Social approach behaviors in oxytocin knockout mice: comparison of two independent lines tested in different laboratory environments

Tyrosinase deficiency impairs social novelty preference in mice

OBJECTIVE

Tyrosinase is a rate-limiting enzyme for the biosynthesis of melanin pigment in peripheral tissues, such as skin and the retina. We recently reported the expression and enzymatic activity of tyrosinase as well as its protective effects against oxidative stress-induced protein damage in the mouse brain. The functional role of tyrosinase in the central nervous system, however, remains largely unknown. In the present study, we investigated the involvement of tyrosinase in social behavior in mice.

METHODS

Pigmented C57BL/10JMsHir (B10) and tyrosinase-deficient albino B10.C- Tyr c /Hir (B10-c) mice were subjected to the three-chamber sociability test to assess sociability and social novelty preference. In addition, we measured the mRNA expression of genes involved in catecholamine metabolism in the hippocampus by real-time quantitative PCR analysis.

RESULTS

The results obtained showed that tyrosinase deficiency impaired social novelty preference, but not sociability in mice. We also found that the hippocampal expression of genes involved in catecholamine metabolism, such as monoamine oxidase A and catechol-O-methyltransferase , were significantly decreased in tyrosinase-deficient B10-c mice.

CONCLUSION

These results suggest that tyrosinase activity is functionally involved in the phenotypic expression of social behavior, particularly social novelty preference, in mice. The present study will advance our understanding of the functional role of tyrosinase in the central nervous system.

Microbiota influence on behavior: Integrative analysis of serotonin metabolism and behavioral profile in germ-free mice

Previous studies on germ-free (GF) animals have described altered anxiety-like and social behaviors together with dysregulations in brain serotonin (5-HT) metabolism. Alterations in circulating 5-HT levels and gut 5-HT metabolism have also been reported in GF mice. In this study, we conducted an integrative analysis of various behaviors as well as markers of 5-HT metabolism in the brain and along the GI tract of GF male mice compared with conventional (CV) ones. We found a strong decrease in locomotor activity, accompanied by some signs of increased anxiety-like behavior in GF mice compared with CV mice. Brain gene expression analysis showed no differences in HTR1A and TPH2 genes. In the gut, we found decreased TPH1 expression in the colon of GF mice, while it was increased in the cecum. HTR1A expression was dramatically decreased in the colon, while HTR4 expression was increased both in the cecum and colon of GF mice compared with CV mice. Finally, SLC6A4 expression was increased in the ileum and colon of GF mice compared with CV mice. Our results add to the evidence that the microbiota is involved in regulation of behavior, although heterogeneity among studies suggests a strong impact of genetic and environmental factors on this microbiota-mediated regulation. While no impact of GF status on brain 5-HT was observed, substantial differences in gut 5-HT metabolism were noted, with tissue-dependent results indicating a varying role of microbiota along the GI tract.

Aging leads to sex-dependent effects on pair bonding and increased number of oxytocin-producing neurons in monogamous prairie voles

Pair bonds powerfully modulate health, which becomes particularly important when facing the detrimental effects of aging. To examine the impact of aging on relationship formation and response to loss, we examined behavior in 6-, 12-, and 18-month male and female prairie voles, a monogamous species that forms mating-based pair bonds. We found that older males (18-months) bonded quicker than younger voles, while similarly aged female voles increased partner directed affiliative behaviors. Supporting sex differences in bonding behaviors, we found that males were more likely to sample both partner and novel voles while females were more likely to display partner preference during the initial 20 minutes of the test. Using partner separation to study loss, we observed an erosion of partner preference only in 12-month females, but an overall decrease in partner-directed affiliation in females across all groups, but not in males. Finally, we found that the number of oxytocin, but not vasopressin, cells in the paraventricular hypothalamus increased during aging. These results establish prairie voles as a novel model to study the effects of normal and abnormal aging on pair bonding.

Highlights

18-month male voles demonstrate accelerated bond formation18-month female voles increase partner-directed huddling after 2 wksBonds erode faster in 12-month female voles after partner separationFemale behavior from partner preference tests is reflected in free interactionThe number of paraventricular hypothalamus oxytocin cells increase during aging.

Age-related differences in affective behaviors in mice: possible role of prefrontal cortical-hippocampal functional connectivity and metabolomic profiles

Introduction

The differential expression of emotional reactivity from early to late adulthood may involve maturation of prefrontal cortical responses to negative valence stimuli. In mice, age-related changes in affective behaviors have been reported, but the functional neural circuitry warrants further investigation.

Methods

We assessed age variations in affective behaviors and functional connectivity in male and female C57BL6/J mice. Mice aged 10, 30 and 60 weeks (wo) were tested over 8 weeks for open field activity, sucrose preference, social interactions, fear conditioning, and functional neuroimaging. Prefrontal cortical and hippocampal tissues were excised for metabolomics.

Results

Our results indicate that young and old mice differ significantly in affective behavioral, functional connectome and prefrontal cortical-hippocampal metabolome. Young mice show a greater responsivity to novel environmental and social stimuli compared to older mice. Conversely, late middle-aged mice (60wo group) display variable patterns of fear conditioning and during re-testing in a modified context. Functional connectivity between a temporal cortical/auditory cortex network and subregions of the anterior cingulate cortex and ventral hippocampus, and a greater network modularity and assortative mixing of nodes was stronger in young versus older adult mice. Metabolome analyses identified differences in several essential amino acids between 10wo mice and the other age groups.

Discussion

The results support differential expression of 'emotionality' across distinct stages of the mouse lifespan involving greater prefrontal-hippocampal connectivity and neurochemistry.

Age-Related Differences in Affective Behaviors in Mice: Possible Role of Prefrontal Cortical-Hippocampal Functional Connectivity and Metabolomic Profiles

The differential expression of emotional reactivity from early to late adulthood may involve maturation of prefrontal cortical responses to negative valence stimuli. In mice, age-related changes in affective behaviors have been reported, but the functional neural circuitry warrants further investigation. We assessed age variations in affective behaviors and functional connectivity in male and female C57BL6/J mice. Mice aged 10, 30 and 60 weeks (wo) were tested over 8 weeks for open field activity, sucrose preference, social interactions, fear conditioning, and functional neuroimaging. Prefrontal cortical and hippocampal tissues were excised for metabolomics. Our results indicate that young and old mice differ significantly in affective behavioral, functional connectome and prefrontal cortical-hippocampal metabolome. Young mice show a greater responsivity to novel environmental and social stimuli compared to older mice. Conversely, late middle-aged mice (60wo group) display variable patterns of fear conditioning and with re-testing with a modified context. Functional connectivity between a temporal cortical/auditory cortex network and subregions of the anterior cingulate cortex and ventral hippocampus, and a greater network modularity and assortative mixing of nodes was stronger in young versus older adult mice. Metabolome analyses identified differences in several essential amino acids between 10wo mice and the other age groups. The results support differential expression of 'emotionality' across distinct stages of the mouse lifespan involving greater prefrontal-hippocampal connectivity and neurochemistry.

The effects of alcohol on anxiety-like, depression-like, and social behavior immediately and a day after binge drinking

The aim of the present study was to determine the effects of binge drinking on anxiety-like, depression-like, and social behavior. The participation of the corticotropin-releasing factor (CRF) receptors (CRF1 and CRF2) in these effects was also investigated. Therefore, male C57BL/6 mice were exposed to drinking in the dark, a classical animal model for binge drinking, and treated intracerebroventricularly (icv) with selective CRF1 antagonist antalarmin or selective CRF2 antagonist astressin2B, immediately or 24 h after binge drinking. After 30 min, the animals were investigated in an elevated plus-maze test and a forced swim test for anxiety-like and depression-like signs, respectively. In addition, mice were tested in a three-chamber social interaction arena for sociability and preference for social novelty. Immediately after binge drinking, mice exposed to alcohol expressed anxiolytic and antidepressant effects, which were reduced by astressin2B, but not antalarmin. Moreover, mice exposed to alcohol showed increased sociability and preference for social novelty immediately after binge drinking. In contrast, 24 h after binge drinking mice exposed to alcohol presented anxiety-like and depression-like signs, which were reversed by antalarmin, but not astressin2B. However, mice exposed to alcohol did not show any significant change in social interaction after 24 h. The present study demonstrates that alcohol exerts different effects on anxiety-like, depression-like, and social behavior immediately and a day after binge drinking, and that the anxiolytic and antidepressant effects produced by binge drinking are mediated by CRF2, whereas the anxiety-like and depression-like signs observed the next day are promoted by CRF1.

The Relevance of Animal Models of Social Isolation and Social Motivation for Understanding Schizophrenia: Review and Future Directions

BACKGROUND AND HYPOTHESES

Social dysfunction in schizophrenia includes symptoms of withdrawal and deficits in social skills, social cognition, and social motivation. Based on the course of illness, with social withdrawal occurring prior to psychosis onset, it is likely that the severity of social withdrawal/isolation contributes to schizophrenia neuropathology.

STUDY DESIGN

We review the current literature on social isolation in rodent models and provide a conceptual framework for its relationship to social withdrawal and neural circuit dysfunction in schizophrenia. We next review preclinical tasks of social behavior used in schizophrenia-relevant models and discuss strengths and limitations of existing approaches. Lastly, we consider new effort-based tasks of social motivation and their potential for translational studies in schizophrenia.

STUDY RESULTS

Social isolation rearing in rats produces profound differences in behavior, pharmacologic sensitivity, and neurochemistry compared to socially reared rats. Rodent models relevant to schizophrenia exhibit deficits in social behavior as measured by social interaction and social preference tests. Newer tasks of effort-based social motivation are being developed in rodents to better model social motivation deficits in neuropsychiatric disorders.

CONCLUSIONS

While experimenter-imposed social isolation provides a viable experimental model for understanding some biological mechanisms linking social dysfunction to clinical and neural pathology in schizophrenia, it bypasses critical antecedents to social isolation in schizophrenia, notably deficits in social reward and social motivation. Recent efforts at modeling social motivation using effort-based tasks in rodents have the potential to quantify these antecedents, identify models (eg, developmental, genetic) that produce deficits, and advance pharmacological treatments for social motivation.

The Role of Corticotropin-Releasing Factor (CRF) and CRF-Related Peptides in the Social Behavior of Rodents

Since the corticotropin-releasing factor (CRF) was isolated from an ovine brain, a growing family of CRF-related peptides has been discovered. Today, the mammalian CRF system consists of four ligands (CRF, urocortin 1 (Ucn1), urocortin 2 (Ucn2), and urocortin 3 (Ucn3)); two receptors (CRF receptor type 1 (CRF1) and CRF receptor type 2 (CRF2)); and a CRF-binding protein (CRF-BP). Besides the regulation of the neuroendocrine, autonomic, and behavioral responses to stress, CRF and CRF-related peptides are also involved in different aspects of social behavior. In the present study, we review the experiments that investigated the role of CRF and the urocortins involved in the social behavior of rats, mice, and voles, with a special focus on sociability and preference for social novelty, as well as the ability for social recognition, discrimination, and memory. In general, these experiments demonstrate that CRF, Ucn1, Ucn2, and Ucn3 play important, but distinct roles in the social behavior of rodents, and that they are mediated by CRF1 and/or CRF2. In addition, we suggest the possible brain regions and pathways that express CRF and CRF-related peptides and that might be involved in social interactions. Furthermore, we also emphasize the differences between the species, strains, and sexes that make translation of these roles from rodents to humans difficult.

A comprehensive assay of social motivation reveals sex-specific roles of autism-associated genes and oxytocin

Social motivation is critical to the development of typical social functioning. Social motivation, specifically one or more of its components (e.g., social reward seeking or social orienting), could be relevant for understanding phenotypes related to autism. We developed a social operant conditioning task to quantify effort to access a social partner and concurrent social orienting in mice. We established that mice will work for access to a social partner, identified sex differences, and observed high test-retest reliability. We then benchmarked the method with two test-case manipulations. Shank3B mutants exhibited reduced social orienting and failed to show social reward seeking. Oxytocin receptor antagonism decreased social motivation, consistent with its role in social reward circuitry. Overall, we believe that this method provides a valuable addition to the assessment of social phenotypes in rodent models of autism and the mapping of potentially sex-specific social motivation neural circuits.

The Role of Oxytocin in Early-Life-Stress-Related Neuropsychiatric Disorders

Early-life stress during critical periods of brain development can have long-term effects on physical and mental health. Oxytocin is a critical social regulator and anti-inflammatory hormone that modulates stress-related functions and social behaviors and alleviates diseases. Oxytocin-related neural systems show high plasticity in early postpartum and adolescent periods. Early-life stress can influence the oxytocin system long term by altering the expression and signaling of oxytocin receptors. Deficits in social behavior, emotional control, and stress responses may result, thus increasing the risk of anxiety, depression, and other stress-related neuropsychiatric diseases. Oxytocin is regarded as an important target for the treatment of stress-related neuropsychiatric disorders. Here, we describe the history of oxytocin and its role in neural circuits and related behaviors. We then review abnormalities in the oxytocin system in early-life stress and the functions of oxytocin in treating stress-related neuropsychiatric disorders.

Early-life propofol exposure does not affect later-life GABAergic inhibition, seizure induction, or social behavior

The early developing brain is especially vulnerable to anesthesia, which can result in long lasting functional changes. We examined the effects of early-life propofol on adult excitatory-inhibitory balance and behavior. Postnatal day 7 male mice were exposed to propofol (250 mg/kg i.p.) and anesthesia was maintained for 2 h; control mice were given the same volume of isotonic saline and treated identically. The behavior and electrophysiology experiments were conducted when the mice were adults. We found that a 2-h neonatal propofol exposure did not significantly reduce paired pulse inhibition, alter the effect of muscimol (3 µM) to inhibit field excitatory postsynaptic potentials or alter the effect of bicuculline (100 µM) to increase the population spike in the CA1 region of hippocampal slices from adult mice. Neonatal propofol did not alter the evoked seizure response to pentylenetetrazol in adult mice. Neonatal propofol did not affect anxiety, as measured in the open field apparatus, depression-like behavior, as measured by the forced swim test, or social interactions with novel mice, in either the three-chamber or reciprocal social tests. These results were different from those with neonatal sevoflurane which demonstrated reduced adult GABAergic inhibition, increased seizure susceptibility and reduced social interaction. Even though sevoflurane and propofol both prominently enhance GABA inhibition, they have unique properties that alter the long-term effects of early-life exposure. These results indicate that clinical studies grouping several general anesthetic agents in a single group should be interpreted with great caution when examining long-term effects.

Twenty years of discoveries emerging from mouse models of autism

More than 100 single gene mutations and copy number variants convey risk for autism spectrum disorder. To understand the extent to which each mutation contributes to the trajectory of individual symptoms of autism, molecular genetics laboratories have introduced analogous mutations into the genomes of laboratory mice and other species. Over the past twenty years, behavioral neuroscientists discovered the consequences of mutations in many risk genes for autism in animal models, using assays with face validity to the diagnostic and associated behavioral symptoms of people with autism. Identified behavioral phenotypes complement electrophysiological, neuroanatomical, and biochemical outcome measures in mutant mouse models of autism. This review describes the history of phenotyping assays in genetic mouse models, to evaluate social and repetitive behaviors relevant to the primary diagnostic criteria for autism. Robust phenotypes are currently employed in translational investigations to discover effective therapeutic interventions, representing the future direction of an intensely challenging research field.

Relationship between the domains of theory of mind, social dysfunction, and oxytocin in schizophrenia

Social dysfunction, manifested by impaired social cognition, is contributing to poorer prognosis of patients with schizophrenia. Growing evidence indicates that oxytocin acts as a neurotransmitter in the regulation of social cognition. It still lacks a thorough understanding of how oxytocin is linked with deficits in social cognition and social functioning in schizophrenia. To this end, we aimed to study the role of plasma oxytocin levels in the relationship between subdomains of social cognition and social dysfunction in patients with schizophrenia. Social Functioning Scale was administered to measure social dysfunction while Faux Pas Recognition Test was used to assess the Theory of Mind (ToM) in 40 patients with schizophrenia and 40 age-matched healthy controls. Patients with schizophrenia exhibited more deficits in ToM, more severe social dysfunction, and had lower plasma oxytocin levels, relative to healthy controls. A pooled correlation analysis of all participants revealed significant effects of plasma oxytocin levels on the ToM and social dysfunction. In patients with schizophrenia, plasma oxytocin levels were positively correlated with the affective but not cognitive component of the ToM, and the effects of plasma oxytocin levels on social functioning were partially mediated by affective ToM. Our findings underscore the importance of oxytocin as a potential predictor of ToM and social functioning in patients with schizophrenia. It may be worthwhile for future studies of oxytocin in schizophrenia to focus on an affected behavioral domain, e.g., social cognition, rather than diagnosis, and the targeted domain should be deconstructed into more detailed subdomains.

Immature olfactory sensory neurons provide behaviourally relevant sensory input to the olfactory bulb

Postnatal neurogenesis provides an opportunity to understand how newborn neurons integrate into circuits to restore function. Newborn olfactory sensory neurons (OSNs) wire into highly organized olfactory bulb (OB) circuits throughout life, enabling lifelong plasticity and regeneration. Immature OSNs form functional synapses capable of evoking firing in OB projection neurons but what contribution, if any, they make to odor processing is unknown. Here, we show that immature OSNs provide odor input to the mouse OB, where they form monosynaptic connections with excitatory neurons. Importantly, immature OSNs respond as selectively to odorants as mature OSNs and exhibit graded responses across a wider range of odorant concentrations than mature OSNs, suggesting that immature and mature OSNs provide distinct odor input streams. Furthermore, mice can successfully perform odor detection and discrimination tasks using sensory input from immature OSNs alone. Together, our findings suggest that immature OSNs play a previously unappreciated role in olfactory-guided behavior.

Socio-behavioral dysfunction in disorders of hypothalamic-pituitary involvement: The potential role of disease-induced oxytocin and vasopressin signaling deficits

Disorders involving hypothalamic and pituitary (HPIT) structures-including craniopharyngioma, Langerhans cell histiocytosis, and intracranial germ cell tumors-can disrupt brain and endocrine function. An area of emerging clinical concern in patients with these disorders is the co-occurring socio-behavioral dysfunction that persists after standard hormone replacement therapy. Although the two neuropeptides most implicated in mammalian social functioning (oxytocin and arginine vasopressin) are of hypothalamic origin, little is known about how disease-induced damage to HPIT structures may disrupt neuropeptide signaling and, in turn, impact patients' socio-behavioral functioning. Here we provide a clinical primer on disorders of HPIT involvement and a review of neuropeptide signaling and socio-behavioral functioning in relevant animal models and patient populations. This collective evidence suggests that neuropeptide signaling disruptions contribute to socio-behavioral deficits experienced by patients with disorders of HPIT involvement. A better understanding of the biological underpinnings of patients' socio-behavioral symptoms is now needed to enable the development of the first targeted pharmacological strategies by which to manage patients' socio-behavioral dysfunction.

Oxytocin, Vasopressin, and Social Behavior: From Neural Circuits to Clinical Opportunities

Oxytocin and vasopressin are peptide hormones secreted from the pituitary that are well known for their peripheral endocrine effects on childbirth/nursing and blood pressure/urine concentration, respectively. However, both peptides are also released in the brain, where they modulate several aspects of social behaviors. Oxytocin promotes maternal nurturing and bonding, enhances social reward, and increases the salience of social stimuli. Vasopressin modulates social communication, social investigation, territorial behavior, and aggression, predominantly in males. Both peptides facilitate social memory and pair bonding behaviors in monogamous species. Here we review the latest research delineating the neural circuitry of the brain oxytocin and vasopressin systems and summarize recent investigations into the circuit-based mechanisms modulating social behaviors. We highlight research using modern molecular genetic technologies to map, monitor activity of, or manipulate neuropeptide circuits. Species diversity in oxytocin and vasopressin effects on social behaviors are also discussed. We conclude with a discussion of the translational implications of oxytocin and vasopressin for improving social functioning in disorders with social impairments, such as autism spectrum disorder.

Oxytocin and cardiometabolic interoception: Knowing oneself affects ingestive and social behaviors

Maintaining homeostasis while navigating one's environment involves accurately assessing and interacting with external stimuli while remaining consciously in tune with internal signals such as hunger and thirst. Both atypical social interactions and unhealthy eating patterns emerge as a result of dysregulation in factors that mediate the prioritization and attention to salient stimuli. Oxytocin is an evolutionarily conserved peptide that regulates attention to exteroceptive and interoceptive stimuli in a social environment by functioning in the brain as a modulatory neuropeptide to control social behavior, but also in the periphery as a hormone acting at oxytocin receptors (Oxtr) expressed in the heart, gut, and peripheral ganglia. Specialized sensory afferent nerve endings of Oxtr-expressing nodose ganglia cells transmit cardiometabolic signals via the Vagus nerve to integrative regions in the brain that also express Oxtr(s). These brain regions are influenced by vagal sensory pathways and coordinate with external events such as those demanding attention to social stimuli, thus the sensations related to cardiometabolic function and social interactions are influenced by oxytocin signaling. This review investigates the literature supporting the idea that oxytocin mediates the interoception of cardiovascular and gastrointestinal systems, and that the modulation of this awareness likewise influences social cognition. These concepts are then considered in relation to Autism Spectrum Disorder, exploring how atypical social behavior is comorbid with cardiometabolic dysfunction.

Social recognition and short-term memory in two taxa of striped mouse with differing social systems

The ability to distinguish between familiar and strange conspecifics is important in group‐living animals and influences the types of interactions between conspecifics. Social systems differ in sister taxa of the striped mouse genus Rhabdomys originating from different environments. Xeric‐adapted R. pumilio displays facultative group‐living whereas the mesic‐adapted R. d. chakae is solitary. We assessed social recognition and attraction to strangers in females of two populations each of R. pumilio and R. d. chakae by means of a social discrimination task. We used a three‐chamber apparatus developed in an established protocol and measured the latency of test females to approach and the duration of their investigation of stimulus females. Differences in social recognition of and preference for unfamiliar conspecifics in group‐living and solitary‐living taxa occurred at the taxon‐level, even though constituent populations occurring kilometers apart showed similar responses. Females differed in the latency (testing phase) and duration of investigation (familiarization and testing phases) inter‐specifically but not intra‐specifically. Female R. pumilio approached stimulus females faster than female R. d. chakae. Female R. pumilio also investigated stimulus females for longer, regardless of stimulus type compared to R. d. chakae, but both taxa spent more time investigating familiar females than novel females and approached the familiar females faster than novel females. Social recognition, short‐term memory, and social preference do not appear to differ between closely related taxa and differences in behavior between the two taxa might be related to inherent personality and social proclivity.

Tested social memory and attraction in two striped mouse taxa (solitary vs. group‐living).

Taxa did not differ in social recognition memory.

Group‐living taxa more motivated to investigate due to personality differences between taxa.

Neonatal exposure to sevoflurane impairs preference for social novelty in C57BL/6 female mice at early-adulthood

Social interaction deficit is core symptom of children with autism, owing to interaction of genetic predisposition and environmental toxins. Sevoflurane could induce neurotoxicity in developing brain in rodent models. This study aims to investigate whether sevoflurane anesthesia in neonatal period could impair social behaviors in male and female mice. Twenty-eight male and thirty-one female mice were randomly assigned to receive 3.0% sevoflurane or 60% oxygen on postnatal day 6. They were tested for social interaction behaviors at one- and two-month-old. In addition, the cortex and hippocampus of neonatal mice undergoing sevoflurane anesthesia were harvested for immunoblotting analysis. As a result, both male and female mice undergoing sevoflurane anesthesia showed strong sociability and weak preference for social novelty at juvenile age. In addition, the male mice developed normal preference for social novelty at early-adulthood; However, the female mice remained weak preference for social novelty. Furthurmore, sevoflurane anesthesia could decrease the levels of PSD95 but not Neuroligin-1 in the hippocampus but not cortex of neonatal mice. In conclusion, sevoflurane anesthesia in neonatal period could disturb development of social memory and impair preference for social novelty in female mice at early-adulthood, with the potential mechanism of decreasing PSD95 expression in the hippocampus of C57BL/6 mice.

PM2.5 as a potential risk factor for autism spectrum disorder: Its possible link to neuroinflammation, oxidative stress and changes in gene expression

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by behavioral deficits including impairments in social communication, social interaction, and repetitive behaviors. Because the etiology of ASD is still largely unknown, there is no cure for ASD thus far. Although it has been established that genetic components play a vital role in ASD development, the influence of epigenetic regulation induced by environmental factors could also contribute to ASD susceptibility. Accumulated evidence has suggested that exposure to atmospheric particulate matter (PM) in polluted air could affect neurodevelopment, thus possibly leading to ASD. Particles with a size of 2.5 μm (PM_2.5) or less have been shown to have negative effects on human health, and could be linked to ASD symptoms in children. This review summarizes evidence from clinical and animal studies to demonstrate the possible linkage between PM_2.5 exposure and the incidence of ASD in children. An attempt was made to explore the possible mechanisms of this linkage, including changes of gene expression, oxidative stress and neuroinflammation induced by PM_2.5 exposure.

Specification of oxytocinergic and vasopressinergic circuits in the developing mouse brain

Oxytocin (OXT) and arginine vasopressin (AVP) support a broad range of behaviors and homeostatic functions including sex-specific and context-appropriate social behaviors. Although the alterations of these systems have been linked with social-related disorders such as autism spectrum disorder, their formation and developmental dynamics remain largely unknown. Using novel brain clearing techniques and 3D imaging, we have reconstructed the specification of oxytocinergic and vasopressinergic circuits in the developing mouse brain with unprecedented cellular resolution. A systematic quantification indicates that OXT and AVP neurons in the hypothalamus display distinctive developmental dynamics and high cellular plasticity from embryonic to early postnatal stages. Our findings reveal new insights into the specification and consolidation of neuropeptidergic systems in the developing CNS.

CAPS2 Deficiency Impairs the Release of the Social Peptide Oxytocin, as Well as Oxytocin-Associated Social Behavior

Ca²⁺-dependent activator protein for secretion 2 (CAPS2) regulates dense-core vesicle (DCV) exocytosis to facilitate peptidergic and catecholaminergic transmitter release. CAPS2 deficiency in mice has mild neuronal effects but markedly impairs social behavior. Rare de novo Caps2 alterations also occur in autism spectrum disorder, although whether CAPS2-mediated release influences social behavior remains unclear. Here, we demonstrate that CAPS2 is associated with DCV exocytosis-mediated release of the social interaction modulatory peptide oxytocin (OXT). CAPS2 is expressed in hypothalamic OXT neurons and localizes to OXT nerve projection and OXT release sites, such as the pituitary. Caps2 KO mice exhibited reduced plasma albeit increased hypothalamic and pituitary OXT levels, indicating insufficient release. OXT neuron-specific Caps2 conditional KO supported CAPS2 function in pituitary OXT release, also affording impaired social interaction and recognition behavior that could be ameliorated by exogenous OXT administered intranasally. Thus, CAPS2 appears critical for OXT release, thereby being associated with social behavior.SIGNIFICANCE STATEMENT The role of the neuropeptide oxytocin in enhancing social interaction and social bonding behavior has attracted considerable public and neuroscientific attention. A central issue in oxytocin biology concerns how oxytocin release is regulated. Our study provides an important insight into the understanding of oxytocin-dependent social behavior from the perspective of the CAPS2-regulated release mechanism.

Maternal anesthesia with sevoflurane during the mid-gestation induces social interaction deficits in offspring C57BL/6 mice

Sevoflurane anesthesia in pregnant mice could induce neurotoxicity in the developing brain and disturb learning and memory in the offspring mice. Whether it could impair social behaviors in the offspring mice is uncertain. Therefore, we assessed the neurobehavioral effect of in-utero exposure to sevoflurane on social interaction behaviors in C57BL/6 mice. The pregnant mice were anesthetized with 2.5% sevoflurane in 100% oxygen for 2 h, and their offspring mice were tested in three-chambered social paradigm, which includes three 10-min sessions of habituation, sociability, and preference for social novelty. At the juvenile age, the offspring mice showed abnormal sociability, as proved by not taking more time sniffing at the stranger 1 mouse compared with the empty enclosure (108.5 ± 25.4 vs. 108.2 ± 44.0 s, P = 0.9876). Meanwhile, these mice showed impaired preference for social novelty, as evidenced by not taking more time sniffing at the stranger 2 compared with the stranger 1 mouse (92.1 ± 52.2 vs. 126.7 ± 50.8 s, P = 0.1502). At the early adulthood, the offspring mice retrieved the normal sociability (145.6 ± 33.2 vs. 76.0 ± 31.8 s, P = 0.0001), but remained the impaired preference for social novelty (100.6 ± 29.3 vs. 118.0 ± 47.9 s, P = 0.3269). Collectively, these results suggested maternal anesthesia with sevoflurane could induce social interaction deficits in their offspring mice. Although the disturbance of sociability could be recoverable, the impairment of preference for social novelty could be long-lasting.

Oxytocin in Schizophrenia: Pathophysiology and Implications for Future Treatment

Schizophrenia is a form of mental disorder that is behaviorally characterized by abnormal behavior, such as social function deficits or other behaviors that are disconnected from reality. Dysregulation of oxytocin may play a role in regulating the expression of schizophrenia. Given oxytocin’s role in social cognition and behavior, a variety of studies have examined the potential clinical benefits of oxytocin in improving the psychopathology of patients with schizophrenia. In this review, we highlight the evidence for the role of endogenous oxytocin in schizophrenia, from animal models to human studies. We further discuss the potential of oxytocin as a therapeutic agent for schizophrenia and its implication in future treatment.

Animal Models of Depression: What Can They Teach Us about the Human Disease?

Depression is apparently the most common psychiatric disease among the mood disorders affecting about 10% of the adult population. The etiology and pathogenesis of depression are still poorly understood. Hence, as for most human diseases, animal models can help us understand the pathogenesis of depression and, more importantly, may facilitate the search for therapy. In this review we first describe the more common tests used for the evaluation of depressive-like symptoms in rodents. Then we describe different models of depression and discuss their strengths and weaknesses. These models can be divided into several categories: genetic models, models induced by mental acute and chronic stressful situations caused by environmental manipulations (i.e., learned helplessness in rats/mice), models induced by changes in brain neuro-transmitters or by specific brain injuries and models induced by pharmacological tools. In spite of the fact that none of the models completely resembles human depression, most animal models are relevant since they mimic many of the features observed in the human situation and may serve as a powerful tool for the study of the etiology, pathogenesis and treatment of depression, especially since only few patients respond to acute treatment. Relevance increases by the fact that human depression also has different facets and many possible etiologies and therapies.

Reconstruction of the Hypothalamo-Neurohypophysial System and Functional Dissection of Magnocellular Oxytocin Neurons in the Brain

The hypothalamo-neurohypophysial system (HNS), comprising hypothalamic magnocellular neuroendocrine cells (MNCs) and the neurohypophysis, plays a pivotal role in regulating reproduction and fluid homeostasis by releasing oxytocin and vasopressin into the bloodstream. However, its structure and contribution to the central actions of oxytocin and vasopressin remain incompletely understood. Using viral tracing and whole-brain imaging, we reconstruct the three-dimensional architecture of the HNS and observe collaterals of MNCs within the brain. By dual viral tracing, we further uncover that subsets of MNCs collaterally project to multiple extrahypothalamic regions. Selective activation of magnocellular oxytocin neurons promote peripheral oxytocin release and facilitate central oxytocin-mediated social interactions, whereas inhibition of these neurons elicit opposing effects. Our work reveals the previously unrecognized complexity of the HNS and provides structural and functional evidence for MNCs in coordinating both peripheral and central oxytocin-mediated actions, which will shed light on the mechanistic understanding of oxytocin-related psychiatric diseases.

Anesthesia plus surgery in neonatal period impairs preference for social novelty in mice at the juvenile age

Anesthetic sevoflurane could induce neurotoxicity in developing brain and cause adverse neurobehavioral outcomes in mice, including inattention, social interaction deficit, and learning and memory impairment. However, there is less data on the effect of anesthesia plus surgery on social interaction behavior. Therefore, we investigated whether the combination of anesthesia and surgical stimulation could induce behavioral and biochemical changes in mice. Firstly, the six-day-old mice were received either 3% sevoflurane anesthesia or abdominal surgery under sevoflurane anesthesia. Then, these mice were scheduled to social interaction test in three-chambered social paradigm at one-month-old. In addition, the brain tissues of neonatal mice were harvested at 24 h after treatment, for measuring the levels of OXTR and NMDAR1 in Western blot analysis. We found that neonatal anesthesia with sevoflurane in a clinically-relevant dosage could not induce social interaction deficit. Nevertheless, anesthesia plus surgery was able to impair preference for social novelty in mice. Moreover, anesthesia plus surgery decreased the levels of OXTR in hippocampus and cortex of mice, as well as NMDAR1 in hippocampus. Collectively, these results suggested that anesthesia plus surgery could impair social novelty preference, but not sociability in mice, and that social memory might be more vulnerable than social affiliation in biological property. Furthermore, reduction in the levels of cortex OXTR and hippocampus NMDAR1 could be associated with social recognition memory in mice.

Oxytocin has sex-specific effects on social behaviour and hypothalamic oxytocin immunoreactive cells but not hippocampal neurogenesis in adult rats

Oxytocin regulates social behaviours, pair bonding and hippocampal neurogenesis but most studies have used adult males. Our study investigated the effects of oxytocin on social investigation and adult hippocampal neurogenesis in male and female rats. Oxytocin has poor penetration of the blood-brain barrier, therefore we tested a nanoparticle drug, TRIOZAN™ (Ovensa Inc.), which permits greater blood-brain-barrier penetration. Adult male and female rats were injected daily (i.p.) for 10 days with either: oxytocin in PBS (0.5 or 1.0 mg/kg), oxytocin in TRIOZAN™ (0.5 or 1.0 mg/kg), or vehicle (PBS) and tested for social investigation. Oxytocin decreased body mass and increased social investigation and number of oxytocin-immunoreactive cells in the supraoptic nucleus (SON) of the hypothalamus in male rats only. In both sexes, oxytocin decreased the number of immature neurons (doublecortin+ cells) in the ventral hippocampus and reduced plasma 17β-estradiol levels in a dose- and delivery-dependent way. Oxytocin in TRIOZAN™ reduced "sedation" observed post-injection and increased certain central effects (oxytocin levels in the hypothalamus and neurogenesis in the ventral hippocampus) relative to oxytocin in PBS, indicating that the nanoparticle may be used as an alternative brain delivery system. We showed that oxytocin has sex-specific effects on social investigation, body mass, "sedation", and the oxytocin system. In contrast, similar effects were observed in both sexes in neurogenesis and plasma 17β-estradiol. Our work suggests that sex differences in oxytocin regulation of brain endpoints is region-specific (hypothalamus versus hippocampus) and that oxytocin does not promote social investigation in females.

Impaired sex preference, but not social and social novelty preferences, following systemic blockade of oxytocin receptors in adult male mice

The hypothalamic neuropeptide oxytocin (OT) is a powerful modulator of mammalian social behavior and its administration was shown to affect various types of social interactions. However, systematic examinations of the role of endogenous OT release in social behavior have heretofore been done only using genetically modified animal models in which the genes encoding either OT or the OT receptor (OTR) were mutated. While such genetic manipulations revealed various behavioral deficits, these deficits may involve developmental or long-term processes and do not prove the participation of acute OT release in the impaired behavior. Here we used a battery of social discrimination tasks to evaluate the effects of acute systemic OTR blockade, using a non-peptide, orally active OTR antagonist (L368,899), on social behavior of adult male C57BL/6 J mice. We found no effect of the pharmacological manipulation on the social preference and social novelty preference behaviors. However, the preference of a male mouse for investigating a female conspecific more than a male (sex preference behavior), was lost by administration of the OTR antagonist. Finally, we found that blocking OTR activity before social defeat prevented the consequent loss of social preference, suggesting a role for OT in the acquisition of aversive social memory. Overall, our results suggest that OT plays a role in modulating the salience of social stimuli and facilitating their memory, as predicted by the social salience theory, rather than in regulating the internal motivation of the subject for social interactions.

Neurobiology of sensory processing in autism spectrum disorder

Altered sensory processing and perception has been one of the characteristics of autism spectrum disorder (ASD). In this chapter, we review the neural underpinnings of sensory abnormalities of ASD by examining the literature on clinical, behavioral and neurobiological evidence that underlies the main patterns of sensory integration function and dysfunction. Furthermore, neural differences in anatomy, function and connectivity of different regions underlying sensory processing are also discussed. We conclude that sensory integration intervention is built on the premise of neuroplasticity to improve function and behavior for individuals with ASD.

Oxytocin receptor signalling modulates novelty recognition but not social preference in zebrafish

Sociality is a complex phenomenon that involves the individual´s motivation to approach their conspecifics, along with social cognitive functions that enable individuals to interact and survive. The nonapeptide oxytocin (OXT) is known to regulate sociality in many species. However, the role of OXT in specific aspects of sociality is still not well understood. In the present study, we investigated the contribution of the OXT receptor (OXTR) signalling in two different aspects of zebrafish social behaviour: social preference, by measuring their motivation to approach a shoal of conspecifics, and social recognition, by measuring their ability to discriminate between a novel and familiar fish, using a mutant zebrafish lacking a functional OXTR. Although oxtr mutant zebrafish displayed normal attraction to a shoal of conspecifics, they exhibited reduced social recognition. We further investigated whether this effect would be social-domain specific by replacing conspecific fish by objects. Although no differences were observed in object approach, oxtr mutant fish also exhibited impaired object recognition. Our findings suggest that OXTR signalling regulates a more general memory recognition of familiar vs novel entities, not only in social but also in a non-social domain, in zebrafish.

Sociability development in mice with cell-specific deletion of the NMDA receptor NR1 subunit gene

Social affiliative behavior is an important component of everyday life in many species and is likely to be disrupted in disabling ways in various neurodevelopmental and neuropsychiatric disorders. Therefore, determining the mechanisms involved in these processes is crucial. A link between N-methyl-d-aspartate (NMDA) receptor function and social behaviors has been clearly established. The cell types in which NMDA receptors are critical for social affiliative behavior, however, remain unclear. Here, we use mice carrying a conditional allele of the NMDA R1 subunit to address this question. Mice bearing a floxed NMDAR1 (NR1) allele were crossed with transgenic calcium/calmodulin-dependent kinase IIα (CaMKIIα)-Cre mice or parvalbumin (PV)-Cre mice targeting postnatal excitatory forebrain or PV-expressing interneurons, respectively, and assessed using the three-chambered Social Approach Test. We found that deletion of NR1 in PV-positive interneurons had no effect on social sniffing, but deletion of NR1 in glutamatergic pyramidal cells resulted in a significant increase in social approach behavior, regardless of age or sex. Therefore, forebrain excitatory neurons expressing NR1 play an important role in regulating social affiliative behavior.

Disrupting the endocannabinoid system in early adolescence negatively impacts sociability

Animal models suggest that the endocannabinoid system (eCS) helps regulate various aspects of social behavior, including play behavior and social reward, during adolescence. Properly tuned endocannabinoid signaling may be a critical developmental component in the emergence of normal adult sociability. In the current experiment, we attempted to pharmacologically disrupt endocannabinoid tone during early adolescence, and then measure the behavioral effects at two subsequent time points. 36 male and 36 female Long Evans rats received daily injections of one of three treatments between post-natal day (PND) 25-39: 1) vehicle treatment, 2) 0.4 mg/kg CP55,940 (a potent CB1/CB2 receptor agonist), or 3) 0.5 mg/kg AM251 (a CB1 receptor antagonist/inverse agonist). Both soon after treatment (PND 40-44) and several weeks later (PND 66-70), subjects were tested in an elevated plus maze (EPM) for anxiety and in a three-chambered apparatus for sociability. For the latter test, the number of entries into each chamber and the amount of time spent investigating each target were measured. Analyses revealed significant main effects of both sex and age on sociability: males expressed greater sociability compared to females, and sociability was higher in adolescence than adulthood. Most importantly, drug treatment (both CP55,940 and AM251) attenuated sociability in adolescence without having a significant effect on anxiety in the EPM. However, this effect did not persist into adulthood. These results indicate that pharmacological disruption of endocannabinoid tone - through either chronic agonism or antagonism of cannabinoid receptors - during early adolescence has a detrimental effect on sociability. This effect may be caused by transient, compensatory alterations in the eCS.

Early Celastrol Administration Prevents Ketamine-Induced Psychotic-Like Behavioral Dysfunctions, Oxidative Stress and IL-10 Reduction in The Cerebellum of Adult Mice

Administration of subanesthetic doses of ketamine during brain maturation represents a tool to mimic an early insult to the central nervous system (CNS). The cerebellum is a key player in psychosis pathogenesis, to which oxidative stress also contributes. Here, we investigated the impact of early celastrol administration on behavioral dysfunctions in adult mice that had received ketamine (30 mg/kg i.p.) at postnatal days (PNDs) 7, 9, and 11. Cerebellar levels of 8-hydroxydeoxyguanosine (8-OHdG), NADPH oxidase (NOX) 1 and NOX2, as well as of the calcium-binding protein parvalbumin (PV), were also assessed. Furthermore, celastrol effects on ketamine-induced alterations of proinflammatory (TNF-α, IL-6 and IL-1β) and anti-inflammatory (IL-10) cytokines in this brain region were evaluated. Early celastrol administration prevented ketamine-induced discrimination index decrease at adulthood. The same was found for locomotor activity elevations and increased close following and allogrooming, whereas no beneficial effects on sniffing impairment were detected. Ketamine increased 8-OHdG in the cerebellum of adult mice, which was also prevented by early celastrol injection. Cerebellar NOX1 levels were enhanced at adulthood following postnatal ketamine exposure. Celastrol per se induced NOX1 decrease in the cerebellum. This effect was more significant in animals that were early administered with ketamine. NOX2 levels did not change. Ketamine administration did not affect PV amount in the cerebellum. TNF-α levels were enhanced in ketamine-treated animals; however, this was not prevented by early celastrol administration. While no changes were observed for IL-6 and IL-1β levels, ketamine determined a reduction of cerebellar IL-10 expression, which was prevented by early celastrol treatment. Our results suggest that NOX inhibition during brain maturation prevents the development of psychotic-like behavioral dysfunctions, as well as the increased cerebellar oxidative stress and the reduction of IL-10 in the same brain region following ketamine exposure in postnatal life. This opens novel neuroprotective opportunities against early detrimental insults occurring during brain development.

Clinical potential of oxytocin in autism spectrum disorder: current issues and future perspectives

The effects of oxytocin on social cognition and behavior have recently attracted considerable attention. In particular, oxytocin has been proposed as a novel therapeutic for psychiatric disorders with social deficits such as autism spectrum disorders. This review provides a brief overview of behavioral and neural responses to oxytocin manipulations in humans and animal models. Although the differences in findings between human and animal studies should be interpreted carefully, shared behavioral phenotypes have been recognized, such as social bonding, social responses, and recognition and usage of social cues. Previous literature suggests that the neural effects of oxytocin in humans and animals overlap in the prefrontal, limbic, and paralimbic cortices. Oxytocin-induced alterations in these regions may indicate a fundamental basis for how oxytocin modulates social behaviors and facilitate the discovery of new pharmaceutical targets for treating social deficits.

Oxytocin receptor knockout prairie voles generated by CRISPR/Cas9 editing show reduced preference for social novelty and exaggerated repetitive behaviors

Behavioral neuroendocrinology has benefited tremendously from the use of a wide range of model organisms that are ideally suited for particular questions. However, in recent years the ability to manipulate the genomes of laboratory strains of mice has led to rapid advances in our understanding of the role of specific genes, circuits and neural populations in regulating behavior. While genome manipulation in mice has been a boon for behavioral neuroscience, the intensive focus on the mouse restricts the diversity in behavioral questions that can be investigated using state-of-the-art techniques. The CRISPR/Cas9 system has great potential for efficiently generating mutants in non-traditional animal models and consequently to reinvigorate comparative behavioral neuroendocrinology. Here we describe the efficient generation of oxytocin receptor (Oxtr) mutant prairie voles (Microtus ochrogaster) using the CRISPR/Cas9 system, and describe initial behavioral phenotyping focusing on behaviors relevant to autism. Oxtr mutant male voles show no disruption in pup ultrasonic vocalization, anxiety as measured by the open field test, alloparental behavior, or sociability in the three chamber test. Mutants did however show a modest elevation in repetitive behavior in the marble burying test, and an impairment in preference for social novelty. The ability to efficiently generate targeted mutations in the prairie vole genome will greatly expand the utility of this model organism for discovering the genetic and circuit mechanisms underlying complex social behaviors, and serves as a proof of principle for expanding this approach to other non-traditional model organisms.

Estrogen and oxytocin involvement in social preference in male mice: a study using a novel long-term social preference paradigm with aromatase, estrogen receptor-α and estrogen receptor-β, oxytocin, and oxytocin receptor knockout male mice

Certain aspects of social behavior help animals make adaptive decisions during encounters with other animals. When mice choose to approach another conspecific, the motivation and preference behind the interaction is not well understood. Estrogen and oxytocin are known to influence a wide array of social behaviors, including social motivation and social preference. The present study investigated the effects of estrogen and oxytocin on social preference using aromatase (ArKO), estrogen receptor (ER) α (αERKO), ERβ (βERKO), oxytocin (OTKO), oxytocin receptor (OTRKO) knockout and their respective wild-type (WT) male mice. Mice were presented with gonadally-intact versus castrated male (IC), intact male versus ovariectomized female (IF), or intact male versus empty cage (IE) stimuli sets for 5 days. ArWT showed no preference for either stimuli in IC and IF and intact male preference in IE, but ArKO mice preferred a castrated male or an ovariectomized female, or had no preference for either stimulus in IC, IF and IE stimuli sets, respectively, suggesting reduced intact male preference. α and β WT mice preferred a castrated male, showed no preference, and preferred an intact male in IC, IF and IE, respectively. αERKO mice displayed similar modified social preference patterns as ArKO, whereas the social preference of βERKO mice remained similar to βWT. OTWT preferred a castrated male whereas OTKO, OTRWT and OTRKO mice failed to show any preference in IC and none showed preference for either stimuli in IF. Collectively, these findings suggest that estrogen regulates social preference in male mice and that impaired social preference in oxytocin-deficient mice may be due to severe deficits in social recognition.

Glutamate, Glutamine and GABA Levels in Rat Brain Measured Using MRS, HPLC and NMR Methods in Study of Two Models of Autism

The disorders of the glutamatergic neurotransmission have been associated with pathogenesis of autism. In this study we evaluated the impact of the in vivo and ex vivo test methodology on measurements of levels of neurotransmitter amino acids in hippocampus of rats for valproic acid- (VPA) and thalidomide- (THAL) induced models of autism. The main goal was to compare the changes in concentrations of glutamate (Glu), glutamine (Gln) and GABA between both autistic groups and the control, measured in vivo and ex vivo in homogenates. The rat pups underwent three in vivo tests: ultrasonic vocalization (USV), magnetic resonance spectroscopy (MRS) and unilateral microdialysis of the hippocampus. Analyses of homogenates of rat hippocampus were performed using high-performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) spectroscopy. For the statistical analysis, we performed univariate and multivariate tests. USV test, which is considered in rodents as an indicator of pathology similar to autism, showed decreased USV in VPA and THAL groups. In vivo MRS studies demonstrated increases of Glu content in male rat's hippocampus in VPA and THAL groups, while the microdialysis, which allows examination of the contents in the extracellular space, detected decreases in the basal level of Gln concentrations in VPA and THAL groups. Ex vivo HPLC studies showed that levels of Glu, Gln and GABA significantly increased in male rat's hippocampus in the VPA and THAL groups, while NMR studies showed increased levels of Gln and GABA in the VPA group. Collectively, these results are consistent with the hypothesis suggesting the role of the glutamatergic disturbances on the pathogenesis of autism. For all methods used, the values of measured changes were in the same direction. The orthogonal partial least square discriminant analysis confirmed that both animal models of autism tested here can be used to trace neurochemical changes in the brain.

The oxytocin system of mice and men-Similarities and discrepancies of oxytocinergic modulation in rodents and primates

Nonapeptides and their respective receptors have been conserved throughout evolution and display astonishing similarities among the animal kingdom. They can be found in worms, birds, fish, amphibians, reptiles and mammals, including rodents, non-human primates and humans. In particular, the neuropeptide oxytocin (OT) has attracted the attention of scientists due to its profound effects on social behavior. However, although both the neuropeptide and its receptor are identical in rodents and primates, the effects of OT vary greatly in the two species. Here, we provide a brief overview about OT's role in the evolution of mammals and provide reasons for the manifold effects of OT within the brain with a particular focus on the discrepancy of OT's effects in rodents and primates. In addition, we suggest new approaches towards improvement of translatability of scientific studies and highlight the most recent advances in animal models for autism spectrum disorder, a disease, in which the normal function of the OT system seems to be impaired.

Impaired social contacts with familiar anesthetized conspecific in CA3-restricted brain-derived neurotrophic factor knockout mice

Familiarity is conveyed by social cues and determines behaviors toward conspecifics. Here, we characterize a novel assay for social behaviors in mice-contacts with anesthetized conspecific-which eliminates reciprocal interactions, including intermale aggression and shows behaviors that are independent of the demonstrator's activity. During the initial 10 minutes (phase-1), the wild-type (WT) subjects contacted the anesthetized conspecifics vigorously regardless of familiarity. During the subsequent 80 minutes (phase-2), however, they contacted more with familiar than unfamiliar conspecifics. We then applied this test to highly aggressive mice with a hippocampal CA3-restricted knockout (KO) of brain-derived neurotrophic factor (BDNF), in which aggression may mask other social behaviors. The KO mice showed less preference for contacting familiar conspecifics than did WT mice during phase-2 but no differences during phase-1. Among nonsocial behaviors, WT mice also spent less time eating in the presence of familiar than with unfamiliar conspecifics, which was not seen in KO mice. In addition, KO mice exhibited reduced pain sensitization. Altogether, these findings suggest that CA3-specific deletion of BDNF results in deficits in circuits that process social cues from familiar conspecifics as well as pain and may underlie empathy-like behaviors.

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.

Specificity in Sociality: Mice and Prairie Voles Exhibit Different Patterns of Peer Affiliation

Social behavior is often described as a unified concept, but highly social (group-living) species exhibit distinct social structures and may make different social decisions. Prairie voles (Microtus ochrogaster) are socially monogamous rodents that often reside in extended family groups, and exhibit robust preferences for familiar social partners (same- and opposite-sex) during extended choice tests, although short-term preferences are not known. Mice (Mus musculus) are gregarious and colonial, but in brief laboratory tests of social preference they typically prefer social novelty. This preference for novel vs. familiar peers may represent a species-specific difference in social decision-making between mice and prairie voles. However, the tests used to measure preferences in each species differ markedly in duration and degree of contact, such that the behaviors cannot be directly compared. We assessed whether social preferences for novelty or familiarity differed between mice and prairie voles of both sexes when assessed with matching protocols: the sociability/social preference test (SPT) typically used in mice (short, no direct contact), and the partner preference test (PPT) used in voles (long, direct contact). A subset of voles also underwent a PPT using barriers (long, no direct contact). In the short SPT, behavior did not differ between species. In the longer test, pronounced partner preferences emerged in prairie voles, but mice exhibited no social preferences and rarely huddled. No sex differences were evident in either test. Direct physical contact was required for partner preferences in huddling time in voles, but preference for the partner chamber was evident with or without contact. Both prairie voles and mice are social, but they exhibit important differences in the specificity and extent of their social behavior. While mice are often used to study social approach and other behaviors, voles are a more suitable species for the study of selective social relationships. Consideration of these differences will be important for studies examining the neural mechanisms supporting different kinds of peer social behavior.

Conditional Deletion of Hippocampal CA2/CA3a Oxytocin Receptors Impairs the Persistence of Long-Term Social Recognition Memory in Mice

Oxytocin (OXT) receptors (OXTRs) are prominently expressed in hippocampal CA2 and CA3 pyramidal neurons, but little is known about its physiological function. As the functional necessity of hippocampal CA2 for social memory processing, we tested whether CA2 OXTRs may contribute to long-term social recognition memory (SRM) formation. Here, we found that conditional deletion of Oxtr from forebrain (Oxtr^-/-) or CA2/CA3a-restricted excitatory neurons in adult male mice impaired the persistence of long-term SRM but had no effect on sociability and preference for social novelty. Conditional deletion of CA2/CA3a Oxtr showed no changes in anxiety-like behavior assessed using the open-field, elevated plus maze and novelty-suppressed feeding tests. Application of a highly selective OXTR agonist [Thr⁴,Gly⁷]-OXT to hippocampal slices resulted in an acute and lasting potentiation of excitatory synaptic responses in CA2 pyramidal neurons that relied on N-methyl-d-aspartate receptor activation and calcium/calmodulin-dependent protein kinase II activity. In addition, Oxtr^-/- mice displayed a defect in the induction of long-term potentiation, but not long-term depression, at the synapses between the entorhinal cortex and CA2 pyramidal neurons. Furthermore, Oxtr deletion led to a reduced complexity of basal dendritic arbors of CA2 pyramidal neurons, but caused no alteration in the density of apical dendritic spines. Considering that the methodologies we have used to delete Oxtr do not rule out targeting the neighboring CA3a region, these findings suggest that OXTR signaling in the CA2/CA3a is crucial for the persistence of long-term SRM.SIGNIFICANCE STATEMENT Oxytocin receptors (OXTRs) are abundantly expressed in hippocampal CA2 and CA3 regions, but there are little known about their physiological function. Taking advantage of the conditional Oxtr knock-out mice, the present study highlights the importance of OXTR signaling in the induction of long-term potentiation at the synapses between the entorhinal cortex and CA2 pyramidal neurons and the persistence of long-term social recognition memory. Thus, OXTRs in the CA2/CA3a may provide a new target for therapeutic approaches to the treatment of social cognition deficits, which are often observed in patients with neuropsychiatric disorders.

Oxytocin enhances observational fear in mice

Empathy is fundamental to human relations, but its neural substrates remain largely unknown. Here we characterize the involvement of oxytocin in the capacity of mice to display emotional state-matching, an empathy-like behavior. When exposed to a familiar conspecific demonstrator in distress, an observer mouse becomes fearful, as indicated by a tendency to freeze and subsequent efforts to escape. Both intranasal oxytocin administration and chemogenetic stimulation of oxytocin neurons render males sensitive to the distress of an unfamiliar mouse. Acute intranasal oxytocin penetrates the brain and enhances cellular activity within the anterior cingulate cortex, whereas chronic administration produces long-term facilitation of observational fear and downregulates oxytocin receptor expression in the amygdala. None of these manipulations affect fear acquired as a result of direct experience with the stressor. Hence, these results implicate oxytocin in observational fear in mice (rather than fear itself) and provide new avenues for examining the neural substrates of empathy.

Oxytocin modulates social behaviours in mammals. Here the authors demonstrate that observational fear, a measure of empathy-like behaviour in rodents, is modulated by oxytocin.

Oxytocin signaling in the medial amygdala is required for sex discrimination of social cues

The neural control of social behaviors in rodents requires the encoding of pheromonal cues by the vomeronasal system. Here we show that the typical preference of male mice for females is eliminated in mutants lacking oxytocin, a neuropeptide modulating social behaviors in many species. Ablation of the oxytocin receptor in aromatase-expressing neurons of the medial amygdala (MeA) fully recapitulates the elimination of female preference in males. Further, single-unit recording in the MeA uncovered significant changes in the sensory representation of conspecific cues in the absence of oxytocin signaling. Finally, acute manipulation of oxytocin signaling in adults is sufficient to alter social interaction preferences in males as well as responses of MeA neurons to chemosensory cues. These results uncover the critical role of oxytocin signaling in a molecularly defined neuronal population in order to modulate the behavioral and physiological responses of male mice to females on a moment-to-moment basis.

Increased aggression and lack of maternal behavior in Dio3-deficient mice are associated with abnormalities in oxytocin and vasopressin systems

Thyroid hormones regulate many aspects of brain development and function, and alterations in the levels of thyroid hormone action lead to abnormal anxiety- and depression-like behaviors. A complement of factors in the brain function independently of circulating levels of hormone to strictly controlled local thyroid hormone signaling. A critical factor is the type 3 deiodinase (DIO3), which is located in neurons and protects the brain from excessive thyroid hormone. Here, we examined whether a local increase in brain thyroid hormone action secondary to DIO3 deficiency is of consequence for social behaviors. Although we did not observe alterations in sociability, Dio3-/- mice of both sexes exhibited a significant increase in aggression-related behaviors and mild deficits in olfactory function. In addition, 85% of Dio3-/- dams manifested no pup-retrieval behavior and increased aggression toward the newborns. The abnormal social behaviors of Dio3-/- mice were associated with sexually dimorphic alterations in the physiology of oxytocin (OXT) and arginine vasopressin (AVP), 2 neuropeptides with important roles in determining social interactions. These alterations included low adult serum levels of OXT and AVP, and an abnormal expression of Oxt, Avp and their receptors in the neonatal and adult hypothalamus. Our results demonstrate that DIO3 is essential for normal aggression and maternal behaviors, and indicate that abnormal local regulation of thyroid hormone action in the brain may contribute to the social deficits associated with neurodevelopmental disorders.

Neuroanatomical Substrates of Rodent Social Behavior: The Medial Prefrontal Cortex and Its Projection Patterns

Social behavior encompasses a number of distinctive and complex constructs that form the core elements of human imitative culture, mainly represented as either affiliative or antagonistic interactions with conspecifics. Traditionally considered in the realm of psychology, social behavior research has benefited from recent advancements in neuroscience that have accelerated identification of the neural systems, circuits, causative genes and molecular mechanisms that underlie distinct social cognitive traits. In this review article, I summarize recent findings regarding the neuroanatomical substrates of key social behaviors, focusing on results from experiments conducted in rodent models. In particular, I will review the role of the medial prefrontal cortex (mPFC) and downstream subcortical structures in controlling social behavior, and discuss pertinent future research perspectives.

Wistar audiogenic rats display abnormal behavioral traits associated with artificial selection for seizure susceptibility

Accumulating evidence from different animal models has contributed to the understanding of the bidirectional comorbidity associations between the epileptic condition and behavioral abnormalities. A strain of animals inbred to enhance seizure predisposition to high-intensity sound stimulation, the Wistar audiogenic rat (WAR), underwent several behavioral tests: forced swim test (FST), open-field test (OFT), sucrose preference test (SPT), elevated plus maze (EPM), social preference (SP), marble burying test (MBT), inhibitory avoidance (IAT), and two-way active avoidance (TWAA). The choice of tests aimed to investigate the correlation between underlying circuits believed to be participating in both WAR's innate susceptibility to sound-triggered seizures and the neurobiological substrates associated with test performance. Comparing WAR with its Wistar counterpart (i.e., resistant to audiogenic seizures) showed that WARs present behavioral despair traits (e.g., increased FST immobility) but no evidence of anhedonic behavior (e.g., increased sucrose consumption in SPT) or social impairment (e.g., no difference regarding juvenile exploration in SP). In addition, tests suggested that WARs are unable to properly evaluate degrees of aversiveness (e.g., performance on OFT, EPM, MBT, IAT, and TWAA). The particularities of the WAR model opens new venues to further untangle the neurobiology underlying the co-morbidity of behavioral disorders and epilepsy. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".