Involvement of the dopamine system in the effect of chronic social isolation during adolescence on social behaviors in male C57 mice

Adolescent social isolation decreases colonic goblet cells and impairs spatial cognition through the reduction of cystine

Negative experiences during adolescence, such as social isolation (SI), bullying, and abuse, increase the risk of psychiatric diseases in adulthood. However, the pathogenesis of psychiatric diseases induced by these factors remain poorly understood. In adolescents, stress affects the intestinal homeostasis in the gut-brain axis. This study determined whether adolescent SI induces behavioral abnormalities by disrupting colonic function. Adolescent mice exposed to SI exhibit spatial cognitive deficits and microglial activation in the hippocampus (HIP). SI decreased the differentiation of mucin-producing goblet cells, which was accompanied by alterations in the composition of the gut microbiota, particularly the depletion of mucin-feeding bacteria. Treatment with rebamipide, which promotes goblet cell differentiation in the colon, attenuated SI-induced spatial cognitive deficits and microglial activation in the HIP and decreased cystine, a downstream metabolite of homocysteine. Treatment with cystine ameliorated SI-induced spatial cognitive deficits and increased microglial C-C motif chemokine ligand 7 (CCL7) levels in the HIP. Inhibition of CCL7 receptors by antagonists of CC motif chemokine receptors 2 (CCR2) and 3 (CCR3) in the HIP prevented spatial cognitive deficits induced by SI. Infusion of CCL7 into the HIP following microglial ablation with clodronate liposome induced spatial cognitive deficits. These findings suggest that adolescent SI decreases serum cystine levels by damaging the colonic goblet cells, resulting in spatial cognitive deficits by triggering microglial activation in the HIP. Our results indicate that increased CCL7 expression in hippocampal microglia may contribute to spatial cognitive deficits by activating CCR2 and CCR3.

Dopamine type II receptors in amygdala along with oxytocin in hypothalamus regulate social behavior in male mandarin voles

The amygdala dopamine (DA) system and hypothalamic oxytocin (OT) play important roles in emotion regulation, and emotions are important in regulating social behavior. However, it is unclear whether DA in the amygdala is involved in the regulation of social behavior, and whether OT in the hypothalamus is also involved in this process. In this study, we examined the release of DA in the medial amygdala (MeA) during different social interactions and the effect of injecting the dopamine II receptor (D2R) agonist quinpirole and the D2R antagonist raclopride into the MeA on social behavior and OT in the paraventricular nucleus (PVN) and supraoptic nucleus (SON), as well as in the blood of male mandarin voles (Microtus mandarinus). The results showed that the DA in the MeA increased in the process of social behavior, and the DA in the face of strangers was higher than that in the face of familiars. In addition, the injection of D2R antagonists in the MeA reduced attacking and escaping behaviors but increased physical contact and investigating behaviors, increased the number of OT-IR neurons in the PVN and SON, and increased OT levels in the blood. While injection of D2R agonists in the MeA increased attacking and escaping behaviors but reduced physical contact and investigating behaviors, it also reduced OT-IR neurons in the SON. In conclusion, D2R in the medial amygdala and oxytocin in the hypothalamus regulate social behavior.

The neuroinflammatory effects of Nociceptin/Orphanin FQ receptor activation can be related to depressive-like behavior

There is limited information on the role of the Nociceptin/Orphanin FQ receptor (NOPR) in neuroinflammation, and there is growing interest in the participation of the NOPR in depression etiology. This study aims to evaluate the neuroinflammatory effects of the NOPR activation in mice submitted to social defeat protocol (SDP). Firstly, male Swiss mice were submitted to the social defeat protocol during 10 or 20 days and treated with the NOPR agonist Ro 65-6570 (1.5 or 2 mg/kg; ip). Subsequently, behavioral tests were applied to evaluate depressive-like behaviors. Finally, inflammatory cytokines were measured in the animals' brains and blood. A meta-analysis, including 11 experiments, was also conducted to evaluate if the NOPR activation contributes to inflammation. The studies' weights, odds ratios, and confidence intervals were used to calculate the average effect size as the main outcome measure. The software SPSS v.29 and R programming language were used to analyze the data. The SDP and/or NOP agonist reduced distance traveled and exploration rate in the open field test. The SDP and/or the NOP agonist also increased immobility time in the tail suspension test, as well as reduced social interaction. Additionally, the NOP agonist increased the concentration of IL-6 and TNF alpha in the hippocampus, as well as reduced the IL-10 concentration in the hippocampus, but not in prefrontal cortex and serum. The SDP increased the concentration of IL-6 and TNF alpha in animals' serum and prefrontal cortex, but not in the hippocampus. The role of NOPR in neuroinflammation was regardless of the social defeat stress in the hippocampus. Meta-analysis also demonstrated the participation of NOPR activation in inducing inflammation in mice models. We suggest that upregulation of NOPR can activate signaling pathways involved in neuroinflammation, contributing to depression etiology.

Social group size alters social behavior and dopaminergic and serotonergic systems

Social behavior is affected by social structure type, but how neural function changes with social type remains unclear. We investigated whether social group size affects social behaviors based on dopamine (DA) and serotonin (5-HT) systems. Four-week-old male mice were housed under different social group sizes: one, two, four, and eight mice per cage (1mpc, 2mpc, 4mpc, 8mpc, respectively). After 4 weeks, social preference, social interaction, and forced swim tests were performed to test sociability and anxiety, respectively, followed by analysis of DA and 5-HT and their metabolites (3,4-dihydroxyphenylacetic acid [DOPAC], 3-methoxytyramine [3-MT], norepinephrine, and 5-hydroxyindoleacetic acid [5-HIAA]). Social interactions and anxious behavior decreased with increased social group size. DA, 3-MT, and 5-HT levels decreased with increasing social group size, whereas DOPAC and 5-HIAA levels increased in the extended mesocorticolimbic system, including the dorsal striatum. Moreover, the increased social group size resulted in increased DOPAC/DA and 5-HIAA/5-HT ratios, accompanied by a decrease in the 3-MT/DA ratio within the extended mesocorticolimbic system. Linear regression analysis also revealed that social group size affects DA and 5-HT turnover. These suggest that social group size may influence behavior and monoamine levels, potentially contributing to DA- and 5-HT-related psychiatric disorders.

Social and nonsocial environmental loss have differential effects on ventral hippocampus-dependent behavior and inhibitory synaptic markers in adult male mice

In humans, psychological loss, whether social or nonsocial, can lead to clinical depression, anxiety disorders, and social memory impairments. Researchers have modeled combined social and nonsocial loss in rodents by transitioning them from social, enriched environments (EE) to individual housing, affecting behaviors related to avoidance, stress coping, and cognitive function. However, it remains unclear if these effects are driven by social or nonsocial loss. We examined the effects of nonsocial loss by housing adult male mice in EE before moving them to standard cages, where they were pair-housed, and compared this to mice experiencing complete social loss. Continuous EE reduced social investigation time while leaving social memory intact, also decreasing avoidance behavior. Nonsocial loss restored social investigation and avoidance behavior to control levels, while social loss impaired social memory and increased avoidance. In rodents, social memory and avoidance require ventral hippocampus (vHIP) neuronal oscillations, which involve parvalbumin-positive (PV+) inhibitory interneurons. We found decreased vHIP PV intensity in the social loss group, with no differences in the nonsocial loss group. Most PV+ cells are surrounded by perineuronal nets (PNNs) concentrating GABAA receptors in their lattice-like holes. Social loss decreased GABAA-δ expression, a subunit associated with extrasynaptic receptors, across PNN+ soma and in PNN holes, while nonsocial loss reduced gephyrin in these regions. These findings suggest social and nonsocial losses differentially affect vHIP function and behavior, with social loss having a more pronounced impact through mechanisms involving PV+ interneurons, PNN structure, and neurotransmitter receptor expression.

Brexpiprazole: A new option in treating agitation in Alzheimer's dementia-Insights from transgenic mouse models

AIM

Brexpiprazole is the first FDA-approved treatment for agitation associated with dementia due to Alzheimer's disease. Agitation in Alzheimer's dementia (AAD) occurs in high prevalence and is a great burden for patients and caregivers. Efficacy, safety, and tolerability of brexpiprazole were demonstrated in the AAD clinical trials. To demonstrate the agitation-ameliorating effect of brexpiprazole in animals, we evaluated brexpiprazole in two AAD mouse models.

METHODS

The resident-intruder test was conducted in 5- to 6-month-old Tg2576 mice, given vehicle or brexpiprazole (0.01 or 0.03 mg/kg) orally 1 h before the test. Locomotor activity was measured in 6-month-old APPSL-Tg mice given vehicle or brexpiprazole (0.01 or 0.03 mg/kg) orally the evening before the start of locomotor measurement for 3 days.

RESULTS

In the resident-intruder test, Tg2576 mice showed significantly higher attack number and shorter latency to first attack compared to non-Tg mice. In the Tg mice, brexpiprazole treatment (0.03 mg/kg) significantly delayed the latency to first attack and showed a trend toward a decrease in attack number. APPSL-Tg mice (≧6 months old) showed significantly higher locomotion during dark period Phase II (Zeitgeber time [ZT] 16-20) and Phase III (ZT20-24) compared to non-Tg mice, correlating with the clinical observations of late afternoon agitation in Alzheimer's disease. Brexpiprazole treatment (0.01 and 0.03 mg/kg) significantly decreased hyperlocomotion during the Phase III in APPSL-Tg mice.

CONCLUSION

The suppression of attack behavior and the reduction of nocturnal hyperlocomotion in these Tg mice may be indicative of the therapeutic effect of brexpiprazole on AAD, as demonstrated in the clinical trials.

Clemastine improves emotional and social deficits in adolescent social isolation mice by reversing demyelination

Adolescence is a critical period for social experience-dependent oligodendrocyte maturation and myelination. Adolescent stress predisposes to cause irreversible changes in brain structure and function with lasting effects on adulthood or beyond. However, the molecular mechanisms linking adolescent social isolation stress with emotional and social competence remain largely unknown. In our study, we found that social isolation during adolescence leads to anxiety-like behaviors, depression-like behaviors, impaired social memory and altered patterns of social ultrasonic vocalizations in mice. In addition, adolescent social isolation stress induces demyelination in the prefrontal cortex and hippocampus of mice, with decreased myelin-related gene expression and disrupted myelin structure. More importantly, clemastine was sufficient to rescue the impairment of emotional and social memory by promoting remyelination. These findings reveal the demyelination mechanism of emotional and social deficits caused by social isolation stress in adolescence, and provides potential therapeutic targets for treating stress-related mental disorders.

Orexins/Hypocretins: Gatekeepers of Social Interaction and Motivation

Ever since the discovery of the brain's orexin/hypocretin system, most research was directed toward unveiling its contribution to the normal functioning of individuals. The investigation of reward-seeking behaviors then gained a lot of attention once the distribution of orexinergic neurons was revealed. Here, we discuss findings on the involvement of orexins in social interaction, a natural reward type. While some studies have succeeded in defining the relationship between orexin and social interaction, the controversy regarding its nature (direct or inverse relation) raises questions about what aspects have been overlooked until now. Upon examining the literature, we identified a research gap concerning conditions influencing the impact of orexins on social behavior expression. In this review, we introduce a number of factors (e.g., stress, orexin's source) that must be considered while studying the role of orexins in social interaction. Furthermore, we refer to published research to investigate the stage at which orexins affect social interaction and we highlight the nucleus accumbens (NAc) shell's role in social interaction and other rewarding behaviors. Finally, the underlying orexin molecular pathway influencing social motivation in particular illnesses is proposed. We conclude that orexin's impact on social interaction is multifactorial and depends on specific conditions available at a time.

The human neuropsychiatric risk gene Drd2 is necessary for social functioning across evolutionary distant species

The Drd2 gene, encoding the dopamine D2 receptor (D2R), was recently indicated as a potential target in the etiology of lowered sociability (i.e., social withdrawal), a symptom of several neuropsychiatric disorders such as Schizophrenia and Major Depression. Many animal species show social withdrawal in response to stimuli, including the vinegar fly Drosophila melanogaster and mice, which also share most human disease-related genes. Here we will test for causality between Drd2 and sociability and for its evolutionary conserved function in these two distant species, as well as assess its mechanism as a potential therapeutic target. During behavioral observations in groups of freely interacting D. melanogaster, Drd2 homologue mutant showed decreased social interactions and locomotor activity. After confirming Drd2's social effects in flies, conditional transgenic mice lacking Drd2 in dopaminergic cells (autoreceptor KO) or in serotonergic cells (heteroreceptor KO) were studied in semi-natural environments, where they could freely interact. Autoreceptor KOs showed increased sociability, but reduced activity, while no overall effect of Drd2 deletion was observed in heteroreceptor KOs. To determine acute effects of D2R signaling on sociability, we also showed that a direct intervention with the D2R agonist Sumanirole decreased sociability in wild type mice, while the antagonist showed no effects. Using a computational ethological approach, this study demonstrates that Drd2 regulates sociability across evolutionary distant species, and that activation of the mammalian D2R autoreceptor, in particular, is necessary for social functioning.

Social environment enrichment alleviates anxiety-like behavior in mice: Involvement of the dopamine system

Rearing environment plays a vital role in maintaining physical and mental health of both animals and humans. Plenty of studies have proved that physical environment enrichment in adolescence has protective effects on emotion, social behavior, learning and memory deficits. However, the following effects of social environment enrichment in adolescence remain largely elusive. Using the paradigm of companion rotation (CR), the present study found that social environment enrichment reduced anxiety-like behaviors of early adult male C57BL/6J mice. CR group also showed significantly higher expression of tyrosine hydroxylase in the ventral tegmental area and dopamine 1 receptor mRNA in the nucleus accumbens shell than control group. Taken together, these findings demonstrate that CR from adolescence to early adulthood can suppress the level of anxiety and upregulate dopaminergic neuron activity in early adult male C57BL/6J mice.

Adolescent social housing protects against adult emotional and cognitive deficits and alters the PFC and NAc transcriptome in male and female C57BL/6J mice

Introduction

Adolescence is a critical period in cognitive and emotional development, characterized by high levels of social interaction and increases in risk-taking behavior including binge drinking. Adolescent exposure to social stress and binge ethanol have individually been associated with the development of social, emotional, and cognitive deficits, as well as increased risk for alcohol use disorder. Disruption of cortical development by early life social stress and/or binge drinking may partly underlie these enduring emotional, cognitive, and behavioral effects. The study goal is to implement a novel neighbor housing environment to identify the effects of adolescent neighbor housing and/or binge ethanol drinking on (1) a battery of emotional and cognitive tasks (2) adult ethanol drinking behavior, and (3) the nucleus accumbens and prefrontal cortex transcriptome.

Methods

Adolescent male and female C57BL/6J mice were single or neighbor housed with or without access to intermittent ethanol. One cohort underwent behavioral testing during adulthood to determine social preference, expression of anxiety-like behavior, cognitive performance, and patterns of ethanol intake. The second cohort was sacrificed in late adolescence and brain tissue was used for transcriptomics analysis.

Results

As adults, single housed mice displayed decreased social interaction, deficits in the novel object recognition task, and increased anxiety-like behavior, relative to neighbor-housed mice. There was no effect of housing condition on adolescent or adult ethanol consumption. Adolescent ethanol exposure did not alter adult ethanol intake. Transcriptomics analysis revealed that adolescent housing condition and ethanol exposure resulted in differential expression of genes related to synaptic plasticity in the nucleus accumbens and genes related to methylation, the extracellular matrix and inflammation in the prefrontal cortex.

Discussion

The behavioral results indicate that social interaction during adolescence via the neighbor housing model may protect against emotional, social, and cognitive deficits. In addition, the transcriptomics results suggest that these behavioral alterations may be mediated in part by dysregulation of transcription in the frontal cortex or the nucleus accumbens.

Fatty acid-binding protein 5 differentially impacts dopamine signaling independent of sex and environment

Epidermal/brain fatty acid-binding protein 5 (FABP5) plays an integral role in the intracellular trafficking of bioactive lipids/endocannabinoids and the subsequent initiation of cellular cascades affecting cannabinoid and dopamine (DA) systems. Social isolation (SI) and environmental enrichment (EE) during adolescence have been shown to impact DA signaling, and, specifically, DA transporter (DAT) and receptor levels of DA type 1 (D1) and 2 (D2); however, the relationship between FABP5, environment and DA signaling remains unclear. The present study quantified DAT and DA receptor levels in male/female FABP5-/- and FABP5+/+ mice raised in either SI or EE. Results showed that FABP5-/- mice had 6.09-8.81% greater D1 levels in striatal sub-regions of the caudal brain, independent of sex or environment. D1 levels were 8.03% greater only in the olfactory tubercle of enrichment-reared animals. In summary, these results supported that FABP5 plays an important function in regulating striatal DA signaling, and this may have important implications as a target with therapeutic potential for various psychiatric disorders.

Social isolation leads to mild social recognition impairment and losses in brain cellularity

Chronic social stress is a significant risk factor for several neuropsychiatric disorders, mainly major depressive disorder (MDD). In this way, patients with clinical depression may display many symptoms, including disrupted social behavior and anxiety. However, like many other psychiatric diseases, MDD has a very complex etiology and pathophysiology. Because social isolation is one of the multiple depression-inducing factors in humans, this study aims to understand better the link between social stress and MDD using an animal model based on social isolation after weaning, which is known to produce social stress in mice. We focused on cellular composition and white matter integrity to establish possible links with the abnormal social behavior that rodents isolated after weaning displayed in the three-chamber social approach and recognition tests. We used the isotropic fractionator method to assess brain cellularity, which allows us to robustly estimate the number of oligodendrocytes and neurons in dissected brain regions. In addition, diffusion tensor imaging (DTI) was employed to analyze white matter microstructure. Results have shown that post-weaning social isolation impairs social recognition and reduces the number of neurons and oligodendrocytes in important brain regions involved in social behavior, such as the anterior neocortex and the olfactory bulb. Despite the limitations of animal models of psychological traits, evidence suggests that behavioral impairments observed in patients might have similar biological underpinnings.

Short-term hindlimb unloading negatively affects dopaminergic transmission in the nigrostriatal system of mice

The nigrostriatal system composed of the dorsal striatum and the substantia nigra (SN) is highly involved in the control of motor behavior. Various extremal and pathological conditions as well as social isolation (SI) may cause an impairment of locomotor function; however, corresponding alterations in the nigrostriatal dopaminergic pathway are far from full understanding. Here, we analyzed the effect of 3-day hindlimb unloading (HU) and SI on the key players of dopamine transmission in the nigrostriatal system of CD1 mice. Three groups of mice were analyzed: group-housed (GH), SI, and HU animals. Our data showed a significant decrease in the expression and phosphorylation of tyrosine hydroxylase (TH) in the SN and dorsal striatum of HU mice that suggested attenuation of dopamine synthesis in response to HU. In the dorsal striatum of HU mice, the downregulation of TH expression was also observed indicating the effect of unloading; however, TH phosphorylation at Ser40 was mainly affected by SI pointing on an impact of isolation too. Expression of dopamine receptors D1 in the dorsal striatum of HU mice was increased suggesting a compensatory response, but the activity of downstream signaling pathways involving protein kinase A and cAMP response element-binding protein was inhibited. At the same time, SI alone did not affect expression of DA receptors and activity of downstream signaling in the dorsal striatum. Obtained data let us to conclude that HU was the main factor which impaired dopamine transmission in the nigrostriatal system but SI made some contribution to its negative effects.

Areas of Convergence and Divergence in Adolescent Social Isolation and Binge Drinking: A Review

Adolescence is a critical developmental period characterized by enhanced social interactions, ongoing development of the frontal cortex and maturation of synaptic connections throughout the brain. Adolescents spend more time interacting with peers than any other age group and display heightened reward sensitivity, impulsivity and diminished inhibitory self-control, which contribute to increased risky behaviors, including the initiation and progression of alcohol use. Compared to adults, adolescents are less susceptible to the negative effects of ethanol, but are more susceptible to the negative effects of stress, particularly social stress. Juvenile exposure to social isolation or binge ethanol disrupts synaptic connections, dendritic spine morphology, and myelin remodeling in the frontal cortex. These structural effects may underlie the behavioral and cognitive deficits seen later in life, including social and memory deficits, increased anxiety-like behavior and risk for alcohol use disorders (AUD). Although the alcohol and social stress fields are actively investigating the mechanisms through which these effects occur, significant gaps in our understanding exist, particularly in the intersection of the two fields. This review will highlight the areas of convergence and divergence in the fields of adolescent social stress and ethanol exposure. We will focus on how ethanol exposure or social isolation stress can impact the development of the frontal cortex and lead to lasting behavioral changes in adulthood. We call attention to the need for more mechanistic studies and the inclusion of the evaluation of sex differences in these molecular, structural, and behavioral responses.