Social isolation induces hyperactivity and exploration in aged female mice

Sex in aging matters: exercise and chronic stress differentially impact females and males across the lifespan

Assessing sex as a biological variable is critical to determining the influence of environmental and lifestyle risks and protective factors mediating behavior and neuroplasticity across the lifespan. We investigated sex differences in affective behavior, memory, and hippocampal neurogenesis following short- or long-term exposure to exercise or chronic mild stress in young and aged mice. Male and female mice were assigned control, running, or chronic stress rearing conditions for 1 month (young) or for 15 months (aged), then underwent a behavioral test battery to assess activity, affective behavior, and memory. Stress exposure into late-adulthood increased hyperactivity in both sexes, and enhanced anxiety-like and depressive-like behavior in aged female, but not male, mice. One month of stress or running had no differential effects on behavior in young males and females. Running increased survival of BrdU-labelled hippocampal cells in both young and aged mice, and enhanced spatial memory in aged mice. These findings highlight the importance of considering sex when determining how aging is differently impacted by modifiable lifestyle factors across the lifespan.

Shifts in naturalistic behaviors induced by early social isolation stress are associated with adult binge-like eating in female rats

Binge eating (BE) is a highly pervasive maladaptive coping strategy in response to severe early life stress such as emotional and social neglect. BE is described as repeated episodes of uncontrolled eating and is tightly linked with comorbid mental health concerns. Despite social stressors occurring at a young age, the onset of BE typically does not occur until adulthood providing an interval for potential therapeutic intervention. Currently, our knowledge of longitudinal noninvasive digital biomarkers predictive of BE needs further development. Monitoring longitudinal impacts of adolescent social isolation stress on naturalistic behaviors in rats will enable the identification of noninvasive digital markers of disease progression to predict adult eating strategies. Recognizing adolescent naturalistic behaviors shaped by social stress informs our understanding of the underlying neurocircuits most effected. This study aimed to monitor and identify longitudinal behavioral shifts to enhance predictive capabilities in a rat model of social isolation stress-induced BE. We placed Paired (n = 12) and Socially Isolated (SI, n = 12) female rats in observational home cages weekly for seven weeks to evaluate the effect of SI on 10 naturalistic behaviors. All 10 naturalistic behaviors were simultaneously detected and tracked using Noldus Ethovision XT automated recognition software. Composite phenotypic z-scores were calculated by standardizing all 10 behaviors. When transitioning into adulthood, all rats underwent conventional emotionality testing and were exposed to a Western-like high fat diet (WD, 43% kcal from fat) to evaluate BE. Longitudinal assessments revealed SI-induced shifts in adolescent phenotypic z-scores and that sniffing, unsupported rearing, jumping, and twitching were the most susceptible to SI. SI increased emotionality compared to the Paired controls. Finally, we identified adolescent twitching as a digital biomarker of adult WD consumption. Our findings suggest that home cage monitoring can detect disrupted naturalistic behaviors associated with maladaptive coping.

Abnormal hippocampal neurogenesis and impaired social recognition memory in two neurodevelopmental models of schizophrenia

Schizophrenia is a mental disorder characterized by cognitive impairments, specifically deficits in social recognition memory (SRM). Abnormal hippocampal neurogenesis has been implicated in these deficits. Due to the pathogenetic heterogeneity of schizophrenia, studying the hippocampal neurogenesis and SRM in two models with prenatal and postnatal defects could enhance our understanding of the developmental aspects of the biological susceptibility to schizophrenia. Here, we examined SRM and hippocampal neurogenesis in two developmental models of schizophrenia: gestational exposure to methylazoxymethanol acetate (MAM) and postweaning social isolation (SI). Our findings revealed that gestational MAM exposure induced a decay of social memory while postweaning SI led to impaired social memory formation and decay. In both models, we observed a correlation between impaired SRM and reduced number, and abnormal differentiation and less complex morphology of hippocampal neurons. These results indicate that aberrant hippocampal neurogenesis may contribute to the deficits of SRM in both models, and these abnormalities may be a shared underlying pathogenic factor in developmental models of schizophrenia, regardless of prenatal and postnatal pathogenesis.

Effects of social housing conditions on ethanol-induced behavioral sensitization in Swiss mice

RATIONALE

In previous animal model studies, it was shown that drug sensitization is dependent upon physical environmental conditions. However, the effects of social housing conditions on drug sensitization is much less known.

OBJECTIVE

The aim of the present study was to investigate the effects of social conditions, through the size of housing groups, on ethanol stimulant effects and ethanol-induced behavioral sensitization in mice.

MATERIALS AND METHODS

Male and female Swiss mice were housed in groups of different sizes (isolated mice, two mice per cage, four mice per cage and eight mice per cage) during a six-week period. A standard paradigm of ethanol-induced locomotor sensitization was then started with one daily injection of 2.5 g/kg ethanol for 8 consecutive days.

RESULTS

The results show that social housing conditions affect the acute stimulant effects of ethanol. The highest stimulant effects were observed in socially isolated mice and then gradually decreased as the size of the group increased. Although the rate of ethanol sensitization did not differ between groups, the ultimate sensitized levels of ethanol-induced stimulant effects were significantly reduced in mice housed in groups of eight.

CONCLUSIONS

These results are consistent with the idea that higher levels of acute and sensitized ethanol stimulant effects are observed in mice housed in stressful housing conditions, such as social isolation.

Antidepressant-like Effect of the Eburnamine-Type Molecule Vindeburnol in Rat and Mouse Models of Ultrasound-Induced Depression

Vindeburnol (VIND, RU24722, BC19), a synthetic molecule derived from the eburnamine-vincamine alkaloid group, has many neuropsychopharmacological effects, but its antidepressant-like effects are poorly understood and have only been described in a few patents. To reliably estimate this effect, vindeburnol was studied in a model of long-term variable-frequency ultrasound (US) exposure at 20-45 kHz in male Wistar rats and BALB/c mice. Vindeburnol was administered chronically for 21 days against a background of simultaneous ultrasound exposure at a dose of 20 mg/kg intraperitoneally (IP). Using four behavioral tests, the sucrose preference test (SPT), the social interaction test (SIT), the open field test (OFT), and the forced swimming test (FST), we found that the treatment with the compound diminished depression-like symptoms in mice and rats. The compound restored the ultrasound-related reduced sucrose consumption to control levels and increased social interaction time in mice and rats compared with those in ultrasound-exposed animals. Vindeburnol showed contraversive results of horizontal and vertical activity in both species and generally did not increase locomotor activity. At the same time, the compound showed a specific effect in the FST, significantly reducing the immobility time. Moreover, we found an increase in norepinephrine, dopamine, and its metabolite levels in the brainstem, as well as an increase in dopamine, 3-methoxytyramine, and 3,4-dihydroxyphenylacetic acid levels in the striatum. We also observed a statistically significant increase in tyrosine hydroxylase (TH) levels in the region containing the locus coeruleus (LC). We suggest that using its distinct chemical structure and pharmacological activity as a starting point could boost antidepressant drug discovery.

Understanding the relationship between loneliness, substance use traits and psychiatric disorders: A genetically informed approach

Loneliness is a common, yet distressing experience associated with adverse outcomes including substance use problems and psychiatric disorders. To what extent these associations reflect genetic correlations and causal relationships is currently unclear. We applied Genomic Structural Equation Modelling (GSEM) to dissect the genetic architecture between loneliness and psychiatric-behavioural traits. Included were summary statistics from 12 genome-wide association analyses, including loneliness and 11 psychiatric phenotypes (range N: 9,537 - 807,553). We first modelled latent genetic factors amongst the psychiatric traits to then investigate potential causal effects between loneliness and the identified latent factors, using multivariate genome-wide association analyses and bidirectional Mendelian randomization. We identified three latent genetic factors, encompassing neurodevelopmental/mood conditions, substance use traits and disorders with psychotic features. GSEM provided evidence of a unique association between loneliness and the neurodevelopmental/mood conditions latent factor. Mendelian randomization results were suggestive of bidirectional causal effects between loneliness and the neurodevelopmental/mood conditions factor. These results imply that a genetic predisposition to loneliness may elevate the risk of neurodevelopmental/mood conditions, and vice versa. However, results may reflect the difficulty of distiguishing between loneliness and neurodevelopmental/mood conditions, which present in similar ways. We suggest, overall, the importance of addressing loneliness in mental health prevention and policy.

Social determinants of health and disease in companion dogs: a cohort study from the Dog Aging Project

Exposure to social environmental adversity is associated with health and survival across many social species, including humans. However, little is known about how these health and mortality effects vary across the lifespan and may be differentially impacted by various components of the environment. Here, we leveraged a relatively new and powerful model for human aging, the companion dog, to investigate which components of the social environment are associated with dog health and how these associations vary across the lifespan. We drew on comprehensive survey data collected on 21,410 dogs from the Dog Aging Project and identified five factors that together explained 33.7% of the variation in a dog's social environment. Factors capturing financial and household adversity were associated with poorer health and lower physical mobility in companion dogs, while factors that captured social support, such as living with other dogs, were associated with better health when controlling for dog age and weight. Notably, the effects of each environmental component were not equal: the effect of social support was 5× stronger than financial factors. The strength of these associations depended on the age of the dog, including a stronger relationship between the owner's age and the dog's health in younger as compared to older dogs. Taken together, these findings suggest the importance of income, stability and owner's age on owner-reported health outcomes in companion dogs and point to potential behavioral and/or environmental modifiers that can be used to promote healthy aging across species.

The impact of loneliness and social isolation on the development of cognitive decline and Alzheimer's Disease

Alzheimer's Disease (AD) is the leading cause of dementia, observed at a higher incidence in women compared with men. Treatments aimed at improving pathology in AD remain ineffective to stop disease progression. This makes the detection of the early intervention strategies to reduce future disease risk extremely important. Isolation and loneliness have been identified among the major risk factors for AD. The increasing prevalence of both loneliness and AD emphasizes the urgent need to understand this association to inform treatment. Here we present a comprehensive review of both clinical and preclinical studies that investigated loneliness and social isolation as risk factors for AD. We discuss that understanding the mechanisms of how loneliness exacerbates cognitive impairment and AD with a focus on sex differences will shed the light for the underlying mechanisms regarding loneliness as a risk factor for AD and to develop effective prevention or treatment strategies.

Influencing cognitive performance via social interactions: a novel therapeutic approach for brain disorders based on neuroanatomical mapping?

Many psychiatric and neurological disorders present deficits in both the social and cognitive domain. In this perspectives article, we provide an overview and the potential of the existence of an extensive neurobiological substrate underlying the close relationship between these two domains. By mapping the rodent brain regions involved in the social and/or cognitive domain, we show that the vast majority of brain regions involved in the cognitive domain are also involved in the social domain. The identified neuroanatomical overlap has an evolutionary basis, as complex social behavior requires cognitive skills, and aligns with the reported functional interactions of processes underlying cognitive and social performance. Based on the neuroanatomical mapping, recent (pre-)clinical findings, and the evolutionary perspective, we emphasize that the social domain requires more focus as an important treatment target and/or biomarker, especially considering the presently limited treatment strategies for these disorders.

The Impact of Social Isolation and Environmental Deprivation on Blood Pressure and Depression-Like Behavior in Young Male and Female Mice

Background: Social isolation (SI) and loneliness are major adult and adolescent health concerns, particularly in the coronavirus disease 2019 (COVID-19) era. Recent prospective cohort studies indicate that older women who experienced both SI and loneliness had a significantly higher risk of cardiovascular disease (CVD). Hypertension, a well-established risk factor for CVD, is more prevalent in elderly women than men. Furthermore, a lack of social relationships is strongly associated with an increased risk of hypertension in middle-aged and elderly women compared to men. Although this has not been extensively studied, adolescents and young adults who experience loneliness or SI may also be at risk for CVD and depression. The purpose of this study was to examine the effect of SI on blood pressure and depression-like behavior in young male and female mice. Methods: Weaned C57BL/6 mice were randomly assigned (n  =  6/group/sex) to either group housing (GH) or SI. Animals in the SI group were housed in individual cages for 8 weeks with no view of other animals. The cages were kept in ventilated racks to prevent pheromone exposure and socially isolated animals had no cage enrichment. Results: SI increased systolic, diastolic, and mean arterial blood pressure in females and elevated heart rate in both sexes. Body weight gain was dramatically increased in socially isolated females but tended to decrease in socially isolated males. In the forced swim test, which detects depression-like behavior, there was no difference between groups in total immobility time. The latency to immobility, however, was significantly decreased in socially isolated females. Serum concentrations of corticosterone and metanephrine did not differ between socially isolated and group-housed females, but corticosterone levels were significantly reduced in socially isolated males. Conclusions: Our results indicate that 8 weeks of SI leads to significant changes in blood pressure and heart rate and mild changes in depression-like behavior in young mice, with females affected more than males.

Age-dependent effects of social isolation on mesolimbic dopamine release

In humans, social isolation is a known risk factor for disorders such as substance use disorder and depression. In rodents, social isolation is a commonly used environmental manipulation that increases the occurrence of behaviors related to these disorders. Age is thought to influence the effects of social isolation, but this predictive relationship is not well-understood. The present study aimed to determine the effects of social isolation on mesolimbic dopamine release at different developmental age points in mice. The experimental ages and their corresponding comparison to human age stages are as follows: 1 month = adolescence, 4 months = mature adulthood, 12 months = middle adulthood, and 18 months = older adult. Mice were socially isolated for 6 weeks during these developmental stages, then in vivo fixed potential amperometry with recording electrodes in the nucleus accumbens was used to measure stimulation-evoked dopamine release, the synaptic half-life of dopamine, dopamine autoreceptor functioning, and the dopaminergic response to cocaine. Isolation altered dopamine functioning in an age-dependent manner. Specifically, isolation increased dopamine release in the adult ages, but not adolescence, potentially due to increased inhibitory effects of dopamine autoreceptors following adolescent social isolation. Regarding the cocaine challenge, isolation increased dopaminergic responses to cocaine in adolescent mice, but not the adult mice. These findings have implications for clinical and experimental settings. Elucidating the relationship between age, social isolation, and neurochemical changes associated with substance use disorder and depression may lead to improvements in preventing and treating these disorders.

PEA-OXA ameliorates allodynia, neuropsychiatric and adipose tissue remodeling induced by social isolation

Chronic social isolation generates a persistent state of stress associated with obesity along with some neuro-endocrine disorders and central behavioral sequelae (eg anxiety, depression, aggression, and allodynia). In this study, we evaluated the effect of social isolation on body weight, depressive- and anxious-aggressive-like behavior, as well as on phenotypic changes of adipocytes from visceral adipose tissue of control (group-housed) or socially isolated (single-housed) male mice. The effect of treatment with pentadecyl-2-oxazoline (PEA-OXA), a natural alpha2 antagonist and histamine H3 protean partial agonist, on these alterations was also evaluated. Single or group-housed mice treated with vehicle or PEA-OXA underwent body weight, mechanical allodynia, anxious-, depressive- and aggressive-like behavior measurements. Proliferation rate, apoptosis, senescence, expression of fat lineage genes, lipid droplets and proinflammatory cytokines were measured on white adipose tissue adipocytes from group- or single-housed mice. Single housed mice developed weight gain, mechanical allodynia at the von Frey test, aggressiveness in the resident intruder test, depression- and anxiety-like behavior in the tail suspension and hole drop tests, respectively. Single housed mice receiving PEA-OXA showed a general resolution of both, physical-metabolic and behavioral alterations associated with social isolation. Furthermore, adipocytes from the adipose tissue of socially isolated mice showed an evident inflamed phenotype (i.e. a reduced rate of proliferation, apoptosis, senescence, and ROS hyper-production together with an increased expression of IL-1β, IL-10, IL-17, and TNF-α and a decrease of IL-6). The treatment with PEA-OXA on adipocytes from single housed mice produced a protective/anti-inflammatory phenotype with an increased expression of brown adipose tissue biomarker. This study confirms that persistent stress caused by social isolation predisposes to obesity and neuropsychiatric disorders. PEA-OXA, through its multi-target activity on alpha2 adrenoceptor and histamine H3 receptors, which have recently aroused great interest in the neuropsychiatric field, reduces weight gain, systemic pro-inflammatory state, allodynia, and affective disorders associated with social isolation.

Voluntary Wheel Running in Old C57BL/6 Mice Reduces Age-Related Inflammation in the Colon but Not in the Brain

Inflammation is considered a possible cause of cognitive decline during aging. This study investigates the influence of physical activity and social isolation in old mice on their cognitive functions and inflammation. The Barnes maze task was performed to assess spatial learning and memory in 3, 9, 15, 24, and 28 months old male C57BL/6 mice as well as following voluntary wheel running (VWR) and social isolation (SI) in 20 months old mice. Inflammatory gene expression was analyzed in hippocampal and colonic samples by qPCR. Cognitive decline occurs in mice between 15 and 24 months of age. VWR improved cognitive functions while SI had negative effects. Expression of inflammatory markers changed during aging in the hippocampus (Il1a/Il6/S100b/Iba1/Adgre1/Cd68/Itgam) and colon (Tnf/Il6/Il1ra/P2rx7). VWR attenuates inflammaging specifically in the colon (Ifng/Il10/Ccl2/S100b/Iba1), while SI regulates intestinal Il1b and Gfap. Inflammatory markers in the hippocampus were not altered following VWR and SI. The main finding of our study is that both the hippocampus and colon exhibit an increase in inflammatory markers during aging, and that voluntary wheel running in old age exclusively attenuates intestinal inflammation. Based on the existence of the gut-brain axis, our results extend therapeutic approaches preserving cognitive functions in the elderly to the colon.