Aggressive behavioral phenotypes in mice

Primed to vocalize: Wild-derived male house mice increase vocalization rate and diversity after a previous encounter with a female

Males in a wide variety of taxa, including insects, birds and mammals, produce vocalizations to attract females. Male house mice emit ultrasonic vocalizations (USVs), especially during courtship and mating, which are surprising complex. It is often suggested that male mice vocalize at higher rates after interacting with a female, but the evidence is mixed depending upon the strain of mice. We conducted a study with wild-derived house mice (Mus musculus musculus) to test whether male courtship vocalizations (i.e., vocalizations emitted in a sexual context) are influenced by a prior direct interaction with a female, and if so, determine how long the effect lasts. We allowed sexually naïve males to directly interact with a female for five minutes (sexual priming), and then we recorded males'vocalizations either 1, 10, 20, or 30 days later when presented with an unfamiliar female (separated by a perforated partition) and female scent. We automatically detected USVs and processed recordings using the Automatic Mouse Ultrasound Detector (A-MUD version 3.2), and we describe our improved version of this tool and tests of its performance. We measured vocalization rate and spectro-temporal features and we manually classified USVs into 15 types to investigate priming effects on vocal repertoire diversity and composition. After sexual priming, males emitted nearly three times as many USVs, they had a larger repertoire diversity, and their vocalizations had different spectro-temporal features (USV length, slope and variability in USV frequency) compared to unprimed controls. Unprimed control males had the most distinctive repertoire composition compared to the primed groups. Most of the effects were found when comparing unprimed to all primed males (treatment models), irrespective of the time since priming. Timepoint models showed that USV length increased 1 day after priming, that repertoire diversity increased 1 and 20 days after priming, and that the variability of USV frequencies was lower 20 and 30 days after priming. Our results show that wild-derived male mice increased the number and diversity of courtship vocalizations if they previously interacted with a female. Thus, the USVs of house mice are not only context-dependent, they depend upon previous social experience and perhaps the contexts of these experiences. The effect of sexual priming on male courtship vocalizations is likely mediated by neuro-endocrine-mechanisms, which may function to advertise males' sexual arousal and facilitate social recognition.

Duplicated dnmt3aa and dnmt3ab DNA Methyltransferase Genes Play Essential and Non-Overlapped Functions on Modulating Behavioral Control in Zebrafish

DNA methylation plays several roles in regulating neuronal proliferation, differentiation, and physiological functions. The major de novo methyltransferase, DNMT3, controls the DNA methylation pattern in neurons according to environmental stimulations and behavioral regulations. Previous studies demonstrated that knockout of Dnmt3 induced mouse anxiety; however, controversial results showed that activation of Dnmt3 causes anxiolytic behavior. Thus, an alternative animal model to clarify Dnmt3 on modulating behavior is crucial. Therefore, we aimed to establish a zebrafish (Danio rerio) model to clarify the function of dnmt3 on fish behavior by behavioral endpoint analyses. We evaluated the behaviors of the wild type, dnmt3aa, and dnmt3ab knockout (KO) fish by the novel tank, mirror biting, predator avoidance, social interaction, shoaling, circadian rhythm locomotor activity, color preference, and short-term memory tests. The results indicated that the dnmt3aa KO fish possessed abnormal exploratory behaviors and less fear response to the predator. On the other hand, dnmt3ab KO fish displayed less aggression, fear response to the predator, and interests to interact with their conspecifics, loosen shoaling formation, and dysregulated color preference index ranking. Furthermore, both knockout fishes showed higher locomotion activity during the night cycle, which is a sign of anxiety. However, changes in some neurotransmitter levels were observed in the mutant fishes. Lastly, whole-genome DNA methylation sequencing demonstrates a potential network of Dnmt3a proteins that is responsive to behavioral alterations. To sum up, the results suggested that the dnmt3aa KO or dnmt3ab KO fish display anxiety symptoms, which supported the idea that Dnmt3 modulates the function involved in emotional control, social interaction, and cognition.

Predicted structural differences of four fertility-related Y-chromosome proteins in Macaca mulatta, M. fascicularis, and their Indochinese hybrids

Species in the genus Macaca typically live in multimale-multifemale social groups with male macaques exhibiting some of the largest testis: body weight ratios among primates. Males are believed to experience intense levels of sperm competition. Several spermatogenesis genes are located on the Y-chromosome and, interestingly, occasional hybridization between two species has led to the introgression of the rhesus macaque (Macaca mulatta) Y-chromosome deep into the range of the long-tailed macaque (M. fascicularis). These observations have led to the prediction that the successful introgression of the rhesus Y-haplotype is due to functional differences in spermatogenesis genes compared to those of the native long-tailed Y-haplotype. We examine here four Y-chromosomal loci-RBMY, XKRY, and two nearly identical copies of CDY-and their corresponding protein sequences. The genes were surveyed in representative animals from north of, south of, and within the rhesus x long-tailed introgression zone. Our results show a series of non-synonymous amino acid substitutions present between the two Y-haplotypes. Protein structure modeling via I-TASSER revealed different folding patterns between the two species' Y-proteins, and functional predictions via TreeSAAP further reveal physicochemical differences as a result of non-synonymous substitutions. These differences inform our understanding of the evolution of primate Y-proteins involved in spermatogenesis and, in turn, have biomedical implications for human male fertility.

Serotonin Signals Overcome Loser Mentality in Drosophila

Traumatic experiences generate stressful neurological effects in the exposed persons and animals. Previous studies have demonstrated that in many species, including Drosophila, the defeated animal has a higher probability of losing subsequent fights. However, the neural basis of this “loser effect” is largely unknown. We herein report that elevated serotonin (5-HT) signaling helps a loser to overcome suppressive neurological states. Coerced activation of 5-HT neurons increases aggression in males and promotes losers to both vigorously re-engage in fights and even defeat the previous winners and regain mating motivation. P1 neurons act upstream and 5-HT1B neurons in the ellipsoid body act downstream of 5-HT neurons to arouse losers. Our results demonstrate an ancient neural mechanism of regulating depressive behavioral states after distressing events.

•

Activating a small subset of serotonin neurons promotes losers to fight

•

Serotonin is necessary and sufficient for modulating aggression in losers

•

The neural circuit for motivating losers includes P1, 5-HT, and 5-HT1B neurons

•

Elevating 5-HT signaling overcomes the depressive behavioral state in losers

Ethanol exposure during the brain growth spurt period increases ethanol-induced aggressive behavior in adolescent male mice

Ethanol exposure during development is associated with deficient social behavior, such as aggressive behavior, and ethanol consumption is associated with violent crimes, thus raising the possibility that individuals with fetal alcohol spectrum disorder may exhibit exacerbated social deficits in response to ethanol exposure. The present study evaluated the effects of ethanol exposure during the brain growth spurt period (i.e., a critical time period during which ethanol's effects are augmented) on aggressive behavior and ethanol-induced aggression during adolescence. From postnatal Day 2 (PD2) to PD8, Swiss mice received either ethanol (5 g/kg, i.p.) or saline on alternate days. On PD39, aggressive behavior was assessed using the resident-intruder paradigm in male mice, and social dominance was investigated using the tube dominance test in both males and females. Testis structure and testosterone levels were evaluated in male mice. Early ethanol exposure increased the gonadosomatic index and the number of Leydig cells. The thickness of the seminiferous tube decreased. No difference in testosterone levels was found. The ethanol-exposed resident mice exhibited increased number and duration of aggressive episodes only when challenged with a low ethanol dose (1 g/kg) before confrontation. Female mice early-exposed to ethanol won more confrontations in the tube dominance test. The present findings suggest a critical brain growth spurt period that is susceptible to ethanol-induced alterations of social dominance behavior in females. Although basal levels of aggression were unaffected, early ethanol exposure resulted in greater susceptibility to ethanol-induced aggression in adolescent male mice.

Rage Against the Machine: Advancing the study of aggression ethology via machine learning

RATIONALE

Aggression, comorbid with neuropsychiatric disorders, exhibits with diverse clinical presentations and places a significant burden on patients, caregivers, and society. This diversity is observed because aggression is a complex behavior that can be ethologically demarcated as either appetitive (rewarding) or reactive (defensive), each with its own behavioral characteristics, functionality, and neural basis that may transition from adaptive to maladaptive depending on genetic and environmental factors. There has been a recent surge in the development of preclinical animal models for studying appetitive aggression-related behaviors and identifying the neural mechanisms guiding their progression and expression. However, adoption of these procedures is often impeded by the arduous task of manually scoring complex social interactions. Manual observations are generally susceptible to observer drift, long analysis times, and poor inter-rater reliability, and are further incompatible with the sampling frequencies required of modern neuroscience methods.

OBJECTIVES

In this review, we discuss recent advances in the preclinical study of appetitive aggression in mice, paired with our perspective on the potential for machine learning techniques in producing automated, robust scoring of aggressive social behavior. We discuss critical considerations for implementing valid computer classifications within behavioral pharmacological studies.

KEY RESULTS

Open-source automated classification platforms can match or exceed the performance of human observers while removing the confounds of observer drift, bias, and inter-rater reliability. Furthermore, unsupervised approaches can identify previously uncharacterized aggression-related behavioral repertoires in model species.

DISCUSSION AND CONCLUSIONS

Advances in open-source computational approaches hold promise for overcoming current manual annotation caveats while also introducing and generalizing computational neuroethology to the greater behavioral neuroscience community. We propose that currently available open-source approaches are sufficient for overcoming the main limitations preventing wide adoption of machine learning within the context of preclinical aggression behavioral research.

Examining the effects of chronic, lake-wide elevated temperatures on behavioural expression in largemouth bass, Micropterus salmoides

Many behaviours have differential fitness consequences across thermal and ecological contexts, indicating that both ecological shifts and warming temperatures induced by climatic change may alter how organisms behave. However, empirical evidence of temperature-driven behavioural selection in natural systems is lacking. We compared behaviours and behavioural syndromes related to activity, exploration, boldness and aggression in populations of largemouth bass (Micropterus salmoides) from ambient lakes to the those from artificially warmed, power plant cooling lakes to investigate changes in behaviours associated with warmer environments. Activity, exploration, boldness and aggression of juvenile largemouth bass were assessed in laboratory conditions using a novel environment assay and a risky situation assay. We found that activity and exploratory behaviours were higher and decreased through first year ontogeny in populations from heated lakes, whereas these behaviours were lower and showed no relationship through ontogeny in populations from ambient lakes. We attribute these differences to the changes in food source availability in heated lakes associated with temperature-driven ecological effects. Bold and aggressive behaviours tended to differ between populations, as did correlations between behaviours, but did not differ between ambient and heated lakes. The findings of this work identify that large ecological changes associated with warming environments, such as food availability, may drive changes in some aspects of behavioural expression in largemouth bass but that other aspects of behavioural expression may be driven by lake-specific factors not related to warming.

Potentiation of Divergent Medial Amygdala Pathways Drives Experience-Dependent Aggression Escalation

Heightened aggression can be serious concerns for the individual and society at large and are symptoms of many psychiatric illnesses, such as post-traumatic stress disorder. The circuit and synaptic mechanisms underlying experience-induced aggression increase, however, are poorly understood. Here we find that prior attack experience leading to an increase in aggressive behavior, known as aggression priming, activates neurons within the posterior ventral segment of the medial amygdala (MeApv). Optogenetic stimulation of MeApv using a synaptic depression protocol suppresses aggression priming, whereas high-frequency stimulation enhances aggression, mimicking attack experience. Interrogation of the underlying neural circuitry revealed that the MeApv mediates aggression priming via synaptic connections with the ventromedial hypothalamus (VmH) and bed nucleus of the stria terminalis (BNST). These pathways undergo NMDAR-dependent synaptic potentiation after attack. Furthermore, we find that the MeApv-VmH synapses selectively control attack duration, whereas the MeApv-BNST synapses modulate attack frequency, both with no effect on social behavior. Synaptic potentiation of the MeApv-VmH and MeApv-BNST pathways contributes to increased aggression induced by traumatic stress, and weakening synaptic transmission at these synapses blocks the effect of traumatic stress on aggression. These results reveal a circuit and synaptic basis for aggression modulation by experience that can be potentially leveraged toward clinical interventions.SIGNIFICANCE STATEMENT Heightened aggression can have devastating social consequences and may be associated with psychiatric disorders, such as post-traumatic stress disorder. The circuit and synaptic mechanisms underlying experience-induced aggression escalation, however, are poorly understood. Here we identify two aggression pathways between the posterior ventral segment of the medial amygdala and its downstream synaptic partners, the ventromedial hypothalamus and bed nucleus of the stria terminalis that undergo synaptic potentiation after attack and traumatic stress to enhance aggression. Notably, weakening synaptic transmission in these circuits blocks aggression priming, naturally occurring aggression, and traumatic stress-induced aggression increase. These results illustrate a circuit and synaptic basis of aggression modulation by experience, which can be potentially targeted for clinical interventions.

The occurrence of tarsal injuries in male mice of C57BL/6N substrains in multiple international mouse facilities

Dislocation in hindlimb tarsals are being observed at a low, but persistent frequency in group-housed adult male mice from C57BL/6N substrains. Clinical signs included a sudden onset of mild to severe unilateral or bilateral tarsal abduction, swelling, abnormal hindlimb morphology and lameness. Contraction of digits and gait abnormalities were noted in multiple cases. Radiographical and histological examination revealed caudal dislocation of the calcaneus and partial dislocation of the calcaneoquartal (calcaneus-tarsal bone IV) joint. The detection, frequency, and cause of this pathology in five large mouse production and phenotyping centres (MRC Harwell, UK; The Jackson Laboratory, USA; The Centre for Phenogenomics, Canada; German Mouse Clinic, Germany; Baylor College of Medicine, USA) are discussed.

Sex-linked roles of the CRF1 and the CRF2 receptor in social behavior

Dysfunctional social behavior is a major clinical feature of mood, anxiety, autism spectrum, and substance-related disorders, and may dramatically contribute to the poor outcome of these diseases. Nevertheless, the mechanisms underlying social behavior deficits are still largely unknown. The corticotropin-releasing factor (CRF) system, a major coordinator of the stress response, has been hypothesized to modulate social behavior. CRF signaling is mediated by two receptor types, termed CRF₁ and CRF₂ . Using the three-chamber task for sociability (i.e., preference for an unfamiliar conspecific vs. an object), this study demonstrates that CRF₂ receptor null mutation (CRF₂ -/-) reduces sociability in female mice but increases it in male mice. Both female and male CRF₂ -/- mice display a preference for social odor cues over neutral cues, indicating that sex- and CRF₂ receptor-dependent sociability is not due to altered olfaction or impaired social cues discrimination. Moreover, treatment with the CRF₁ receptor-preferring antagonist, antalarmin, consistently induces sociability in non-social mice but disrupts it in social mice, independently of CRF₂ receptor deficiency. Sex, CRF₂ receptor deficiency, or antalarmin affect locomotor activity during the three-chamber test. However, throughout the study CRF₁ and CRF₂ receptor-linked sociability is independent of locomotor activity. The present findings highlight major functions for the CRF system in the regulation of social behavior. Moreover, they provide initial evidence of sex-linked roles for the CRF₁ and the CRF₂ receptor, emphasizing the importance of sex as a major biological variable to be taken into consideration in preclinical and clinical studies.

Stress in groups: Lessons from non-traditional rodent species and housing models

A major feature of life in groups is that individuals experience social stressors of varying intensity and type. Social stress can have profound effects on health, social behavior, and ongoing relationships. Relationships can also buffer the experience of exogenous stressors. Social stress has most commonly been investigated in dyadic contexts in mice and rats that produce intense stress. Here we review findings from studies of diverse rodents and non-traditional group housing paradigms, focusing on laboratory studies of mice and rats housed in visible burrow systems, prairie and meadow voles, and mole-rats. We argue that the use of methods informed by the natural ecology of rodent species provides novel insights into the relationship between social stress, behavior and physiology. In particular, we describe how this ethologically inspired approach reveals how individuals vary in their experience of and response to social stress, and how ecological and social contexts impact the effects of stress. Social stress induces adaptive changes, as well as long-term disruptive effects on behavior and physiology.

Serotonin in Animal Cognition and Behavior

Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator of nervous systems in both invertebrates and vertebrates. It has been proposed for several decades that it impacts animal cognition and behavior. In spite of a completely distinct organization of the 5-HT systems across the animal kingdom, several lines of evidence suggest that the influences of 5-HT on behavior and cognition are evolutionary conserved. In this review, we have selected some behaviors classically evoked when addressing the roles of 5-HT on nervous system functions. In particular, we focus on the motor activity, arousal, sleep and circadian rhythm, feeding, social interactions and aggressiveness, anxiety, mood, learning and memory, or impulsive/compulsive dimension and behavioral flexibility. The roles of 5-HT, illustrated in both invertebrates and vertebrates, show that it is more able to potentiate or mitigate the neuronal responses necessary for the fine-tuning of most behaviors, rather than to trigger or halt a specific behavior. 5-HT is, therefore, the prototypical neuromodulator fundamentally involved in the adaptation of all organisms across the animal kingdom.

Reward Circuitry and Motivational Deficits in Social Anxiety Disorder: What Can Be Learned From Mouse Models?

Social anxiety disorder (SAD) is a common and serious psychiatric condition that typically emerges during adolescence and persists into adulthood if left untreated. Prevailing interventions focus on modulating threat and arousal systems but produce only modest rates of remission. This gap in efficacy suggests that most mainstream treatment concepts do not sufficiently target core processes involved in the onset and maintenance of SAD. This idea has further driven the development of new theoretical models that target dopamine (DA)-driven reward circuitry and motivational deficits that appear to be systematically altered in SAD. Most of the available data linking systemic alterations in DA neurobiology to SAD in humans, although abundant, remains at the level of correlational evidence. Accordingly, the purpose of this brief review is to critically evaluate the relevance of experimental work in rodent models that link details of DA function to symptoms of social anxiety. We conclude that, despite certain systematic limitations inherent in animal models, these approaches provide useful insights into human biomarkers of social anxiety including that (1) adolescence may serve as a critical period for the convergence of neurobiological and environmental factors that modify future expectations about social reward through experience dependent changes in DA-ergic circuitry, (2) females may show unique susceptibility to social anxiety symptoms when encountering relational instability that influences DA-related neural processes, and (3) separate from fear and arousal systems, the functional neurobiology of central DA systems contribute uniquely to susceptibility and maintenance of anhedonic factors relevant to human models of SAD.

Ultrasonic signals associated with different types of social behavior of mice

Communication plays an integral role in human social dynamics and is impaired in several neurodevelopmental disorders. Mice are used to study the neurobiology of social behavior; however, the extent to which mouse vocalizations influence social dynamics has remained elusive because it is difficult to identify the vocalizing animal among mice involved in a group interaction. By tracking the ultrasonic vocal behavior of individual mice and using an algorithm developed to group phonically similar signals, we showed that distinct patterns of vocalization emerge as male mice perform specific social actions. Mice dominating other mice were more likely to emit different vocal signals than mice avoiding social interactions. Furthermore, we showed that the patterns of vocal expression influence the behavior of the socially engaged partner but do not influence the behavior of other animals in the cage. These findings clarify the function of mouse communication by revealing a communicative ultrasonic signaling repertoire.

Reduced social investigation and increased injurious behavior in transgenic 5xFAD mice

Social withdrawal and agitation/aggression are common behavioral and psychological symptoms of dementia presented by Alzheimer's disease (AD) patients, with males exhibiting more aggressive behaviors than females. Some transgenic mouse models of AD also exhibit social withdrawal and aggression, but many of these models only recapitulate the early stages of the disease. By comparison, the 5xFAD mouse model of AD exhibits rapid, progressive neurodegeneration, and is suitable for modeling cognitive and behavioral deficits at early, mid-, and late-stage disease progression. Anecdotal reports suggest that transgenic 5xFAD males exhibit high levels of aggression compared to wild-type controls, but to date, indirect genetic effects in this strain have not been studied. We measured home-cage behaviors in 5xFAD males housed in three different group-housing conditions (transgenic-only, wild-type only, and mixed-genotype) and social approach behaviors when exposed to a novel free-roaming or restrained, wild-type or transgenic conspecific. Transgenic-only home cages required earlier separation due to injuries arising from aggression compared to wild-type-only or mixed-genotype cages, despite no obvious increase in the frequency of aggressive behaviors. Transgenic 5xFAD males and females also spent less time investigating free-roaming conspecifics compared to wild-type controls, but they showed normal investigation of restrained conspecifics; the genotype of the conspecific did not affect approach behavior, and there was no aggression observed in transgenic males. These findings provide evidence in an animal model that amyloid pathology ultimately leads to avoidance of novel social stimuli, and that frequent interactions between individuals exhibiting an AD phenotype further exacerbates aggressive behaviors.

Temporal microstructure of dyadic social behavior during relationship formation in mice

Socially competent animals must learn to modify their behavior in response to their social partner in a contextually appropriate manner. Dominant-subordinate relationships are a particularly salient social context for mice. Here we observe and analyze the microstructure of social and non-social behaviors as 21 pairs of outbred CD-1 male mice (Mus Musculus) establish dominant-subordinate relationships during daily 20-minute interactions for five consecutive days in a neutral environment. Firstly, using a Kleinberg burst detection algorithm, we demonstrate aggressive and subordinate interactions occur in bursting patterns followed by quiescent periods rather than being uniformly distributed across social interactions. Secondly, we identify three phases of dominant-subordinate relationship development (pre-, middle-, and post-resolution) by utilizing two statistical methods to identify stability in aggressive and subordinate behavior across these bursts. Thirdly, using First Order Markov Chains we find that dominant and subordinate mice show distinct behavioral transitions, especially between tail rattling and other aggressive/subordinate behaviors. Further, dominant animals engaged in more digging and allogrooming behavior and were more likely to transition from sniffing their partner's body to head, whereas subordinates were more likely to transition from head sniffing to side-by-side contact. Lastly, we utilized a novel method (Forward Spike Time Tiling Coefficient) to assess how individuals respond to the behaviors of their partner. We found that subordinates decrease their tail rattling and aggressive behavior in response to aggressive but not subordinate behavior exhibited by dominants and that tail rattling in particular may function to deescalate aggressive behavior in pairs. Our findings demonstrate that CD-1 male mice rapidly establish dominance relationships and modify their social and non-social behaviors according to their current social status. The methods that we detail also provide useful tools for other researchers wishing to evaluate the temporal dynamics of rodent social behavior.

Chronic non-discriminatory social defeat is an effective chronic stress paradigm for both male and female mice

Stress-related mood disorders are more prevalent in females than males, yet preclinical chronic stress paradigms were developed in male rodents and are less effective in female rodents. Here we characterize a novel chronic non-discriminatory social defeat stress (CNSDS) paradigm that results in comparable stress effects in both sexes. Male and female C57BL/6J mice were simultaneously introduced into the home cage of resident CD-1 aggressors for 10 daily 5-min sessions. CD-1 aggressors attacked males and females indiscriminately, resulting in stress resilient and susceptible subpopulations in both sexes. CD-1 aggressors attacked C57BL/6J male intruders faster and more frequently than female intruders. However, CNSDS similarly induced negative valence behaviors in SUS mice of both sexes relative to RES and CNTRL mice. Furthermore, SUS male and female mice displayed similar increases in plasma corticosterone levels following CNSDS exposure relative to pre-stress exposure levels. The estrous cycle did not impact CD-1 attack behavior or negative valence behaviors. Thus, CNSDS induces chronic stress behavioral and neuroendocrine effects in both male and female C57BL/6J mice and allows direct comparisons between sexes. Adoption of this modified social defeat paradigm will help advance the initiative to include female rodents in preclinical chronic stress research.

Fighting Females: Neural and Behavioral Consequences of Social Defeat Stress in Female Mice

BACKGROUND

Despite the twofold higher prevalence of major depressive and posttraumatic stress disorders in women compared with men, most clinical and preclinical studies have focused on male subjects. We used an ethological murine model to study several cardinal symptoms of affective disorders in the female targets of female aggression.

METHODS

Intact Swiss Webster (CFW) female resident mice were housed with castrated male mice and tested for aggression toward female intruders. For 10 days, aggressive CFW female residents defeated C57BL/6J (B6) female intruders during 5-minute encounters. Measures of corticosterone, c-Fos activation in hypothalamic and limbic structures, and species-typical behaviors were collected from defeated and control females. Ketamine (20 mg/kg) was tested for its potential to reverse stress-induced social deficits.

RESULTS

Housed with a castrated male mouse, most intact resident CFW females readily attacked unfamiliar B6 female intruders, inflicting >40 bites in a 5-minute encounter. Compared with controls, defeated B6 females exhibited elevated plasma corticosterone and increased c-Fos activation in the medial amygdala, ventral lateral septum, ventromedial hypothalamus, and hypothalamic paraventricular nucleus. Chronically defeated females also showed vigilance-like behavior and deficits in social interactions, novel object investigation, and nesting. The duration of social interactions increased 24 hours after chronically defeated female mice received a systemic dose of ketamine.

CONCLUSIONS

These findings demonstrate that CFW female mice living with male conspecifics can be used as aggressive residents in an ethological model of female social defeat stress. These novel behavioral methods will encourage further studies of sex-specific neural, physiological, and behavioral adaptations to chronic stress and the biological bases for interfemale aggression.

Correlation between GRIK2 rs6922753, rs2227283 polymorphism and aggressive behaviors with Bipolar Mania in the Chinese Han population

OBJECTIVES

Animal studies have shown that glutamate receptor ionotropic kainate 2 (GRIK2) gene knockout mice are more impulsive and aggressive. This study aims to verify whether the rs6922753 and rs2227283 polymorphisms of the GRIK2 gene are associated with both aggressive behavior and bipolar mania in the Chinese Han population.

METHODS

Polymerase chain reaction (PCR) was applied in the genotype rs6922753 and rs2227283 polymorphisms of the GRIK2 gene in 201 bipolar manic patients with aggressive behaviors, 198 bipolar manic patients without aggressive behaviors, and 132 healthy controls. The Modified Overt Aggression Scale (MOAS) was used to evaluate aggressive behavior in patients with bipolar mania.

RESULTS

No correlation was found between aggressive behavior and the rs6922753 polymorphism in the three groups. The A/A genotype and A allele of the rs2227283 polymorphism were found significantly more frequently in patients with aggressive behavior than in healthy controls (p = .004 and p = .013, respectively) and in patients with nonaggressive behavior (p = .002 and p = .018, respectively). The A/A genotype and A allele were associated with an increased risk of aggressive behavior.

CONCLUSION

This study suggests that the rs2227283 polymorphism of the GRIK2 gene is related to aggressive behaviors in bipolar manic patients and that the A/A genotype and A allele may increase the risk of the aggressive behavior in bipolar manic patients.

Cannabidiol attenuates aggressive behavior induced by social isolation in mice: Involvement of 5-HT1A and CB1 receptors

Long-term single housing increases aggressive behavior in mice, a condition named isolation-induced aggression or territorial aggression, which can be attenuated by anxiolytic, antidepressant, and antipsychotic drugs. Preclinical and clinical findings indicate that cannabidiol (CBD), a non-psychotomimetic compound from Cannabis sativa, has anxiolytic, antidepressant, and antipsychotic properties. Few studies, however, have investigated the effects of CBD on aggressive behaviors. Here, we investigated whether CBD (5, 15, 30, and 60 mg/kg; i.p.) could attenuate social isolation-induced aggressive behavior in the resident-intruder test. Male Swiss mice (7-8 weeks) were single-housed for 10 days (resident mice) to induce aggressive behaviors, while conspecific mice of same sex and age (intruder mice) were group-housed. During the test, the intruder was placed into the resident's home-cage and aggressive behaviors initiated by the resident, including the latency for the first attack, number of attacks, and total duration of aggressive encounters, were recorded. The involvement of 5-HT1A and CB1 receptors (CB1R) in the effects of CBD was also investigated. All tested CBD doses induced anti-aggressive effects, indicated by a decrease in the number of attacks. CBD, at intermediary doses (15 and 30 mg/kg), also increased latency to attack the intruder and decreased the duration of aggressive encounters. No CBD dose interfered with locomotor behavior. CBD anti-aggressive effects were attenuated by the 5-HT1A receptor antagonist WAY100635 (0.3 mg/kg) and the CB1 antagonist AM251 (1 mg/kg), suggesting that CBD decreases social isolation-induced aggressive behaviors through a mechanism associated with the activation of 5-HT1A and CB1 receptors. Also, CBD decreased c-Fos protein expression, a neuronal activity marker, in the lateral periaqueductal gray (lPAG) in social-isolated mice exposed to the resident-intruder test, indicating a potential involvement of this brain region in the drug effects. Taken together, our findings suggest that CBD may be therapeutically useful to treat aggressive behaviors that are usually associated with psychiatric disorders.

Sex-Dimorphic Behavioral Alterations and Altered Neurogenesis in U12 Intron Splicing-Defective Zrsr1 Mutant Mice

Mutant mice with respect to the splicing factor Zrsr1 present altered spermatogenesis and infertility. To investigate whether Zrsr1 is involved in the homeostatic control that the hypothalamus exerts over reproductive functions, we first analyzed both differential gene and isoform expression and alternative splicing alterations in Zrsr1 mutant (Zrsr1^mu) hypothalamus; second, we analyzed the spontaneous and social behavior of Zrsr1^mu mice; and third, we analyzed adult cell proliferation and survival in the Zrsr1^mu hypothalamus. The Zrsr1^mu hypothalamus showed altered expression of genes and isoforms related to the glutathione metabolic process, synaptonemal complex assembly, mRNA transport, and altered splicing events involving the enrichment of U12-type intron retention (IR). Furthermore, increased IR in U12-containing genes related with the prolactin, progesterone, and gonadotropin-releasing hormone (GnRH) reproductive signaling pathway was observed. This was associated with a hyperactive phenotype in both males and females, with an anxious phenotype in females, and with increased social interaction in males, instead of the classical aggressive behavior. In addition, Zrsr1^mu females but not males exhibited reduced cell proliferation in both the hypothalamus and the subventricular zone. Overall, these results suggest that Zrsr1 expression and function are relevant to organization of the hypothalamic cell network controlling behavior.

TMEM16B Calcium-Activated Chloride Channels Regulate Action Potential Firing in Lateral Septum and Aggression in Male Mice

The lateral septum (LS) plays an important role in regulating aggression. It is well recognized that LS lesions lead to a dramatic increase in aggressive behaviors. A better understanding of LS neurophysiology and its functional output is therefore important to assess LS involvement in regulating aggression. The LS is a heterogeneous structure that maintains inputs and outputs with multiple brain regions, and is also divided into subregions that innervate one another. Thus, it is challenging to identify the exact cell type and projections for characterization. In this study, we determined the expression pattern of the calcium-activated chloride channel, TMEM16B, in the LS of both male and female mice. We then investigated the physiological contribution of the calcium-activated chloride channel to LS neuronal signaling. By performing whole-cell patch-clamp recording, we showed that TMEM16B alters neurotransmitter release at the hippocampal-LS synapse, and regulates spike frequency and spike frequency adaptation in subpopulations of LS neurons. We further demonstrated that loss of TMEM16B function promotes lengthened displays of aggressive behaviors by male mice during the resident intruder paradigm. In conclusion, our findings suggest that TMEM16B function contributes to neuronal excitability in subpopulations of LS neurons and the regulation of aggression in male mice.SIGNIFICANCE STATEMENT Aggression is a behavior that arose evolutionarily from the necessity to compete for limited resources and survival. One particular brain region involved in aggression is the lateral septum (LS). In this study, we characterized the expression of the TMEM16B calcium-activated chloride channel in the LS and showed that TMEM16B regulates the action potential firing frequency of LS neurons. We discovered that loss of TMEM16B function lengthens the displays of aggressive behaviors in male mice. These findings suggest that TMEM16B plays an important role in regulating LS neuronal excitability and behaviors associated with LS function, thereby contributing to our understanding of how the LS may regulate aggression.

Persistent increase of I.V. cocaine self-administration in a subgroup of C57BL/6J male mice after social defeat stress

RATIONALE

Stressful life experiences can persistently increase the motivation for, and consumption of, intensely rewarding stimuli, like cocaine, over time. In rodents, intermittent versus continuous exposure to social stress engenders opposing changes to reward-related behavior, as measured by consumption of sucrose and cocaine.

OBJECTIVE

The present study examines if the effects of intermittent versus continuous social stress on cocaine self-administration in mice parallel those seen in rats.

METHODS

Both forms of social stress involve a brief daily physical confrontation with an aggressive resident for 10 consecutive days. Continuous social stress involves constant visual and olfactory exposure to an aggressive resident via habitation in a protected portion of the resident's home cage, while exposure to an aggressive resident during intermittent social stress is limited to a single, physical encounter per day. Implementing a femoral vein catheterization method for the first time in mice, we determined divergent changes to intravenous cocaine self-administration.

RESULTS

Modestly increased cocaine self-administration after intermittent social stress was confirmed. In a subset of animals, continuous social stress in mice substantially increased cocaine self-administration and sucrose intake. By stark contrast, another subpopulation had substantial attenuation of cocaine self-administration and sucrose intake after continuous social stress.

CONCLUSIONS

Bimodal divergence in responding for rewarding stimuli including cocaine after social stress experience likely reflects two opposing forms of coping to continuous social stress that promote either a sensitization or attenuation of reward-seeking.

Translational models of adaptive and excessive fighting: an emerging role for neural circuits in pathological aggression

Aggression is a phylogenetically stable behavior, and attacks on conspecifics are observed in most animal species. In this review, we discuss translational models as they relate to pathological forms of offensive aggression and the brain mechanisms that underlie these behaviors. Quantifiable escalations in attack or the development of an atypical sequence of attacks and threats is useful for characterizing abnormal variations in aggression across species. Aggression that serves as a reinforcer can be excessive, and certain schedules of reinforcement that allow aggression rewards also allow for examining brain and behavior during the anticipation of a fight. Ethological attempts to capture and measure offensive aggression point to two prominent hypotheses for the neural basis of violence. First, pathological aggression may be due to an exaggeration of activity in subcortical circuits that mediate adaptive aggressive behaviors as they are triggered by environmental or endogenous cues at vulnerable time points. Indeed, repeated fighting experiences occur with plasticity in brain areas once considered hardwired. Alternatively, a separate "violence network" may converge on aggression circuitry that disinhibits pathological aggression (for example, via disrupted cortical inhibition). Advancing animal models that capture the motivation to commit pathological aggression remains important to fully distinguish the neural architecture of violence as it differs from adaptive competition among conspecifics.

Social isolation as a promising animal model of PTSD comorbid suicide: neurosteroids and cannabinoids as possible treatment options

Post-traumatic stress disorder (PTSD) is a psychiatric condition characterized by drastic alterations in mood, emotions, social abilities and cognition. Notably, one aspect of PTSD, particularly in veterans, is its comorbidity with suicide. Elevated aggressiveness predicts high-risk to suicide in humans and despite the difficulty in reproducing a complex human suicidal behavior in rodents, aggressive behavior is a well reproducible behavioral trait of suicide. PTSD animal models are based on a peculiar phenotype, including exaggerated fear memory and impaired fear extinction associated with neurochemical dysregulations in the brain circuitry regulating emotion. The endocannabinoid and the neurosteroid systems regulate emotions and stress responses, and recent evidence shows these two systems are interrelated and critically compromised in neuropsychiatric disorders. For instance, levels of the neurosteroid, allopregnanolone, as well as those of the endocannabinoids, anandamide and its congener, palmitoylethanolamide are decreased in PTSD. Similarly, the endocannabinoid system and neurosteroid biosynthesis are altered in suicidal individuals. Selective serotonin reuptake inhibitors (SSRIs), the only FDA-approved treatments for PTSD, fail to help half of the treatment-seeking patients. This highlights the need for developing biomarker-based efficient therapies. One promising alternative to SSRIs points to stimulation of allopregnanolone biosynthesis as a treatment and a valid end-point to predict treatment response in PTSD patients. This review highlights running findings on the role of the endocannabinoid and neurosteroid systems in PTSD and suicidal behavior both in a preclinical and clinical perspective. A specific focus is given to predictive PTSD/suicide animal models. Ultimately, we discuss the idea that disruption of neurosteroid and endocannabinoid biosynthesis may offer a novel promising biomarker axis to develop new treatments for PTSD and, perhaps, suicidal behavior.

Contrasting characteristic behaviours among common laboratory mouse strains

Mice are widely used to model wide-ranging human neurological disorders, from development to degenerative pathophysiology. Behavioural and molecular characteristics of these mouse models are influenced by the genetic background of each strain. Among the most commonly used strains, the inbred C57BL/6J, BALB/c, CBA and 129SvEv lines and the CD1 outbred line are particularly predominant. Despite their prevalence, comparative performance of these strains on many standard behavioural tests commonly used to assess neurological conditions remains diffusely and indirectly accessible in the literature. Given that independent studies may be conducted with mice of differing genetic backgrounds, any variation in characteristic behavioural responses of specific strains should be delineated in order to properly interpret results among studies. Thus, in the present study, we aimed to characterize these commonly used mice strains through several standard behavioural tests. Here, we found that animals from different genetic background strains exhibited varying behavioural patterns when assessed for sociability/novelty, memory function, and negative behaviours like despair and stress calls. These results suggest that genetic variation among strains may be responsible-in part-for strain-specific behavioural phenotypes and potential predisposition to some neurological disorders.

Identification of Cholecystokinin by Genome-Wide Profiling as Potential Mediator of Serotonin-Dependent Behavioral Effects of Maternal Separation in the Amygdala

Converging evidence suggests a role of serotonin (5-hydroxytryptamine, 5-HT) and tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme of 5-HT synthesis in the brain, in modulating long-term, neurobiological effects of early-life adversity. Here, we aimed at further elucidating the molecular mechanisms underlying this interaction, and its consequences for socio-emotional behaviors, with a focus on anxiety and social interaction. In this study, adult, male Tph2 null mutant (Tph2 -/-) and heterozygous (Tph2 +/-) mice, and their wildtype littermates (Tph2 +/+) were exposed to neonatal, maternal separation (MS) and screened for behavioral changes, followed by genome-wide RNA expression and DNA methylation profiling. In Tph2 -/- mice, brain 5-HT deficiency profoundly affected socio-emotional behaviors, i.e., decreased avoidance of the aversive open arms in the elevated plus-maze (EPM) as well as decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Tph2 +/- mice showed an ambiguous profile with context-dependent, behavioral responses. In the EPM they showed similar avoidance of the open arm but decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Notably, MS effects on behavior were subtle and depended on the Tph2 genotype, in particular increasing the observed avoidance of EPM open arms in wildtype and Tph2 +/- mice when compared to their Tph2 -/- littermates. On the genomic level, the interaction of Tph2 genotype with MS differentially affected the expression of numerous genes, of which a subset showed an overlap with DNA methylation profiles at corresponding loci. Remarkably, changes in methylation nearby and expression of the gene encoding cholecystokinin, which were inversely correlated to each other, were associated with variations in anxiety-related phenotypes. In conclusion, next to various behavioral alterations, we identified gene expression and DNA methylation profiles to be associated with TPH2 inactivation and its interaction with MS, suggesting a gene-by-environment interaction-dependent, modulatory function of brain 5-HT availability.

Social Defeat Stress (SDS) in Mice: Using Swiss Mice as Resident

[Abstract] Due to the high prevalence and great economic impact of depression, studies with animal models have been increasingly used to identify neurobiological mechanisms associated with this disorder. However, many animal models use stressful conditions that are not consistent with what we observe in the modern human world. Examples are the chronic unpredictable stress and the electric shock model used in rodents. It’s well established the social stress as the major cause of depressive disorder in human, in this way a social defeat stress model was recently standardized and can induce depressive-like behavior of social avoidance, a typical human depressive behavior. In this model, mice are exposed on consecutive days to an aggressor mouse, suffering brief periods of physical aggression followed by longer periods of visual and olfactory (sensory) contact and, as a consequence, a relationship of social submission is characterized. Thus, the objective of this work is to describe a social defeat stress protocol using swiss mice as resident, also describing valuable procedural suggestions that will help researchers to reproduce the model easily.

Animal Models of (or for) Aggression Reward, Addiction, and Relapse: Behavior and Circuits

Inappropriate and pathological aggression plays a leading role in the suffering and death of millions of people, and further places an untenable strain on the caregivers and families of those afflicted. In some cases, such as addictive drugs, aggression can be highly rewarding (appetitive) and continually pursued despite short- and long-term negative consequences. Similarly, recidivism (relapse) rates for repeat violent offenders are as high as relapse rates for drug addicts. Appetitive aggression and relapse to aggression seeking can be modeled in mice studies using conditioned place preference and self-administration procedures followed by a period of abstinence and subsequent tests for relapse to aggression preference and aggression seeking. These procedures allow for the study of the mechanisms that control the appetitive versus the consummatory (attack) phases of aggressive behavior. In this review, we first discuss the behavioral procedures developed to probe appetitive aggression in mouse models, spanning from Pavlovian to operant tasks, and we also describe the recently proposed phenomenon of "aggression addiction." Next, we discuss the pharmacological and circuit mechanisms of aggression conditioned place preference and aggression self-administration, seeking, and relapse, highlighting mechanistic congruence and divergence between appetitive and consummatory phases of aggression. We conclude by discussing clinical implications of the studies reviewed.

Gradient of Parvalbumin- and Somatostatin-Expressing Interneurons Across Cingulate Cortex Is Differentially Linked to Aggression and Sociability in BALB/cJ Mice

Successfully navigating social interactions requires the precise and balanced integration of social and environmental cues. When such flexible information integration fails, maladaptive behavioral patterns arise, including excessive aggression, empathy deficits, and social withdrawal, as seen in disorders such as conduct disorder and autism spectrum disorder. One of the main hubs for the context-dependent regulation of behavior is cingulate cortex, specifically anterior cingulate cortex (ACC) and midcingulate cortex (MCC). While volumetric abnormalities of ACC and MCC have been demonstrated in patients, little is known about the exact structural changes responsible for the dysregulation of behaviors such as aggression and social withdrawal. Here, we demonstrate that the distribution of parvalbumin (PV) and somatostatin (SOM) interneurons across ACC and MCC differentially predicts aggression and social withdrawal in BALB/cJ mice. BALB/cJ mice were phenotyped for their social behavior (three-chamber task) and aggression (resident-intruder task) compared to control (BALB/cByJ) mice. In line with previous studies, BALB/cJ mice behaved more aggressively than controls. The three-chamber task revealed two sub-groups of highly-sociable versus less-sociable BALB/cJ mice. Highly-sociable BALB/cJ mice were as aggressive as the less-sociable group-in fact, they committed more acts of socially acceptable aggression (threats and harmless bites). PV and SOM immunostaining revealed that a lack of specificity in the distribution of SOM and PV interneurons across cingulate cortex coincided with social withdrawal: both control mice and highly-sociable BALB/cJ mice showed a differential distribution of PV and SOM interneurons across the sub-areas of cingulate cortex, while for less-sociable BALB/cJ mice, the distributions were near-flat. In contrast, both highly-sociable and less-sociable BALB/cJ mice had a decreased concentration of PV interneurons in MCC compared to controls, which was therefore linked to aggressive behavior. Together, these results suggest that the dynamic balance of excitatory and inhibitory activity across ACC and MCC shapes both social and aggressive behavior.

Injections to pregnant mice produce prenatal stress that affects aggressive behavior in their adult male offspring

Maternal stress could reprogram the developing fetal nervous system. A common target of maternal glucocorticoids is fetal neuro-endocrine axis. In the present study, pregnant mice were exposed to stress by injection and their male offspring were tested for sexual and aggressive behaviors in adult life. Three groups of pregnant mice were exposed to stress by sham syringe injection. The first group was injected on days 13, 14, and 15 p.c., the second group was injected on days 17 and 18 p.c., and the third group was injected daily from days 13 to 18 p.c. while control mice were not injected. Male offspring that were exposed to stress on days 13-18 p.c. and 17-18 p.c. were less aggressive and had lower blood testosterone levels in comparison to the control group. In male sexual behavior, there were no statistically significant differences between the groups. Body weight differed significantly with groups injected on days 13-18 p.c. and 13-15 p.c. having significantly higher body weight in adult life than the other two groups. After behavioral testing, brains were processed for immunohistochemical staining with antibodies against vasopressin (AVP) and calbindin (CALB). The expression of AVP and CALB in the lateral septum and in the preoptic area, respectively, did not differ between groups, suggesting that these two masculinization markers were not affected by prenatal stress. Present study therefore shows that even presumably mild and short prenatal stress weakens aggressive behavior of adult male mice, possibly due to reduced testosterone levels in blood.

The Role of Estrogen Receptor β (ERβ) in the Establishment of Hierarchical Social Relationships in Male Mice

Acquisition of social dominance is important for social species including mice, for preferential access to foods and mates. Male mice establish social rank through agonistic behaviors, which are regulated by gonadal steroid hormone, testosterone, as its original form and aromatized form. It is well known that estrogen receptors (ERs), particularly ER α (ERα), mediate effects of aromatized testosterone, i.e., 17β-estradiol, but precise role played by ER β (ERβ) is still unclear. In the present study, we investigated effects of ERβ gene disruption on social rank establishment in male mice. Adult male ERβ knockout (βERKO) mice and their wild type (WT) littermates were paired based on genotype- and weight-matched manner and tested against each other repeatedly during 7 days experimental period. They underwent 4 trials of social interaction test in neutral cage (homogeneous set test) every other day. Along repeated trials, WT but not βERKO pairs showed a gradual increase of agonistic behaviors including aggression and tail rattling, and a gradual decrease of latency to social rank determination in tube test conducted after each trial of the social interaction test. Analysis of behavioral transition further suggested that WT winners in the tube test showed one-sided aggression during social interaction test suggesting WT pairs went through a process of social rank establishment. On the other hand, a dominant-subordinate relationship in βERKO pairs was not as apparent as that in WT pairs. Moreover, βERKO mice showed lower levels of aggressive behavior than WT mice in social interaction tests. These findings collectively suggest that ERβ may play a significant role in the establishment and maintenance of hierarchical social relationships among male mice.

Postnatal TrkB ablation in corticolimbic interneurons induces social dominance in male mice

The tight balance between synaptic excitation and inhibition (E/I) within neocortical circuits in the mammalian brain is important for complex behavior. Many loss-of-function studies have demonstrated that brain-derived neurotrophic factor (BDNF) and its cognate receptor tropomyosin receptor kinase B (TrkB) are essential for the development of inhibitory GABAergic neurons. However, behavioral consequences of impaired BDNF/TrkB signaling in GABAergic neurons remain unclear, largely due to confounding motor function deficits observed in previous animal models. In this study, we generated conditional knockout mice (TrkB cKO) in which TrkB was ablated from a majority of corticolimbic GABAergic interneurons postnatally. These mice showed intact motor coordination and movement, but exhibited enhanced dominance over other mice in a group-housed setting. In addition, immature fast-spiking GABAergic neurons of TrkB cKO mice resulted in an E/I imbalance in layer 5 microcircuits within the medial prefrontal cortex (mPFC), a key region regulating social dominance. Restoring the E/I imbalance via optogenetic modulation in the mPFC of TrkB cKO mice normalized their social dominance behavior. Taken together, our results provide strong evidence for a role of BDNF/TrkB signaling in inhibitory synaptic modulation and social dominance behavior in mice.

Learning to actively cope with stress in female mice

Repeated exposure to a same-sex resident stranger enhances subsequent indications of active coping that generalize across multiple contexts in intruder male mice. Here we investigate female mice for comparable learning to cope training effects. Stress coping research focused on females is important because stress related mood and anxiety disorders are more prevalent in women than men. Female mice were monitored for coping behavior in open-field, object-exploration, and tail-suspension tests conducted after repeated exposure to a same-sex resident stranger. During repeated exposure sessions of training staged in the resident's home cage, behavioral measures of aggression and risk assessment were collected and plasma measures of the stress hormone corticosterone were obtained from separate samples of mice. Repeated exposure to a same-sex resident stranger subsequently enhanced active coping behavior exemplified by diminished freezing and increased center entries in the open-field, shorter object-exploration latencies, and a tendency toward decreased immobility on tail-suspension tests. Open-field locomotion considered as an index of non-specific activity was not increased by repeated sessions of exposure and did not correlate significantly with any measure of active coping. During repeated sessions of exposure to a same-sex resident stranger, risk assessment behavior and consistent but limited aggression occurred and corticosterone responses increased over repeated sessions. Exposure to a same-sex resident stranger is mildly stressful and promotes learning to actively cope in mice assessed in three different contexts.

Mice With Partial Deletion of Y-Heterochromatin Exhibits Stress Vulnerability

The role of Y chromosome in sex determination and male fertility is well established. It is also known that infertile men are prone to psychological disturbances. Earlier studies in the laboratory identified genes expressed in testes that are putatively regulated by Y chromosome in man and mouse. With the availability of a Y-deleted mouse model, that is subfertile, we studied the effect of a partial deletion of Y-chromosomal heterochromatin on mouse behavior when compared to its wild type. The partial Y-deleted mice exhibited anxiety like phenotype under stress when different anxiety (open field test and elevated plus maze, EPM test) and depression related tests (tail suspension and force swim) were performed. The mutant mice also showed reduction in hippocampal neurogenesis and altered expression of neurogenesis markers such as Nestin, Sox2, Gfap, NeuroD1 and Dcx using quantitative real time PCR (qPCR) analysis. The genes with altered expression contained short stretches of homology to Y-derived transcripts only in their Untranslated Regions (UTRs). Our study suggests putative regulation of these genes by the Y chromosome in mouse brain altering stress related behavior.

Animal models of social stress: the dark side of social interactions

Social stress occurs in all social species, including humans, and shape both mental health and future interactions with conspecifics. Animal models of social stress are used to unravel the precise role of the main stress system - the HPA axis - on the one hand, and the social behavior network on the other, as these are intricately interwoven. The present review aims to summarize the insights gained from three highly useful and clinically relevant animal models of psychosocial stress: the resident-intruder (RI) test, the chronic subordinate colony housing (CSC), and the social fear conditioning (SFC). Each model brings its own focus: the role of the HPA axis in shaping acute social confrontations (RI test), the physiological and behavioral impairments resulting from chronic exposure to negative social experiences (CSC), and the neurobiology underlying social fear and its effects on future social interactions (SFC). Moreover, these models are discussed with special attention to the HPA axis and the neuropeptides vasopressin and oxytocin, which are important messengers in the stress system, in emotion regulation, as well as in the social behavior network. It appears that both nonapeptides balance the relative strength of the stress response, and simultaneously predispose the animal to positive or negative social interactions.

Altered Amygdala Excitation and CB1 Receptor Modulation of Aggressive Behavior in the Neuroligin-3R451C Mouse Model of Autism

Understanding neuronal mechanisms underlying aggression in patients with autism spectrum disorder (ASD) could lead to better treatments and prognosis. The Neuroligin-3 (NL3)^R451C mouse model of ASD has a heightened aggressive phenotype, however the biological mechanisms underlying this behavior are unknown. It is well established that NL3^R451C mice have imbalanced excitatory and inhibitory synaptic activity in the hippocampus and somatosensory cortex. The amygdala plays a role in modulating aggressive behavior, however potential changes in synaptic activity in this region have not previously been assessed in this model. We investigated whether aggressive behavior is robustly present in mice expressing the R451C mutation, following back-crossing onto a congenic background strain. Endocannabinoids influence social interaction and aggressive behavior, therefore we also studied the effects of cannabinoid receptor 1 (CB1) agonist on NL3^R451C mice. We report that NL3^R451C mice have increased amplitude of miniature excitatory postsynaptic currents (EPSCs) with a concomitant decrease in the amplitude of inhibitory postsynaptic currents (IPSCs) in the basolateral amygdala. Importantly, we demonstrated that NL3^R451C mice bred on a C57Bl/6 background strain exhibit an aggressive phenotype. Following non-sedating doses (0.3 and 1.0 mg/kg) of the CB1 receptor agonist WIN55,212-2 (WIN), we observed a significant reduction in aggressive behavior in NL3^R451C mice. These findings demonstrate altered synaptic activity in the basolateral amygdala and suggest that the NL3^R451C mouse model is a useful preclinical tool to understand the role of CB1 receptor function in aggressive behavior.

Quantitative trait locus mapping and analysis of heritable variation in affiliative social behavior and co-occurring traits

Humans exhibit broad heterogeneity in affiliative social behavior. Twin and family studies show that individual differences in core dimensions of social behavior are heritable, yet there are knowledge gaps in understanding the underlying genetic and neurobiological mechanisms. Animal genetic reference panels (GRPs) provide a tractable strategy for examining the behavioral and genetic architecture of complex traits. Here, using males from 50 mouse strains from the BXD GRP, 4 domains of affiliative social behavior-social approach, social recognition, direct social interaction (DSI) (partner sniffing) and vocal communication-were examined in 2 widely used behavioral tasks-the 3-chamber and DSI tasks. There was continuous and broad variation in social and nonsocial traits, with moderate to high heritability of social approach sniff preference (0.31), ultrasonic vocalization (USV) count (0.39), partner sniffing (0.51), locomotor activity (0.54-0.66) and anxiety-like behavior (0.36). Principal component analysis shows that variation in social and nonsocial traits are attributable to 5 independent factors. Genome-wide mapping identified significant quantitative trait loci for USV count on chromosome (Chr) 18 and locomotor activity on Chr X, with suggestive loci and candidate quantitative trait genes identified for all traits with one notable exception-partner sniffing in the DSI task. The results show heritable variation in sociability, which is independent of variation in activity and anxiety-like traits. In addition, a highly heritable and ethological domain of affiliative sociability-partner sniffing-appears highly polygenic. These findings establish a basis for identifying functional natural variants, leading to a new understanding typical and atypical sociability.

Effect of Environmental Enrichment on Aggression in BALB/cJ and BALB/cByJ Mice Monitored by Using an Automated System

Aggression among mice remains a common undesirable problem in laboratory settings, and animal welfare and scientific outcomes may become compromised depending on the severity of aggression. This study evaluated the effect of cage enrichment comprising a bilevel, mounted 'mezzanine' compared with a cotton square or shelter on intracage male aggression over a 6-wk period. Our first study involved home-cage behavioral challenges to male mice from a high-aggression substrain (BALB/cJ) and low-aggression substrain (BALB/cByJ). Aggressive interactions and locomotor activity were scored manually and then compared with measures of activity obtained by using a continuous automated home-cage monitoring system, the Digital Ventilated Caging (DVC) system. BALB/cJ mice exhibited similar levels of aggression across housing conditions, whereas BALB/cByJ mice had lower aggression when housed with a mezzanine. In the second study, videorecordings and continuous DVC automated measures were collected over 24 h and divided into 12-h light and dark phases. BALB/cByJ mice—but not BALB/cJ—mice had increased aggressive behaviors during the dark phase. However, the DVC detected higher activity levels during the dark phase, compared with the light phase, in both substrains. Elevated activity levels recorded by the DVC correlated with fighting bouts and high levels of locomotion. These results show that a bilevel structural form of enrichment reduces aggression, depending on the BALB/c substrain, and confirms higher aggression levels in the BALB/cJ substrain. In addition, our findings provide evidence that the DVC is effective in identifying mouse cages with patterns of high activity levels, signaling possible aggression incidences, thus potentially allowing for early intervention and consequently improving animal welfare.

A hypothalamic circuit for the circadian control of aggression

“Sundowning” in dementia and Alzheimer’s disease is characterized by early evening agitation and aggression. While such periodicity suggests a circadian origin, whether the circadian clock directly regulates aggressive behavior is unknown. We demonstrate that a daily rhythm in aggression propensity in male mice is gated by GABAergic subparaventricular zone (SPZ^GABA) neurons, the major postsynaptic targets of the central circadian clock, the suprachiasmatic nucleus (SCN). Optogenetic mapping revealed that SPZ^GABA neurons receive input from vasoactive intestinal polypeptide SCN neurons and innervate neurons in the ventrolateral part of the ventromedial hypothalamus (VMHvl) known to regulate aggression. Additionally, VMH-projecting dorsal SPZ neurons are more active during early day than early night, and acute chemogenetic inhibition of SPZ^GABA transmission phase-dependently increases aggression. Finally, SPZ^GABA-recipient central VMH neurons directly innervate VMHvl neurons and activation of this intra-VMH circuit drove attack behavior. Altogether, we reveal a functional polysynaptic circuit by which the SCN clock regulates aggression.

A quantitative trait locus on chromosome 1 modulates intermale aggression in mice

Aggression between male conspecifics is a complex social behavior that is likely modulated by multiple gene variants. In this study, the BXD recombinant inbred mouse strains (RIS) were used to map quantitative trait loci (QTLs) underlying behaviors associated with intermale aggression. Four hundred and fifty-seven males from 55 strains (including the parentals) were observed at an age of 13 ± 1 week in a resident-intruder test following 10 days of isolation. Attack latency was measured directly within a 10-minute time period and the test was repeated 24 hours later. The variables we analyzed were the proportion of attacking males in a given strain as well as the attack latency (on days 1 and 2, and both days combined). On day 1, 29% of males attacked, and this increased to 37% on day 2. Large strain differences were obtained for all measures of aggression, indicating substantial heritability (intraclass correlations 0.10-0.18). We identified a significant QTL on chromosome (Chr) 1 and suggestive QTLs on mouse Chrs 1 and 12 for both attack and latency variables. The significant Chr 1 locus maps to a gene-sparse region between 82 and 88.5 Mb with the C57BL/6J allele increasing aggression and explaining about 18% of the variance. The most likely candidate gene modulating this trait is Htr2b which encodes the serotonin 2B receptor and has been implicated in aggressive and impulsive behavior in mice, humans and other species.

The effect of aggression I: The increases of metabolic cost and mobilization of fat reserves in male striped hamsters

Aggression can benefit individuals by enhancing their dominance and thereby their ability to acquire and retain resources that increase survival or fitness. Engaging in aggressive behavior costs energy and how animals manage their energy budget to accommodate aggression remains unclear. We conducted three experiments to examine changes in physiological, behavioral and hormonal markers indicative of energy budget in male striped hamsters subject to resident-intruder aggression tests. Body temperature, metabolic rate and serum corticosterone levels significantly increased in resident hamsters immediately after the introduction of intruders. Energy intake did not change, but the metabolic rate of residents increased by 16.1% after 42-days of repeated encounters with intruders. Residents had significantly decreased body fat content and serum thyroxine (T₄) levels, and a considerably elevated tri-iodothyronine (T₃)/T₄ ratio compared to a control group that had no intruders. Attack latency considerably shortened, and the number of attack bouts and total duration of attacks, significantly increased in residents on day 42 compared to day 1 of experiments. These findings may suggest that the conversion of T₄ to T₃ is involved in defensive aggression behavior. The mobilization of fat reserves resulting in lean body mass is probably common response to the increased metabolic cost of aggression in small mammals. Aggressive behavior, which is important for the successful acquisition and defense of resources, may be of significance for adaptation and evolution of metabolic rate.

Cell-type-specific role for nucleus accumbens neuroligin-2 in depression and stress susceptibility

Behavioral coping strategies are critical for active resilience to stress and depression; here we describe a role for neuroligin-2 (NLGN-2) in the nucleus accumbens (NAc). Neuroligins (NLGN) are a family of neuronal postsynaptic cell adhesion proteins that are constituents of the excitatory and inhibitory synapse. Importantly, NLGN-3 and NLGN-4 mutations are strongly implicated as candidates underlying the development of neuropsychiatric disorders with social disturbances such as autism, but the role of NLGN-2 in neuropsychiatric disease states is unclear. Here we show a reduction in NLGN-2 gene expression in the NAc of patients with major depressive disorder. Chronic social defeat stress in mice also decreases NLGN-2 selectively in dopamine D1-positive cells, but not dopamine D2-positive cells, within the NAc of stress-susceptible mice. Functional NLGN-2 knockdown produces bidirectional, cell-type-specific effects: knockdown in dopamine D1-positive cells promotes subordination and stress susceptibility, whereas knockdown in dopamine D2-positive cells mediates active defensive behavior. These findings establish a behavioral role for NAc NLGN-2 in stress and depression; provide a basis for targeted, cell-type specific therapy; and highlight the role of active behavioral coping mechanisms in stress susceptibility.

Behavioral Assays in the Study of Olfaction: A Practical Guide

Olfaction is a fundamental sense in most animal species. In mammals, the olfactory system comprises several subpopulations of sensory neurons located throughout the nasal cavity, which detect a variety of chemostimuli, including odorants, intraspecies and interspecies chemical communication cues. Some of these compounds are important for regulating innate and learned behaviors, and endocrine changes in response to other animals in the environment. With a particular focus on laboratory rodent species, this chapter provides a comprehensive description of the most important behavioral assays used for studying the olfactory system, and is meant to be a practical guide for those who study olfaction-mediated behaviors or who have an interest in deciphering the molecular, cellular, or neural mechanisms through which the sense of smell controls the generation of adaptive behavioral outputs.

Screening of the Behavioral Tests for Monitoring Agonistic Behavior of Layer Chicks

Chicken agonistic behavior, a type of social behavior related to threatening and fighting, is among the most serious problems in the poultry industry. However, due to luck of effective models for investigating the brain mechanisms of the behavior, no effective measures have been taken. This study, therefore, aimed to select the behavioral tests available for monitoring chicken agonistic behavior. Two behavioral tests, resident-intruder (R-I) test and social interaction (SI) test, were performed for 10 minutes in 10 pairs of male layer chicks at 8, 12, 16, 20, and 24 days of age, and total agonistic frequencies (TAF: Sum of the frequencies of agonistic displays like pecking, biting, kicking, threatening, and leaping) and latency (the period of time from the beginning of the behavioral test to the occurrence of the first agonistic behavior) were measured as indices of agonistic behavior. Two-way repeated measures ANOVA revealed significant differences in TAF and latency between aggressors and opponents in both the behavioral tests. In the R-I test, the TAF of aggressors significantly increased from 8 to 20 days of age, and the latency significantly decreased from 8 to 24 days of age. In the SI test, however, the TAF of aggressors significantly increased and the latency significantly decreased only from 16 to 20 days of age. When the criterion of high agonistic behavior was defined as the TAF, where aggressors showed more than 30 times of TAF and the opponents did less than one-third TAF of aggressors, the aggression establishment rate (AER), which is equal to the number of aggressors showing high agonistic behavior per total behavioral trials, was significantly higher in the R-I test than in the SI test. These results suggest that the R-I test, rather than the SI test, is an effective tool for monitoring agonistic behavior of layer chicks.