Early-life lipopolysaccharide exposure potentiates forebrain expression of NLRP3 inflammasome proteins and anxiety-like behavior in adolescent rats

Evidence and Mechanism of Bile Acid-Mediated Gut-Brain Axis in Anxiety and Depression

Bidirectional communication between the brain and gastrointestinal tract, called the gut-brain axis, is linked with our emotions. Intestinal lipids, hormones, and molecules, such as bile acids (BAs), impact our mood, motivation, and emotions via the gut-brain axis. BAs are synthesized from cholesterol in the liver and serve as a regulator of lipid metabolism and hormonal secretion in the intestine. Human studies have indicated that the alteration of plasma BA levels is associated with depression and anxiety. Several possible mechanisms, such as BA receptor-dependent and receptor-independent mechanisms, have been reported for emotional control. Animal studies have indicated that the deletion of BA receptors shows behavioral abnormalities. BAs regulate gut hormones, glucagon-like peptide-1 secretion, bioactive lipids, oleoylethanolamide, and the immune system function, which influences neural activities. Thus, BAs act as an emotional regulator. This review aims to summarize the following: clinical evidence of BA concentration linked to mental disorders, including depression and anxiety; and animal studies of BA-related signaling correlated with its neurobehavioral effect supporting its mechanism. We will also discuss future research required for further neurobehavioral treatment.

Exogenous monosodium glutamate exacerbates lipopolysaccharide-induced neurobehavioral deficits, oxidative damage, neuroinflammation, and cholinergic dysfunction in rat brain

Extensive experimental evidence points to neuroinflammation and oxidative stress as major pathogenic events that initiate and drive the neurodegenerative process. Monosodium glutamate (MSG) is a widely used food additive in processed foods known for its umami taste-enhancing properties. However, concerns about its potential adverse effects on the brain have been raised. Thus, the present study investigated the impact of MSG on lipopolysaccharide (LPS)-induced neurotoxicity in rat brains. Wistar rats weighing between 180 g and 200 g were randomly allocated into four groups: control (received distilled water), MSG (received 1.5 g/kg/day), LPS (received 250 µg/kg/day), and LPS + MSG (received LPS, 250 µg/kg, and MSG, 1.5 g/kg). LPS was administered intraperitoneally for 7 days while MSG was administered orally for 14 days. Our results showed that MSG exacerbated LPS-induced impairment in locomotor and exploratory activities in rats. Similarly, MSG exacerbated LPS-induced oxidative stress as evidenced by increased levels of malondialdehyde (MDA) with a concomitant decrease in levels of superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione-s-transferase (GST) in the brain tissue. In addition, MSG potentiated LPS-induced neuroinflammation, as indicated by increased levels of pro-inflammatory cytokines such as interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) as well as myeloperoxidase (MPO) and nitric oxide (NO) in the brain. Moreover, MSG aggravated LPS-induced cholinergic dysfunction, as demonstrated by increased activity of acetylcholinesterase (AChE) in the brain. Further, we found a large number of degenerative neurons widespread in hippocampal CA1, CA3 regions, cerebellum, and cortex according to H&E staining. Taken together, our findings suggest that MSG aggravates LPS-induced neurobehavioral deficits, oxidative stress, neuroinflammation, cholinergic dysfunction, and neurodegeneration in rat brains.

Hydrogen sulfide ameliorates lipopolysaccharide-induced anxiety-like behavior by inhibiting checkpoint kinase 1 activation in the hippocampus of mice

Hydrogen sulfide (H2S), an endogenous gasotransmitter, exhibits the anxiolytic roles through its anti-inflammatory effects, although its underlying mechanisms remain largely elusive. Emerging evidence has documented that cell cycle checkpoint kinase 1 (Chk1)-regulated DNA damage plays an important role in the neurodegenerative diseases; however, there are few relevant reports on the research of Chk1 in neuropsychiatric diseases. Here, we aimed to investigate the regulatory role of H2S on Chk1 in lipopolysaccharide (LPS)-induced anxiety-like behavior focusing on inflammasome activation in the hippocampus. Cystathionine γ-lyase (CSE, a H2S-producing enzyme) knockout (CSE-/-) mice displayed anxiety-like behavior and activation of inflammasome-mediated inflammatory responses, manifesting by the increase levels of interleukin-1β (IL-1β), IL-6, and ionized calcium-binding adaptor molecule-1 (Iba-1, microglia marker) expression in the hippocampus. Importantly, expression of p-Chk1 and γ-H2AX (DNA damage marker) levels were also increased in the hippocampus of CSE-/- mice. LPS treatment decreased the expression of CSE and CBS while increased p-Chk1 and γ-H2AX levels and inflammasome-activated neuroinflammation in the hippocampus of mice. Moreover, p-Chk1 and γ-H2AX protein levels and cellular immunoactivity were significantly increased while CSE and CBS were markedly decreased in cultured BV2 cells followed by LPS treatment. Treatment of mice with GYY4137, a donor of H2S, inhibited LPS-induced increased in p-Chk1 and γ-H2AX levels, mitigated inflammasome activation and inflammatory responses as well as amelioration of anxiety-like behavior. Notably, SB-218078, a selective Chk1 inhibitor treatment attenuated the effect of LPS on inflammasome activation and inflammatory responses and the induction of anxiety-like behavior. Finally, STAT3 knockdown with AAV-STAT3 shRNA alleviated LPS-induced anxiety-like behavior and inhibited inflammasome activation in the hippocampus, and blockade of NLRP3 with MCC950 attenuated neuroinflammation induction and ameliorated LPS-induced anxiety-like behavior. Overall, this study indicates that downregulation of Chk1 activity by H2S activation may be considered as a valid strategy for preventing the progression of LPS-induced anxiety-like behavior.

Bile acid signalling and its role in anxiety disorders

Anxiety disorder is a prevalent neuropsychiatric disorder that afflicts 7.3%~28.0% of the world's population. Bile acids are synthesized by hepatocytes and modulate metabolism via farnesoid X receptor (FXR), G protein-coupled receptor (TGR5), etc. These effects are not limited to the gastrointestinal tract but also extend to tissues and organs such as the brain, where they regulate emotional centers and nerves. A rise in serum bile acid levels can promote the interaction between central FXR and TGR5 across the blood-brain barrier or activate intestinal FXR and TGR5 to release fibroblast growth factor 19 (FGF19) and glucagon-like peptide-1 (GLP-1), respectively, which in turn, transmit signals to the brain via these indirect pathways. This review aimed to summarize advancements in the metabolism of bile acids and the physiological functions of their receptors in various tissues, with a specific focus on their regulatory roles in brain function. The contribution of bile acids to anxiety via sending signals to the brain via direct or indirect pathways was also discussed. Different bile acid ligands trigger distinct bile acid signaling cascades, producing diverse downstream effects, and these pathways may be involved in anxiety regulation. Future investigations from the perspective of bile acids are anticipated to lead to novel mechanistic insights and potential therapeutic targets for anxiety disorders.

The interplay between sexual abuse and inflammation, oxidative stress, and DNA damage in drug-naïve panic disorder patients

Although accumulating evidence suggests an interplay between child abuse and inflammatory processes and the pathophysiology of mental disorders, few studies have investigated the cellular mechanisms related to this matter. Furthermore, no studies to date have evaluated cytokine, oxidative stress, and DNA damage levels in drug-naïve panic disorder (PD) patients and their possible association with childhood trauma. The aim of the present study was to determine the levels of the proinflammatory interleukin (IL)-1B, the oxidative stress marker TBARS, and  8-hydroxy-2' -deoxyguanosine (8-OHdG; representing DNA damage) in drug-naïve PD patients compared to controls. Furthermore, this investigation aimed to determine whether early-life trauma could predict peripheral levels of the previously mentioned markers in unmedicated PD patients. This work showed that drug-naïve PD patients presented elevated levels of TBARS and IL-1B but not 8-OHdG compared to healthy controls. In addition, sexual abuse during childhood was associated with increased levels of IL-1B in PD patients. Our findings suggest that the microglial NLRP3 inflammasome complex might be activated in drug-naïve PD patients. This study is the first to associated sexual abuse with increased levels of IL-1B in drug-naïve PD patients and to demonstrate that this population presents high concentrations of oxidative stress and inflammation markers but not DNA damage markers when compared to healthy controls. Independent replication of these findings would support further clinical trials of inflammasome inhibitory drugs in PD patients, which could lead to effective novel treatments for people with PD and contribute to elucidating pathophysiological differences depending on trauma exposure in the immune disturbances accompanying PD.

The influence of long-term housing in enriched environment on behavior of normal rats and subjected to neonatal pro-inflammatory challenge

It is well known that neonatal pro-inflammatory challenge (NPC) acquire a predisposition to the development of a number of neuropsychiatric diseases: depression, anxiety disorders, autism, attention deficit hyperactivity disorder. Symptoms of these diseases can manifest themselves in adulthood and adolescent after repeated exposure to negative influences. Preventing the development of the negative consequences of NPC is one of the main tasks for researchers. The exposure to an enriched environment (EE) was shown to have anxiolytic, anti-depressive, and pro-cognitive effects. The present work was aimed to investigate the effects of the long-term EE on anxious-depressive and conditioned fear behavior in normal male and female rats and subjected to NPC. The NPC was induced by subcutaneous administration of lipopolysaccharide (LPS, 50 μg/kg) on 3d and 5th PNDs. The control animals received saline (SAL). The rats were placed in the EE from 25 to 120 PND. Animals housed in the standard conditions (STAND) served as controls. In adult female and male rats of the STAND groups, LPS did not affect the anxiety, depressive-like behavior and conditioned fear. The EE increased motor and search activity in males and females. In the open field, the EE reduced anxiety in males of the SAL and LPS groups and in females of SAL groups compared to the STAND housed animals. In the elevated plus maze, the EE decreased anxiety only in males of the SAL group. In the sucrose preference test, the EE did not change sucrose consumption in males and females of SAL and LPS groups, while, in the forced swimming test, the EE reduced depressive-like behavior in females of both SAL and LPS groups. The enrichment decreased the contextual conditioned fear in male and female of SAL groups, but not of the LPS group, and did not affect the cue conditioned fear. The corticosterone reactivity to the forced swimming stress increased in males of the EE groups. The basal level of IL-1beta in blood serum decreased in males of the SAL-EE group. Thus, the EE reduced anxiety in males, depressive-like behavior in females, and contextual conditioned fear in males and females compared to the STAND housed animals. Although the NPC did not affect these behaviors in the STAND groups, LPS prevented the beneficial EE effects on anxiety and conditioned fear. The opposing effects of LPS were dependent on sex and type of testing.

(m-CF3-PhSe)2 benefits against anxiety-like phenotype associated with synaptic plasticity impairment and NMDAR-mediated neurotoxicity in young mice exposed to a lifestyle model

Lifestyle habits including energy-dense foods and ethanol intake are associated with anxiety disorders. m-Trifluoromethyl-diphenyl diselenide [(m-CF₃-PhSe)₂] has been reported to modulate serotonergic and opioidergic systems and elicit an anxiolytic-like phenotype in animal models. This study investigated if the modulation of synaptic plasticity and NMDAR-mediated neurotoxicity contributes to the (m-CF₃-PhSe)₂ anxiolytic-like effect in young mice exposed to a lifestyle model. Swiss male mice (25-days old) were subjected to a lifestyle model, an energy-dense diet (20:20% lard: corn syrup) from the postnatal day (PND) 25-66 and sporadic ethanol (2 g/kg) (3 x a week, intragastrically, i.g.) from PND 45 to 60. From PND 60 to 66, mice received (m-CF₃-PhSe)₂ (5 mg/kg/day; i.g). The corresponding vehicle (control) groups were carried out. After, mice performed anxiety-like behavioral tests. Mice exposed only to an energy-dense diet or sporadic ethanol did not show an anxiety-like phenotype. (m-CF₃-PhSe)₂ abolished the anxiety-like phenotype in young mice exposed to a lifestyle model. Anxious-like mice showed increased levels of cerebral cortical NMDAR2A and 2B, NLRP3 and inflammatory markers, and decreased contents of synaptophysin, PSD95, and TRκB/BDNF/CREB signaling. (m-CF₃-PhSe)₂ reversed cerebral cortical neurotoxicity, the increased levels of NMDA2A and 2B, and decreased levels of synaptic plasticity-related signaling in the cerebral cortex of young mice exposed to a lifestyle model. In conclusion, the (m-CF₃-PhSe)₂ anxiolytic-like effect was associated with the modulation of NMDAR-mediated neurotoxicity and synaptic plasticity in the cerebral cortex of young mice exposed to the lifestyle model.

CB2R activation ameliorates late adolescent chronic alcohol exposure-induced anxiety-like behaviors during withdrawal by preventing morphological changes and suppressing NLRP3 inflammasome activation in prefrontal cortex microglia in mice

BACKGROUND

Chronic alcohol exposure (CAE) during late adolescence increases the risk of anxiety development. Alcohol-induced prefrontal cortex (PFC) microglial activation, characterized by morphological changes and increased associations with neurons, plays a critical role in the pathogenesis of anxiety. Alcohol exposure increases NLRP3 inflammasome expression, increasing cytokine secretion by activated microglia. Cannabinoid type 2 receptor (CB2R), an essential receptor of the endocannabinoid system, regulates microglial activation and neuroinflammatory reactions. We aimed to investigate the role of CB2R activation in ameliorating late adolescent CAE-induced anxiety-like behaviors and microglial activation in C57BL/6J mice.

METHODS

Six-week-old C57BL/6J mice were acclimated for 7 days and then were administered alcohol by gavage (4 g/kg, 25 % w/v) for 28 days. The mice were intraperitoneally injected with the specific CB2R agonist AM1241 1 h before alcohol treatment. Anxiety-like behaviors during withdrawal were assessed by open field test and elevated plus maze test 24 h after the last alcohol administration. Microglial activation, microglia-neuron interactions, and CB2R and NLRP3 inflammasome-related molecule expression in the PFC were measured using immunofluorescence, immunohistochemical, qPCR, and Western blotting assays. Microglial morphology was evaluated by Sholl analysis and the cell body-to-total cell size index. Additionally, N9 microglia were activated by LPS in vitro, and the effects of AM1241 on NLRP3 and N9 microglial activation were investigated.

RESULTS

After CAE, mice exhibited severe anxiety-like behaviors during withdrawal. CAE induced obvious microglia-neuron associations, and increased expression of microglial activation markers, CB2R, and NLRP3 inflammasome-related molecules in the PFC. Microglia also showed marked filament retraction and reduction and cell body enlargement after CAE. AM1241 treatment ameliorated anxiety-like behaviors in CAE model mice, and it prevented microglial morphological changes, reduced microglial activation marker expression, and suppressed the microglial NLRP3 inflammasome activation and proinflammatory cytokine secretion induced by CAE. AM1241 suppressed the LPS-induced increase in NLRP3 inflammasome-related molecules, IL-1β release, and M1 phenotype markers (iNOS and CD86) in N9 cell, which was reversed by CB2R antagonist treatment.

CONCLUSIONS

CAE caused anxiety-like behaviors in late adolescent mice at least partly by inducing microglial activation and increasing microglia-neuron associations in the PFC. CB2R activation ameliorated these effects by preventing morphological changes and suppressing NLRP3 inflammasome activation in PFC microglia.

Once induced, it lasts for a long time: the structural and molecular signatures associated with depressive-like behavior after neonatal immune activation

Adverse factors such as stress or inflammation in the neonatal period can affect the development of certain brain structures and have negative delayed effects throughout the lifespan of an individual, by reducing cognitive abilities and increasing the risk of psychopathologies. One possible reason for these delayed effects is the neuroinflammation caused by neonatal immune activation (NIA). Neuroinflammation can lead to disturbances of neurotransmission and to reprogramming of astroglial and microglial brain cells; when combined, the two problems can cause changes in the cytoarchitecture of individual regions of the brain. In addition, neuroinflammation may affect the hypothalamic-pituitary-adrenal (HPA) axis and processes of oxidative stress, thereby resulting in higher stress reactivity. In our review, we tried to answer the questions of whether depressive-like behavior develops after NIA in rodents and what the molecular mechanisms associated with these disorders are. Most studies indicate that NIA does not induce depressive-like behavior in a steady state. Nonetheless, adult males (but not females or adolescents of both sexes) with experience of NIA exhibit marked depressive-like behavior when exposed to aversive conditions. Analyses of molecular changes have shown that NIA leads to an increase in the amount of activated microglia and astroglia in the frontal cortex and hippocampus, an increase in oxidative-stress parameters, a change in stress reactivity of the HPA axis, and an imbalance of cytokines in various regions of the brain, but not in blood plasma, thus confirming the local nature of the inflammation. Therefore, NIA causes depressive-like behavior in adult males under aversive testing conditions, which are accompanied by local inflammation and have sex- and age-specific effects.

Intrauterine Inflammation Leads to Select Sex- and Age-Specific Behavior and Molecular Differences in Mice

Sex-specific differences in behavior have been observed in anxiety and learning in children exposed to prenatal inflammation; however, whether these behaviors manifest differently by age is unknown. This study assesses possible behavioral changes due to in utero inflammation as a function of age in neonatal, juvenile, and adult animals and presents potential molecular targets for observed differences. CD-1 timed pregnant dams were injected in utero with lipopolysaccharide (LPS, 50 μg/animal) or saline at embryonic day 15. No differences in stress responses were measured by neonatal ultrasonic vocalizations between LPS- and saline-exposed groups of either sex. By contrast, prenatal inflammation caused a male-specific increase in anxiety in mature but not juvenile animals. Juvenile LPS-exposed females had decreased movement in open field testing that was not present in adult animals. We additionally observed improved memory retrieval after in utero LPS in the juvenile animals of both sexes, which in males may be related to a perseverative phenotype. However, there was an impairment of long-term memory in only adult LPS-exposed females. Finally, gene expression analyses revealed that LPS induced sex-specific changes in genes involved in hippocampal neurogenesis. In conclusion, intrauterine inflammation has age- and sex-specific effects on anxiety and learning that may correlate to sex-specific disruption of gene expression associated with neurogenesis in the hippocampus.

Chronic lipopolysaccharide impairs motivation when delivered to the ventricles, but not when delivered peripherally in male rats

Increased neuroinflammation relative to controls is observed in major depression. Moreover, depressive disorders are significantly elevated in conditions which increase neuroinflammation (e.g., brain injury, Parkinson's disease, Alzheimer's disease). To better understand the relationship between neuroinflammation and depression, additional research is needed. The current set of studies made use of the progressive ratio (PR) task in male rats, a stable measure of motivation which can be evaluated daily and thus is ideally suited for examining a potential role for chronic neuroinflammation in depressive-like behavior. Lipopolysaccharide (LPS) was used to induce an inflammatory response. Experiment 1 confirmed prior acute LPS administration experiments for sensitivity of the PR task, with a large effect at 2 mg/kg, a partial effect at 1 mg/kg, and no effect at 0.5 mg/kg. Experiment 2 evaluated a dose-response of continuous s.c. LPS infusion but found no significant elevation in brain cytokines after 14 days at any doses of 0.1, 0.5, 1, or 2 mg/kg/week. Experiment 3 assessed motivation during continuous s.c. infusion of a large 5 mg/kg/week LPS dose and found no significant impairments in motivation, but transient decreases in rates of lever pressing (i.e., only motoric deficits). Experiment 4 measured motivation during continuous ICV infusion of 10.5 μg/kg/week LPS and found significantly decreased motivation without changes to rates of lever pressing (i.e., only motivational deficits). Together these results suggest that the PR task is efficient for evaluating models of chronic inflammation, and that the adaptive response to chronic LPS exposure, even when delivered centrally, may necessitate alternative strategies for generating long-term neuroinflammation.

Outer membrane vesicles of Porphyromonas gingivalis trigger NLRP3 inflammasome and induce neuroinflammation, tau phosphorylation, and memory dysfunction in mice

Background

Porphyromonas gingivalis (Pg), the keystone pathogen in chronic periodontitis, is reported to initiate Alzheimer’s disease pathologies in preclinical studies. However, the specific mechanisms and signaling pathways acting on the brain still need to be further explored. Outer membrane vesicles are derived from Gram-negative bacteria and contain many virulence factors of bacteria. We hypothesized that outer membrane vesicles are an important weapon of Porphyromonas gingivalis to initiate Alzheimer’s disease pathologies.

Methods

The outer membrane vesicles of Porphyromonas gingivalis (Pg OMVs, 4 mg/kg) or saline were delivered to 14-month-old mice by oral gavage every other day for eight weeks. Behavioral alterations were assessed by the open field test, Morris water maze, and Y-maze test. Blood–brain barrier permeability, neuroinflammation, tau phosphorylation, and NLRP3 inflammasome-related protein were analyzed.

Results

Pg OMVs impaired memory and learning ability of mice and decreased tight junction–related gene expression ZO-1, occludin, claudin-5, and occludin protein expression in the hippocampus. Pg OMVs could be detected in the hippocampus and cortex three days after oral gavage. Furthermore, Pg OMVs activated both astrocytes and microglia and elevated IL-1β, tau phosphorylation on the Thr231 site, and NLRP3 inflammasome–related protein expression in the hippocampus. In in vitro studies, Pg OMV (5 µg/ml) stimulation increased the mRNA and immunofluorescence of NLRP3 in BV2 microglia, which were significantly inhibited by the NLRP3 inhibitor MCC950. In contrast, the tau phosphorylation in N2a neurons was enhanced after treatment with conditioned media from Pg OMV-stimulated microglia, which was attenuated after pretreatment with MCC950.

Conclusions

These results indicate that Pg OMVs prompt memory dysfunction, neuroinflammation, and tau phosphorylation and trigger NLRP3 inflammasome in the brain of middle-aged mice. We propose that Pg OMVs play an important role in activating neuroinflammation in the AD-like pathology triggered by Porphyromonas gingivalis, and NLRP3 inflammasome activation is a possible mechanism.

SARS-CoV-2 consequences for mental health: Neuroinflammatory pathways linking COVID-19 to anxiety and depression

The coronavirus disease 2019 (COVID-19) pandemic has been linked to an increased prevalence of mental health disorders, particularly anxiety and depression. Moreover, the COVID-19 pandemic has caused stress in people worldwide due to several factors, including fear of infection; social isolation; difficulty in adapting to new routines; lack of coping methods; high exposure to social media, misinformation, and fake reports; economic impact of the measures implemented to slow the contagion and concerns regarding the disease pathogenesis. COVID-19 patients have elevated levels of pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, and tumor necrosis factor-α, and other inflammation-related factors. Furthermore, invasion of the central nervous system by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may potentially contribute to neuroinflammatory alterations in infected individuals. Neuroinflammation, a consequence of psychological stress due to the COVID-19 pandemic, may also play a role in the development of anxiety and depressive symptoms in the general population. Considering that neuroinflammation plays a significant role in the pathophysiology of depression and anxiety, this study investigated the effects of SARS-CoV-2 on mental health and focused on the impact of the COVID-19 pandemic on the neuroinflammatory pathways.

Agmatine as a novel candidate for rapid-onset antidepressant response

Major depressive disorder (MDD) is a disabling and highly prevalent mood disorder as well as a common cause of suicide. Chronic stress, inflammation, and intestinal dysbiosis have all been shown to play crucial roles in the pathophysiology of MDD. Although conventional antidepressants are widely used in the clinic, they can take weeks to months to produce therapeutic effects. The discovery that ketamine promotes fast and sustaining antidepressant responses is one of the most important breakthroughs in the pharmacotherapy of MDD. However, the adverse psychomimetic/dissociative and neurotoxic effects of ketamine discourage its chronic use. Therefore, agmatine, an endogenous glutamatergic modulator, has been postulated to elicit fast behavioral and synaptogenic effects by stimulating the mechanistic target of rapamycin complex 1 signaling pathway, similar to ketamine. However, recent evidence has demonstrated that the modulation of the NLR family pyrin domain containing 3 inflammasome and gut microbiota, which have been shown to play a crucial role in the pathophysiology of MDD, may also participate in the antidepressant-like effects of both ketamine and agmatine. This review seeks to provide evidence about the mechanisms that may underlie the fast antidepressant-like responses of agmatine in preclinical studies. Considering the anti-inflammatory properties of agmatine, it may also be further investigated as a useful compound for the management of MDD associated with a pro-inflammatory state. Moreover, the fast antidepressant-like response of agmatine noted in animal models should be investigated in clinical studies.

NLRP3 Inflammasome Is Involved in Cocaine-Mediated Potentiation on Behavioral Changes in CX3CR1-Deficient Mice

Microglia, the primary immunocompetent cells of the brain, are suggested to play a role in the development of drug addiction. Previous studies have identified the microglia-derived pro-inflammatory factor IL1β can promote the progression of cocaine addiction. Additionally, the activation status of microglia and “two-hit hypothesis” have been proposed in the field of drug addiction to explain how early life stress (ELS) could significantly increase the incidence of drug addiction in later life. However, the mechanisms underlying microglia prime and full activation and their roles in drug addiction remain greatly unexplored. Here, we employed CX3CR1-GFP mice (CX3CR1 functional deficiency, CX3CR1−/−) to explore whether primed microglia could potentiate cocaine-mediated behavioral changes and the possible underlying mechanisms. CX3CR1−/− mice revealed higher hyperlocomotion activity and conditional place preference than wild-type (WT) mice did under cocaine administration. In parallel, CX3CR1−/− mice showed higher activity of NLR family pyrin domain-containing 3 (NLRP3) inflammasome than WT mice. Interestingly, CX3CR1 deficiency itself could prime NLRP3 signaling by increasing the expression of NLPR3 and affect lysosome biogenesis under basal conditions. Taken together, our findings demonstrated that the functional status of microglia could have an impact on cocaine-mediated reward effects, and NLRP3 inflammasome activity was associated with this phenomenon. This study was consistent with the two-hit hypothesis and provided solid evidence to support the involvement of microglia in drug addiction. Targeting the NLRP3 inflammasome may represent a novel therapeutic approach for ameliorating or blocking the development of drug addiction.

Neonatal Exposure to Bacterial Lipopolysaccharide Affects Behavior and Expression of Ionotropic Glutamate Receptors in the Hippocampus of Adult Rats after Psychogenic Trauma

According to the two-hit hypothesis of psychoneuropathology formation, infectious diseases and other pathological conditions occurring during the critical periods of early ontogenesis disrupt normal brain development and increase its susceptibility to stress experienced in adolescence and adulthood. It is believed that these disorders are associated with changes in the functional activity of the glutamatergic system in the hippocampus. Here, we studied expression of NMDA (GluN1, GluN2a, GluN2b) and AMPA (GluA1, GluA2) glutamate receptor subunits, as well as glutamate transporter EAAT2, in the ventral and dorsal regions of the hippocampus of rats injected with LPS during the third postnatal week and then subjected to predator stress (contact with a python) in adulthood. The tests were performed 25 days after the stress. It was found that stress altered protein expression in the ventral, but not in the dorsal hippocampus. Non-stressed LPS-treated rats displayed lower levels of the GluN2b protein in the ventral hippocampus vs. control animals. Stress significantly increased the content of GluN2b in the LPS-treated rats, but not in the control animals. Stress also affected differently the exploratory behavior of LPS-injected and control rats. Compared to the non-stressed animals, stressed control rats demonstrated a higher locomotor activity during the 1st min of the open field test, while the stressed LPS-injected rats displayed lower locomotor activity than the non-stressed rats. In addition, LPS-treated stressed and non-stressed rats spent more time in the open arms of the elevated plus maze and demonstrated reduced blood levels of corticosterone. To summarize the results of our study, exposure to bacterial LPS in the early postnatal ontogenesis affects the pattern of stress-induced changes in the behavior and hippocampal expression of genes coding for ionotropic glutamate receptor subunits after psychogenic trauma suffered in adulthood.

Dexmedetomidine alleviates neuroinflammation, restores sleep disorders and neurobehavioral abnormalities in rats with minimal hepatic encephalopathy

The occurrence and progress of minimal hepatic encephalopathy (MHE) is closely related to the inflammatory response; however, inflammation contributes to behavioral abnormalities and sleep disorders. Dexmedetomidine has anti-inflammatory effects against various diseases. Whether dexmedetomidine improves MHE and the underlying mechanism is yet unclear. The present study aimed to explore the effects of dexmedetomidine on sleep structure, neurobehavior, and brain morphology of MHE rats and investigate its underlying mechanism. A rat MHE model was established by intraperitoneal injection of thioacetamide (TAA). Dexmedetomidine or yohimbine was administered intraperitoneally to investigate the role of α2 adrenoreceptor in the protection conferred by dexmedetomidine. The 24-h sleep, neurobehavioral changes, the liver function, blood ammonia and morphological changes of the liver and brain were assessed. Also, the microglia, astrocytes, neurons, the expression of pro-inflammatory factors (IL-1β, TNF-α, IL-18), and NLRP3 inflammasomes were detected. The results showed that marked sleep disorders, cognitive impairment, anxiety, abnormal liver function and pathological damage of liver and brain were detected in the MHE rats. The microglia in the prefrontal cortex was highly activated along with the increased expression of pro-inflammatory factors and NLRP3 inflammasomes. Interestingly, dexmedetomidine improved above indicators, however, yohimbine significantly abolished the protection of dexmedetomidine. These findings showed that dexmedetomidine restored the changes in the sleep disorders and neurobehavior in rats and reduced brain damage. The mechanism might be partially related to the activation of α2 adrenergic receptors, reduction of neuroinflammatory response, and inhibition of the activation of microglia and NLRP3/Caspase1 signaling pathway.

Paeoniflorin ameliorates neuropathic pain-induced depression-like behaviors in mice by inhibiting hippocampal neuroinflammation activated via TLR4/NF-κB pathway

Neuropathic pain (NP) that contributes to the comorbidity between pain and depression is a clinical dilemma. Neuroinflammatory responses are known to have potentially important roles in the initiation of NP and depressive mood. In this study, we aimed to investigate the effects of paeoniflorin (PF) on NP-induced depression-like behaviors by targeting the hippocampal neuroinflammation through the toll-like receptor 4 (TLR4)/nuclear factor-kappa B (NF-κB) signaling pathway. We used a murine model of NP caused by unilateral sciatic nerve cuffing (Cuff). PF was injected intraperitoneally once a day for a total of 14 days. Pain and depression-like behavior changes were evaluated via behavioral tests. Pathological changes in the hippocampus of mice were observed by H&E staining. The levels of proinflammatory cytokines in the hippocampus were detected using ELISA. Activated microglia were measured by immunohistochemical staining. The TLR4/NF-κB signaling pathway-associated protein expression in the hippocampus was detected by western blotting. We found that the PF could significantly alleviate Cuff-induced hyperalgesia and depressive behaviors, lessen the pathological damage to the hippocampal cell, reduce proinflammatory cytokines levels, and inhibit microglial over-activation. Furthermore, PF downregulated the expression levels of TLR4/NF-κB signaling pathway-related proteins in the hippocampus. These results indicate that PF is an effective drug for improving the comorbidity between NP and depression.

Exposure to bacterial lipopolysaccharidein early life affects the expression of ionotropic glutamate receptor genes and is accompanied by disturbances in long-term potentiation and cognitive functions in young rats

Infections in childhood play an essential role in the pathogenesis of cognitive and psycho-emotional disorders. One of the possible mechanisms of these impairments is changes in the functional properties of NMDA and AMPA glutamate receptors in the brain. We suggest that bacterial infections during the early life period, which is critical for excitatory synapse maturation, can affect the subunit composition of NMDA and AMPA receptors. In the present study, we investigated the effect of repetitive lipopolysaccharide (LPS) intraperitoneal (i.p.) administration (25 μg/kg/day on P14, 16, and 18), mimicking an infectious disease, on the expression of subunits of NMDA and AMPA receptors in young rats. We revealed a substantial decrease of GluN2B subunit expression in the hippocampus at P23 using Western blot analysis and real-time polymerase chain reaction assay. Moderate changes were also found in GluN1, GluN2A, and GluA1 mRNA expression. The LPS-treated rats exhibited decreased exploratory and locomotor activity in the open field test and the impairment of spatial learning in the Morris water maze. Behavioral impairments were accompanied by a significant reduction in long-term hippocampal synaptic potentiation. Our data indicate that LPS-treatment in the critical period for excitatory synapse maturation alters ionotropic glutamate receptor gene expression, disturbs synaptic plasticity, and alters behavior.

Microglial Function in the Effects of Early-Life Stress on Brain and Behavioral Development

The putative effects of early-life stress (ELS) on later behavior and neurobiology have been widely investigated. Recently, microglia have been implicated in mediating some of the effects of ELS on behavior. In this review, findings from preclinical and clinical literature with a specific focus on microglial alterations induced by the exposure to ELS (i.e., exposure to behavioral stressors or environmental agents and infection) are summarized. These studies were utilized to interpret changes in developmental trajectories based on the time at which the stress occurred, as well as the paradigm used. ELS and microglial alterations were found to be associated with a wide array of deficits including cognitive performance, memory, reward processing, and processing of social stimuli. Four general conclusions emerged: (1) ELS interferes with microglial developmental programs, including their proliferation and death and their phagocytic activity; (2) this can affect neuronal and non-neuronal developmental processes, which are dynamic during development and for which microglial activity is instrumental; (3) the effects are extremely dependent on the time point at which the investigation is carried out; and (4) both pre- and postnatal ELS can prime microglial reactivity, indicating a long-lasting alteration, which has been implicated in behavioral abnormalities later in life.

Neurodegenerative Susceptibility During Maternal Nutritional Programing: Are Central and Peripheral Innate Immune Training Relevant?

Maternal overnutrition modulates body weight, development of metabolic failure and, potentially, neurodegenerative susceptibility in the offspring. Overnutrition sets a chronic pro-inflammatory profile that integrates peripheral and central immune activation nodes, damaging neuronal physiology and survival. Innate immune cells exposed to hypercaloric diets might experience trained immunity. Here, we address the role of maternal overnutrition as a trigger for central and peripheral immune training and its contribution to neurodegeneration and the molecular nodes implicated in the Nod-like receptor protein 3 (NLRP3) inflammasome pathway leading to immune training. We propose that maternal overnutrition leads to peripheral or central immune training that favor neurodegenerative susceptibility in the offspring.

Fatigue, Sleep, and Autoimmune and Related Disorders

Profound and debilitating fatigue is the most common complaint reported among individuals with autoimmune disease, such as systemic lupus erythematosus, multiple sclerosis, type 1 diabetes, celiac disease, chronic fatigue syndrome, and rheumatoid arthritis. Fatigue is multi-faceted and broadly defined, which makes understanding the cause of its manifestations especially difficult in conditions with diverse pathology including autoimmune diseases. In general, fatigue is defined by debilitating periods of exhaustion that interfere with normal activities. The severity and duration of fatigue episodes vary, but fatigue can cause difficulty for even simple tasks like climbing stairs or crossing the room. The exact mechanisms of fatigue are not well-understood, perhaps due to its broad definition. Nevertheless, physiological processes known to play a role in fatigue include oxygen/nutrient supply, metabolism, mood, motivation, and sleepiness-all which are affected by inflammation. Additionally, an important contributing element to fatigue is the central nervous system-a region impacted either directly or indirectly in numerous autoimmune and related disorders. This review describes how inflammation and the central nervous system contribute to fatigue and suggests potential mechanisms involved in fatigue that are likely exhibited in autoimmune and related diseases.

Involvement of the microglial NLRP3 inflammasome in the anti-inflammatory effect of the antidepressant clomipramine

BACKGROUND

Depression has recently been referred to as a neuroimmune disease because it is characterized by inflammatory changes in the cerebral cortex and hippocampus. Studies have demonstrated that microglial activation plays a crucial role in releasing inflammatory cytokines in the central nervous system (CNS), thereby contributing to depression, the mechanism underlying which remains unclear.

METHODS

First, we examined microglial activation and inflammatory changes in C57BL/6 male mice injected with lipopolysaccharide (LPS; 1 mg/kg), which leads to depressive behaviors in mice that were attenuated by the antidepressant clomipramine. Second, we utilized a BV2 cell line and primary microglial cultures to determine the inflammatory response in vitro, and the effects of clomipramine exerted on the inflammatory response using real-time polymerase chain reaction and ELISA. Third, we utilized NLRP3 (NOD-like receptor protein 3) knock-out (KO) mice to prove that NLRP3 is involved in the effects of clomipramine.

RESULTS

The results showed that LPS injection induced depressive-like behaviors in mice, as assessed using several behavioral tests including body weight, and forced swimming and tail suspension tests. The LPS-induced expression of interleukin-1beta (IL-1β), IL-6, and tumor necrosis factor alpha could be downregulated by clomipramine pre-treatment both in vivo and in vitro. The inhibitory effect of clomipramine on the LPS-induced increase in cytokines was found at both the protein and gene levels. Clomipramine significantly reduced the LPS-induced increase in NLRP3 gene expression in BV2 cells. Furthermore, we utilized NLRP3 KO mice to explore whether NLPP3 was involved in this process and found that clomipramine treatment inhibits the LPS-induced increased expression of IL-1β.

CONCLUSION

These results imply that clomipramine could attenuate depressive behaviors and neuroinflammation induced by LPS via partial regulation of NLRP3.

Targeting NLRP3 Inflammasome in the Treatment of CNS Diseases

Central nervous system (CNS) is one of the largest killers of people’s health all over the world. The overactivation of the immune and inflammatory responses is considered as an important factor, contributing to the pathogenesis and progression of CNS disorders. Among all kinds of immune and inflammatory reaction, the inflammasome, a complex of proteins, has been drawn increasingly attention to by researchers. The initiation and activation of the inflammasome is involved in the onset of various kinds of diseases. The NLRP3 inflammasome, the most studied member of the inflammasome, is closely associated with many kinds of CNS disorders. Here in this review, the roles of the NLRP3 inflammasome in the pathogenesis and progression of several well-known CNS diseases would be discussed, including cerebrovascular diseases, neurodegenerative diseases, multiple sclerosis, depression as well as other CNS disorders. In addition, several therapeutic strategies targeting on the NLRP3 inflammasome for the treatment of CNS disorders would be described in this review.

Simulated viral infection in early-life alters brain morphology, activity and behavior in zebra finches (Taeniopygia guttata)

Early-life immune challenges (ELIC) have long-term effects on adult behavior and brain development. ELIC studies on birds are still few, but they are epidemiologically crucial since birds are important hosts of many mosquito-borne viruses. In this study, we administered a viral infection mimicking agent, Polyinosinic: polycytidylic acid (Poly I:C), to nestling zebra finches on post-hatch day 14. When birds became sexually mature, their general activity (i.e., hopping, feeding behavior) and mosquito defense behaviors (i.e., hops, head movements, pecks, wing movements, foot movements, and scratches) were measured. Following behavioral trials, brains of male birds were collected for anatomical and histochemical analyses. Poly I:C challenge had sex-dependent effects on general activity and mosquito defense behaviors. When compared to control females, Poly I:C challenged females hopped and fed less often in their general activities, but hopped more often in the presence of mosquitoes. Poly I:C challenged males did not differ from control males in any behaviors. Brain analysis revealed that the nucleus taeniae of the amygdala (TnA) of Poly I:C challenged males were smaller in volume yet had more neurons expressing immediate-early gene proteins compared with controls, suggesting a more active TnA. These results suggest that immune challenges early in the life could have long-term effects on behaviors and brains of zebra finches, which may influence disease spread and fitness of individual birds.

Pistacia lentiscus oil attenuates memory dysfunction and decreases levels of biomarkers of oxidative stress induced by lipopolysaccharide in rats

Pistacia lentiscus L. is a well-known medicinal plant that has been used for its antioxidant, anti-inflammatory, neuroprotective, and hepatoprotective effects. However, the neuroprotective effect of Pistacia lentiscus oil (PLo) of has not been reported. The present study was designed to examine the neuroprotective and hepatoprotective effects of PLo aigainst lipopolysaccharide (LPS)-induced memory impairment and oxidative damage in rats. Twenty-four adult male Wistar rats were equally divided into three groups. The first group was kept as a control. In the second group, LPS was given at the single dose of 1 mg/kg intraperitoneally (i.p.). In the third group, PLo (3.3 mL/kg; per orally (p.o.)) was administered daily for 15 days, and challenged with LPS (1 mg/kg; i.p. injection two h before behavioral test). Thereafter, memory was assessed using spatial object recognition test. Cholinesterase activity and oxidative stress response were estimated in brain tissues and liver. PLo attenuated LPS-induced memory impairment in spatial object recognition test (p < 0.05). LPS treatment caused significant oxidative damage via induction of lipid peroxidation and reductions antioxidant defense system potency in the brain tissue and liver. Moreover, LPS increased brain activity of acetylcholinesterase and butyrylcholinesterase activity in the liver. The present results suggest that the beneficial effects of PLo on memory impairment of LPS-treated rats may be due to its protective effects against oxidative stress damage presumably via its antioxidant property.

Acute inhalation of combustion smoke triggers neuroinflammation and persistent anxiety-like behavior in the mouse

CONTEXT

Acute inhalation of combustion smoke triggers neurologic sequelae in survivors. Due to the challenges posed by heterogeneity of smoke exposures in humans, mechanistic links between acute smoke inhalation and neuropathologic sequelae have not been systematically investigated.

METHODS

Here, using mouse model of acute inhalation of combustion smoke, we studied longitudinal neurobehavioral manifestations of smoke exposures and molecular/cellular changes in the mouse brain.

RESULTS

Immunohistochemical analyses at eight months post-smoke, revealed hippocampal astrogliosis and microgliosis accompanied by reduced myelination. Elevated expression of proinflammatory cytokines was also detected. Longitudinal testing in different neurobehavioral paradigms in the course of post-smoke recovery, revealed lasting anxiety-like behavior. The examined paradigms included the open field exploration/anxiety testing at two, four and six months post-smoke, which detected decreases in total distance traveled and time spent in the central arena in the smoke-exposed compared to sham-control mice, suggestive of dampened exploratory activity and increased anxiety-like behavior. In agreement with reduced open field activity, cued fear conditioning test revealed increased freezing in response to conditioned auditory stimulus in mice after acute smoke inhalation. Similarly, elevated plus maze testing demonstrated lesser presence in open arms of the maze, consistent with anxiety-like behavior, for the post-smoke exposure mice.

CONCLUSIONS

Taken together, our data demonstrate for the first time persistent neurobehavioral manifestations of acute inhalation of combustion smoke and provide new insights into long-term progression of events initiated by disrupted brain oxygenation that might contribute to lasting adverse sequelae in survivors of smoke inhalation injuries.