Centrally injected arginine vasopressin (AVP) facilitates social memory in rats

Sex differences in the structure and function of the vasopressin system in the ventral pallidum are associated with the sex-specific regulation of social play behavior in juvenile rats

Vasopressin (AVP) regulates various social behaviors, often in sex-specific ways, including social play behavior, a rewarding behavior displayed primarily by juveniles. Here, we examined whether and how AVP acting in the brain's reward system regulates social play behavior in juvenile rats. Specifically, we focused on AVP signaling in the ventral pallidum (VP), a brain region that is a part of the reward system. First, we examined the organization of the VP-AVP system in juvenile rats and found sex differences, with higher density of both AVP-immunoreactive fibers and AVP V1a receptor (V1aR) binding in males compared to females while females show a greater number of V1aR-expressing cells compared to males. We further found that, in both sexes, V1aR-expressing cells co-express a GABA marker to a much greater extent (approx. 10 times) than a marker for glutamate. Next, we examined the functional involvement of V1aR-expressing VP cells in social play behavior. We found that exposure to social play enhanced the proportion of activated V1aR-expressing VP cells in males only. Finally, we showed that infusion of a specific V1aR antagonist into the VP increased social play behaviors in juvenile male rats while decreasing these behaviors in juvenile female rats. Overall, these findings reveal structural and functional sex differences in the AVP-V1aR system in the VP that are associated with the sex-specific regulation of social play behavior.

Therapeutic potential of vasopressin in the treatment of neurological disorders

Vasopressin (VP) is a nonapeptide made of nine amino acids synthesized by the hypothalamus and released by the pituitary gland. VP acts as a neurohormone, neuropeptide and neuromodulator and plays an important role in the regulation of water balance, osmolarity, blood pressure, body temperature, stress response, emotional challenges, etc. Traditionally VP is known to regulate the osmolarity and tonicity. VP and its receptors are widely expressed in the various region of the brain including cortex, hippocampus, basal forebrain, amygdala, etc. VP has been shown to modulate the behavior, stress response, circadian rhythm, cerebral blood flow, learning and memory, etc. The potential role of VP in the regulation of these neurological functions have suggested the therapeutic importance of VP and its analogues in the management of neurological disorders. Further, different VP analogues have been developed across the world with different pharmacotherapeutic potential. In the present work authors highlighted the therapeutic potential of VP and its analogues in the treatment and management of various neurological disorders.

Selective breeding of rats for high (HAB) and low (LAB) anxiety-related behaviour: A unique model for comorbid depression and social dysfunctions

Animal models of selective breeding for extremes in emotionality are a strong experimental approach to model psychopathologies. They became indispensable in order to increase our understanding of neurobiological, genetic, epigenetic, hormonal, and environmental mechanisms contributing to anxiety disorders and their association with depressive symptoms or social deficits. In the present review, we extensively discuss Wistar rats selectively bred for high (HAB) and low (LAB) anxiety-related behaviour on the elevated plus-maze. After 30 years of breeding, we can confirm the prominent differences between HAB and LAB rats in trait anxiety, which are accompanied by consistent differences in depressive-like, social and cognitive behaviours. We can further confirm a single nucleotide polymorphism in the vasopressin promotor of HAB rats causative for neuropeptide overexpression, and show that low (or high) anxiety and fear levels are unlikely due to visual dysfunctions. Thus, HAB and LAB rats continue to exist as a reliable tool to study the multiple facets underlying the pathology of high trait anxiety and its comorbidity with depression-like behaviour and social dysfunctions.

Exocrine scent marking: Coordinative role of arginine vasopressin in the systemic regulation of social signaling behaviors

Arginine vasopressin (AVP) is a neurohypophysial hormone that coordinatively regulates central socio-emotional behavior and peripheral control of antidiuretic fluid homeostasis. Most mammals, including rodents, utilize exocrine or urine-contained scent marking as a social signaling tool that facilitates social adaptation. The exocrine scent marking behavior is postulated to fine-tune sensory and cognitive abilities to recognize key social features via exocrine/urinary olfactory cues and subsequently control exocrine deposition or urinary marking through the mediation of osmotic fluid balance. AVP is implicated as a major player in controlling both recognition and signaling responses. This review provides constructive hypotheses on the coordinative processes of the AVP neurohypophysial circuits in the systemic regulations of fluid control and social-communicative behavior, via the expression of exocrine scent marking, and further emphasizes a potential role of AVP in a common mechanism underlying social communication in rodents.

Oxytocin and vasopressin in the hippocampus

Oxytocin (OXT) and vasopressin (AVP) are related neuropeptides that exert a wide range of effects on general health, homeostasis, development, reproduction, adaptability, cognition, social and nonsocial behaviors. The two peptides are mainly of hypothalamic origin and execute their peripheral and central physiological roles via OXT and AVP receptors, which are members of the G protein-coupled receptor family. These receptors, largely distributed in the body, are abundantly expressed in the hippocampus, a brain region particularly vulnerable to stress exposure and various lesions. OXT and AVP have important roles in the hippocampus, by modulating important processes like neuronal excitability, network oscillatory activity, synaptic plasticity, and social recognition memory. This chapter includes an overview regarding OXT and AVP structure, synthesis, receptor distribution, and functions, focusing on their relationship with the hippocampus and mechanisms by which they influence hippocampal activity. Brief information regarding hippocampal structure and susceptibility to lesions is also provided. The roles of OXT and AVP in neurodevelopment and adult central nervous system function and disorders are highlighted, discussing their potential use as targeted therapeutic tools in neuropsychiatric diseases.

Individual variation in the attribution of incentive salience to social cues

Research on the attribution of incentive salience to drug cues has furthered our understanding of drug self-administration in animals and addiction in humans. The influence of social cues on drug-seeking behavior has garnered attention recently, but few studies have investigated how social cues gain incentive-motivational value. In the present study, a Pavlovian conditioned approach (PCA) procedure was used to identify rats that are more (sign-trackers; STs) or less (goal-trackers; GTs) prone to attribute incentive salience to food reward cues. In Experiment 1, a novel procedure employed social ‘peers’ to compare the tendency of STs and GTs to attribute incentive salience to social reward cues as well as form a social-conditioned place preference. In Experiment 2, social behavior of STs and GTs was compared using social interaction and choice tests. Finally, in Experiment 3, levels of plasma oxytocin were measured in STs and GTs seven days after the last PCA training session, because oxytocin is known to modulate the mesolimbic reward system and social behavior. Compared to GTs, STs attributed more incentive salience to social-related cues and exhibited prosocial behaviors (e.g., social-conditioned place preference, increased social interaction, and social novelty-seeking). No group differences were observed in plasma oxytocin levels. Taken together, these experiments demonstrate individual variation in the attribution of incentive salience to both food- and social-related cues, which has important implications for the pathophysiology of addiction.

From adolescence to late aging: A comprehensive review of social behavior, alcohol, and neuroinflammation across the lifespan

The passage of time dictates the pace at which humans and other organisms age but falls short of providing a complete portrait of how environmental, lifestyle and underlying biological processes contribute to senescence. Two fundamental features of the human experience that change dramatically across the lifespan include social interactions and, for many, patterns of alcohol consumption. Rodent models show great utility for understanding complex interactions among aging, social behavior and alcohol use and abuse, yet little is known about the neural changes in late aging that contribute to the natural decline in social behavior. Here, we posit that aging-related neuroinflammation contributes to the insipid loss of social motivation across the lifespan, an effect that is exacerbated by patterns of repeated alcohol consumption observed in many individuals. We provide a comprehensive review of (i) neural substrates crucial for the expression of social behavior under non-pathological conditions; (ii) unique developmental/lifespan vulnerabilities that may contribute to the divergent effects of low-and high-dose alcohol exposure; and (iii) aging-associated changes in neuroinflammation that may sit at the intersection between social processes and alcohol exposure. In doing so, we provide an overview of correspondence between lifespan/developmental periods between common rodent models and humans, give careful consideration to model systems used to aptly probe social behavior, identify points of coherence between human and animal models, and point toward a multitude of unresolved issues that should be addressed in future studies. Together, the combination of low-dose and high-dose alcohol effects serve to disrupt the normal development and maintenance of social relationships, which are critical for both healthy aging and quality of life across the lifespan. Thus, a more complete understanding of neural systems-including neuroinflammatory processes-which contribute to alcohol-induced changes in social behavior will provide novel opportunities and targets for promoting healthy aging.

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

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

Oxytocin and vasopressin in the rodent hippocampus

The role of the hippocampus in social memory and behavior is under intense investigation. Oxytocin (Oxt) and vasopressin (Avp) are two neuropeptides with many central actions related to social cognition. Oxt- and Avp-expressing fibers are abundant in the hippocampus and receptors for both peptides are seen throughout the different subfields, suggesting that Oxt and Avp modulate hippocampal-dependent processes. In this review, we first focus on the anatomical sources of Oxt and Avp input to the hippocampus and consider the distribution of their corresponding receptors in different hippocampal subfields and neuronal populations. We next discuss the behavioral outcomes related to social memory seen with perturbation of hippocampal Oxt and Avp signaling. Finally, we review Oxt and Avp modulatory mechanisms in the hippocampus that may underlie the behavioral roles for both peptides.

Prairie Voles as a Model for Understanding the Genetic and Epigenetic Regulation of Attachment Behaviors

Over a lifetime, humans build relationships with family, friends, and partners that are critically important for our mental and physical health. Unlike commonly used laboratory mice and rats, Microtine rodents provide a unique model to study the neurobiology underlying pair bonding and the selective attachments that form between adults. Comparisons between monogamous prairie voles and the closely related but nonmonogamous meadow and montane voles have revealed that brain-region-specific neuropeptide receptor patterning modulates social behavior between and within species. In particular, diversity in vasopressin 1a receptor (V1aR) distribution has been linked to individual and species differences in monogamy-related behaviors such as partner preference, mate guarding, and space use. Given the importance of differential receptor expression for regulating social behavior, a critical question has emerged: What are the genetic and epigenetic mechanisms that underlie brain-region-specific receptor patterns? This review will summarize what is known about how the vasopressin (AVP)-V1aR axis regulates social behaviors via signaling in discrete brain regions. From this work, we propose that brain-region-specific regulatory mechanisms facilitate robust evolvability of V1aR expression to generate diverse sociobehavioral traits. Translationally, we provide a perspective on how these studies have contributed to our understanding of human social behaviors and how brain-region-specific regulatory mechanisms might be harnessed for targeted therapies to treat social deficits in psychiatric disorders such as depression, complicated grief, and autism spectrum disorder.

Neuroanatomical distribution and function of the vasopressin V1B receptor in the rat brain deciphered using specific fluorescent ligands

It is now accepted that vasopressin, through V_1A/V_1B receptors, centrally regulates cognitive functions such as memory, affiliation, stress, fear and depression. However, the respective roles of these receptor isoforms and their contribution to stress-related pathologies remain uncertain. The development of new therapeutic treatments requires a precise knowledge of the distribution of these receptors within the brain, which has been so far hampered by the lack of selective V_1B markers. In the present study, we have determined the pharmacological properties of three new potent rat V_1B fluorescent ligands and demonstrated that they constitute valuable tools for simultaneous visualization and activation of native V_1B receptors in living rat brain tissue. Thus, d[Leu⁴,Lys-Alexa 647)⁸]VP (analogue 3), the compound with the best affinity-selectivity/fluorescence ratio for the V_1B receptor emerged as the most promising. The rat brain regions most concerned by stress such as hippocampus, olfactory bulbs, cortex and amygdala display the highest V_1B fluorescent labelling with analogue 3. In the hippocampus CA₂, V_1B receptors are located on glutamatergic, not GABAergic neurones, and are absent from astrocytes. Using AVP-EGFP rats, we demonstrate the presence of V_1B autoreceptors on AVP-secreting neurones not only in the hypothalamus, but also sparsely in the hippocampus. Finally, using both electrophysiology and visualization of ERK phosphorylation, we show analogue 3-induced activation of the V_1B receptor in situ. This will help to analyse expression and functionality of V_1B receptors in the brain and contribute to further explore the AVPergic circuitry in normal and pathological conditions.

Targeted activation of the hippocampal CA2 area strongly enhances social memory

Social cognition enables individuals to understand others' intentions. Social memory is a necessary component of this process, for without it, subsequent encounters are devoid of any historical information. The CA2 area of the hippocampus, particularly the vasopressin 1b receptor (Avpr1b) expressed there, is necessary for memory formation. We used optogenetics to excite vasopressin terminals, originating from the hypothalamic paraventricular nucleus, in the CA2 of mice. This markedly enhanced their social memory if the stimulation occurred during memory acquisition, but not retrieval. This effect was blocked by an Avpr1b antagonist. Finally, this enhanced memory is resistant to the social distraction of an introduced second mouse, important for socially navigating populations of individuals. Our results indicate the CA2 can increase the salience of social signals. Targeted pharmacotherapy with Avpr1b agonists or deep brain stimulation of the CA2 are potential avenues of treatment for those with declining social memory as in various dementias.

Oxytocin (OT) and arginine-vasopressin (AVP) act on OT receptors and not AVP V1a receptors to enhance social recognition in adult Syrian hamsters (Mesocricetus auratus)

Social recognition is a fundamental requirement for all forms of social relationships. A majority of studies investigating the neural mechanisms underlying social recognition in rodents have investigated relatively neutral social stimuli such as juveniles or ovariectomized females over short time intervals (e.g., 2h). The present study developed a new testing model to study social recognition among adult males using a potent social stimulus. Flank gland odors are used extensively in social communication in Syrian hamsters and convey important information such as dominance status. We found that the recognition of flank gland odors after a 3min exposure lasted for at least 24h, substantially longer than the recognition of other social cues in rats and mice. Intracerebroventricular injections of OT and AVP prolonged the recognition of flank gland odor for up to 48h. Selective OTR but not V1aR agonists, mimicked these enhancing effects of OT and AVP. Similarly, selective OTR but not V1aR antagonists blocked recognition of the odors after 20min. In contrast, the recognition of non-social stimuli was not blocked by either the OTR or the V1aR antagonists. Our findings suggest both OT and AVP enhance social recognition via acting on OTRs and not V1aRs and that the recognition enhancing effects of OT and AVP are limited to social stimuli.

Sex-Dependent Effects of Prenatal Stress on Social Memory in Rats: A Role for Differential Expression of Central Vasopressin-1a Receptors

Prenatal stress (PNS) affects a number of traits in the offspring, including stress axis regulation, emotionality and cognition; however, much less is known about the effects of PNS on social memory and the underlying central mechanisms. In the present study, we investigated social preference, social memory under basal and stress conditions and olfactory memory for social and nonsocial odours in the adult offspring of dams exposed to social stress during late pregnancy. Given the key roles that the central oxytocin and vasopressin systems play in facilitating social memory, we further investigated the effects of PNS on the central expression of mRNA for oxytocin (Oxtr) and vasopressin-1a (Avpr1a) receptors. PNS did not affect social preference in either sex; however, social memory was impaired under basal conditions in PNS females but not PNS males. Accordingly, Avpr1a mRNA expression in the lateral septum and bed nucleus of stria terminalis (BNST) was unaltered in males but was significantly lower in PNS females compared to controls. No differences in Oxtr mRNA expression were detected between control and PNS offspring in either sex in any of the brain regions examined. Social memory deficits in PNS females persisted when social odours were used; however, this does not appear to be a result of impaired olfaction because memory for nonsocial odours was similar in control and PNS females. Under acute stress conditions, deficits in social memory were observed in both male and female control offspring; however, PNS males were unaffected. Moreover, acute stress facilitated social memory in PNS females and this was associated with an up-regulation of Avpr1a mRNA in the lateral septum and BNST. Our data support a role for altered signalling via central Avpr1a in PNS-induced sex-dependent changes in social memory and may have implications for understanding the aetiology of neurodevelopmental disorders characterised by social behaviour deficits in humans.

Correspondence of DNA Methylation Between Blood and Brain Tissue and Its Application to Schizophrenia Research

Given the difficulty of procuring human brain tissue, a key question in molecular psychiatry concerns the extent to which epigenetic signatures measured in more accessible tissues such as blood can serve as a surrogate marker for the brain. Here, we aimed (1) to investigate the blood-brain correspondence of DNA methylation using a within-subject design and (2) to identify changes in DNA methylation of brain-related biological pathways in schizophrenia.We obtained paired blood and temporal lobe biopsy samples simultaneously from 12 epilepsy patients during neurosurgical treatment. Using the Infinium 450K methylation array we calculated similarity of blood and brain DNA methylation for each individual separately. We applied our findings by performing gene set enrichment analyses (GSEA) of peripheral blood DNA methylation data (Infinium 27K) of 111 schizophrenia patients and 122 healthy controls and included only Cytosine-phosphate-Guanine (CpG) sites that were significantly correlated across tissues.Only 7.9% of CpG sites showed a statistically significant, large correlation between blood and brain tissue, a proportion that although small was significantly greater than predicted by chance. GSEA analysis of schizophrenia data revealed altered methylation profiles in pathways related to precursor metabolites and signaling peptides.Our findings indicate that most DNA methylation markers in peripheral blood do not reliably predict brain DNA methylation status. However, a subset of peripheral data may proxy methylation status of brain tissue. Restricting the analysis to these markers can identify meaningful epigenetic differences in schizophrenia and potentially other brain disorders.

Vasopressin and oxytocin receptor systems in the brain: Sex differences and sex-specific regulation of social behavior

The neuropeptides vasopressin (VP) and oxytocin (OT) and their receptors in the brain are involved in the regulation of various social behaviors and have emerged as drug targets for the treatment of social dysfunction in several sex-biased neuropsychiatric disorders. Sex differences in the VP and OT systems may therefore be implicated in sex-specific regulation of healthy as well as impaired social behaviors. We begin this review by highlighting the sex differences, or lack of sex differences, in VP and OT synthesis in the brain. We then discuss the evidence showing the presence or absence of sex differences in VP and OT receptors in rodents and humans, as well as showing new data of sexually dimorphic V1a receptor binding in the rat brain. Importantly, we find that there is lack of comprehensive analysis of sex differences in these systems in common laboratory species, and we find that, when sex differences are present, they are highly brain region- and species-specific. Interestingly, VP system parameters (VP and V1aR) are typically higher in males, while sex differences in the OT system are not always in the same direction, often showing higher OT expression in females, but higher OT receptor expression in males. Furthermore, VP and OT receptor systems show distinct and largely non-overlapping expression in the rodent brain, which may cause these receptors to have either complementary or opposing functional roles in the sex-specific regulation of social behavior. Though still in need of further research, we close by discussing how manipulations of the VP and OT systems have given important insights into the involvement of these neuropeptide systems in the sex-specific regulation of social behavior in rodents and humans.

Association between polymorphisms in NOS3 and KCNH2 and social memory

Social memory, including the ability to recognize faces and voices, is essential for social relationships. It has a large heritable component, but the knowledge about the contributing genes is sparse. The genetic variation underlying inter-individual differences in social memory was investigated in an exploratory sample (n = 55), genotyped with a chip comprising approximately 200,000 single nucleotide polymorphisms (SNPs), and in a validation sample (n = 582), where 30 SNPs were targeted. In the exploratory study face identity recognition was measured. The validation study also measured vocal sound recognition, as well as recognition of faces and vocal sounds combined (multimodal condition). In the exploratory study, the 30 SNPs that were associated with face recognition at p_uncorrected < 0.001 and located in genes, were chosen for further study. In the validation study two of these SNPs showed significant associations with recognition of faces, vocal sounds, and multimodal stimuli: rs1800779 in the gene encoding nitric oxide synthase 3 (NOS3) and rs3807370 in the gene encoding the voltage-gated channel, subfamily H, member 2 (KCNH2), in strong linkage disequilibrium with each other. The uncommon alleles were associated with superior performance, and the effects were present for men only (p < 0.0002). The exploratory study also showed a weaker but significant association with (non-emotional) word recognition, an effect that was independent of the effect on face recognition. This study demonstrates evidence for an association between NOS3 and KCNH2 SNPs and social memory.

WAY 267,464, a non-peptide oxytocin receptor agonist, impairs social recognition memory in rats through a vasopressin 1A receptor antagonist action

RATIONALE

Recent in vitro studies suggest that the oxytocin receptor (OTR) agonist WAY 267,464 has vasopressin 1A receptor (V1AR) antagonist effects. This might limit its therapeutic potential due to the positive involvement of the V1AR in social behavior.

OBJECTIVES

The objective of this study was to assess functional V1AR antagonist-like effects of WAY 267,464 in vivo using a test of social recognition memory.

METHODS

Adult experimental rats were tested for their recognition of a juvenile conspecific rat that they had briefly met 30 or 120 min previously. The modulatory effects of vasopressin (AVP), the selective V1AR antagonist SR49059, and WAY 267,464 were examined together with those of the selective OTR antagonist Compound 25 (C25). Drugs were administered immediately after the first meeting.

RESULTS

Control rats showed recognition of juveniles at a 30 min, but not a 120 min retention interval. AVP (0.005, but not 0.001 mg/kg intraperitoneal (i.p.)) improved memory such that recognition was evident after 120 min. This was prevented by pretreatment with SR49059 (1 mg/kg) and WAY 267,464 (10, 30, and 100 mg/kg). Given alone, SR49059 (1 mg/kg) and WAY 267,464 (30 and 100 mg/kg) impaired memory at a 30 min retention interval. The impairment with WAY 267,464 was not prevented by C25 (5 mg/kg), suggesting V1AR rather than OTR mediation of the effect. Given alone, C25 also impaired memory.

CONCLUSIONS

These results highlight a tonic role for endogenous AVP (and oxytocin) in social recognition memory and indicate that WAY 267,464 functions in vivo as a V1AR antagonist to prevent the memory-enhancing effects of AVP.

Endogenous vasopressin, innate anxiety, and the emission of pro-social 50-kHz ultrasonic vocalizations during social play behavior in juvenile rats

Although the involvement of the neuropeptide arginine vasopressin (AVP) in rodent social interaction is already extensively characterized, little is known about its role in social communication. Rats communicate in the ultrasonic range by means of ultrasonic vocalizations (USV). Depending on developmental stage and affective state, rats emit various distinct types of USV, with appetitive 50-kHz USV being induced by positive social interactions, like juvenile social play, probably serving an affiliative communicative function, namely to (re)establish or induce social proximity. In rats and mice selectively bred for low (LAB) and high (HAB) anxiety-related behavior, the emission of isolation-induced distress USV during maternal deprivation as pups correlates with innate high levels of hypothalamic AVP availability. Moreover, male LAB and HAB rats express deficits in social approach towards conspecifics, together with high and/or abnormal forms of aggression when confronted with harmless opponents, possibly due to a lack of social communication skills. The aim of this study was therefore (1) to investigate and characterize social play behavior and concomitant pro-social 50-kHz USV emission in male and female, juvenile LAB and HAB rats and to compare them to non-selected Wistar (NAB) rats; and (2) to link these findings pharmacologically to the central AVP system via applying an AVP 1a receptor (V1aR) antagonist (0.75 μg; Manning compound) or synthetic AVP (1 ng) into the lateral ventricle of male juvenile NAB rats. Our results show that reduced social play behavior in highly anxious male and female, juvenile HAB rats is accompanied by low amounts of pro-social 50-kHz USV, as compared to respective LAB and NAB rats, possibly reflecting a lack of positive affective states in expectation of or following social interactions in these individuals. Secondly, although synthetic AVP did not alter social play behavior and pro-social 50-kHz USV, we demonstrated for the first time that a blockade of the central AVP system not only reduces juvenile social play behavior, but at the same time pro-social 50-kHz USV emission rates, indicating an involvement of the social neuropeptide in regulating affiliative communication in rodents.

Examining the Role of Vasopressin in the Modulation of Parental and Sexual Behaviors

Vasopressin (VP) and VP-like neuropeptides are evolutionarily stable peptides found in all vertebrate species. In non-mammalian vertebrates, vasotocin (VT) plays a role similar to mammalian VP, whereas mesotocin and isotocin are functionally similar to mammalian oxytocin (OT). Here, we review the involvement of VP in brain circuits, synaptic plasticity, evolution, and function, highlighting the role of VP in social behavior. In all studied species, VP is encoded on chromosome 20p13, and in mammals, VP is produced in specific hypothalamic nuclei and released by the posterior pituitary. The role of VP is mediated by the stimulation of the V1a, V1b, and V2 receptors as well as the oxytocinergic and purinergic receptors. VT and VP functions are usually related to osmotic and cardiovascular homeostasis when acting peripherally. However, these neuropeptides are also critically involved in the central modulation of social behavior displays, such as pairing recognition, pair-bonding, social memory, sexual behavior, parental care, and maternal and aggressive behavior. Evidence suggests that these effects are primarily mediated by V1a receptor in specific brain circuits that provide important information for the onset and control of social behaviors in normal and pathological conditions.

Social bonding: regulation by neuropeptides

Affiliative social relationships (e.g., among spouses, family members, and friends) play an essential role in human society. These relationships affect psychological, physiological, and behavioral functions. As positive and enduring bonds are critical for the overall well-being of humans, it is not surprising that considerable effort has been made to study the neurobiological mechanisms that underlie social bonding behaviors. The present review details the involvement of the nonapeptides, oxytocin (OT), and arginine vasopressin (AVP), in the regulation of social bonding in mammals including humans. In particular, we will discuss the role of OT and AVP in the formation of social bonds between partners of a mating pair as well as between parents and their offspring. Furthermore, the role of OT and AVP in the formation of interpersonal bonding involving trust is also discussed.

Social preference and maternal defeat-induced social avoidance in virgin female rats: sex differences in involvement of brain oxytocin and vasopressin

BACKGROUND

Research concerning non-reproductive sociability in rodents is mainly restricted to assessing the effects of oxytocin (OXT) and arginine-vasopressin (AVP) in male rats and mice. Comparable studies on natural social preference and social avoidance in females are substantially lacking.

NEW METHOD

Here, we adapted a behavioral paradigm for monitoring social preference of female rats consisting of two consecutive exposures to either non-social or social stimuli. Further, to induce stimulus-specific social avoidance, female rats were exposed to a single 10-min maternal defeat by a lactating dam.

RESULTS

Social preference towards same-sex conspecifics in female rats was shown to be independent of the estrous cycle and even more pronounced than in male rats. Intracerebroventricular (icv) application of OXT, AVP, or their selective receptor antagonists or agonists, did not alter naturally-occurring social preference in female rats. Stimulus-specific social avoidance could be induced by prior exposure to a lactating rat: an effect that could not be reversed/overcome by icv OXT.

COMPARISON WITH EXISTING METHOD(S)

The female social preference paradigm for rats established in this study detected subtle sex differences in social preference behavior of rats. Further, stimulus-specific social deficits could be induced in female rats using an acute exposure to social defeat - as previously observed in male rodents.

CONCLUSIONS

Female rats show strong social preference behavior, which can be prevented by social defeat, but does not seem to be regulated by the OXT or AVP systems. Accordingly, icv application of synthetic OXT does not reverse maternal defeat-induced social avoidance in female rats.

Acute prosocial effects of oxytocin and vasopressin when given alone or in combination with 3,4-methylenedioxymethamphetamine in rats: involvement of the V1A receptor

The neuropeptides oxytocin (OT) and vasopressin (AVP) are recognized for their modulation of social processes in humans when delivered peripherally. However, there is surprisingly little evidence for acute social effects of peripherally administered OT or AVP in animal models. On the other hand, the party drug 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') has powerful prosocial effects in rats that appear to occur through stimulation of central OT release. Here, we directly compared the social effects of peripherally administered OT and AVP with those of MDMA, and examined a possible role for the vasopressin 1A receptor (V1AR) in the observed prosocial effects. Adult male Long-Evans rats were tested in a social interaction paradigm after OT (0.1, 0.25, 0.5, and 1 mg/kg, intraperitoneal (IP)), AVP (0.001, 0.0025, 0.005, 0.01, and 0.1 mg/kg, IP), and MDMA (2.5, 5 mg/kg, IP), or combined low doses of OT and MDMA, or AVP and MDMA. The effects of pretreatment with the non-peptide OT receptor antagonist compound 25 (C25; 5 mg/kg, IP) and the V1AR antagonist SR49059 (1 mg/kg, IP) were also examined. OT (0.5 mg/kg), AVP (0.01 mg/kg), and MDMA (5 mg/kg) potently increased 'adjacent lying', where rats meeting for the first time lie passively next to each other. C25 did not inhibit adjacent lying induced by OT, whereas SR49059 inhibited adjacent lying induced by MDMA (5 mg/kg), OT (0.5 mg/kg), and AVP (0.01 mg/kg). Interestingly, when ineffective doses of OT and MDMA, or AVP and MDMA, were combined, a robust increase in adjacent lying was observed. These results show for the first time acute prosocial effects of peripherally injected OT and AVP in laboratory rats, and suggest a commonality of action of OT, AVP, and MDMA in stimulating social behavior that involves V1ARs.

Vasopressin infusion into the lateral septum of adult male rats rescues progesterone-induced impairment in social recognition

It is well established that social recognition memory is mediated, in part, by arginine vasopressin (AVP). AVP cells within the bed nucleus of the stria terminalis (BST) and medial amygdala (MeA) send AVP-ergic projections to the lateral septum (LS). We have demonstrated that progesterone treatment decreases AVP immunoreactivity within the BST, the MeA and the LS, and that progesterone treatment impairs social recognition. These data suggested that progesterone may impair social recognition memory by decreasing AVP. In the present experiment, we hypothesized that infusions of AVP into the LS would rescue the progesterone-induced impairment in social recognition within adult male rats. One week after adult male rats underwent cannula surgery, they were given systemic injections of either a physiological dose of progesterone or oil control for 3 days. Four hours after the last injection, we tested social recognition memory using the social discrimination paradigm, a two-trial test that is based on the natural propensity for rats to be highly motivated to investigate novel conspecifics. Immediately after the first exposure to a juvenile, each animal received bilateral infusions of either AVP or artificial cerebrospinal fluid into the LS. Our results show that, as expected, control animals exhibited normal social discrimination. In corroboration with our previous results, animals given progesterone have impaired social discrimination. Interestingly, animals treated with progesterone and AVP exhibited normal social discrimination, suggesting that AVP treatment rescued the impairment in social recognition caused by progesterone. These data also further support a role for progesterone in modulating vasopressin-dependent behavior within the male brain.

Intranasal application of vasopressin fails to elicit changes in brain immediate early gene expression, neural activity and behavioural performance of rats

Intranasal administration has been widely used to investigate the effects of the neuropeptides vasopressin and oxytocin on human behaviour and neurological disorders, although exactly what happens when these neuropeptides are administered intranasally is far from clear. In particular, it is not clear whether a physiological significant amount of peptide enters the brain to account for the observed effects. In the present study, we investigated whether the intranasal administration of vasopressin and oxytocin to rats induces the expression of the immediate-early gene product Fos in brain areas that are sensitive to centrally-administered peptide, whether it alters neuronal activity in the way that centrally-administered peptide does, and whether it affects behaviour in the ways that are expected from studies of centrally-administered peptide. We found that, whereas i.c.v. injection of very low doses of vasopressin or oxytocin increased Fos expression in several distinct brain regions, intranasal administration of large doses of the peptides had no significant effect. By contrast to the effects of vasopressin applied topically to the main olfactory bulb, we saw no changes in the electrical activity of olfactory bulb mitral cells after intranasal vasopressin administration. In addition, vasopressin given intranasally had no significant effects on social recognition or short-term recognition memory. Finally, intranasal infusions of vasopressin had no significant effects on the parameters monitored on the elevated plus maze, a rodent model of anxiety. Our data obtained in rats suggest that, after intranasal administration, significant amounts of vasopressin and oxytocin do not reach areas in the brain at levels sufficient to change immediate early gene expression, neural activity or behaviour in the ways described for central administration of the peptides.

Rapid and reversible impairments of short- and long-term social recognition memory are caused by acute isolation of adult rats via distinct mechanisms

Mammalian social organizations require the ability to recognize and remember individual conspecifics. This social recognition memory (SRM) can be examined in rodents using their innate tendency to investigate novel conspecifics more persistently than familiar ones. Here we used the SRM paradigm to examine the influence of housing conditions on the social memory of adult rats. We found that acute social isolation caused within few days a significant impairment in acquisition of short-term SRM of male and female rats. Moreover, SRM consolidation into long-term memory was blocked following only one day of social isolation. Both impairments were reversible, but with different time courses. Furthermore, only the impairment in SRM consolidation was reversed by systemic administration of arginine-vasopressin (AVP). In contrast to SRM, object recognition memory was not affected by social isolation. We conclude that acute social isolation rapidly induces reversible changes in the brain neuronal and molecular mechanisms underlying SRM, which hamper its acquisition and completely block its consolidation. These changes occur via distinct, AVP sensitive and insensitive mechanisms. Thus, acute social isolation of rats swiftly causes changes in their brain and interferes with their normal social behavior.

Vasopressin eliminates the expression of familiar odor bias in neonatal female mice through V1aR

V1aR has a well established role in the neural regulation of adult mammalian social behavior. The role of V1aR in developmentally emerging social behavior is less well understood. We mapped V1aR at post-natal day 8 (P8) and demonstrate developmentally-specific expression in the neocortex and hippocampus. We tested the ability of male and female C57BL/6J mice to show orienting bias to a familiar odor at this age. We demonstrate that females, but not males, show an orienting bias for odors previously paired with the mother, which is eliminated by V1aR signaling.

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.

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.

Evidence for a role of oxytocin receptors in the long-term establishment of dominance hierarchies

Exposure to stress can affect the establishment of dominance hierarchies. In our model, a social hierarchy established by two male rats during a first encounter is not maintained 1 week later. If one of the two rats is stressed, the stressed rat becomes subordinate and the hierarchy that is formed is maintained. In this study, we investigated the changes in the expression of oxytocin (Otr) and vasopressin (V1aR) receptor genes in the medial amygdala (MeA) and the lateral septum (LS) in the hours following hierarchy establishment under both stressed and basal conditions. We found that the potentiation of a social hierarchy induced by stress is accompanied by social status- and region-specific changes in the expression of Otr mRNA in the MeA 3 h after the social encounter. At this time point, no evidence was found for the regulation of V1aR mRNA in any of the brain regions examined. Results from pharmacological experiments involving the microinfusion of a specific OTR antagonist immediately after the acquisition of a subordinate status under basal, non-stress conditions suggested a role for this receptor in the MeA on the long-term establishment of the subordinate status. Altogether, these findings highlight a role for the oxytocinergic system in the mechanisms through which stress facilitates the long-term establishment of a social hierarchy.

Vasopressin modulates social recognition-related activity in the left temporoparietal junction in humans

The neuropeptide vasopressin is a key molecular mediator of social behavior in animals and humans, implicated in anxiety and autism. Social recognition, the ability to assess the familiarity of others, is essential for appropriate social interactions and enhanced by vasopressin; however, the neural mechanisms mediating this effect in humans are unknown. Using functional magnetic resonance imaging (fMRI) and an implicit social recognition matching task, we employed a double-blinded procedure in which 20 healthy male volunteers self-administered 40 UI of vasopressin or placebo intranasally, 45 min before performing the matching task in the scanner. In a random-effects fMRI analysis, we show that vasopressin induces a regionally specific alteration in a key node of the theory of mind network, the left temporoparietal junction, identifying a neurobiological mechanism for prosocial neuropeptide effects in humans that suggests novel treatment strategies.

Neuroendocrine actions of organohalogens: thyroid hormones, arginine vasopressin, and neuroplasticity

Organohalogen compounds are global environmental pollutants. They are highly persistent, bioaccumulative, and cause adverse effects in humans and wildlife. Because of the widespread use of these organohalogens in household items and consumer products, indoor contamination may be a significant source of human exposure, especially for children. One significant concern with regard to health effects associated with exposure to organohalogens is endocrine disruption. This review focuses on PCBs and PBDEs as old and new organohalogens, respectively, and their effects on two neuroendocrine systems; thyroid hormones and the arginine vasopressin system (AVP). Regarding neuroendocrine effects of organohalogens, there is considerable information on the thyroid system as a target and evidence is now accumulating that the AVP system and associated functions are also susceptible to disruption. AVP-mediated functions such as osmoregulation, cardiovascular function as well as social behavior, sexual function and learning/memory are discussed. For both thyroid and AVP systems, the timing of exposure seems to play a major role in the outcome of adverse effects. The mechanism of organohalogen action is well understood for the thyroid system. In comparison, this aspect is understudied in the AVP system but some similarities in neural processes, shown to be targeted by these pollutants, serve as promising possibilities for study. One challenge in understanding modes of action within neuroendocrine systems is their complexity stemming, in part, from interdependent levels of organization. Further, because of the interplay between neuroendocrine and neural functions and behavior, further investigation into organohalogen-mediated effects is warranted and may yield insights with wider scope. Indeed, the current literature provides scattered evidence regarding the role of organohalogen-induced neuroendocrine disruption in the neuroplasticity related to both learning functions and brain structure but future studies are needed to establish the role of endocrine disruption in nervous system function and development.

Central vasopressin V1a receptor activation is independently necessary for both partner preference formation and expression in socially monogamous male prairie voles

The neuropeptide arginine vasopressin (AVP) modulates a variety of species-specific social behaviors. In socially monogamous male prairie voles, AVP acts centrally via vasopressin V1a receptor (V1aR) to facilitate mating induced partner preferences. The display of a partner preference requires at least 2 temporally distinct processes: social bond formation as well as its recall, or expression. Studies to date have not determined in which of these processes V1aR acts to promote partner preferences. Here, male prairie voles were administered intracerebroventricularly a V1aR antagonist (AVPA) at different time points to investigate the role of V1aR in social bond formation and expression. Animals receiving AVPA prior to cohabitation with mating or immediately prior to partner preference testing failed to display a partner preference, while animals receiving AVPA immediately after cohabitation with mating and control animals receiving vehicle at all 3 time points displayed partner preferences. These results suggest that V1aR signaling is necessary for both the formation and expression of partner preferences and that these processes are dissociable.

Memória de reconhecimento social em ratos

O paradigma intruso-residente vem sendo intensamente empregado em estudos para avaliar a memória de reconhecimento social em roedores. Tipicamente, ratos adultos (residentes) são expostos a dois encontros de 5 minutos cada com um mesmo intruso juvenil ou com juvenis diferentes; o intervalo entre encontros é usualmente 30 minutos. A quantidade de comportamentos sociais do residente, no segundo encontro, em relação a um intruso familiar é substancialmente menor do que o observado no primeiro encontro, o que não ocorre quando o segundo encontro envolve um juvenil novo; esse resultado caracteriza memória de reconhecimento social. Neste estudo discutimos achados recentes sobre os tipos de comportamentos usualmente incluídos nas categorias social e não-social, a influência da fase temporal, a interferência de rotinas laboratoriais na memória de reconhecimento social, modalidades sensoriais usualmente empregadas por roedores no processamento de informações na memória social e alternativas adicionais para o estudo da socialidade em roedores.

Neuroendocrinology of social information processing in rats and mice

We reviewed oxytocin (OT), arginine-vasopressin (AVP) and gonadal hormone involvement in various modes of social information processing in mice and rats. Gonadal hormones regulate OT and AVP mediation of social recognition and social learning. Estrogens foster OT-mediated social recognition and the recognition and avoidance of parasitized conspecifics via estrogen receptor (ER) alpha (ERalpha) and ERbeta. Testosterone and its metabolites, including estrogens, regulate social recognition in males predominantly via the AVP V1a receptor. Both OT and AVP are involved in the social transmission of food preferences and ERalpha has inhibitory, while ERbeta has enhancing, roles. OT also enhances mate copying by females. ERalpha mediates the sexual, and ERbeta the recognition, aspects of the risk-taking enhancing effects of females on males. Thus, androgens and estrogens control social information processing by regulating OT and AVP. This control is finely tuned for different forms of social information processing.

Neural correlates of scent marking behavior in C57BL/6J mice: detection and recognition of a social stimulus

Mice show urinary scent marking behavior as a form of social communication. Marking to a conspecific stimulus mouse or odor varies with stimulus familiarity, indicating discrimination of novel and familiar animals. This study investigated Fos immunoreactivity in inbred C57BL/6J (C57) males following scent marking behavior in response to detection of a social stimulus, or discrimination between a familiar and an unfamiliar conspecific. In Experiment 1 C57 mice were exposed for four daily trials to an empty chamber; on a test day they were exposed to the same chamber or to a male CD-1 mouse in that chamber. Increased scent marking to the CD-1 mouse was associated with increased Fos-immunoreactive cells in the basolateral amygdala, medial amygdala, and dorsal and ventral premammillary nuclei. In Experiment 2 C57 mice were habituated to a CD-1 male for 4 consecutive days and, on the 5th day, exposed to the same CD-1 male, or to a novel CD-1 male. Mice exposed to a novel CD-1 displayed a significant increase in scent marking compared to their last exposure to the familiar stimulus, indicating discrimination of the novelty of this social stimulus. Marking to the novel stimulus was associated with enhanced activation of several telencephalic, as well as hypothalamic and midbrain, structures in which activation had not been seen in the detection paradigm (Experiment 1). These included medial prefrontal and piriform cortices, and lateral septum; the paraventricular nuclei, ventromedial nuclei, and lateral area of the hypothalamus, and the ventrolateral column of the periaqueductal gray. These data suggest that a circumscribed group of structures largely concerned with olfaction is involved in detection of a conspecific olfactory stimulus, whereas discrimination of a novel vs. a familiar conspecific stimulus engages a wider range of forebrain structures encompassing higher-order processes and potentially providing an interface between cognitions and emotions.

Treatment of autism spectrum disorders: neurotransmitter signaling pathways involved in motivation and reward as therapeutic targets

BACKGROUND

There is a growing body of literature describing the etiology of autism spectrum disorders (ASD). Some of the targets suggested belong to neurochemical transmitter pathways implicated in the behavior and motivation reward pathway.

OBJECTIVE

To examine data linking potential targets to ASD and the feasibility of developing drugs targeting these pathways. While the inhibitors are mostly being developed for other indications, it is beneficial to examine them to determine the responsiveness of the targets to small-molecule modulation.

METHODS

A search in Medline and Scifinder for articles concerning relevant targets in the context of ASD and their relation to the reward signaling pathway.

RESULTS

There is evidence suggesting that behaviors controlled by these targets are related to behaviors exhibited by individuals with ASD. The targets appear to be involved in neurotransmitter pathways controlling motivation and reward, further implicating this system in ASD. Sufficient research has been conducted to identify lead compounds for discovering agents for treatment of ASD.

Arginine vasopressin V1a receptor antagonist impairs maternal memory in rats

Primiparous female rats rapidly respond to foster pups following an extended separation from pups after an initial maternal experience. This consolidation of maternal behavior has been referred to as maternal memory. The neurochemical regulation of maternal memory is not clearly understood. One neuropeptide that may mediate maternal memory is arginine vasopressin (AVP), a neuropeptide which is modulated around the time of parturition and has an established role in learning and memory processes. Thus, the present studies examine the possible involvement of AVP in the establishment of maternal memory in female rats. Pregnant rats were implanted with chronic cannulae connected to subcutaneous osmotic minipumps filled with a V1a receptor antagonist [d(CH2)5Tyr(Me)AVP, 0.1-12.5 ng/h] or saline vehicle which were chronically infused either into the lateral ventricles or bilaterally into the medial amygdala beginning on day 18 of gestation. Both the osmotic pumps and the newborn pups were removed 24 h following parturition. The effects of the V1a antagonist treatments on social recognition and maternal behavior were measured following parturition and maternal memory was assessed following a ten day separation from pups. Whereas none of the AVP treatments affected the initial establishment of maternal behavior postpartum, maternal memory was impaired in rats infused into the amygdala with the AVP antagonist (1.25 and 12.5 ng/h). Social recognition was not impaired by intracerebroventricular infusion of either the 0.1 or 1.0 ng/h dose of the V1a antagonist. The present results suggest a role for medial amygdaloid V1a receptors in the establishment of maternal memory.

The vasopressin system--from antidiuresis to psychopathology

Vasopressin is a neuropeptide with multiple functions. In addition to its predominantly antidiuretic action after peripheral secretion from the posterior pituitary, it seems to fulfill--together with its receptor subtype--all requirements for a neuropeptide system critically involved in higher brain functions, including cognitive abilities and emotionality. Following somatodendritic and axonal release in distinct brain areas, vasopressin acts as a neuromodulator and neurotransmitter in multiple and varying modes of interneuronal communication. Accordingly, changes in vasopressin expression and release patterns may have wide-spread consequences. As shown in mice, rats, voles, and humans, central vasopressin release along a continuum may be beneficial to the individual, serving to adjust physiology and behavior in stressful scenarios, possibly at the potential expense of increasing susceptibility to disease. Indeed, if over-expressed and over-released, it may contribute to hyper-anxiety and depression-like behaviors. A vasopressin deficit, in turn, may cause signs of both diabetes insipidus and total hypo-anxiety. The identification of genetic polymorphisms underlying these phenomena does not only explain individual variation in social memory and emotionality, but also help to characterize potential targets for therapeutic interventions. The capability of both responding to stressful stimuli and mediating genetic polymorphisms makes the vasopressin system a key mediator for converging (i.e., environmentally and genetically driven) behavioral regulation.

Oxytocin, vasopressin and pair bonding: implications for autism

Understanding the neurobiological substrates regulating normal social behaviours may provide valuable insights in human behaviour, including developmental disorders such as autism that are characterized by pervasive deficits in social behaviour. Here, we review the literature which suggests that the neuropeptides oxytocin and vasopressin play critical roles in modulating social behaviours, with a focus on their role in the regulation of social bonding in monogamous rodents. Oxytocin and vasopressin contribute to a wide variety of social behaviours, including social recognition, communication, parental care, territorial aggression and social bonding. The effects of these two neuropeptides are species-specific and depend on species-specific receptor distributions in the brain. Comparative studies in voles with divergent social structures have revealed some of the neural and genetic mechanisms of social-bonding behaviour. Prairie voles are socially monogamous; males and females form long-term pair bonds, establish a nest site and rear their offspring together. In contrast, montane and meadow voles do not form a bond with a mate and only the females take part in rearing the young. Species differences in the density of receptors for oxytocin and vasopressin in ventral forebrain reward circuitry differentially reinforce social-bonding behaviour in the two species. High levels of oxytocin receptor (OTR) in the nucleus accumbens and high levels of vasopressin 1a receptor (V1aR) in the ventral pallidum contribute to monogamous social structure in the prairie vole. While little is known about the genetic factors contributing to species-differences in OTR distribution, the species-specific distribution pattern of the V1aR is determined in part by a species-specific repetitive element, or 'microsatellite', in the 5' regulatory region of the gene encoding V1aR (avpr1a). This microsatellite is highly expanded in the prairie vole (as well as the monogamous pine vole) compared to a very short version in the promiscuous montane and meadow voles. These species differences in microsatellite sequence are sufficient to change gene expression in cell culture. Within the prairie vole species, intraspecific variation in the microsatellite also modulates gene expression in vitro as well as receptor distribution patterns in vivo and influences the probability of social approach and bonding behaviour. Similar genetic variation in the human AVPR1A may contribute to variations in human social behaviour, including extremes outside the normal range of behaviour and those found in autism spectrum disorders. In sum, comparative studies in pair-bonding rodents have revealed neural and genetic mechanisms contributing to social-bonding behaviour. These studies have generated testable hypotheses regarding the motivational systems and underlying molecular neurobiology involved in social engagement and social bond formation that may have important implications for the core social deficits characterizing autism spectrum disorders.

Cytotoxic lesions of the hippocampus do not impair social recognition memory in socially housed rats

Socially housed mice with cytotoxic lesions of the hippocampus do not exhibit social recognition memory 30 min following exposure to a juvenile mouse, however the social recognition memory of singly housed rats is unimpaired. The present study tests the hypothesis that social housing of rats could render social recognition memory hippocampally dependent as seen for mice. Rats were housed with juveniles or with adults. Two social recognition one-animal tests paralleling those used with mice were carried out. Seven social discrimination two-animal tests were also given. Sham operated and hippocampally lesioned rats had normal social memory at 30 min whether socially housed for 24, 48 h, 7 or 8 days prior to testing. These findings support other results indicating the hippocampus proper is not required for normal social memory in rats. In a final experiment, rats socially housed in groups of three since weaning, were tested for 30 min and 24 h social memory. Unlike mice, rats socially housed throughout life exhibited social memory only at 30 min, but not at 24 h. Manipulations that extend social memory in rats may be required to render social memory hippocampally dependent or rats and mice may differ in the neural mediation of social memory.

The V1a vasopressin receptor is necessary and sufficient for normal social recognition: a gene replacement study

Vasopressin modulates many social and nonsocial behaviors, including emotionality. We have previously reported that male mice with a null mutation in the V1a receptor (V1aR) exhibit a profound impairment in social recognition and changes in anxiety-like behavior. Using site-specific injections of a V1aR-specific antagonist, we demonstrate that the lateral septum, but not the medial amygdala, is critical for social recognition. Reexpressing V1aR in the lateral septum of V1aR knockout mice (V1aRKO) using a viral vector resulted in a complete rescue of social recognition. Furthermore, overexpression of the V1aR in the lateral septum of wild-type (wt) mice resulted in a potentiation of social recognition behavior and a mild increase in anxiety-related behavior. These results demonstrate that the V1aR in the lateral septum plays a critical role in the neural processing of social stimuli required for complex social behavior.

Sexual dimorphism in the vasopressin system: Lack of an altered behavioral phenotype in female V1a receptor knockout mice

Previous findings with an AVP V1a receptor knockout mouse (V1aRKO) demonstrate a significant role for this receptor in anxiety-like behavior in males. Here we report the lack of anxiety-like effects of the null mutation in female mice. V1aRKO females performed normally on all tests for anxiety-like behavior. This sex difference may be due to the sexual dimorphism in the extra-hypothalamic vasopressin system, with males having significantly more vasopressin fibers in this system.

Behavioral deficits in adult rats treated neonatally with glutamate

The present study evaluated long-term behavioral consequences of neonatal monosodium-l-glutamate (MSG) treatment in rats. The pups received MSG (3 mg/g sc) daily from postnatal day (PD) 5-12. Data from an automatic activity monitor showed that locomotion of MSG-treated females and males aged 56 and 84 days was significantly reduced. Beginning PD 120, three behavioral tests were performed. As compared to the controls, in the elevated plus maze test, modified to evaluate the adaptive form of spatial memory, MSG-treated animals of both sex had significantly prolonged start and transfer latencies. In the social recognition test, assessing olfactory working memory, MSG-treated males displayed a reduced interest in the juvenile conspecific as the stimulus partner during both the initial exposure and re-exposure performed 30 min later. In the open field test, a significant decrease in the habituation rate was found in MSG-treated animals. Sex-dependent differences in behavioral performance were suggested in the open field and elevated plus maze tests. Behavioral changes are discussed in light of the deficits in perception and processing of visual and olfactory stimuli.

Neurochemical regulation of pair bonding in male prairie voles

Pair bonding represents social attachment between mates and is common among monogamous animals. The prairie vole (Microtus ochrogaster) is a monogamous rodent in which mating facilitates pair bond formation. In this review, we first discuss how prairie voles have been used as an excellent model for neurobiological studies of pair bonding. We then primarily focus on male prairie voles to summarize recent findings from neuroanatomical, neurochemical, cellular, molecular, and behavioral studies implicating vasopressin (AVP), oxytocin (OT), and dopamine (DA) in the regulation of pair bonding. Possible interactions among these neurochemicals in the regulation of pair bonding, the brain areas important for pair bond formation, and potential sexually dimorphic mechanisms underlying pair bonding are also discussed. As analogous social bonds are formed by humans, investigation of the neurochemical regulation of pair bond formation in prairie voles may be beneficial for our understanding of the mechanisms associated with normal and abnormal social behaviors in humans.

Central nitric oxide synthase inhibition disrupts maternal behavior in the rat

Blocking nitric oxide (NO) production, by 3rd ventricle administration of a nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 250 microg/5 microl, postpartum [pp]) decreased milk ejections in Day 10 pp rats. On Day 4 pp, L-NAME treatment eliminated pup retrieval and at both stages of lactation suppressed maternal aggression. Fewer rats treated with L-NAME on Day 10 pp retrieved 4-day-old pups than controls, although all nursed older litters. Following exposure to a mobile intruder, Fos expression was lower in the medial preoptic area and the bed nucleus of the stria terminalis in L-NAME-treated rats than in controls but was lower in the medial amygdala only following exposure to an anaesthetized intruder. Thus, the elevated levels of NO observed in lactation may contribute to the mechanism(s) that mediate maternal behavior and aggression.