Impact of social isolation on the oxytocinergic system: A systematic review and meta-analysis of rodent data

Early maternal separation potentiates the impact of later social isolation in inducing depressive-like behavior via oxidative stress in adult rats

RATIONALE

Individuals who have experienced early life stress (ELS) are more vulnerable to later life stress induced depression, which might attribute to ELS potentiated impact of later life stress. The presumption and neurobiological mechanisms involved require further validation and elucidation.

OBJECTIVES

To investigate impact of pre-weaning maternal separation (MS) on post-weaning social isolation (SI) in inducing depressive-like behavior, and involvement of central oxidative stress, glutamatergic and brain-derived neurotrophic factor (BDNF)/tyrosine kinase receptor B (TrkB) signaling in the process.

METHODS

Male offspring were exposed to MS, SI or maternal separation and social isolation (MSSI) stress, respectively. Subjects were treated with saline, antioxidant diallyl disulfide (DADS) (30 mg/kg, i.g.) or antidepressant fluoxetine (10 mg/kg, i.p.), for two weeks before behavioral tests in adolescents or adults. Depressive-like behavior was assessed with sucrose preference, forced swim and tail suspension tests. Concentrations of 4-hydroxynonenal (4-HNE), glutathione and superoxide dismutase in hippocampus and serum, and hippocampal protein expressions of glutamate transporter 1 (GLT-1), BDNF and TrkB were assessed by western blotting analysis.

RESULTS

MSSI, rather than MS or SI, induced significant depressive-like behavior, in adults but not adolescents. Consistently, only MSSI significantly elevated 4-HNE, whereas inhibited GLT-1, BDNF and TrkB in adult hippocampus. MSSI induced behavioral and biochemical abnormalities in adults were reversed by DADS or fluoxetine treatment.

CONCLUSIONS

Early MS age-dependently potentiates later SI impact in inducing depressive-like behavior in male rats, through elevating oxidative stress and interrupting glutamatergic and BDNF/TrkB signaling in the brain. Results further suggest antioxidant treatment as a promising anti-depressant avenue.

Oxytocin signaling in the ventral tegmental area mediates social isolation-induced craving for social interaction

BACKGROUND

Social interaction is crucial for mental health across animal species. Social experiences, especially in early-life stages, strongly influence brain function and social behavior later in life. Acute social isolation (SI) increases motivation to seek social interaction, but little is known about its underlying neuronal and circuitry mechanisms. Here, we focus on oxytocin signaling in the ventral tegmental area (VTA), a vital node of the brain's reward network, as a potential mechanism for SI-induced craving for social interaction.

METHODS

Adolescent (4-week-old) or adult (14-week-old) male C57BL/6J mice underwent a 1-week SI. Free interaction, object exploration, three-chamber social approach, and habituation tests were used to assess social and non-social behavior changes. Viral vectors were used to decipher the underlying neural circuitry, and chemogenetic techniques were applied to modify neuronal activity.

RESULTS

We found that in male C57BL/6J mice, SI during adolescence, but not adulthood, leads to increased craving for social interaction and object exploration, accompanied by impaired social habituation, social novelty preference, and social recognition memory (SRM). SI-induced craving for social interaction and SRM deficit is still observed upon regrouping. Through cell-type-specific manipulations with designer receptors exclusively activated by designer drugs (DREADD), we show that oxytocin neurons in the paraventricular nucleus of the hypothalamus (PVN) are crucial for SI-induced social behavior changes. Chemogenetic activation of PVN oxytocin neurons recapitulates social behavior changes observed in SI mice, whereas chemogenetic inhibition of oxytocin neurons prevents social behavior changes caused by SI. Moreover, we found that dopaminergic neurons in the VTA mediate SI-induced craving for social interaction through their projections to the medial prefrontal cortex (mPFC), but not to the nucleus accumbens. Injection of a specific oxytocin receptor antagonist L368,899 into the VTA or chemical lesions of dopaminergic axon terminals in the mPFC with local application of 6-hydroxydopamine ameliorates SI-induced social behavior changes.

CONCLUSIONS

These findings suggest that adolescent SI has enduring effects on social behaviors in male mice through an oxytocinergic modulation of the VTA-to-mPFC dopaminergic circuit activity.

Moderate caloric restriction improved body composition and maintains bone quality of rats under hypoestrogenism

BACKGROUND

Hypoestrogenism state is associated with weight and adiposity gain, generally impairing bone properties. Caloric restriction (CR) is the main intervention to promote weight loss, however, a chronic severe restriction can lead to bone loss and malnutrition.

OBJECTIVE

To analyse whether 12 weeks of moderate CR could reduce body mass and adiposity without worsening the bone health of ovariectomized (OVX) rats.

METHODS

CR and OVX-CR animals were subject daily to receive a 20 % less amount of food based on the last week's consumption of C and OVX groups. Body mass and food intake were recorded weekly, and spontaneous physical activity (SPA) biweekly. The adipose tissue of subcutaneous (SAT), peritoneal (PTAT), and perigonadal (PGAT) portions was collected to record its mass, and the right femur was collected to determine bone volume (BV), density (BD), mineral density (BMD), and percentage of mineral material (MM).

RESULTS

Ovariectomy increased body mass and food intake (p < 0.001), SAT, PTAT, and PGAT mass (p < 0.01), decreased BV (p < 0.05), BD and BMD (p < 0.01) and MM (p < 0.05), but not affected SPA (p = 0.16). CR decreased body mass, SAT, PTAT, and PGAT mass (p < 0.01) and BMD and MM (p < 0.05), but did not affect BV (p = 0.90), BD (p = 0.24), and SPA (p = 0.25). Therefore, post-hoc data did not demonstrate significant differences in these bone parameters between the OVX and OVX-RC groups (p > 0.05).

CONCLUSION

12 weeks of 20 % of CR is enough to prevent weight and fat gain in the hypoestrogenism state without causing additional worsening in bone properties.

Salivary hormones in depression: the future in diagnosis and treatment

Depression is associated with a significant burden on individuals, families, and communities. It leads to impaired social and occupational functioning, increased disability, decreased quality of life, and higher mortality rates, often due to suicide. A recent estimate from the World Health Organization (WHO) states that over 280 million people of all ages suffer from depression, which equals approximately 3.8% of the world population. Despite effective treatments for mental disorders, a dire treatment gap persists. This treatment gap could be reduced by effective and available diagnostic methods that have the potential to aid in depression diagnosis, stratification of patient subgroups, and treatment monitoring. In this regard, salivary hormones have been studied as potential markers for different types and etiologies of depression due to the convenience of non-invasive sample collection and their correlation with certain aspects of mood and mental health. The literature suggests they can help clinicians assess an individual's stress response, hormonal imbalances, and treatment response, leading to more personalized and effective interventions. In this review, we offer an up-to-date look at all studied salivary hormones associated with depression, including Cortisol, Melatonin, Oxytocin, Serotonin, Dehydroepiandrosterone, Testosterone, Progesterone, and Estradiol.

Isolated during adolescence: long-term impact on social behavior, pain sensitivity, and the oxytocin system in male and female rats

BACKGROUND

Adolescent social isolation (ASI) has profound long-term effects on behavioral and neural development. Despite this, the specific long-term impact of ASI during different adolescent stages and across sexes remain underexplored.

METHODS

Our study addresses this gap by examining the effects of early- and late- adolescent social isolation on both male and female rats. Rats were either isolated (or group-housed) starting from PD 21 (early) or PD 42 (late) for three weeks and then rehoused into groups. In adulthood (PD 90), rats underwent a battery of tests: elevated plus-maze, open field, novel object recognition, social interaction and social recognition memory and hotplate tests. Finally, we analyzed oxytocin receptor binding in several regions in the brains of a second cohort of rats.

RESULTS

Both, male and female rats from the late adolescent social isolation (LASI) groups spent significantly less time interacting in the social interaction test. Additionally, we observed a general decrease in social recognition memory regardless of sex. Both male ASI groups demonstrated heightened thermal pain sensitivity, while the opposite was observed in early adolescent social isolation (EASI) female rats. In the brain, we observed changes in oxytocin receptor (OTR) binding in the paraventricular nucleus of the hypothalamus (PVN) and paraventricular nucleus of the thalamus (PVT) and central amygdala (CeA) with the largest changes in EASI and LASI female rats.

CONCLUSION

Our model demonstrates long-lasting alterations on behavior and oxytocin receptor binding levels following ASI providing insights into the long-term effects of ASI in a time- and sex-specific manner.

Pair bond quality influences social conditioned place preference expression, passive coping behavior, and central oxytocin receptor expression following partner loss in male prairie voles

The dissolving of social bonds is disruptive and leads to increased stress responsivity and a strong desire for reunion. The oxytocin (OXT) system is critical for the formation of social attachments, such as pair bonds, and is also involved in social recognition, social memory, and social vigilance. Therefore, long-term changes in the OXT system resulting from cohabitation and pair bonding may contribute to reunion-seeking behavior. Here, we employed social conditioned place preference (SCPP) and the forced swim test (FST) to examine sensitivity to partner-associated contexts and passive stress coping following a period of partner separation. We found that opposite-sex cohabitation led to SCPP formation only in male prairie voles with a strong preference for their partner, and this SCPP was maintained following short-term loss of a pair bonded partner. Furthermore, pair bonded males that were separated from their partner displayed more passive stress-coping than those that were not bonded to their lost partner, suggesting that differences in prairie vole mating tactics (i.e. formation of a bond or not) influence the behavioral response to partner separation. Finally, we found changes in OXTR binding that may reflect variation in loss-related behavioral phenotypes based on different mating strategies.

Association between RMTg Neuropeptide Genes and Negative Effect during Alcohol Withdrawal in Mice

Alcohol use disorders (AUDs) frequently co-occur with negative mood disorders, such as anxiety and depression, exacerbating relapse through dopaminergic dysfunction. Stress-related neuropeptides play a crucial role in AUD pathophysiology by modulating dopamine (DA) function. The rostromedial tegmental nucleus (RMTg), which inhibits midbrain dopamine neurons and signals aversion, has been shown to increase ethanol consumption and negative emotional states during abstinence. Despite some stress-related neuropeptides acting through the RMTg to affect addiction behaviors, their specific roles in alcohol-induced contexts remain underexplored. This study utilized an intermittent voluntary drinking model in mice to induce negative effect behavior 24 h into ethanol (EtOH) abstinence (post-EtOH). It examined changes in pro-stress (Pnoc, Oxt, Npy) and anti-stress (Crf, Pomc, Avp, Orx, Pdyn) neuropeptide-coding genes and analyzed their correlations with aversive behaviors. We observed that adult male C57BL/6J mice displayed evident anxiety, anhedonia, and depression-like symptoms at 24 h post-EtOH. The laser-capture microdissection technique, coupled with or without retrograde tracing, was used to harvest total ventral tegmental area (VTA)-projecting neurons or the intact RMTg area. The findings revealed that post-EtOH consistently reduced Pnoc and Orx levels while elevating Crf levels in these neuronal populations. Notably, RMTg Pnoc and Npy levels counteracted ethanol consumption and depression severity, while Crf levels were indicative of the mice's anxiety levels. Together, these results underscore the potential role of stress-related neuropeptides in the RMTg in regulating the negative emotions related to AUDs, offering novel insights for future research.

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.

Environmental enrichment accelerates the acquisition of schedule-induced drinking in rats

Environmental enrichment (EE) provides an improvement in the housing conditions of experimental animals, such as laboratory rats, with greater physical and social stimulation through toys and company in the home cages. Its use is known to influence performance of experimental protocols, but these effects have not been well determined in the schedule-induced drinking (SID) procedure. The main goal of this study was to investigate the effects of EE on the acquisition of SID in 24 12-week-old male Wistar rats, divided into two groups, a group with EE housed with toys and companions, and a group without enrichment in individual housing conditions without toys (social isolation and no environmental enrichment, INEE). A total of 25 sessions, under a fixed time 30 s food reinforcement schedule and with access to water in the experimental chambers were carried out. Sessions lasted 30 min. The results showed that the EE group developed faster the excessive drinking pattern of SID, and drank to higher levels, than the INEE group. The greater development of SID in the EE group contradicts the view of schedule-induced behavior as linked to stress reduction and better suits with the conception of induction related to positive reinforcement.

Recent Developments in Protein Lactylation in PTSD and CVD: Novel Strategies and Targets

In 1938, Corneille Heymans received the Nobel Prize in physiology for discovering that oxygen sensing in the aortic arch and carotid sinus was mediated by the nervous system. The genetics of this process remained unclear until 1991 when Gregg Semenza while studying erythropoietin, came upon hypoxia-inducible factor 1, for which he obtained the Nobel Prize in 2019. The same year, Yingming Zhao found protein lactylation, a posttranslational modification that can alter the function of hypoxia-inducible factor 1, the master regulator of cellular senescence, a pathology implicated in both post-traumatic stress disorder (PTSD) and cardiovascular disease (CVD). The genetic correlation between PTSD and CVD has been demonstrated by many studies, of which the most recent one utilizes large-scale genetics to estimate the risk factors for these conditions. This study focuses on the role of hypertension and dysfunctional interleukin 7 in PTSD and CVD, the former caused by stress-induced sympathetic arousal and elevated angiotensin II, while the latter links stress to premature endothelial cell senescence and early vascular aging. This review summarizes the recent developments and highlights several novel PTSD and CVD pharmacological targets. They include lactylation of histone and non-histone proteins, along with the related biomolecular actors such as hypoxia-inducible factor 1α, erythropoietin, acid-sensing ion channels, basigin, and Interleukin 7, as well as strategies to delay premature cellular senescence by telomere lengthening and resetting the epigenetic clock.

Can Translational Social Neuroscience Research Offer Insights to Mitigate Structural Racism in the United States?

Social isolation and conflict due to structural racism may result in human suffering and loneliness across the life span. Given the rising prevalence of these problems in the United States, combined with disruptions experienced during the COVID-19 pandemic, the neurobiology of affiliative behaviors may offer practical solutions to the pressing challenges associated with structural racism. Controlled experiments across species demonstrate that social connections are critical to survival, although strengthening individual resilience is insufficient to address the magnitude and impact of structural racism. In contrast, the multilevel construct of social resilience, defined by the power of groups to cultivate, engage in, and sustain positive relationships that endure and recuperate from social adversities, offers unique insights that may have greater impact, reach, and durability than individual-level interventions. Here, we review putative social resilience-enhancing interventions and, when available, their biological mediators, with the hope to stimulate discovery of novel approaches to mitigate structural racism. We first explore the social neuroscience principles underlying psychotherapy and other psychiatric interventions. Then, we explore translational efforts across species to tailor treatments that increase social resilience, with context and cultural sensitivity in mind. Finally, we conclude with some practical future directions for understudied areas that may be essential for progress in biological psychiatry, including ethical ways to increase representation in research and developing social paradigms that inform dynamics toward or away from socially resilient outcomes.

Mitigating the impact of adolescence isolation on the development of social anxiety: A potential role for oxytocin

Exposure to isolation can lead to the development of social anxiety disorder (SAD), which affects 13% of Americans. There are sex differences in the prevalence of anxiety disorders, as women experience higher rates of SAD relative to men. Importantly, isolation experienced during adolescence increases the likelihood of developing SAD in adulthood. Unfortunately, the current treatments for SAD are only effective in 50–65% of patients. As such, it is critical to identify therapeutic targets for the treatment and prevention of SAD, particularly in women. Here, we discuss the links between childhood isolation and adulthood SAD. Next, we examine the preclinical models used to study the impact of isolation on social anxiety-like behaviors in rodents. Increasing evidence from both clinical and pre-clinical studies suggests oxytocin signaling is a potential target to modify social anxiety-like behaviors. We present the evidence that sex hormones influence the oxytocin system. Finally, we highlight future directions for both clinical and pre-clinical studies to further evaluate the efficacy of oxytocin as a treatment for isolation-induced SAD.

Pharmacological Strategies for Suicide Prevention Based on the Social Pain Model: A Scoping Review

Suicidal behaviour is a public health problem whose magnitude is both substantial and increasing. Since many individuals seek medical treatment following a suicide attempt, strategies aimed at reducing further attempts in this population are a valid and feasible secondary prevention approach. An evaluation of the available evidence suggests that existing treatment approaches have a limited efficacy in this setting, highlighting the need for innovative approaches to suicide prevention. Existing research on the neurobiology of social pain has highlighted the importance of this phenomenon as a risk factor for suicide, and has also yielded several attractive targets for pharmacological strategies that could reduce suicidality in patients with suicidal ideation or a recent attempt. In this paper, the evidence related to these targets is synthesized and critically evaluated. The way in which social pain is related to the “anti-suicidal” properties of recently approved treatments, such as ketamine and psilocybin, is examined. Such strategies may be effective for the short-term reduction in suicidal ideation and behaviour, particularly in cases where social pain is identified as a contributory factor. These pharmacological approaches may be effective regardless of the presence or absence of a specific psychiatric diagnosis, but they require careful evaluation.

Regulation of oxytocin receptor gene expression in obsessive-compulsive disorder: a possible role for the microbiota-host epigenetic axis

BACKGROUND

Obsessive-compulsive disorder (OCD) is a prevalent and severe clinical condition. Robust evidence suggests a gene-environment interplay in its etiopathogenesis, yet the underlying molecular clues remain only partially understood. In order to further deepen our understanding of OCD, it is essential to ascertain how genes interact with environmental risk factors, a cross-talk that is thought to be mediated by epigenetic mechanisms. The human microbiota may be a key player, because bacterial metabolites can act as epigenetic modulators. We analyzed, in the blood and saliva of OCD subjects and healthy controls, the transcriptional regulation of the oxytocin receptor gene and, in saliva, also the different levels of major phyla. We also investigated the same molecular mechanisms in specific brain regions of socially isolated rats showing stereotyped behaviors reminiscent of OCD as well as short chain fatty acid levels in the feces of rats.

RESULTS

Higher levels of oxytocin receptor gene DNA methylation, inversely correlated with gene expression, were observed in the blood as well as saliva of OCD subjects when compared to controls. Moreover, Actinobacteria also resulted higher in OCD and directly correlated with oxytocin receptor gene epigenetic alterations. The same pattern of changes was present in the prefrontal cortex of socially-isolated rats, where also altered levels of fecal butyrate were observed at the beginning of the isolation procedure.

CONCLUSIONS

This is the first demonstration of an interplay between microbiota modulation and epigenetic regulation of gene expression in OCD, opening new avenues for the understanding of disease trajectories and for the development of new therapeutic strategies.