Chronic exposure to exogenous glucocorticoids primes microglia to pro-inflammatory stimuli and induces NLRP3 mRNA in the hippocampus

Maternal viral infection during pregnancy elicits anti-social behavior in neonatal piglet offspring independent of postnatal microglial cell activation

Maternal infection during pregnancy increases risk for neurodevelopmental disorders and reduced stress resilience in offspring, but the mechanisms are not fully understood. We hypothesized that piglets born from gilts infected with a respiratory virus during late gestation would exhibit aberrant microglia activity, cognitive deficits and reduced sociability. Pregnant gilts were inoculated with porcine reproductive and respiratory syndrome virus (PRRSV; 5×10⁵ TCID₅₀ of live PRRSV) or saline at gestational day 76. Gilts infected with PRRSV exhibited fever (p<0.01) and reduced appetite (p<0.001) for 2weekspost-inoculation and were PRRSV-positive at parturition. Piglets born from infected and control gilts were weaned at postnatal day (PD) 1 and assigned to two groups. Group 1 was challenged with lipopolysaccharide (LPS, 5μg/kg body weight i.p.) or saline on PD 14 and tissues were collected. Group 2 was tested in a T-maze task to assess spatial learning and in a 3-chamber arena with unfamiliar conspecifics to assess social behavior from PD 14-27. Microglia (CD11b⁺ CD45^low) isolated from Group 2 piglets at PD 28 were challenged ex vivo with LPS; a subset of cells was analyzed for MHCII expression. Maternal infection did not affect offspring circulating TNFα, IL-10, or cortisol levels basally or 4h post-LPS challenge. While performance in the T-maze task was not affected by maternal infection, both sociability and preference for social novelty were decreased in piglets from infected gilts. There was no effect of maternal infection on microglial MHCII expression or LPS-induced cytokine production. Taken together, these results suggest the reduced social behavior elicited by maternal infection is not due to aberrant microglia activity postnatally.

Hyperleptinemia in Neonatally Overfed Female Rats Does Not Dysregulate Feeding Circuitry

Neonatal overfeeding during the first weeks of life in male rats is associated with a disruption in the peripheral and central leptin systems. Neonatally overfed male rats have increased circulating leptin in the first 2 weeks of life, which corresponds to an increase in body weight compared to normally fed counterparts. These effects are associated with a short-term disruption in the connectivity of neuropeptide Y (NPY), agouti-related peptide (AgRP), and pro-opiomelanocortin (POMC) neurons within the regions of the hypothalamus responsible for control of energy balance and food intake. Female rats that are overfed during the first weeks of their life experience similar changes in circulating leptin levels as well as in their body weight. However, it has not yet been studied whether these metabolic changes are associated with the same central effects as observed in males. Here, we hypothesized that hyperleptinemia associated with neonatal overfeeding would lead to changes in central feeding circuitry in females as it does in males. We assessed hypothalamic NPY, AgRP, and POMC gene expression and immunoreactivity at 7, 12, or 14 days of age, as well as neuronal activation in response to exogenous leptin in neonatally overfed and control female rats. Neonatally overfed female rats were hyperleptinemic and were heavier than controls. However, these metabolic changes were not mirrored centrally by changes in hypothalamic NPY, AGRP, and POMC fiber density. These findings are suggestive of sex differences in the effects of neonatal overfeeding and of differences in the ability of the female and male central systems to respond to changes in the early life nutritional environment.

An Integrated Review of Psychological Stress in Parkinson's Disease: Biological Mechanisms and Symptom and Health Outcomes

Parkinson's disease (PD) is characterized by complex symptoms and medication-induced motor complications that fluctuate in onset, severity, responsiveness to treatment, and disability. The unpredictable and debilitating nature of PD and the inability to halt or slow disease progression may result in psychological stress. Psychological stress may exacerbate biological mechanisms believed to contribute to neuronal loss in PD and lead to poorer symptom and health outcomes. The purpose of this integrated review is to summarize and appraise animal and human research studies focused on biological mechanisms, symptom, and health outcomes of psychological stress in PD. A search of the electronic databases PubMed/Medline and CINAHL from 1980 to the present using the key words Parkinson's disease and stress, psychological stress, mental stress, and chronic stress resulted in 11 articles that met inclusion criteria. The results revealed significant associations between psychological stress and increased motor symptom severity and loss of dopamine-producing neurons in animal models of PD and between psychological stress and increased symptom severity and poorer health outcomes in human subjects with PD. Further research is needed to fully elucidate the underlying biological mechanisms responsible for these relationships, for the ultimate purpose of designing targeted interventions that may modify the disease trajectory.

The antidepressant-like effects of pioglitazone in a chronic mild stress mouse model are associated with PPARγ-mediated alteration of microglial activation phenotypes

BACKGROUND

Discoveries that microglia-mediated neuroinflammation is involved in the pathological process of depression provided a new strategy for novel antidepressant therapy. Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor regulating inflammation and microglial polarization and, therefore, a potential target for resolving depressive disorders. Our hypothesis was that antidepressant effects could be achieved through anti-inflammatory and neuroprotective activities by PPARγ-dependent microglia-modulating agents.

METHODS

Chronic mild stress (CMS) treatment was performed on C57BL/6 mice for 6 weeks. After 3 weeks with the CMS procedure, depressive-like behaviors were evaluated by sucrose preference (SP), tail suspension test (TST), forced swimming test (FST), and locomotor activity. Pioglitazone was administered intragastrically once per day for 3 weeks at different doses. Neuroinflammatory cytokines were determined by real time-PCR (RT-PCR), enzyme-linked immunosorbent assay (ELISA), and western blot. The activated microglial state was confirmed by immunohistochemistry. N9 microglial cells were subjected to lipopolysaccharide, pioglitazone, and GW9662 to discuss the phenotype of activated microglia by RT-PCR, ELISA, and western blot.

RESULTS

It was demonstrated that the PPARγ agonist pioglitazone (2.5 mg/kg) ameliorated depression-like behaviors in CMS-treated mice, as indicated by body weight (BW), the SP test, the FST, and the TST. The amelioration of the depression was blocked by the PPARγ antagonist GW9662. The expression of M1 markers (IL-1β, IL-6, TNFα, iNOS, and CCL2) increased, and the gene expression of M2 markers (Ym1, Arg1, IL-4, IL-10, and TGFβ) decreased in the hippocampus of the stress-treated mice. Pioglitazone significantly inhibited the increased numbers and morphological alterations of microglia in the hippocampus, reduced the elevated expression of microglial M1 markers, and increased the downgraded expression of microglial M2 markers in C57BL/6 mice exposed to CMS. In an in vitro experiment, pioglitazone reversed the imbalance of M1 and M2 inflammatory cytokines, which is correlated with the inhibition of nuclear factor kB activation and is expressed in LPS-stimulated N9 microglial cells.

CONCLUSIONS

We showed that pioglitazone administration induce the neuroprotective phenotype of microglia and ameliorate depression-like behaviors in CMS-treated C57BL/6 mice. These data suggested that the microglia-modulating agent pioglitazone present a beneficial choice for depression.

Clock Genes in Glia Cells: A Rhythmic History

Circadian rhythms are periodic patterns in biological processes that allow the organisms to anticipate changes in the environment. These rhythms are driven by the suprachiasmatic nucleus (SCN), the master circadian clock in vertebrates. At a molecular level, circadian rhythms are regulated by the so-called clock genes, which oscillate in a periodic manner. The protein products of clock genes are transcription factors that control their own and other genes’ transcription, collectively known as “clock-controlled genes.” Several brain regions other than the SCN express circadian rhythms of clock genes, including the amygdala, the olfactory bulb, the retina, and the cerebellum. Glia cells in these structures are expected to participate in rhythmicity. However, only certain types of glia cells may be called “glial clocks,” since they express PER-based circadian oscillators, which depend of the SCN for their synchronization. This contribution summarizes the current information about clock genes in glia cells, their plausible role as oscillators and their medical implications.

Forced treadmill exercise can induce stress and increase neuronal damage in a mouse model of global cerebral ischemia

Physical exercise is known to be a beneficial factor by increasing the cellular stress tolerance. In ischemic stroke, physical exercise is suggested to both limit the brain injury and facilitate behavioral recovery. In this study we investigated the effect of physical exercise on brain damage following global cerebral ischemia in mice. We aimed to study the effects of 4.5 weeks of forced treadmill running prior to ischemia on neuronal damage, neuroinflammation and its effect on general stress by measuring corticosterone in feces. We subjected C57bl/6 mice (n = 63) to either treadmill running or a sedentary program prior to induction of global ischemia. Anxious, depressive, and cognitive behaviors were analyzed. Stress levels were analyzed using a corticosterone ELISA. Inflammatory and neurological outcomes were analyzed using immunohistochemistry, multiplex electrochemoluminescence ELISA and Western blot. To our surprise, we found that forced treadmill running induced a stress response, with increased anxiety in the Open Field test and increased levels of corticosterone. In accordance, mice subjected to forced exercise prior to ischemia developed larger neuronal damage in the hippocampus and showed higher cytokine levels in the brain and blood compared to non-exercised mice. The extent of neuronal damage correlated with increased corticosterone levels. To compare forced treadmill with voluntary wheel running, we used a different set of mice that exercised freely on running wheels. These mice did not show any anxiety or increased corticosterone levels. Altogether, our results indicate that exercise pre-conditioning may not be beneficial if the animals are forced to run as it can induce a detrimental stress response.

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Enforcement to run results in anxious behavior.

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Mice that are forced to run have elevated levels of corticosterone.

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Enforcement to run results in more neuronal death in hippocampus.

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Corticosterone levels correlates with the neuronal damage in hippocampus.

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Increased corticosterone and anxiety is not seen in mice that run voluntarily.

Glucocorticoids Mediate Short-Term High-Fat Diet Induction of Neuroinflammatory Priming, the NLRP3 Inflammasome, and the Danger Signal HMGB1

The impact of the foods we eat on metabolism and cardiac physiology has been studied for decades, yet less is known about the effects of foods on the CNS, or the behavioral manifestations that may result from these effects. Previous studies have shown that long-term consumption of high-fat foods leading to diet-induced obesity sensitizes the inflammatory response of the brain to subsequent challenging stimuli, causing deficits in the formation of long-term memories. The new findings reported here demonstrate that short-term consumption of a high-fat diet (HFD) produces the same outcomes, thus allowing the examination of mechanisms involved in this process long before obesity and associated comorbidities occur. Rats fed an HFD for 3 d exhibited increases in corticosterone, the inflammasome-associated protein NLRP3 (nod-like receptor protein 3), and the endogenous danger signal HMGB1 (high-mobility group box 1) in the hippocampus. A low-dose (10 μg/kg) lipopolysaccharide (LPS) immune challenge potentiated the neuroinflammatory response in the hippocampus of rats fed the HFD, and caused a deficit in the formation of long-term memory, effects not observed in rats fed regular chow. The blockade of corticosterone action with the glucocorticoid receptor antagonist mifepristone prevented the NLRP3 and HMGB1 increases in unchallenged animals, normalized the proinflammatory response to LPS, and prevented the memory impairment. These data suggest that short-term HFD consumption increases vulnerability to memory disruptions caused by an immune challenge by upregulating important neuroinflammatory priming and danger signals in the hippocampus, and that these effects are mediated by increases in hippocampal corticosterone.

High glucocorticoid levels during gestation activate the inflammasome in hippocampal oligodendrocytes of the offspring

Exposure to high levels of glucocorticoids (GCs) during early life induces long-lasting neuroinflammation. GCs induce rapid degranulation of mast cells, which release proinflammatory molecules promoting activation of microglia and astrocytes. The possible involvement of oligodendrocytes, however, remains poorly understood. It was studied whether high GC levels during gestation activates the inflammasome in hippocampal oligodendrocytes of mouse offspring. Oligodendrocytes of control pups showed expression of inflammasome components (NLRP3, ACS, and caspase-1) and their levels were increased by prenatal administration of dexamethasone (DEX), a synthetic GC. These cells also showed high levels of IL-1β and TNF-α, revealing activation of the inflammasome. Moreover, they showed increased levels of the P2X₇ receptor and pannexin1, which are associated to inflammasome activation. However, levels of connexins either were not affected (Cx29) or reduced (Cx32 and Cx47). Nonetheless, the functional states of pannexin1 and connexin hemichannels were elevated and directly associated to functional P2X₇ receptors. As observed in DEX-treated brain slices, hemichannel activity first increased in hippocampal mast cells and later in microglia and macroglia. DEX-induced oligodendrocyte hemichannel activity was mimicked by urocortin-II, which is a corticotropin-releasing hormone receptor (CRHR) agonist. Response to DEX and urocortin-II was inhibited by antalarmin (a CRHR blocker) or by mast cells or microglia inhibitors. The increase in hemichannel activity persisted for several weeks after birth and cross-fostering with a control mother did not reverse this condition. It is proposed that activation of the oligodendrocyte inflammasome might be relevant in demyelinating diseases associated with early life exposure to high GC levels. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 625-642, 2017.

Glucocorticoid-Potentiated Spinal Microglia Activation Contributes to Preoperative Anxiety-Induced Postoperative Hyperalgesia

Clinically, preoperative anxiety adversely affected postoperative hyperalgesia. As stress-induced glucocorticoids (GCs) were reported to sensitize the activation of microglia, the present study investigated whether and how GCs and microglia played in the process of preoperative anxiety-induced postoperative hyperalgesia. The study used an animal model that exposed rats to single prolonged stress (SPS) procedure to induce preoperative anxiety-like behaviors 24 h before the plantar incisional surgery. Behavioral testing revealed that preoperative SPS enhanced the mechanical allodynia induced by plantar incision. SPS was also found to induce elevated circulating corticosterone levels, potentiate the activation of spinal microglia, and increase the expression of spinal proinflammatory cytokines. Inhibition of microglia by pretreatment with minocycline attenuated the SPS-enhanced mechanical allodynia, and this was accompanied by decreased activation of spinal microglia and expression of proinflammatory cytokines. Another experiment was conducted by administering RU486, the GC receptor (GR) antagonist, to rats. The results showed that RU486 suppressed SPS-induced and SPS-potentiated proinflammatory activation of spinal microglia and revealed analgesic effects. Together, these data indicated that inhibition of stress-induced GR activation attenuated the preoperative anxiety-induced exacerbation of postoperative pain, and the suppression of spinal microglia activation may underlie this anti-hyperalgesia effect. Pending further studies, these findings suggested that GR and spinal microglia may play important roles in the development of preoperative anxiety-induced postoperative hyperalgesia and may serve as novel targets to prevent this phenomenon.

Beneficial impact of intracerebroventricular fractalkine administration on behavioral and biochemical changes induced by prenatal stress in adult rats: Possible role of NLRP3 inflammasome pathway

Several lines of evidence indicate that adverse experience in early life may be a triggering factor for pathological inflammatory processes and lead to the development of depression. Fractalkine (CX3CL1), a chemokine, plays an important role not only in the migration, differentiation and proliferation of neuronal and glial cells but also in the regulation of neuronal-microglial signaling and the production of pro-inflammatory factors. In the present study, we examined the impact of a prenatal stress procedure on the expression of fractalkine in the hippocampus and frontal cortex of young and adult male rats. Furthermore, we measured the age-dependent effect of stress during pregnancy on the expression of pro-inflammatory factors IL-1β, IL-18, TNF-α, IL-6, and CCL2 in both brain structures. Next, to illustrate the link between fractalkine signaling and the behavioral and biochemical changes induced by prenatal stress, adult prenatally stressed offspring were injected intracerebroventricularly (icv) with exogenous fractalkine. We reported that prenatal stress leads to long-lasting deficits in fractalkine signaling and enhanced inflammatory activation. The study demonstrates that icv administration of fractalkine attenuates the behavioural changes evoked by prenatal stress procedure in adult animals. Moreover, fractalkine administration, exhibits anti-inflammatory action, mainly in the frontal cortex of adult prenatally stressed rats. The effect of fractalkine is related to inhibition of NLRP3 inflammasome. However, its action on the other members of NOD-like receptor family (NLR) cannot be excluded. These findings provide new in vivo evidence that the behavioral and inflammatory disturbances observed in adult prenatally stressed rats may be related to long-lasting malfunctions in fractalkine signaling.

Stress-induced neuroinflammatory priming is time of day dependent

Circadian rhythms are endogenous cycles of physiology and behavior that align with the daily rotation of the planet and resulting light-dark cycle. The circadian system ensures homeostatic balance and regulates many aspects of physiology, including the stress response and susceptibility to and/or severity of stress-related sequelae. Both acute and chronic stressors amplify neuroinflammatory responses to a subsequent immune challenge, however it is not known whether circadian timing of the stressor regulates the priming response. Here, we test whether stress-induced neuroinflammatory priming is regulated by the circadian system. As has been previously shown, exposure to 100 inescapable tails shocks (IS) increased hippocampal cytokines following a subsequent inflammatory challenge. However, this effect was limited to animals that experienced the stressor during the light phase. Rats exposed to stress during the dark phase did not alter inflammatory potential following lipopolysaccharide (LPS) challenge. To determine whether microglia might be involved in diurnal differences in neuroinflammatory priming, microglia were isolated 24h after stress that occurred either during the middle of the light or dark phase. Only microglia isolated from animals stressed during the light phase demonstrated an exaggerated inflammatory response when treated ex vivo with LPS. To determine possible circadian dependency of microglia responsiveness to glucocorticoids - the likely proximal mediator for stress associated neuroinflammatory priming - microglia were isolated during the middle of the light or dark phase and treated ex vivo with corticosterone. Glucocorticoids treatment downregulated CX3CR1 and CD200R, two genes involved in microglial inflammatory "off" signaling; however, there was no effect of time of day on expression of either gene. Importantly, while absolute concentrations of corticosterone were comparable following IS during the light and dark phase, the magnitude of change in corticosterone was greater during the light phase. This work highlights the importance of studying circadian rhythms to elucidate biological mechanisms of stress.

Stress-induced neuroinflammation is mediated by GSK3-dependent TLR4 signaling that promotes susceptibility to depression-like behavior

Most psychiatric and neurological diseases are exacerbated by stress. Because this may partially result from stress-induced inflammation, we examined factors involved in this stress response. After a paradigm of inescapable foot shock stress that causes learned helplessness depression-like behavior, eighteen cytokines and chemokines increased in mouse hippocampus, peaking 6-12h after stress. A 24h prior pre-conditioning stress accelerated the rate of stress-induced hippocampal cytokine and chemokine increases, with most reaching peak levels after 1-3h, often without altering the maximal levels. Toll-like receptor 4 (TLR4) was involved in this response because most stress-induced hippocampal cytokines and chemokines were attenuated in TLR4 knockout mice. Stress activated glycogen synthase kinase-3 (GSK3) in wild-type mouse hippocampus, but not in TLR4 knockout mice. Administration of the antidepressant fluoxetine or the GSK3 inhibitor TDZD-8 reduced the stress-induced increases of most hippocampal cytokines and chemokines. Stress increased hippocampal levels of the danger-associated molecular pattern (DAMP) protein high mobility group box 1 (HMGB1), activated the inflammatory transcription factor NF-κB, and the NLRP3 inflammasome. Knockdown of HMGB1 blocked the acceleration of cytokine and chemokine increases in the hippocampus caused by two successive stresses. Fluoxetine treatment blocked stress-induced up-regulation of HMGB1 and subsequent NF-κB activation, whereas TDZD-8 administration attenuated NF-κB activation downstream of HMGB1. To test if stress-induced cytokines and chemokines contribute to depression-like behavior, the learned helplessness model was assessed. Antagonism of TNFα modestly reduced susceptibility to learned helplessness induction, whereas TLR4 knockout mice were resistant to learned helplessness. Thus, stress-induces a broad inflammatory response in mouse hippocampus that involves TLR4, GSK3, and downstream inflammatory signaling, and these stress responses contribute to susceptibility to depression-like behavior in mice.

Forced swimming sabotages the morphological and synaptic maturation of newborn granule neurons and triggers a unique pro-inflammatory milieu in the hippocampus

Recent experimental data suggest that mood disorders are related to inflammatory phenomena and have led to the "inflammatory hypothesis of depression". Given that the hippocampus is one of the most affected areas in these disorders, we used a model of acute stress (the Porsolt test) to evaluate the consequences of forced swimming on two crucial events related to the pathophysiology of major depression: the functional maturation of newborn granule neurons; and the hippocampal inflammatory milieu. Using PSD95:GFP-expressing retroviruses, we found that forced swimming selectively alters the dendritic morphology of newborn neurons and impairs their connectivity by reducing the number and volume of their postsynaptic densities. In addition, acute stress triggered a series of morphological changes in microglial cells, together with an increase in microglial CD68 expression, thus suggesting the functional and morphological activation of this cell population. Furthermore, we observed an intriguing change in the hippocampal inflammatory milieu in response to forced swimming. Importantly, the levels of several molecules affected by acute stress (such as Interleukin-6 and eotaxin) have been described to also be altered in patients with depression and other mood disorders.

Microglial M1/M2 polarization and metabolic states

Microglia are critical nervous system-specific immune cells serving as tissue-resident macrophages influencing brain development, maintenance of the neural environment, response to injury and repair. As influenced by their environment, microglia assume a diversity of phenotypes and retain the capability to shift functions to maintain tissue homeostasis. In comparison with peripheral macrophages, microglia demonstrate similar and unique features with regards to phenotype polarization, allowing for innate immunological functions. Microglia can be stimulated by LPS or IFN-γ to an M1 phenotype for expression of pro-inflammatory cytokines or by IL-4/IL-13 to an M2 phenotype for resolution of inflammation and tissue repair. Increasing evidence suggests a role of metabolic reprogramming in the regulation of the innate inflammatory response. Studies using peripheral immune cells demonstrate that polarization to an M1 phenotype is often accompanied by a shift in cells from oxidative phosphorylation to aerobic glycolysis for energy production. More recently, the link between polarization and mitochondrial energy metabolism has been considered in microglia. Under these conditions, energy demands would be associated with functional activities and cell survival and thus, may serve to influence the contribution of microglia activation to various neurodegenerative conditions. This review examines the polarization states of microglia and their relationship to mitochondrial metabolism. Additional supporting experimental data are provided to demonstrate mitochondrial metabolic shifts in primary microglia and the BV-2 microglia cell line induced under LPS (M1) and IL-4/IL-13 (M2) polarization.

Stress and neuroinflammation: a systematic review of the effects of stress on microglia and the implications for mental illness

RATIONALE

Psychosocial stressors are a well-documented risk factor for mental illness. Neuroinflammation, in particular elevated microglial activity, has been proposed to mediate this association. A number of preclinical studies have investigated the effect of stress on microglial activity. However, these have not been systematically reviewed before.

OBJECTIVES

This study aims to systematically review the effects of stress on microglia, as indexed by the histological microglial marker ionised calcium binding adaptor molecule 1 (Iba-1), and consider the implications of these for the role of stress in the development of mental disorders.

METHODS

A systematic review was undertaken using pre-defined search criteria on PubMed and EMBASE. Inclusion and data extraction was agreed by two independent researchers after review of abstracts and full text.

RESULTS

Eighteen studies met the inclusion criteria. These used seven different psychosocial stressors, including chronic restraint, social isolation and repeated social defeat in gerbils, mice and/or rats. The hippocampus (11/18 studies) and prefrontal cortex (13/18 studies) were the most frequently studied areas. Within the hippocampus, increased Iba-1 levels of between 20 and 200 % were reported by all 11 studies; however, one study found this to be a duration-dependent effect. Of those examining the prefrontal cortex, ∼75 % found psychosocial stress resulted in elevated Iba-1 activity. Elevations were also consistently seen in the nucleus accumbens, and under some stress conditions in the amygdala and paraventricular nucleus.

CONCLUSIONS

There is consistent evidence that a range of psychosocial stressors lead to elevated microglial activity in the hippocampus and good evidence that this is also the case in other brain regions. These effects were seen with early-life/prenatal stress, as well as stressors in adulthood. We consider these findings in terms of the two-hit hypothesis, which proposes that early-life stress primes microglia, leading to a potentiated response to subsequent stress. The implications for understanding the pathoaetiology of mental disorders and the development of new treatments are also considered.

Stress-induced neuroinflammatory priming: A liability factor in the etiology of psychiatric disorders

Stress and glucocorticoids (GCs) have universally been considered to be anti-inflammatory, however in recent years, stress and GCs have been found to exert permissive effects (immunological priming) on neuroinflammatory processes. This phenomenon of priming is characterized by prior stress or GC exposure potentiating the neuroinflammatory response to a subsequent immune challenge. A considerable body of evidence is discussed here that supports this permissive effect of stress and GCs.

In light of this evidence, a mechanism of neuroinflammatory priming is proposed involving a signal cascade in the brain involving danger-associated molecular patterns (HMGB-1) and inflammasomes (NLRP3), which results in an exaggerated or amplified neuroinflammatory response and subsequently, the amplification of the physiological and behavioral sequelae of this response (i.e. sickness). Finally, we explore the notion that stressor-induced sensitization of the neuroimmune microenvironment may predispose individuals to psychiatric disorders, in which exaggerated innate immune/inflammatory responses in the brain are now thought to play a key role.

Enriched environment decreases microglia and brain macrophages inflammatory phenotypes through adiponectin-dependent mechanisms: Relevance to depressive-like behavior

Regulation of neuroinflammation by glial cells plays a major role in the pathophysiology of major depression. While astrocyte involvement has been well described, the role of microglia is still elusive. Recently, we have shown that Adiponectin (ApN) plays a crucial role in the anxiolytic/antidepressant neurogenesis-independent effects of enriched environment (EE) in mice; however its mechanisms of action within the brain remain unknown. Here, we show that in a murine model of depression induced by chronic corticosterone administration, the hippocampus and the hypothalamus display increased levels of inflammatory cytokines mRNA, which is reversed by EE housing. By combining flow cytometry, cell sorting and q-PCR, we show that microglia from depressive-like mice adopt a pro-inflammatory phenotype characterized by higher expression levels of IL-1β, IL-6, TNF-α and IκB-α mRNAs. EE housing blocks pro-inflammatory cytokine gene induction and promotes arginase 1 mRNA expression in brain-sorted microglia, indicating that EE favors an anti-inflammatory activation state. We show that microglia and brain-macrophages from corticosterone-treated mice adopt differential expression profiles for CCR2, MHC class II and IL-4recα surface markers depending on whether the mice are kept in standard environment or EE. Interestingly, the effects of EE were abolished when cells are isolated from ApN knock-out mouse brains. When injected intra-cerebroventricularly, ApN, whose level is specifically increased in cerebrospinal fluid of depressive mice raised in EE, rescues microglia phenotype, reduces pro-inflammatory cytokine production by microglia and blocks depressive-like behavior in corticosterone-treated mice. Our data suggest that EE-induced ApN increase within the brain regulates microglia and brain macrophages phenotype and activation state, thus reducing neuroinflammation and depressive-like behaviors in mice.

Tris-(2,3-Dibromopropyl) Isocyanurate, a New Emerging Pollutant, Impairs Cognition and Provokes Depression-Like Behaviors in Adult Rats

Tris-(2,3-dibromopropyl) isocyanurate (TDBP-TAZTO), an emerging brominated flame retardant, possesses the characteristics of candidate persistent organic pollutants and has displayed toxicity to fish and rodents. TDBP-TAZTO can pass through the blood brain barrier and accumulate in brain. However, the neurotoxicity of TDBP-TAZTO has not yet studied in rodents. We hypothesize that TDBP-TAZTO could induce the neurotoxicity in rat hippocampal neurons. The male adult rats were exposed to TDBP-TAZTO of 5 and 50 mg/kg by gavage, daily for 6 months. TDBP-TAZTO resulted in cognitive impairment and depression-like behaviors, which may be related with TDBP-TAZTO-induced hypothalamic-pituitary-adrenal axis hyperactivation, upregulation of inflammatory and oxidative stress markers, overexpression of pro-apoptotic proteins, downexpression of neurogenesis-related proteins in hippocampus, and hippocampal neurons damage in DG, CA1 and CA3 areas. Our findings suggested that TDBP-TAZTO induces significant hippocampal neurotoxicity, which provokes cognitive impairment and depression-like behaviors in adult rats. Therefore, this research will contribute to evaluate the neurotoxic effects of TDBP-TAZTO in human.

Relevance of chronic stress and the two faces of microglia in Parkinson's disease

This review is aimed to highlight the importance of stress and glucocorticoids (GCs) in modulating the inflammatory response of brain microglia and hence its potential involvement in Parkinson’s disease (PD). The role of inflammation in PD has been reviewed extensively in the literature and it is supposed to play a key role in the course of the disease. Historically, GCs have been strongly associated as anti-inflammatory hormones. However, accumulating evidence from the peripheral and central nervous system have clearly revealed that, under specific conditions, GCs may promote brain inflammation including pro-inflammatory activation of microglia. We have summarized relevant data linking PD, neuroinflamamation and chronic stress. The timing and duration of stress response may be critical for delineating an immune response in the brain thus probably explain the dual role of GCs and/or chronic stress in different animal models of PD.

Stress sounds the alarmin: The role of the danger-associated molecular pattern HMGB1 in stress-induced neuroinflammatory priming

High mobility group box-1 (HMGB1) is an endogenous danger signal or alarmin that mediates activation of the innate immune response including chemotaxis and pro-inflammatory cytokine release. HMGB1 has been implicated in the pathophysiology of several neuroinflammatory conditions including ischemia, traumatic brain injury, seizure and chronic ethanol use. In the present review, the unique structural and functional properties of HMGB1 will be explored including its affinity for multiple pattern recognition receptors (TLR2/TLR4), redox sensitivity and adjuvant-like properties. In light of recent evidence suggesting that HMGB1 may also mediate stress-induced sensitization of neuroinflammatory responses, mechanisms of HMGB1 action in neuroinflammatory priming are explored. A model of neuroinflammatory priming is developed wherein glucocorticoids induce synthesis and release of HMGB1 from microglia, which signals through TLR2/TLR4, thereby priming the NLRP3 inflammasome. We propose that if GCs reach a critical threshold as during a fight/flight response, they may thus function as an alarmin by inducing HMGB1, thereby preparing an organism's innate immune system (NLRP3 inflammasome priming) for subsequent immune challenges such as injury, trauma or infection, which are more likely to occur during a fight/flight response. In doing so, GCs may confer a significant survival advantage by enhancing the central innate immune and sickness response to immune challenges.

The permissive role of glucocorticoids in neuroinflammatory priming: mechanisms and insights

PURPOSE OF REVIEW

Glucocorticoids have been universally regarded as anti-inflammatory; however, a considerable number of studies now demonstrate that under some conditions, glucocorticoids are capable of potentiating neuroinflammatory processes (i.e. priming), a permissive function of glucocorticoids. The present review addresses recent evidence that provides insight into the mechanism(s) of glucocorticoid-induced neuroinflammatory priming.

RECENT FINDINGS

Glucocorticoids have been found to prime inflammasomes [i.e. nucleotide-binding domain, leucine-rich repeat, pyrin domain containing proteins-3 (NLRP3)], which are intracellular multiprotein complexes that mediate proinflammatory processes. Inflammasomes are activated by products of stressed or damaged cells. Interestingly, these products (damage-associated molecular patterns) are induced by stress and mediate stress-induced neuroinflammatory priming.

SUMMARY

In light of these findings, we propose a model of glucocorticoid-induced neuroinflammatory priming whereby stress and glucocorticoids induce cellular damage/stress in the brain, the products of which prime the NLRP3 inflammasome. Thus, glucocorticoid-induced priming of the NLRP3 inflammasome may mediate the potentiated neuroinflammatory response to a subsequent proinflammatory immune challenge. We propose that during a flight-or-flight response, available energy stores should be diverted to defensive behaviours, and it might be after the emergency is over that resources should be shifted to recuperation and host defense against infection. This is the scenario that would be promoted by elevated glucocorticoids reducing ongoing inflammation while simultaneously priming the NLRP3 inflammasome.

Identification of Spinal Cord MicroRNA and Gene Signatures in a Model of Chronic Stress-Induced Visceral Hyperalgesia in Rat

Introduction

Animal studies have shown that stress could induce epigenetic and transcriptomic alterations essential in determining the balance between adaptive or maladaptive responses to stress. We tested the hypothesis that chronic stress in rats deregulates coding and non-coding gene expression in the spinal cord, which may underline neuroinflammation and nociceptive changes previously observed in this model.

Methods

Male Wistar rats were exposed to daily stress or handled, for 10 days. At day 11, lumbar spinal segments were collected and processed for mRNA/miRNA isolation followed by expression profiling using Agilent SurePrint Rat Exon and Rat miRNA Microarray platforms. Differentially expressed gene lists were generated using the dChip program. Microarrays were analyzed using the Ingenuity Pathways Analysis (IPA) tool from Ingenuity Systems. Multiple methods were used for the analysis of miRNA-mRNA functional modules. Quantitative real time RT-PCR for Interleukin 6 signal transducer (gp130), the Signal Transducer And Activator Of Transcription 3 (STAT3), glial fibrillary acidic protein and mir-17-5p were performed to confirm levels of expression.

Results

Gene network analysis revealed that stress deregulated different inflammatory (IL-6, JAK/STAT, TNF) and metabolic (PI3K/AKT) signaling pathways. MicroRNA array analysis revealed a signature of 39 deregulated microRNAs in stressed rats. MicroRNA-gene network analysis showed that microRNAs are regulators of two gene networks relevant to inflammatory processes. Specifically, our analysis of miRNA-mRNA functional modules identified miR-17-5p as an important regulator in our model. We verified miR-17-5p increased expression in stress using qPCR and in situ hybridization. In addition, we observed changes in the expression of gp130 and STAT3 (involved in intracellular signaling cascades in response to gp130 activation), both predicted targets for miR-17-5p. A modulatory role of spinal mir17-5p in the modulation of visceral sensitivity was confirmed in vivo.

Conclusion

Using an integrative high throughput approach, our findings suggest a link between miR-17-5p increased expression and gp130/STAT3 activation providing new insight into the possible mechanisms mediating the effect of chronic stress on neuroinflammation in the spinal cord.

Role of corticosteroids for fatigue in advanced incurable cancer: is it a 'wonder drug' or 'deal with the devil'

PURPOSE OF REVIEW

Although corticosteroids are frequently used to palliate cancer-related symptoms, limited published studies are available. This review summarizes recent literature on the impact of corticosteroids on fatigue, its related symptoms, and the role of 'corticosteroid rotation' in improving the corticosteroid-related side-effects.

RECENT FINDINGS

Only two placebo-controlled double-blinded randomized studies on corticosteroid for symptoms that met the inclusion criteria were published recently, despite the criteria being extended beyond 2012 and 2013. These two studies confirm the preliminary data that corticosteroids significantly improve cancer-related fatigue and anorexia with no significant side-effects. However, these studies were of short duration. Long-term use of corticosteroids is not recommended at this time because of the risk of debilitating side-effects and no evidence of its long-term benefits. Recent studies also suggest the possible role of corticosteroid rotation or switching similar to opioid rotation for improvement of these side-effects.

SUMMARY

Recent studies confirm the short-term benefit of corticosteroids for the symptomatic treatment of cancer-related fatigue and anorexia cachexia in advanced incurable cancer. Future studies are needed to determine the optimal dose, type, and the role of corticosteroid rotation so as to optimize long-term efficacy and minimize side-effects.

The role of inflammation and microglial activation in the pathophysiology of psychiatric disorders

Psychiatric disorders, including major depressive disorder (MDD), bipolar disorder (BD) and schizophrenia, affect a significant percentage of the world population. These disorders are associated with educational difficulties, decreased productivity and reduced quality of life, but their underlying pathophysiological mechanisms are not fully elucidated. Recently, studies have suggested that psychiatric disorders could be considered as inflammatory disorders, even though the exact mechanisms underlying this association are not known. An increase in inflammatory response and oxidative stress may lead to inflammation, which in turn can stimulate microglia in the brain. Microglial activation is roused by the M1 phenotype, which is associated with an increase in interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). On the contrary, M2 phenotype is associated with a release of anti-inflammatory cytokines. Thus, it is possible that the inflammatory response from microglial activation can contribute to brain pathology, as well as influence treatment responses. This review will highlight the role of inflammation in the pathophysiology of psychiatric disorders, such as MDD, BD, schizophrenia, and autism. More specifically, the role of microglial activation and associated molecular cascades will also be discussed as a means by which these neuroinflammatory mechanisms take place, when appropriate.

Corticosterone primes the neuroinflammatory response to DFP in mice: potential animal model of Gulf War Illness

Gulf War Illness (GWI) is a multi‐symptom disorder with features characteristic of persistent sickness behavior. Among conditions encountered in the Gulf War (GW) theater were physiological stressors (e.g., heat/cold/physical activity/sleep deprivation), prophylactic treatment with the reversible AChE inhibitor, pyridostigmine bromide (PB), the insect repellent, N,N‐diethyl‐meta‐toluamide (DEET), and potentially the nerve agent, sarin. Prior exposure to the anti‐inflammatory glucocorticoid, corticosterone (CORT), at levels associated with high physiological stress, can paradoxically prime the CNS to produce a robust proinflammatory response to neurotoxicants and systemic inflammation; such neuroinflammatory effects can be associated with sickness behavior. Here, we examined whether CORT primed the CNS to mount neuroinflammatory responses to GW exposures as a potential model of GWI. Male C57BL/6 mice were treated with chronic (14 days) PB/ DEET, subchronic (7–14 days) CORT, and acute exposure (day 15) to diisopropyl fluorophosphate (DFP), a sarin surrogate and irreversible AChE inhibitor. DFP alone caused marked brain‐wide neuroinflammation assessed by qPCR of tumor necrosis factor‐α, IL6, chemokine (C‐C motif) ligand 2, IL‐1β, leukemia inhibitory factor, and oncostatin M. Pre‐treatment with high physiological levels of CORT greatly augmented (up to 300‐fold) the neuroinflammatory responses to DFP. Anti‐inflammatory pre‐treatment with minocycline suppressed many proinflammatory responses to CORT+DFP. Our findings are suggestive of a possible critical, yet unrecognized interaction between the stressor/environment of the GW theater and agent exposure(s) unique to this war. Such exposures may in fact prime the CNS to amplify future neuroinflammatory responses to pathogens, injury, or toxicity. Such occurrences could potentially result in the prolonged episodes of sickness behavior observed in GWI. Gulf War (GW) veterans were exposed to stressors, prophylactic medicines and, potentially, nerve agents in theater. Subsequent development of GW Illness, a persistent multi‐symptom disorder with features characteristic of sickness behavior, may be caused by priming of the CNS resulting in exaggerated neuroinflammatory responses to pathogens/insults. Nerve agent, diisopropyl fluorophosphate (DFP), produced a neuroinflammatory response that was exacerbated by pre‐treatment with levels of corticosterone simulating heightened stressor conditions. While prophylactic treatments reduced DFP‐induced neuroinflammation, this effect was negated when those treatments were combined with corticosterone.

Neuroinflammation in the normal aging hippocampus

A consequence of normal aging is a greater susceptibility to memory impairments following an immune challenge such as infection, surgery, or traumatic brain injury. The neuroinflammatory response, produced by these challenges results in increased and prolonged production of pro-inflammatory cytokines in the otherwise healthy aged brain. Here we discuss the mechanisms by which long-lasting elevations in pro-inflammatory cytokines in the hippocampus produce memory impairments. Sensitized microglia are a primary source of this exaggerated neuroinflammatory response and appear to be a hallmark of the normal aging brain. We review the current understanding of the causes and effects of normal aging-induced microglial sensitization, including dysregulations of the neuroendocrine system, potentiation of neuroinflammatory responses following an immune challenge, and the impairment of memories. We end with a discussion of therapeutic approaches to prevent these deleterious effects.

Microglia inflammatory responses are controlled by an intrinsic circadian clock

The circadian system regulates many physiological functions including inflammatory responses. For example, mortality caused by lipopolysaccharide (LPS) injection varies depending on the time of immunostimulation in mammals. The effects of more subtle challenges on the immune system and cellular mechanisms underlying circadian differences in neuroinflammatory responses are not well understood. Here we show that adult male Sprague-Dawley rats injected with a sub-septic dose of LPS during the light phase displayed elevated sickness behaviors and hippocampal cytokine production compared to rats injected during the dark phase. Microglia are the primary central nervous system (CNS) immune cell type and may mediate diurnal differences in sickness response, thus we explored whether microglia demonstrate temporal variations in inflammatory factors. Hippocampal microglia isolated from adult rats rhythmically expressed inflammatory factors and circadian clock genes. Microglia displayed robust rhythms of TNFα, IL1β and IL6 mRNA, with peak cytokine gene expression occurring during the middle of the light phase. Microglia isolated during the light phase were also more reactive to immune stimulation; such that, ex vivo LPS treatment induced an exaggerated cytokine response in light phase-isolated microglia. Treating microglia with corticosterone ex vivo induced expression of the circadian clock gene Per1. However, microglia isolated from adrenalectomized rats maintained temporal differences in clock and inflammatory gene expression. This suggests circadian clock gene expression in microglia is entrained by, but oscillates in the absence of, glucocorticoids. Taken together, these findings demonstrate that microglia possess a circadian clock that influences inflammatory responses. These results indicate time-of-day is an important factor to consider when planning inflammatory interventions such as surgeries or immunotherapies.

Neuroimmune mechanisms of stress: sex differences, developmental plasticity, and implications for pharmacotherapy of stress-related disease

The last decade has witnessed profound growth in studies examining the role of fundamental neuroimmune processes as key mechanisms that might form a natural bridge between normal physiology and pathological outcomes. Rooted in core concepts from psychoneuroimmunology, this review utilizes a succinct, exemplar-driven approach of several model systems that contribute significantly to our knowledge of the mechanisms by which neuroimmune processes interact with stress physiology. Specifically, we review recent evidence showing that (i) stress challenges produce time-dependent and stressor-specific patterns of cytokine/chemokine expression in the CNS; (ii) inflammation-related genes exhibit unique expression profiles in males and females depending upon individual, cooperative or antagonistic interactions between steroid hormone receptors (estrogen and glucocorticoid receptors); (iii) adverse social experiences incurred through repeated social defeat engage a dynamic process of immune cell migration from the bone marrow to brain and prime neuroimmune function and (iv) early developmental exposure to an inflammatory stimulus (carageenin injection into the hindpaw) has a lasting influence on stress reactivity across the lifespan. As such, the present review provides a theoretical framework for understanding the role that neuroimmune mechanisms might play in stress plasticity and pathological outcomes, while at the same time pointing toward features of the individual (sex, developmental experience, stress history) that might ultimately be used for the development of personalized strategies for therapeutic intervention in stress-related pathologies.

Greater glucocorticoid receptor activation in hippocampus of aged rats sensitizes microglia

Healthy aging individuals are more likely to suffer profound memory impairments following an immune challenge than are younger adults. These challenges produce a brain inflammatory response that is exaggerated with age. Sensitized microglia found in the normal aging brain are responsible for this amplified response, which in turn interferes with processes involved in memory formation. Here, we examine factors that may lead aging to sensitize microglia. Aged rats exhibited higher corticosterone levels in the hippocampus, but not in plasma, throughout the daytime (diurnal inactive phase). These elevated hippocampal corticosterone levels were associated with increased hippocampal 11β-hydroxysteroid dehydrogenase type 1 protein expression, the enzyme that catalyzes glucocorticoid formation and greater hippocampal glucocorticoid receptor (GR) activation. Intracisternal administration of mifepristone, a GR antagonist, effectively reduced immune-activated proinflammatory responses, specifically from hippocampal microglia and prevented Escherichia coli-induced memory impairments in aged rats. Voluntary exercise as a therapeutic intervention significantly reduced total hippocampal GR expression. These data strongly suggest that increased GR activation in the aged hippocampus plays a critical role in sensitizing microglia.

Thinking outside the brain for cognitive improvement: Is peripheral immunomodulation on the way?

Cognitive impairment is a devastating condition commonly observed with normal aging and neurodegenerative disorders such as Alzheimer's Disease (AD). Although major efforts to prevent or slow down cognitive decline are largely focused within the central nervous system (CNS), it has become clear that signals from the systemic milieu are closely associated with the dysfunctional brain. In particular, the bidirectional crosstalk between the CNS and peripheral immune system plays a decisive role in shaping neuronal survival and function via neuroimmune, neuroendocrinal and bioenergetic mechanisms. Importantly, it is emerging that some neuroprotective and cognition-strengthening drugs may work by targeting the brain-periphery interactions, which could be intriguingly achieved without entering the CNS. We describe here how recent advances in dissecting cognitive deficits from a systems-perspective have contributed to a non-neurocentric understanding of its pathogenesis and treatment strategy. We also discuss the therapeutic and diagnostic implications of these exciting progresses and consider some key issues in the clinical translation. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.