Reduced hypothalamic vasopressin secretion underlies attenuated adrenocorticotropin stress responses in pregnant rats

Maternal glucocorticoids do not directly mediate the effects of maternal social stress on the fetus

Stress during pregnancy negatively affects the fetus and increases the risk for affective disorders in adulthood. Excess maternal glucocorticoids are thought to mediate fetal programming; however, whether they exert their effects directly or indirectly remains unclear. During pregnancy, protective mechanisms including maternal hypothalamic-pituitary-adrenal (HPA) axis hyporesponsiveness and placental 11β-hydroxysteroid dehydrogenase (11βHSD) type 2, which inactivates glucocorticoids, limit mother-to-fetus glucocorticoid transfer. However, whether repeated stress negatively impacts these mechanisms is not known. Pregnant rats were exposed to repeated social stress on gestational days (GD) 16-20 and several aspects of HPA axis and glucocorticoid regulation, including concentrations of glucocorticoids, gene expression for their receptors (Nr3c1, Nr3c2), receptor chaperones (Fkbp51, Fkbp52) and enzymes that control local glucocorticoid availability (Hsd11b1, Hsd11b2), were investigated in the maternal, placental and fetal compartments on GD20. The maternal HPA axis was activated following stress, though the primary driver was vasopressin, rather than corticotropin-releasing hormone. Despite the stress-induced increase in circulating corticosterone in the dams, only a modest increase was detected in the circulation of female fetuses, with no change in the fetal brain of either sex. Moreover, there was no change in the expression of genes that mediate glucocorticoid actions or modulate local concentrations in the fetal brain. In the placenta labyrinth zone, stress increased Hsd11b2 expression only in males and Fkbp51 expression only in females. Our results indicate that any role glucocorticoids play in fetal programming is likely indirect, perhaps through sex-dependent alterations in placental gene expression, rather than exerting effects via direct crossover into the fetal brain.

Allopregnanolone: An overview on its synthesis and effects

Allopregnanolone, a 3α,5α-progesterone metabolite, acts as a potent allosteric modulator of the γ-aminobutyric acid type A receptor. In the present review, the synthesis of this neuroactive steroid occurring in the nervous system is discussed with respect to physiological and pathological conditions. In addition, its physiological and neuroprotective effects are also reported. Interestingly, the levels of this neuroactive steroid, as well as its effects, are sex-dimorphic, suggesting a possible gender medicine based on this neuroactive steroid for neurological disorders. However, allopregnanolone presents low bioavailability and extensive hepatic metabolism, limiting its use as a drug. Therefore, synthetic analogues or a different therapeutic strategy able to increase allopregnanolone levels have been proposed to overcome any pharmacokinetic issues.

Involvement of neurosteroids in the control of hypothalamic-pituitary-adrenal axis activity in pregnant sheep under basal and stressful conditions

Neurosteroids are synthesized locally in the brain, where they can modify neuronal functionality depending on the physiological state. A high correlation was demonstrated between the increasing activity of the hypothalamic-pituitary-adrenal (HPA) axis and allopregnanolone (AL) concentration in the cerebrospinal fluid in sheep during pregnancy. Therefore, the present study tested the hypothesis that blocking neurosteroid synthesis in the brain of a pregnant sheep would affect HPA axis activity under both basal and stressful conditions. Two groups of sheep in the fourth month of gestation (n = 7 each) were subjected to the following treatments: 1) intracerebroventricular (icv) infusion of vehicle for three days (C) and then icv infusion of finasteride (a total of 100 μg/240 μL/day) for three days (F), one week apart, and 2) icv infusion of vehicle for three days and application of stressful stimuli (isolation and partial movement restriction) on the third day (S), and subsequently icv infusion of finasteride for three days and application of stressful stimuli on the third day (SF), one week apart. On the third days of the experiment, a 4-h push-pull perfusion of the infundibular nucleus/median eminence and blood sampling were performed. Mean perfusate corticotropin-releasing hormone (CRH), plasma adrenocorticotropin (ACTH) and cortisol concentrations were significantly higher in sheep treated with finasteride, stress and finasteride in combination with stress compared to controls. The highest hormone concentrations in Groups F, S and SF, were recorded during the first 60 min; however, significant increases in CRH and ACTH levels were observed in Group SF towards the end of the experiment. It can be concluded that neurosteroids may be an essential component of the mechanism controlling HPA axis activity in pregnant sheep, not only under stress-free conditions, but more importantly, also by inhibiting the neuroendocrine response to stressors.

Physiopathological role of the enzymatic complex 5α-reductase and 3α/β-hydroxysteroid oxidoreductase in the generation of progesterone and testosterone neuroactive metabolites

The enzymatic complex 5α-reductase (5α-R) and 3α/3β-hydroxysteroid oxidoreductase (HSOR) is expressed in the nervous system, where it transforms progesterone (PROG) and testosterone (T) into neuroactive metabolites. These metabolites regulate myelination, brain maturation, neurotransmission, reproductive behavior and the stress response. The expression of 5α-R and 3α-HSOR and the levels of PROG and T reduced metabolites show regional and sex differences in the nervous system and are affected by changing physiological conditions as well as by neurodegenerative and psychiatric disorders. A decrease in their nervous tissue levels may negatively impact the course and outcome of some pathological events. However, in other pathological conditions their increased levels may have a negative impact. Thus, the use of synthetic analogues of these steroids or 5α-R modulation have been proposed as therapeutic approaches for several nervous system pathologies. However, further research is needed to fully understand the consequences of these manipulations, in particular with 5α-R inhibitors.

Maternal distress, HPA activity, and antenatal interventions: A systematic review

BACKGROUND

Elevated antenatal distress has been associated with negative outcomes for both mothers and, as a result, their infants. One mechanism hypothesised to underlie these associations is the maternal hypothalamic pituitary adrenal (HPA) axis. Though research has examined whether biopsychosocial antenatal interventions can reduce maternal HPA activity, only one review has summarized the nature of findings to date. The present study examined randomised control trials (RCTs) specifically; our primary aim was to assess the effectiveness of antenatal interventions in reducing HPA activity in pregnant women, our secondary aim was to examine whether antenatal interventions reduced maternal self-report of depression and/or anxiety.

METHODS

This study systematically reviewed RCTs that evaluated biopsychosocial interventions that reported subjective and objective markers of maternal distress in pregnant women within the clinical population.

RESULTS

Eight studies met inclusion criteria and women were in their second or third trimester. HPA-activity was primarily assessed through salivary cortisol (n = 7) and self-reported maternal distress was assessed using a variety of validated screening measures. Included trials demonstrated significant methodological heterogeneity and small sample sizes, poor treatment adherence, and poor reliability in cortisol measurement indicated low methodological quality.

CONCLUSIONS

Due to the high heterogeneity across studies, small sample sizes, and unreliable sampling methods, firm conclusions about the efficacy and effectiveness of antenatal interventions cannot be drawn. Despite this, interventions which targeted pregnancy-specific influencers of maternal mood were more likely to result in reduced depression and anxiety symptomatology as reported by mothers.

Mom doesn't care: When increased brain CRF system activity leads to maternal neglect in rodents

Mothers are the primary caregivers in mammals, ensuring their offspring's survival. This strongly depends on the adequate expression of maternal behavior, which is the result of a concerted action of "pro-maternal" versus "anti-maternal" neuromodulators such as the oxytocin and corticotropin-releasing factor (CRF) systems, respectively. When essential peripartum adaptations fail, the CRF system has negative physiological, emotional and behavioral consequences for both mother and offspring often resulting in maternal neglect. Here, we provide an elaborate and unprecedented review on the implications of the CRF system in the maternal brain. Studies in rodents have advanced our understanding of the specific roles of brain regions such as the limbic bed nucleus of the stria terminalis, medial preoptic area and lateral septum even in a CRF receptor subtype-specific manner. Furthermore, we discuss potential interactions of the CRF system with other neurotransmitters like oxytocin and noradrenaline, and present valuable translational aspects of the recent research.

Giving a good start to a new life via maternal brain allostatic adaptations in pregnancy

Successful pregnancy requires adjustments to multiple maternal homeostatic mechanisms, governed by the maternal brain to support and enable survival of the growing fetus and placenta. Such adjustments fit the concept of allostasis (stability through change) and have a cost: allostatic load. Allostasis is driven by ovarian, anterior pituitary, placental and feto-placental hormones acting on the maternal brain to promote adaptations that support the pregnancy and protect the fetus. Many women carry an existing allostatic load into pregnancy, from socio-economic circumstances, poor mental health and in 'developed' countries, also from obesity. These pregnancies have poorer outcomes indicating negative interactions (failing allostasis) between pre-pregnancy and pregnancy allostatic loads. Use of animal models, such as adult prenatally stressed female offspring with abnormal neuroendocrine, metabolic and behavioural phenotypes, to probe gene expression changes, and epigenetic mechanisms in the maternal brain in adverse pregnancies are discussed, with the prospect of ameliorating poor pregnancy outcomes.

Early-life adversity-induced long-term epigenetic programming associated with early onset of chronic physical aggression: Studies in humans and animals

Objectives: To examine whether chronic physical aggression (CPA) in adulthood can be epigenetically programmed early in life due to exposure to early-life adversity. Methods: Literature search of public databases such as PubMed/MEDLINE and Scopus. Results: Children/adolescents susceptible for CPA and exposed to early-life abuse fail to efficiently cope with stress that in turn results in the development of CPA later in life. This phenomenon was observed in humans and animal models of aggression. The susceptibility to aggression is a complex trait that is regulated by the interaction between environmental and genetic factors. Epigenetic mechanisms mediate this interaction. Subjects exposed to stress early in life exhibited long-term epigenetic programming that can influence their behaviour in adulthood. This programming affects expression of many genes not only in the brain but also in other systems such as neuroendocrine and immune. Conclusions: The propensity to adult CPA behaviour in subjects experienced to early-life adversity is mediated by epigenetic programming that involves long-term systemic epigenetic alterations in a whole genome.

Neuroendocrinology and Adaptive Physiology of Maternal Care

Parental care is critical for offspring survival in many species. In mammals, parental care is primarily provided through maternal care, due to obligate pregnancy and lactation constraints, although some species also show paternal and alloparental care. These behaviors are driven by specialized neural circuits that receive sensory, cortical, and hormonal input to generate a coordinated and timely change in behavior, and sustain that behavior through activation of reward pathways. Importantly, the hormonal changes associated with pregnancy and lactation also act to coordinate a broad range of physiological changes to support the mother and enable her to adapt to the demands of these states. This chapter will review the neural pathways that regulate maternal behavior, the hormonal changes that occur during pregnancy and lactation, and how these two facets merge together to promote both young-directed maternal responses (including nursing and grooming) and young-related responses (including maternal aggression and other physiological adaptions to support the development of and caring for young). We conclude by examining how experimental animal work has translated into knowledge of human parenting, particularly in regards to maternal mental health issues.

The psychoneuroimmunology of pregnancy

Pregnancy is associated with a number of significant changes in maternal physiology. Perhaps one of the more notable changes is the significant alteration in immune function that occurs during pregnancy. This change in immune function is necessary to support a successful pregnancy, but also creates a unique period of life during which a female is susceptible to disease and, as we'll speculate here, may also contribute to mental health disorders associated with pregnancy and the postpartum period. Here, we review the known changes in peripheral immune function that occur during pregnancy and the postpartum period, while highlighting the impact of hormones during these times on immune function, brain or neural function, as well as behavior. We also discuss the known and possible impact of pregnancy-induced immune changes on neural function during this time and briefly discuss how these changes might be a risk factor for perinatal anxiety or mood disorders.

The Neuroendocrinology of the Microbiota-Gut-Brain Axis: A Behavioural Perspective

The human gut harbours trillions of symbiotic bacteria that play a key role in programming different aspects of host physiology in health and disease. These intestinal microbes are also key components of the gut-brain axis, the bidirectional communication pathway between the gut and the central nervous system (CNS). In addition, the CNS is closely interconnected with the endocrine system to regulate many physiological processes. An expanding body of evidence is supporting the notion that gut microbiota modifications and/or manipulations may also play a crucial role in the manifestation of specific behavioural responses regulated by neuroendocrine pathways. In this review, we will focus on how the intestinal microorganisms interact with elements of the host neuroendocrine system to modify behaviours relevant to stress, eating behaviour, sexual behaviour, social behaviour, cognition and addiction.

Inability to suppress the stress-induced activation of the HPA axis during the peripartum period engenders deficits in postpartum behaviors in mice

The stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis is normally suppressed during pregnancy. Dysregulation of the HPA axis has been proposed to play a role in postpartum depression. However, direct investigation into the relationship between the HPA axis and postpartum depression has been hindered by the lack of useful animal models. Building on our discovery of a role for the K+/Cl-co-transporter, KCC2, in the GABAergic regulation of CRH neurons in the paraventricular nucleus of the hypothalamus (PVN), critical for mounting the body's physiological response to stress, we assessed the role of KCC2 in the regulation of the HPA axis during pregnancy and the postpartum period. Here we demonstrate that the normal suppression of the stress-induced activation of the HPA axis during the peripartum period involves maintenance of KCC2 in the PVN. Mice lacking KCC2 specifically in corticosterone-releasing hormone (CRH) neurons, which govern the activity of the HPA axis (KCC2/Crh mice), exhibit dysregulation of the HPA axis and abnormal postpartum behaviors. Loss of KCC2 specifically in CRH neurons in the PVN is sufficient to reproduce the depression-like phenotype and deficits in maternal behaviors during the postpartum period. Similarly, chemogenetic activation of CRH neurons in the PVN is sufficient to induce abnormal postpartum behaviors and chemogenetic silencing of CRH neurons in the PVN can ameliorate abnormal postpartum behaviors observed in KCC2/Crh mice. This study demonstrates that dysregulation of the HPA axis is sufficient to induce abnormal postpartum behaviors and deficits in maternal behaviors in mice, providing empirical support for a role of HPA axis dysfunction in the pathophysiology of postpartum depression.

Arginine vasopressin (AVP): a review of its historical perspectives, current research and multifunctional role in the hypothalamo-hypophysial system

INTRODUCTION

This publication reviews the function of arginine vasopressin and focuses on the morphologic and functional correlation between the hormone and its effect on stress, the hypophysial-adrenocortical axis, neuroimmune responses, renal function and corticotroph pituitary tumors.

MATERIALS AND METHODS

A literature review was performed using various search engines for information regarding the morphology and the multifunctional role of arginine vasopressin.

RESULTS

Although a large number of studies were published discussing these interactions, there are several important areas that are still obscure.

CONCLUSION

The questions of how does arginine vasopressin affect the morphology and function of these various areas, and how does the secretion of ACTH and adrenocortical hormones influence the morphology of arginine vasopressin-producing cells and their hormone secretion requires further investigation.

Neuroactive steroids and stress axis regulation: Pregnancy and beyond

The hypothalamo-pituitary-adrenal (HPA) axis plays a critical role in regulating responses to stress and long term dysregulation of the HPA axis is associated with higher rates of mood disorders. There are circumstances where the HPA axis is more or less responsive to stress. For example, during late pregnancy ACTH and corticosterone responses to stress are markedly suppressed, whereas in offspring born to mothers that experienced repeated stress during pregnancy, the HPA axis is hyper-responsive to stress. Neuroactive steroids such as allopregnanolone, tetrahydrodeoxycorticosterone (THDOC) and androstanediol can modulate HPA axis activity and concentrations of some neuroactive steroids in the brain are altered during pregnancy and following stress. Thus, here altered neurosteroidogenesis is proposed as a mechanism that could underpin the dynamic changes in HPA axis regulation typically observed in late pregnant and in prenatally stressed individuals. In support of this hypothesis, evidence in rats demonstrates that elevated levels of allopregnanolone in pregnancy induce a central inhibitory opioid mechanism that serves to minimize stress-induced HPA axis activity. Conversely, in prenatally stressed rodents, where HPA axis stress responses are enhanced, evidence indicates the capacity of the brain for neurosteroidogenesis is reduced. Understanding the mechanisms involved in adaptations in HPA axis regulation may provide insights for manipulating stress sensitivity and for developing therapies for stress-related disorders in humans.

Using animal models to study post-partum psychiatric disorders

The post-partum period represents a time during which all maternal organisms undergo substantial plasticity in a wide variety of systems in order to ensure the well-being of the offspring. Although this time is generally associated with increased calmness and decreased stress responses, for a substantial subset of mothers, this period represents a time of particular risk for the onset of psychiatric disorders. Thus, post-partum anxiety, depression and, to a lesser extent, psychosis may develop, and not only affect the well-being of the mother but also place at risk the long-term health of the infant. Although the risk factors for these disorders, as well as normal peripartum-associated adaptations, are well known, the underlying aetiology of post-partum psychiatric disorders remains poorly understood. However, there have been a number of attempts to model these disorders in basic research, which aim to reveal their underlying mechanisms. In the following review, we first discuss known peripartum adaptations and then describe post-partum mood and anxiety disorders, including their risk factors, prevalence and symptoms. Thereafter, we discuss the animal models that have been designed in order to study them and what they have revealed about their aetiology to date. Overall, these studies show that it is feasible to study such complex disorders in animal models, but that more needs to be done in order to increase our knowledge of these severe and debilitating mood and anxiety disorders.

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.

Hormonal influences on neuroimmune responses in the CNS of females

Particular reproductive stages such as lactation impose demands on the female. To cope with these demands, her physiology goes through numerous adaptations, for example, attenuation of immune and stress responses. Hormonal fluctuation during lactation exerts a strong influence, inducing neuroplasticity in the hypothalamus and extrahypothalamic regions, and diminishing the stress and inflammatory responses. Thus, hormones confer decreased vulnerability to the female brain. This mini-review focuses on the adaptations of the immune and stress response during maternity, and on the neuroprotective actions of progesterone and prolactin and their effects on inflammation. The importance of pregnancy and lactation as experimental models to study immune responses and disease is also highlighted.

Neuroendocrinology of pregnancy and parturition

During pregnancy, the maternal brain drives a series of adaptive mechanisms that are fundamental for allowing fetal growth and development, protecting both mother and fetus from adverse programming and timing of parturition. This neuroendocrine concept is even more complex as fetal brain and placenta also participate as regulators of maternal-placental-fetal physiology. The placenta is now seen as a neuroendocrine organ, acting as a source of several neuroactive factors that may exert their biologic effects either locally or by entering maternal and fetal circulation, thus acting in an autocrine, paracrine, and endocrine manner. A variety of hypothalamic neurohormones (GnRH, GHRH, somatostatin, CRH, oxytocin) are expressed in the placenta. When stress occurs during pregnancy, the maternal, fetal, and placental hypothalamic-pituitary-adrenal (HPA) axes are activated to stimulate a series of responses contributing to maintain physiologic conditions while at the same time avoiding the adverse effects of stress on the mother and offspring. However, when stress is excessive, a number of obstetric complications may occur, such as preterm birth, pre-eclampsia and intrauterine growth restriction, related to an impairment of the placental adaptive response.

Stress responsiveness of the hypothalamic-pituitary-adrenal axis: age-related features of the vasopressinergic regulation

The hypothalamic-pituitary-adrenal (HPA) axis plays a key role in adaptation to environmental stresses. Parvicellular neurons of the hypothalamic paraventricular nucleus secrete corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) into pituitary portal system; CRH and AVP stimulate adrenocorticotropic hormone (ACTH) release through specific G-protein-coupled membrane receptors on pituitary corticotrophs, CRHR1 for CRH and V1b for AVP; the adrenal gland cortex secretes glucocorticoids in response to ACTH. The glucocorticoids activate specific receptors in brain and peripheral tissues thereby triggering the necessary metabolic, immune, neuromodulatory, and behavioral changes to resist stress. While importance of CRH, as a key hypothalamic factor of HPA axis regulation in basal and stress conditions in most species, is generally recognized, role of AVP remains to be clarified. This review focuses on the role of AVP in the regulation of stress responsiveness of the HPA axis with emphasis on the effects of aging on vasopressinergic regulation of HPA axis stress responsiveness. Under most of the known stressors, AVP is necessary for acute ACTH secretion but in a context-specific manner. The current data on the AVP role in regulation of HPA responsiveness to chronic stress in adulthood are rather contradictory. The importance of the vasopressinergic regulation of the HPA stress responsiveness is greatest during fetal development, in neonatal period, and in the lactating adult. Aging associated with increased variability in several parameters of HPA function including basal state, responsiveness to stressors, and special testing. Reports on the possible role of the AVP/V1b receptor system in the increase of HPA axis hyperactivity with aging are contradictory and requires further research. Many contradictory results may be due to age and species differences in the HPA function of rodents and primates.

Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics

We recently reviewed the status of peptide and nonpeptide agonists and antagonists for the V(1a), V(1b) and V(2) receptors for arginine vasopressin (AVP) and the oxytocin receptor for oxytocin (OT). In the present review, we update the status of peptides and nonpeptides as: (i) research tools and (ii) therapeutic agents. We also present our recent findings on the design of fluorescent ligands for V(1b) receptor localisation and for OT receptor dimerisation. We note the exciting discoveries regarding two novel naturally occurring analogues of OT. Recent reports of a selective VP V(1a) agonist and a selective OT agonist point to the continued therapeutic potential of peptides in this field. To date, only two nonpeptides, the V(2) /V(1a) antagonist, conivaptan and the V(2) antagonist tolvaptan have received Food and Drug Administration approval for clinical use. The development of nonpeptide AVP V(1a), V(1b) and V(2) antagonists and OT agonists and antagonists has recently been abandoned by Merck, Sanofi and Pfizer. A promising OT antagonist, Retosiban, developed at Glaxo SmithKline is currently in a Phase II clinical trial for the prevention of premature labour. A number of the nonpeptide ligands that were not successful in clinical trials are proving to be valuable as research tools. Peptide agonists and antagonists continue to be very widely used as research tools in this field. In this regard, we present receptor data on some of the most widely used peptide and nonpeptide ligands, as a guide for their use, especially with regard to receptor selectivity and species differences.

Neuroendocrine control of maternal stress responses and fetal programming by stress in pregnancy

The major changes in highly dynamic neuroendocrine systems that are essential for establishing and maintaining pregnancy are outlined from studies on rodents. These changes optimise the internal environment to provide the life support system for the placenta, embryo and fetus. These include automatic prevention of further pregnancy, blood volume expansion, increased appetite and energy storage. The brain regulates these changes, in response to steroid (estrogens, progesterone) and peptide (lactogens, relaxin) hormone signals. Activation of inhibitory endogenous opioid mechanisms in the brain in late pregnancy restrains premature secretion of oxytocin, and attenuates hypothalamo-pituitary-adrenal (HPA) responses to stress. This opioid mechanism is activated by allopregnanolone, a neuroactive progesterone metabolite. The significance of reduced HPA axis responses in shifting maternal metabolic balance, and in protecting the fetuses from adverse programming of HPA axis stress responsiveness and anxious behaviour in later life is critically discussed. Experimental studies showing sex-dependent fetal programming by maternal stress or glucocorticoid exposure in late pregnancy are reviewed. The possibility of over-writing programming in offspring through neurosteroid administration is discussed. The impact of maternal stress on placental function is considered in the context of reconciling studies that show offspring programming by stress in very early or late pregnancy produce similar phenotypes in the offspring.

Prenatal Stress Alters Progestogens to Mediate Susceptibility to Sex-Typical, Stress-Sensitive Disorders, such as Drug Abuse: A Review

Maternal-offspring interactions begin prior to birth. Experiences of the mother during gestation play a powerful role in determining the developmental programming of the central nervous system. In particular, stress during gestation alters developmental programming of the offspring resulting in susceptibility to sex-typical and stress-sensitive neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. However, neither these effects, nor the underlying mechanisms, are well understood. Our hypothesis is that allopregnanolone, during gestation, plays a particularly vital role in mitigating effects of stress on the developing fetus and may mediate, in part, alterations apparent throughout the lifespan. Specifically, altered balance between glucocorticoids and progestogens during critical periods of development (stemming from psychological, immunological, and/or endocrinological stressors during gestation) may permanently influence behavior, brain morphology, and/or neuroendocrine-sensitive processes. 5α-reduced progestogens are integral in the developmental programming of sex-typical, stress-sensitive, and/or disorder-relevant phenotypes. Prenatal stress (PNS) may alter these responses and dysregulate allopregnanolone and its normative effects on stress axis function. As an example of a neurodevelopmental, neuropsychiatric, and/or neurodegenerative process, this review focuses on responsiveness to drugs of abuse, which is sensitive to PNS and progestogen milieu. This review explores the notion that allopregnanolone may effect, or be influenced by, PNS, with consequences for neurodevelopmental-, neuropsychiatric-, and/or neurodegenerative- relevant processes, such as addiction.

Allopregnanolone and suppressed hypothalamo-pituitary-adrenal axis stress responses in late pregnancy in the rat

In rats, late pregnancy is associated with suppressed hypothalamo-pituitary-adrenal (HPA) axis responses to a variety of stressful stimuli. The result is reduced corticosterone secretion following stress, which is considered to give some protection to the fetuses from adverse glucocorticoid programming and limits the catabolic effect of corticosterone, hence minimizing maternal energy expenditure. We have used a model of immune challenge in which systemic administration of the cytokine, interleukin-1β (IL-1β), allows study of the mechanisms underlying HPA axis hyporesponsiveness in late pregnancy. Suppressed responsiveness of parvocellular paraventricular nucleus (pPVN) corticotropin-releasing hormone neurons, and hence the HPA axis, following IL-1β in late pregnancy is evidently explained by presynaptic inhibition of noradrenaline release in the pPVN, owing to increased endogenous opioid peptide enkephalin production in brainstem nucleus tractus solitarii neurons. Allopregnanolone, a neurosteroid metabolite of progesterone, signals the pregnancy status of the animal to the brain and stimulates opioid production in the brainstem. In this review, we discuss the supporting evidence for these mechanisms.

Resetting the dynamic range of hypothalamic-pituitary-adrenal axis stress responses through pregnancy

The hypothalamic-pituitary-adrenal (HPA) axis plays a key role in the neuroendocrine response to stress. Dynamic changes in HPA axis regulation and hence HPA responsivity occur over the lifetime of an animal. This article focuses on two extremes of the spectrum. The first occurs naturally during pregnancy when stress responses are dampened. The second, at the opposite end of the scale, occurs in offspring of mothers who were exposed to stress during pregnancy and display exaggerated HPA axis stress responses. Reduced glucocorticoid output in response to stress in pregnancy may have important consequences for conserving energy supply to the foetus(es), in modulating immune system adaptations and in protecting against adverse foetal programming by glucocorticoids. Understanding the mechanisms underpinning this adaptation in pregnancy may provide insights for manipulating HPA axis responsiveness in later life, particularly in the context of resetting HPA axis hyperactivity associated with prenatal stress exposure, which may underlie several major pathologies, including cardiovascular disease, diabetes mellitus type 2, obesity, cognitive decline and mood disorders.

Baby on board: do responses to stress in the maternal brain mediate adverse pregnancy outcome?

Stress and adverse environmental surroundings result in suboptimal conditions in a pregnant mother such that she may experience poor pregnancy outcome including complete pregnancy failure and preterm labor. Furthermore her developing baby is at risk of adverse programming, which confers susceptibility to long term ill health. While some mechanisms at the feto-maternal interface underlying these conditions are understood, the underlying cause for their adverse adaptation is often not clear. Progesterone plays a key role at many levels, including control of neuroendocrine responses to stress, procuring the required immune balance and controlling placental and decidual function, and lack of progesterone can explain many of the unwanted consequences of stress. How stress that is perceived by the mother inhibits progesterone secretion and action is beginning to be investigated. This overview of maternal neuroendocrine responses to stress throughout pregnancy analyses how they interact to compromise progesterone secretion and precipitate undesirable effects in mother and offspring.

Prolactin-releasing peptide regulates the cardiovascular system via corticotrophin-releasing hormone

Prolactin-releasing peptide (PrRP)-producing neurones are known to be localised mainly in the medulla oblongata and to act as a stress mediator in the central nervous system. In addition, central administration of PrRP elevates the arterial pressure and heart rate. However, the neuronal pathway of the cardiovascular effects of PrRP has not been revealed. In the present study, we demonstrate that PrRP-immunoreactive neurones projected to the locus coeruleus (LC) and the paraventricular nucleus (PVN) of the hypothalamus. The c-fos positive neurones among the noradrenaline cells in the LC, and the parvo- and magnocellular neurones in the PVN, were increased after central administration of PrRP. The arterial pressure and heart rate were both elevated after i.c.v. administration of PrRP. Previous studies have demonstrated that PrRP stimulated the neurones in the PVN [i.e. oxytocin-, vasopressin- and corticotrophin-releasing hormone (CRH)-producing neurones], which suggests that PrRP may induce its cardiovascular effect via arginine vasopressin (AVP) or CRH. Although the elevation of blood pressure and heart rate elicited by PrRP administration were not inhibited by an AVP antagonist, they were completely suppressed by treatment with a CRH antagonist. Thus, we conclude that PrRP stimulated CRH neurones in the PVN and that CRH might regulate the cardiovascular system via the sympathetic nervous system.

Central opioid inhibition of neuroendocrine stress responses in pregnancy in the rat is induced by the neurosteroid allopregnanolone

The hypothalamus-pituitary-adrenal (HPA) axis is the major neuroendocrine stress response system. Corticotropin-releasing hormone (CRH) neurons in the parvocellular paraventricular nucleus (pPVN) play a key role in coordinating responses of this system to stressors. The cytokine interleukin-1beta (IL-1beta), mimicking infection, robustly activates these CRH neurons via a noradrenergic input arising from the nucleus tractus solitarii (NTS). In late pregnancy, HPA axis responses to stressors, including IL-1beta, are attenuated by a central opioid mechanism that auto-inhibits noradrenaline release in the PVN. Here we show that the neuroactive progesterone metabolite allopregnanolone induces these changes in HPA responsiveness to IL-1beta in pregnancy. In late pregnancy, inhibition of 5alpha-reductase (an allopregnanolone-synthesizing enzyme) with finasteride restored HPA axis responses (rapidly increased pPVN CRH mRNA expression, ACTH, and corticosterone secretion) to IL-1beta. Conversely, allopregnanolone reduced HPA responses in virgin rats. In late pregnancy, activity of the allopregnanolone-synthesizing enzymes (5alpha-reductase and 3alpha-hydroxysteroid dehydrogenase) was increased in the hypothalamus as was mRNA expression in the NTS and PVN. Naloxone, an opioid antagonist, restores HPA axis responses to IL-1beta in pregnancy but had no additional effect after finasteride, indicating a causal connection between allopregnanolone and the endogenous opioid mechanism. Indeed, allopregnanolone induced opioid inhibition over HPA responses to IL-1beta in virgin rats. Furthermore, in virgin rats, allopregnanolone treatment increased, whereas in pregnant rats finasteride decreased proenkephalin-A mRNA expression in the NTS. Thus, in pregnancy, allopregnanolone induces opioid inhibition over HPA axis responses to immune challenge. This novel opioid-mediated mechanism of allopregnanolone action may alter regulation of other brain systems in pregnancy.

Social buffering in rats: prolactin attenuation of active interaction

Stress may result when the present environment is interpreted as threatening, and stress is known to increase the prolactin-secretory response. In the present study, rats (N=83) were exposed to a conditioned-fear paradigm (environment paired with footshock), and on testing day, rats were exposed to the experimental chamber without shock while alone (Alone n=16), with an object (Object n=17), with a euthanized conspecific (Euthanized n=16), or with a social partner (Social n=19). The control group (Control n=15) was exposed to the experimental chamber but was never shocked. The Control group had significantly lower levels of prolactin than the Alone, Object, and Euthanized groups; however, the Control group's levels of prolactin were not significantly different than that of the Social group, which was significantly lower than that for the Alone group. Social interaction decreased fear independent of the distraction provided by a stimulus in the chamber. Active touch appeared to be crucial for social buffering to occur.

Adaptive responses of the maternal hypothalamic-pituitary-adrenal axis during pregnancy and lactation

Over the past 40 years, it has been recognised that the maternal hypothalamic-pituitary-adrenal (HPA) axis undergoes adaptations through pregnancy and lactation that might contribute to avoidance of adverse effects of stress on the mother and offspring. The extent of the global adaptations in the HPA axis has been revealed and the underlying mechanisms investigated within the last 20 years. Both basal, including the circadian rhythm, and stress-induced adrenocorticotrophic hormone and glucocorticoid secretory patterns are altered. Throughout most of pregnancy, and in lactation, these changes predominantly reflect reduced drive by the corticotropin-releasing factor (CRF) neurones in the parvocellular paraventricular nucleus (pPVN). An accompanying profound attenuation of HPA axis responses to a wide variety of psychological and physical stressors emerges after mid-pregnancy and persists until the end of lactation. Central to this suppression of stress responsiveness is reduced activation of the pPVN CRF neurones. This is consequent on the reduced effectiveness of the stimulation of brainstem afferents to these CRF neurones (for physical stressors) and of altered processing by limbic structures (for emotional stressors). The mechanism of reduced CRF neurone responses to physical stressors in pregnancy is the suppression of noradrenaline release in the PVN by an up-regulated endogenous opioid mechanism, which is induced by neuroactive steroid produced from progesterone. By contrast, in lactation suckling the young provides a neural stimulus that dampens the HPA axis circadian rhythm and reduces stress responses. Reduced noradrenergic input activity is involved in reduced stress responses in lactation, although central prolactin action also appears important. Such adaptations limit the adverse effects of excess glucocorticoid exposure on the foetus(es) and facilitate appropriate metabolic and immune responses.

The expectant brain: adapting for motherhood

A successful pregnancy requires multiple adaptations of the mother's physiology to optimize fetal growth and development, to protect the fetus from adverse programming, to provide impetus for timely parturition and to ensure that adequate maternal care is provided after parturition. Many of these adaptations are organized by the mother's brain, predominantly through changes in neuroendocrine systems, and these changes are primarily driven by the hormones of pregnancy. By contrast, adaptations in the mother's brain during lactation are maintained by external stimuli from the young. The changes in pregnancy are not necessarily innocuous: they may predispose the mother to post-partum mood disorders.

Relations between mechanisms of CNS arousal and mechanisms of stress

In the centennial year of the birth of Hans Selye, this review compares his classical concepts of stress with a modern approach to mechanisms of CNS arousal. Relations between the two concepts are described. Neuroanatomical, neurophysiological, and functional genomic mechanisms underlying CNS arousal are briefly reviewed. Controls over stress responses and arousal are compared to particular concepts of control system engineering. Understanding these two systems is of crucial importance because their dysregulation is associated with large numbers of disease states.

Hypothalamic-pituitary-adrenal axis hyporesponsiveness to restraint stress in mice deficient for large-conductance calcium- and voltage-activated potassium (BK) channels

Stress activates the hypothalamic-pituitary-adrenal (HPA) axis, releasing ACTH from the anterior pituitary gland and glucocorticoids from the adrenal cortex. Stress also activates the sympathetic nervous system, evoking adrenaline release from the adrenal medulla. Large-conductance calcium- and voltage-activated potassium (BK) channels have been implicated in regulation of cellular excitability in these systems. Here, we examine the functional role of BK channels in HPA axis regulation in vivo using female mice genetically deficient (BK(-/-)) for the pore-forming subunits of BK channels. BK(-/-) phenotype in the HPA was confirmed by immunohistochemistry, Western blot analysis, and corticotrope patch-clamp recording. Restraint stress-induced plasma concentrations of ACTH and corticosterone were significantly blunted in BK(-/-) mice compared with wild type (WT) controls. This stress hyporesponsiveness was associated with reduced activation of hypothalamic paraventricular nucleus (PVN) neurons. Basal expression of CRH, but not arginine vasopressin mRNA in the PVN was significantly lower in BK(-/-) mice compared with WT controls. Total anterior pituitary ACTH peptide content, but not proopiomelanocortin mRNA expression or corticotrope number, was significantly reduced in BK(-/-) mice compared with WT. However, anterior pituitary corticotropes from BK(-/-) mice fully supported ACTH output, releasing a significantly greater proportion of stored ACTH in response to secretagogue in vitro compared with WT. These results support an important role for BK channels in both the neural circuitry and endocrine output of the HPA axis and indicate that the stress hyporesponsiveness in BK(-/-) mice primarily results from reduced activation of hypothalamic PVN neurosecretory neurons.

ACTH cells of pituitary pars distalis of viscacha (Lagostomus maximus maximus): immunohistochemical study in relation to season, sex, and growth

Corticotrope or ACTH cells were immunohistochemically identified in the pituitary pars distalis (PD) of viscacha by using the polyclonal antiserum against ACTH (1-24). The localization, distribution, shape, percentage immunopositive area, and major cellular and nuclear diameters of these cells were analyzed by image analysis in adult male viscachas captured in their natural habitat during the year and after the chronic administration of melatonin. The same parameters were analyzed in immature male and adult female viscachas. ACTH cells in adult males were mainly localized in the dorsal and cephalic regions of PD. They were isolated, forming small groups and in contact with follicular structures and blood vessels. They were pleomorphic, with some of them being polygonal, oval, round, and others, stellate with cytoplasmic extensions. The percentage immunopositive area and the major cellular diameter showed seasonal variations with lower values during June and July (early winter). A decrease in the percentage immunopositive area was observed after the administration of melatonin in adult male animals. ACTH cells of immature animals differed from the adults' cells in their distribution, shape, pattern of immunolabeling, and percentage immunopositive area. These parameters in adult females did not vary in relation to adult males at the same time of the year although. However, the cells in females were smaller in size during April-May. In pregnant viscachas (June-July), these parameters did not show significant differences with the results of non-pregnant females (April-May). This suggests that the environmental stressors do not exert the same influence on the hypothalamo-pituitary-adrenal axis in adult male and female viscachas, probably due to the physiological changes caused by pregnancy. Our results in adult male viscacha demonstrate that the morphology of the ACTH cells varies according to the different seasonal conditions, thus participating in the adaptation process of this rodent to the environment. The elevated levels of melatonin during winter months might inhibit the synthesis of ACTH, probably when affecting some secretagogue of this pituitary hormone. Moreover, the morphological variations observed between adult and immature male viscachas and between both sexes suggest that the steroid gonadal hormones might act on the development and activity of ACTH cells.

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.

Fetal programming of hypothalamo-pituitary-adrenal function: prenatal stress and glucocorticoids

Prenatal stress (PS) and maternal exposure to exogenous glucocorticoids can lead to permanent modification of hypothalamo-pituitary-adrenal (HPA) function and stress-related behaviour. Both of these manipulations lead to increased fetal exposure to glucocorticoids. Glucocorticoids are essential for many aspects of normal brain development, but exposure of the fetal brain to an excess of glucocorticoids can have life-long effects on neuroendocrine function. Both endogenous glucocorticoid and synthetic glucocorticoid exposure have a number of rapid effects in the fetal brain, including modification of neurotransmitter systems and transcriptional machinery. Such fetal exposure permanently alters HPA function in prepubertal, postpubertal and ageing offspring, in a sex-dependent manner. Prenatal stress and exogenous glucocorticoid manipulation also lead to the modification of behaviour, brain and organ morphology, as well as altered regulation of other endocrine systems. It is also becoming increasingly apparent that the timing of exposure to PS or synthetic glucocorticoids has tremendous effects on the nature of the phenotypic outcome. Permanent changes in endocrine function will ultimately impact on health in both human and animal populations.

Position 4 analogues of [deamino-Cys1] arginine vasopressin exhibit striking species differences for human and rat V2/V1b receptor selectivity

Arginine vasopressin (AVP) mediates a wide variety of biological actions by acting on three distinct G-protein coupled receptors, termed V(1a) (vascular), V(1b) (pituitary) and V(2) (renal). It also binds to the oxytocin (OT) receptor. As part of a program aimed at the design of selective agonists for the human V(1b) receptor, we recently reported the human V(1b), V(1a), V(2) and OT receptor affinities of the following position 4 substituted analogues of [deamino-Cys(1)] arginine vasopressin (dAVP)-(1) d[Leu(4)]AVP, (2) d[Orn(4)]AVP, (3) d[Lys(4)]AVP, (4) d[Har(4)]AVP, (5) d[Arg(4)]AVP, (6) d[Val(4)]AVP, (7) d[Ala(4)]AVP, (8) d[Abu(4)]AVP, (9) d[Nva(4)]AVP, (10) d[Nle(4)]AVP, (11) d[Ile(4)]AVP, (12) d[Phe(4)]AVP, (13) d[Asn(4)]AVP, (14) d[Thr(4)]AVP: (15) d[Dap(4)]AVP. With the exception of Nos. 7 and 12, all peptides exhibit very high affinities for the human V(1b) receptor. Furthermore, peptides 1-4 exhibit high selectivities for the human V(1b) receptor with respect to the V(1a), V(2) and OT receptors and, with d[Cha(4)]AVP, in functional tests, are the first high affinity selective agonists for the human V(1b) receptor (Cheng LL et al., J. Med. Chem. 47: 2375-2388, 2004). We report here the pharmacological properties of peptides 1-4, 5 (from a resynthesis), 7, 9-13, 15 in rat bioassays (antidiuretic, vasopressor and oxytocic) (in vitro: no Mg(++)) with those previously reported for peptides 5, 6, 8, 14. We also report the rat V(1b), V(1a), V(2) and OT receptor affinities of peptides 1-5 and the rat V(2) receptor affinities for peptides: 7-15.The antidiuretic activities in units/mg of peptides 1-15, are: 1=378; 2=260; 3=35; 4=505; 5=748; 6=1150; 7=841; 8=1020; 9=877; 10=1141; 11=819, 12=110; 13=996; 14=758; 15=1053. Peptides 1-4 exhibit respectively the following rat and human (in brackets) V(2) receptor affinities: 1=3.1 nm (245 nm); 2=3.4 nm (1125 nm); 3=24.6 nm (11,170 nm); 4=0.6 nm (1386 nm). Their rat V(1b) receptor affinities are 1=0.02 nm; 2=0.45 nm; 3=9.8 nm; 4=0.32 nm. Their rat V(1a) receptor affinities are 1=1252 nm; 2=900 nm; 3=1478 nm; 4=32 nm. Their rat oxytocin (OT) receptor affinities are 1=481 nm; 2=997 nm; 3=5042 nm; 4=2996 nm. All four peptides have high affinities and selectivities for the rat V(1b) receptor with respect to the rat V(1a) and OT receptors. However, in contrast to their high selectivity for the human V(1b) receptor with respect to the human V(2) receptor, they are not selective for the V(1b) receptor with respect to the V(2) receptor in the rat. These findings confirm previous observations of profound species differences between the rat and human V(2) receptors. Peptides 1-4 are promising leads to the design of the first high affinity selective agonists for the rat V(1b) receptor.

Endogenous opioids and attenuated hypothalamic-pituitary-adrenal axis responses to immune challenge in pregnant rats

In late pregnant rats, the hypothalamic-pituitary-adrenal (HPA) axis is hyporesponsive to psychogenic stressors. Here, we investigated attenuated HPA responses to an immune challenge and a role for endogenous opioids. ACTH and corticosterone were assayed in blood samples from virgin and 21 d pregnant rats before and after endotoxin [lipopolysaccharide (LPS); 1 microg/kg, i.v.], interleukin-1beta (IL-1beta; 500 ng/kg, i.v.), or vehicle. In virgins, plasma ACTH concentrations increased 1 h after LPS and 15 min after IL-1beta, as did corticosterone, with no responses in pregnant rats. In situ hybridization revealed increased corticotrophin releasing hormone (CRH) mRNA expression in the dorsomedial parvocellular paraventricular nucleus (pPVN) and increased anterior pituitary pro-opiomelanocortin mRNA expression 4 h after IL-1beta in virgins; these responses were absent in pregnant rats. In contrast, immunocytochemistry showed that Fos expression was similarly increased in the nucleus tractus solitarius (NTS) A2 region in virgin and pregnant rats 90 min and 4 h after IL-1beta. Naloxone pretreatment (5 mg/kg, i.v.) restored ACTH and pPVN CRH mRNA responses after IL-1beta in pregnant rats but reduced the CRH mRNA response in virgins without affecting ACTH. Proenkephalin-A and mu-opioid receptor mRNA expression in the NTS was significantly increased in the pregnant rats, indicating upregulated brainstem opioid mechanisms. IL-1beta increased noradrenaline release in the PVN of virgin, but not pregnant, rats. However, naloxone infused directly into the PVN increased noradrenaline release after IL-1beta in pregnant rats. Thus, the HPA axis responses to immune signals are suppressed in pregnancy at the level of pPVN CRH neurons through an opioid mechanism, possibly acting by preterminal autoinhibition of NTS projections to the pPVN.

Measuring stress responses in postpartum mothers: perspectives from studies in human and animal populations

Reduced hypothalamic-pituitary-adrenal (HPA) responses to stress during the last week of pregnancy and lactation have been consistently observed in rat studies. Several contributing factors have been proposed for this phenomenon in lactation, including the suckling stimulus from the pups, hormones (oxytocin and prolactin) and opioids, a decrease in the ability of noradrenaline to potentiate hypothalamic responses and changes in pituitary responsiveness to ACTH secretagogues (AVP and CRF). In contrast to this vast literature using the rat model, only few studies have addressed this issue in the human population. The consensus is that women engaging in breastfeeding activities exhibit reduced anxiety, although the reductions in neuroendocrine and autonomic responses to stressors are variable, in part because of the different nature of the stressors used. Further work is required to investigate how additional factors, such as maternal parity or emotional salience of the stressor can affect stress responsiveness in postpartum women. Here, we review first the findings regarding stress responsiveness during lactation in both rat and human studies, and then discuss potential research avenues and methodological issues that could be the lead to future research protocols in human subjects. Knowing the reciprocal relationship in the mother-infant dyad, it is clear that investigation of the mechanisms regulating stress responses and mental health in postpartum mothers can only be beneficial to the development of the infant.