Chronic social isolation induces NF-κB activation and upregulation of iNOS protein expression in rat prefrontal cortex

Increased stress vulnerability in the offspring of socially isolated rats: Behavioural, neurochemical and redox dysfunctions

Stressful events during pregnancy impact on the progeny neurodevelopment. However, little is known about preconceptional stress effects. The rat social isolation represents an animal model of chronic stress inducing a variety of dysfunctions. Moreover, social deprivation during adolescence interferes with key neurodevelopmental processes. Here, we investigated the development of behavioural, neurochemical and redox alterations in the male offspring of socially isolated female rats before pregnancy, reared in group (GRP) or in social isolation (ISO) from weaning until young-adulthood. To this aim, females were reared in GRP or in ISO conditions, from PND21 to PND70, when they were mated. Their male offspring was housed in GRP or ISO conditions through adolescence and until PND70, when passive avoidance-PA, novel object recognition-NOR and open field-OF tests were performed. Levels of noradrenaline (NA), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), glutamate (GLU) and GABA were assessed in the prefrontal cortex (PFC). Moreover, cortical ROS levels were quantified, as well as NF-kB and the NADPH oxidase NOX2 expression, redox status (expressed as GSH:GSSG ratio) and SOD1 amount. A significant decrease of the latency time in the PA was observed in the offspring of ISO females. In the NOR test, while a significant increase in the exploratory activity towards the novel object was observed in the offspring of GRP females, no significant differences were found in the offspring of ISO females. No significant differences were found in the OF test among experimental groups. Theoffspring of ISO females showed increased NA and 5-HIAA levels, whereas in the offspring persistently housed in isolation condition from weaninguntil adulthood, we detected reduced 5-HT levels and ehnanced 5-HIAA amount. No significant changes in GLU concentrations were detected, while decreased GABA content was observed in the offspring of ISO females exposed to social isolation. Increased ROS levels as well as reduced NF-κB, NOX2 expression were detected in the offspring of ISO females. This was accompanied by reduced redox status and enhanced SOD1 levels. In conclusion, our results suggest that female exposure to chronic social stress before pregnancy might have a profound influence on the offspring neurodevelopment in terms of cognitive, neurochemical and redox-related alterations, identifying this specific time window for possible preventive and therapeutic strategies.

Role of Acorus calamus in preventing depression, anxiety, and oxidative stress in long-term socially isolated rats

Background and Aim

Social isolation stress (SIS) and individual housing have been shown to cause abnormal cognitive insufficiencies, altered anxiety levels, and signs of psychiatric diseases. Acorus calamus (AC), commonly known as Sweet Flag, has been widely used in India to treat neurological, metabolic, and respiratory disorders, indicating its potential therapeutic value. This study aimed to determine the antidepressant and antioxidative effects of AC on rats subjected to long-term, social isolation-induced stress.

Materials and Methods

This study involved 2-month-old male rats (24) weighing approximately 180-200 g bred in-house. The rats were divided into four groups (n = 6): Group 1 received saline, Group 2 received SIS, Group 3 received only 50 mg/kg AC, and Group 4 received 50 mg/kg AC and SIS for 6 weeks. After this, behavioral, biochemical, and neuronal assays were conducted.

Results

Behavioral experiments showed significantly higher activity levels (p < 0.001) in AC-treated rats than in the SIS group. In addition, rats subjected to SIS with AC treatment exhibited enhanced total antioxidants, superoxide dismutase, and neuronal assays compared to rats subjected to SIS alone.

Conclusion

Acorus calamus treatment improved the antidepressant and antioxidant potential against SIS in rat brain tissue. Moreover, we proved that AC can effectively reverse the neurotoxicity induced by SIS in animal models. As we battle against the coronavirus disease 2019 pandemic and social isolation, AC could be considered a supplementary treatment to alleviate depressive-like symptoms in our present-day lifestyle.

How Oxidative Stress Induces Depression?

Depression increasingly affects a wide range and a large number of people worldwide, both physically and psychologically, which makes it a social problem requiring prompt attention and management. Accumulating clinical and animal studies have provided us with substantial insights of disease pathogenesis, especially central monoamine deficiency, which considerably promotes antidepressant research and clinical treatment. The first-line antidepressants mainly target the monoamine system, whose drawbacks mainly include slow action and treatment resistant. The novel antidepressant esketamine, targeting on central glutamatergic system, rapidly and robustly alleviates depression (including treatment-resistant depression), whose efficiency is shadowed by potential addictive and psychotomimetic side effects. Thus, exploring novel depression pathogenesis is necessary, for seeking more safe and effective therapeutic methods. Emerging evidence has revealed vital involvement of oxidative stress (OS) in depression, which inspires us to pursue antioxidant pathway for depression prevention and treatment. Fully uncovering the underlying mechanisms of OS-induced depression is the first step towards the avenue, thus we summarize and expound possible downstream pathways of OS, including mitochondrial impairment and related ATP deficiency, neuroinflammation, central glutamate excitotoxicity, brain-derived neurotrophic factor/tyrosine receptor kinase B dysfunction and serotonin deficiency, the microbiota-gut-brain axis disturbance and hypothalamic-pituitary-adrenocortical axis dysregulation. We also elaborate on the intricate interactions between the multiple aspects, and molecular mechanisms mediating the interplay. Through reviewing the related research progress in the field, we hope to depict an integral overview of how OS induces depression, in order to provide fresh ideas and novel targets for the final goal of efficient treatment of the disease.

Astragalin attenuates depression-like behaviors and memory deficits and promotes M2 microglia polarization by regulating IL-4R/JAK1/STAT6 signaling pathway in a murine model of perimenopausal depression

RATIONALE

Neuroinflammation can be alleviated via M2 microglia polarization, which could promote the recovery of perimenopausal depression. Astragalin (AST) possesses anti-neuroinflammatory activity. However, the effects of AST on perimenopausal depression and the molecular mechanism in regulating microglia polarization remained unknown.

OBJECTIVES

The purpose was to investigate the effects of AST on mice with simulated perimenopausal depression through regulating microglia polarization. It was aimed to clarify the molecular mechanism related to the interleukin-4 receptor (IL-4R)/janus kinase (JAK) 1/signal transducer and activator of transcription (STAT) 6 signaling pathway.

METHODS

The ovariectomy (OVX)/chronic unpredictable mild stress (CUMS)-induced murine model of perimenopausal depression was established and treated with AST. Then the depression-like behaviors and cognitive ability of mice were examined. After that, we detected the markers of microglia polarization and its regulatory signals. In addition, lipopolysaccharides (LPS)/adenosine triphosphate (ATP)-induced inflammatory BV2 model were used to verify the potential molecular mechanism.

RESULTS

AST alleviated perimenopausal depression-like behaviors and memory deficits. AST alleviated microglia activation and increased Ki67-positive cells in dentate gyrus (DG). The viability of BV2 decreased by LPS/ATP was raised by AST. Moreover, both in vivo and in vitro, AST switched microglia from M1 phenotype caused by OVX/CUMS or LPS/ATP to M2 phenotype. The IL-4R/JAK1/STAT6 signaling was restored, and the levels of inducible nitric oxide synthase (iNOS), nuclear NF-KappaB-p65 were reduced by AST. Importantly, AST showed prevention against the ubiquitination modification and degradation of STAT6.

CONCLUSIONS

Our results revealed new insights into molecular mechanism associated with microglia polarization in the effect of AST on the mouse model of perimenopausal depression.

Inducible Nitric Oxide Synthase Inhibition in the Medial Prefrontal Cortex Attenuates the Anxiogenic-Like Effect of Acute Restraint Stress via CB1 Receptors

Stress exposure can result in several proinflammatory alterations in the brain, including overexpression of the inducible isoform of nitric oxide synthase (iNOS) in the medial prefrontal cortex (mPFC). These changes may be involved in the development of many psychiatric conditions. However, it is unknown if iNOS in mPFC plays a significant role in stress-induced behavioral changes. The endocannabinoid (ECB) system is also influenced by stress. Its activation seems to be a counter regulatory mechanism to prevent or decrease the stress-mediated neuroinflammatory consequences. However, it is unclear if the ECB system and iNOS interact to influence stress consequences. This study aimed to test the hypothesis that the anti-stress effect of iNOS inhibition in mPFC involves the local ECB system, particularly the CB₁ cannabinoid receptors. Male Wistar rats with guide cannula aimed at the mPFC were submitted to acute restraint stress (RS) for 2 h. In the following morning, rats received bilateral microinjections of vehicle, AM251 (CB₁ antagonist; 100 pmol), and/or 1400W (iNOS selective inhibitor; 10^-4, 10^-3, or 10^-2 nmol) into the prelimbic area of mPFC (PL-mPFC) before being tested in the elevated plus-maze (EPM). iNOS inhibition by 1400W prevented the anxiogenic-like effect observed in animals submitted to RS. The drug did not promote behavior changes in naive animals, demonstrating a stress-dependent effect. The 1400W-anti-stress effect was prevented by local pretreatment with AM251. Our data suggest that iNOS inhibition may facilitate the local endocannabinoid signaling, attenuating stress effects.

Duration of Social Isolation Affects Production of Nitric Oxide in the Rat Brain

Social isolation deprives rodents of social interactions that are critical for normal development of brain and behavior. Several studies have indicated that postweaning isolation rearing may affect nitric oxide (NO) production. The aim of this study was to compare selected behavioral and biochemical changes related to NO production in the brain of rats reared in social isolation for different duration. At the age of 21 days, male Sprague Dawley rats were randomly assigned into four groups reared in isolation or socially for 10 or 29 weeks. At the end of the rearing, open-field and prepulse inhibition (PPI) tests were carried out. Furthermore, in several brain areas we assessed NO synthase (NOS) activity, protein expression of nNOS and iNOS isoforms and the concentration of conjugated dienes (CD), a marker of oxidative damage and lipid peroxidation. Social isolation for 10 weeks resulted in a significant decrease in PPI, which was accompanied by a decrease in NOS activity in the cerebral cortex and the cerebellum, an increase in iNOS in the hippocampus and an increase in CD concentration in cortex homogenate. On the other hand, a 29 week isolation had an opposite effect on NOS activity, which increased in the cerebral cortex and the cerebellum in animals reared in social isolation, accompanied by a decrease in CD concentration. The decrease in NOS activity after 10 weeks of isolation might have been caused by chronic stress induced by social isolation, which has been documented in previous studies. The increased oxidative state might result in the depleted NO bioavailability, as NO reacts with superoxide radical creating peroxynitrite. After 29 weeks of isolation, this loss of NO might be compensated by the subsequent increase in NOS activity.

Chronic stress followed by social isolation promotes depressive-like behaviour, alters microglial and astrocyte biology and reduces hippocampal neurogenesis in male mice

Unpredictable chronic mild stress (UCMS) is one of the most commonly used, robust and translatable models for studying the neurobiological basis of major depression. Although the model currently has multiple advantages, it does not entirely follow the trajectory of the disorder, whereby depressive symptomology can often present months after exposure to stress. Furthermore, patients with depression are more likely to withdraw in response to their stressful experience, or as a symptom of their depression, and, in turn, this withdrawal/isolation can further exacerbate the stressful experience and the depressive symptomology. Therefore, we investigated the effect(s) of 6 weeks of UCMS followed by another 6 weeks of social isolation (referred to as UCMSI), on behaviour, corticosterone stress responsivity, immune system functioning, and hippocampal neurogenesis, in young adult male mice. We found that UCMSI induced several behavioural changes resembling depression but did not induce peripheral inflammation. However, UCMSI animals showed increased microglial activation in the ventral dentate gyrus (DG) of the hippocampus and astrocyte activation in both the dorsal and ventral DG, with increased GFAP-positive cell immunoreactivity, GFAP-positive cell hypertrophy and process extension, and increased s100β-positive cell density. Moreover, UCMSI animals had significantly reduced neurogenesis in the DG and reduced levels of peripheral vascular endothelial growth factor (VEGF) - a trophic factor produced by astrocytes and that stimulates neurogenesis. Finally, UCMSI mice also had normal baseline corticosterone levels but a smaller increase in corticosterone following acute stress, that is, the Porsolt Swim Test. Our work gives clinically relevant insights into the role that microglial and astrocyte functioning, and hippocampal neurogenesis may play in the context of stress, social isolation and depression, offering a potentially new avenue for therapeutic target.

nNOS-mediated protein-protein interactions: promising targets for treating neurological and neuropsychiatric disorders

Neurological and neuropsychiatric disorders are one of the leading causes of disability worldwide and affect the health of billions of people. Nitric oxide (NO), a free gas with multitudinous bioactivities, is mainly produced from the oxidation of L-arginine by neuronal nitric oxide synthase (nNOS) in the brain. Inhibiting nNOS benefits a variety of neurological and neuropsychiatric disorders, including stroke, depression and anxiety disorders, post-traumatic stress disorder, Parkinson's disease, Alzheimer's disease, chronic pain, and drug addiction. Due to critical roles of nNOS in learning and memory and synaptic plasticity, direct inhibition of nNOS may cause severe side effects. Importantly, interactions of several proteins, including post-synaptic density 95 (PSD-95), carboxy-terminal PDZ ligand of nNOS (CAPON) and serotonin transporter (SERT), with the PSD/Disc-large/ZO-1 homologous (PDZ) domain of nNOS have been demonstrated to influence the subcellular distribution and activity of the enzyme in the brain. Therefore, it will be a preferable means to interfere with nNOS-mediated protein-protein interactions (PPIs), which do not lead to undesirable effects. Herein, we summarize the current literatures on nNOS-mediated PPIs involved in neurological and neuropsychiatric disorders, and the discovery of drugs targeting the PPIs, which is expected to provide potential targets for developing novel drugs and new strategy for the treatment of neurological and neuropsychiatric disorders.

Long-Term Impact of Social Isolation and Molecular Underpinnings

Prolonged periods of social isolation can have detrimental effects on the physiology and behavior of exposed individuals in humans and animal models. This involves complex molecular mechanisms across tissues in the body which remain partly identified. This review discusses the biology of social isolation and describes the acute and lasting effects of prolonged periods of social isolation with a focus on the molecular events leading to behavioral alterations. We highlight the role of epigenetic mechanisms and non-coding RNA in the control of gene expression as a response to social isolation, and the consequences for behavior. Considering the use of strict quarantine during epidemics, like currently with COVID-19, we provide a cautionary tale on the indiscriminate implementation of such form of social isolation and its potential damaging and lasting effects in mental health.

Fluoxetine increases hippocampal neural survival by improving axonal transport in stress-induced model of depression male rats

INTRODUCTION

Axonal transport deficit is a key mechanism involved in neurodegenerative conditions. Fluoxetine, a commonly used antidepressant for treatment of depression, is known to regulate several important structural and neurochemical aspects of hippocampal functions. However, the mechanisms underlying these effects are still poorly understood. This study aimed to investigate the effects of chronic fluoxetine treatment on axonal transport in the hippocampus of rat stress-induced model of depression.

METHODS

We have analyzed the effects of chronic fluoxetine treatment (20 mg/kg/day, 24 days) on immobility behavior (forced swimming test), hippocampal iNOS (inflammatory factor) expression (RT-PCR) as well as hippocampal BDNF, kinesin and dynein expression (RT-PCR) and hippocampal neuronal survival (Nissl staining).

RESULTS

This study provided evidence that fluoxetine could effectively suppress iNOS expression following unpredictable chronic mild stress (P < 0.01), increase hippocampal BDNF (P < 0.01), kinesin (P < 0.05) and dynein (P < 0.01) gene expression, and control neuronal death in CA1 (P < 0.01) and CA3 regions (P < 0.01) of the hippocampus and thereby improve immobility behavior (P < 0.001).

CONCLUSION

Based on the findings of this study, we concluded the neuroprotective effect of fluoxetine may be due to its ability to improve axonal transmission, followed by increased energy supply and neurotrophin concentration and function.

Ovariectomy Induces Microglial Cell Activation and Inflammatory Response in Rat Prefrontal Cortices to Accelerate the Chronic Unpredictable Stress-Mediated Anxiety and Depression

Perimenopausal women are associated with increased risks of depression and anxiety, which may be potentially related to the lack of ovarian hormone with antidepression activity in the body. However, the precise mechanism remains unclear so far. This study first adopted the Sprague-Dawley (SD) female rats to construct the ovariectomy (OVX) combined with a chronic unpredictable stress (CUS) model. Then, a series of behavioral experimental results revealed that the ovariectomized rats receiving CUS had remarkably elevated anxiety and depression behaviors relative to those in sham group rats, and the sucrose preference rate in the sucrose preference test (SPT) was evidently reduced. In elevated plus maze test (EPM) experiment, the open arm entry time and open arm duration were decreased. In the open field test (OFT), the number of line crossings, rearing number, center square entries, and center square duration were reduced; the grooming time was extended; and the number of fecal particles in rats was increased. In the forced swimming test (FST), the rat immobility rate was increased, while the numbers of swimming and crawling were decreased. Afterwards, we discovered that OVX downregulated the serum levels of estradiol and corticosterone in rats. Thereafter, IF results suggested that OVX dramatically induced the increasing of the number of activated microglial cells in prefrontal cortices and the level of M1-type marker iNOS. Finally, PCR results demonstrated that, compared with the sham group, the proinflammatory and prooxidative genes, such as IL-1β, IL-6, TNF-α, iNOS, and CX3CR1, were upregulated in the prefrontal cortices of OVX rats after CUS stimulation, whereas the anti-inflammatory factor Arg1 and microglial cell negative regulatory factor CD200 were downregulated. To sum up, OVX enhances the CUS-mediated anxiety and depression phenomena in rats, and its mechanism may be related to inducing the activation and polarization of microglial cells in the prefrontal cortex of animal and to accelerating the inflammatory response.

PDTC Alleviates Depressive Symptoms and Colon Tissue Injury via Inhibiting NO Overproduction in CUMS Rats

Background

The accumulated evidence demonstrates that stress plays an important role in the pathogenesis of depression that is associated with intestinal dysfunctions. However, the mechanisms remain unresolved.

Methods

A total of 40 male Wistar rats were obtained and randomly divided into four equal-sized group: control, PDTC + chronic and unpredictable mild stress (CUMS), FLX + CUMS, and CUMS. Western blotting and qRT-PCR were used to examine the levels of nitric oxide (NO), nuclear factor kappa beta (NF-κB), inducible nitric oxide synthase (iNOS), and iNOS mRNA in spinal cord L1-2 and colon.

Key Results

Chronic and unpredictable mild stress increased the serum CORT level, decreased body weight and sucrose preference, and altered OFT performance, while increased levels of NO, iNOS mRNA, iNOS and NF-κB protein in colon and spinal cord were accompanied by histopathological changes in colon. Pretreatment with an NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC), reversed these effects. Fluoxetine failed to prevent NO increase in both spinal cord and colon, while the iNOS protein level, although not statistically significantly increased compared to control, was not decreased compared to CUMS. Also, fluoxetine failed to prevent histological changes.

Conclusion

In conclusion, the NF-κB/iNOS pathway may be involved in the mechanism of CUMS-induced depressive-like behavior and colon tissue injury.

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.

Fucoxanthin prevents lipopolysaccharide-induced depressive-like behavior in mice via AMPK- NF-κB pathway

Fucoxanthin (FX), a natural carotenoid abundant in edible brown seaweeds, has been shown the great anti-oxidant, anti-inflammatory and anti-diabetic effects in vivo and in vitro. The present study was designed to investigate the effects of FX on lipopolysaccharide (LPS)-induced behavioral defects in mice. In depressive behavior tests, the increased immobility time of forced swimming test and tail suspension test by LPS treatment in mice, which were significantly reversed by FX treatment (200 mg/kg, i.g.). In anxiety behavior tests, LPS injection was neither influence the anxiety-related parameters in marble burying test nor that in elevated plus maze test. Interestingly, anxiolytic effects were observed in single FX treated control and LPS-induced mice groups. FX treatment also reversed LPS-induced body weight loss and food intake decreases. Biochemical analysis indicated that FX inhibited LPS-induced overexpression of pro-inflammatory cytokines (IL-1β, IL-6 and TNF-α), as well as iNOS and COX-2 in the hippocampus, frontal cortex and hypothalamus, via the modulation of AMPK-NF-κB signaling pathway.

Hippocampal nuclear factor kappa B accounts for stress-induced anxiety behaviors via enhancing neuronal nitric oxide synthase (nNOS)-carboxy-terminal PDZ ligand of nNOS-Dexras1 coupling

Anxiety disorders are associated with a high social burden worldwide. Recently, increasing evidence suggests that nuclear factor kappa B (NF-κB) has significant implications for psychiatric diseases, including anxiety and depressive disorders. However, the molecular mechanisms underlying the role of NF-κB in stress-induced anxiety behaviors are poorly understood. In this study, we show that chronic mild stress (CMS) and glucocorticoids dramatically increased the expression of NF-κB subunits p50 and p65, phosphorylation and acetylation of p65, and the level of nuclear p65 in vivo and in vitro, implicating activation of NF-κB signaling in chronic stress-induced pathological processes. Using the novelty-suppressed feeding (NSF) and elevated-plus maze (EPM) tests, we found that treatment with pyrrolidine dithiocarbamate (PDTC; intra-hippocampal infusion), an inhibitor of NF-κB, rescued the CMS- or glucocorticoid-induced anxiogenic behaviors in mice. Microinjection of PDTC into the hippocampus reversed CMS-induced up-regulation of neuronal nitric oxide synthase (nNOS), carboxy-terminal PDZ ligand of nNOS (CAPON), and dexamethasone-induced ras protein 1 (Dexras1) and dendritic spine loss of dentate gyrus (DG) granule cells. Moreover, over-expression of CAPON by infusing LV-CAPON-L-GFP into the hippocampus induced nNOS-Dexras1 interaction and anxiety-like behaviors, and inhibition of NF-κB by PDTC reduced the LV-CAPON-L-GFP-induced increases in nNOS-Dexras1 complex and anxiogenic-like effects in mice. These findings indicate that hippocampal NF-κB mediates anxiogenic behaviors, probably via regulating the association of nNOS-CAPON-Dexras1, and uncover a novel approach to the treatment of anxiety disorders.

Depression and sterile inflammation: Essential role of danger associated molecular patterns

Stress is a major risk factor for psychiatric disorder including major depressive disorder (MDD) and can induce inflammation, which is known to be dysregulated in depression. Several clinical and pre-clinical studies have demonstrated a strong association between depressive symptoms and the expression of factors that increase inflammation. Conversely, administration of anti-inflammatory agents has been shown to ameliorate depressive symptoms, demonstrating the importance of inflammation as a mediator of depression. Although it is clear that inflammation plays a role in the pathophysiology of depression, the mechanism by which inflammation is activated in mood disorders remains unclear. To address this issue, studies have investigated the role of pattern recognition receptor (PRR) activation in stress-induced inflammation and mood disorders. However, the identification of the endogenous factors, referred to as danger-associated molecular patterns (DAMP) that activate these receptors remains understudied. Here we review the role of DAMPs in depression and highlight the clinical evidence for elevation of DAMP signaling in MDD patients and in pre-clinical animal stress models of depression.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Chronic stress from adolescence to aging in the prefrontal cortex: A neuroimmune perspective

The development of the organism is a critical variable which influences the magnitude, duration, and reversibility of the effects of chronic stress. Such factors are relevant to the prefrontal cortex (PFC), as this brain region is the last to mature, the first to decline, and is highly stress-sensitive. Therefore, this review will examine the intersection between the nervous system and immune system at glutamatergic synapses in the PFC across three developmental periods: adolescence, adulthood, and aging. Glutamatergic synapses are tightly juxtaposed with microglia and astrocytes, and each of these cell types exhibits their own developmental trajectory. Not only does chronic stress differentially impact each of these cell types across development, but chronic stress also alters intercellular communication within this quad-partite synapse. These observations suggest that developmental shifts in both neural and immune function across neurons, microglia, and astrocytes mediate shifting effects of chronic stress on glutamatergic transmission.

Early enriched physical environment reverses impairments of the hippocampus, but not medial prefrontal cortex, of socially-isolated mice

Early social isolation (SI) produces a variety of emotional, behavioral and cognitive abnormalities. Conversely, environmental enrichment (EE), a complicated social and physical construct, offers beneficial effects on brain plasticity and development. However, whether or not exclusive physical EE is sufficient to reverse the adverse consequences of early SI remains unclear. Here we reported that 1month-old solitary mice housed in the EE for 8weeks corrected spatial cognitive dysfunction, but did not ameliorate social interaction deficits and increased anxiety-like behavior. Pathological analyses revealed that the enriched environment decreased cellular apoptosis, synaptic protein loss, myelination defect and microglial activation in the hippocampus, but not medial prefrontal cortex (mPFC) of mice housed singly. Moreover, increased nuclear factor-kappaB and interleukin-1β levels, and downregulation of brain-derived neurotrophic factor signaling pathway were normalized in the hippocampus rather than mPFC of these animals. Our results revealed a brain region-specific effectiveness of physical EE in remediating brain impairment of adolescent SI mice, with a complete reversal of hippocampus-dependent cognitive dysfunctions, but without mitigation of mPFC associated anxiety and social interaction defects. This finding emphasizes the irreplaceable role of social life for the early brain development.

Prefrontal cortical glutathione-dependent defense and proinflammatory mediators in chronically isolated rats: Modulation by fluoxetine or clozapine

Chronic psychosocial stress modulates brain antioxidant systems and causes neuroinflammation that plays a role in the pathophysiology of depression. Although the antidepressant fluoxetine (FLX) represents the first-line treatment for depression and the atypical antipsychotic clozapine (CLZ) is considered as a second-line treatment for psychotic disorders, the downstream mechanisms of action of these treatments, beyond serotonergic or dopaminergic signaling, remain elusive. We examined behavioral changes, glutathione (GSH)-dependent defense and levels of proinflammatory mediators in the prefrontal cortex (PFC) of adult male Wistar rats exposed to 21days of chronic social isolation (CSIS). We also tested the ability of FLX (15mg/kg/day) or CLZ (20mg/kg/day), applied during CSIS, to prevent stress-induced changes. CSIS caused depressive- and anxiety-like behaviors, compromised GSH-dependent defense, and induced nuclear factor-kappa B (NF-κB) activation with a concomitant increase in cytosolic levels of proinflammatory mediators cyclooxigenase-2, interleukin-1beta and tumor necrosis factor-alpha in the PFC. NF-κB activation and proinflammatory response in the PFC were not found in CSIS rats treated with FLX or CLZ. In contrast, only FLX preserved GSH content in CSIS rats. CLZ not only failed to protect against CSIS-induced GSH depletion, but it diminished its levels when applied to non-stressed rats. In conclusion, prefrontal cortical GSH depletion and the proinflammatory response underlying depressive- and anxiety-like states induced by CSIS were prevented by FLX. The protective effect of CLZ, which was equally effective as FLX on the behavioral level, was limited to proinflammatory components. Hence, different mechanisms underlie the protective effects of these two drugs in CSIS rats.

The effects of fisetin on lipopolysaccharide-induced depressive-like behavior in mice

Major depressive disorder (MDD) involves a series of pathological changes including the inflammation and increased cytokine levels. Fisetin, a natural flavonoid, has anti-inflammatory and antioxidant, and also has been shown in our previous studies to exert anti-depressant-like properties. The present study aimed to investigate the effect of fisetin on lipopolysaccharide (LPS)-induced depressive-like behavior and inflammation in mice. The results suggested that the immobility time in the forced swimming test (FST) and tail suspension test (TST) were increased at 6 h, 12 h and 24 h after LPS injection (0.83 mg/kg). However, only the group of 24 h treatment did not show any effect on locomotion counts. Pretreatment with fisetin at doses of 20, 40 and 80 mg/kg (p.o.) for 7 days reversed LPS-induced alterations of the immobility time in both of these two tests. Further neurochemical assays suggested that pretreatment with fisetin reversed LPS-induced overexpression of pro-inflammatory cytokine (IL-1β, IL-6 and TNF-α) in the hippocampus and the prefrontal cortex (PFC). Moreover, higher dose of fisetin effectively antagonized iNOS mRNA expression and nitrite levels via the modulation of NF-κB in the hippocampus and PFC. Taken together, fisetin may be an effective therapeutic agent for LPS-induced depressive-like behaviors, which is due to its anti-inflammatory property.

Nitric oxide in the prelimbic medial prefrontal cortex is involved in the anxiogenic-like effect induced by acute restraint stress in rats

Neurons containing the neuronal nitric oxide synthase (nNOS) enzyme are located in brain areas related to defensive behavior, such as the ventromedial prefrontal cortex (vMPFC). Rats exposed to a live predator (a cat) present anxiety-like behavior and an increased number of nNOS-positive neurons in this brain area one-week later. Moreover, stress-related behavioral changes in rodents can be prevented by systemic or local vMPFC nNOS inhibition. In the present study we investigated if acute restraint stress (RS)-induced delayed (one-week) anxiogenic-like effect was associated with increased nNOS expression or activity in the vMPFC. Furthermore, we also tested if local pharmacological nNOS inhibition would prevent stress-induced behavioral changes. Male Wistar rats were submitted to RS for 3h and tested in the elevated plus maze (EPM) 24h or 7 days later. Two hours after the EPM test, their brains were removed, processed and nNOS expression in the vMPFC was evaluated by immunohistochemistry. Another group of animals was used for measuring NO metabolites (NOx; an indirect measure of NOS activity) immediately after the EPM test, 24h after RS. Independent groups had guide cannula implanted bilaterally into the prelimbic (PL) portion of vMPFC. Five to six days after surgery, the animals were submitted to RS and 24h later received local administration of the nNOS inhibitor, N-propyl-l-arginine (NPLA; 0.04 nmol). They were tested in the EPM 10 min later. RS-induced anxiogenic-like effect was accompanied by increased nNOS expression in the PL (p<0.05), but not in the infralimbic (IL) vMPFC, both 24h and 7 days after RS. Moreover, open-arm exploration of the EPM was negatively correlated with nNOS expression (p<0.05) and NOx levels (p<0.05) in the PL. The anxiogenic-like effect observed 24h after RS was prevented by NPLA (p<0.05). Our results suggest that RS-induced anxiogenic-like effect might depend on increased nNOS-mediated signaling in the PL MPFC.

Pycnogenol Protects Against Rotenone-Induced Neurotoxicity in PC12 Cells Through Regulating NF-κB-iNOS Signaling Pathway

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by dopaminergic neurons degeneration and oxidative damage may underlie this process. However, there are still no efficient drugs to cure the disease. Pycnogenol (PYC) isolated from the procyanidin-rich French maritime pine (Pinus maritime) bark has shown various antioxidant activities in previous studies. In this study, we explored its effect against rotenone (Rot)-induced neurotoxicity and the underlying mechanisms in PC12 cells. Using Rot-induced cell model of PD, we found that PYC treatment significantly increased cell viability and decreased cell apoptosis in Rot-treated PC12 cells in a dose-dependent manner. Furthermore, data showed that PYC markedly reduced inducible nitric oxide synthase (iNOS)-nitric oxide (NO) signaling in Rot-treated PC12 cells. Pretreatment with the iNOS-specific inhibitor significantly attenuated Rot-induced neurotoxicity. Moreover, PYC was found to be capable of reducing Rot-induced NF-κB activation. Blocking NF-κB signaling with its inhibitor mimicked the biological effect of PYC on Rot-induced iNOS and NO expression levels, as well as neurotoxicity in PC12 cells, suggesting that the NF-κB-iNOS signaling pathway was likely to participate in the PYC-mediated protective progress. Our results suggest that PYC protects against Rot-induced neurotoxicity in PC12 cells, and the mechanism may be associated with the downregulation of NF-κB-iNOS signaling pathway.

Neurodevelopmental Animal Models Reveal the Convergent Role of Neurotransmitter Systems, Inflammation, and Oxidative Stress as Biomarkers of Schizophrenia: Implications for Novel Drug Development

Schizophrenia is a life altering disease with a complex etiology and pathophysiology, and although antipsychotics are valuable in treating the disorder, certain symptoms and/or sufferers remain resistant to treatment. Our poor understanding of the underlying neuropathological mechanisms of schizophrenia hinders the discovery and development of improved pharmacological treatment, so that filling these gaps is of utmost importance for an improved outcome. A vast amount of clinical data has strongly implicated the role of inflammation and oxidative insults in the pathophysiology of schizophrenia. Preclinical studies using animal models are fundamental in our understanding of disease development and pathology as well as the discovery and development of novel treatment options. In particular, social isolation rearing (SIR) and pre- or postnatal inflammation (PPNI) have shown great promise in mimicking the biobehavioral manifestations of schizophrenia. Furthermore, the "dual-hit" hypothesis of schizophrenia states that a first adverse event such as genetic predisposition or a prenatal insult renders an individual susceptible to develop the disease, while a second insult (e.g., postnatal inflammation, environmental adversity, or drug abuse) may be necessary to precipitate the full-blown syndrome. Animal models that emphasize the "dual-hit" hypothesis therefore provide valuable insight into understanding disease progression. In this Review, we will discuss SIR, PPNI, as well as possible "dual-hit" animal models within the context of the redox-immune-inflammatory hypothesis of schizophrenia, correlating such changes with the recognized monoamine and behavioral alterations of schizophrenia. Finally, based on these models, we will review new therapeutic options, especially those targeting immune-inflammatory and redox pathways.

One is the deadliest number: the detrimental effects of social isolation on cerebrovascular diseases and cognition

The deleterious effects of chronic social isolation (SI) have been recognized for several decades. Isolation is a major source of psychosocial stress and is associated with an increased prevalence of vascular and neurological diseases. In addition, isolation exacerbates morbidity and mortality following acute injuries such as stroke or myocardial infarction. In contrast, affiliative social interactions can improve organismal function and health. The molecular mechanisms underlying these effects are unknown. Recently, results from large epidemiological trials and pre-clinical studies have revealed several potential mediators of the detrimental effects of isolation. At least three major biological systems have been implicated: the neuroendocrine (HPA) axis, the immune system, and the autonomic nervous system. This review summarizes studies examining the relationship between isolation and mortality and the pathophysiological mechanisms underlying SI. Cardiovascular, cerebrovascular, and neurological diseases including atherosclerosis, myocardial infarction, ischemic stroke and Alzheimer's disease are given special emphasis in the context of SI. Sex differences are highlighted and studies are separated into clinical and basic science for clarity.

The neuroendocrinology of social isolation

Social isolation has been recognized as a major risk factor for morbidity and mortality in humans for more than a quarter of a century. Although the focus of research has been on objective social roles and health behavior, the brain is the key organ for forming, monitoring, maintaining, repairing, and replacing salutary connections with others. Accordingly, population-based longitudinal research indicates that perceived social isolation (loneliness) is a risk factor for morbidity and mortality independent of objective social isolation and health behavior. Human and animal investigations of neuroendocrine stress mechanisms that may be involved suggest that (a) chronic social isolation increases the activation of the hypothalamic pituitary adrenocortical axis, and (b) these effects are more dependent on the disruption of a social bond between a significant pair than objective isolation per se. The relational factors and neuroendocrine, neurobiological, and genetic mechanisms that may contribute to the association between perceived isolation and mortality are reviewed.

Pair housing reverses post-stroke depressive behavior in mice

Social isolation (SI) has been linked epidemiologically to high rates of morbidity and mortality following stroke. In contrast, strong social support enhances recovery and lowers stroke recurrence. However, the mechanism by which social support influences stroke recovery has not been adequately explored. The goal of this study was to examine the effect of post-stroke pair housing and SI on behavioral phenotypes and chronic functional recovery in mice. Young male mice were paired for 14 days before a 60 min transient middle cerebral artery occlusion (MCAO) or sham surgery and assigned to various housing environments immediately after stroke. Post-stroke mice paired with either a sham or stroke partner showed significantly higher (P<0.05) sociability after MCAO than isolated littermates. Sociability deficits worsened over time in isolated animals. Pair-housed mice showed restored sucrose consumption (P<0.05) and reduced immobility in the tail suspension test compared to isolated cohorts. Pair-housed stroked mice demonstrated significantly reduced cerebral atrophy after 6 weeks (17.5 ± 1.5% in PH versus 40.8 ± 1.3% in SI; P<0.001). Surprisingly, total brain arginase-1, a marker of a M2 "alternatively activated" myeloid cells was higher in isolated mice. However, a more detailed assessment of cellular expression showed a significant increase in the number of microglia that co-labeled with arginase-1 in the peri-infarct region in PH stroke mice compared to SI mice. Pair housing enhances sociability and reduces avolitional and anhedonic behavior. Pair housing reduced serum IL-6 and enhanced peri-infarct microglia arginase-1 expression. Social interaction reduces post-stroke depression and improves functional recovery.

Antidepressant-like effect of nitric oxide synthase inhibitors and sildenafil against lipopolysaccharide-induced depressive-like behavior in mice

Inflammation, oxidative and nitrosative stress underlie depression being assessed in rodents by the systemic administration of lipopolysacharide (LPS). There is an increasing body of evidence of an involvement of nitric oxide (NO) pathway in depression, but this issue was not investigated in LPS-induced model. Thus, herein we evaluated the effects of NO-pathway-modulating drugs, named aminoguanidine, l-NAME, sildenafil and l-arginine, on the behavioral (forced swimming test [FST], sucrose preference [SPT] and prepulse inhibition [PPI] of the startle) and neurochemical (glutathione [GSH], lipid peroxidation, IL-1β) alterations in the prefrontal cortex, hippocampus and striatum as well as in BDNF levels in the hippocampus 24h after LPS (0.5mg/kg, i.p.) administration, a time-point related to depressive-like behavior. Twenty-four hours post LPS there was an increase in immobility time in the FST, decrease in sucrose preference and PPI levels accompanied by a decrease in GSH levels and an increase in lipid peroxidation, IL-1β and hippocampal BDNF levels suggestive of a depressive-like state. The pretreatment with the NOS inhibitors, l-NAME and aminoguanidine as well as sildenafil prevented the behavioral and neurochemical alterations induced by LPS, although sildenafil and l-NAME were not able to prevent the increase in hippocampal BDNF levels induced by LPS. The iNOS inhibitor, aminoguanidine, and imipramine prevented all behavioral and neurochemical alterations induced by LPS. l-arginine did not prevent the alterations in immobility time, sucrose preference and GSH induced by LPS. Taken together our results show that the NO-cGMP pathway is important in the modulation of the depressive-like alterations induced by LPS.

Protective effect of Hsp70i against chronic social isolation stress in the rat hippocampus

Stress-related glucocorticoids and glutamate release has been implicated in depression. Glutamate neurotoxicity is mediated, in part, by the production of nitric oxide via nitric oxide synthase (NOS) isoforms and mitochondrial damage. We previously reported that chronic social isolation stress triggers proapoptotic signaling in the rat prefrontal cortex, but not in the hippocampus. Given that the hippocampus is highly sensitive to stress, we examined signaling cascades underlying the hippocampal cellular protection through the NOS pathway, antioxidant capacity and heat shock protein (Hsp) expression. We investigated neuronal (nNOS) and inducible (iNOS) protein levels, subcellular protein distributions of nuclear factor-κB (NF-κB), CuZnSOD and MnSOD activity, reduced glutathione (GSH), stress-inducible Hsp70 (Hsp70i) protein expression and serum corticosterone (CORT) levels of rats exposed to 21 days of chronic social isolation, an animal model of depression, alone or in combination with 2 h of acute immobilization or cold stress (combined stress). Both acute stressors elevated CORT, with lesser magnitude increase in chronically isolated rats exposed to novel acute stress as compared to acute stressors alone, indicating compromised HPA axis activity. Acute cold decreased nuclear CuZnSOD activity and stimulated NF-κB nuclear translocation. Chronic social isolation resulted in no activation of NF-κB, but led to decreased GSH, iNOS and increased nNOS and Hsp70i levels, alterations that remained following combined stressors. Decreased mitochondrial MnSOD activity after combined stressors suggests compromised detoxifying capacity. These data indicate that Hsp70i upregulation may provide hippocampal cellular protection against chronic social isolation stress mediated by downregulation of iNOS protein expression through suppression of NF-κB activation.