Chronic Social Isolation Stress Research Articles

Impact of prolonged chronic social isolation stress on behavior and multifractal complexity of metabolic rate in Octodon degus

Social interaction can improve animal performance through the prevention of stress-related events, the provision of security, and the enhancement of reproductive output and survival. We investigated the effects of prolonged chronic social isolation stress on behavioral, cognitive, and physiological performance in the social, long-lived rodent Octodon degus. Degu pups were separated into two social stress treatments: control (CTRL) and chronically isolated (CI) individuals from post-natal and post-weaning until adulthood. We quantified anxiety-like behavior and cognitive performance with a battery of behavioral tests. Additionally, we measured their basal metabolic rate (BMR) and analyzed the multifractal properties of the oxygen consumption time series using Multifractal Detrended Fluctuation Analysis, a well-known method for assessing the fractal characteristics of biological signals. Our results showed that CI induced a significant increase in anxiety-like behaviors and led to a reduction in social and working memory in male degus. In addition, CI-treated degus reduced the multifractal complexity of BMR compared to CTRL, which implies a decrease in the ability to respond to environmental stressors and, as a result, an unhealthy state. In contrast, we did not observe significant effects of social stress on BMR. Multivariate analyses showed a clear separation of behavior and physiological variables into two clusters, corresponding to CI and CTRL degus. This study provides novel insights into the effects of prolonged chronic social isolation stress on behavior, cognitive performance, and metabolic complexity in this rodent animal model. To the best of our knowledge, it is the first study to integrate cognitive-behavioral performance and multifractal dynamics of a physiological signal in response to prolonged social isolation. These findings highlight the importance of social interactions for the well-being and overall performance of social animals.

Betaine prevents cognitive dysfunction by suppressing hippocampal microglial activation in chronic social isolated male mice.

Chronic social isolation (SI) stress, which became more prevalent during the COVID-19 pandemic, contributes to abnormal behavior, including mood changes and cognitive impairment. Known as a functional nutrient, betaine has potent antioxidant and anti-inflammatory properties in vivo. However, whether betaine can alleviate the abnormal behavior induced by chronic SI in mice remains unknown. In this study, we investigated the efficacy of betaine in the treatment of behavioral changes and its underlying mechanism. Three-week-old male mice were randomly housed for 8 weeks in either group housing (GH) or SI. The animals were divided into normal saline-treated GH, normal saline-treated SI, and betaine-treated SI groups in the sixth week. The cognitive and depression-like behavior was determined in the eighth week. We found that long-term betaine administration improved cognitive behavior in SI mice but failed to prevent depression-like behavior. Moreover, long-term betaine administration inhibited hippocampal microglia over-activation and polarized microglia toward the M2 phenotype, which effectively inhibited the expression of inflammatory factors in SI mice. Finally, the protective effect of betaine treatment in SI mice might not be due to altered activity of the hypothalamic-pituitary-adrenal axis. Collectively, our findings reveal that betaine can improve SI-induced cognitive impairment, thus providing an alternative natural source for the prevention of memory loss caused by SI or loneliness.

Effects of agonists of melanocortin receptors MC3Rand MC4R on components of sexual behavior of male rats reared in condition of chronic social isolation

BACKGROUND: Melanocortins, including -melanocyte stimulating hormone (MSH), MSH, MSH, and adrenocorticotropic hormone, are products of the polypeptide proopiomelanocortin. These products bind to five different melanocortin receptors (MC1R, MC2R, MC3R, MC4R, MC5R) with different affinities. Because MC3R and MC4R are the major types expressed in the brain, these two receptors can mediate the behavioral effects of melanocortins.
 AIM: To clear the role of MC4R in sexual behavior of male rats after social isolation from relatives.
 MATERIALS AND METHODS: In the present study, 40 naive male Wistar rats divided into 4 groups, 3 of which were grown in complete social and partial sensory isolation, received saline, PT-141 (MC3R/MC4R agonist) in a dose 0.3 g intraperitoneally, and THIQ (MC4R agonist) in a dose 0.1 g intranasally. Behavioral effects were registered in a cage with unreachable reinforcement in which the female in the estrus phase, separated by a transparent perforated barrier, was kept for 10 minutes under red light. Blood samples were taken 30 minutes after administration from the tail vein. Testosterone concentrations were measured by ELISA.
 RESULTS: Social isolation had no significant effect on latent time before trying to reach a female but decreased the time spent near the cage with a female. Isolated animals did not show genital grooming acts during the experiment. PT-141 decreased the latent time to approach the female, increased the time spent near the female cage, and stimulated genital grooming. THIQ decreased the latent time to reach a female and stimulated genital grooming. At the same time, THIQ had no effect on the time spent near the female cage. Social isolation reduced testosterone levels more than twice compared to controls. The administration of PT-141 and THIQ had no effect on testosterone concentration.
 CONCLUSIONS: The results confirm the depressing effect of chronic social isolation stress on the sexual motivation of male rats and demonstrate different roles of melanocortin MC3R/MC4R receptors in the realization of sexual behavior.

Resveratrol attenuates chronic social isolation stress-induced affective disorders: Involvement of NF-κB/NLRP3 axis.

Social isolation stress (SIS) is associated with affective disorders (i.e., anxiety and depression) in adults. In a preclinical study, we aimed to investigate the effects of resveratrol (RV) on the mood swings of rats exposed to SIS. Animals were randomized into six different groups, including control: healthy animals received normal saline (NS) as a vehicle; SIS + NS: SIS animals received NS; SIS + FL: SIS animals received fluoxetine (10 mg/kg/i.p.); SIS + RV20, SIS + RV40, and SIS + RV80: SIS animals received RV (20, 40, and 80 mg/kg/i.p). SIS was induced for 4weeks, then animals were treated with NS, FL, and RV for 4weeks. Rats were evaluated by the behavioral tests, including the elevated plus-maze, tail suspension test, the open field test, and forced-swimming test, for mood alterations and nuclear factor kappa B(NF-κB) levels, along with NLRP3,apoptosis-associated speck-like(ASC), and proCaspase-1 were determined in the hippocampus. Behavioral tests confirmed that exposing the animals to SIS caused anxiety and depression. The highest concentrations of NLRP3, proCaspase-1, ASC, and NF-κB, were confirmed in the SIS + NS group. Compared to FL, RV showed antidepressant potential according to the behavioral tests. In particular, the administration of RV (20, 40, and 80 mg/kg) revered the NF-κB/NLRP3 axis cascade in rats exposed to chronic SIS. Our findings revealed that RV attenuated anxiety and depression of SIS-exposed rats via regulation of NF-κB/NLRP3 signaling pathways. RV can be used as a potential anxiolytic agent and antidepressant.

Resveratrol exerts anxiolytic-like effects through anti-inflammatory and antioxidant activities in rats exposed to chronic social isolation

Emerging evidence has confirmed resveratrol's (RES) antioxidant, anti-inflammatory, and antidepressant effects. The beneficial effects of RES were confirmed for several emotional and cognitive deficits. This research aimed to assess the impacts of RES on behavior and hippocampal levels of anti-inflammatory and pro-inflammatory factors in rats exposed to chronic social isolation (SI) stress, which is known to induce mental disorders such as depressive-like behavior. The animals were treated by RES (20, 40, or 80 mg/kg/intraperitoneally) for 28 days following a 28-day exposure to stress. Behavioral tests, including the forced swim test (FST), open-field test (OFT), tail suspension test (TST), and sucrose preference test (SPT), assessed depressive symptoms. Finally, the animals were sacrificed, and molecular studies (qPCR and ELISA) were performed. Exposure of animals to SI dramatically increased the immobility of animals in TST and FST, enhanced the time spent in the open-field peripheral zone of the OFT, and reduced the sucrose preference rate. In addition, SI increased serum levels of corticosterone and hippocampal content of MDA, whereas it reduced hippocampal SOD and CAT activities. Moreover, SI upregulated the expression of IL-10, IL-18, and IL-1β and downregulated the expression of TGF-β in the hippocampus. RES treatment (40 & 80 mg/kg) significantly improved the behavioral alterations through the modulation of neuroinflammation and oxidative stress. The 20 mg/kg RES dose was inefficient for treating SI-induced depressive-like behavior. These results indicated that RES attenuated depressive-like behavior in prolonged stressed animals. These properties might be associated with RES-mediated improvements in serum corticosterone and hippocampal inflammatory and oxidative stress biomarkers.

Chronic Social Isolation Stress and Crowding in Rats Have Different Effects on Learning an Operant Behavior and the State of the Hypothalamo-Hypophyseal-Adrenocortical System

Chronic Social Isolation Stress and Crowding in Rats Have Different Effects on Learning an Operant Behavior and the State of the Hypothalamo-Hypophyseal-Adrenocortical System

Molecular and cellular mechanisms for differential effects of chronic social isolation stress in males and females.

Chronic social isolation stress during adolescence induces susceptibility for neuropsychiatric disorders. Here we show that 5-week post-weaning isolation stress induces sex-specific behavioral abnormalities and neuronal activity changes in the prefrontal cortex (PFC), basal lateral amygdala (BLA), and ventral tegmental area (VTA). Chemogenetic manipulation, optogenetic recording, and in vivo calcium imaging identify that the PFC to BLA pathway is causally linked to heightened aggression in stressed males, and the PFC to VTA pathway is causally linked to social withdrawal in stressed females. Isolation stress induces genome-wide transcriptional alterations in a region-specific manner. Particularly, the upregulated genes in BLA of stressed males are under the control of activated transcription factor CREB, and CREB inhibition in BLA normalizes gene expression and reverses aggressive behaviors. On the other hand, neuropeptide Hcrt (Hypocretin/Orexin) is among the top-ranking downregulated genes in VTA of stressed females, and Orexin-A treatment rescues social withdrawal. These results have revealed molecular mechanisms and potential therapeutic targets for stress-related mental illness.

Reactivity of the Thyroid System to Short-Term Stress in Wistar Rats with Visceral Obesity and Restricted Social Activity.

The obesity problem requires a study of its pathophysiological consequences affecting hormonal regulation and organism’s reactivity to extreme exposures. The study was aimed first to examine the effect of a high-calorie diet and social isolation, as well as their combination for 4 months, on the development of obesity, its metabolic and behavioral sequelae, features of the thyroid status, while at the second stage, to assess the reaction of hormonal indices of the thyroid status to short-term stress in rats. The experiments were carried out on male Wistar rats and at the first stage focused on the effects of a high-calorie diet and social isolation, as well as their combinations for 4 months. At the end of the experiment, behavioral reactions, metabolic syndrome indices, thyroid status, and cortisol levels were evaluated. At the second stage, the animals were exposed to short-term acute stress, and the shifts in the hormonal indices were recorded one hour later versus the initial background. A high-calorie diet led to the development of metabolic syndrome, signs of depression, increased thyroid-stimulating hormone (TSH), thyroxine and triiodothyronine serum levels, as well as iodothyronine deiodinase type 1 (D1) activity, in the rat liver. At the same time, there was a decrease in thyroperoxidase activity and an increase in thyroid levels of triglycerides and malondialdehyde. The physiological response to stress in the control rat group included an increase in cortisol and TSH serum levels, however, against the background of a high-calorie diet, no cortisol release into the bloodstream was recorded. Social isolation did not alter normal reactivity of the adrenal cortex, but reduced TSH release in response to acute stress, since the initial level of this hormone was slightly elevated against the background of chronic social isolation stress. Thus, excessive nutrition and the deficit of social activities in male Wistar rats led to significant changes in the organism’s reactivity to acute stress.

Early social isolation stress increases addiction vulnerability to heroin and alters c-Fos expression in the mesocorticolimbic system.

Adverse psychosocial factors during early childhood or adolescence compromise neural structure and brain function, inducing susceptibility for many psychiatric disorders such as substance use disorder. Nevertheless, the mechanisms underlying early life stress-induced addiction vulnerability is still unclear, especially for opioids. To address this, we used a mouse heroin self-administration model to examine how chronic early social isolation (ESI) stress (5 weeks, beginning at weaning) affects the behavioral and neural responses to heroin during adulthood. We found that ESI stress did not alter the acquisition for sucrose or heroin self-administration, nor change the motivation for sucrose on a progressive ratio schedule. However, ESI stress induced an upward shift of heroin dose-response curve in female mice and increased motivation and seeking for heroin in both sexes. Furthermore, we examined the neuronal activity (measured by c-Fos expression) within the key brain regions of the mesocorticolimbic system, including the prelimbic cortex (PrL), infralimbic cortex (IL), nucleus accumbens (NAc) core and shell, caudate putamen, and ventral tegmental area (VTA). We found that ESI stress dampened c-Fos expression in the PrL, IL, and VTA after 14-day forced abstinence, while augmented the neuronal responses to heroin-predictive context and cue in the IL and NAc core. Moreover, ESI stress disrupted the association between c-Fos expression and attempted infusions during heroin-seeking test in the PrL. These data indicate that ESI stress leads to increased seeking and motivation for heroin, and this may be associated with distinct changes in neuronal activities in different subregions of the mesocorticolimbic system.

Sex-specific effects of social isolation stress and ketamine on hippocampal plasticity

Chronic social isolation stress (SIS) induces lasting negative effects on the brain, including memory deficits, cognitive impairments, and mood alterations such as depression and anxiety. All these symptoms, at least in part, reflect reduced hippocampal function. In both clinical and preclinical studies, subanesthetic doses of the NMDA receptor antagonist, ketamine (KET), was shown to have rapid and lasting antidepressant effects. Animal studies have shown that biological sex and levels of gonadal hormones alter the behavioral effects of KET, with ovarian hormones increasing sensitivity to the antidepressant-like effects of KET. Since the hippocampus plays a key role in mediating some of the effects of SIS, and considering that KET at low doses has been shown to rescue some of the behavioral deficits of isolation rearing this study aimed to assess the effects of isolation stress on pre- and post-synaptic hippocampal functions in male and female rats reared in SIS, as well as determine whether some of the physiological deficits can be rescued with a single injection of sub-anesthetic doses of KET. To do this, Sprague-Dawley rats were raised from weaning in either social isolation or with same-sex cage mate for 5 to 7 weeks. Male and female rats in either diestrus of proestrus received a single injection of KET (0, 2.5, or 5.0 mg/kg) three hours prior to termination and collection of acute hippocampal slices for ex vivo electrophysiological field potential recordings. Long-term potentiation (LTP) and paired pulse facilitation (PPF) outputs were assessed in a canonical CA3-CA1 dorsal hippocampal circuit. Our data show that SIS inhibits hippocampal LTP without affecting PPF in male rats, an effect that was rescued by KET. In female rats, isolation stress did not alter LTP, but did reduce PPF - especially when females were tested in diestrus-, an effect that was rescued by KET at the highest dose. Our data thus suggest sex differences in the contribution of pre-and postsynaptic hippocampal compartments in response to stress and KET.

Melatonin improves learning and memory of mice with chronic social isolation stress via an interaction between microglia polarization and BDNF/TrkB/CREB signaling pathway

Chronic social isolation stress (SIS) could impair learning and memory-related behaviors. Herein, we investigated the efficacy of Melatonin in treatment of memory despair and also its possible underlying mechanism of action in an animal model of SIS. For this purpose, mice were allocated to two opposing conditions, including social condition (SC) and isolated condition (IC), for five weeks. The study consisted of three groups, including saline-treated SC, saline-treated IC, Melatonin-treated IC (10 mg/kg/day for five successive days). At the end of the isolation period, mice underwent three neurobehavioral tests: passive avoidance (PA), Morris water maze (MWM), and Y maze (YM) tests. Hippocampus samples were obtained and the expressions of BDNF, TrkB, phosphorylated TrkB (pTrkB), CREB, phosphorylated CREB (pCREB), as well as M1 and M2 microglia were assessed. Interpreting the behavioral tests, we found that isolated mice showed lower freezing response in the PA test, lower number of novel arm visits in the YM, and higher escape latency and less time spent in the target quadrant in the MWM, when compared to SC rodents (P values < 0.001). The isolated group had higher M1/M2 relative ratio (P < 0.001), as well as lower concentrations of BDNF mRNA (p < 0.001) and protein (P < 0.001), TrkB protein (P = 0.035), CREB mRNA (P < 0.001) and protein (P = 0.012), pTrkB (P < 0.001), and pCREB (P = 0.035). However, Melatonin relatively reversed the behavioral, cellular, and molecular effects of SIS. Taken together, melatonin therapy could alleviate memory impairment through switching microglial polarization from M1 to M2 phenotype along with altered expression and function in the BDNF/TrkB/CREB signaling pathway.

Neural circuits and activity dynamics underlying sex-specific effects of chronic social isolation stress

Exposure to prolonged stress in critical developmental periods induces heightened vulnerability to psychiatric disorders, which may have sex-specific consequences. Here we investigate the neuronal circuits mediating behavioral changes in mice after chronic adolescent social isolation stress. Escalated aggression is exhibited in stressed males, while social withdrawal is shown in stressed females. Invivo multichannel recordings of free-moving animals indicate that pyramidal neurons in prefrontal cortex (PFC) from stressed males exhibit the significantly decreased spike activity during aggressive attacks, while PFC pyramidal neurons from stressed females show a blunted increase of discharge rates during sociability tests. Chemogenetic and electrophysiological evidence shows that PFC hypofunctioning and BLA principal neuron hyperactivity contribute to the elevated aggression in stressed males, while PFC hypofunctioning and VTA dopamine neuron hypoactivity contribute to the diminished sociability in stressed females. These results establish a framework for understanding the circuit and physiological mechanisms underlying sex-specific divergent effects of stress.

“Live together, die alone”: The effect of re-socialization on behavioural performance and social-affective brain-related proteins after a long-term chronic social isolation stress

Loneliness affects group-living mammals triggering a cascade of stress-dependent physiological disorders. Indeed, social isolation stress is a major risk factor for several neuropsychiatric disorders including anxiety and depression. Furthermore, social isolation has a negative impact on health and fitness. However, the neurobiological consequences of long-term chronic social isolation stress (LTCSIS) manifested during the adulthood of affected individuals are not fully understood. Our study assessed the impact of LTCSIS and social buffering (re-socialization) on the behavioural performance and social-affective brain-related proteins in diurnal, social, and long-lived Octodon degus (degus). Thereby, anxiety-like and social behaviour, and social recognition memory were assessed in male and female animals subjected to a variety of stress-inducing treatments applied from post-natal and post-weaning until their adulthood. Additionally, we evaluated the relationship among LTCSIS, Oxytocin levels (OXT), and OXT-Ca2+-signalling proteins in the hypothalamus, the hippocampus, and the prefrontal cortex. Our findings suggest that LTCSIS induces anxiety like-behaviour and impairs social novelty preference whereas sociability is unaffected. On the other hand, re-socialization can revert both isolation-induced anxiety and social memory impairment. However, OXT and its signalling remained reduced in the abovementioned brain areas, suggesting that the observed changes in OXT-Ca2+ pathway proteins were permanent in male and female degus. Based on these findings, we conclude degus experience social stress differently, suggesting the existence of sex-related mechanisms to cope with specific adaptive challenges.

Hippocampal synaptoproteomic changes of susceptibility and resilience of male rats to chronic social isolation

The susceptibility of an individual to chronic social isolation (CSIS) stress may cause major depression (MD) whereby some individuals are resistant to the stress. Recent studies relate MD with altered expression of synaptic proteins in specific brain regions. To explore the neurobiological underpinnings and identify candidate biomarkers of susceptibility or resilience to CSIS, a comparative proteomic approach was used to map hippocampal synaptic protein alterations of rats exposed to 6 weeks of CSIS, an animal model of depression. This model generates two stress-response phenotypes: CSIS-sensitive (depressive-like behaviour) and CSIS-resilience assessed by means of sucrose preference and forced swim tests. Our aim was to characterize the synaptoproteome changes representative of potential long-term changes in protein expression underlying susceptibility or resilience to stress. Proteomic data showed increased expression of glycolytic enzymes, the energy-related mitochondrial proteins, actin cytoskeleton, signalling transduction and synaptic transmission proteins in CSIS-sensitive rats. Protein levels of glutamate-related enzymes such as glutamate dehydrogenase and glutamine synthetase were also increased. CSIS-resilient rats showed similar proteome changes, however with a weaker increase compared to CSIS-sensitive rats. The main difference was observed in the level of protein expression of vesicle-mediated transport proteins. Nonetheless, only few proteins were uniquely up-regulated in the CSIS-resilient rats, whereby Cytochrome b-c1 complex subunit 2, mitochondrial (Uqcrc2) and Voltage-dependent anion-selective channel protein 1 (Vdac1) were uniquely down-regulated. Identified altered activated pathways and potential protein biomarkers may help us better understand the molecular mechanisms underlying synaptic neurotransmission in MD or resilience, crucial for development of new therapeutics.

Tianeptine Enhances Energy-related Processes in the Hippocampal Non-synaptic Mitochondria in a Rat Model of Depression

Tianeptine (Tian) has been widely used in treating mood and anxiety disorders, and recently as a nootropic to improve cognitive performance. However, its mechanisms of action are insufficiently clear. We used a comparative proteomic approach to identify sub-proteome changes in hippocampal cytosol and non-synaptic mitochondria (NSM) following chronic Tian treatment (3 weeks, 10 mg/kg/day) of adult male Wistar rats and rats exposed to chronic social isolation stress (CSIS) (6 weeks), an animal model of depression. Behavioural assessment of depressive and anxiety-like behaviours was based on sucrose preference, forced swim test and marble burying. Selected differently expressed proteins were validated by Western blot and/or immunohistochemical analysis. Tian normalized the behavioural alternations induced by CSIS, indicating its antidepressant and anxiolytic efficacy. Proteomic data showed that Tian increased the expression of proteasome system elements and redox system enzymes, enhanced energy metabolism and increased glyceraldehyde-3-phosphate dehydrogenase expression bound to NSM in control rats. Tian-treatment of CSIS-stressed rats resulted in a minor suppression of the increase in proteasome elements and antioxidative enzymes, except for an increase in Cu-Zn superoxide dismutase, and increased the level of Lactate dehydrogenase. Our results indicate on an increased NSM functionality in controls and suppression of the CSIS-induced impairment of NSM functionality by Tian treatment as well as on the CSIS-caused discrepancy in Tian effects relative to controls.

Chronic Isolation Stress Affects Subsequent Crowding Stress-Induced Brain Nitric Oxide Synthase (NOS) Isoforms and Hypothalamic-Pituitary-Adrenal (HPA) Axis Responses

The nitric oxide (NO) pathway in the brain is involved in response to psychosocial stressors. The aim of this study was to elucidate the role of nNOS and iNOS in the prefrontal cortex (PFC), hippocampus (HIP), and hypothalamus (HYPO) during social isolation stress (IS), social crowding stress (CS), and a combined IS + CS. In the PFC, 3 days of CS increased iNOS but not nNOS protein level. In the HIP and HYPO, the levels of nNOS and iNOS significantly increased after 3 days of CS. In the PFC, IS alone (11 days) enhanced iNOS protein level following 3 days of CS and increased nNOS level in the HIP and HYPO after 14 days of CS. By contrast, in the HIP, IS abolished the subsequent CS-induced increase in nNOS in the HIP and strongly elevated iNOS level after 7 days of CS. In the HYPO, prior IS inhibited nNOS protein level induced by subsequent CS for 3 days, but increased nNOS protein level after longer exposure times to CS. Isolation stress strongly upregulated plasma interleukin-1β (IL-1β) and adrenocorticotropic hormone (ACTH) levels while corticosterone (CORT) level declined. We show that the modulatory action of the NO pathway and ACTH/CORT adaptation to chronic social isolation stress is dependent on the brain structure and nature and duration of the stressor. Our results indicate that isolation is a robust natural stressor in social animals; it enhances the NO pathway in the PFC and abolishes subsequent social CS-induced NOS responses in the HIP and HYPO.

Chronic Social Isolation Stress during Peri-Adolescence Alters Presynaptic Dopamine Terminal Dynamics via Augmentation in Accumbal Dopamine Availability.

Chronic peri-adolescent stress in humans increases risk to develop a substance use disorder during adulthood. Rats reared in social isolation during peri-adolescence (aSI; 1 rat/cage) period show greater ethanol and cocaine intake compared to group housed (aGH; 4 rats/cage) rats. In addition, aSI rats have a heightened dopamine response in the nucleus accumbens (NAc) to rewarding and aversive stimuli. Furthermore, single pulse electrical stimulation in slices containing NAc core elicits greater dopamine release in aSI rats. Here, we further investigated dopamine release kinetics and machinery following aSI. Dopamine release, across a wide range of stimulation intensities and frequencies, was significantly greater in aSI rats. Interestingly, subthreshold intensity stimulations also resulted in measurable dopamine release in accumbal slices from aSI but not aGH rats. Extracellular [Ca2+] manipulations revealed augmented calcium sensitivity of dopamine release in aSI rats. The readily releasable pools of dopamine, examined by bath application of Ro-04-1284/000, a vesicular monoamine transporter 2 (VMAT2) inhibitor, were depleted faster in aGH rats. Western blot analysis of release machinery proteins (VMAT2, Synaptogyrin-3, Syntaxin-1, and Munc13-3) showed no difference between the two groups. Tyrosine hydroxylase (TH) protein expression levels, however, were elevated in aSI rats. The greater dopamine release could potentially be explained by higher levels of TH, the rate-limiting step for dopamine synthesis. This augmented responsivity of the dopamine system and heightened dopamine availability post-aSI may lead to an increased risk of addiction vulnerability.

Oxytocin modulates the expression of norepinephrine transporter, β3-adrenoceptors and muscarinic M2 receptors in the hearts of socially isolated rats

Social stress produces behavioral alterations, and autonomic and cardiac dysfunction in animals. In addition to the well-known roles of oxytocin on birth and maternal bonding, recent evidence shows that this neuropeptide possesses cardio-protective properties. However less is known about its role in the regulation of the autonomic nervous system. The direct influence of oxytocin on the cardiac catecholamine synthesizing enzyme, transport beta-adrenoceptors and muscarinic receptors in animals exposed to chronic social isolation stress has not yet been studied. In this study, we examined the influence of peripheral chronic oxytocin treatment on anxiety-related behavior, the morphology and content of epinephrine and norepinephrine, mRNA and protein levels of tyrosine hydroxylase (TH), norepinephrine transporter (NET) and receptors <beta> 3 (β3-AR) and muscarinic 2 (M2 MR) in the right and left cardiac atrium and ventricle of chronically socially isolated male rats. Our results show that oxytocin treatment exhibits an anxiolytic effect, decreases the heart/body weight ratio and prevents the hypertrophy of cardiomyocytes in the wall of the left ventricle of stressed rats. Epinephrine and TH protein levels were unchanged after prolonged oxytocin treatment. Peripheral oxytocin administration led to the enhancement of gene expression of β3-AR in both atria, NET protein in the left ventricle and gene expression of M2 MR in the right atrium and the left ventricle of chronically socially isolated rats. The study provides evidence that oxytocin treatment in chronically socially isolated animals enhances norepinephrine uptake and expression of cardio-inhibitory receptors in cardiac tissues, which could have a beneficial effect on the cardiovascular system under the increased activity of the sympathoneural system.

The Neuropeptide Tac2 Controls a Distributed Brain State Induced by Chronic Social Isolation Stress

Chronic social isolation causes severe psychological effects in humans, but their neural bases remain poorly understood. 2weeks (but not 24hr) of social isolation stress (SIS) caused multiple behavioral changes in mice and induced brain-wide upregulation of the neuropeptide tachykinin 2 (Tac2)/neurokinin B (NkB). Systemic administration of an Nk3R antagonist prevented virtually all of the behavioral effects of chronic SIS. Conversely, enhancing NkB expression and release phenocopied SIS in group-housed mice, promoting aggression and converting stimulus-locked defensive behaviors to persistent responses. Multiplexed analysis of Tac2/NkB function in multiple brain areas revealed dissociable, region-specific requirements for both the peptide and its receptor in different SIS-induced behavioral changes. Thus, Tac2 coordinates a pleiotropic brain state caused by SIS via a distributed mode of action. These data reveal the profound effects of prolonged social isolation on brain chemistry and function and suggest potential new therapeutic applications for Nk3R antagonists.

Social Isolation Co-opts Fear and Aggression Circuits

Social isolation is a stressful condition that often leads to maladaptive behaviors. In this issue of Cell, Zelikowsky etal. find that chronic social isolation stress triggers an increase in neuronal tachykinin signaling across distinct brain regions that mediate fear and aggression, elucidating the neural basis of these maladaptive responses.