Intracranial dialysis measurement of oxytocin, monoamine and uric acid release from the olfactory bulb and substantia nigra of sheep during parturition, suckling, separation from lambs and eating

Monitoring oxytocin signaling in the brain: More than a love story

More than any other neuropeptide, oxytocin (OXT) is attracting the attention of neurobiologists, psychologists, psychiatrists, evolutionary biologists and even economists. It is often called a "love hormone" due to its many prosocial functions described in vertebrates including mammals and humans, especially its ability to support "bonding behaviour". Oxytocin plays an important role in female reproduction, as it promotes labour during parturition, enables milk ejection in lactation and is essential for related reproductive behaviours. Therefore, it particularly attracts the interest of many female researchers. In this short narrative review I was invited to provide a personal overview on my scientific journey closely linked to my research on the brain OXT system and the adventures associated with starting my research career behind the Iron Curtain.

Species differences in the effect of oxytocin on maternal behavior: A model incorporating the potential for allomaternal contributions

Oxytocin has historically been linked to processes involved with maternal behavior. However, the relative importance of oxytocin for maternal behavior widely varies among mammalian species, from indispensable to apparently nonessential. This review proposes a new model in which the relative importance of oxytocin for mothering across species is explained by an evolutionary pressure which we term "allomaternal potential", or the degree to which other conspecifics are capable and likely to assist with caregiving. It is notable that in animals where allomaternal potential is high (i.e., many quality helpers are available), oxytocin is decoupled from mothering. However, in animals where allomaternal potential is low (i.e., conspecifics refuse to, or do not provide, quality help), oxytocin is crucial for mothering. We posit that this relationship is a form of kin selection, whereby oxytocin is a signal that leads mothers to preferentially dispense resources to their own young when quality helpers are unlikely.

The Onset of Maternal Behavior in Sheep and Goats: Endocrine, Sensory, Neural, and Experiential Mechanisms

In sheep and goats, the onset of maternal behavior at parturition is characterized by a first phase called maternal responsiveness during which the mother is attracted to any newborn. In a second phase, called maternal selectivity, the mother establishes a selective bond with her young so that she only accepts it at suckling. After a description of the behavioral expression of both phases, this chapter reviews the physiological, sensory, and neural mechanisms involved. These two behavioral processes are synchronized with parturition by the vaginocervical stimulation induced by the expulsion of the newborn. Olfactory cues provided by the neonate are involved in maternal responsiveness and selectivity. Oxytocin supported by estrogens is the key factor for maternal responsiveness. The neural network involved in maternal responsiveness is mainly hypothalamic and is different from the circuitry involved in selectivity, which mainly concerns olfactory processing regions. Visual and auditory cues are necessary for offspring recognition at a distance. This multisensory recognition suggests that mothers form a mental image of their young. Maternal experience renders mothers more responsive to maternally relevant physiology and to young-related sensory inputs.

Regulation of maternal behavior, social isolation responses, and postpartum estrus by steroid hormones and vaginocervical stimulation in sheep

Periparturient ewes display several activities in addition to maternal behavior per se. They isolate themselves from the flock and, contrary to other mammals, do not show postpartum estrus. We investigated the possibility of a common hormonal control of maternal behavior, prepartum social isolation responses, and absence of postpartum estrus. We quantified responses to flockmate separation and incidence of sexual receptivity at several reproductive stages in intact ewes (Experiment 1). Responses to social isolation were lowest in preparturient ewes and at pregnancy day 149, intermediate at pregnancy day 147 and highest at day 136 and in non-pregnant ewes (P < 0.05 between the 3 levels). In a second experiment, we quantified the same parameters and maternal behavior in 1) ovariectomized ewes receiving medroxyprogesterone acetate only (ovxMPA); 2) ovariectomized ewes receiving MPA + estradiol benzoate (ovxSHORT); 3) intact ewes receiving a longer MPA + estradiol dipropionate treatment, before and after vaginocervical stimulation (VCS). Before VCS no steroid treatments decreased social isolation responses and maternal behavior was scarce or absent. Following VCS and interaction with lamb, maternal responses in the ovxSHORT group increased while social isolation responses decreased. Sexual receptivity occurred in non-pregnant ewes and in ovxSHORT group. Conclusion: some hormonal treatments +VCS can effectively induce maternal behavior and reduce social isolation responses. Long-term progestin treatment can inhibit postpartum estrus.

Descended Social Anxiety Disorder and Craving in Women Heroin Dependence Through Exercise Alerts Plasma Oxytocin Levels

Purpose: This study explored the association between peripheral blood oxytocin (OT) and social anxiety disorder (SAD) and cue-induced cravings in female heroin addicts. The effect of exercise on alleviation of SAD and OT levels was also explored. Methods: A total of 72 females with heroin dependence were assigned to three groups based on SAD severity. The three groups were Non-SAD control, SAD control, and SAD exercise groups. Subjects in the SAD exercise group underwent aerobic exercise and resistance training for 8 weeks (60 min/day, 5 days/week). Enzyme-linked immunosorbent assay analysis and Liebowitz Social Anxiety Scale (LSAS) scores were used to determine plasma OT concentration and SAD, respectively. Cue-induced craving was assessed using Visual Analog Scale (VAS) and Desires for Drug Questionnaire (DDQ). Mixed-effect analysis of variance and Pearson correlation analysis were used to explore the effect and correlation between different parameters. Results: OT levels in the SAD exercise group were significantly high after exercise (p < 0.01). LSAS, VAS, and DDQ ("Desire and Intention" and "Negative reinforcement") scores in the SAD exercise group were significantly lower after exercise (p < 0.01). Plasma OT level was negatively correlated with LSAS score (r = -0.534, p < 0.001), VAS score (r = -0.609, p < 0.001), "Desire and Intention" score (r = -0.555, p < 0.001), and "Negative reinforcement" score (r = -0.332, p < 0.01) and positively correlated with the "control" score (r = 0.258, p < 0.05). LSAS was positively correlated with VAS score (r = 0.588, p < 0.001) and "Desire and Intention" score (r = 0.282, p < 0.05). Conclusions: The findings of the present study indicate that plasma OT is a potential peripheral biomarker for prediction of the severity of social anxiety in female heroin withdrawal patients. Aerobic exercise combined with resistance training plus incremental load for 8 weeks can increase plasma OT levels and significantly reduce severity of SAD and cue-induced cravings in female heroin addicts.

Brain oxytocin: how puzzle stones from animal studies translate into psychiatry

The neuropeptide oxytocin has attracted great attention of the general public, basic neuroscience researchers, psychologists, and psychiatrists due to its profound pro-social, anxiolytic, and "anti-stress" behavioral and physiological effects, and its potential application for treatment of mental diseases associated with altered socio-emotional competence. During the last decade, substantial progress has been achieved in understanding the complex neurobiology of the oxytocin system, including oxytocinergic pathways, local release patterns, and oxytocin receptor distribution in the brain, as well as intraneuronal oxytocin receptor signaling. However, the picture of oxytocin actions remains far from being complete, and the central question remains: "How does a single neuropeptide exert such pleotropic actions?" Although this phenomenon, typical for many of about 100 identified neuropeptides, may emerge from the anatomical divergence of oxytocin neurons, their multiple central projections, distinct oxytocin-sensitive cell types in different brain regions, and multiple intraneuronal signaling pathways determining the specific cellular response, further basic studies are required. In conjunction, numerous reports on positive effects of intranasal application of oxytocin on human brain networks controlling socio-emotional behavior in health and disease require harmonic tandems of basic researchers and clinicians. During the COVID-19 crisis in 2020, oxytocin research seems central as question of social isolation-induced inactivation of the oxytocin system, and buffering effects of either activation of the endogenous system or intranasal application of synthetic oxytocin need to be thoroughly investigated.

Dopaminergic modulation of olfactory-evoked motor output in sea lampreys (Petromyzon marinus L.)

Detection of chemical cues is important to guide locomotion in association with feeding and sexual behavior. Two neural pathways responsible for odor-evoked locomotion have been characterized in the sea lamprey (Petromyzon marinus L.), a basal vertebrate. There is a medial pathway originating in the medial olfactory bulb (OB) and a lateral pathway originating from the rest of the OB. These olfactomotor pathways are present throughout the life cycle of lampreys, but olfactory-driven behaviors differ according to the developmental stage. Among possible mechanisms, dopaminergic (DA) modulation in the OB might explain the behavioral changes. Here, we examined DA modulation of olfactory transmission in lampreys. Immunofluorescence against DA revealed immunoreactivity in the OB that was denser in the medial part (medOB), where processes were observed close to primary olfactory afferents and projection neurons. Dopaminergic neurons labeled by tracer injections in the medOB were located in the OB, the posterior tuberculum, and the dorsal hypothalamic nucleus, suggesting the presence of both intrinsic and extrinsic DA innervation. Electrical stimulation of the olfactory nerve in an in vitro whole-brain preparation elicited synaptic responses in reticulospinal cells that were modulated by DA. Local injection of DA agonists in the medOB decreased the reticulospinal cell responses whereas the D2 receptor antagonist raclopride increased the response amplitude. These observations suggest that DA in the medOB could modulate odor-evoked locomotion. Altogether, these results show the presence of a DA innervation within the medOB that may play a role in modulating olfactory inputs to the motor command system of lampreys.

Demographic, sampling- and assay-related confounders of endogenous oxytocin concentrations: A systematic review and meta-analysis

Studies on endogenous oxytocin concentrations are often criticized for the debatable comparability between specimens and the variation in reported values. We performed meta-regressions on k = 229 studies (n = 12 741 participants), testing whether specimen, extraction, sex, age, time of day, or fasting instructions influenced oxytocin measurements. Predicted oxytocin concentrations differed depending on specimen and extraction: Measurements were extremely high in unextracted blood, compared to extracted blood and other specimens. Measurements were higher in samples with more female participants and higher age. Instructions not to smoke before sampling were correlated with higher oxytocin in unextracted samples. There was no impact of instructions to refrain from eating, drinking, consume caffeine, alcohol or exercising. Oxytocin concentrations increased from morning to afternoon. Our results showed that oxytocin is differentially reflected in blood, saliva, urine and cerebrospinal fluid. Extraction impacts oxytocin measurements, particularly in blood. Considering relevant confounders might increase comparability between studies.

Tracking oxytocin functions in the rodent brain during the last 30 years: From push-pull perfusion to chemogenetic silencing

A short overview is provided of the last 30 years of oxytocin (and vasopressin) research performed in our laboratories, starting with attempts to monitor the release of this nonapeptide in the rodent brain during physiological conditions such as suckling in the lactating animal. Using push-pull perfusion and microdialysis approaches, release patterns in hypothalamic and limbic brain regions could be characterised to occur from intact neuronal structures, to be independent of peripheral secretion into blood, and to respond differentially to various stimuli, particularly those related to reproduction and stress. Parallel efforts focused on the functional impact of central oxytocin release, including neuroendocrine and behavioural effects mediated by nonapeptide receptor interactions and subsequent intraneuronal signalling cascades. The use of a variety of sophisticated behavioural paradigms to manipulate central oxytocin release, along with pharmacological, genetic and pharmacogenetic approaches, revealed multiple consequences on social behaviours, particularly social fear.

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.

Role of oxytocin in parental behaviour

Both animal and human studies have provided conclusive evidence that oxytocin (OXT) acts in the brain (eg, medial preoptic area, ventral tegmental area, nucleus accumbens) to promote parental behaviour under different reproductive and physiological conditions. OXT appears to accelerate and strengthen the neural process that makes newborns attractive or rewarding. Furthermore, OXT reduces stress/anxiety and might improve mood and well being, resulting in indirect benefits for parents. However, OXT also plays a role in the development of species reproductive and social strategies, making some species or individuals more prone to display caring activities in nonreproductive contexts. There are important differences in the development of the OXT system and its regulation by gonadal hormones that can make individuals or species very different. Those intra- and interspecific differences in the OXT system have been associated with differences in parental behaviour. For example, differences in OXT levels in body fluids and genetic variants for the OXT and OXT receptor genes have been associated with variability in parental mood and behaviour in humans. Thus, OXT has received much attention as a potential therapeutic agent for affective, emotional and behavioural problems. Despite many preliminary studies indicating promising findings, several unknown aspects of the OXT system remain to be addressed before we can achieve a complete understanding of its function in the brain. The enormous interest that this area of study has attracted in the last decade will likely continually contribute to advancing our understanding of the role of OXT in parental behaviour and other behavioural and physiological functions.

Dopaminergic Neurones in the Main Olfactory Bulb: An Overview from an Electrophysiological Perspective

The olfactory bulb (OB), the first center processing olfactory information, is characterized by a vigorous life-long activity-dependent plasticity responsible for a variety of odor-evoked behavioral responses. It hosts the more numerous group of dopaminergic (DA) neurones in the central nervous system, cells strategically positioned at the entry of the bulbar circuitry, directly in contact with the olfactory nerve terminals, which play a key role in odor processing and in the adaptation of the bulbar network to external conditions. Here, we focus mainly on the electrophysiological properties of DA interneurones, reviewing findings concerning their excitability profiles in adulthood and in different phases of adult neurogenesis. We also discuss dynamic changes of the DA interneurones related to environmental stimuli and their possible functional implications.

Plasma Oxytocin Levels in Untreated Adult Obsessive-Compulsive Disorder Patients

BACKGROUND AND AIM

Given the paucity of information on the possible role of oxytocin (OT) in the pathophysiology of obsessive-compulsive disorder (OCD), our study aimed at evaluating plasma OT levels in a group of 44 OCD outpatients, as compared with a similar group of healthy control subjects. At the same time, the relationships between OT and clinical features and romantic attachment characteristics were examined as well.

METHODS

Diagnosis was assessed according to DSM-IV-TR criteria, while the OCD severity was measured by means of the Y-BOCS rating scale. All patients were drug free and not depressed. The romantic attachment was assessed by means of the Italian version of the 'Experiences in Close Relationships' questionnaire. Plasma OT levels were evaluated by means of a standard RIA kit.

RESULTS

The main findings of our study showed that OT levels were increased in OCD patients, as compared with healthy subjects, and negatively related to symptom severity. Positive relationships were detected between OT levels and the fearful-avoidant and dismissing styles of romantic attachments, but only in male OCD patients.

CONCLUSION

Taken together, these findings suggest that OT may play a role in OCD pathophysiology and also in the romantic attachment of patients with gender specificity.

Puerperal and parental experiences alter rat preferences for pup odors via changes in the oxytocin system

In the rat, induction of maternal behavior depends on the parity of the female. For example, nulliparous (NP) females need longer exposure to pups than multiparous (MP) or lactating (L) females to exhibit similar maternal behavior. In this study, we investigated the role of brain oxytocin in the approaching behavior of these female rats. Olfactory preferences for pup odors were examined for 8 consecutive days. Each preference test was followed by direct overnight exposure to pups. On the 8th day, MP and L, but not NP females showed robust pup-odor preferences. After the behavioral test, half of the females were exposed to pups for 2 h, whereas the other half were not. The females were then sacrificed to analyze brain oxytocin (OXT) and vasopressin (AVP) activities by cFos immunohistochemistry and to quantify their receptor mRNA expression using real-time PCR. In the paraventricular nucleus (PVN), the percentage of cFos-positive OXT neurons was significantly larger in MP and L females than in NP females after pup exposure. No significant differences were found in cFos expression in OXT neurons of the supraoptic nucleus (SON) or in AVP neurons of either the PVN or SON. Expression of OXT receptor mRNA in the medial preoptic area and amygdala of the control groups was also higher in MP females than in NP females. Finally, we demonstrated that infusion of OXT into the lateral ventricle of NP females promoted preferences for pup odors. These results indicate that puerperal and parental experiences enhance the responsiveness of OXT neurons in the PVN to pup stimuli and establish olfactory preferences for these odors in a parity-dependent manner.

Validity of physiological biomarkers for maternal behavior in cows--a comparison of beef and dairy cattle

The objectives of the present study were to evaluate the suitability of potential biomarkers for maternal ability in cattle, and in addition to test the hypothesis that dairy cows have a less pronounced motherliness than beef cows. Therefore, maternal behavior of 20 Simmental beef-type (S) and 20 German Black Pied (dairy-type) Cattle (BP) was assessed on the 2nd and again on the 3rd day of the calf's life. Measurements included the frequency of interactions between cow and calf, the cow's willingness to defend her calf, the overall maternal behavior, saliva cortisol, saliva oxytocin, heart rate, and thermal images of the eye (ET). Mixed model analysis revealed that BP had significantly (P<0.05) higher oxytocin (88.6±9.2 vs. 62.8±9.2 pg/ml saliva) and cortisol (1.3±0.1 vs. 1.0±0.1 ng/ml saliva) levels, but lower heart rates (80.0±2.0 vs. 95.8±2.0bpm) than S cows. Simmental (beef) cows showed more defensive behavior (3.5±0.2 vs. 2.7±0.2 scores), but fewer total interactions between cow and calf (8.1±1.4 vs. 13.8±1.4), compared to BP (dairy). However, with the exception of heart rate and overall maternal behavior, breed differences tended to diminish from the 2nd to the 3rd day of the calf's life. Repeatabilities ranged from 9±23% (ET) to 77±7% (maternal behavior measured on a visual analogue scale), and correlations between physiological parameters and behavior differed between breeds and were generally at a low level. In conclusion, beef cows do not seem to be per se more maternal compared to dairy cows, and the assessed parameters are of limited use as biomarkers for maternal behavior.

Oxytocin and vasopressin modulation of the neural correlates of motivation and emotion: results from functional MRI studies in awake rats

Oxytocin and vasopressin modulate a range of species typical behavioral functions that include social recognition, maternal-infant attachment, and modulation of memory, offensive aggression, defensive fear reactions, and reward seeking. We have employed novel functional magnetic resonance mapping techniques in awake rats to explore the roles of these neuropeptides in the maternal and non-maternal brain. Results from the functional neuroimaging studies that are summarized here have directly and indirectly confirmed and supported previous findings. Oxytocin is released within the lactating rat brain during suckling stimulation and activates specific subcortical networks in the maternal brain. Both vasopressin and oxytocin modulate brain regions involved unconditioned fear, processing of social stimuli and the expression of agonistic behaviors. Across studies there are relatively consistent brain networks associated with internal motivational drives and emotional states that are modulated by oxytocin and vasopressin. This article is part of a Special Issue entitled Oxytocin and Social Behav.

Maternal aggression in rodents: brain oxytocin and vasopressin mediate pup defence

The most significant social behaviour of the lactating mother is maternal behaviour, which comprises maternal care and maternal aggression (MA). The latter is a protective behaviour of the mother serving to defend the offspring against a potentially dangerous intruder. The extent to which the mother shows aggressive behaviour depends on extrinsic and intrinsic factors, as we have learned from studies in laboratory rodents. Among the extrinsic factors are the pups' presence and age, as well as the intruders' sex and age. With respect to intrinsic factors, the mothers' innate anxiety and the prosocial brain neuropeptides oxytocin (OXT) and arginine vasopressin (AVP) play important roles. While OXT is well known as a maternal neuropeptide, AVP has only recently been described in this context. The increased activities of these neuropeptides in lactation are the result of remarkable brain adaptations peripartum and are a prerequisite for the mother to become maternal. Consequently, OXT and AVP are significantly involved in mediating the fine-tuned regulation of MA depending on the brain regions. Importantly, both neuropeptides are also modulators of anxiety, which determines the extent of MA. This review provides a detailed overview of the role of OXT and AVP in MA and the link to anxiety.

Intranasal administration of oxytocin: behavioral and clinical effects, a review

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The mechanisms behind the effects of IN-applied substances need more attention.

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The mechanisms involved in the brain-distribution of IN-OT are completely unexplored.

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The possibly cascading effects of IN-OT on the intrinsic OT-system require serious investigation.

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IN-OT induces clear and specific changes in neural activation.

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IN-OT is a promising approach to treat certain clinical symptoms.

The intranasal (IN-) administration of substances is attracting attention from scientists as well as pharmaceutical companies. The effects are surprisingly fast and specific. The present review explores our current knowledge about the routes of access to the cranial cavity. ‘Direct-access-pathways’ from the nasal cavity have been described but many additional experiments are needed to answer a variety of open questions regarding anatomy and physiology.

Among the IN-applied substances oxytocin (OT) has an extensive history. Originally applied in women for its physiological effects related to lactation and parturition, over the last decade most studies focused on their behavioral ‘prosocial’ effects: from social relations and ‘trust’ to treatment of ‘autism’.

Only very recently in a microdialysis study in rats and mice, the ‘direct-nose-brain-pathways’ of IN-OT have been investigated directly, implying that we are strongly dependent on results obtained from other IN-applied substances. Especially the possibility that IN-OT activates the ‘intrinsic’ OT-system in the hypothalamus as well needs further clarification.

We conclude that IN-OT administration may be a promising approach to influence human communication but that the existing lack of information about the neural and physiological mechanisms involved is a serious problem for the proper understanding and interpretation of the observed effects.

Oxytocin and vasopressin in rodent behaviors related to social dysfunctions in autism spectrum disorders

Autism spectrum disorders (ASD) and social anxiety disorder involve various forms of social deficits like impaired affiliative behavior, social cognition and social approach. Although the neurobiological underpinnings of these disorders are largely unknown, rodent and human studies suggest an involvement of the evolutionary highly conserved oxytocin (OXT) and vasopressin (AVP), as these neuropeptides modulate various aspects of mammalian social behaviors. In this review we summarize the current knowledge regarding the involvement of brain OXT and AVP in rodent social behaviors related to social dysfunctions in ASD. Starting with an introduction into the neurobiology of the central OXT and AVP systems (neuroanatomy, central release, receptor distribution) we describe the distinct roles OXT and AVP play in basic social behaviors in rodents, i.e. affiliative behavior (pair-bonding and maternal behavior), social cognition (social memory), and social approach (social preference or social avoidance). The regulatory capacity of OXT and AVP to modulate social behaviors in various rodent species implies a high translational potential, in particular that dys-regulations in the brain neuropeptide systems may underlie social dysfunctions in ASD. It also suggests that the brain OXT and AVP systems are promising pharmacotherapeutic targets to improve social behaviors and to reverse social deficits.

A link between oxytocin and serotonin in humans: supporting evidence from peripheral markers

Pharmacological studies indicate a functional interaction between the serotonergic and oxytocinergic systems. In particular, some selective serotonin (5-HT) reuptake inhibitors, such as citalopram and fluvoxamine, seem to exert part of their antidepressant effects through oxytocin (OT) release. Further, the administration of fenfluramine, a serotonergic agonist, to healthy subjects increases plasma OT levels. Interestingly, immunocytochemical and double-immunofluorescent techniques revealed a high degree of overlap between 5-HT transporter (SERT)-labeled fibers and OT-containing cells in the paraventricular and supraoptic nuclei of primate hypothalamus. These findings suggest that the influence of 5-HT on OT system might be mediated by SERT. In this study, we explored the possible existence of a link between OT and SERT in human subjects, by means of two peripheral markers, the platelet SERT, as measured by [³H]-paroxetine ([³H]-Par) binding, and plasma OT levels. As far as [³H]-Par binding parameters are concerned, the Bmax (mean ± SD, fmol/mg protein) was 1155 + 130 and the Kd (mean ± SD, nM) was 1.31 ± 0.61. The OT plasma levels (mean ± SD, pg/ml) were 1.14 ± 1.07. A significant and positive correlation was found between plasma OT levels and Kd values (correlation coefficient: r: 0.466, p = .038). This result represents the first evidence of an interaction between OT and SERT, as measured by [³H]-Par binding, at peripheral levels in humans. Given the several activities mediated by both OT and 5-HT, such a relationship might provide new perspectives and insights into psychiatric disorders and/or social relationship disturbances, as well as novel treatment strategies overcoming and/or integrating the serotonergic paradigm.

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.

Oxytocin, vasopressin and estrogen receptor gene expression in relation to social recognition in female mice

Inter- and intra-species differences in social behavior and recognition-related hormones and receptors suggest that different distribution and/or expression patterns may relate to social recognition. We used qRT-PCR to investigate naturally occurring differences in expression of estrogen receptor-alpha (ERα), ER-beta (ERβ), progesterone receptor (PR), oxytocin (OT) and receptor, and vasopressin (AVP) and receptors in proestrous female mice. Following four 5 min exposures to the same two conspecifics, one was replaced with a novel mouse in the final trial (T5). Gene expression was examined in mice showing high (85-100%) and low (40-60%) social recognition scores (i.e., preferential novel mouse investigation in T5) in eight socially-relevant brain regions. Results supported OT and AVP involvement in social recognition, and suggest that in the medial preoptic area, increased OT and AVP mRNA, together with ERα and ERβ gene activation, relate to improved social recognition. Initial social investigation correlated with ERs, PR and OTR in the dorsolateral septum, suggesting that these receptors may modulate social interest without affecting social recognition. Finally, increased lateral amygdala gene activation in the LR mice may be associated with general learning impairments, while decreased lateral amygdala activity may indicate more efficient cognitive mechanisms in the HR mice.

Maternal neglect: oxytocin, dopamine and the neurobiology of attachment

Maternal neglect, including physical and emotional neglect, is a pervasive public health challenge with serious long-term effects on child health and development. I provide an overview of the neurobiological basis of maternal caregiving, aiming to better understand how to prevent and respond to maternal neglect. Drawing from both animal and human studies, key biological systems are identified that contribute to maternal caregiving behaviour, focusing on the oxytocinergic and dopaminergic systems. Mesocorticolimbic and nigrostriatal dopamine pathways contribute to the processing of infant-related sensory cues leading to a behavioural response. Oxytocin may activate the dopaminergic reward pathways in response to social cues. Human neuroimaging studies are summarised that demonstrate parallels between animal and human maternal caregiving responses in the brain. By comparing different patterns of human adult attachment, we gain a clearer understanding of how differences in maternal brain and endocrine responses may contribute to maternal neglect. For example, in insecure/dismissing attachment, which may be associated with emotional neglect, we see reduced activation of the mesocorticolimbic dopamine reward system in response to infant face cues, as well as decreased peripheral oxytocin response to mother-infant contact. We are currently testing whether the administration of intranasal oxytocin, as part of a randomised placebo controlled trial, may reverse some of these neurological differences, and potentially augment psychosocial and behavioural interventions for maternal neglect.

Reproductive experience affects parental retrieval behaviour associated with increased plasma oxytocin levels in wild-type and CD38-knockout mice

The transition to motherhood results in a number of hormonal, neurological and behavioural changes necessary to ensure offspring growth. Once motherhood is established, further neurological and behavioural changes may result in long-term memory in mothering. Recent research has shown that postpartum motherhood enhances both nurturing behaviour and oxytocin activities. The transmembrane glycoprotein, CD38, is expressed on many neuronal cells and has been shown to play a role in social behaviours through stimulating the release of oxytocin in the hypothalamus. The present study was performed to investigate the effects of reproductive experience (primi- and multiparity, dams and sires) on the degree of parental behaviour, such as retrieval. Comparisons were performed between wild-type (Cd38 (+/+) ) and Cd38 knockout (Cd38 (-/-) ) mice of the ICR strain. Multiparous Cd38 (-/-) dams retrieved pups much faster than primiparous mice, whereas there were no significant differences between primi- and multiparous Cd38 (+/+) dams. Plasma oxytocin levels were significantly increased in multiparous dams of both genotypes. In addition, oxytocin levels in the hypothalamus and pituitary were lower in Cd38 (-/-) than in wild-type mice. ADP-ribosyl cyclase activity in the hypothalamus, but not in the pituitary, was slightly increased in Cd38 (+/+) dams. In an identical test, 40% of first-time Cd38 (+/+) sires showed retrieval. The time required to retrieval was shorter in second-time Cd38 (+/+) sires. Both first- and second-time Cd38 (-/-) sires showed only 10% retrieval behaviour. These results indicate that parental behaviour is improved by reproductive experience, especially in Cd38 (-/-) dams, and suggest that these effects may be a result of increased oxytocin levels.

Oxytocin and social motivation

Humans are fundamentally social creatures who are ‘motivated’ to be with others. In this review we examine the role of oxytocin (OT) as it relates to social motivation. OT is synthesized in the brain and throughout the body, including in the heart, thymus, gastrointestinal tract, as well as reproductive organs. The distribution of the OT receptor (OTR) system in both the brain and periphery is even more far-reaching and its expression is subject to changes over the course of development. OTR expression is also sensitive to changes in the external environment and the internal somatic world. The OT system functions as an important element within a complex, developmentally sensitive biobehavioral system. Other elements include sensory inputs, the salience, reward, and threat detection pathways, the hypothalamic-pituitary-gonadal axis, and the hypothalamic-pituitary-adrenal stress response axis. Despite an ever expanding scientific literature, key unresolved questions remain concerning the interplay of the central and peripheral components of this complex biobehavioral system that dynamically engages the brain and the body as humans interact with social partners over the course of development.

Relationships among estrogen receptor, oxytocin and vasopressin gene expression and social interaction in male mice

The incidence of social disorders such as autism and schizophrenia is significantly higher in males, and the presentation more severe, than in females. This suggests the possible contribution of sex hormones to the development of these psychiatric disorders. There is also evidence that these disorders are highly heritable. To contribute toward our understanding of the mechanisms underlying social behaviors, particularly social interaction, we assessed the relationship of social interaction with gene expression for two neuropeptides, oxytocin (OT) and arginine vasopressin (AVP), using adult male mice. Social interaction was positively correlated with: oxytocin receptor (OTR) and vasopressin receptor (V1aR) mRNA expression in the medial amygdala; and OT and AVP mRNA expression in the paraventricular nucleus of the hypothalamus (PVN). When mice representing extremes of social interaction were compared, all of these mRNAs were more highly expressed in high social interaction mice than in low social interaction mice. OTR and V1aR mRNAs were highly correlated with estrogen receptor α (ERα) mRNA in the medial amygdala, and OT and AVP mRNAs with estrogen receptor β (ERβ) mRNA in the PVN, indicating that OT and AVP systems are tightly regulated by estrogen receptors. A significant difference in the level of ERα mRNA in the medial amygdala between high and low social interaction mice was also observed. These results support the hypothesis that variations of estrogen receptor levels are associated with differences in social interaction through the OT and AVP systems, by upregulating gene expression for those peptides and their receptors.

The advantage of social living: brain neuropeptides mediate the beneficial consequences of sex and motherhood

Living in social groups is clearly beneficial for many species, often resulting in increased survival, enhanced fitness of the group, and progression of brain development and cognitive abilities. The development of the social brain has been promoted on the basis (i) of activation of reward centres by social stimuli, (ii) of positive consequences of close social interactions on emotionality (which is reinforcing by itself) and on general fitness, and (iii) of negative health consequences in the absence or as a result of sudden interruption of social interactions. For example, social interactions as seen between mother and child or between mating partners have beneficial effects on the mental and physical health state, in particular on adaptive processes related to emotional and physiological stress coping in both sexes. Here, the neurobiological basis of social behaviour, in particular the involvement of the brain neuropeptides, oxytocin and prolactin, in mediating such positive health effects will be discussed.

Oxytocin levels in social anxiety disorder

Oxytocin is a neuropeptide recently associated with social behavior in animals and humans, but the study of its function in populations with social deficits such as autism, schizophrenia, and social anxiety disorder has only recently begun. We measured plasma oxytocin in 24 patients with Generalized Social Anxiety Disorder (GSAD) and 22 healthy controls using an enzyme-linked immunosorbent assay. There were no significant differences in oxytocin level (pg/mL) between patients (M=163.0, SD=109.4) and controls (M=145.0, SD=52.9, z=0.21, P=0.8). Within the GSAD sample, however, higher social anxiety symptom severity adjusted for age and gender was associated with higher oxytocin level (R2=0.21, beta=0.014, SE=0.006, t=2.18, P=0.04). In addition, dissatisfaction with social relationships was associated with higher oxytocin levels (R2=0.18, beta=-0.20, SE=0.10, t=-2.01, P=0.05). Our data provide preliminary support for a link between social anxiety severity and plasma oxytocin. These findings may suggest a possible role for oxytocin as a facilitator of social behavior, an effect which may not be fully utilized in individuals with severe social anxiety.

Oxytocin facilitates the induction of long-term potentiation in the accessory olfactory bulb

When female mice are mated, they form a memory to the pheromonal signal of their male partner. Several lines of evidence indicate that the neural changes underlying this memory occur in the accessory olfactory bulb (AOB) at the first stage of the vomeronasal system. The formation of this memory depends on the mating-induced release of noradrenaline in the AOB. In addition to noradrenaline, the neuropeptide oxytocin (OT) is also released within the central nervous system during mating. Because OT has been implicated in social memory and its receptors are expressed in the AOB, we hypothesized that OT might promote the strength of synaptic transmission from mitral to granule cells in the AOB. To test this hypothesis, we analyzed the lateral olfactory tract-evoked field potential that represents the granule cell response to mitral cell activation and its plasticity in parasagittal slices of the AOB. Of the 10-, 20-, 50-, and 100-Hz stimulations tested, the 100-Hz stimulation was optimal for inducing long-term potentiation (LTP). OT paired with 100-Hz stimulation that only produced short-term potentiation enhanced LTP induction in a dose-dependent manner. OT-paired LTP was blocked by both the selective OT antagonist desGly-NH(2),d(CH(2))(5)[Tyr(Me)(2),Thr(4)]-ornithine vasotocin and the N-methyl-d-aspartate (NMDA) receptor antagonist dl-2-amino-5-phosphonovaleric acid. These results indicate that OT can function as a gate to modulate the establishment of NMDA receptor-dependent LTP at the mitral-to-granule cell synapse in the AOB.

Receptors and neural effects of oxytocin in the rodent hypothalamus and preoptic region

Vasopressin and oxytocin are produced in and secreted from not only hypothalamo-hypophysial neurons which shed their products into the circulation to act as hormones or releasing factors, but also from neurons whose axons form tracts which remain within the central nervous system. Using tritiated or radioiodinated ligands, binding sites for vasopressin and for oxytocin have been detected by in vitro autoradiography. In the rat hypothalamus binding sites for vasopressin are present in the suprachiasmatic, sigmoid and arcuate nuclei, and oxytocin receptors in the area of the ventromedial nucleus. Electrophysiological evidence obtained using single cell recordings in slices suggests that oxytocin-binding sites present in the ventromedial hypothalamus and in the bed nucleus of the stria terminalis mostly represent functional, neuronal receptors. The expression of these receptors (but not of the vasopressin receptors) depends on gonadal steroid hormones, as does that of uterine and mammary gland oxytocin receptors. Modifications of the hormonal status associated with, for example, puberty and lactation cause 'up-regulation' of central and peripheral oxytocin receptors. The central administration of oxytocin facilitates (and the administration of oxytocin agonists inhibits) maternal behaviour and the milk ejection reflex, therefore the hormonal and neural actions of oxytocin appear to be complementary in ensuring the birth and development of the offspring.

Effects of an acute stressor on blood pressure and heart rate in rats pretreated with intracerebroventricular oxytocin injections

Oxytocin induces a long-lasting reduction of blood pressure in rats. The aim of the present study was to investigate the effects of an acute stressor on blood pressure and heart rate in rats previously exposed to repeated administration of intracerebroventricular (ICV) oxytocin. For this purpose oxytocin (0.3 microg, ICV) was administered to male rats once a day during 5 days. Blood pressure and heart rate were measured before and after treatment. In addition, blood pressure and heart rate were measured during 30 min after exposure to 10s of noise from an alarm clock. The oxytocin treatment reduced blood pressure significantly (systolic: 108+/-4.6 vs. 121+/-1.8, p<0.01, diastolic: 96+/-5.1 vs. 108+/-3.0, p<0.01), whereas heart rate remained unchanged. In contrast, systolic and diastolic blood pressure increased significantly after the exposure to the ringing alarm clock in the oxytocin-treated rats (p<0.05), and became equal to the blood pressure in controls. In addition, heart rate increased and stayed significantly higher in the oxytocin-treated rats compared to the controls during the 30 min observation period (ANOVA p<0.01). Twenty-four hours later, blood pressure was again significantly lower in the oxytocin-treated rats compared to controls (p<0.01). In conclusion, oxytocin decreased blood pressure without changing pulse rate. However, when the oxytocin-treated rats were subjected to the unexpected noise from a ringing alarm clock blood pressure and heart rate increased significantly. No such effect was observed in the control group. Thus repeated oxytocin treatment can, in spite of decreasing blood pressure during basal conditions, increase cardiovascular reactivity to some types of stressors.

Neuropeptidergic regulation of affiliative behavior and social bonding in animals

Social relationships are essential for maintaining human mental health, yet little is known about the brain mechanisms involved in the development and maintenance of social bonds. Animal models are powerful tools for investigating the neurobiological mechanisms regulating the cognitive processes leading to the development of social relationships and for potentially extending our understanding of the human condition. In this review, we discuss the roles of the neuropeptides oxytocin and vasopressin in the regulation of social bonding as well as related social behaviors which culminate in the formation of social relationships in animal models. The formation of social bonds is a hierarchical process involving social motivation and approach, the processing of social stimuli and formation of social memories, and the social attachment itself. Oxytocin and vasopressin have been implicated in each of these processes. Specifically, these peptides facilitate social affiliation and parental nurturing behavior, are essential for social recognition in rodents, and are involved in the formation of selective mother-infant bonds in sheep and pair bonds in monogamous voles. The convergence of evidence from these animal studies makes oxytocin and vasopressin attractive candidates for the neural modulation of human social relationships as well as potential therapeutic targets for the treatment of psychiatric disorders associated with disruptions in social behavior, including autism.

A relationship between oxytocin and anxiety of romantic attachment

The formation of social bonding is fundamental for several animals, including humans, for its relevant and obvious impact upon reproduction and, thus, survival of the species. Recent data would suggest that oxytocin might be one of the mediators of this process. Given the paucity of data on the possible involvement of oxytocin in human attachment, the present study was aimed to explore the possible relationships between the plasma levels of this neuropeptide and romantic attachment in healthy subjects. Forty-five healthy subjects who volunteered for the study, were included in the study. The romantic attachment was assessed using the Italian version of the so-called "Experiences in Close Relationships" (ECR), a self-report questionnaire for measuring this parameter in adults. The results showed that attachment anxiety and oxytocin are positively linked in romantic attachment to a statistically significant degree (r = 0.30, p = 0.04), that is, the higher the oxytocin levels the higher the score on the anxiety scale of the ECR. The authors suggest the hypothesis that this link represents one of the biological processes resulting in those rewarding emotions related to romantic attachment.

Maternal responsiveness and maternal selectivity in domestic sheep and goats: The two facets of maternal attachment

Sheep and goats rapidly establish an exclusive relationship with their neonate following contact with it during a sensitive period of maternal responsiveness induced by the physiological events occurring at parturition. The data concerning the sensory, physiological, and neurobiological factors involved in the activation of both maternal responsiveness and the establishment of selective nursing indicates that these processes are activated simultaneously by the combined action of two main factors, the prepartum rise in circulating estrogen and the vaginocervical stimulation (VCS) caused by fetus expulsion. On the one hand, these two factors act on a neural network including the main olfactory system (MOB), the medial preoptic area (MPOA), and the paraventricular nucleus of the hypothalamus (PVN) to induce maternal responsiveness towards any neonate. The intracerebral release of oxytocin (OT) from the PVN, and the triggering of olfactory attraction for amniotic fluid (AF) are key elements in this process. On the other hand, VCS at birth also sets the MOB ready to memorize the individual odor of the neonate, through the release of peptides and neurotransmitters (noradrenaline and acetylcholine). In addition to the MOB, the network involved in recognition mainly includes the medial and cortical amygdala. Across consolidation processes, reorganization occurs in the network engaged in lamb recognition. Whether this memorization may be potentiated by other sensory cues is not known. The identification of the chemosensory compounds involved in the attraction for AF and in the recognition of the neonate is important for understanding the mechanisms of maternal attachment.

Antagonistic interaction between adenosine A2A and dopamine D2 receptors modulates the social recognition memory in reserpine-treated rats

Increasing evidence suggests that antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the basal ganglia are involved in the control of motor activity. However, there are few studies investigating this interaction in other brain regions and its role in additional functions. In the present study, we evaluated whether reserpine-treated rats (1.0 mg/kg, i.p.) exhibit altered social recognition memory abilities. The effects of acute administration of the dopamine receptor agonists 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3 benzazepine (SKF 38393, dopamine D(1) receptor agonist) and quinpirole (dopamine D(2) receptor agonist), together with the adenosine receptor antagonists caffeine (non-selective), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, adenosine A(1) receptor antagonist) and 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)phenol (ZM241385, adenosine A(2A) receptor antagonist), were also investigated. Twenty-four hours after treatment, reserpine-treated rats exhibited a significant disruption in the ability to recognize a juvenile rat after a short period of time. These animals did not show any motor deficit. The social recognition disruption induced by reserpine was reversed by acute treatment with quinpirole (0.05-0.1 mg/kg, i.p.), caffeine (10.0-30.0 mg/kg, i.p.) or ZM241385 (0.5-1.0 mg/kg, i.p.), but not with SKF 38393 (0.5-3.0 mg/kg, i.p.) or DPCPX (0.5-3.0 mg/kg, i.p.). Moreover, a synergistic response was observed following the co-administration of 'non-effective' doses of ZM241385 (0.1 mg/kg, i.p.) and quinpirole (0.01 mg/kg, i.p.). These results reinforce and extend the notion of antagonistic interactions between adenosine and dopamine receptors, and demonstrate, for the first time, that the blockade of adenosine A(2A) receptors and the activation of dopamine D(2) receptors can reverse the social recognition deficits induced by reserpine in rats.

Release of oxytocin in the rat central amygdala modulates stress-coping behavior and the release of excitatory amino acids

Previous experiments have indicated that the release of oxytocin (OXT) occurs in various hypothalamic and extrahypothalamic brain areas. In the present study, we investigated in male rats whether swim stress triggers the release of OXT in the central amygdala (CeA), a key area in processing emotions and stress responses. Further, we examined the physiological significance of OXT released within the CeA for behavioral responses during forced swimming as well as effects on the local release of selected amino acids including glutamate, aspartate, arginine, taurine, and GABA, which are thought to modulate processing of emotions. Exposure to a 10-min forced swimming session caused a significant increase in OXT release (200%, p<0.01) within, but not outside, the CeA as monitored by microdialysis. Administration of the OXT receptor antagonist des-Gly-NH2d(CH2)5(Tyr(Me)2Thr4)OVT via inverse microdialysis into the amygdala before and during exposure to swimming reduced the floating time by 55% (p<0.05) and increased the swimming time by 29% (p<0.05) indicative of a more active stress-coping strategy. Simultaneously, local administration of the OXT receptor antagonist caused a significant increase in the stress-induced release of the excitatory amino acids glutamate and aspartate, whereas the basal release of these amino acids remained unchanged. Taken together, these findings demonstrate a significant activation of the oxytocinergic system in the CeA in response to swim stress. Furthermore, our data indicate that OXT receptor-mediated mechanisms within the amygdala are involved in the generation of passive stress-coping strategies, which might be mediated at least in part via its inhibitory influence on the local release of excitatory amino acids during stress.

Facilitation of short-term social memory by ethanol in rats is mediated by dopaminergic receptors

Ethanol is a drug that has apparently opposite effects on memory processes depending on when it is given relative to the task, as well as the nature of the task under study. Recently, we demonstrated that acute low doses of ethanol (0.5 and 1.0 g/kg, i.p.) improve the short-term social memory in rats in a specific and time-dependent manner, and that this action is, at least in part, related to opioid, but not to muscarinic receptors. In the present study, we evaluated whether this positive effect of ethanol on the short-term memory of rats is related to a reducing impact of interference during the task through two different procedures: the introduction of an unfamiliar juvenile rat or the placing of the adult rat in the open field during the inter-exposure interval. The actions of reserpine (0.4 and 0.8 mg/kg, s.c.), haloperidol (0.05 and 0.2 mg/kg, i.p.), the D2 receptor antagonist sulpiride (20.0 and 50.0 mg/kg, i.p.) and the D1 receptor antagonist SCH 23390 (0.01 and 0.03 mg/kg, s.c.) and their interaction with ethanol (1.0 g/kg, i.p.) in relation to short-term memory were also studied. The administration of ethanol (1.0 g/kg, i.p.), immediately after the end of the first presentation, did not reduce the effect on social memory of the introduction of an unfamiliar juvenile or placing the adult rat in the open field during the inter-exposure interval. The facilitatory effect of ethanol on social memory was inhibited by the pretreatment with reserpine and it was antagonized by the administration of haloperidol or sulpiride, but not by SCH 23390. These results indicate that the facilitation of short-term social memory by ethanol is not related to a reduction in the deleterious impact of interference and that this action of ethanol is mediated, at least in part, by D2 receptors, but not by D1 dopaminergic receptors.

Changes in cerebrospinal fluid neurochemistry during pregnancy

BACKGROUND

Little is known about changes in brain function that may occur during pregnancy. Studies in rodents and sheep suggest that several brain neurotransmitter and neurohormonal systems known to modulate anxiety may be altered during pregnancy.

METHODS

Cerebrospinal fluid (CSF) and plasma samples were obtained from 21 women (during weeks 38-39 of pregnancy) who were undergoing elective cesarean section and from 22 healthy nonpregnant women.

RESULTS

The CSF levels of g-aminobutyric acid (GABA) and 3-methoxy-4-hydroxyphenylglycolwere reduced in pregnant women. There were no changes in CSF glutamate, 5-hydroxyindoleactic acid, and homovanillic acid. There was a large increase in CSF prolactin in pregnant women and also a trend toward an elevation in CSF oxytocin. Levels of prolactin, but not oxytocin, in CSF and plasma were correlated in pregnant women.

CONCLUSIONS

These results suggest that pregnancy alters regulation of brain GABA, norepinephrine, and prolactin, which may play a role in changes in vulnerability to anxiety and depression during pregnancy and postpartum. Prolactin circulating in the bloodstream seems to be the major source of CSF prolactin during pregnancy.

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

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

Molecular approaches to aggregation behavior and social attachment

In many animal species individuals aggregate to live in groups. A range of experimental approaches in different animals, including studies of social feeding in nematodes, maternal behavior in rats and sheep, and pair-bonding in voles, are providing insights into the neural bases for these behaviors. These studies are delineating multiple neural circuits and gene networks in the brain that interact in ways that are as yet poorly understood to coordinate social behavior.

Involvement of the brain oxytocin system in stress coping: interactions with the hypothalamo-pituitary-adrenal axis

In response to various ethologically relevant stressors, oxytocin is released not only from neurohypophysial terminals into the blood, but also within distinct brain regions, for example the hypothalamic supraoptic and paraventricular nuclei, the septum and the amygdala in dependence on the quality and intensity of the stressor. Thus, oxytocin secretory activity may accompany the response of the hypothalamo-pituitary-adrenal (HPA) axis to a given stressor. In the present chapter, I try to summarize our efforts to reveal the physiological significance of intracerebrally released oxytocin in rats with respect to the regulation of the HPA axis under basal and stress conditions as well as with respect to behavioural stress responses. The effects of oxytocin appear to depend on the brain region studied and the state of activity of the animal (basal versus stress). In order to reveal interactions between the oxytocin system and the HPA axis, preliminary results are presented pointing towards a differential action of glucocorticoids on intracerebral and peripheral oxytocin release.

Maternal olfaction differentially modulates oxytocin and prolactin release during suckling in goats

In postparturient goats, olfactory recognition of the young allows the establishment of a selective bond between the mother and her kids. Once this bond is formed, the mother rejects alien young that attempt to suckle. We tested whether the development of the maternal selective bond in goats modulates prolactin (PRL) and oxytocin (OT) release in response to suckling. On day 37 of lactation, serial blood samples were taken during nursing of the mother's own or alien kid(s) in 10 intact/selective goats and in 10 goats rendered anosmic/nonselective through prepartum peripheral ZnSO(4) irrigation. Spontaneous nursing behavior was also studied weekly from day 7 to 30 of lactation, at which time milk production was measured. Maternal selectivity had no effect on PRL release, in contrast to OT release, which was significantly affected by this factor. Intact mothers released OT only when nursing their own kids, but not with aliens, while anosmic/nonselective dams showed an increase in OT levels regardless of the identity of the kids. In addition to these effects on maternal selectivity, the amplitude of the response of both hormones was lower in anosmic mothers than in intact mothers. Finally, nursing behavior and milk production were not significantly affected by anosmia. We conclude that maternal selective behavior in goats, which relies on the individual olfactory signature of the kid, modulates the OT, but not the PRL, response to suckling. In addition, perception of the smell of the young appears to have a general facilitatory effect, independent of the kid's identity, on the release of both hormones.

Energy conservation in stressed rats exposed to an oxytocin-injected cage mate

In previous studies we found indications of stress reduction in saline-injected rats when exposed to an oxytocin (OT)-injected cage mate. Olfactory impairment and OT antagonist treatment abolished the effects. This suggested an olfactorily mediated oxytocinergic stress-inhibitory mechanism. To test this hypothesis bodyweight, tail skin temperatures, food-intake and plasma ACTH and corticosterone concentrations were analysed. Suppressed weight loss and decreased stress-hormone release was found in saline-injected rats exposed to an OT-injected cage mate, but not in OT antagonist-injected rats, supporting the hypothesis. Our results suggest that OT in a stressed animal can inhibit the olfactory stress cues emitted, and that the olfactory cues from the stressed animal can influence an OT in pathway in the odour recipient animals to reduce stress effects.

Vaginocervical stimulation releases oxytocin within the spinal cord in rats

Vaginocervical stimulation (VS) significantly elevated the concentration of oxytocin (OT) in spinal cord superfusates of 8 intact urethane-anesthetized rats measured 10-15 min after VS (median [interquartile range]: 1.7 [1.00-3.37] pg/ml) compared to that measured 10-15 min before VS (1.1 [1.01-1.40] pg/ml). When VS was administered once (n = 8), it produced a 55% increase over baseline values; when administered a second time 45 min later (n = 6), it produced only a 22% increase over pre-VS values. The effects of estrogen on the VS-induced release of OT were then investigated using ovariectomized rats that were treated either with estradiol benzoate (EB; 10 microg/100 g bw) (n = 6) or with an oil vehicle (n = 6) subcutaneously for 3 days. The EB treatment significantly elevated the basal levels of OT released into spinal cord superfusates above vehicle control levels. Within 5-10 min after the onset of VS, OT concentrations in the superfusates were significantly higher in EB-treated than in vehicle-treated rats. The vehicle-treated rats did not show a significant elevation in OT concentration following VS. To rule out the possibility that the posterior pituitary gland was the source of this OT, the effect of hypophysectomy (HYPOX) was assessed on the VS-induced release of OT into spinal cord superfusates and plasma. The concentration of OT in spinal cord superfusates of both the HYPOX (n = 5) and intact rats (n = 6) increased significantly from 5.8 [4.4-6.5] pg/ml pre-VS to 7.9 [6.7-10.3] pg/ml immediately after VS, and from 4.4 [3.8-5] pg/ml pre-VS to 5.1 [4.6-5.7] pg/ml immediately after VS, respectively. There was no significant difference in baseline levels of OT in cerebrospinal fluid between the two groups. By contrast, plasma OT levels, while significantly elevated in response to VS from 3.42 [2.9-5.34] pg/ml baseline to 7.25 [5.33-15.77] pg/ml in the intact group, failed to respond significantly to VS in the HYPOX group (n = 5). The present findings provide evidence of a direct estrogen-dependent release of OT within the spinal cord in response to VS, presumably via descending oxytocinergic neurons.