Imipramine attenuates neuroinflammatory signaling and reverses stress-induced social avoidance

Trained immunity: General and emerging concepts

Over the past decade, compelling evidence has unveiled previously overlooked adaptive characteristics of innate immune cells. Beyond their traditional role in providing short, non-specific protection against pathogens, innate immune cells can acquire antigen-agnostic memory, exhibiting increased responsiveness to secondary stimulation. This long-term de-facto innate immune memory, also termed trained immunity, is mediated through extensive metabolic rewiring and epigenetic modifications. While the upregulation of trained immunity proves advantageous in countering immune paralysis, its overactivation contributes to the pathogenesis of autoinflammatory and autoimmune disorders. In this review, we present the latest advancements in the field of innate immune memory followed by a description of the fundamental mechanisms underpinning trained immunity generation and different cell types that mediate it. Furthermore, we explore its implications for various diseases and examine current limitations and its potential therapeutic targeting in immune-related disorders.

Involvement of indoleamine 2, 3-dioxygenase (IDO) and brain-derived neurotrophic factor (BDNF) in the neuroprotective mechanisms of ferulic acid against depressive-like behaviour

OBJECTIVE

Ferulic acid (FA) is a common food ingredient that is abundantly present in various routinely consumed food and beverages. Like many cinnamic acid derivatives, FA produces wide-ranging effects in a dose-dependent manner and various studies link FA consumption with reduced risk of depressive disorders. The aim of this study was to exploit the neuroprotective mechanisms of FA including indoleamine 2,3-dioxygenase (IDO), brain-derived neurotrophic factor (BDNF), and other pro-inflammatory cytokines by employing lipopolysaccharide (LPS)-induced depressive-like behaviour model.

METHODS

C57BL/6J male mice were divided into 4 groups consisting of saline (SAL), LPS, FA and Imipramine (IMI). Animals were pretreated orally with FA (10 mg/kg) and IMI (10 mg/kg) for 21 days once daily and all groups except SAL were challenged with LPS (0.83 mg/kg) intraperitoneally on day 21.

RESULTS

LPS administration produced a biphasic change in the behaviour of the animals where the animals lost a significant weight and express high immobility time at 24 h. Proinflammatory cytokines including, TNF-α, IL-6, IL-1β, and IFN-γ were significantly increased along with increased lipid peroxidation and reduced BDNF. Furthermore, the increased kynurenine to tryptophan ratio was indicative of elevated IDO activity.

CONCLUSION

The results of this study emphasise that low dose of FA is effective in attenuating depressive-like behaviour by modulating IDO, BDNF and reducing neuroinflammation.

Oxidative Stress in Depression: The Link with the Stress Response, Neuroinflammation, Serotonin, Neurogenesis and Synaptic Plasticity

Depression is a prevalent, complex, and highly debilitating disease. The full comprehension of this disease is still a global challenge. Indeed, relapse, recurrency, and therapeutic resistance are serious challenges in the fight against depression. Nevertheless, abnormal functioning of the stress response, inflammatory processes, neurotransmission, neurogenesis, and synaptic plasticity are known to underlie the pathophysiology of this mental disorder. The role of oxidative stress in disease and, particularly, in depression is widely recognized, being important for both its onset and development. Indeed, excessive generation of reactive oxygen species and lack of efficient antioxidant response trigger processes such as inflammation, neurodegeneration, and neuronal death. Keeping in mind the importance of a detailed study about cellular and molecular mechanisms that are present in depression, this review focuses on the link between oxidative stress and the stress response, neuroinflammation, serotonergic pathways, neurogenesis, and synaptic plasticity's imbalances present in depression. The study of these mechanisms is important to lead to a new era of treatment and knowledge about this highly complex disease.

Intranasal interferon-beta alleviates anxiety and depressive-like behaviors by modulating microglia polarization in an Alzheimer's disease model

Alzheimer's disease (AD) patients frequently experience neuropsychiatric symptoms (NPS), which are linked to a lower quality of life and a faster rate of disease progression. A growing body of research indicates that several microglial phenotypes control the inflammatory response and are crucial in the pathophysiology of AD-related NPS. Given the crucial role played by inflammatory mediators produced by microglia in developing of NPS, interferon-beta (IFNβ), a cytokine with anti-inflammatory capabilities, maybe a successful treatment for NPS caused by AD. In this investigation, using a rat model of AD, we examined the impact of intranasal treatment of IFNβ on anxious/depressive-like behavior and microglial M1/M2 polarization. The rat hippocampus was bilaterally injected with lentiviruses harboring mutant human amyloid precursor protein. Rats were given recombinant IFNβ1a (68,000 IU/rat) via the intranasal route, starting on day 23 following viral infection and continuing until day 49. On days 47-49, the elevated plus maze, forced swim, and tail suspension tests were applied to measure anxiety- and depressive-like behavior. Additionally, qPCR was utilized to quantify the expression of M1 markers (CD68, CD86, and CD40) and M2 markers (Ym1, CD206, Arg1, GDNF, BDNF, and SOCS1). Our findings demonstrated that decreased M2 marker expression is accompanied by anxious/depressive-like behavior when the mutant human APP gene is overexpressed in the hippocampus. In the rat model of AD, IFNβ therapy reduces anxious/depressive-like behaviors, at least in part by polarizing microglia towards M2. Therefore, IFNβ may be a viable therapeutic drug for reducing NPS in the context of AD.

IL-1 Receptor-1 on Vglut2 + neurons in the hippocampus is critical for neuronal and behavioral sensitization after repeated social stress

Myriad findings connect stress and inflammation to mood disorders. Social defeat in mice promotes the convergence of neuronal, central inflammatory (microglia), and peripheral immune (monocytes) pathways causing anxiety, social avoidance, and "stress-sensitization." Stress-sensitization results in augmented inflammation and the recurrence of anxiety after re-exposure to social stress. Different cell compartments, including neurons, may be uniquely sensitized by social defeat-induced interleukin-1 (IL-1) signaling. Therefore, the aim of this study was to determine if glutamatergic neuronal IL-1 receptor signaling was essential in promoting stress-sensitization after social defeat. Here, wild-type (IL-1R1+/+) mice and mice with IL-1 receptor-1 deleted selectively in glutamatergic neurons (Vglut2-IL-1R1-/-) were stress-sensitized by social defeat (6-cycles) and then exposed to acute defeat (1-cycle) at day 30. Acute defeat-induced neuronal activation (ΔFosB and phospo-CREB) in the hippocampus of stress-sensitized mice was dependent on neuronal IL-1R1. Moreover, acute defeat-induced social withdrawal and working memory impairment in stress-sensitized mice were also dependent on neuronal IL-1R1. To address region and time dependency, an AAV2-IL-1 receptor antagonist construct was administered into the hippocampus after sensitization, but prior to acute defeat at day 30. Although stress-sensitized mice had increased hippocampal pCREB and decreased working memory after stress re-exposure, these events were not influenced by AAV2-IL-1 receptor antagonist. Hippocampal ΔFosB induction and corresponding social withdrawal in stress-sensitized mice after stress re-exposure were prevented by the AAV2-IL-1 receptor antagonist. Collectively, IL-1 signaling in glutamatergic neurons of the hippocampus was essential in neuronal-sensitization after social defeat and the recall of social withdrawal.

The neuroimmunology of social-stress-induced sensitization

Myriad clinical findings provide links between chronic stressors, inflammation, and mood disorders. Furthermore, traumatic or chronic exposure to psychological stressors may promote stress sensitization, in which individuals have long-term complications, including increased vulnerability to subsequent stressors. Post-traumatic stress disorder (PTSD) is a clinically relevant example of stress sensitization. PTSD alters neuronal circuitry and mood; however, the mechanisms underlying long-term stress sensitization within this disorder are unclear. Rodent models of chronic social defeat recapitulate several key physiological, immunological, and behavioral responses associated with psychological stress in humans. Repeated social defeat (RSD) uniquely promotes the convergence of neuronal, central inflammatory (microglial), and peripheral immune (monocyte) pathways, leading to prolonged anxiety, social withdrawal, and cognitive impairment. Moreover, RSD promotes stress sensitization, in which mice are highly sensitive to subthreshold stress exposure and recurrence of anxiety weeks after the cessation of stress. Therefore, the purpose of this Review is to discuss the influence of social-defeat stress on the immune system that may underlie stress sensitization within three key cellular compartments: neurons, microglia, and monocytes. Delineating the mechanisms of stress sensitization is critical in understanding and treating conditions such as PTSD.

Effect of the dietary intake of fish oil on psycho-social behavioral disorder caused by social-defeat stress

Exposure to psychosocial stress is a risk factor for human diseases such as depression. Social defeat stress (SDS) is a well-known rodent model of human psychosocial stress, and animals exposed to SDS show social avoidance behavior. Fish oil, which is rich in docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), is expected to decrease the risk of depressive disorders. In this study, we determined whether fish oil affects the social behavior of SDS-exposed mice and measured serotonin levels and expression of genes related to tryptophan (TRP) metabolism in the hippocampus. The experimental animals were fed a diet containing fish oil during SDS exposure. For the fish oil treatment, experimental mice were fed a diet containing fish oil at low (L-FO), middle (M-FO), and high (H-FO) concentrations. The control group was supplemented with an equivalent amount of canola oil (no fish oil: N-FO). After the SDS protocol, we performed a social interaction test and assessed the sociality of experimental mice. In the N-FO group, SDS-exposed mice showed negative social interactions compared with non-stressed mice. The L-FO and H-FO groups showed negative social interactions after SDS exposure; however, the M-FO group did not exhibit negative social behavior. The serotonin levels of SDS-exposed mice were lower than those of non-stressed mice in the N-FO group. In contrast with these results in the N-FO group, there was no difference in serotonin levels between SDS-exposed and non-stressed mice in the FO groups. In addition, the expression of genes related to TRP metabolism in SDS-exposed mice increased in the N-FO group, but not in the FO group. These results suggest that fish oil improves the psychosocial behavioral disorders caused by SDS. This improvement could be explained by the increase in serotonin synthesis in the hippocampus.

Time of exposure to social defeat stress during childhood and adolescence and redox dysregulation on long-lasting behavioral changes, a translational study

Traumatic events during childhood/early adolescence can cause long-lasting physiological and behavioral changes with increasing risk for psychiatric conditions including psychosis. Genetic factors and trauma (and their type, degree of repetition, time of occurrence) are believed to influence how traumatic experiences affect an individual. Here, we compared long-lasting behavioral effects of repeated social defeat stress (SD) applied during either peripuberty or late adolescence in adult male WT and Gclm-KO mice, a model of redox dysregulation relevant to schizophrenia. As SD disrupts redox homeostasis and causes oxidative stress, we hypothesized that KO mice would be particularly vulnerable to such stress. We first found that peripubertal and late adolescent SD led to different behavioral outcomes. Peripubertal SD induced anxiety-like behavior in anxiogenic environments, potentiated startle reflex, and increased sensitivity to the NMDA-receptor antagonist, MK-801. In contrast, late adolescent SD led to increased exploration in novel environments. Second, the long-lasting impact of peripubertal but not late adolescent SD differed in KO and WT mice. Peripubertal SD increased anxiety-like behavior in anxiogenic environments and MK-801-sensitivity mostly in KO mice, while it increased startle reflex in WT mice. These suggest that a redox dysregulation during peripuberty interacts with SD to remodel the trajectory of brain maturation, but does not play a significant role during later SD. As peripubertal SD induced persisting anxiety- and fear-related behaviors in male mice, we then investigated anxiety in a cohort of 89 early psychosis male patients for whom we had information about past abuse and clinical assessment during the first year of psychosis. We found that a first exposure to physical/sexual abuse (analogous to SD) before age 12, but not after, was associated with higher anxiety at 6-12 months after psychosis onset. This supports that childhood/peripuberty is a vulnerable period during which physical/sexual abuse in males has wide and long-lasting consequences.

The Relationship between Stress, Inflammation, and Depression

A narrative review about the relationship between stress, inflammation, and depression is made as follows: Chronic stress leads to various stress-related diseases such as depression. Although most human diseases are related to stress exposure, the common pathways between stress and pathophysiological processes of different disorders are still debatable. Chronic inflammation is a crucial component of chronic diseases, including depression. Both experimental and clinical studies have demonstrated that an increase in the levels of pro-inflammatory cytokines and stress hormones, such as glucocorticoids, substantially contributes to the behavioral alterations associated with depression. Evidence suggests that inflammation plays a key role in the pathology of stress-related diseases; however, this link has not yet been completely explored. In this study, we aimed to determine the role of inflammation in stress-induced diseases and whether a common pathway for depression exists. Recent studies support pharmacological and non-pharmacological treatment approaches significantly associated with ameliorating depression-related inflammation. In addition, major depression can be associated with an activated immune system, whereas antidepressants can exert immunomodulatory effects. Moreover, non-pharmacological treatments for major depression (i.e., exercise) may be mediated by anti-inflammatory actions. This narrative review highlights the mechanisms underlying inflammation and provides new insights into the prevention and treatment of stress-related diseases, particularly depression.

Antidepressants Usage and Risk of Pneumonia Among Elderly Patients With the Parkinson's Disease: A Population-Based Case-Control Study

The patients with Parkinson's disease (PD) are associated with a higher risk of pneumonia. Antidepressants exert an anticholinergic effect in varying degrees and various classes of antidepressants also can produce a different effect on immune function. The relationship between the risk of pneumonia and the use of antidepressants among elderly patients with PD is unknown. The study investigated the risk of pneumonia associated with the use of antidepressants in elderly patients with PD. This case-control study was based on data from the longitudinal health insurance database in Taiwan. We analyzed the data of 551,975 elderly patients with PD between 2002 and 2018. To reduce the potential confounding caused by unbalanced covariates in non-experimental settings, we used propensity score matching to include older patients without pneumonia to serve as the comparison. The antidepressants in the study included tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), serotonin, and norepinephrine reuptake inhibitors (SNRIs). The conditional logistic regression was used to investigate the association between antidepressants and pneumonia. Control variables in the study included sex, age, income level, urbanization, Charlson comorbidity index score, and comorbidities related to pneumonia. In terms of TCAs users, compared with patients not receiving TCAs, current users had a lower risk of incident pneumonia (adjusted odds ratio [aOR] = 0.86, 95% CI = 0.82–0.90) and recent users (aOR = 0.83, 95% CI = 0.80–0.87). In terms of MAOIs users, current users had a lower risk of incident pneumonia (aOR = 0.88, 95% CI = 0.83–0.93), recent users (aOR = 0.89, 95% CI = 0.85–0.93). In terms of SSRIs users, current users had a higher risk of incident pneumonia (a OR = 1.13, 95% CI = 1.01–1.17), recent users (aOR = 1.01, 95% CI = 1.06–1.13), and past users (aOR = 1.19, 95% CI = 1.17–1.21). In terms of SNRIs users, past users had a higher risk of incident pneumonia (aOR = 1.07, 95% CI = 1.03–1.10). The incident pneumonia is associated with the use of individuals of different classes of antidepressants. The use of TCAs (such as, amitriptyline and imipramine) had a lower odds of incident pneumonia. The use of MAOIs (such as, selegiline and rasagiline) had a lower odds of pneumonia during recent use. The use of SSRIs (such as, fluoxetine, sertraline, escitalopram, paroxetine, and citalopram) and SNRIs (such as, milnacipran, and venlafaxine) had a higher odds of incident pneumonia.

Kampo formulas alleviate aging-related emotional disturbances and neuroinflammation in male senescence-accelerated mouse prone 8 mice

Aging-induced neuroinflammation, also known as neuroinflammaging, plays a pivotal role in emotional disturbances, including depression and anxiety, in older individuals, thereby leading to cognitive dysfunction. Although numerous studies have focused on therapeutic strategies for cognitive impairment in older individuals, little research has been performed on treating its preceding emotional disturbances. Here, we examined whether Kampo formulas (kososan [KS], nobiletin-rich kososan [NKS], and hachimijiogan [HJG]) can ameliorate aging-induced emotional disturbances and neuroinflammation in mice. The depression-like behaviors observed in SAMP8 mice, relative to normally aging SAMR1 mice, were significantly prevented by treatment with Kampo formulas for 13 weeks. Western blot analysis revealed that hippocampal neuroinflammation was significantly abrogated by Kampo formulas. KS and NKS also significantly attenuated the hippocampal neuroinflammatory priming induced by lipopolysaccharide (LPS, 0.33 mg/kg, i.p.) challenge in SAMP8 mice. Hippocampal IL-1β, IL-6, and MCP-1 levels were significantly decreased in NKS-treated SAMP8 mice. KS and NKS showed significantly reduced tau accumulation in the brains of SAMP8 mice. RNA-sequencing revealed that each Kampo formula led to unique dynamics of hippocampal gene expression and appeared to abrogate hippocampal inflammatory responses. HJG significantly blocked the LPS-induced increase in serum IL-6 and MCP-1. These results suggest that Kampo formulas would be useful for treating aging-induced depression, in part by regulating neuroinflammatory pathways. This finding may pave the way for the development of therapeutic strategies for aging-related emotional disturbances, which may contribute to the prevention of cognitive dysfunction in older individuals.

Neuroinflammation and Mitochondrial Dysfunction Link Social Stress to Depression

Major depressive disorder is a debilitating mental illness and a leading cause of global disease burden. While many etiological factors have been identified, social stress is a highly prevalent causative factor for the onset of depression. Unfortunately, rates of depression continue to increase around the world, and the recent COVID-19 pandemic has further exacerbated this mental health crisis. Though several therapeutic strategies are available, nearly 50% of patients who receive treatment never reach remission. The exact mechanisms by which social stress exposure promotes the development of depression are unclear, making it challenging to develop novel and more effective therapeutics. However, accumulating evidence points to a role for stress-induced neuroinflammation, particularly in treatment-resistant patients. Moreover, recent evidence has expanded the concept of the pathogenesis of depression to mitochondrial dysfunction, suggesting that the combined effects of social stress on mitochondria and inflammation may synergize to facilitate stress-related depression. In this chapter, we review evidence for neuroinflammation and mitochondrial dysfunction in the pathogenesis of social stress-induced depression and discuss these in the context of novel therapeutic targets for the treatment of depression.

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

Microglial Inflammatory-Metabolic Pathways and Their Potential Therapeutic Implication in Major Depressive Disorder

Increasing evidence supports the notion that neuroinflammation plays a critical role in the etiology of major depressive disorder (MDD), at least in a subset of patients. By virtue of their capacity to transform into reactive states in response to inflammatory insults, microglia, the brain's resident immune cells, play a pivotal role in the induction of neuroinflammation. Experimental studies have demonstrated the ability of microglia to recognize pathogens or damaged cells, leading to the activation of a cytotoxic response that exacerbates damage to brain cells. However, microglia display a wide range of responses to injury and may also promote resolution stages of inflammation and tissue regeneration. MDD has been associated with chronic priming of microglia. Recent studies suggest that altered microglial morphology and function, caused either by intense inflammatory activation or by senescence, may contribute to depression and associated impairments in neuroplasticity. In this context, modifying microglia phenotype by tuning inflammatory pathways might have important translational relevance to harness neuroinflammation in MDD. Interestingly, it was recently shown that different microglial phenotypes are associated with distinct metabolic pathways and analysis of the underlying molecular mechanisms points to an instrumental role for energy metabolism in shaping microglial functions. Here, we review various canonical pro-inflammatory, anti-inflammatory and metabolic pathways in microglia that may provide new therapeutic opportunities to control neuroinflammation in brain disorders, with a strong focus on MDD.

Gut Bacteria and Neuropsychiatric Disorders

Bacteria in the gut microbiome plays an intrinsic part in immune activation, intestinal permeability, enteric reflex, and entero-endocrine signaling. Apart from physiological and structural changes brought about by gut bacteria on entero-epithelial cells and mucus layers, a vast number of signals generated in the gastro-intestinal tract (GIT) reaches the brain via the vagus nerve. Research on the gut–brain axis (GBA) has mostly been devoted to digestive functions and satiety. Less papers have been published on the role gut microbiota play in mood, cognitive behavior and neuropsychiatric disorders such as autism, depression and schizophrenia. Whether we will be able to fully decipher the connection between gut microbiota and mental health is debatable, especially since the gut microbiome is diverse, everchanging and highly responsive to external stimuli. Nevertheless, the more we discover about the gut microbiome and the more we learn about the GBA, the greater the chance of developing novel therapeutics, probiotics and psychobiotics to treat gastro-intestinal disorders such as inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), but also improve cognitive functions and prevent or treat mental disorders. In this review we focus on the influence gut bacteria and their metabolites have on neuropsychiatric disorders.

Genetic Ablation of the Inducible Form of Nitric Oxide in Male Mice Disrupts Immature Neuron Survival in the Adult Dentate Gyrus

Inducible nitric oxide synthase (iNOS) is an enzyme upregulated in the brain during neuroimmune stimuli which is associated with an oxidative and pro-inflammatory environment in several brain regions, including the hippocampal formation and the prefrontal cortex. The dentate gyrus of the hippocampal formation is the site of a process known as adult hippocampal neurogenesis (AHN). Although many endogenous and extrinsic factors can modulate AHN, the exact participation of specific proinflammatory mediators such as iNOS in these processes remains to be fully elucidated. Here, we investigated how the total genetic ablation of iNOS impacts the hippocampal neurogenic niche and microglial phenotype and if these changes are correlated to the behavioral alterations observed in iNOS knockout (K.O.) mice submitted or not to the chronic unpredictable stress model (CUS - 21 days protocol). Contrary to our initial hypothesis, at control conditions, iNOS K.O. mice displayed no abnormalities on microglial activation in the dentate gyrus. However, they did exhibit impaired newborn cells and immature neuron survival, which was not affected by CUS. The reduction of AHN in iNOS K.O. mice was accompanied by an increased positive coping response in the tail suspension test and facilitation of anxiety-like behaviors in the novelty suppressed feeding. Next, we investigated whether a pro-neurogenic stimulus would rescue the neurogenic capacity of iNOS K.O. mice by administering in control and CUS groups the antidepressant escitalopram (ESC). The chronic treatment with ESC could not rescue the neurogenic capacity or the behavioral changes observed in iNOS K.O. mice. Besides, in the ventromedial prefrontal (vmPFC) cortex there was no change in the expression or the chronic activation of PV neurons (evaluated by double labeling PV with FOSB) in the prelimbic (PrL) or infralimbic subregions. FOSB expression, however, increased in the PrL of iNOS K.O. mice. Our results suggest that iNOS seems essential for the survival of newborn cells and immature neurons in the hippocampus and seem to partially explain the anxiogenic-like behavior observed in iNOS K.O. mice. On the other hand, the iNOS ablation appears to result in increased activity of the PrL which could explain the antidepressant-like behaviors of iNOS K.O mice.

The Relationship Between the Gut Microbiome-Immune System-Brain Axis and Major Depressive Disorder

Major depressive disorder (MDD) is a prominent cause of disability worldwide. Current antidepressant drugs produce full remission in only about one-third of MDD patients and there are no biomarkers to guide physicians in selecting the best treatment for individuals. There is an urgency to learn more about the etiology of MDD and to identify new targets that will lead to improved therapy and hopefully aid in predicting and preventing MDD. There has been extensive interest in the roles of the immune system and the gut microbiome in MDD and in how these systems interact. Gut microbes can contribute to the nature of immune responses, and a chronic inflammatory state may lead to increased responsiveness to stress and to development of MDD. The gut microbiome-immune system-brain axis is bidirectional, is sensitive to stress and is important in development of stress-related disorders such as MDD. Communication between the gut and brain involves the enteric nervous system (ENS), the autonomic nervous system (ANS), neuroendocrine signaling systems and the immune system, and all of these can interact with the gut microbiota. Preclinical studies and preliminary clinical investigations have reported improved mood with administration of probiotics and prebiotics, but large, carefully controlled clinical trials are now necessary to evaluate their effectiveness in treating MDD. The roles that several gut microbe-derived molecules such as neurotransmitters, short chain fatty acids and tryptophan play in MDD are reviewed briefly. Challenges and potential future directions associated with studying this important axis as it relates to MDD are discussed.

Highlighting Immune System and Stress in Major Depressive Disorder, Parkinson's, and Alzheimer's Diseases, with a Connection with Serotonin

There is recognition that both stress and immune responses are important factors in a variety of neurological disorders. Moreover, there is an important role of several neurotransmitters that connect these factors to several neurological diseases, with a special focus in this paper on serotonin. Accordingly, it is known that imbalances in stressors can promote a variety of neuropsychiatric or neurodegenerative pathologies. Here, we discuss some facts that link major depressive disorder, Alzheimer’s, and Parkinson’s to the stress and immune responses, as well as the connection between these responses and serotonergic signaling. These are important topics of investigation which may lead to new or better treatments, improving the life quality of patients that suffer from these conditions.

High risk of drug toxicity in social isolation stress due to liver dysfunction: Role of oxidative stress and inflammation

BACKGROUND

Previous studies have shown that social isolation stress (SIS) could associate with several systemic diseases; however, the role of SIS on liver dysfunction has yet to be established. This study aimed to investigate the effect of SIS on liver function and possible drug toxicity through liver inflammation and oxidative stress.

METHODS

Male Naval Medical Research Institute mice in two groups of SIS and control were treated with typical anti-depressant and anxiolytic agents including fluoxetine, norfluoxetine, desipramine, and imipramine in both groups. Then blood concentrations (or their active metabolites) of these drugs were assessed. Liver function test, including aspartate transaminase (AST), alanine aminotransferase (ALT), total bilirubin, and conjugated bilirubin), oxidative activity, inflammatory cytokines, and the gene expression of cytochrome P450 enzymes were assessed.

RESULTS

We observed that the liver enzymes including AST and ALT was slightly higher in SIS animals. The blood concentrations of fluoxetine, norfluoxetine, desipramine, and imipramine were significantly higher in SIS animals. The gene expression of CYP1A2, CYP2A6, CYP2C9, CYP2C29, and CYP2D were significantly decreased in SIS animals. Our results showed that SIS animals had significantly higher level of tumor necrosis factor-α, interleukin-1β, and interleukin-6. SIS could significantly decrease the activity of antioxidant agent (Glutathione).

CONCLUSION

We hypothesized that SIS could induce liver dysfunction and decrease the rate of drug clearance through liver inflammation and oxidative stress; therefore, the blood concentration of anti-depressant/anxiolytic agents should closely monitor in SIS due to the high toxicity of these agents.

Acute Diallyl Disulfide Administration Prevents and Reveres Lipopolysaccharide-Induced Depression-Like Behaviors in Mice via Regulating Neuroinflammation and Oxido-Nitrosative Stress

Neuroinflammation and oxidative stress play critical roles in pathogenesis of depression. Diallyl disulfide (DADS), an active compound in garlic oil, has been shown to exhibit obvious anti-inflammatory and anti-oxidative activities. Preliminary evidence indicates that depression is associated with high levels of pro-inflammatory cytokines and oxidative markers, suggesting that inhibition of neuroinflammatory response and oxidative stress may be beneficial for depression interruption. Here, we investigated the antidepressant effect of DADS as well as it mechanisms in a depression-like model induced by lipopolysaccharide (LPS). Similarly to imipramine (10 mg/kg), a clinical antidepressant, DADS (40 or 80 mg/kg), which was administered 1 h before LPS treatment (pre-LPS) or 1.5 h and 23.5 h after LPS treatment (post-LPS), prevented and reversed LPS (100 μg/kg)-induced increase in immobility time in the tail suspension test (TST) and forced swim test (FST) in mice. Mechanistic studies revealed that DADS pre-treatment or post-treatment at the dose of 40 and 80 mg/kg prevented and reversed (i) LPS-induced increases in interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and nitric oxide (NO) levels in the hippocampus and prefrontal cortex, (ii) LPS-induced increases in contents of malondialdehyde (MDA), a parameter reflecting high levels of oxidative stress, and (iii) LPS-induced decreases in contents of GSH, a marker reflecting weakened anti-oxidative ability, in the hippocampus and prefrontal cortex in mice. These results indicate that DADS is comparable to imipramine in effectively ameliorating LPS-induced depression-like behaviors in mice, providing a potential value for DADS in prevention and/or therapy of depression.

Neonatal Proinflammatory Stress and Expression of Neuroinflammation-Associated Genes in the Rat Hippocampus

Differential effect of the neonatal proinflammatory stress (NPS) on the development of neuroinflammation in the hippocampus and induction of the depressive-like behavior in juvenile and adult male and female rats was studied. NPS induction by bacterial lipopolysaccharide in the neonatal period upregulated expression of the Il6 and Tnf mRNAs accompanied by the development of depressive-like behavior in the adult male rats. NPS increased expression of the mRNAs for fractalkine and its receptor in the ventral hippocampus of the juvenile male rats, but did not affect expression of mRNAs for the proinflammatory cytokines and soluble form of fractalkine. NPS downregulated expression of fractalkine mRNA in the dorsal hippocampus of juvenile males. No significant effects of NPS were found in the female rats. Therefore, the NPS induces long-term changes in the expression of neuroinflammation-associated genes in different regions of the hippocampus, which ultimately leads to the induction of neuroinflammation and development of depressive-like behavior in male rats.

Trained Immunity: Reprogramming Innate Immunity in Health and Disease

Traditionally, the innate and adaptive immune systems are differentiated by their specificity and memory capacity. In recent years, however, this paradigm has shifted: Cells of the innate immune system appear to be able to gain memory characteristics after transient stimulation, resulting in an enhanced response upon secondary challenge. This phenomenon has been called trained immunity. Trained immunity is characterized by nonspecific increased responsiveness, mediated via extensive metabolic and epigenetic reprogramming. Trained immunity explains the heterologous effects of vaccines, which result in increased protection against secondary infections. However, in chronic inflammatory conditions, trained immunity can induce maladaptive effects and contribute to hyperinflammation and progression of cardiovascular disease, autoinflammatory syndromes, and neuroinflammation. In this review we summarize the current state of the field of trained immunity, its mechanisms, and its roles in both health and disease.

Inflammation-driven brain and gut barrier dysfunction in stress and mood disorders

Regulation of emotions is generally associated exclusively with the brain. However, there is evidence that peripheral systems are also involved in mood, stress vulnerability vs. resilience, and emotion‐related memory encoding. Prevalence of stress and mood disorders such as major depression, bipolar disorder, and post‐traumatic stress disorder is increasing in our modern societies. Unfortunately, 30%–50% of individuals respond poorly to currently available treatments highlighting the need to further investigate emotion‐related biology to gain mechanistic insights that could lead to innovative therapies. Here, we provide an overview of inflammation‐related mechanisms involved in mood regulation and stress responses discovered using animal models. If clinical studies are available, we discuss translational value of these findings including limitations. Neuroimmune mechanisms of depression and maladaptive stress responses have been receiving increasing attention, and thus, the first part is centered on inflammation and dysregulation of brain and circulating cytokines in stress and mood disorders. Next, recent studies supporting a role for inflammation‐driven leakiness of the blood–brain and gut barriers in emotion regulation and mood are highlighted. Stress‐induced exacerbated inflammation fragilizes these barriers which become hyperpermeable through loss of integrity and altered biology. At the gut level, this could be associated with dysbiosis, an imbalance in microbial communities, and alteration of the gut–brain axis which is central to production of mood‐related neurotransmitter serotonin. Novel therapeutic approaches such as anti‐inflammatory drugs, the fast‐acting antidepressant ketamine, and probiotics could directly act on the mechanisms described here improving mood disorder‐associated symptomatology. Discovery of biomarkers has been a challenging quest in psychiatry, and we end by listing promising targets worth further investigation.

Evolutionary Significance of the Neuroendocrine Stress Axis on Vertebrate Immunity and the Influence of the Microbiome on Early-Life Stress Regulation and Health Outcomes

Stress is broadly defined as the non-specific biological response to changes in homeostatic demands and is mediated by the evolutionarily conserved neuroendocrine networks of the hypothalamus-pituitary-adrenal (HPA) axis and the sympathetic nervous system. Activation of these networks results in transient release of glucocorticoids (cortisol) and catecholamines (epinephrine) into circulation, as well as activation of sympathetic fibers innervating end organs. These interventions thus regulate numerous physiological processes, including energy metabolism, cardiovascular physiology, and immunity, thereby adapting to cope with the perceived stressors. The developmental trajectory of the stress-axis is influenced by a number of factors, including the gut microbiome, which is the community of microbes that colonizes the gastrointestinal tract immediately following birth. The gut microbiome communicates with the brain through the production of metabolites and microbially derived signals, which are essential to human stress response network development. Ecological perturbations to the gut microbiome during early life may result in the alteration of signals implicated in developmental programming during this critical window, predisposing individuals to numerous diseases later in life. The vulnerability of stress response networks to maladaptive development has been exemplified through animal models determining a causal role for gut microbial ecosystems in HPA axis activity, stress reactivity, and brain development. In this review, we explore the evolutionary significance of the stress-axis system for health maintenance and review recent findings that connect early-life microbiome disturbances to alterations in the development of stress response networks.

Cannabinoid receptor 1 signalling modulates stress susceptibility and microglial responses to chronic social defeat stress

Psychosocial stress is one of the main environmental factors contributing to the development of psychiatric disorders. In humans and rodents, chronic stress is associated with elevated inflammatory responses, indicated by increased numbers of circulating myeloid cells and activation of microglia, the brain-resident immune cells. The endocannabinoid system (ECS) regulates neuronal and endocrine stress responses via the cannabinoid receptor 1 (CB1). CB1-deficient mice (Cnr1^-/-) are highly sensitive to stress, but if this involves altered inflammatory responses is not known. To test this, we exposed Cnr1^+/+ and Cnr1^-/- mice to chronic social defeat stress (CSDS). Cnr1^-/- mice were extremely sensitive to a standard protocol of CSDS, indicated by an increased mortality rate. Therefore, a mild CSDS protocol was established, which still induced a behavioural phenotype in susceptible Cnr1^-/- mice. These mice also showed altered glucocorticoid levels after mild CSDS, suggesting dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. Mild CSDS induced weak myelopoiesis in the periphery, but no recruitment of myeloid cells to the brain. In contrast, mild CSDS altered microglial activation marker expression and morphology in Cnr1^-/- mice. These microglial changes correlated with the severity of the behavioural phenotype. Furthermore, microglia of Cnr1^-/- mice showed increased expression of Fkbp5, an important regulator of glucocorticoid signalling. Overall, the results confirm that CB1 signalling protects the organism from the physical and emotional harm of social stress and implicate endocannabinoid-mediated modulation of microglia in the development of stress-related pathologies.

Tetragonia tetragonioides Relieves Depressive-Like Behavior through the Restoration of Glial Loss in the Prefrontal Cortex

Tetragonia tetragonioides, which is a halophyte and grows widely in Asian-Pacific regions, has been used for the treatment of digestive disorders in traditional oriental medicine. This study examined the potential antidepressant effect of Tetragonia tetragonioides in an astroglial degeneration model of depression, which was established based on the postmortem study of depressive patients' brain presenting diminished astrocytes in the prefrontal cortex. C57BL/6 male mice were exposed to glial ablation in the prefrontal cortex by the administration of the gliotoxin, L-alpha-aminoadipic acid (L-AAA) to induce depression. Tetragonia tetragonioides at doses of 100 mg/kg and 300 mg/kg, imipramine at a dose of 15 mg/kg, and distilled water were orally administrated to mice for 18 days. Behavioral tests including the open field test (OFT), sucrose preference test (SPT), forced swimming test (FST), and tail suspension test (TST) were carried out after 2 days of L-AAA injection. The expression levels of GFAP and NeuN in the prefrontal cortex were determined by immunohistochemistry. Mice subjected to glial ablation in the prefrontal cortex displayed decreased sucrose consumption in SPT and increased immobility time in FST and TST. Treatment with imipramine and Tetragonia tetragonioides remarkably ameliorated the behavioral despair induced by L-AAA. In addition, immunohistochemistry analysis showed that treatment with Tetragonia tetragonioides significantly restored the glial loss as indicated by the elevated GFAP expression level. These findings suggest that Tetragonia tetragonioides exerts an antidepressant effect through the restoration of glial loss under conditions of depression and can be a candidate for an antidepressant agent.

How stress physically re-shapes the brain: Impact on brain cell shapes, numbers and connections in psychiatric disorders

Severe stress is among the most robust risk factors for the development of psychiatric disorders. Imaging studies indicate that life stress is integral to shaping the human brain, especially regions involved in processing the stress response. Although this is likely underpinned by changes to the cytoarchitecture of cellular networks in the brain, we are yet to clearly understand how these define a role for stress in human psychopathology. In this review, we consolidate evidence of macro-structural morphometric changes and the cellular mechanisms that likely underlie them. Focusing on stress-sensitive regions of the brain, we illustrate how stress throughout life may lead to persistent remodelling of the both neurons and glia in cellular networks and how these may lead to psychopathology. We support that greater translation of cellular alterations to human cohorts will support parsing the psychological sequalae of severe stress and improve our understanding of how stress shapes the human brain. This will remain a critical step for improving treatment interventions and prevention outcomes.

Antidepressant Effect of Paeoniflorin Is Through Inhibiting Pyroptosis CASP-11/GSDMD Pathway

Nod-like receptor protein 3 (NLRP3)-associated neuroinflammation mediated by activated microglia is involved in the pathogenesis of depression. The role of the pore-forming protein gasdermin D (GSDMD), a newly identified pyroptosis executioner downstream of NLRP3 inflammasome mediating inflammatory programmed cell death, in depression has not been well defined. Here, we provide evidence that paeoniflorin (PF), a monoterpene glycoside compound derived from Paeonia lactiflora, ameliorated reserpine-induced mouse depression-like behaviors, characterized as increased mobility time in tail suspension test and forced swimming test, as well as the abnormal alteration of synaptic plasticity in the depressive hippocampus. The molecular docking simulation predicted that PF would interact with C-terminus of GSDMD. We further demonstrated that PF administration inhibited the enhanced expression of GSDMD which mainly distributed in microglia, along with the proteins involved in pyroptosis signaling transduction including caspase (CASP)-11, CASP-1, NLRP3, and interleukin (IL)-1β in the hippocampus of mice treated with reserpine. And also, PF prevented lipopolysaccharide (LPS) and adenosine triphosphate (ATP)-induced pyroptosis in murine N9 microglia in vitro, evidenced by inhibiting the expression of CASP-11, NLRP3, CASP-1 cleavage, as well as IL-1β. Furthermore, VX-765, an effective and selective inhibitor for CASP-1 activation, reduced the expression of inflammasome and pyroptosis-associated proteins in over-activated N9 and also facilitated PF-mediated inhibition of pyroptosis synergistically. Collectively, the data indicated that PF exerted antidepressant effects, alleviating neuroinflammation through inhibiting CASP-11-dependent pyroptosis signaling transduction induced by over-activated microglia in the hippocampus of mice treated with reserpine. Thus, GSDMD-mediated pyroptosis in activated microglia is a previously unrecognized inflammatory mechanism of depression and represents a unique therapeutic opportunity for mitigating depression given PF administration.

From "Leaky Gut" to Impaired Glia-Neuron Communication in Depression

In the last three decades, the robust scientific data emerged, demonstrating that the immune-inflammatory response is a fundamental component of the pathophysiology of major depressive disorder (MDD). Psychological stress and various inflammatory comorbidities contribute to such immune activation. Still, this is not uncommon that patients with depression do not have defined inflammatory comorbidities, and alternative mechanisms of immune activation need to take place. The gastrointestinal (GI) tract, along with gut-associated lymphoid tissue (GALT), constitutes the largest lymphatic organ in the human body and forms the biggest surface of contact with the external environment. It is also the most significant source of bacterial and food-derived antigenic material. There is a broad range of reciprocal interactions between the GI tract, intestinal microbiota, increased intestinal permeability, activation of immune-inflammatory response, and the CNS that has crucial implications in brain function and mental health. This intercommunication takes place within the microbiota-gut-immune-glia (MGIG) axis, and glial cells are the main orchestrator of this communication. A broad range of factors, including psychological stress, inflammation, dysbiosis, may compromise the permeability of this barrier. This leads to excessive bacterial translocation and the excessive influx of food-derived antigenic material that contributes to activation of the immune-inflammatory response and depressive psychopathology. This chapter summarizes the role of increased intestinal permeability in MDD and mechanisms of how the "leaky gut" may contribute to immune-inflammatory response in this disorder.

Microglia in neurodegenerative diseases

A major feature of neurodegeneration is disruption of central nervous system homeostasis, during which microglia play diverse roles. In the central nervous system, microglia serve as the first line of immune defense and function in synapse pruning, injury repair, homeostasis maintenance, and regulation of brain development through scavenging and phagocytosis. Under pathological conditions or various stimulations, microglia proliferate, aggregate, and undergo a variety of changes in cell morphology, immunophenotype, and function. This review presents the features of microglia, especially their diversity and ability to change dynamically, and reinterprets their role as sensors for multiple stimulations and as effectors for brain aging and neurodegeneration. This review also summarizes some therapeutic approaches for neurodegenerative diseases that target microglia.

Behavioral and neurological improvement by Cydonia oblonga fruit extract in chronic immobilization stress rats

It is known that chronic stress is a contributing factor to several physical and mental diseases. In this study, we examined the effect of hydroethanolic extract of Cydonia oblonga fruit (HECO, 300 mg/kg) in chronically immobilized rats on physiological and behavioral parameters by the open field test (OFT), sucrose preference test (SPT), and forced swimming test (FST) and on neurological alterations by analysis of the hippocampal neurogenesis. A daily 6 hr exposure to chronic immobilization stress (CIS) for 21 consecutive days induced anxiety- and depressive-like behaviors in rats' concomitant with decreased weight gain and increased plasma corticosterone (CORT) levels, rats also showed atrophy in the CA3 subregion of the hippocampus and a decreased number of Ki67 and DCX positive cells in the dentate gyrus (DG). Treatment with HECO successfully suppressed the physiological and behavioral markers of the CIS and prevents the structural abnormality and the impaired cell proliferation in the hippocampus. Moreover, the daily administration of HECO improved the mood function in normal rats. Taking together, our findings demonstrate, for the first time, the anti-depressive effect of C. oblonga fruit by enhancing the hippocampal neurogenesis in the rat model of depression.

Microglial activation in the dorsal striatum participates in anxiety-like behavior in Cyld knockout mice

CYLD lysine 63 deubiquitinase (CYLD), that is mainly involved in immune responses and inflammation, is expressed at high levels in the brain, especially in the dorsal striatum, but its physiological function of CYLD in the brain remains unexplored. The present study investigated the effect of Cyld gene knockout on behavior relevant to the dorsal striatum, such as motor activity and depression-like and anxiety-like behavior. Microglia and the pro-inflammatory cytokines including interleukin (IL)-1 β and tumor necrosis factor (TNF)- α were evaluated in the dorsal striatum to elucidate the underlying mechanism. Cyld knockout (Cyld^-/-) mice exhibited anxiety-like behavior, but not motor deficits or depression-like behavior. Microglia were activated and the mRNA levels of IL-1 β and TNF- α were increased in the dorsal striatum of Cyld^-/- mice compared to Cyld^+/+ mice. The microglial modulator minocycline partially reversed the anxiety-like behavior, microglial activation and increase in IL-1 β and TNF- α mRNA and protein levels in the dorsal striatum of Cyld^-/- mice. Collectively, these results suggest that Cyld knockout leading to microglial activation promotes IL-1 β and TNF- α expression and acts as a critical pathway in the pathophysiology of anxiety.

Emotional Impairments and Neuroinflammation are Induced in Male Mice Invulnerable to Repeated Social Defeat Stress

Prolonged stress triggers neuroinflammation, which plays a significant role in the development of depression; however, stressed people do not always suffer from depression because of individual differences in stress vulnerability. Negative cognitive bias (NCB) toward pessimistic judgment often underlies depressive episodes. However, a relationship between stress vulnerability, neuroinflammation, and NCB remains elusive. In addition, an animal model with all the traits would be a powerful tool for studying the etiology of depression and its therapeutic approaches. Accordingly, this study evaluated the effect of stress vulnerability on neuroinflammation and depression-related behaviors, including NCB in males, using a modified version of repeated social defeat stress (mRSDS) paradigm, a validated animal model of psychosocial stress. Exposure to mRSDS, consisting of 5 min of social defeat by unfamiliar CD-1 aggressor mice for five consecutive days, caused NCB, which co-occurred with depressive- and anxiety-like behaviors, and neuroinflammation in male BALB/c mice. Treatment with minocycline, an antibiotic with anti-inflammatory property, blocked mRSDS-induced depressive-like behaviors and neuroinflammation, but not NCB, indicating the limited effect of an anti-inflammatory intervention. In addition, marked differences were found in neuroinflammatory profiles and hippocampal gene expression patterns between resilient and unstressed mice, as well as between susceptible and resilient mice. Therefore, mice resilient to mRSDS are indeed not intact. Our findings provide insights into the unique features of the mRSDS model in male BALB/c mice, which could be used to investigate the etiological mechanisms underlying depression as well as bridge the gap in the relationship between stress vulnerability, neuroinflammation, and NCB in males.

Sex Differences in the Inflammatory Consequences of Stress: Implications for Pharmacotherapy

Women are at significantly greater risk of developing stress-related disorders such as depression. The increased risk begins during puberty and continues throughout life until menopause, suggesting a role for ovarian hormones in this increased susceptibility. Importantly, inflammation has been gaining momentum in its role in the pathogenesis of depression. Herein, clinical and preclinical studies have been reviewed to better understand how sex differences within the immune system may contribute to exaggerated risk of depression in females. First, studies that investigate the ability of psychologic stress episodes to engage the inflammatory systems both in the brain and periphery are reviewed with a special focus on sex-specific effects. Moreover, studies are discussed that identify whether imbalanced inflammatory milieu contributes to the development of depression in males versus females and whether these effects are regulated by estradiol. Importantly, we propose a locus coeruleus-norepinephrine-cytokine circuit as a conduit through which stress could increase stress susceptibly in females. Finally, the anti-inflammatory capacity of traditional and nontraditional antidepressants is investigated, with the goal of providing a better understanding of pharmacotherapeutics to enhance strategies to personalize antidepressant treatments between the sexes. The studies reviewed herein strongly support the need for further studies to elucidate whether females are especially sensitive to anti-inflammatory compounds as adjuvants to traditional therapies. SIGNIFICANCE STATEMENT: Women have hve an increased risk of developing stress-related disorders such as depression. In this review, literature from clinical and preclinical studies are integrated to define sex differences in stress-induced inflammatory responses as a potential source for the etiology of sex differences in depressive disorders. Moreover, the anti-inflammatory capacity of traditional and nontraditional antidepressants is reviewed to inform on potential pharmacotherapeutic strategies to personalize antidepressant therapy in a sex-dependent manner.

Beyond the looking glass: recent advances in understanding the impact of environmental exposures on neuropsychiatric disease

The etiologic pathways leading to neuropsychiatric diseases remain poorly defined. As genomic technologies have advanced over the past several decades, considerable progress has been made linking neuropsychiatric disorders to genetic underpinnings. Interest and consideration of nongenetic risk factors (e.g., lead exposure and schizophrenia) have, in contrast, lagged behind heritable frameworks of explanation. Thus, the association of neuropsychiatric illness to environmental chemical exposure, and their potential interactions with genetic susceptibility, are largely unexplored. In this review, we describe emerging approaches for considering the impact of chemical risk factors acting alone and in concert with genetic risk, and point to the potential role of epigenetics in mediating exposure effects on transcription of genes implicated in mental disorders. We highlight recent examples of research in nongenetic risk factors in psychiatric disorders that point to potential shared biological mechanisms-synaptic dysfunction, immune alterations, and gut-brain interactions. We outline new tools and resources that can be harnessed for the study of environmental factors in psychiatric disorders. These tools, combined with emerging experimental evidence, suggest that there is a need to broadly incorporate environmental exposures in psychiatric research, with the ultimate goal of identifying modifiable risk factors and informing new treatment strategies for neuropsychiatric disease.

Inflammation-induced behavioral changes is driven by alterations in Nrf2-dependent apoptosis and autophagy in mouse hippocampus: Role of fluoxetine

Inflammation has been associated with the progression of many neurological diseases. Peripheral inflammation has also been vaguely linked to depression-like symptoms in animal models, but the underlying pathways that orchestrate inflammation-induced behavioral or molecular changes in the brain are still elusive. We have recently shown that intraperitoneal injections of lipopolysaccharide (LPS) to Swiss albino mice triggers systemic inflammation, leading to an activated immune response along with changes in monoamine levels in the brain. Herein we pinpoint the fundamental pathways linking peripheral inflammation and depression-like behavior in a mouse model, thereby identifying suitable targets of intervention to combat the situation. We show that LPS-induced peripheral inflammation provoked a depression-like behavior in mice and a distinct pro-inflammatory bias in the hippocampus, as evident from increased microglial activation and elevated levels of pro-inflammatory cytokines IL-6 and TNF-α, and activation of NFκB-p65 pathway. Significant alterations in Nrf2-dependent cellular redox status, coupled with altered autophagy and increased apoptosis were noticed in the hippocampus of LPS-exposed mice. We and others have previously shown that, fluoxetine (an anti-depressant) has effective anti-inflammatory and antioxidant properties by virtue of its abilities to regulate NFκB and Nrf2 signaling. We observed that treatment with fluoxetine or the Nrf2 activator tBHQ (tert-butyl hydroquinone), could reverse depression-like-symptoms and mitigate alterations in autophagy and cell death pathways in the hippocampus by activating Nrf2-dependent gene expressions. Taken together, the data suggests that systemic inflammation potentiates Nrf2-dependent changes in cell death and autophagy pathway in the hippocampus, eventually leading to major pathologic sequelae associated with depression. Therefore, targeting Nrf2 could be a novel approach in combatting depression and ameliorating its associated pathogenesis.

Inflammation as the Potential Basis in Depression

There is growing evidence of the association between inflammation and stress-related disorders including depression. The positive correlation between the increased levels of inflammatory cytokines observed in patients with other diseases and the byproduct of the depressive symptoms may be caused by chronic stress. Increased neuroinflammatory responses are capable of activating microglia and astrocytes, which leads to release pro-inflammatory cytokines. Moreover, elevated levels of inflammatory cytokines such as tumor necrosis factor-alpha, interleukin (IL)-1, and IL-6 are causally related to various aspects of depression such as the behavioral symptomatology. Eventually, these elevated cytokines aggravate and propagate neuroinflammation, impairing brain functions. Thus, activated astrocytes and microglia may be potential mediators in neuroinflammatory processes contributing to the development of depression.

Mesenchymal Stromal Cells Modulate Peripheral Stress-Induced Innate Immune Activation Indirectly Limiting the Emergence of Neuroinflammation-Driven Depressive and Anxiety-like Behaviors

BACKGROUND

Hyperactivation of innate immunity has been implicated in the etiology of mood disorders, including major depressive disorder (MDD). Mesenchymal stromal cells (MSCs) have demonstrated potent immunomodulatory capabilities in the context of chronic inflammatory disease and injury but have yet to be evaluated in stress-based preclinical models of MDD. We sought to test the ability of intravenous MSCs to modulate innate immune activation and behavioral patterns associated with repeated social defeat (RSD).

METHODS

Murine RSD-induced innate immune activation as well as depressive and anxiety-like behaviors were assessed in unstressed, RSD, and RSD + human MSC groups. Biodistribution and fate studies were performed to inform potential mechanisms of action.

RESULTS

MSCs reduced stress-induced circulating proinflammatory cytokines, monocytes, neuroinflammation, and depressive and anxiety-like behaviors. Biodistribution analyses indicated that infused MSCs distributed within peripheral organs without homing to the brain. Murine neutrophils targeted MSCs in the lungs within hours of administration. MSCs and recipient neutrophils were cleared by recipient macrophages promoting a switch toward a regulatory phenotype and systemic resolution of inflammation.

CONCLUSIONS

Peripheral delivery of MSCs modulates central nervous system inflammatory processes and aberrant behavioral patterns in a stress-based rodent model of MDD and anxiety. Recent studies suggest that host immune cell-mediated phagocytosis of MSCs in vivo can trigger an immunomodulatory cascade, resulting in resolution of inflammation. Our data suggest that similar mechanisms may protect distal organs, including the brain, from systemic, stress-induced proinflammatory spikes and may uncover unexpected targets in the periphery for novel or adjunct treatment for a subset of patients with MDD.

Potential Role of Gut Microbiota in Induction and Regulation of Innate Immune Memory

The gut microbiota significantly regulates the development and function of the innate and adaptive immune system. The attribute of immunological memory has long been linked only with adaptive immunity. Recent evidence indicates that memory is also present in the innate immune cells such as monocytes/macrophages and natural killer cells. These cells exhibit pattern recognition receptors (PRRs) that recognize microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) expressed by the microbes. Interaction between PRRs and MAMPs is quite crucial since it triggers the sequence of signaling events and epigenetic rewiring that not only play a cardinal role in modulating the activation and function of the innate cells but also impart a sense of memory response. We discuss here how gut microbiota can influence the generation of innate memory and functional reprogramming of bone marrow progenitors that helps in protection against infections. This article will broaden our current perspective of association between the gut microbiome and innate memory. In the future, this knowledge may pave avenues for development and designing of novel immunotherapies and vaccination strategies.

Interleukin-4 signalling pathway underlies the anxiolytic effect induced by 3-deoxyadenosine

RATIONALE

Converging evidence suggests that neuroimmunity plays an important role in the pathophysiology of anxiety. Interleukin (IL)-4 is a key cytokine regulating neuroimmune functions in the central nervous system. More efficient anxiolytics with neuro-immune mechanisms are urgently needed.

OBJECTIVE

To determine whether 3'-deoxyadenosine (3'-dA) exerts an anxiolytic effect and to examine the role of IL-4 in the anxiolytic effect of 3'-dA in mice.

METHODS

We investigated the effects of 3'-dA on anxiety-like behaviors using elevated plus maze (EPM) or light-dark box (LDB) tests after 45 min or 5 days of treatment. Expression of IL-4, IL-10, IL-1β, TNF-α, and IL-6 in the prefrontal cortex (PFC) was detected by Western blot and/or double immunostaining. Intracerebroventricular injection of RIL-4Rα (an IL-4-specific inhibitor) and intraperitoneal injection of 3'-dA or imipramine were co-administered, followed by EPM test.

RESULTS

3'-dA exhibited a stronger and faster anxiolytic effect than imipramine in behavioral tests. Furthermore, 3'-dA enhanced IL-4 expression after 45 min or 5 days, TNF-α and IL-1β expression decreased significantly after a 5-day treatment with 3'-dA, and IL-10 expression increased after a 5-day treatment with 3'-dA or imipramine in the PFC. IL-4 was expressed in neurons and in some astrocytes and microglia. IL-4 expression showed a strong positive correlation with reduced anxiety behaviors. RIL-4Rα completely blocked the anxiolytic effects induced by 3'-dA and imipramine.

CONCLUSIONS

This study identifies a novel and common anxiolytic IL-4 signaling pathway and provides an innovative drug with a novel neuro-immune mechanism for treating anxiety disorder.

Possible involvement of metformin in downregulation of neuroinflammation and associated behavioural changes in mice

Metformin (MET), a biguanide oral hypoglycaemic agent, recently has been shown to be effective in various conditions other than type-2 diabetes including cancer, stroke, weight reduction, and polycystic ovarian syndrome, to name a few. MET has also possessed antioxidant and antiinflammatory properties by activation of AMPK . This study was aimed at evaluating the effects of MET on lipopolysaccharide (LPS)-induced systemic and neuroinflammation, oxidative stress, and behavioural changes. The study consisted of six groups, where three selected doses of MET (100, 200, and 300 mg/kg) were employed in male Swiss albino mice, with one group of imipramine (IMI), saline, and LPS each. Systemic inflammation was induced by injecting LPS (1.5 mg/kg) by intraperitoneal route. A battery of behavioural tests including open field, forced swim, and tail suspension tests were employed to assess the impact of systemic inflammation on exploratory behaviour and learned helplessness. LPS induced significant immobility with profound symptoms of sickness behaviour. Furthermore, LPS led to significant increase in serum and brain proinflammatory cytokines TNF-α and IL-6; and also increased lipid peroxidation with reduced glutathione levels. Pretreatment of the animals with 100 and 200 mg/kg of MET significantly reduced both systemic and central inflammatory markers along with protecting against LPS-induced oxidative stress. The higher dose, 300 mg/kg of MET was not effective against most of LPS-induced biochemical changes. Our preliminary results from this study suggest the antiinflammatory and neuroprotective effects of MET in LPS-induced model of sickness behaviour and neuroinflammation.

The Role of Inflammation in Depression and Fatigue

Depression and fatigue are conditions responsible for heavy global societal burden, especially in patients already suffering from chronic diseases. These symptoms have been identified by those affected as some of the most disabling symptoms which affect the quality of life and productivity of the individual. While many factors play a role in the development of depression and fatigue, both have been associated with increased inflammatory activation of the immune system affecting both the periphery and the central nervous system (CNS). This is further supported by the well-described association between diseases that involve immune activation and these symptoms in autoimmune disorders, such as multiple sclerosis and immune system activation in response to infections, like sepsis. Treatments for depression also support this immunopsychiatric link. Antidepressants have been shown to decrease inflammation, while higher levels of baseline inflammation predict lower treatment efficacy for most treatments. Those patients with higher initial immune activation may on the other hand be more responsive to treatments targeting immune pathways, which have been found to be effective in treating depression and fatigue in some cases. These results show strong support for the hypothesis that depression and fatigue are associated with an increased activation of the immune system which may serve as a valid target for treatment. Further studies should focus on the pathways involved in these symptoms and the development of treatments that target those pathways will help us to better understand these conditions and devise more targeted treatments.

Higher Peripheral Inflammatory Signaling Associated With Lower Resting-State Functional Brain Connectivity in Emotion Regulation and Central Executive Networks

BACKGROUND

Researchers document bidirectional pathways linking peripheral inflammation and neural circuitries subserving emotion processing and regulation. To extend this work, we present results from two independent studies examining the relationship between inflammation and resting-state functional connectivity (rsFC), as measured by functional magnetic resonance imaging.

METHODS

Study 1 involved 90 rural African American young adults, 25 years of age (52% female), and study 2 involved 82 urban African American youths, 13 to 14 years of age (66% female). Both studies measured circulating inflammatory biomarkers (C-reactive protein, interleukin 6, interleukin 10, tumor necrosis factor alpha), and the measures were averaged to form a composite. Study 2 also enumerated classical monocytes, a key leukocyte subpopulation involved in immune-to-brain signaling. All participants completed a resting-state functional magnetic resonance imaging scan.

RESULTS

Consistent with our prediction, higher scores on the inflammatory composite were associated with lower rsFC within an emotion regulation network in study 1, controlling for sex. Study 2 replicated study 1, showing that higher scores on the inflammatory composite were associated with lower rsFC within the emotion regulation network, controlling for sex, age, and pubertal status, and found a similar pattern for rsFC within a central executive network. Study 2 also found that higher numbers of classical monocytes were associated with lower rsFC within both the emotion regulation and central executive networks. There was no relationship between rsFC in the anterior salience or default mode networks with inflammation in either study.

CONCLUSIONS

With these findings, we document relationships between peripheral inflammation and rsFC within an emotion regulation and central executive network and replicate these associations with the emotion regulation network across two independent samples.

Potential exerkines for physical exercise-elicited pro-cognitive effects: Insight from clinical and animal research

A sedentary lifestyle is now known as a critical risk factor for accelerated aging-related neurodegenerative disorders. In contract, having regular physical exercise has opposite effects. Clinical findings have suggested that physical exercise can promote brain plasticity, particularly the hippocampus and the prefrontal cortex, that are important for learning and memory and mood regulations. However, the underlying mechanisms are still unclear. Animal studies reveal that the effects of physical exercise on promoting neuroplasticity could be mediated by different exerkines derived from the peripheral system and the brain itself. This book chapter summarizes the recent evidence from clinical and pre-clinical studies showing the emerging mediators for exercise-promoted brain health, including myokines secreted from skeletal muscles, adipokines from adipose tissues, and other factors secreted from the bone and liver.

Repeated social defeat in female mice induces anxiety-like behavior associated with enhanced myelopoiesis and increased monocyte accumulation in the brain

Anxiety and mood disorders affect both men and women. The majority of experimental models of stress, however, are completed using only male animals. For repeated social defeat (RSD), a rodent model, this is due to the inherent difficulty in eliciting male aggression toward female mice. To address this limitation, a recent study showed that a DREADD-based activation of the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl) was effective in inducing aggressive behavior in male mice towards females in a social defeat paradigm. Therefore, the goal of this study was to determine if this modified version of RSD in females elicited behavioral, physiological, and immune responses similar to those reported in males. Here, we show that female mice subjected to RSD with the male DREADD aggressor developed anxiety-like behavior and social avoidance. These behavioral alterations coincided with enhanced neuronal and microglial activation in threat-appraisal regions of the brain. Moreover, stressed female mice had an enhanced peripheral immune response characterized by increased myelopoiesis, release of myeloid cells into circulation, and monocyte accumulation in the spleen and brain. These results are consistent with previously reported findings that male mice exposed to RSD exhibited increased fear and threat appraisal responses, enhanced myelopoiesis, myeloid cell release and trafficking, and anxiety-like behavior. These findings validate that RSD is a relevant model to study stress responses in female mice.

The Influence of Microglial Elimination and Repopulation on Stress Sensitization Induced by Repeated Social Defeat

BACKGROUND

Stress is associated with an increased prevalence of anxiety and depression. Repeated social defeat (RSD) stress in mice increases the release of monocytes from the bone marrow that are recruited to the brain by microglia. These monocytes enhance inflammatory signaling and augment anxiety. Moreover, RSD promotes stress sensitization, in which exposure to acute stress 24 days after cessation of RSD causes anxiety recurrence. The purpose of this study was to determine whether microglia were critical to stress sensitization and exhibited increased reactivity to subsequent acute stress or immune challenge.

METHODS

Mice were exposed to RSD, microglia were eliminated by colony-stimulating factor 1 receptor antagonism (PLX5622) and allowed to repopulate, and responses to acute stress or immune challenge (lipopolysaccharide) were determined 24 days after RSD sensitization.

RESULTS

Microglia maintained a unique messenger RNA signature 24 days after RSD. Moreover, elimination of RSD-sensitized microglia prevented monocyte accumulation in the brain and blocked anxiety recurrence following acute stress (24 days). When microglia were eliminated prior to RSD and repopulated and mice were subjected to acute stress, there was monocyte accumulation in the brain and anxiety in RSD-sensitized mice. These responses were unaffected by microglial elimination/repopulation. This may be related to neuronal sensitization that persisted 24 days after RSD. Following immune challenge, there was robust microglial reactivity in RSD-sensitized mice associated with prolonged sickness behavior. Here, microglial elimination/repopulation prevented the amplified immune reactivity ex vivo and in vivo in RSD-sensitized mice.

CONCLUSIONS

Microglia and neurons remain sensitized weeks after RSD, and only the immune reactivity component of RSD-sensitized microglia was prevented by elimination/repopulation.