Effects of social isolation on glucocorticoid regulation in social mammals

Effects of social environments on male primate HPG and HPA axis developmental programming

Developmental plasticity is particularly important for humans and other primates because of our extended period of growth and maturation, during which our phenotypes adaptively respond to environmental cues. The hypothalamus-pituitary-gonadal (HPG) and hypothalamus-pituitary-adrenal (HPA) axes are likely to be principal targets of developmental "programming" given their roles in coordinating fitness-relevant aspects of the phenotype, including sexual development, adult reproductive and social strategies, and internal responses to the external environment. In social animals, including humans, the social environment is believed to be an important source of cues to which these axes may adaptively respond. The effects of early social environments on the HPA axis have been widely studied in humans, and to some extent, in other primates, but there are still major gaps in knowledge specifically relating to males. There has also been relatively little research examining the role that social environments play in developmental programming of the HPG axis or the HPA/HPG interface, and what does exist disproportionately focuses on females. These topics are likely understudied in males in part due to the difficulty of identifying developmental milestones in males relative to females and the general quiescence of the HPG axis prior to maturation. However, there are clear indicators that early life social environments matter for both sexes. In this review, we examine what is known about the impact of social environments on HPG and HPA axis programming during male development in humans and nonhuman primates, including the role that epigenetic mechanisms may play in this programming. We conclude by highlighting important next steps in this research area.

Association of social health with all-cause mortality and cause-specific mortality: A population-based cohort study

BACKGROUND

Previous studies only focused on the individual social factors, without considering the overall social health patterns. The present study aimed to develop an integrated social health score (SHS) and investigate its associations with all-cause, cardiovascular disease (CVD), cancer mortality.

METHODS

A total of 330,716 participants (mean age 56.3 years; 52.4 % female) from UK Biobank was included between 2006 and 2010, and thereafter followed up to 2021. SHS was calculated by using information on social connections, social engagement and social support. Cox proportional hazards models was used to estimate the hazard ratios and 95 % confidence intervals (CIs) of the association between SHS and all-cause and cause-specific mortality and the 4-way decomposition was used to quantify the mediating effect of lifestyle factors.

RESULTS

During a median follow-up period of 12.4 years, 37,897 death cases were recorded, including 4347 CVD and 10,380 cancer cases. The SHS was inversely associated with the risks of all-cause, CVD and cancer mortality in a dose-dependent manner (P for trend <0.001). The association between SHS with all-cause mortality was mediated by lifestyle factors including diet score, smoking status and alcohol consumption.

CONCLUSION

Integrated SHS was inversely associated with risks of all-cause, CVD and cancer mortality, and the associations were partially mediated by lifestyle factors. Our study highlights the importance of maintaining high levels of social health by jointly enhancing social involvement, expanding social networks, and cultivating enduring intimate relationships across the life course.

Neuroimmune modulation by tryptophan derivatives in neurological and inflammatory disorders

The nervous and immune systems are highly developed, and each performs specialized physiological functions. However, they work together, and their dysfunction is associated with various diseases. Specialized molecules, such as neurotransmitters, cytokines, and more general metabolites, are essential for the appropriate regulation of both systems. Tryptophan, an essential amino acid, is converted into functional molecules such as serotonin and kynurenine, both of which play important roles in the nervous and immune systems. The role of kynurenine metabolites in neurodegenerative and psychiatric diseases has recently received particular attention. Recently, we found that hyperactivity of the kynurenine pathway is a critical risk factor for septic shock. In this review, we first outline neuroimmune interactions and tryptophan derivatives and then summarized the changes in tryptophan metabolism in neurological disorders. Finally, we discuss the potential of tryptophan derivatives as therapeutic targets for neuroimmune 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.

The role of sociability in social instability stress: Behavioral, neuroendocrine and monoaminergic effects

Extensive literature has reported a link between social stress and mental health. In this complex relationship, individual strategies for coping with social stress are thought to have a possible modulating effect, with sociability being a key factor. Despite the higher incidence of affective disorders in females and sex-related neurochemical differences, female populations have been understudied. The aim of the present study was, therefore, to analyze the behavioral, neuroendocrine, and neurochemical effects of stress in female OF1 mice, paying special attention to social connectedness (female mice with high vs low sociability). To this end, subjects were exposed to the Chronic Social Instability Stress (CSIS) model for four weeks. Although female mice exposed to CSIS had increased arousal, there was no evidence of depressive-like behavior. Neither did exposure to CSIS affect corticosterone levels, although it did increase the MR/GR ratio by decreasing GR expression. Female mice exposed to CSIS had higher noradrenaline and dopamine levels in the hippocampus and striatum respectively, with a lower monoaminergic turnover, resulting in an increased arousal. CSIS increased serotonin levels in both the hippocampus and striatum. Similarly, CSIS was found to reduce kynurenic acid, 3-HK, and IDO and iNOS enzyme levels in the hippocampus. Interestingly, the observed decrease in IDO synthesis and the increased serotonin and dopamine levels in the striatum were only found in subjects with high sociability. These highly sociable female mice also had significantly lower levels of noradrenaline in the striatum after CSIS application. Overall, our model has produced neuroendocrine and neurochemical but not behavioral changes, so it has not allowed us to study sociability in depth. Therefore, a model that induces both molecular and behavioral phenotypes should be applied to determine the role of sociability.

Social isolation and loneliness: Undervalued risk factors for disease states and mortality

Social isolation and loneliness are two common but undervalued conditions associated with a poor quality of life, decreased overall health and mortality. In this review, we aim to discuss the health consequences of social isolation and loneliness. We first provide the potential causes of these two conditions. Then, we explain the pathophysiological processes underlying the effects of social isolation and loneliness in disease states. Afterwards, we explain the important associations between these conditions and different non-communicable diseases, as well as the impact of social isolation and loneliness on health-related behaviours. Finally, we discuss the current and novel potential management strategies for these conditions. Healthcare professionals who attend to socially isolated and/or lonely patients should be fully competent in these conditions and assess their patients thoroughly to detect and properly understand the effects of isolation and loneliness. Patients should be offered education and treatment alternatives through shared decision-making. Future studies are needed to understand the underlying mechanisms better and to improve the treatment strategies for both social isolation and loneliness.

A systematic review and meta-analysis of 90 cohort studies of social isolation, loneliness and mortality

The associations between social isolation, loneliness and the risk of mortality from all causes, cardiovascular disease (CVD) and cancer are controversial. We systematically reviewed prospective studies on the association between social isolation, loneliness and mortality outcomes in adults aged 18 years or older, as well as studies on these relationships in individuals with CVD or cancer, and conducted a meta-analysis. The study protocol was registered with PROSPERO (reg. no. CRD42022299959). A total of 90 prospective cohort studies including 2,205,199 individuals were included. Here we show that, in the general population, both social isolation and loneliness were significantly associated with an increased risk of all-cause mortality (pooled effect size for social isolation, 1.32; 95% confidence interval (CI), 1.26 to 1.39; P < 0.001; pooled effect size for loneliness, 1.14; 95% CI, 1.08 to 1.20; P < 0.001) and cancer mortality (pooled effect size for social isolation, 1.24; 95% CI, 1.19 to 1.28; P < 0.001; pooled effect size for loneliness, 1.09; 95% CI, 1.01 to 1.17; P = 0.030). Social isolation also increased the risk of CVD mortality (1.34; 95% CI, 1.25 to 1.44; P < 0.001). There was an increased risk of all-cause mortality in socially isolated individuals with CVD (1.28; 95% CI, 1.10 to 1.48; P = 0.001) or breast cancer (1.51; 95% CI, 1.34 to 1.70; P < 0.001), and individuals with breast cancer had a higher cancer-specific mortality owing to social isolation (1.33; 95% CI, 1.02 to 1.75; P = 0.038). Greater focus on social isolation and loneliness may help improve people's well-being and mortality risk.

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.

Predictors of Psychological Strain and Allostatic Load in Teachers: Examining the Long-Term Effects of Biopsychosocial Risk and Protective Factors Using a LASSO Regression Approach

Teacher stress significantly challenges teachers' health, teaching quality, and students' motivation and achievement. Thus, it is crucial to identify factors that effectively prevent it. Using a LASSO regression approach, we examined which factors predict teachers' psychological strain and allostatic load over two years. The study included 42 teachers (28 female, Mage = 39.66, SD = 11.99) and three measurement time points: At baseline, we assessed teachers' (a) self-reports (i.e., on personality, coping styles, and psychological strain), (b) behavioral data (i.e., videotaped lessons), and (c) allostatic load (i.e., body mass index, blood pressure, and hair cortisol concentration). At 1- and 2-year follow-ups, psychological strain and allostatic load biomarkers were reassessed. Neuroticism and perceived student disruptions at baseline emerged as the most significant risk factors regarding teachers' psychological strain two years later, while a positive core self-evaluation was the most important protective factor. Perceived support from other teachers and the school administration as well as adaptive coping styles were protective factors against allostatic load after two years. The findings suggest that teachers' psychological strain and allostatic load do not primarily originate from objective classroom conditions but are attributable to teachers' idiosyncratic perception of this environment through the lens of personality and coping strategies.

Oxytocin prevents dysregulation of the acute stress response and glucocorticoid-induced oxidative stress in chronically isolated prairie voles

Chronic social isolation can lead to dysregulation of many physiological and psychological processes, including the ability to respond to acute stressors. Previous work in our lab reported that six weeks of social isolation in prairie voles (Microtus ochrogaster) caused increased glucocorticoid levels, oxidative damage, telomere degradation and anhedonia, and that oxytocin treatment prevented all of these changes. Following these results, we investigated how chronic social isolation with and without oxytocin treatment affected glucocorticoid (CORT) and oxidative stress responses to an acute stressor, a 5-min resident-intruder (R-I) test at the end of the social isolation period. To investigate the effect of a brief acute stressor on CORT and oxidative stress, baseline blood samples were collected following six weeks of social isolation, 24-hrs before the R-I test. Two more blood samples were collected 15-min after the end of the R-I test, and again 25-min later to measure peak and recovery responses, respectively. Isolated animals had higher baseline, peak, recovery, and integrated levels of CORT and reactive oxygen metabolites (ROMs, a measure of oxidative stress), compared to animals that did not experience isolation. Importantly, oxytocin treatment throughout the isolation period prevented these elevations in CORT and ROMs. No significant changes were observed in total antioxidant capacity (TAC). Levels of CORT and ROMs at the peak and recovery time points were positively correlated. These data show that acute stress in chronically isolated prairie voles, then, is associated with increased glucocorticoid-induced oxidative stress (GiOS), and that oxytocin mitigates isolation-induced dysregulation of glucocorticoid and oxidative stress acute stress responses.

Chronic Early-Life Social Isolation Enhances Spatial Memory in Male and Female Rats

Social adversity during childhood and adolescence can alter brain development in ways that may increase the likelihood of many prominent mental illnesses. To determine the underlying mechanisms, several animal models have been developed, such as Chronic Early-Life Social Isolation (CELSI), which sees rats isolated for several weeks after weaning. Although such a paradigm does cause many consistent changes in adult behaviour, one area where uncertainty exists concerns its effect upon hippocampal-dependent learning and memory. To help sort out how CELSI affects spatial learning and memory, male and female siblings from 15 Sprague-Dawley rat litters were stratified by sex and then randomly assigned to either group-housing (3 animals/cage), or social isolation (1 animal/cage) for 7 weeks. Spatial learning and memory were then tested over 5 days using the Morris water maze. Next, the animals were euthanised, and stress-sensitive biometrics, including serum corticosterone levels, were collected. Lastly, to determine whether CELSI affected neural cell density, the expression of key neuronal and glial proteins (such as PSD-95 and GFAP, respectively) was assessed in isolated hippocampal tissue using immunoblotting. Notably, both male and female rats that had experienced post-weaning social isolation displayed stronger spatial learning and memory abilities than their group-housed counterparts. As well, socially isolated male rats exhibited a clear increase in expression of PSD-95. However, housing condition did not seem to affect either stress-sensitive biometrics, or hippocampal GFAP expression. Our results support the possibility that CELSI may enhance some aspects of hippocampal-dependent behaviour in a fashion similar among male and female rats.

Persistence of social isolation and mortality: 10-year follow-up of the Guangzhou Biobank Cohort study

Although social isolation has been associated with higher mortality risk, the associations of persistence of social isolation with mortality are unclear. We investigated the prospective associations of intermittent and persistent social isolation with all-cause and cause-specific mortality, considering the social contact types (face-to-face and non-face-to-face). 30,518 participants were recruited in 2003-2008 initially and 18,104 participants with re-assessed social isolation information in 2008-2012 were followed up to Dec 2019 in Guangzhou Biobank Cohort Study (GBCS). During an average of 9.7 years of follow-up, 2,119 deaths occurred. The isolation at baseline survey, isolation at second survey and persistent isolation were positively associated with all-cause mortality in the minimal adjusted model (adjusted hazard ratio (AHR) =1.24, 95% CI 1.12-1.38, 1.11, 1.00-1.23 and 1.23, 1.05-1.43, respectively). Totally 47.2% of the risk was explained by health status, SEP, and biological, behavioural and psychological factors. Persistent isolation from face-to-face with co-inhabitants, versus no isolation, was associated with higher risks of all-cause (HR=1.40, 1.09-1.81) and CVD (subdistribution hazard ratio (SHR)=1.92, 1.31-2.81) mortality in fully adjusted model. Our study showed that intermittent and persistent isolation were generally associated with higher risks of mortality, and the risks were even higher in those with persistent face-to-face isolation with co-inhabitants.

Social isolation produces a sex- and brain region-specific alteration of microglia state

Social isolation is a profound form of psychological stress that impacts the mental health of a large proportion of society. Other experimental models of stress have demonstrated a microglia response that serves either a protective or pathological function. However, the effect of adult social isolation on microglia has not been thoroughly investigated. We measured microglia territory, branching, end points and phagocytic-lysosomal activity in group housed C57Bl/6 mice and mice that were socially isolated for 2 weeks. Our results show that the dorsomedial hypothalamus and hippocampal CA2 region of adult male mice undergo increased microglia volume, territory and endpoints following social isolation, whereas females exhibit this increase in the hypothalamus only. Males exhibited decreases in the phagocytic-lysosomal marker CD68 in microglia in these regions, whereas females showed an increase in CD68 in the hypothalamus suggesting sexually dimorphic and brain region-specific change in microglia state in response to social isolation. The prefrontal cortex, central amygdala, nucleus accumbens shell and visual cortex did not exhibit changes in microglia structure in either male or female mice. These data show that microglia in different brain regions undergo a distinct response to social isolation which may account for changes in cognition and behaviour associated with this prevalent form of psychological stress.

Group or individual housing does not reduce socio-sexual and reproductive responses in anestrous goats during the first contact with the photo-stimulated buck

Herein, we examined the effect of the type of housing (individual vs group housing) of anestrus female goats on plasma cortisol concentrations and socio-sexual behaviors during first contact with photo-stimulated male goats and determined the impact on sexual and reproductive responses after 15 d of contact with photo-stimulated males. Therefore, after weaning, 10 females each were individually and group-housed, respectively. Ten bucks were rendered sexually active by exposure to artificially long days (16 h of light and 8 h of darkness per day) for 2.5 mo. During the naturally increasing photoperiod, 15-mo-old females were exposed and maintained with males. On day 0, during the first contact with males, individually- and group-housed females exhibited similar plasma cortisol concentrations (22.6 vs 27.4 ng/mL, respectively). Likewise, socio-sexual behaviors did not differ between examined groups on day 0. Moreover, the interval from first contact with males to the first estrus, short and normal ovulatory cycles, ovulation, fertility, and prolificacy rates were similar between differently housed females. Furthermore, mounting attempts did not differ between males in contact with either female group. In conclusion, individually- and group-housed anestrus females displayed elevated and similar plasma cortisol concentrations during first contact with photo-stimulated males and similar socio-sexual behaviors and reproductive responses when exposed to photo-stimulated males.

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.

The neural circuits of monogamous behavior

The interest in studying the neural circuits related to mating behavior and mate choice in monogamous species lies in the parallels found between human social structure and sexual behavior and that of other mammals that exhibit social monogamy, potentially expanding our understanding of human neurobiology and its underlying mechanisms. Extensive research has suggested that social monogamy, as opposed to non-monogamy in mammals, is a consequence of the neural encoding of sociosensory information from the sexual partner with an increased reward value. Thus, the reinforced value of the mate outweighs the reward value of mating with any other potential sexual partners. This mechanism reinforces the social relationship of a breeding pair, commonly defined as a pair bond. In addition to accentuated prosocial behaviors toward the partner, other characteristic behaviors may appear, such as territorial and partner guarding, selective aggression toward unfamiliar conspecifics, and biparental care. Concomitantly, social buffering and distress upon partner separation are also observed. The following work intends to overview and compare known neural and functional circuits that are related to mating and sexual behavior in monogamous mammals. We will particularly discuss reports on Cricetid rodents of the Microtus and Peromyscus genus, and New World primates (NWP), such as the Callicebinae subfamily of the titi monkey and the marmoset (Callithrix spp.). In addition, we will mention the main factors that modulate the neural circuits related to social monogamy and how that modulation may reflect phenotypic differences, ultimately creating the widely observed diversity in social behavior.

The effects of social isolation stress and discrimination on mental health

Social isolation and discrimination are growing public health concerns associated with poor physical and mental health. They are risk factors for increased morbidity and mortality and reduced quality of life. Despite their detrimental effects on health, there is a lack of knowledge regarding translation across the domains of experimental research, clinical studies, and real-life applications. Here, we review and synthesize evidence from basic research in animals and humans to clinical translation and interventions. Animal models indicate that social separation stress, particularly in early life, activates the hypothalamic-pituitary-adrenal axis and interacts with monoaminergic, glutamatergic, and GABAergic neurotransmitter systems, inducing long-lasting reductions in serotonin turnover and alterations in dopamine receptor sensitivity. These findings are of particular importance for human social isolation stress, as effects of social isolation stress on the same neurotransmitter systems have been implicated in addictive, psychotic, and affective disorders. Children may be particularly vulnerable due to lasting effects of social isolation and discrimination stress on the developing brain. The effects of social isolation and loneliness are pronounced in the context of social exclusion due to discrimination and racism, during widespread infectious disease related containment strategies such as quarantine, and in older persons due to sociodemographic changes. This highlights the importance of new strategies for social inclusion and outreach, including gender, culture, and socially sensitive telemedicine and digital interventions for mental health care.

Dynamic influences on the neural encoding of social valence

Social signals can serve as potent emotional triggers with powerful impacts on processes from cognition to valence processing. How are social signals dynamically and flexibly associated with positive or negative valence? How do our past social experiences and present social standing shape our motivation to seek or avoid social contact? We discuss a model in which social attributes, social history, social memory, social rank and social isolation can flexibly influence valence assignment to social stimuli, termed here as 'social valence'. We emphasize how the brain encodes each of these four factors and highlight the neural circuits and mechanisms that play a part in the perception of social attributes, social memory and social rank, as well as how these factors affect valence systems associated with social stimuli. We highlight the impact of social isolation, dissecting the neural and behavioural mechanisms that mediate the effects of acute versus prolonged periods of social isolation. Importantly, we discuss conceptual models that may account for the potential shift in valence of social stimuli from positive to negative as the period of isolation extends in time. Collectively, this Review identifies factors that control the formation and attribution of social valence - integrating diverse areas of research and emphasizing their unique contributions to the categorization of social stimuli as positive or negative.

Social Isolation Stress Modulates Pregnancy Outcomes and the Inflammatory Profile of Rat Uterus

Prenatal stressors have been linked to adverse pregnancy outcomes; including preterm birth (PTB). Recent work demonstrates that social isolation in mothers represents a silent stressor contributing to PTB risk. Here; we investigate the association of inflammatory and stress markers with PTB risk in Long–Evans rats exposed to social isolation stress (SIS) during preconception and pregnancy across four generations (F0-F3). Gestational length; blood glucose; corticosterone levels; and maternal and offspring weights were assessed in two SIS paradigms: transgenerational (TG) and multigenerational (MG) exposure. Maternal uterine tissues were collected 21 days after the dams gave birth. Exposure to SIS reduced pregnancy lengths in the parental generation and neonatal birth weights in the F1 and F2 generations. Interleukin (IL)-1β (Il1b) mRNA levels increased in F0 animals but decreased in the offspring of both stress lineages. Protein levels of IL-1β decreased in the TG lineage. Corticotrophin-releasing hormone receptor 1 (Crhr1) expression decreased in SIS-exposed F0 animals and increased in the TG-F2 and MG-F1 offspring. Expression of enzyme 11-β hydroxysteroid dehydrogenase-2 (11bHSD2) was enhanced in F1 animals. These findings suggest SIS has adverse consequences on the F0 mothers; but their F1–F3 progeny may adapt to this chronic stress; thus supporting the fetal programming hypothesis.

Characterization of Longitudinal Testosterone, Cortisol, and Musth in Male Asian Elephants (Elephas maximus), Effects of Aging, and Adrenal Responses to Social Changes and Health Events

Simple Summary

The conservation of endangered species and sustainability of managed populations requires good health and welfare of individuals. Male elephants experience a biological phenomenon called “musth”, characterized by a suite of physiological and behavioral changes which serve to facilitate copulation between the sexes, but can also pose unique challenges under human care. This study characterized longitudinal patterns of circulating testosterone and cortisol in relation to musth in four adult Asian elephant bulls and assessed age-related effects on musth activity and adrenal responses to social changes and clinical health events. All bulls exhibited regular annual musth cycles, and there was no clear evidence of chronically elevated cortisol secretion outside of musth. Circulating cortisol covaried positively with testosterone and musth, highlighting intrinsic patterns that should be considered when evaluating the impact of changes on adrenal glucocorticoid activity. Testosterone decreased with age in sexually mature bulls, whereas age-related changes in cortisol varied across individuals, with the three older bulls showing the greatest rate of change during musth. Changes in cortisol were associated with treatment for Mycobacterium tuberculosis in two bulls, but not with activation of disease. In contrast to physiological factors, there was no evidence that social changes involving male herdmates impacted adrenal glucocorticoid activity in the short term. This study highlights the importance of longitudinal hormone monitoring to track changes in physiological function and identify factors that may influence welfare, which is important for making more informed decisions on how to manage male elephants under varying degrees of human care.

Abstract

The conservation of endangered species and sustainability of managed populations requires considerations to ensure the health and welfare of individuals. Male elephants experience a biological phenomenon called “musth”, which is characterized by increased testosterone production, temporal gland secretion and urine dribbling, heightened aggression and sexual behavior, and therefore can pose unique challenges for human safety and animal welfare. This study characterized longitudinal (9 to 22 years) patterns of circulating testosterone and cortisol in relation to musth in four adult Asian elephant bulls spanning ages from 12 to 54 years. Age-related effects on musth activity and adrenal responses to social changes and clinical health events were also examined. All bulls exhibited regular annual musth cycles. Circulating cortisol covaried positively with testosterone and musth, highlighting intrinsic patterns that should be considered when evaluating the impact of social, health, and environmental changes on adrenal glucocorticoid activity. Except for an end-of-life cortisol increase in one bull, there was no clear evidence of chronically elevated cortisol secretion outside of musth in any individual. Testosterone decreased with age in sexually mature bulls, whereas age-related changes in cortisol varied across individuals, with the three older bulls showing the greatest rate of change during musth versus inter-musth periods. In contrast to physiological factors, there was no evidence of social factors, such as addition of a new male and death of male herdmates, impacting adrenal glucocorticoid activity in these bulls in the short term. Changes in cortisol were associated with treatment for Mycobacterium tuberculosis (M. tb) in two bulls, increasing after start of treatment and decreasing with cessation of treatment, but were not clearly associated with activation of disease. This study highlights the importance of longitudinal hormone monitoring to track changes in physiological function and responses to social, health, and environmental change in elephant bulls, which is important for making more informed decisions on how to manage male elephants under varying degrees of human care to ensure welfare and safety.

The immunomodulatory effects of social isolation in mice are linked to temperature control

Living in isolation is considered an emerging societal problem that negatively affects the physical wellbeing of its sufferers in ways that we are just starting to appreciate. This study investigates the immunomodulatory effects of social isolation in mice, utilising a two-week program of sole cage occupancy followed by the testing of immune-inflammatory resilience to bacterial sepsis. Our results revealed that mice housed in social isolation showed an increased ability to clear bacterial infection compared to control socially housed animals. These effects were associated with specific changes in whole blood gene expression profile and an increased production of classical pro-inflammatory cytokines. Interestingly, equipping socially isolated mice with artificial nests as a substitute for their natural huddling behaviour reversed the increased resistance to bacterial sepsis. Together these results suggest that the control of body temperature through social housing and huddling behaviour are important factors in the regulation of the host immune response to infection in mice and might provide another example of the many ways by which living conditions influence immunity.

Early Life Exposure to a Diet With a Supramolecular Lipid Structure Close to That of Mammalian Milk Improves Early Life Growth, Skeletal Development, and Later Life Neurocognitive Function in Individually and Socially Housed Male C57BL/6J Mice

Breastfeeding (duration) can be positively associated with infant growth outcomes as well as improved cognitive functions during childhood and later life stages. (Prolonged) exposure to optimal lipid quantity and quality, i.e., the supramolecular structure of lipids, in mammalian milk, may contribute to these beneficial effects through nutritional early-life programming. In this pre-clinical study, we exposed male C57BL/6J mice from post-natal Days 16 to 42 (i.e., directly following normal lactation), to a diet with large lipid droplets coated with bovine milk fat globule membrane-derived phospholipids, which mimic more closely the supramolecular structure of lipid droplets in mammalian milk. We investigated whether exposure to this diet could affect growth and brain development-related parameters. As these outcomes are also known to be affected by the post-weaning social environment in mice, we included both individually housed and pair-wise housed animals and studied whether effects of diet were modulated by the social environment. After Day 42, all the animals were fed standard semi-synthetic rodent diet. Growth and body composition were assessed, and the mice were subjected to various behavioral tests. Individual housing attenuated adolescent growth, reduced femur length, and increased body fat mass. Adult social interest was increased due to individual housing, while cognitive and behavioral alterations as a result of different housing conditions were modest. The diet increased adolescent growth and femur length, increased lean body mass, reduced adolescent anxiety, and improved adult cognitive performance. These effects of diet exposure were comparable between individually and socially housed mice. Hence, early life exposure to a diet with lipid droplets that mimic the supramolecular structure of those in mammalian milk may improve adolescent growth and alters brain function in both socially and individually housed mice. These findings suggest that lipid structure in infant milk formula may be a relevant target for nutritional solutions, targeting both healthy infants and infants facing growth challenges.

Pre-pandemic Psychobiological Features Predict Impact of COVID-19 Confinement on Loneliness

The coronavirus disease 2019 (COVID-19) pandemic led to various government-imposed limitations on social interaction and strict home confinement. Such involuntary social-distancing policies can exacerbate feelings of loneliness and alter emotional well-being. Dysregulation of the hypothalamic-pituitary-adrenocortical (HPA) axis is a potential mechanism for loneliness' deleterious health effects. In this study, we explored whether pre-pandemic diurnal cortisol output (AUC G ), a measure of HPA axis function, may predict the propensity to changes in loneliness during long-term COVID-19 home confinement and if extraversion would moderate this relationship. This association has been explored by analysing the impact of COVID-19 pandemic and strict home confinement on social and emotional loneliness in 45 Spanish young adults. Diurnal cortisol levels were measured from five saliva samples obtained across a day just before the pandemic, and data about participants' perceived loneliness, empathic state, extraversion, and prospective volunteering were obtained both before and during the confinement. Participants' social and family loneliness increased during long-term strict home confinement, while prospective volunteering tendencies and extraversion decreased. Importantly, after adjusting for relevant confounders, moderation analyses revealed that in young adults with high pre-pandemic extraversion, a higher AUC G predicted a larger increase in social loneliness during confinement, while in individuals with low extraversion, AUC G was negatively related to change in loneliness. Our findings highlight the utility of pre-pandemic diurnal cortisol output in predicting the social impact of COVID-19 home confinement, presenting this hormone as a potential biomarker for a priori identification of at-risk groups during public health crises.

Approach motivation and loneliness: Individual differences and parasympathetic activity

Loneliness, or perceived social isolation, is linked to a number of negative long‐term effects on both mental and physical health. However, how an individual responds to feeling lonely may influence their risk for later negative health outcomes. Here, we sought to clarify what influences variability in individuals' motivated responses to loneliness. Specifically, we assessed whether resting parasympathetic activity, a physiological marker linked to flexible adaptation, facilitates increased approach‐oriented behaviors. Seventy‐four adult participants underwent a conditioning paradigm assessing how they approach and avoid rewards and threats. Individuals with higher levels of loneliness and high resting parasympathetic activity were more likely to demonstrate approach behaviors. We discuss these findings in terms of the role resting parasympathetic activity may play in facilitating adaptive responses to feeling socially isolated.

How an individual responds to feeling lonely may influence their risk for later negative health outcomes. The current study provides new insight into what may influence variability in responses to feeling lonely. Individuals with higher resting parasympathetic activity and high levels of loneliness demonstrate increased approach‐oriented behaviors. This suggests loneliness increases approach motivations but only in the presence of other markers of adaptive responding, like high resting parasympathetic activity.

Temporal associations among loneliness, anxiety, and depression during the COVID-19 pandemic period

Numerous studies have reported that individuals' loneliness, anxiety, and depression levels increased during the COVID-19 pandemic period. However, reciprocal associations among loneliness, anxiety, and depression, as well as gender differences in these associations, have not been investigated. Therefore, temporal associations among loneliness, anxiety, and depression and gender differences in these associations were examined in a longitudinal study during the COVID-19 pandemic period. The loneliness, anxiety, and depression levels of 458 university students were evaluated at three timepoints (T1, T2, and T3) during the COVID-19 pandemic period in China. The timepoints were separated by 1 month. Cross-lagged panel designs were used to examine reciprocal associations among loneliness, anxiety, and depression as well as the stability and gender differences of these associations. Cross-lagged panel analysis revealed that T1 depression positively predicted T2 anxiety and loneliness, T1 loneliness positively predicted T2 depression, T2 anxiety positively predicted T3 depression, T2 depression positively predicted T3 anxiety and loneliness, T2 loneliness positively predicted T3 depression, and T1 loneliness positively predicted T3 anxiety through the mediating role of T2 depression. No gender differences were observed in the cross-lagged associations. During the COVID-19 pandemic period, loneliness and depression predicted each other across time, and loneliness predicted anxiety across time, mediated by depression. No gender differences were observed in the cross-lagged associations among loneliness, anxiety, and depression.

Successive approximation of horses to their first work on a treadmill: The effect of previous loading into a trailer

A horse learning about the entrance to narrow, cage-shaped places may be challenging both for the horses as well as for the owners. For some behaviors, such as loading into a trailer or climbing onto a treadmill, the final behavioral goal can be achieved by working towards it in stages. This study compared the successive approximation of horses to their first work on a treadmill with horses hardly ever loaded (HE L) and regularly loaded (R L) into a trailer. Fourteen horses were divided into two groups (HE L n = 7 and R L n = 7) based on their experiences of entering into a trailer. All horses were taught using four stages of successive approximation. The average lead time was longer in the HE L than in the R L group, both in the first (HE L: 33.8 ± 12.4 s; R L: 17.6 ± 12.9 s; p = 0.035) and last stages (HE L: 12.0 ± 10.3 s; R L: 3.7 ± 1.0 s; p = 0.032) of trials. With the subsequent repetitions of each step, the heart rate decreased in both groups. Very few behaviors indicating fear or unwillingness ("rearing," "sideways," and "backwards") were observed. Horses that were regularly loaded exhibited signs of relaxation. The successive approximation of horses to the first work on a treadmill differed and may depend on the previous experiences with loading and travelling in the confined space of a trailer.

Post-weaning Social Isolation in Male and Female Prairie Voles: Impacts on Central and Peripheral Immune System

The socially monogamous prairie vole (Microtus ochrogaster) offers a unique opportunity to examine the impacts of adolescent social isolation on the brain, immune system, and behavior. In the current study, male and female prairie voles were randomly assigned to be housed alone or with a same-sex cagemate after weaning (i.e., on postnatal day 21-22) for a 6-week period. Thereafter, subjects were tested for anxiety-like and depressive-like behaviors using the elevated plus maze (EPM) and Forced Swim Test (FST), respectively. Blood was collected to measure peripheral cytokine levels, and brain tissue was processed for microglial density in various brain regions, including the Nucleus Accumbens (NAcc), Medial Amygdala (MeA), Central Amygdala (CeA), Bed Nucleus of the Stria Terminalis (BNST), and Paraventricular Nucleus of the Hypothalamus (PVN). Sex differences were found in EPM and FST behaviors, where male voles had significantly lower total arm entries in the EPM as well as lower latency to immobility in the FST compared to females. A sex by treatment effect was found in peripheral IL-1β levels, where isolated males had a lower level of IL-1β compared to cohoused females. Post-weaning social isolation also altered microglial density in a brain region-specific manner. Isolated voles had higher microglial density in the NAcc, MeA, and CeA, but lower microglial density in the dorsal BNST. Cohoused male voles also had higher microglial density in the PVN compared to cohoused females. Taken together, these data suggest that post-weaning social housing environments can alter peripheral and central immune systems in prairie voles, highlighting a potential role for the immune system in shaping isolation-induced alterations to the brain and behavior.

Affective Neuroscience of Loneliness: Potential Mechanisms underlying the Association between Perceived Social Isolation, Health, and Well-Being

Loneliness, or the subjective feeling of social isolation, is an important social determinant of health. Loneliness is associated with poor physical health, including higher rates of cardiovascular disease and dementia, faster cognitive decline, and increased risk of mortality, as well as disruptions in mental health, including higher levels of depression, anxiety, and negative affect. Theoretical accounts suggest loneliness is a complex cognitive and emotional state characterized by increased levels of inflammation and affective disruptions. This review examines affective neuroscience research on social isolation in animals and loneliness in humans to better understand the relationship between perceptions of social isolation and the brain. Loneliness associated increases in inflammation and neural changes consistent with increased sensitivity to social threat and disrupted emotion regulation suggest interventions targeting maladaptive social cognitions may be especially effective. Work in animal models suggests the neural changes associated with social isolation may be reversible. Therefore, ameliorating loneliness may be an actionable social determinant of health target. However, more research is needed to understand how loneliness impacts healthy aging, explore the role of inflammation as a potential mechanism in humans, and determine the best time to deliver interventions to improve physical health, mental health, and well-being across a diverse array of populations.

Regulation of stress response on the hypothalamic-pituitary-gonadal axis via gonadotropin-inhibitory hormone

Under stressful condition, reproductive function is impaired due to the activation of various components of the hypothalamic-pituitaryadrenal (HPA) axis, which can suppress the activity of the hypothalamic-pituitary-gonadal (HPG) axis at multiple levels. A hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH) is a key negative regulator of reproduction that governs the HPG axis. Converging lines of evidence have suggested that different stress types and their duration, such as physical or psychological, and acute or chronic, can modulate the GnIH system. To clarify the sensitivity and reactivity of the GnIH system in response to stress, we summarize and critically review the available studies that investigated the effects of various stressors, such as restraint, nutritional/metabolic and social stress, on GnIH expression and/or its neuronal activity leading to altered HPG action. In this review, we focus on GnIH as the potential novel mediator responsible for stress-induced reproductive dysfunction.

Social Isolation Is Associated With Rapid Kidney Function Decline and the Development of Chronic Kidney Diseases in Middle-Aged and Elderly Adults: Findings From the China Health and Retirement Longitudinal Study (CHARLS)

Background: There is limited evidence on the relationship between social isolation and renal outcomes. To address this gap, this study estimated the prospective relationship of social isolation with rapid kidney function decline and the development of chronic kidney disease (CKD) in middle-aged and elderly Chinese with normal kidney function. Methods: We analyzed data from 3,031 participants aged ≥ 45 years with baseline estimated glomerular filtration rates (eGFR) ≥ 60 ml/min/1.73 m². All data were obtained from the 2011 and 2015 waves of the Chinese Longitudinal Study of Health and Retirement (CHARLS). eGFR was estimated based on a combination of serum creatinine and cystatin C. The primary outcome was rapid decline in renal function, as defined by an eGFR decrease of > 5 ml/min/1.73 m² per year, while the secondary outcome was the development of CKD, as defined by an eGFR decrease to a level < 60 ml/min/1.73 m². Results: During the follow-up of 4 years, 258 (8.5%) participants experienced a rapid decline in renal function, while 87 (2.9%) developed CKD. In the fully adjusted model, high social isolation was significantly related to an increased risk of experiencing a rapid decline in renal function (OR 1.805, 95% CI 1.310-2.487) and CKD onset (OR 1.842, 95% CI 1.084-3.129). Among the five components of social isolation, being unmarried, not participating in social activities, and living alone independently predicted declined renal function. Conclusions: Social isolation is significantly associated with the risk of rapid eGFR decline and CKD onset in middle-aged and older adults with normal kidney function in China.

Basic behavioral science research priorities in minority health and health disparities

Achieving health equity among disparity populations has been a national, regional, and local priority for several years. Health promotion and disease prevention behaviors play an important role in achieving health equity; the first generation of behavioral science studies in minority health and health disparities have provided important insights about the nature and distribution of risk exposure behaviors in disparity populations. Interventions have also been developed to enhance health promotion and disease prevention behaviors using behavioral counseling, tailored health communications, and interventions that are developed collaboratively with community stakeholders. Although intervention development and evaluation are components of transdisciplinary translational behavior research, discovery science is a critical first step in translational research. Consistent with this, conceptual models and frameworks of minority health and health disparities have evolved to include multilevel determinants that include basic behavioral mechanisms such as stress responses and stress reactivity that have physiological, psychological, and behavioral components that are relevant to minority health and health disparities. This report describes priorities, opportunities, and barriers to conducting transdisciplinary translational behavioral research during the next generation of minority health and health disparities research.

Loneliness: An Immunometabolic Syndrome

Loneliness has been defined as an agonizing encounter, experienced when the need for human intimacy is not met adequately, or when a person’s social network does not match their preference, either in number or attributes. This definition helps us realize that the cause of loneliness is not merely being alone, but rather not being in the company we desire. With loneliness being introduced as a measurable, distinct psychological experience, it has been found to be associated with poor health behaviors, heightened stress response, and inadequate physiological repairing activity. With these three major pathways of pathogenesis, loneliness can do much harm; as it impacts both immune and metabolic regulation, altering the levels of inflammatory cytokines, growth factors, acute-phase reactants, chemokines, immunoglobulins, antibody response against viruses and vaccines, and immune cell activity; and affecting stress circuitry, glycemic control, lipid metabolism, body composition, metabolic syndrome, cardiovascular function, cognitive function and mental health, respectively. Taken together, there are too many immunologic and metabolic manifestations associated with the construct of loneliness, and with previous literature showcasing loneliness as a distinct psychological experience and a health determinant, we propose that loneliness, in and of itself, is not just a psychosocial phenomenon. It is also an all-encompassing complex of systemic alterations that occur with it, expanding it into a syndrome of events, linked through a shared network of immunometabolic pathology. This review aims to portray a detailed picture of loneliness as an “immunometabolic syndrome”, with its multifaceted pathology.

Effects of social tolerance on stress: hair cortisol concentrations in the tolerant Tonkean macaques (Macaca tonkeana) and the despotic long-tailed macaques (Macaca fascicularis)

Group living is a source of stress and an individuals' social environment has been shown to have a significant effect on its health and well-being. However, little is known about how different social organizations affect the stress levels of their members. Is living in a hierarchical society more or less stressful than living in a more tolerant structure? Here, we assess cortisol concentrations in the hair of two macaque species with radically different dominance styles: the egalitarian Tonkean macaque (Macaca Tonkeana) and the despotic long-tailed macaques (Macaca fascicularis). Hair was sampled in winter and again in late spring in two mixed-sex groups of 22 Tonkean macaques and 9 long-tailed macaques; Hair cortisol concentrations were significantly higher in the egalitarian Tonkean macaques than in the despotic long-tailed macaques, ranging from 161.13 to 938.8 pg/mg (mean ± SD 349.67 ± 126.22) and from 134.46 to 339.86 pg/mg (mean ± SD 231.2 ± 44.24), respectively. There was no difference between male and female cortisol concentrations, but hair cortisol increased with age in males. Dominance rank certainty was lower among female Tonkean macaques compared to long-tailed macaques. Our results suggest that species differences in dominance styles may translate into differences in long-term cortisol concentrations. We suggest that the higher cortisol concentrations in Tonkean macaques could be linked to the instability and lack of predictability and control around social relationships.

Acute cannabidiol treatment attenuates ethanol-induced place preference and reduces aggressivity in group-housed male rats

Alcohol abuse is a widespread cause of aggressive and impulsive behaviors that impact the users as well as their entourage. However, only a few medications are effective. Recently, cannabidiol has been reported to improve mood disorders and recovery from substance abuse, yet the psychopharmacologic effects of cannabidiol in ethanol-induced drug reward and aggressivity remain unexplored. In the present study, we investigated the effects of cannabidiol on ethanol-induced place preference and aggressivity in individually and group-housed male rats using the conditioned place preference test, and intruder evoc aggression test, respectively. The obtained results showed that ethanol significantly increased locomotor activity, induced conditioned place preference in all animals, and, specifically, increased aggressivity in individually housed rats. These behavioural impairments induced by ethanol were associated with decreased glucocorticoid and mineralocorticoid receptors transcription in the prefrontal cortex. Notwithstanding, cannabidiol at a dose of 10 mg/kg significantly inhibited Et-OH-induced place preference in group-housed, but not in individually housed rats, and markedly inhibited the aggressive behaviour. These findings suggest that ethanol-induced behavioural impairments are dependent on the housing condition that may affect corticosterone receptors expression and subsequently the animal responsivity to cannabidiol treatment.

Putative neural consequences of captivity for elephants and cetaceans

The present review assesses the potential neural impact of impoverished, captive environments on large-brained mammals, with a focus on elephants and cetaceans. These species share several characteristics, including being large, wide-ranging, long-lived, cognitively sophisticated, highly social, and large-brained mammals. Although the impact of the captive environment on physical and behavioral health has been well-documented, relatively little attention has been paid to the brain itself. Here, we explore the potential neural consequences of living in captive environments, with a focus on three levels: (1) The effects of environmental impoverishment/enrichment on the brain, emphasizing the negative neural consequences of the captive/impoverished environment; (2) the neural consequences of stress on the brain, with an emphasis on corticolimbic structures; and (3) the neural underpinnings of stereotypies, often observed in captive animals, underscoring dysregulation of the basal ganglia and associated circuitry. To this end, we provide a substantive hypothesis about the negative impact of captivity on the brains of large mammals (e.g., cetaceans and elephants) and how these neural consequences are related to documented evidence for compromised physical and psychological well-being.

Exploring the Relationship Between Social Ties and Resilience From Evolutionary Framework

This conceptual paper examines the necessity and importance of social bonds and networks in building resilience to fight the COVID-19. Resilience is a quality that energizes an individual’s actions and acts as a buffer to stressful events. The current article is intended to explore the evolutionary programmed behavior of the human mind to make social ties and structure. Humans have a strong need to connect and relate with other individuals by developing cooperation and perspective-taking. The ability to make social connections, group living, and sharing resources had a selective advantage in coping with physical and psychological stress. Social bonds provide resilience to people’s approach while making adjustments and adapting to situations, thus presents fitness benefits at both group and individual levels. An attempt has also been made to address how social isolation as a strategy to contain the infection adversely influence body homeostasis. Finally, this article recommends health practitioners, clinicians, and researchers to encourage research on the impact of social isolation/social interaction on mental and physical health indicators.

Targeting GluN2B/NO Pathway Ameliorates Social Isolation-Induced Exacerbated Attack Behavior in Mice

Exacerbated attack behavior has a profound socioeconomic impact and devastating social consequences; however, there is no satisfactory clinical management available for an escalated attack behavior. Social isolation (SI) is widespread during this pandemic and may exert detrimental effects on mental health, such as causing heightened attack behavior. To explore the therapeutic approaches that alleviate the SI-induced heightened attack behavior, we utilized pharmacological methods targeting the GluN2B/NO signaling pathway during the attack behavior. Ifenprodil and TAT-9C peptide targeting GluN2B showed that the inhibition of GluN2B mitigated the SI-induced escalated attack behavior and the SI-induced aberrant nitric oxide (NO) level in the brain. Additionally, the potentiation of the NO level by L-arginine reversed the effects of the inhibition of GluN2B. Moreover, we showed that high doses of L-NAME and 7-NI and subeffective doses of L-NAME in combination with ifenprodil or TAT-9C or subeffective doses of 7-NI plus ifenprodil or TAT-9C all decreased the SI-induced escalated attack behavior and reduced the NO level, further supporting the idea that GluN2B/NO signaling is a crucial modulator of the escalated attack behavior.

The Impact of Stress on Health in Childhood and Adolescence in the Era of the COVID-19 Pandemic

BACKGROUND

The ongoing pandemic of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is posing many challenges to global health. Efforts from the whole scientific community have shed light on the pathogenetic mechanisms and the clinical features of SARS-CoV-2 infection as well as on potential therapeutic strategies.

SUMMARY

The consequences of stress related to social isolation and anxiety generated by the pandemic on mental and physical health are collateral effects that are yet poorly investigated.

The Long-Term Public Health Impact of Social Distancing on Brain Health: Topical Review

Social distancing has been a critical public health measure for the COVID-19 pandemic, yet a long history of research strongly suggests that loneliness and social isolation play a major role in several cognitive health issues. What is the true severity and extent of risks involved and what are potential approaches to balance these competing risks? This review aimed to summarize the neurological context of social isolation and loneliness in population health and the long-term effects of social distancing as it relates to neurocognitive aging, health, and Alzheimer’s disease and related dementias. The full scope of the underlying causal mechanisms of social isolation and loneliness in humans remains unclear partly because its study is not amenable to randomized controlled trials; however, there are many detailed experimental and observational studies that may provide a hypothesis-generating theoretical framework to better understand the pathophysiology and underlying neurobiology. To address these challenges and inform future studies, we conducted a topical review of extant literature investigating associations of social isolation and loneliness with relevant biological, cognitive, and psychosocial outcomes, and provide recommendations on how to approach the need to fill key knowledge gaps in this important area of research.

An Improved Method for Individual Tracking of Voluntary Wheel Running in Pair-housed Juvenile Mice

Rodent cages equipped with access to a voluntary running wheel are commonly used to study the effects of aerobic physical activity on physiology and behavior. Notable discoveries in exercise neurobiology, including the key role of brain-derived neurotrophic factor (BDNF) in neural plasticity and cognition, have been made using rodents housed with voluntary running wheels. A major advantage of using home-cage running wheels over treadmills is the elimination of stress potentially associated with forced running. In addition, voluntary wheel running may simulate a more natural running pattern in laboratory mice. Singly housing mice with voluntary running wheels is traditionally employed to obtain exact quantitation of the distance ran; however, social isolation stress is often ignored to obtain precise running distances. Moreover, voluntary exercise studies in adolescent mice must consider the neurodevelopmental implications of isolation stress. In this protocol, we wean 21-day-old mouse pups directly into running wheel-equipped cages and pair-house them to reduce the impact of social isolation and other developmentally specific factors that could adversely affect their behavior or development. Individual running distances are obtained from each mouse in the cage using a radio-frequency identification (RFID) ear tag and a hidden antenna placed directly under the running wheel. We have demonstrated that voluntary running during a specific juvenile-adolescent developmental period can improve hippocampal memory when tested during adolescence ( Ivy et al., 2020 ). Individual exercise tracking of group-housed mice can enable future studies to precisely correlate the amount of exercise with readouts such as cell-specific gene expression, epigenetic mechanisms, serum biomarkers, and behavior, in an intra-individual manner. Graphic abstract: Figure 1.Illustration of the dual RFID and Vital View system for individual mouse running in a pair-housed cage.

The Preventive Role of Spirulina Platensis (Arthrospira Platensis) in Immune and Oxidative Insults in a Stress-induced Rat Model

Introduction

There is a balance between oxidative stress, antioxidant capacity and immune response. Their roles in physiological and behavioural mechanisms are important for the maintenance of the organism's internal equilibrium. This study aimed to evaluate the antioxidant effects of the exogenous alga Spirulina platensis (Arthrospira platensis) in a stress-induced rat model, and to describe its possible mechanism of action.

Material and Methods

Thirty-six adult male Sprague Dawley rats were separated into four groups: control (C), stress (S), S. platensis (Sp), and S. platensis + stress (SpS). The rats in groups Sp and SpS were fed with 1,500 mg/kg b.w./day Spirulina platensis for 28 days. All rats were exposed to prolonged light phase conditions (18 h light : 6 h dark) for 14 days. The SpS and S groups were exposed to stress by being kept isolated and in a crowded environment. Blood samples were obtained by puncturing the heart on the 28th day. The effect of stress on serum corticosterone, oxidative stress markers (TOS, TAC, PON1, OSI) and immunological parameters (IL-2, IL-4, IFN-ɣ) were tested. Also, the brain, heart, intestines (duodenum, ileum, and colon), kidney, liver, spleen, and stomach of the rats were weighed.

Results

Serum corticosterone levels were higher in the S group than in the C group, and significantly lower in the SpS group than in the S group. Mean total antioxidant capacity were lower in the S group than in the C group, and Spirulina reversed this change. Although not significantly different, IL-2 was lower in the S group than in the C group. However, in the SpS group, IL-2 increased due to Spirulina platensis mitigating effects of stress.

Conclusion

Male rats fed a diet with Spirulina platensis could experience significantly milder physiological changes during stress, although stress patterns may be different. Exogenous antioxidant supplements merit further investigation in animals and humans where the endogenous defence mechanism against stress may not be sufficient.

Early social deprivation does not affect cortisol response to acute and chronic stress in zebrafish

Social isolation is a well-established technique for inducing early adversity but, in rodent models, the need of parental care makes it difficult to distinguish the effects of social deprivation from the consequences of nutritional deficiencies. Zebrafish do not require parental care, allowing separation of social deprivation from nutritional deprivation, and have emerged as a promising model to study ontogeny of normal and pathological behaviors relevant for human neuropsychological disorders. Previous reports of life-long isolation in zebrafish showed some consistency with mammalian literature, depicting later social deficits and locomotor hyperactivity. However, unlike reports of higher anxiety and stress behavior in isolated rodents and primates, behavioral responses were tapered in isolated fish. To examine whether life-long developmental isolation has a dampening effect on zebrafish endocrine stress response, we applied stressors to zebrafish siblings that were either isolated or socialized, and compared their whole-body cortisol levels with non-stressed control siblings kept in low-housing densities. Utilizing previously validated paradigms (exposure to novel tank and unpredictable chronic stress), we exposed separate groups (n = 9-14, mixed-sex) of social and isolated zebrafish to acute and chronic stressors and measured their cortisol levels. A univariate ANOVA and post-hoc Tukey's HSD tests confirmed that compared to socially raised control fish, developmental isolation did not increase baseline cortisol levels in zebrafish. Additionally, compared to the non-stressed condition, application of both acute and chronic stressors significantly increased cortisol levels in isolated fish and, to a similar degree, to socially raised fish. Our findings suggest that zebrafish isolation studies may help separate effects of social deprivation from nonsocial aspects of early adversity. These studies further substantiate the use of developmental isolation in zebrafish, particularly with acute and chronic stress paradigms, for modeling neuropsychological disorders.LAY SUMMARYA difficult childhood can make humans react more frequently or severely to later stress and modeling this effect in animals can help explain how and why early stress affects subsequent mental and physical health. Early social isolation does not affect later response to stressful situations in adult zebrafish, providing us with a model of psychiatric disorders that allows separation of effects of poor physical environments (lacking food, shelter, etc.) from poor social environments (lack of appropriate socialization).