Oxytocin actions within the supraoptic and paraventricular nuclei: differential effects on peripheral and intranuclear vasopressin release

Oxytocin: A Multi-Functional Biomolecule with Potential Actions in Dysfunctional Conditions; From Animal Studies and Beyond

Oxytocin is a hormone secreted from definite neuroendocrine neurons located in specific nuclei in the hypothalamus (mainly from paraventricular and supraoptic nuclei), and its main known function is the contraction of uterine and/or mammary gland cells responsible for parturition and breastfeeding. Among the actions of the peripherally secreted oxytocin is the prevention of different degenerative disorders. These actions have been proven in cell culture and in animal models or have been tested in humans based on hypotheses from previous studies. This review presents the knowledge gained from the previous studies, displays the results from oxytocin intervention and/or treatment and proposes that the well described actions of oxytocin might be connected to other numerous, diverse actions of the biomolecule.

Early-life oxytocin attenuates the social deficits induced by caesarean-section delivery in the mouse

The oxytocin (OXT) system has been strongly implicated in the regulation of social behaviour and anxiety, potentially contributing to the aetiology of a wide range of neuropathologies. Birth by Caesarean-section (C-section) results in alterations in microbiota diversity in early-life, alterations in brain development and has recently been associated with long-term social and anxiety-like behaviour deficits. In this study, we assessed whether OXT intervention in the early postnatal period could reverse C-section-mediated effects on behaviour, and physiology in early life and adulthood. Following C-section or per vaginum birth, pups were administered with OXT (0.2 or 2 μg/20 μl; s.c.) or saline daily from postnatal days 1-5. We demonstrate that early postnatal OXT treatment has long-lasting effects reversing many of the effects of C-section on mouse behaviour and physiology. In early-life, high-dose OXT administration attenuated C-section-mediated maternal attachment impairments. In adulthood, low-dose OXT restored social memory deficits, some aspects of anxiety-like behaviour, and improved gastrointestinal transit. Furthermore, as a consequence of OXT intervention in early life, OXT plasma levels were increased in adulthood, and dysregulation of the immune response in C-section animals was attenuated by both doses of OXT treatment. These findings indicate that there is an early developmental window sensitive to manipulations of the OXT system that can prevent lifelong behavioural and physiological impairments associated with mode of birth.

Do nonapeptides regulate parental care depending on experience in zebra finches?

Recent research suggests that the nonapeptide neurohormones regulate parental behaviors in a diverse array of vertebrates. However, it remains unclear how these neurohormones regulate parental care among birds, especially those which exhibit biparental care, or whether hormonal effects are contingent on a bird's previous experience as a parent. We measured the effects of nonapeptides on parental behaviors by peripherally injecting, over three treatment days, a short-acting nonapeptide receptor antagonist (OTA) or a saline control into breeding pairs of zebra finches (Taeniopygia guttata) that either did or did not have previous parental experience. We then compared how the duration of parental behaviors changed over the five days of observation (including one day before and two days after injections were administered). To compare treatment effects on parental outcomes, we also measured chick growth and mortality rates for each pair. There was a nearly significant interaction between treatment and experience for the amount of time birds spent in the nest, with time in the nest declining across the experiment inexperienced and experienced OTA birds. There was also a significant treatment by trial day interaction for nest guarding and a treatment by experience by trial day interaction for nest maintenance. Chicks reared by parents that received the OTA had significantly lower growth rates than chicks reared by control parents and, among experienced birds, higher mortality relative to control birds. Together, these results provide some support for the hypothesis that nonapeptides play a role in regulating parental outcomes and some parental behaviors in both experienced and inexperienced zebra finches.

The Supraoptic Nucleus of the Hypothalamus Modulates Autonomic, Neuroendocrine, and Behavioral Responses to Acute Restraint Stress in Rats

AIMS

Acute restraint stress (RS) has been reported to cause neuronal activation in the supraoptic nucleus of the hypothalamus (SON). The aim of the study was to evaluate the role of SON on autonomic (mean arterial pressure [MAP], heart rate [HR], and tail temperature), neuroendocrine (corticosterone, oxytocin, and vasopressin plasma levels), and behavioral responses to RS.

METHODS

Guide cannulas were implanted bilaterally in the SON of male Wistar rats for microinjection of the unspecific synaptic blocker cobalt chloride (CoCl2, 1 mM) or vehicle (artificial cerebrospinal fluid, 100 nL). A catheter was introduced into the femoral artery for MAP and HR recording. Rats were subjected to RS, and it was studied the effect of microinjection of CoCl2 or vehicle into the SON on pressor and tachycardic responses, drop in tail temperature, plasma oxytocin, vasopressin, and corticosterone levels, and anxiogenic-like effect induced by RS.

RESULTS

SON pretreatment with CoCl2 reduced the RS-induced MAP and HR increase, without affecting the RS-evoked tail temperature decrease. Microinjection of CoCl2 into areas surrounding the SON did not affect RS-induced increase in MAP and HR, reinforcing the idea that SON influences RS-evoked cardiovascular responses. Also, SON pretreatment with CoCl2 reduced RS-induced increase in corticosterone and oxytocin, without affecting vasopressin plasma levels, suggesting its involvement in RS-induced neuroendocrine responses. Finally, the CoCl2 microinjection into SON inhibited the RS-caused delayed anxiogenic-like effect.

CONCLUSION

The results indicate that SON is an important component of the neural pathway that controls autonomic, neuroendocrine, and behavioral responses induced by RS.

Committed for the long haul: Do nonapeptides regulate long-term pair maintenance in zebra finches?

The nonapeptides (oxytocin, vasopressin, and their non-mammalian homologs) regulate a number of social behaviors across vertebrates including monogamous pair bonds in mammals. Recent work on zebra finches has shown an important role for these neurohormones in establishing avian pair bonds as well. However, studies on the role of nonapeptides in maintaining pair bonds after pair formation are lacking. The goal of the present study was to investigate the effects of an oxytocin receptor antagonist (OTA) on pair maintenance behaviors in the monogamous zebra finch. I injected established zebra finch pairs over three days with either 5 μg of an OTA or a vehicle control, and separated the partners for one hour, after which partners were reunited and their reunion recorded on video for 30 min. Videos were then coded to measure singing, affiliative (allopreening, clumping, following), and aggressive (pecking) behaviors. These behaviors were also measured both on the day before injections to establish a pre-treatment level and two days after the last injection. Control and antagonist treated birds did not differ in the amount of time spent clumping or the frequency of pecking across the experiment. However, both male and female zebra finches that received OTA significantly reduced the amount of time spent following their partner. Females given the OTA treatment reduced allopreening and males given the OTA treatment reduced the frequency of singing bouts directed at their partners relative to controls. These results suggest that the nonapeptides play a role in regulating some, but not all, pair maintenance behaviors in experienced zebra finches.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Neuropeptide S Activates Paraventricular Oxytocin Neurons to Induce Anxiolysis

Neuropeptides, such as neuropeptide S (NPS) and oxytocin (OXT), represent potential options for the treatment of anxiety disorders due to their potent anxiolytic profile. In this study, we aimed to reveal the mechanisms underlying the behavioral action of NPS, and present a chain of evidence that the effects of NPS within the hypothalamic paraventricular nucleus (PVN) are mediated via actions on local OXT neurons in male Wistar rats. First, retrograde studies identified NPS fibers originating in the brainstem locus coeruleus, and projecting to the PVN. FACS identified prominent NPS receptor expression in PVN-OXT neurons. Using genetically encoded calcium indicators, we further demonstrated that NPS reliably induces a transient increase in intracellular Ca2+ concentration in a subpopulation of OXT neurons, an effect mediated by NPS receptor. In addition, intracerebroventricular (i.c.v.) NPS evoked a significant somatodendritic release of OXT within the PVN as assessed by microdialysis in combination with a highly sensitive radioimmunoassay. Finally, we could show that the anxiolytic effect of NPS seen after i.c.v. or intra-PVN infusion requires responsive OXT neurons of the PVN and locally released OXT. Thus, pharmacological blockade of OXT receptors as well as chemogenetic silencing of OXT neurons within the PVN prevented the effect of synthetic NPS. In conclusion, our results indicate a significant role of the OXT system in mediating the effects of NPS on anxiety, and fill an important gap in our understanding of brain neuropeptide interactions in the context of regulation of emotional behavior within the hypothalamus.SIGNIFICANCE STATEMENT Given the rising scientific interest in neuropeptide research in the context of emotional and stress-related behaviors, our findings demonstrate a novel intrahypothalamic mechanism involving paraventricular oxytocin neurons that express the neuropeptide S receptor. These neurons respond with transient Ca2+ increase and somatodendritic oxytocin release following neuropeptide S stimulation. Thereby, oxytocin neurons seem essential for neuropeptide S-induced anxiolysis, as this effect was blocked by pharmacological and chemogenetic inhibition of the oxytocin system.

Regional c-Fos expression induced by peripheral oxytocin administration is prevented by the vasopressin 1A receptor antagonist SR49059

Peripherally administered oxytocin induces a wide range of behavioural and physiological effects that are thought to be mediated by the oxytocin receptor (OTR). However, oxytocin also has considerable affinity for the vasopressin 1A receptor (V_1AR), such that various oxytocinergic effects may in fact be mediated by the V_1AR rather than the OTR. Here we used c-Fos immunohistochemistry to determine the extent to which the regional pattern of neuronal activation produced by peripheral oxytocin involves the V_1AR. Male Wistar rats were administered oxytocin (1mg/kg, IP) alone, or following pre-treatment with the V_1AR antagonist SR49059 (1mg/kg, IP), and were assessed for locomotor activity changes and for c-Fos expression across a number of brain regions. Oxytocin reduced the distance travelled by rats during a 70min test session, and this inhibitory behavioural effect was prevented by SR49059. Consistent with previous reports, oxytocin increased c-Fos expression in a number of brain regions. In several of these regions-the supraoptic and paraventricular (PVN) nuclei of the hypothalamus, locus coeruleus and nucleus of the solitary tract-the c-Fos response was prevented by SR49059 pre-treatment. Notably, SR49059 inhibited the c-Fos activation in oxytocin-synthesising magnocellular neurons in the PVN. However, c-Fos expression in the central amygdala to oxytocin was unaffected by SR49059. The current findings add to an increasing body of research suggesting that many of the functional effects of oxytocin may be V_1AR mediated.

The neurobiology of mammalian parenting and the biosocial context of human caregiving

This article is part of a Special Issue "Parental Care". Research on the neurobiology of attachment, pioneered by scholars in the generation that followed the discovery of social bonding, examined the biological basis of mammalian parenting through systematic experiments in animal models and their application to theories on human attachment. This paper argues for the need to construct a theory on the neurobiology of human attachment that integrates findings in animal models with human neuroscience research to formulate concepts based on experimental, not only extrapolative data. Rosenblatt's (2003) three characteristics of mammalian parenting - rapid formation of attachment, behavioral synchrony, and mother-offspring attachment as basis of social organization - are used to guide discussion on mammalian-general versus human-specific attributes of parental care. These highlight specific components of attachment in rodents, primates, and humans that chart the evolution from promiscuous, nest-bound, olfactory-based bonds to exclusive, multi-sensory, and representation-based attachments. Following, three continua are outlined in parental behavior, hormones, and brain, each detailing the evolution from rodents to humans. Parental behavior is defined as a process of trophallaxis - the reciprocal multisensory exchange that supports approach orientation and enables collaboration in social species - and includes human-specific features that enable behavioral synchrony independent of tactile contact. The oxytocin system incorporates conserved and human-specific components and is marked by pulsatile activity and dendritic release that reorganize neural networks on the basis of species-specific attachment experiences. Finally, the subcortical limbic circuit underpinning mammalian mothering extends in humans to include multiple cortical networks implicated in empathy, mentalizing, and emotion regulation that enable flexible, goal-directed caregiving. I conclude by presenting a philosophical continuum from Hobbes to Lorenz, which illustrates how research on the neurobiology of attachment can put in the forefront the social-collaborative elements in human nature and afford a new perspective on the mind-brain polarity.

Inborn stress reactivity shapes adult behavioral consequences of early-life maternal separation stress

Early-life experience strongly impacts neurodevelopment and stress susceptibility in adulthood. Maternal separation (MS), an established model of early-life adversity, has been shown to negatively impact behavioral and endocrine responses to stress in adulthood. However, the impact of MS in rats with heightened inborn stress susceptibility has not been fully explored. To address this issue we conducted MS in Wistar-Kyoto (WKY) rats, an animal model of comorbid depression and anxiety, and Wistar rats, which share a similar genetic background with WKYs. WKY and Wistar pups experienced either 180-min daily MS or 15-min separation (neonatal handling) during the first two postnatal weeks, and were tested for depressive- and anxiety- like behaviors in adulthood. Exposure to early-life MS in WKY rats decreased anxiety- and depressive- like behaviors, leading to increased exploration on the open field test (OFT), enhanced social interaction, and diminished immobility on the forced swim test. MS had an opposite effect in Wistar offspring, leading to enhanced anxiety-like behaviors, such as reduced OFT exploration and decreased social interaction. These findings are consistent with the match/mismatch theory of disease and the predictive adaptive response, which suggests that early life stress exposure can confer adaptive value in later life within certain individuals. Our data supports this theory, showing that early-life MS has positive and perhaps adaptive effects within stress-vulnerable WKY offspring. Future studies will be required to elucidate the neurobiological underpinnings of contrasting behavioral effects of MS on WKY vs. Wistar offspring.

Aerosolized oxytocin increases cerebrospinal fluid oxytocin in rhesus macaques

Intranasal (IN) administration is a widely used method for examining the effect of oxytocin (OT) on social behavior and cognition in healthy subjects and psychiatric populations. IN-OT in humans enhances trust, emotional perception, and empathetic behavior and is under investigation as a potential pharmacotherapy to enhance social functioning in a variety of neuropsychiatric disorders, including autism spectrum disorders (ASD). Nonhuman primates (NHP) are an important model for understanding the effect of OT on social cognition, its neural mechanisms, and the development of IN-OT as a pharmacotherapy for treating social deficits in humans. However, NHP and even some human populations, such as very young infants and children, cannot easily follow the detailed self-administration protocol used in the majority of human IN-OT studies. Therefore, we evaluated the efficacy of several OT-administration routes for elevating central OT concentrations in rhesus macaques. First, we examined the effect of IN and intravenous (IV) routes of OT administration on concentrations of OT and vasopressin (AVP) in plasma and lumbar CSF. Second, we examined these same measures in monkeys after an aerosolized (AE) OT delivery route. All three administration routes significantly increased plasma OT concentrations, but only the AE-OT route significantly increased concentrations of CSF OT. No route affected concentrations of AVP in plasma or CSF. This study confirms that the AE route is the most effective method for increasing central OT concentrations in monkeys, and may also be an effective route, alternative to IN, for administering OT to some human populations.

Beyond labor: the role of natural and synthetic oxytocin in the transition to motherhood

Emerging research raises questions that synthetic oxytocin during childbirth may alter the endogenous oxytocin system and influence maternal stress, mood, and behavior. Endogenous oxytocin is a key component in the transition to motherhood, affecting molecular pathways that buffer stress reactivity, support positive mood, and regulate healthy mothering behaviors (including lactation). Synthetic oxytocin is widely used throughout labor and postpartum care in modern birth. Yet research on the implications beyond labor of maternal exposure to perinatal synthetic oxytocin is rare. In this article, we review oxytocin-related biologic pathways and behaviors associated with the transition to motherhood and evidence supporting the need for further research on potential effects of intrapartum oxytocin beyond labor. We include a primer on oxytocin at the molecular level.

Stability and dynamics of forebrain vasopressin receptor and oxytocin receptor during pregnancy in prairie voles

During pregnancy, females undergo several physiologically driven changes that facilitate adaptive behaviours and prepare the mother to care for her developing offspring. The nonapeptide hormone oxytocin is best recognised for its involvement in mammalian pregnancy and has been tightly associated with maternal care, in addition to its roles in pregnancy, parturition and lactation. A closely-related nonapeptide hormone, arganine vasopressin, has received considerably less attention for its role in pregnancy, although it has recently been implicated in modulating maternal care and aggression. In the present study, we examined the expression patterns of receptors for oxytocin (OXTR) and vasopressin (V1aR) over the course of pregnancy, ranging from non-mated virgin to immediately postpartum female prairie voles (Microtus ochrogaster). Unexpectedly, we found that OXTR was highly stable in all measured structures in the forebrain. V1aR was also stable throughout most of the brain. Two exceptions to this were found in the ventral pallidum (VPall) and the paraventricular nucleus of the hypothalamus (PVN); both significantly correlated with the length of time that females were pregnant. Changes in the PVN may reflect functional feedback in vasopressin release, or preparatory changes for ensuing maternal behaviour. The results also indicate an unappreciated role for VPall V1aR in pregnancy, which may relate to the function of the VPall in hedonic 'liking' and motivational 'wanting.' Taken together, our data indicate that, with a few compelling exceptions, nonapeptide dynamics during prairie vole pregnancy are largely limited to changes in the synthesis and release of oxytocin and vasopressin, and not the receptors to which they bind.

Peripheral vasopressin but not oxytocin relates to severity of acute psychosis in women with acutely-ill untreated first-episode psychosis

BACKGROUND

In women with chronic schizophrenia, higher levels of peripheral oxytocin have been associated with lower levels of positive but not negative symptoms. Sex-specific associations between endogenous levels of oxytocin (OT) and arginine vasopressin (AVP) with clinical symptoms and cognition in untreated early course patients have not been examined.

METHOD

Clinical ratings and neuropsychological testing were performed in thirty-eight acutely ill, unmedicated first-episode schizophrenia patients (14 women, 24 men). Serum hormone assays were obtained in patients and thirty-eight demographically similar healthy controls.

RESULTS

Patients demonstrated increased AVP levels compared to controls (p = 0.01). Higher AVP levels were associated with greater positive symptoms (r = 0.58, p = 0.03) and worse verbal learning (r = -0.63, p = 0.02) in female, but not male, patients. OT levels did not statistically differ between patients and controls, and were unrelated to clinical symptoms or cognition in patients.

CONCLUSION

Results suggest an association of endogenous AVP with increased positive symptom severity and worse cognition in untreated female, but not male, schizophrenia patients. Findings support the role of neuroendocrine alterations in acute psychosis and the importance of examining sex-specific neuroendocrine alterations early in the course of schizophrenia.

Presence of oxytocin, vasopressin and atrial natriuretic peptide and their modification in rat hypothalamic paraventricular nucleus during resistance training

Many studies have demonstrated the physiological effects of oxytocin (OT), atrial natriuretic peptide (ANP) and vasopressin (VP) in the homoeostasis of body fluids during physical exercise. However, a little information is available about the related immunohistochemical changes in hypothalamic magnocellular neurosecretory system during and after the training. The aim of the present work was to study the immunohistochemical changes in OT, ANP and VP levels in the hypothalamic paraventricular nucleus during and after resistance exercise protocol. Three groups of Wistar rats were trained by a rung ladder protocol for 15, 30 and 45 days, respectively; a fourth group was left to rest for 15 days after the training. Finally, four sedentary groups were used as controls. The results show that resistance training induces a significant reduction in the percentage of OT-positive neurons, compared with sedentary controls. In contrast, this protocol did not induce any change in VP levels, and ANP levels did not change significantly. However, VP increased after the resting period of 15 days. Our work shows that neurons of the paraventricular nucleus are involved in body fluid homoeostasis during and after resistance exercise. The functional significance of these changes in OT and VP levels, during and after the protocol, needs to be further investigated.

Both oxytocin and vasopressin are mediators of maternal care and aggression in rodents: from central release to sites of action

In the mammalian peripartum period, the activity of both the brain oxytocin and vasopressin system is elevated as part of the physiological adaptations occurring in the mother. This is reflected by increased expression and intracerebral release of oxytocin and vasopressin, as well as increased neuropeptide receptor expression and binding. In this review we discuss the functional role of the brain oxytocin and vasopressin system in the context of maternal behavior, specifically maternal care and maternal aggression in rodents. In order to enable the identification of significant and peptide-specific contributions to the display of maternal behavior, various complementary animal models of maternal care and/or maternal aggression were studied, including rats selectively bred for differences in anxiety-related behavior (HAB and LAB dams), monitoring of local neuropeptide release during ongoing maternal behavior, and local pharmacological or genetic manipulations of the neuropeptide systems. The medial preoptic area was identified as a major site for oxytocin- and vasopressin-mediated maternal care. Furthermore, both oxytocin and vasopressin release and receptor activation in the central amygdala and the bed nucleus of the stria terminalis play an important role for maternal aggression. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Salubrious effects of oxytocin on social stress-induced deficits

Social relationships are a fundamental aspect of life, affecting social, psychological, physiological, and behavioral functions. While positive social interactions can attenuate stress and promote health, the social environment can also be a major source of stress when it includes social disruption, confrontation, isolation, or neglect. Social stress can impair the basal function and stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis, impairing function of multiple biological systems and posing a risk to mental and physical health. In contrast, social support can ameliorate stress-induced physiological and immunological deficits, reducing the risk of subsequent psychological distress and improving an individual's overall well-being. For better clinical treatment of these physiological and mental pathologies, it is necessary to understand the regulatory mechanisms of stress-induced pathologies as well as determine the underlying biological mechanisms that regulate social buffering of the stress system. A number of ethologically relevant animal models of social stress and species that form strong adult social bonds have been utilized to study the etiology, treatment, and prevention of stress-related disorders. While undoubtedly a number of biological pathways contribute to the social buffering of the stress response, the convergence of evidence denotes the regulatory effects of oxytocin in facilitating social bond-promoting behaviors and their effect on the stress response. Thus, oxytocin may be perceived as a common regulatory element of the social environment, stress response, and stress-induced risks on mental and physical health. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

On the role of volume transmission and receptor-receptor interactions in social behaviour: focus on central catecholamine and oxytocin neurons

This article is focused on understanding the mechanisms for the interactions between the central catecholamine (CA) and oxytocin (OXY) neurons and their relevance for brain function especially social behaviour in the field of pair bonding. Such a topic is analysed under two perspectives namely the intercellular communication modes between CA and OXT neurons and the molecular integrative mechanisms at the plasma membrane level between their respective decoding systems. As a matter of fact, recent observations strongly indicate a major role of volume transmission and receptor-receptor interactions in the CA/OXT neuron interplay in the brain control of social behaviour and pair bonding. This article is part of a Special Issue entitled: Brain Integration.

Estrogen receptors and the regulation of neural stress responses

It is now well established that estrogens can influence a panoply of physiological and behavioral functions. In many instances, the effects of estrogens are mediated by the 'classical' actions of two different estrogen receptors (ERs), ERα or ERβ. ERα and ERβ appear to have opposing actions in the control of stress responses and modulate different neurotransmitter or neuropeptide systems. Studies elucidating the molecular mechanisms for such regulatory processes are currently in progress. Furthermore, the use of ERα and ERβ knockout mouse lines has allowed the exploration of the importance of these receptors in behavioral responses such as anxiety-like and depressive-like behaviors. This review examines some of the recent advances in our knowledge of hormonal control of neuroendocrine and behavioral responses to stress and underscore the importance of these receptors as future therapeutic targets for control of stress-related signaling pathways.

Are behavioral effects of early experience mediated by oxytocin?

Early experiences can alter adaptive emotional responses necessary for social behavior as well as physiological reactivity in the face of challenge. In the highly social prairie vole (Microtus ochrogaster), manipulations in early life or hormonal treatments specifically targeted at the neuropeptides oxytocin (OT) and arginine vasopressin (AVP), have long-lasting, often sexually dimorphic, consequences for social behavior. Here we examine the hypothesis that behavioral changes associated with differential early experience, in this case handling the family during the first week of life, may be mediated by changes in OT or AVP or their brain receptors. Four early treatment groups were used, differing only in the amount of manipulation received during the first week of life. MAN1 animals were handled once on post-natal day 1; MAN1 treatment produces a pattern of behavior usually considered typical of this species, against which other groups were compared. MAN1-7 animals were handled once a day for post-natal days 1-7, MAN 7 animals were handled once on post-natal day 7, and MAN0 animals received no handling during the first week of life. When tested following weaning, males in groups that had received manipulation during the first few days of life (MAN1 and MAN1-7) displayed higher alloparenting than other groups. Neuroendocrine measures, including OT receptor binding and OT and AVP immunoreactivity, varied by early treatment. In brain areas including the nucleus accumbens, bed nucleus of stria terminalis and lateral septum, MAN0 females showed increased OT receptor binding. MAN1 animals also displayed higher numbers of immunoreactive OT cell bodies in the supraoptic nucleus. Taken together these findings support the broader hypothesis that experiences in the first few days of life, mediated in part by sexually dimorphic changes in neuropeptides, especially in the receptor for OT, may have adaptive consequences for sociality and emotion regulation.

Vasopressin administration into the paraventricular nucleus normalizes plasma oxytocin and corticosterone levels in Brattleboro rats

Adult male rats of the Brattleboro strain were used to investigate the impact of the congenital absence of vasopressin on plasma adrenocorticotropin, corticosterone, and oxytocin concentrations as well as the release pattern of oxytocin within the hypothalamic paraventricular nucleus (PVN), in response to a 10-min forced swimming session. Measurement of adrenocorticotropin in plasma samples collected via chronically implanted jugular venous catheters revealed virtually identical stress responses for vasopressin-lacking Brattleboro (KO) and intact control animals. In contrast, plasma corticosterone and oxytocin levels were found to be significantly elevated 105 min after onset of the stressor in KO animals only. Microdialysis samples collected from the extracellular fluid of the PVN showed significantly higher levels of oxytocin both under basal conditions and in response to stressor exposure in KO vs. intact control animals accompanied by elevated oxytocin mRNA levels in the PVN of KO rats. These findings suggest that the increased oxytocin levels in the PVN caused by the congenital absence of vasopressin may contribute to normal adrenocorticotropin stress responses in KO animals. However, whereas the stressor-induced elevation of plasma oxytocin in KO rats may be responsible for their maintained corticosterone levels, oxytocin seems unable to fully compensate for the lack of vasopressin. This hypothesis was tested by retrodialyzing synthetic vasopressin into the PVN area concomitantly with blood sampling in KO animals. Indeed, this treatment normalized plasma oxytocin and corticosterone levels 105 min after forced swimming. Thus, endogenous vasopressin released within the PVN is likely to act as a paracrine signal to facilitate the return of plasma oxytocin and corticosterone to basal levels after acute stressor exposure.

Post-weaning social isolation alters anxiety-related behavior and neurochemical gene expression in the brain of male prairie voles

Stressful social experiences early in life, such as maternal separation and social isolation, have enduring effects on the development of the brain and behavior. In the present study in socially monogamous male prairie voles (Microtus ochrogaster), we found that following 6 weeks of social isolation after weaning males spent more time in the closed arms and less time in the open arms during an elevated plus maze (EPM) test, moved more frequently from central to peripheral squares in an open field test, and diminished their preferences for the empty chamber during a two-chamber affiliation test, compared to control males that were housed with siblings. This increased behavioral anxiety in socially isolated males was also associated with enhanced mRNA expression for vasopressin (AVP), oxytocin (OT), corticotrophin releasing factor (CRF), and tyrosine hydroxylase (TH) in the paraventricular nucleus of the hypothalamus (PVN). Together, these data illustrate the importance of the post-weaning social environment on anxiety-related behavior and suggest a potential role of neurochemical systems in the PVN in the regulation of this behavior.

The role of the arginine vasopressin Avp1b receptor in the acute neuroendocrine action of antidepressants

In times of stress the hypothalamic-pituitary-adrenal (HPA) axis is activated and releases two neurohormones, corticotropin-releasing hormone (Crh) and arginine vasopressin (Avp), to synergistically stimulate the secretion of adrenocorticotropin hormone (ACTH) from the anterior pituitary, culminating in a rise in circulating glucocorticoids. Avp mediates its actions at the Avp V1b receptor (Avpr1b) present on pituitary corticotropes. Dysregulation of the stress response is associated with the pathophysiology of depression and a major treatment involves increasing the availability of monamines at the synaptic cleft. Acute administration of selective serotonin reuptake inhibitors (SSRI) and tricyclic antidepressants (TCA) has previously been shown to activate the HPA axis. The present study was undertaken to evaluate the involvement of the Avpr1b in the HPA axis response to acute SC administration of an SSRI (fluoxetine 10mg/kg) and a TCA (desipramine 10mg/kg). We measured plasma ACTH and corticosterone (CORT) levels and neuropeptide mRNA expression in the hypothalamic paraventricular nucleus (PVN) of Avpr1b knockout (KO) mice and wild-type controls. Fluoxetine and desipramine administration significantly attenuated plasma ACTH and CORT levels in male and female Avpr1b KO mice when compared to their wild-type counterparts. Avp, oxytocin (Oxt) and Crh mRNA expression in the PVN did not change in fluoxetine-treated male Avpr1b KO or wild-type mice. In contrast, fluoxetine treatment increased PVN Avp mRNA levels in female Avpr1b wild type but not KO animals. PVN Oxt mRNA levels increased in fluoxetine-treated female mice of both genotypes. The data suggests that the Avpr1b is required to drive the HPA axis response to acute antidepressant treatment and provides further evidence of a sexual dichotomy in the regulation of PVN Avp/Oxt gene expression following antidepressant administration.

Stimuli and consequences of dendritic release of oxytocin within the brain

The brain oxytocin system has served as a distinguished model system in neuroendocrinology to study detailed mechanisms of intracerebral release, in particular of somatodendritic release, and its behavioural and neuroendocrine consequences. It has been shown that oxytocin is released within various brain regions, but evidence for dendritic release is limited to the main sites of oxytocin synthesis, i.e. the hypothalamic SON (supraoptic nucleus) and PVN (paraventricular nucleus). In the present paper, stimuli of dendritic release of oxytocin and the related neuropeptide vasopressin are discussed, including parturition and suckling, i.e. the period of a highly activated brain oxytocin system. Also, exposure to various pharmacological, psychological or physical stressors triggers dendritic oxytocin release, as monitored by intracerebral microdialysis within the SON and PVN during ongoing behavioural testing. So far, dendritic release of the neuropeptide has only been demonstrated within the hypothalamus, but intracerebral oxytocin release has also been found within the central amygdala and the septum in response to various stimuli including stressor exposure. Such a locally released oxytocin modulates physiological and behavioural reproductive functions, emotionality and hormonal stress responses, as it exerts, for example, pro-social, anxiolytic and antistress actions within restricted brain regions. These discoveries make oxytocin a promising neuromodulator of the brain for psychotherapeutic intervention and treatment of numerous psychiatric illnesses, for example, anxiety-related diseases, social phobia, autism and postpartum depression.

Extracellular amino acid levels in the paraventricular nucleus and the central amygdala in high- and low-anxiety dams rats during maternal aggression: regulation by oxytocin

Brain oxytocin (OT) regulates aspects of emotionality and stress coping including maternal behavior and maternal aggression. Maternal aggression correlates with the amount of OT released within the paraventricular nucleus (PVN) and the central amygdala (CeA). OT, a key neurotransmitter or neuromodulator, is likely to modulate other neurotransmitter systems. Here, we investigated the dynamic changes in extracellular concentrations of the amino acids aspartate, glutamate, gamma-aminobutyric acid (GABA), serine, histidine, arginine and taurine in the PVN and CeA in lactating rats bred for high (HAB) and low (LAB) anxiety-related behavior under basal conditions and during maternal aggression. Further, to determine whether local OT is involved in the regulation of amino acid release we infused a selective OT receptor antagonist (OTA) via local retrodialysis. Within the CeA, HAB and LAB dams differed in the basal release of glutamate and arginine. Infusion of a selective OTA increased the concentrations of glutamate and aspartate in LAB dams and GABA in HAB dams. In OTA-treated HAB and LAB dams taurine levels increased during maternal aggression. Within the PVN, the highly-aggressive HAB dams showed a more pronounced increase in aspartate and serine levels; the latter being attenuated by local OTA administration. However, OTA did not affect the level of any amino acid in the LAB dams. Thus, the extracellular concentrations of selected amino acids differed between lactating HAB and LAB dams under both basal conditions and following maternal aggression. The effects of OT within the CeA and PVN on maternal aggressive behavior might be related to its regulation of local amino acid release.

Direct evidence that stimulation of neuropeptide Y Y5 receptor activates hypothalamo-pituitary-adrenal axis in conscious rats via both corticotropin-releasing factor- and arginine vasopressin-dependent pathway

An abundance of data suggests a crucial role of neuropeptide Y (NPY) as an activator of the hypothamamo-pituitary-adrenal (HPA) axis. However, there is quite limited evidence regarding receptors that mediate this response. Here, we address the possibility that Y(5) receptor subtype may be responsible for NPY-induced activation of HPA axis. For this purpose, the effects of an intracerebroventricular injection of Y(5)-selective agonist, [cPP(1-7), NPY(19-23), Ala(31), Aib(32), Gln(34)]-human pancreatic polypeptide (hPP), on circulating ACTH and corticosterone in conscious rats were evaluated. Central injection of hPP (100 pmol) produced significant increases in plasma ACTH and corticosterone compared with artificial cerebrospinal fluid, and previous treatment with a novel Y(5)-selective antagonist, FMS586 [3-(9-isopropyl-6,7,8,9-tetrahydro-5H-carbazol-3-yl)-1-methyl-1-(2-pyridin-4-yl-ethyl)-urea hydrochloride] (25 mg/kg, po), completely blocked these alterations. Pretreatment with corticotropin-releasing factor (CRF) receptor antagonist (astressin, 10-50 microg/rat, iv) or arginine vasopressin (AVP) receptor antagonist ([deamino-Pen(1), O-Me-Tyr(2), Arg(8)] vasopressin; 3-30 microg/rat, iv) differentially suppressed these increases by 70-80 or 40-50%, respectively. The combined treatment showed no additive effect of these antagonists. Furthermore, an exogenous AVP (0.3 microg/rat, iv)-induced HPA activation was fully inhibited by astressin, suggesting a convergent pathway of AVP receptor signals onto CRF neurons. Central injection of hPP also evoked marked up-regulation of mRNA expression for CRF and AVP in the hypothalamus, which, likewise, were completely reversed by FMS586. Our observations provide the first evidence that selective stimulation of Y(5) receptor provokes activation of the HPA axis and its downstream pathway is chiefly composed of both CRF (primary regulator) and AVP (subordinate to the former) with distinct relative contribution.

Social stress induces hypothalamo-pituitary-adrenal axis responses in lactating rats bred for high trait anxiety

Hypothalamo-pituitary-adrenal axis responses to various stressors are typically attenuated during lactation, including in rats selectively bred for high or low anxiety. As high-anxiety dams are more aggressive towards intruders than low-anxiety dams during maternal defence, we investigated their hypothalamo-pituitary-adrenal axis responses to this social stress. Maternal defence induced elevated stress responses in high-anxiety dams only; nerve-growth-factor-induced gene B mRNA expression in the parvocellular paraventricular nucleus and adrenocorticotropin hormone secretory responses were substantially enhanced after maternal defence. In contrast, secretory responses to a non-social stress (elevated platform) were not different between high- and low-anxiety dams. Thus, responsiveness of the stress axis in lactation is dependent upon the innate level of anxiety of the dam and, as a consequence, her reactiveness to social threat.

Effects of Morris water maze testing on the neuroendocrine stress response and intrahypothalamic release of vasopressin and oxytocin in the rat

Adult male Wistar rats were trained in the Morris water maze (MWM) on 3 consecutive days to find a visible platform. Concomitantly, microdialysis samples from the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei were collected in order to monitor local release of the neuropeptides vasopressin (AVP) and oxytocin (OXT), respectively, during controllable swim stress. Additionally, a separate set of animals was equipped with chronic jugular venous catheters to collect blood samples for analyzing plasma concentrations of corticotropin (ACTH) and corticosterone during training in the MWM. As measured by microdialysis, swimming in the MWM caused a significantly increased release of AVP within the PVN and of OXT within the SON on each of the 3 test sessions. In contrast to OXT in the SON, basal AVP concentrations in the PVN tended to rise from day to day. Plasma ACTH and corticosterone were found to be similarly elevated in response to MWM exposure on each of the test sessions. Taken together, these data demonstrate that testing in the MWM is not only associated with a significant activation of the hypothalamo-pituitary-adrenal axis but also with an intrahypothalamic release of AVP and OXT. If compared with findings using repeated forced swimming as an uncontrollable stressor (Wotjak, C.T., Ganster, J., Kohl, G., Holsboer, F., Landgraf, R., Engelmann, M., 1998. Dissociated central and peripheral release of vasopressin, but not oxytocin, in response to repeated swim stress: new insights into the secretory capacities of peptidergic neurons. Neuroscience 85, 1209-1222), the present results suggest that (1) similarities in the release profiles of AVP in the PVN and plasma hormone levels are fairly independent from the controllability of the stressor and seem, thus, to primarily relate to the physical demands of the task, whereas (2) the different intra-SON OXT release profiles might be linked to the controllability of the stressor.