone or progesterone withdrawal on NTS or SON neurons are not mediated through the classical PR. Upregulation of PR expression in the MPA during pregnancy and parturition may relate to the onset of maternal behavior and/or regulation of GnRH neuronal activity.

Brain preparations for maternity--adaptive changes in behavioral and neuroendocrine systems during pregnancy and lactation. An overview

Pregnancy, parturition and lactation comprise a continuum of adaptive changes necessary for the development and maintenance of the offspring. The endocrine changes that are driven by the conceptus and are essential for the maintenance of pregnancy and are involved in the preparations for motherhood are outlined. These changes include large increases in the secretion of sex steroid hormones, and the secretion of peptide hormones that are unique to pregnancy. The ability of these pregnancy hormones to alter several aspects of brain function in pregnancy is considered, and the adaptive importance of some of these changes is discussed, for example in metabolic and body fluid adjustments, and the induction of maternal behavior. The importance of sex steroids in determining the timing of the various adaptive changes in preparing for parturition and maternal behavior is emphasized, and the concept that the actions of prolactin and oxytocin, quintessential mammalian motherhood neuropeptides, can serve to coordinate a spectrum of adaptive changes is discussed. The part played by oxytocin neurons and their regulatory mechanisms is reviewed to illustrate how neural systems involved in maternity are prepared in pregnancy via changes in phenotype, synaptic organization and in the relative importance of their different inputs, to function optimally when needed. For oxytocin neurons secreting from the posterior pituitary, important in parturition and essential in lactation, these changes include mechanisms to restrain their premature activation, and adaptations to support synchronized burst firing for pulsatile oxytocin secretion in response to stimulation via afferents from the birth canal, olfactory system or suckled nipples. Within the brain, expression of oxytocin receptors permits centrally released oxytocin to facilitate the expression of maternal behavior. Changes in other neuroendocrine systems are similarly extensive, leading to lactation, suppression of ovulation, reduced stress responses and increased appetite; these changes in lactation are driven by the suckling stimulus. The possible link between these adaptations and changes in cognition and mood in pregnancy and post partum are considered, as well as the dysfunctions that lead to common problems of depression and puerperal psychoses.