Borja García-Bueno

Stress as a neuroinflammatory condition in brain: damaging and protective mechanisms.

Several neuropsychiatric diseases are related with stress (posttraumatic stress disorder, major depressive disorder, anxiety disorders, schizophrenia) and stress exposure modifies the onset and evolution of some neurological diseases (neurodegenerative diseases). It is accepted that brain inflammatory responses contribute to cell damage during these illnesses. Studies carried out with some stress protocols (physical, psychological or mixed) show a pro-inflammatory response in the brain and other systems mainly characterized by a complex release of several inflammatory mediators such as cytokines, prostanoids, free radicals and transcription factors. This review considers the current status of knowledge of stress-induced inflammation in the brain. Interestingly, anti-inflammatory pathways are also activated in brain in response to stress, constituting a possible endogenous mechanism of defence against excessive inflammation. The possibility of pharmacological modulation of these pathways to prevent the accumulation of pro-inflammatory mediators and subsequent brain damage in stress and in stress-related neuropsychological conditions is also reviewed. This dual response elicited by stress in brain, both pro- and anti-inflammatory deserves further attention in order to understand pathophysiological changes as well as possible new therapeutic approaches of stress-related neuropsychopathologies.

Stress-induced neuroinflammation: mechanisms and new pharmacological targets

Stress is triggered by numerous unexpected environmental, social or pathological stimuli occurring during the life of animals, including humans, which determine changes in all of their systems. Although acute stress is essential for survival, chronic, long-lasting stress can be detrimental. In this review, we present data supporting the hypothesis that stress-related events are characterized by modifications of oxidative/nitrosative pathways in the brain in response to the activation of inflammatory mediators. Recent findings indicate a key role for nitric oxide (NO) and an excess of pro-oxidants in various brain areas as responsible for both neuronal functional impairment and structural damage. Similarly, cyclooxygenase-2 (COX-2), another known source of oxidants, may account for stress-induced brain damage. Interestingly, some of the COX-2-derived mediators, such as the prostaglandin 15d-PGJ2 and its peroxisome proliferator-activated nuclear receptor PPARgamma, are activated in the brain in response to stress, constituting a possible endogenous anti-inflammatory mechanism of defense against excessive inflammation. The stress-induced activation of both biochemical pathways depends on the activation of the N-methyl-D-aspartate (NMDA) glutamate receptor and on the activation of the transcription factor nuclear factor kappa B (NFkappaB). In the case of inducible NO synthase (iNOS), release of the cytokine TNF-alpha also accounts for its expression. Different pharmacological strategies directed towards different sites in iNOS or COX-2 pathways have been shown to be neuroprotective in stress-induced brain damage: NMDA receptor blockers, inhibitors of TNF-alpha activation and release, inhibitors of NFkappaB, specific inhibitors of iNOS and COX-2 activities and PPARgamma agonists. This article reviews recent contributions to this area addressing possible new pharmacological targets for the treatment of stress-induced neuropsychiatric disorders.

CCL2/MCP-1 modulation of microglial activation and proliferation

BackgroundMonocyte chemoattractant protein (CCL2/MCP-1) is a chemokine that attracts cells involved in the immune/inflammatory response. As microglia are one of the main cell types sustaining inflammation in brain, we proposed here to analyze the direct effects of MCP-1 on cultured primary microglia.MethodsPrimary microglia and neuronal cultures were obtained from neonatal and embryonic Wistar rats, respectively. Microglia were incubated with different concentrations of recombinant MCP-1 and LPS. Cell proliferation was quantified by measuring incorporation of bromodeoxyuridine (BrdU). Nitrite accumulation was measured using the Griess assay. The expression and synthesis of different proteins was measured by RT-PCR and ELISA. Cell death was quantified by measuring release of LDH into the culture medium.ResultsMCP-1 treatment (50 ng/ml, 24 h) did not induce morphological changes in microglial cultures. Protein and mRNA levels of different cytokines were measured, showing that MCP-1 was not able to induce proinflammatory cytokines (IL-1β, IL6, MIP-1α), either by itself or in combination with LPS. A similar lack of effect was observed when measuring inducible nitric oxide synthase (NOS2) expression or accumulation of nitrites in the culture media as a different indicator of microglial activation. MCP-1 was also unable to alter the expression of different trophic factors that were reduced by LPS treatment. In order to explore the possible release of other products by microglia and their potential neurotoxicity, neurons were co-cultured with microglia: no death of neurons could be detected when treated with MCP-1. However, the presence of MCP-1 induced proliferation of microglia, an effect opposite to that observed with LPS.ConclusionThese data indicate that, while causing migration and proliferation of microglia, MCP-1 does not appear to directly activate an inflammatory response in this cell type, and therefore, other factors may be necessary to cause the changes that result in the neuronal damage commonly observed in situations where MCP-1 levels are elevated.

Stress-induced neuroinflammation: role of the Toll-like receptor-4 pathway.

BACKGROUND Stressful challenges are associated with variations in immune parameters, including increased innate immunity/inflammation. Among possible mechanisms through which brain monitors peripheral immune responses, toll-like receptors (TLRs) recently emerged as the first line of defense against invading microorganisms. Their expression is modulated in response to pathogens and other environmental stresses. METHODS Taking into account this background, the present study aimed to elucidate whether the toll-like receptor-4 (TLR-4) signaling pathway is activated after repeated restraint/acoustic stress exposure in mice prefrontal cortex (PFC), the potential regulatory mechanism implicated (i.e., bacterial translocation), and its role in conditions of stress-induced neuroinflammation, using a genetic strategy: C3H/HeJ mice with a defective response to lipopolysaccharide stimulation of TLR-4. RESULTS Stress exposure upregulates TLR-4 pathway in mice PFC. Stress-induced inflammatory nuclear factor κB activation, upregulation of the proinflammatory enzymes nitric oxide synthase and cyclooxygenase type 2, and cellular oxidative/nitrosative damage are reduced when the TLR-4 pathway is defective. Conversely, TLR-4 deficient mice presented higher levels of the anti-inflammatory nuclear factor peroxisome proliferator activated receptor-gamma after stress exposure than control mice. The series of experiments using antibiotic intestinal decontamination also suggest a role for bacterial translocation on TLR-4 activation in PFC after stress exposure. CONCLUSIONS Taken together, all the data presented here suggest a bifunctional role of TLR-4 signaling pathway after stress exposure by triggering neuroinflammation at PFC level and regulating gut barrier function/permeability. Furthermore, our data suggest a possible protective role of antibiotic decontamination in stress-related pathologies presenting increased intestinal permeability (leaky gut) such as depression, showing a potential therapeutic target that deserves further consideration.

Stress-Induced Oxidative Changes in Brain

Numerous systems and organs are affected by stress. In this review we will focus on the effects in brain. Some of the most impressive effects of the stress in brain are the atrophy of hippocampal dendrites or even the reduction of the hippocampal size observed in brains from subjects exposed to severe or chronic stress. Obviously, before reaching this point of damage there are many other processes taking place in the stressed CNS. The release of glucocorticoids is one of the first features of the stress response. Glucocorticoids can result in neurotoxicity through different mechanisms, including modifications in the energy metabolism or via an increase in excitatory amino acids such as glutamate in the extracellular space. Glutamate can induce neuronal excitotoxicity. This sequence of events leads to the activation of TNFalpha convertase (TACE) and TNFalpha release in brain of rats subjected to restraint stress. One of the multiple effects exerted by this cytokine is to initiate the translocation of the transcription factor NFkappaB to neuronal nuclei. NFkappaB activation results in the induction of iNOS and COX2, two enzymes responsible for a great portion of the neurological damage produced in models of stress.

Peroxisome proliferator-activated receptor gamma activation decreases neuroinflammation in brain after stress in rats

BACKGROUND A growing body of evidence has demonstrated that peroxisome proliferator-activated receptor gamma (PPARgamma) play a role in brain inflammatory conditions because various PPARgamma ligands inhibit proinflammatory mediators, such as cytokines (tumor necrosis factor alpha [TNFalpha]) and inducible nitric oxide synthase (NOS-2). As has been previously shown, immobilization stress and stress-related neuropsychologic conditions are followed by accumulation of oxidative/nitrosative mediators in brain after the release of cytokines, nuclear factor kappaB activation, and NOS-2 and cyclooxygenase 2 (COX-2) expression in the brain. METHODS To assess whether PPARgamma activation can modify the accumulation of oxidative/nitrosative species seen in brain after stress, and to study the mechanisms by which this effect is achieved, young-adult male Wistar rats (control and immobilized during 6 hours) were injected (IP) with the high-affinity ligand rosiglitazone (RS) at the onset of stress. RESULTS Stress increased PPARgamma expression in cortical neurons and glia as assessed by Western blot and immunohistochemistry. In stressed animals, RS (1-3 mg/kg) decreased stress-induced increases in NOS-2 activity. On the other hand, the PPARgamma ligand decreased stress-induced malondialdehyde (an indicator of lipid peroxidation) accumulation in cortex and prevented oxidation of the main antioxidant glutathione. The mechanisms involved in the antioxidative properties of RS in stress involve nuclear factor KB blockade (by preventing stress-induced IkappaBalpha decrease) and inhibition of TNFalpha release in stressed animals. At the doses tested, RS did not decrease COX-2 expression and prostaglandin E2 release during stress. Finally, RS also decreased chronic (repeated immobilization for 21 days) stress-induced accumulation of oxidative/nitrosative mediators. CONCLUSIONS Taken together, these findings suggest a role for this antiinflammatory pathway in the brain response to stress and the possibility of pharmacologic modulation for preventing accumulation of oxidative/nitrosative species and subsequent brain damage in stress-related neuropsychologic conditions.

Origin and consequences of brain Toll-like receptor 4 pathway stimulation in an experimental model of depression

BackgroundThere is a pressing need to identify novel pathophysiological pathways relevant to depression that can help to reveal targets for the development of new medications. Toll-like receptor 4 (TLR-4) has a regulatory role in the brains response to stress. Psychological stress may compromise the intestinal barrier, and increased gastrointestinal permeability with translocation of lipopolysaccharide (LPS) from Gram-negative bacteria may play a role in the pathophysiology of major depression.MethodsAdult male Sprague-Dawley rats were subjected to chronic mild stress (CMS) or CMS+intestinal antibiotic decontamination (CMS+ATB) protocols. Levels of components of the TLR-4 signaling pathway, of LPS and of different inflammatory, oxidative/nitrosative and anti-inflammatory mediators were measured by RT-PCR, western blot and/or ELISA in brain prefrontal cortex. Behavioral despair was studied using Porsolts test.ResultsCMS increased levels of TLR-4 and its co-receptor MD-2 in brain as well as LPS and LPS-binding protein in plasma. In addition, CMS also increased interleukin (IL)-1β, COX-2, PGE2 and lipid peroxidation levels and reduced levels of the anti-inflammatory prostaglandin 15d-PGJ2 in brain tissue. Intestinal decontamination reduced brain levels of the pro-inflammatory parameters and increased 15d-PGJ2, however this did not affect depressive-like behavior induced by CMS.ConclusionsOur results suggest that LPS from bacterial translocation is responsible, at least in part, for the TLR-4 activation found in brain after CMS, which leads to release of inflammatory mediators in the CNS. The use of Gram-negative antibiotics offers a potential therapeutic approach for the adjuvant treatment of depression.

Pro-/Anti-inflammatory Dysregulation in Patients With First Episode of Psychosis: Toward an Integrative Inflammatory Hypothesis of Schizophrenia

BACKGROUND Schizophrenia is a chronic syndrome of unknown etiology, predominantly defined by signs of psychosis. The onset of the disorder occurs typically in late adolescence or early adulthood. Efforts to study pathophysiological mechanisms in early stages of the disease are crucial in order to prompt intervention. METHODS Case-control study of first-episode psychotic (FEP) patients and matched controls. We recruited 117 patients during the first year after their FEP according to the DSM-IV criteria and recruited 106 gender-, race-, and age-matched controls between September 2010 and June 2011. RESULTS Biochemical studies carried out in peripheral mononuclear blood cells (PMBC) and plasma evidence a significant increase in intracellular components of a main proinflammatory pathway, along with a significant decrease in the anti-inflammatory ones. Multivariate logistic regression analyses identified the expression of inducible isoforms of nitric oxide synthase and cyclooxygenase in PMBC and homocysteine plasma levels as the most reliable potential risk factors and the inhibitor of the inflammatory transcription factor NFκB, IκBα, and the anti-inflammatory prostaglandin 15d-PGJ2 as potential protection factors. DISCUSSION Taken as a whole, the results of this study indicate robust phenotypical differences at the cellular machinery level in PMBC of patients with FEP. Although more scientific evidence is needed, the determination of multiple components of pro- and anti-inflammatory cellular pathways including the activity of nuclear receptors has interesting potential as biological markers and potential risk/protective factors for FEP. Due to its soluble nature, a notable finding in this study is that the anti-inflammatory mediator 15d-PGJ2 might be used as plasmatic biomarker for first episodes of psychosis.

Effects of peroxisome proliferator-activated receptor gamma agonists on brain glucose and glutamate transporters after stress in rats.

Repeated stress causes an energy-compromised status in the brain, with a decrease in glucose utilization by the brain cells, which might account for excitotoxicity processes seen in this condition. In fact, brain glucose metabolism mechanisms are impaired in some neurodegenerative disorders, including stress-related neuropsychopathologies. More recently, it has been demonstrated that some synthetic peroxisome proliferator-activated receptor gamma (PPARγ) agonists increase glucose utilization in rat cortical slices and astrocytes, as well as inhibit brain oxidative damage after repeated stress, which add support for considering these drugs as potential neuroprotective agents. To assess if stress causes glucose utilization impairment in the brain and to study the mechanisms by which this effect is achieved, young-adult male Wistar rats (control and immobilized for 6 h during 7 or 14 consecutive days, S7, S14) were i.p. injected with the natural ligand 15-deoxy-Δ-12,14-prostaglandin J2 (PGJ2, 120 μg/kg) or the high-affinity ligand rosiglitazone (RG, 3 mg/kg) at the onset of stress. Repeated immobilization during 1 or 2 weeks produces a decrease in brain cortical synaptosomal glucose uptake, and this effect was prevented by treatment with both natural and synthetic PPARγ ligands by restoring protein expression of the neuronal glucose transporter, GLUT-3 in membrane fractions. On the other hand, treatment with PPARγ ligands prevents stress-induced ATP loss in rat brain. Finally, repeated immobilization stress also produces a decrease in brain cortical synaptosomal glutamate uptake, and this effect was prevented by treatment with PPARγ ligands by restoring synaptosomal protein expression of the glial glutamate transporter, EAAT2. In summary, our results demonstrate that 15d-PGJ2 and the thiazolidinedione rosiglitazone increase neuronal glucose metabolism, restore brain ATP levels and prevent the impairment in glutamate uptake mechanisms induced by exposure to stress, suggesting that this class of drugs may be therapeutically useful in conditions in which brain glucose levels or availability are limited after exposure to stress.

Inflammation in schizophrenia: A question of balance.

In the past decade, there has been renewed interest in immune/inflammatory changes and their associated oxidative/nitrosative consequences as key pathophysiological mechanisms in schizophrenia and related disorders. Both brain cell components (microglia, astrocytes, and neurons) and peripheral immune cells have been implicated in inflammation and the resulting oxidative/nitrosative stress (O&NS) in schizophrenia. Furthermore, down-regulation of endogenous antioxidant and anti-inflammatory mechanisms has been identified in biological samples from patients, although the degree and progression of the inflammatory process and the nature of its self-regulatory mechanisms vary from early onset to full-blown disease. This review focuses on the interactions between inflammation and O&NS, their damaging consequences for brain cells in schizophrenia, the possible origins of inflammation and increased O&NS in the disorder, and current pharmacological strategies to deal with these processes (mainly treatments with anti-inflammatory or antioxidant drugs as add-ons to antipsychotics).