AMPA-kainate subtypes of glutamate receptor in rat cerebral microglia

Voltage-gated proton channel HV1 in microglia

Microglia are brain resident immune cells and their functions are implicated in both the normal and diseased brain. Microglia express a plethora of ion channels, including K(+) channels, Na(+) channels, TRP channels, Cl(-) channels, and proton channels. These ion channels play critical roles in microglial proliferation, migration, and production/release of cytokines, chemokines, and neurotoxic or neurotrophic substances. Among microglial ion channels, the voltage-gated proton channel HV1 is a recently cloned ion channel that rapidly removes protons from depolarized cytoplasm and is highly expressed in the immune system. However, the function of microglial HV1 in the brain is poorly understood. Recent studies showed that HV1 is selectively expressed in microglia but not neurons in the brain. At the cellular level, microglial HV1 regulates intracellular pH and aids in NADPH oxidase-dependent generation of reactive oxygen species. In a mouse model of middle cerebral artery occlusion, microglial HV1 contributes to neuronal cell death and ischemic brain damage. This review discusses the discovery, properties, regulation, and pathophysiology of microglial HV1 proton channel in the brain.

Purinergic and glutamatergic receptors on astroglia

Astroglial cells express many neurotransmitter receptors; the receptors to glutamate and ATP being the most abundant. Here, we provide a concise overview on the expression and main properties of astroglial glutamate receptors (ionotropic receptors represented by AMPA and NMDA subtypes) and metabotropic (mainly mGluR5 and mGluR3 subtypes) and purinoceptors (adenosine receptors of A1, A2A, A2B, and A3 types, ionotropic P2X1/5 and P2X7 subtypes, and metabotropic P2Y purinoceptors). We also discuss the role of these receptors in glial physiology and pathophysiology.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

Glial modulators as potential treatments of psychostimulant abuse

Glia (including astrocytes, microglia, and oligodendrocytes), which constitute the majority of cells in the brain, have many of the same receptors as neurons, secrete neurotransmitters and neurotrophic and neuroinflammatory factors, control clearance of neurotransmitters from synaptic clefts, and are intimately involved in synaptic plasticity. Despite their prevalence and spectrum of functions, appreciation of their potential general importance has been elusive since their identification in the mid-1800s, and only relatively recently have they been gaining their due respect. This development of appreciation has been nurtured by the growing awareness that drugs of abuse, including the psychostimulants, affect glial activity, and glial activity, in turn, has been found to modulate the effects of the psychostimulants. This developing awareness has begun to illuminate novel pharmacotherapeutic targets for treating psychostimulant abuse, for which targeting more conventional neuronal targets has not yet resulted in a single, approved medication. In this chapter, we discuss the molecular pharmacology, physiology, and functional relationships that the glia have especially in the light in which they present themselves as targets for pharmacotherapeutics intended to treat psychostimulant abuse disorders. We then review a cross section of preclinical studies that have manipulated glial processes whose behavioral effects have been supportive of considering the glia as drug targets for psychostimulant-abuse medications. We then close with comments regarding the current clinical evaluation of relevant compounds for treating psychostimulant abuse, as well as the likelihood of future prospects.

Microglia during development and aging

Microglia are critical nervous system-specific cells influencing brain development, maintenance of the neural environment, response to injury, and repair. They contribute to neuronal proliferation and differentiation, pruning of dying neurons, synaptic remodeling and clearance of debris and aberrant proteins. Colonization of the brain occurs during gestation with an expansion following birth with localization stimulated by programmed neuronal death, synaptic pruning, and axonal degeneration. Changes in microglia phenotype relate to cellular processes including specific neurotransmitter, pattern recognition, or immune-related receptor activation. Upon activation, microglia cells have the capacity to release a number of substances, e.g., cytokines, chemokines, nitric oxide, and reactive oxygen species, which could be detrimental or beneficial to the surrounding cells. With aging, microglia shift their morphology and may display diminished capacity for normal functions related to migration, clearance, and the ability to shift from a pro-inflammatory to an anti-inflammatory state to regulate injury and repair. This shift in microglia potentially contributes to increased susceptibility and neurodegeneration as a function of age. In the current review, information is provided on the colonization of the brain by microglia, the expression of various pattern recognition receptors to regulate migration and phagocytosis, and the shift in related functions that occur in normal aging.

Expression, subunit composition, and function of AMPA-type glutamate receptors are changed in activated microglia; possible contribution of GluA2 (GluR-B)-deficiency under pathological conditions

Microglia express AMPA (α-amino-hydroxy-5-methyl-isoxazole-4-propionate)-type of glutamate (Glu) receptors (AMPAR), which are highly Ca(2+) impermeable due to the expression of GluA2. However, the functional importance of AMPAR in microglia remains to be investigated, especially under pathological conditions. As low expression of GluA2 was reported in some neurodegenerative diseases, GluA2(-/-) mice were used to show the functional change of microglial AMPARs in response to Glu or kainate (KA). Here we found that Glu-induced currents in the presence of 100 μM cyclothiazide, an inhibitor of AMPAR desensitization, showed time-dependent decrease after activation of microglia with lipopolysaccharide (LPS) in GluA2(+/+) microglia, but not in GluA2(-/-) microglia. Upon activation of microglia, expression level of GluA2 subunits significantly increased, while expression of GluA1, A3 and A4 subunits on membrane surface significantly decreased. These results suggest that nearly homomeric GluA2 subunits were the main reason for low conductance of AMPAR in activated microglia. Increased expression of GluA2 in microglia was also detected partially in brain slices from LPS-injected mice. Cultured microglia from GluA2(-/-) mice showed higher Ca(2+) -permeability, consequently inducing significant increase in the release of proinflammatory cytokine, such as TNF-α. The conditioning medium from KA-treated GluA2(-/-) microglia had more neurotoxic effect on wild type cultured neurons than that from KA-treated GluA2(+/+) microglia. These results suggest that membrane translocation of GluA2-containing AMPARs in activated microglia has functional importance and thus, dysfunction or decreased expression of GluA2 may accelerate Glu neurotoxicity via excess release of proinflammatory cytokines from microglia.

Microglial calcium signaling in the adult, aged and diseased brain

Microglial cells are the resident immune cells of the CNS. They mediate innate immune response of the brain to injury, inflammation and neurodegenerative diseases. Apart from their role in disease they are critically involved in the development and plasticity-driven reorganization of neuronal networks and the homeostatic maintenance of brain tissue. Accumulating in vitro evidence suggests that executive functions of microglia are coupled to the intracellular Ca(2+) signaling of these cells. So far, however, very little is known about microglial Ca(2+) signaling in situ or in vivo, both in the healthy and in the diseased brain. Here, we summarize the recent in vivo/in situ findings and compare the properties of surveillant microglia in these preparations with those of microglia in vitro. The data suggest that surveillant microglia rarely show spontaneous Ca(2+) transients, express fewer functional receptors directly coupled to changes in the intracellular free Ca(2+) concentration on their surface, but vividly respond with Ca(2+) transients to cell or tissue damage in their microenvironment. Interestingly, some of these properties microglia share with monocytes engrafting in the brain under pathological conditions.

Intrathecal lamotrigine attenuates antinociceptive morphine tolerance and suppresses spinal glial cell activation in morphine-tolerant rats

Glial cells play a critical role in morphine tolerance, resulting from repeated administration of morphine. Both the development and the expression of tolerance are suppressed by the analgesic lamotrigine. This study investigated the relationship between the ability of lamotrigine to maintain the antinociceptive effect of morphine during tolerance development and glial cell activation in the spinal cord. In a rat model, morphine (15 µg) was intrathecally injected once daily for 7 days to induce morphine tolerance. Lamotrigine (200 µg) was co-administered with morphine either for 7 days or the first or last 3 days of this 7 day period. Thermal nociception was measured. OX-42 and GFAP immunoreactivity, indicating spinal microglial and astrocytic activation were evaluated on day 8. Tolerance developed after 7 days of intrathecal morphine administration; however, this was completely blocked and reversed by co-administration of lamotrigine. When lamotrigine was coinjected with morphine on days 5-7, the morphine effect was partially restored. Glial cell activation increased with the development of morphine tolerance but was clearly inhibited in the presence of lamotrigine. These results suggest that, in association with the suppression of spinal glial cell activity, intrathecally coadministered lamotrigine attenuates antinociceptive tolerance to morphine.

Vitreous mediators in retinal hypoxic diseases

The causes of retinal hypoxia are many and varied. Under hypoxic conditions, a variety of soluble factors are secreted into the vitreous cavity including growth factors, cytokines, and chemokines. Cytokines, which usually serve as signals between neighboring cells, are involved in essentially every important biological process, including cell proliferation, inflammation, immunity, migration, fibrosis, tissue repair, and angiogenesis. Cytokines and chemokines are multifunctional mediators that can direct the recruitment of leukocytes to sites of inflammation, promote the process, enhance immune responses, and promote stem cell survival, development, and homeostasis. The modern particle-based flow cytometric analysis is more direct, stable and sensitive than the colorimetric readout of the conventional ELISA but, similar to ELISA, is influenced by vitreous hemorrhage, disruption of the blood-retina barrier, and high serum levels of a specific protein. Finding patterns in the expression of inflammatory cytokines specific to a particular disease can substantially contribute to the understanding of its basic mechanism and to the development of a targeted therapy.

A brief overview of multitalented microglia

Microglia are the resident immune cells of the central nervous system, and accumulating data demonstrates a vast array of tasks in the healthy and injured brain. Microglia participate in both innate and adaptive immune responses. These cells contribute to the brain homeostasis, including the regulation of cell death, synapse elimination, neurogenesis, and neuronal surveillance. However, microglia can also become activated and/or deregulated in the context of neurodegenerative diseases, brain injuries, and cancer and thereby contribute to disease severity. As a consequence of these developments, microglia have attracted substantial attention on themselves.

IL-6 receptor is a possible target against growth of metastasized lung tumor cells in the brain

In the animal model of brain metastasis using human lung squamous cell carcinoma-derived cells (HARA-B) inoculated into the left ventricle of the heart of nude mice, metastasized tumor cells and brain resident cells interact with each other. Among them, tumor cells and astrocytes have been reported to stimulate each other, releasing soluble factors from both sides, subsequently promoting tumor growth significantly. Among the receptors for soluble factors released from astrocytes, only IL-6 receptor (IL-6R) on tumor cells was up-regulated during the activation with astrocytes. Application of monoclonal antibody against human IL-6R (tocilizumab) to the activated HARA-B cells, the growth of HARA-B cells stimulated by the conditioned medium of HARA-B/astrocytes was significantly inhibited. Injecting tocilizumab to animal models of brain metastasis starting at three weeks of inoculation of HARA-B cells, two times a week for three weeks, significantly inhibited the size of the metastasized tumor foci. The up-regulated expression of IL-6R on metastasized lung tumor cells was also observed in the tissue from postmortem patients. These results suggest that IL-6R on metastasized lung tumor cells would be a therapeutic target to inhibit the growth of the metastasized lung tumor cells in the brain.

The role of non-synaptic extracellular glutamate

Although there are some mechanisms which allow the direct crossing of substances between the cytoplasm of adjacent cells (gap junctions), most substances use the extracellular space to diffuse between brain cells. The present work reviews the behavior and functions of extracellular glutamate (GLU). There are two extracellular pools of glutamate (GLU) in the brain, a synaptic pool whose functions in the excitatory neurotransmission has been widely studied and an extrasynaptic GLU pool although less known nonetheless is gaining attention among a growing number of researchers. Evidence accumulated over the last years shows a number of mechanisms capable of releasing glial GLU to the extracellular medium, thus modulating neurons, microglia and oligodendrocytes, and regulating the immune response, cerebral blood flow, neuronal synchronization and other brain functions. This new scenario is expanding present knowledge regarding the role of GLU in the brain under different physiological and pathological conditions. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Differential effects of domoic acid and E. coli lipopolysaccharide on tumor necrosis factor-alpha, transforming growth factor-beta1 and matrix metalloproteinase-9 release by rat neonatal microglia: evaluation of the direct activation hypothesis

The excitatory amino acid domoic acid is the causative agent of amnesic shellfish poisoning in humans. The in vitro effects of domoic acid on rat neonatal brain microglia were compared with E. coli lipopolysaccharide (LPS), a known activator of microglia mediator release over a 4 to 24 hour observation period. LPS [3 ng/mL] but not domoic acid [1mM] stimulated a statistically significant increase in TNF-α mRNA and protein generation. Furthermore, both LPS and domoic acid did not significantly affect TGF-β1 gene expression and protein release. Finally, an in vitro exposure of microglia to LPS resulted in statistically significant MMP-9 expression and release, thus extending and confirming our previous observations. However, in contrast, no statistically significant increase in MMP-9 expression and release was observed after domoic acid treatment. Taken together our observations do not support the hypothesis that a short term (4 to 24 hours) in vitro exposure to domoic acid, at a concentration toxic to neuronal cells, activates rat neonatal microglia and the concomitant release of the pro-inflammatory mediators tumor necrosis factor-α (TNF-α) and matrix metalloproteinases-9 (MMP-9), as well as the anti-inflammatory cytokine transforming growth factor β1 (TGF-β1).

Metabotropic glutamate receptors inhibit microglial glutamate release

Pro-inflammatory stimuli evoke an export of glutamate from microglia that is sufficient to contribute to excitotoxicity in neighbouring neurons. Since microglia also express various glutamate receptors themselves, we were interested in the potential feedback of glutamate on this system. Several agonists of mGluRs (metabotropic glutamate receptors) were applied to primary rat microglia, and the export of glutamate into their culture medium was evoked by LPS (lipopolysaccharide). Agonists of group-II and -III mGluR ACPD [(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid] and L-AP4 [L-(+)-2-amino-4-phosphonobutyric acid] were both capable of completely blocking the glutamate export without interfering with the production of NO (nitric oxide); the group-I agonist tADA (trans-azetidine-2,4-dicarboxylic acid) was ineffective. Consistent with the possibility of feedback, inhibition of mGluR by MSPG [(R,S)-α-2-methyl-4sulfonophenylglycine] potentiated glutamate export. As the group-II and -III mGluR are coupled to Gα_i-containing G-proteins and the inhibition of adenylate cyclase, we explored the role of cAMP in this effect. Inhibition of cAMP-dependent protein kinase [also known as protein kinase A (PKA)] by H89 mimicked the effect of ACPD, and the mGluR agonist had its actions reversed by artificially sustaining cAMP through the PDE (phosphodiesterase) inhibitor IBMX (isobutylmethylxanthine) or the cAMP mimetic dbcAMP (dibutyryl cAMP). These data indicate that mGluR activation attenuates a potentially neurotoxic export of glutamate from activated microglia and implicate cAMP as a contributor to this aspect of microglial action.

Psychostimulant abuse and neuroinflammation: emerging evidence of their interconnection

During the past two decades, there has been a tremendous expansion of knowledge regarding the neurobiological effects of substance abuse and how these effects impact behavior. At the same time, there has been a profound change in our understanding of the way in which the central nervous system responds to noxious stimuli. Most often referred to as the innate immune response (IIR), this defense mechanism is activated by a number of agents (toxic, microbial, ischemic) and has been implicated in the progression of a number of neurodegenerative diseases. We review evidence that psychostimulants of abuse (cocaine, methamphetamine, ecstasy) are associated with activation of the IIR. We first present background on what is currently known about the IIR including some of the cellular elements involved (microglia, astrocytes, vascular endothelial cells), key receptor pathways, and primary inflammatory cytokines (IL-1β, IL-6, TNF-α). We then present a variety of protein and gene expression data taken from animal studies that show increased expression of various components of the IIR following acute or repeated psychostimulant administration. Collectively the data indicate an association of psychostimulant use with IIR activation in the brain even at exposures not traditionally associated with neurotoxicity. Thus, the gradually escalating deleterious effects of psychostimulant use could in part involve neuroinflammatory mechanisms. Finally, we offer one hypothesis of a possible mechanism by which psychostimulants result in IIR activation and discuss the potential therapeutic implications of these findings for treatment of the recovering addict.

Functions of microglia in the central nervous system--beyond the immune response

Microglia cells are the immune cells of the central nervous system and consequently play important roles in brain infections and inflammation. Recent in vivo imaging studies have revealed that in the resting healthy brain, microglia are highly dynamic, moving constantly to actively survey the brain parenchyma. These active microglia can rapidly respond to pathological insults, becoming activated to induce a range of effects that may contribute to both pathogenesis, or to confer neuronal protection. However, interactions between microglia and neurons are being recognized as important in shaping neural circuit activity under more normal, physiological conditions. During development and neurogenesis, microglia interactions with neurons help to shape the final patterns of neural circuits important for behavior and with implications for diseases. In the mature brain, microglia can respond to changes in sensory activity and can influence neuronal activity acutely and over the long term. Microglia seem to be particularly involved in monitoring the integrity of synaptic function. In this review, we discuss some of these new insights into the involvement of microglia in neural circuits.

The biphasic role of microglia in Alzheimer's disease

Neuroinflammation is involved in the pathogenesis of Alzheimer's disease (AD). Microglia, macrophage-like resident immune cells in the brain, play critical roles in the inflammatory aspects of AD. Microglia may be activated by oligomeric and fibrillar species of amyloid β (Aβ) that are constituents of senile plaques and by molecules derived from degenerated neurons, such as purines and chemokines, which enhance their migration and phagocytosis. The main neurotoxic molecules produced by activated microglia may be reactive oxygen species, glutamate, and inflammatory cytokines such as tumor-necrosis-factor-α and interleukin- (IL-) 1β These molecules differentially induce neurotoxicity. Aβ itself directly damages neurons. In terms of neuroprotective properties, microglia treated with fractalkine or IL-34 attenuate Aβ neurotoxicity by Aβ clearance and the production of antioxidants. Therefore, regulation of the microglial role in neuroprotection may be a useful therapeutic strategy for AD.

Neuroprotective roles of the P2Y(2) receptor

Purinergic signaling plays a unique role in the brain by integrating neuronal and glial cellular circuits. The metabotropic P1 adenosine receptors and P2Y nucleotide receptors and ionotropic P2X receptors control numerous physiological functions of neuronal and glial cells and have been implicated in a wide variety of neuropathologies. Emerging research suggests that purinergic receptor interactions between cells of the central nervous system (CNS) have relevance in the prevention and attenuation of neurodegenerative diseases resulting from chronic inflammation. CNS responses to chronic inflammation are largely dependent on interactions between different cell types (i.e., neurons and glia) and activation of signaling molecules including P2X and P2Y receptors. Whereas numerous P2 receptors contribute to functions of the CNS, the P2Y(2) receptor is believed to play an important role in neuroprotection under inflammatory conditions. While acute inflammation is necessary for tissue repair due to injury, chronic inflammation contributes to neurodegeneration in Alzheimer's disease and occurs when glial cells undergo prolonged activation resulting in extended release of proinflammatory cytokines and nucleotides. This review describes cell-specific and tissue-integrated functions of P2 receptors in the CNS with an emphasis on P2Y(2) receptor signaling pathways in neurons, glia, and endothelium and their role in neuroprotection.

Glutamate potentiates lipopolysaccharide-stimulated interleukin-10 release from neonatal rat spinal cord astrocytes

Interleukin-10 (IL-10) has important anti-inflammatory effects and can be protective in inflammatory conditions, such as chronic pain and infection. Exploring factors that modulate IL-10 levels may provide insight into pathomechanisms of inflammatory conditions and may provide a method of neuroprotection during these conditions. Lipopolysaccharide (LPS) stimulation of astrocytes is a source of IL-10; hence, it is of interest to investigate factors that modulate this process. Glutamate is present in increased concentrations in inflammatory conditions, and astrocytes also express glutamate receptors. The present study, therefore, investigated whether glutamate modulates LPS stimulation of IL-10 release from neonatal spinal cord astrocytes. Enzyme-linked immunosorbent assays (ELISAs) were used to quantify IL-10 release from cultured neonatal spinal cord astrocytes, and reverse transcriptase-polymerase chain reaction (RT-PCR) was used to measure IL-10 mRNA expression. Glutamate (1 mM) significantly increased LPS (1 μg/ml)-stimulated IL-10 release from astrocytes by 166% and significantly upregulated IL-10 mRNA levels. Glutamate synergistically signaled through metabotropic glutamate receptor subgroups and the phospholipase C signaling pathway. Spinal cord astrocytes may, therefore, play a larger anti-inflammatory role than first thought in situations where glutamate and a high concentration of Toll-like receptor 4 (TLR4) agonists are present.

Combinations of ketamine and atropine are neuroprotective and reduce neuroinflammation after a toxic status epilepticus in mice

Epileptic seizures and status epilepticus (SE) induced by the poisoning with organophosphorus nerve agents (OP), like soman, are accompanied by neuroinflammation whose role in seizure-related brain damage (SRBD) is not clear. Antagonists of the NMDA glutamate ionotropic receptors are currently among the few compounds able to arrest seizures and provide neuroprotection even during refractory status epilepticus (RSE). Racemic ketamine (KET), in combination with atropine sulfate (AS), was previously shown to counteract seizures and SRBD in soman-poisoned guinea-pigs. In a mouse model of severe soman-induced SE, we assessed the potentials of KET/AS combinations as a treatment for SE/RSE-induced SRBD and neuroinflammation. When starting 30min after soman challenge, a protocol involving six injections of a sub-anesthetic dose of KET (25mg/kg) was evaluated on body weight loss, brain damage, and neuroinflammation whereas during RSE, anesthetic protocols were considered (KET 100mg/kg). After confirming that during RSE, KET injection was to be repeated despite some iatrogenic deaths, we used these proof-of-concept protocols to study the changes in mRNA and related protein contents of some inflammatory cytokines, chemokines and adhesion molecules in cortex and hippocampus 48h post-challenge. In both cases, the KET/AS combinations showed important neuroprotective effects, suppressed neutrophil granulocyte infiltration and partially suppressed glial activation. KET/AS could also reduce the increase in mRNA and related pro-inflammatory proteins provoked by the poisoning. In conclusion, the present study confirms that KET/AS treatment has a strong potential for SE/RSE management following OP poisoning. The mechanisms involved in the reduction of central neuroinflammation remain to be studied.

Regulation of microglia by ionotropic glutamatergic and GABAergic neurotransmission

Recent studies have indicated that constitutive functions of microglia in the healthy adult central nervous system (CNS) involve immune surveillance, synapse maintenance and trophic support. These functions have been related to the ramified structure of 'resting' microglia and the prominent motility in their processes that provide extensive coverage of the entire extracellular milleu. In this review, we examine how external signals, and in particular, ionotropic neurotransmission, regulate features of microglial morphology and process motility. Current findings indicate that microglial physiology in the healthy CNS is constitutively and reciprocally regulated by endogenous ionotropic glutamatergic and GABAergic neurotransmission. These influences do not act directly on microglial cells but indirectly via the activity-dependent release of ATP, likely through a mechanism involving pannexin channels. Microglia in the 'resting' state are not only dynamically active, but also constantly engaged in ongoing communication with neuronal and macroglial components of the CNS in a functionally relevant way.

Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity

Microglia are resident cells of the brain involved in regulatory processes critical for development, maintenance of the neural environment, injury and repair. They belong to the monocytic-macrophage lineage and serve as brain immune cells to orchestrate innate immune responses; however, they are distinct from other tissue macrophages due to their relatively quiescent phenotype and tight regulation by the CNS microenvironment. Microglia actively survey the surrounding parenchyma and respond rapidly to changes such that any disruption to neural architecture or function can contribute to the loss in regulation of the microglia phenotype. In many models of neurodegeneration and neurotoxicity, early events of synaptic degeneration and neuronal loss are accompanied by an inflammatory response including activation of microglia, perivascular monocytes, and recruitment of leukocytes. In culture, microglia have been shown to be capable of releasing several potentially cytotoxic substances, such as reactive oxygen intermediates, nitric oxide, proteases, arachidonic acid derivatives, excitatory amino acids, and cytokines; however, they also produce various neurotrophic factors and quench damage from free radicals and excitotoxins. As the primary source for pro-inflammatory cytokines, microglia are implicated as pivotal mediators of neuroinflammation and can induce or modulate a broad spectrum of cellular responses. Neuroinflammation should be considered as a balanced network of processes whereby subtle modifications can shift the cells toward disparate outcomes. For any evaluation of neuroinflammation and microglial responses, within the framework of neurotoxicity or degeneration, one key question in determining the consequence of neuroinflammation is whether the response is an initiating event or the consequence of tissue damage. As examples of environmental exposure-related neuroinflammation in the literature, we provide an evaluation of data on manganese and diesel exhaust particles.

The yin and yang of microglia

Microglia, the resident immune cells of the mammalian central nervous system (CNS), play a pivotal role in both physiological and pathological conditions such as the restoration of CNS integrity and the progression of neurodegenerative disorders. Extensive data have been published that describe neuroinflammation by microglial activation to have detrimental consequences on the developing and mature brain. On the other hand, a properly directed and limited inflammatory response is known to be a natural healing process after an insult in several other tissues. Thus, it is not surprising that research results illustrating benefits of neuroinflammation have been emerging over the past decade. Inflammation-mediated benefits for CNS outcomes include mechanisms such as neuroprotection, mobilization of neural precursors for repair, remyelination and axonal regeneration. Here, we review data that highlight the dual aspects of microglia with a focus on the developing brain, i.e. as aggressors potentiating damage and as helpers in the recovery process following CNS damage.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Glutamate signaling in the pathophysiology and therapy of prenatal insults

Birth asphyxia and hypoxia-ischemia (HI) are important factors affecting the normal development and maturation of the central nervous system (CNS). Depending on the maturity of the brain, HI-induced damage at different ages is region-selective, the white matter (WM) peripheral to the lateral ventricles being selectively vulnerable to damage in premature infants. As a squeal of primary or secondary HI in the preterm infant, the brain injury comprises periventricular leukomalasia (PVL), accompanied by neuronal and axonal damage, which affects several brain regions. Premature delivery and improved neonatal intensive care have led to a survival rate of about 75% to 90% of infants weighting under 1500g both in Europe and in the United States. However, about 5-10% of these survivors exhibit cerebral palsy (CP), and many have cognitive, behavioral, attentional or socialization deficits. In this review, we first shortly discuss developmental changes in the expression of the excitatory glutamate receptors (GluRs), and then in more detail elucidate the contribution of GluRs to oligodendrocyte (OL) damage both in experimental models and in preterm human infants. Finally, therapeutic interventions targeted at GluRs at the young age are discussed in the light of results obtained from recent experimental HI animal models and from humans.

Evidence for neuroinflammatory and microglial changes in the cerebral response to sleep loss

STUDY OBJECTIVES

Sleep loss has pro-inflammatory effects, but the roles of specific cell populations in mediating these effects have not been delineated. We assessed the modulation of the electroencephalographic and molecular responses to sleep deprivation (S-DEP) by minocycline, a compound that attenuates microglial activation occurring in association with neuroinflammatory events.

DESIGN

Laboratory rodents were subjected to assessment of sleep and wake in baseline and sleep deprived conditions.

PARTICIPANTS

Adult male CD-1 mice (30-35 g) subjected to telemetric electroencephalography.

INTERVENTIONS

Minocycline was administered daily. Mice were subjected to baseline data collection on the first day of minocycline administration and, on subsequent days, 2 S-DEP sessions, 1 and 3 h in duration, followed by recovery sleep. Following EEG studies, mice were euthanized either at the end of a 3 h S-DEP or as time-of day controls for sampling of brain messenger RNAs. Gene expression was measured by real-time polymerase chain reaction.

MEASUREMENTS AND RESULTS

Minocycline-treated mice exhibited a reduction in time spent asleep, relative to saline-treated mice, in the 3-h interval immediately after administration. S-DEP resulted in an increase in EEG slow wave activity relative to baseline in saline-treated mice. This response to S-DEP was abolished in animals subjected to chronic minocycline administration. S-DEP suppressed the expression of the microglial-specific transcript cd11b and the neuroinflammation marker peripheral benzodiazepine receptor, in the brain at the mRNA level. Minocycline attenuated the elevation of c-fos expression by S-DEP. Brain levels of pro-inflammatory cytokine mRNAs interleukin-1β (il-1β), interleukin-6 (il-6), and tumor necrosis factor-α (tnfα) were unaffected by S-DEP, but were elevated in minocycline-treated mice relative to saline-treated mice.

CONCLUSIONS

The anti-neuroinflammatory agent minocycline prevents either the buildup or expression of sleep need in rodents. The molecular mechanism underlying this effect is not known, but it is not mediated by suppression of il-1β, il-6, and tnfα at the transcript level.

Regulation of glutamate release by α7 nicotinic receptors: differential role in methamphetamine-induced damage to dopaminergic and serotonergic terminals

Regulation of glutamate release is an important underlying mechanism in mediating excitotoxic events such as damage to dopamine (DA) and serotonin (5-HT) neurons observed after exposure to methamphetamine (Meth). One way to regulate glutamate release may be through the modulation of α7 nicotinic acetylcholine (nACh) receptors. Meth administration is known to increase acetylcholine release; however, it is unknown whether Meth increases glutamate release and causes long-term damage to both DA and 5-HT terminals through the activation of α7 nACh receptors. To test this hypothesis, the α7 nACh receptor antagonist, methyllycaconitine (MLA), was administered before the administration of repeated doses of Meth while simultaneously monitoring extracellular striatal glutamate with in vivo microdialysis. In addition, the subsequent long-term decreases in markers of dopaminergic and serotonergic terminals, including DA reuptake transporter (DAT), serotonin reuptake transporter (SERT), vesicular monoamine transporter-2, vesicular DA, and vesicular 5-HT content in the rat striatum, were measured. The results show that MLA pretreatment prevented Meth-induced increases in striatal glutamate and protected against the subsequent long-term decreases in striatal DAT and vesicular DA content without affecting the hyperthermia produced by Meth. In contrast, the Meth-induced decreases in striatal SERT immunoreactivity and vesicular 5-HT content were not affected by MLA. This suggests that the α7 nACh receptor differentially mediates glutamate-dependent damage to DA but not 5-HT terminals in a manner that is independent of hyperthermia. Furthermore, antagonism of α7 nACh receptors may be a possible therapeutic strategy for decreasing extracellular glutamate and preventing the excitotoxic damage observed in other DA-related neurodegenerative disorders.

Microglial morphology and dynamic behavior is regulated by ionotropic glutamatergic and GABAergic neurotransmission

PURPOSE

Microglia represent the primary resident immune cells in the CNS, and have been implicated in the pathology of neurodegenerative diseases. Under basal or "resting" conditions, microglia possess ramified morphologies and exhibit dynamic surveying movements in their processes. Despite the prominence of this phenomenon, the function and regulation of microglial morphology and dynamic behavior are incompletely understood. We investigate here whether and how neurotransmission regulates "resting" microglial morphology and behavior.

METHODS

We employed an ex vivo mouse retinal explant system in which endogenous neurotransmission and dynamic microglial behavior are present. We utilized live-cell time-lapse confocal imaging to study the morphology and behavior of GFP-labeled retinal microglia in response to neurotransmitter agonists and antagonists. Patch clamp electrophysiology and immunohistochemical localization of glutamate receptors were also used to investigate direct-versus-indirect effects of neurotransmission by microglia.

RESULTS

Retinal microglial morphology and dynamic behavior were not cell-autonomously regulated but are instead modulated by endogenous neurotransmission. Morphological parameters and process motility were differentially regulated by different modes of neurotransmission and were increased by ionotropic glutamatergic neurotransmission and decreased by ionotropic GABAergic neurotransmission. These neurotransmitter influences on retinal microglia were however unlikely to be directly mediated; local applications of neurotransmitters were unable to elicit electrical responses on microglia patch-clamp recordings and ionotropic glutamatergic receptors were not located on microglial cell bodies or processes by immunofluorescent labeling. Instead, these influences were mediated indirectly via extracellular ATP, released in response to glutamatergic neurotransmission through probenecid-sensitive pannexin hemichannels.

CONCLUSIONS

Our results demonstrate that neurotransmission plays an endogenous role in regulating the morphology and behavior of "resting" microglia in the retina. These findings illustrate a mode of constitutive signaling between the neural and immune compartments of the CNS through which immune cells may be regulated in concert with levels of neural activity.

Expression of N-methyl D-aspartate receptor subunits in amoeboid microglia mediates production of nitric oxide via NF-κB signaling pathway and oligodendrocyte cell death in hypoxic postnatal rats

The present study was focused on identifying the expression of N-methyl D-aspartate receptor (NMDAR) subunits on activated microglia and to determine their role in the pathogenesis of periventricular white matter damage (PWMD) in neonatal rats following hypoxia. One day old wistar rats were subjected to hypoxia (5% O(2) ; 95% N(2) ) and the mRNA and protein expression of NMDAR subunits (NR1, NR2A-D, and NR3A) in the periventricular white matter (PWM) was determined at different time points (3,24 h, 3, 7, and 14 days) following hypoxic exposure. Immunoexpression of NR1 and NR2A-D was localized in amoeboid microglial cells (AMC) suggesting the presence of functional NMDARs in them. The expression of NMDAR in primary microglial cultures was ascertained by RT-PCR analysis and double immunofluorescence studies. The functionality of the microglial NMDAR in cultured microglial cells was examined by monitoring calcium movements in cells with fura-2. In primary microglial cultures, hypoxia induced the nuclear translocation of NF-κB which was suppressed by administration of MK801, an NMDAR antagonist. MK801 also down regulated the hypoxia-induced expression of tumor necrosis factor-α, interleukin-1β, inducible nitric oxide synthase (iNOS), and nitric oxide (NO) production by microglia which may be mediated by the NF-κB signaling pathway. NO produced by microglia is known to cause death of oligodendrocytes in the developing PWM. In this connection, pharmacological agents such as MK801, BAY (NF-κB inhibitor), and 1400w (iNOS inhibitor) proved to be beneficial since they reduced the hypoxia-induced iNOS expression, NO production, and a corresponding reduction in the death of oligodendrocytes following hypoxia.

Microglial activation in stroke: therapeutic targets

Microglial activation is an early response to brain ischemia and many other stressors. Microglia continuously monitor and respond to changes in brain homeostasis and to specific signaling molecules expressed or released by neighboring cells. These signaling molecules, including ATP, glutamate, cytokines, prostaglandins, zinc, reactive oxygen species, and HSP60, may induce microglial proliferation and migration to the sites of injury. They also induce a nonspecific innate immune response that may exacerbate acute ischemic injury. This innate immune response includes release of reactive oxygen species, cytokines, and proteases. Microglial activation requires hours to days to fully develop, and thus presents a target for therapeutic intervention with a much longer window of opportunity than acute neuroprotection. Effective agents are now available for blocking both microglial receptor activation and the microglia effector responses that drive the inflammatory response after stroke. Effective agents are also available for targeting the signal transduction mechanisms linking these events. However, the innate immune response can have beneficial as well deleterious effects on outcome after stoke, and a challenge will be to find ways to selectively suppress the deleterious effects of microglial activation after stroke without compromising neurovascular repair and remodeling.

Beta-adrenoceptor mediated surgery-induced production of pro-inflammatory cytokines in rat microglia cells

Immunological changes initiated by major operative injury may result in inflammatory responses in both peripheral and central nervous system, which may lead to organ dysfunction. Recent studies indicate that beta-adrenergic receptors (beta-ARs) may mediate production of pro-inflammatory cytokines in the brain. In the present study propranolol (beta-AR antagonist), but not prazosin (alpha1-AR antagonist), antagonized surgical trauma induced pro-inflammatory cytokine production in microglia cells isolated from rats. beta-AR activation in the absence of pro-inflammatory stimuli increased IL-1beta, TNF-alpha and IL-6 mRNA and protein expressions in the primary microglia cell culture. Isoproterenol (beta-AR agonist) treatment induced a time- and concentration-dependent increase of IL-1beta in cells. Both ERK1/2 and P38 MAPK inhibitor, but not PKA and JNK1/2 inhibitor abrogated isoproterenol-induced IL-1beta and IL-6 production in microglia cells. In conclusion, the results suggest that beta-ARs possess pro-inflammatory properties by modulating the functions of microglia cell.

Activation of microglia by neuronal activity: results from a new in vitro paradigm based on neuronal-silicon interfacing technology

Cognition and behavior primarily arise from the communication that occurs between brain cells. By using photoconductive stimulation to trigger localized regions of neuronal action potentials and astrocyte Ca(2+) waves in dissociated rat hippocampal cultures, we can directly study microglia behavior in response to physiological and pathological levels of activity. Connections between neurons can be modified by microglia, which regulate gap junctions and synapses through secretion of proteins such as cytokines, proteases and neurotrophic factors. Activated microglia participate in bidirectional communication with the excitable tissues that they support. Through feedback from the many ion channels and surface receptors they express, microglia are informed of neuronal and astrocytic activity that may indicate disruption in the homeostasis of the CNS. Such disturbances alert microglia to locations of such activity and promote their transformation into a reactive state, in which they perform adaptive functions that can be either neuroprotective, neurotoxic, or neuromodulatory. Under physiological conditions, normal brain activity has the effect of suppressing microglia inflammatory responses. This report summarizes available data about the interaction of microglia and brain activity and presents a new in vitro paradigm to study the mechanisms involved. We propose that photoconductive stimulation is a powerful tool for studying the cellular and molecular mechanisms underlying the dynamic interactions between neurons, astrocytes and microglia.

P2X7 regenerative-loop potentiation of glutamate synaptic transmission by microglia and astrocytes

P2X7 purinergic receptors have been implicated in chronic neuropathic and neuroinflammatory pain as well as in depression. These receptors are predominantly found in the central nervous system on microglial cells and on glutamatergic nerve terminals. Here, we develop hypotheses concerning mechanisms by which transient high-frequency impulse firing in glutamatergic terminals, such as occurs in nociceptor terminals accompanying neuropathic/neuroinflammatory pain, can lead to long-lasting changes in neural network function that is mediated by surrounding glial cells. The hypothesis consists of two parts. In the first, glutamate released by low-frequency (2Hz) terminal action potentials is insufficient to generate postsynaptic action potentials, but these are generated by brief high-frequency input bursts. Glutamate released by these bursts is partly removed by transporters on the enveloping astrocyte processes and also excites AMPA receptors on these processes, which then release ATP. This ATP is partly metabolised to adenosine, which acts on presynaptic A1 receptors to inhibit glutamate release. The remaining ATP acts on the presynaptic P2X7 receptors to facilitate glutamate release by both the high-frequency burst of action potentials as well as by a continuous low-frequency (2Hz) action potential firing that occurs in the absence of a neuropathic/neuroinflammatory insult. The positive feedback of terminal glutamate release, triggering astrocyte ATP release and leading to further glutamate release through activation of P2X7 receptors, is then sufficient to allow the normal low-frequency (2Hz) action potentials to now elicit postsynaptic action potentials after the insult is removed. In the second part of this model, the high concentration of ATP derived from astrocytes at the terminal attracts microglia by chemotaxis. The P2X7 receptors on these microglia are then engaged, resulting in microglia secreting the cytokine TNFalpha. This acts on postsynaptic TNF-R1 receptors to increase the number of AMPA receptors there, thus enhancing the efficacy of synaptic transmission. The TNFalpha also acts on presynaptic TNF-R1 to increase the amount of glutamate released by each nerve terminal impulse. Experimental tests can be made of this hypothesis that P2X7 receptors on the presynaptic terminal and those on the microglia synergistically act to ensure feedback pathways that reset to a high level the efficacy of synaptic transmission, thus ensuring chronic neuropathic/neuroinflammatory pain even when the initial insult has subsided.

Neuronal injury induces the release of pro-interleukin-1beta from activated microglia in vitro

Microglia activated after brain injury, are a major source of the pro-inflammatory cytokine interleukin-1 (IL-1), which is known to further exacerbate damage. However, the mechanisms that control IL-1 release in acute neuronal injury are unknown and the purpose of this study was to test the hypothesis that neuronal injury induces IL-1beta release from microglial cells. Here we report that lipopolysaccharide (LPS)-activated rat microglia co-cultured with healthy rat neurons express pro-IL-1beta, which in the absence of cell death accumulates in the cells. Treatment of co-cultures with the excitotoxin N-methyl-D-aspartate (NMDA) induced neuronal cell death leading to the appearance of pro-IL-1beta in the culture supernatant. This effect was reversed by the NMDA receptor antagonist MK-801, and was neuron-dependent, since NMDA had no effect on cell death or pro-IL-1beta release in mixed glial cell cultures. In addition, we show that pro-IL-1beta release from LPS-treated mixed glia or LPS-treated microglia is significantly reduced in the presence of conditioned medium from healthy co-cultures or neuronal cultures respectively. These results demonstrate that injured neurons promote the release of pro-IL-1beta from microglia, possibly by regulating microglial cell viability, and suggest an important alternative mechanism of IL-1beta release that occurs in response to neuronal injury.

Neuron-glia crosstalk gets serious: role in pain hypersensitivity

PURPOSE OF REVIEW

Recent studies show that peripheral injury activates both neuronal and nonneuronal or glial components of the peripheral and central cellular circuitry. The subsequent neuron-glia interactions contribute to pain hypersensitivity. This review will briefly discuss novel findings that have shed light on the cellular mechanisms of neuron-glia interactions in persistent pain.

RECENT FINDINGS

Two fundamental questions related to neuron-glia interactions in pain mechanisms have been addressed: what are the signals that lead to central glial activation after injury and how do glial cells affect central nervous system neuronal activity and promote hyperalgesia?

SUMMARY

Evidence indicates that central glial activation depends on nerve inputs from the site of injury and release of chemical mediators. Hematogenous immune cells may migrate to/infiltrate the brain and circulating inflammatory mediators may penetrate the blood-brain barrier to participate in central glial responses to injury. Inflammatory cytokines such as interleukin-1beta released from glia may facilitate pain transmission through its coupling to neuronal glutamate receptors. This bidirectional neuron-glia signaling plays a key role in glial activation, cytokine production and the initiation and maintenance of hyperalgesia. Recognition of the contribution of the mutual neuron-glia interactions to central sensitization and hyperalgesia prompts new treatment for chronic pain.

Role of microglial IKKbeta in kainic acid-induced hippocampal neuronal cell death

Microglial cells are activated during excitotoxin-induced neurodegeneration. However, the in vivo role of microglia activation in neurodegeneration has not yet been fully elucidated. To this end, we used Ikkbeta conditional knockout mice (LysM-Cre/Ikkbeta(F/F)) in which the Ikkbeta gene is specifically deleted in cells of myeloid lineage, including microglia, in the CNS. This deletion reduced IkappaB kinase (IKK) activity in cultured primary microglia by up to 40% compared with wild-type (Ikkbeta(F/F)), and lipopolysaccharide-induced proinflammatory gene expression was also compromised. Kainic acid (KA)-induced hippocampal neuronal cell death was reduced by 30% in LysM-Cre/Ikkbeta(F/F) mice compared with wild-type mice. Reduced neuronal cell death was accompanied by decreased KA-induced glial cell activation and subsequent expression of proinflammatory genes such as tumour necrosis factor (TNF)-alpha and interleukin (IL)-1beta. Similarly, neurons in organotypic hippocampal slice cultures (OHSCs) from LysM-Cre/Ikkbeta(F/F) mouse brain were less susceptible to KA-induced excitotoxicity compared with wild-type OHSCs, due in part to decreased TNF-alpha and IL-1beta expression. Based on these data, we concluded that IKK/nuclear factor-kappaB dependent microglia activation contributes to KA-induced hippocampal neuronal cell death in vivo through induction of inflammatory mediators.

Charge compensation for NADPH oxidase activity in microglia in rat brain slices does not involve a proton current

The membrane properties of isolated cultured microglia have been extensively studied but it is important to understand their properties in situ, where they protect the brain against infection, but also contribute to neurodegenerative diseases. Microglia and macrophages attack bacteria by generating reactive oxygen species, a process which involves NADPH oxidase pumping electrons out across the cell membrane. The resulting inward current evokes a depolarization, which would inhibit the activity of the NADPH oxidase if there were no charge-compensating current which moves positive charge out across the membrane. The mechanism of this charge compensation is controversial. In neutrophils and in cultured microglia a depolarization-activated H(+) conductance has been proposed to provide charge compensation, and also to remove protons generated intracellularly by the NADPH oxidase. Alternatively, a depolarization-activated K(+) conductance has been proposed to mediate charge compensation. Here we show that in microglia, either in the resting state or when activated by the bacterial coat component lipopolysaccharide, both in acute and in cultured hippocampal slices, no significant H(+) current is detectable. This implies that the membrane properties of microglia in their normal cellular environment differ from those of cultured microglia (similarly, microglia generated a current in response to ATP but, unlike in culture, not to glutamate or GABA). Furthermore, the K(+) current (Kv1.3) that is activated by lipopolysaccharide is inactivated by depolarization, making it unsuitable for mediating charge compensation on a long time scale at positive voltages. Instead, charge compensation may be mediated by a previously undescribed non-selective cation current.

A comparison of spinal Iba1 and GFAP expression in rodent models of acute and chronic pain

The treatment of acute and chronic pain is still deficient. The modulation of glial cells may provide novel targets to treat pain. We hypothesize that astrocytes and microglia participate in the initiation and maintenance of both, acute surgical and chronic neuropathic pain. Rats underwent paw incision, L5 nerve exposure or L5 nerve transection surgery. Behavioral mechanical allodynia was assessed using von Frey filaments. Immunohistochemistry was performed using anti-ionized calcium binding adaptor protein, Iba-1 (microglia), and anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) on day 1, 4 and 7 after surgery. Following paw incision and at spinal L5 segment GFAP expression was increased in laminae I-II and Iba1 in deep laminae on day 1, in the entire dorsal horn on day 4 and dissipated on day 7 after paw incision in parallel with the allodynia. L5 nerve transection induced mechanical allodynia from day 1 to 7 which correlated with Iba-1 increases on day 1, 4 (entire dorsal horn) and day 7 after nerve injury (deep laminae of the dorsal horn) at spinal L5 segment. Conversely, GFAP increased at later time points from day 4 (deep laminae) and on day 7 (entire dorsal horn). Our data demonstrates that astrocytes (GFAP expression) play a role in the initiation of acute pain and the maintenance of chronic pain while Iba-1 increases closely correlated with the early phase of neuropathic pain. Iba1 and GFAP increased rostrally, at L3 segment, after paw incision (day 4) and only Iba1 increased following L5 nerve transection (day 7).

Calcium and glial cell death

Calcium (Ca2+) homeostasis is crucial for development and survival of virtually all types of cells including glia of the central nervous system (CNS). Astrocytes, oligodendrocytes and microglia, the major glial cell types in the CNS, are endowed with a rather sophisticated array of Ca2+-permeable receptors and channels, as well as store-operated channels and pumps, all of which determine Ca2+ homeostasis. In addition, glial cells detect functional activity in neighbouring neurons and respond to it by means of Ca2+ signals that can modulate synaptic interactions. Like in neurons, Ca2+ overload resulting from dysregulation of channels and pumps can be deleterious to glia. In this review, we summarize recent advances in the understanding Ca2+ homeostasis in glial cells, the consequences of its alteration in cell demise as well as in neurological and psychiatric disorders that experience glial cell loss.

Autoantibodies against opioid or glutamate receptors are associated with changes in morphine reward and physical dependence in mice

BACKGROUND

Possible interactions between nervous and immune systems during opioid addiction remain elusive. Recombinant mu-delta opioid receptors (MDOR) and the glutamate receptor 1 (GluR1) subunit of amino-3-hydroxy-5-methyl-4-isoxazole propionic acid glutamate receptors are involved in acute and chronic effects of morphine. Elevated levels of autoantibodies (aAbs) to these receptors were demonstrated in heroin human addicts and in animal models. This study characterized the role of aAbs to these receptors in behavioral modulations recruited during opioid tolerance and sensitization.

METHODS AND FINDINGS

Male CD-1 mice, immunized with either MDOR or GluR1 peptide fragments (80 microg intraperitoneal (i.p.)), were examined for spontaneous behavior and response to morphine (5 mg/kg i.p.). Spontaneous home-cage activity, novelty-induced self-grooming and morphine-induced hyperactivity were higher in GluR1 mice compared to Vehicle subjects, whereas MDOR immunization was associated with an increased morphine-induced conditioned place preference. In response to escalating doses of morphine (from 10 to 60 mg/kg i.p., twice daily) and naloxone-precipitated withdrawal (1 mg/kg subcutaneous), GluR1 mice exhibited a more marked stereotyped sniffing behavior and less body tremors compared to Vehicle subjects, whereas less sniffing and teeth chattering were found in MDOR mice. The expected downregulation of mu receptor binding sites, induced by chronic morphine in vehicle subjects, was completely absent following MDOR immunization.

CONCLUSIONS

These findings indicate an altered response to morphine-related reinforcing and aversive effects in MDOR mice and altered coping with the environment in GluR1 mice. Circulating aAbs to specific neuroreceptors may alter the response to opiates and play a role as determinants of vulnerability to opiate addiction.

Resting microglial motility is independent of synaptic plasticity in mammalian brain

Microglia are well known for their roles in brain injuries and infections. However, there is no function attributes to resting microglia thus far. Here we performed a combination of simultaneous electrophysiology and time-lapse confocal imaging in green fluorescent protein-labeled microglia in acute hippocampal slices. In contrast to CA1 neurons, microglia showed no spontaneous or evoked synaptic currents. Neither glutamate- nor GABA-induced current/chemotaxis of microglia was detected. Strong tetanic stimulation of Schaffer-collateral pathways that induce CA1 long-term potentiation did not affect microglial motilities. Our results suggest that microglia are highly reserved for neuronal protective function but not synaptic plasticity in the brain.

Role of nonsynaptic communication in regulating the immune response

The discovery of nonsynaptic communication in the 1960s and 1970s was an important milestone in investigating the function of the nervous system, and it revolutionized our view about information transmission between neurons. In addition, nonsynaptic communication has a practical importance not only within the nervous system, but in the communication between the peripheral nervous system and other organ systems. Nonsynaptic communication takes place in different immune organs, which are innervated by sympathetic nerve terminals. In addition, the function of microglia, one of the immunocompetent cell types of the brain, can also be affected by neurotransmitters released from axon varicosities. The various functions of immune cells are modulated by released neurotransmitters without any direct synaptic contact between nerve endings and targeted immune cells requiring only functional neurotransmitter receptors on immune cells. Here, we briefly overview the role of the various receptor subtypes mediating nonsynaptic modulation of the function of immunocompetent cells both in the periphery and in the central nervous system.

Immunological aspects in the neurobiology of suicide: elevated microglial density in schizophrenia and depression is associated with suicide

OBJECTIVES

Suicide has a high prevalence in patients with schizophrenia and affective disorder. Our recent postmortem study [Steiner J, Mawrin C, Ziegeler A, Bielau H, Ullrich O, Bernstein HG, Bogerts B. Distribution of HLA-DR-positive microglia in schizophrenia reflects impaired cerebral lateralization. Acta Neuropathologica (Berl) 2006;112:305-16.] revealed increased microglial densities in two schizophrenic patients who had committed suicide. Therefore, the hypothesis of microglial activation during acute psychosis was proposed. Alternatively, "suicide" could be a diagnosis-independent factor leading to microgliosis.

METHODS

To clarify this question, microglial HLA-DR expression was analyzed by immunohistochemistry in the dorsolateral prefrontal cortex (DLPFC), anterior cingulate cortex (ACC), mediodorsal thalamus (MD) and hippocampus of 16 schizophrenics, 14 depressed patients with affective disorder and 10 matched controls. A subgroup of six schizophrenics and seven patients with affective disorder who committed suicide was included.

RESULTS

ANOVA revealed no effect of diagnosis on microglial density (DLPFC: P=0.469; ACC: P=0.349; MD: P=0.569; hippocampus: P=0.497). However, significant microgliosis was observed in the DLPFC (P=0.004), ACC (P=0.012) and MD (P=0.004) of suicide patients. A similar trend was seen in the hippocampus (P=0.057).

CONCLUSION

In conclusion, immunological factors may play a hitherto underestimated role in suicide. First, microglial activation might be interpreted as a consequence of presuicidal stress. Second, one might speculate a causal link between microglial activation and suicidal behaviour, such as neuroendocrine factors, cytokines, and nitric oxide, which are released from microglial cells and are known to modulate noradrenergic or serotonergic neurotransmission and thus may trigger suicidality.

Developmental stage of oligodendrocytes determines their response to activated microglia in vitro

Background

Oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes are both lost in central nervous system injury and disease. Activated microglia may play a role in OPC and oligodendrocyte loss or replacement, but it is not clear how the responses of OPCs and oligodendrocytes to activated microglia differ.

Methods

OPCs and microglia were isolated from rat cortex. OPCs were induced to differentiate into oligodendrocytes with thyroid hormone in defined medium. For selected experiments, microglia were added to OPC or oligodendrocyte cultures. Lipopolysaccharide was used to activate microglia and microglial activation was confirmed by TNFα ELISA. Cell survival was assessed with immunocytochemistry and cell counts. OPC proliferation and oligodendrocyte apoptosis were also assessed.

Results

OPCs and oligodendrocytes displayed phenotypes representative of immature and mature oligodendrocytes, respectively. Activated microglia reduced OPC survival, but increased survival and reduced apoptosis of mature oligodendrocytes. Activated microglia also underwent cell death themselves.

Conclusion

Activated microglia may have divergent effects on OPCs and mature oligodendrocytes, reducing OPC survival and increasing mature oligodendrocyte survival. This may be of importance because activated microglia are present in several disease states where both OPCs and mature oligodendrocytes are also reacting to injury. Activated microglia may simultaneously have deleterious and helpful effects on different cells after central nervous system injury.