The limited success in understanding the pathophysiology of major depression may result from excessive focus on the dysfunctioning of neurons, as compared with other types of brain cells. Therefore, ...we examined the role of dynamic alterations in microglia activation status in the development of chronic unpredictable stress (CUS)-induced depressive-like condition in rodents. We report that following an initial period (2-3 days) of stress-induced microglial proliferation and activation, some microglia underwent apoptosis, leading to reductions in their numbers within the hippocampus, but not in other brain regions, following 5 weeks of CUS exposure. At that time, microglia displayed reduced expression of activation markers as well as dystrophic morphology. Blockade of the initial stress-induced microglial activation by minocycline or by transgenic interleukin-1 receptor antagonist overexpression rescued the subsequent microglial apoptosis and decline, as well as the CUS-induced depressive-like behavior and suppressed neurogenesis. Similarly, the antidepressant drug imipramine blocked the initial stress-induced microglial activation as well as the CUS-induced microglial decline and depressive-like behavior. Treatment of CUS-exposed mice with either endotoxin, macrophage colony-stimulating factor or granulocyte-macrophage colony-stimulating factor, all of which stimulated hippocampal microglial proliferation, partially or completely reversed the depressive-like behavior and dramatically increased hippocampal neurogenesis, whereas treatment with imipramine or minocycline had minimal or no anti-depressive effects, respectively, in these mice. These findings provide direct causal evidence that disturbances in microglial functioning has an etiological role in chronic stress-induced depression, suggesting that microglia stimulators could serve as fast-acting anti-depressants in some forms of depressive and stress-related conditions.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Highlights • Following challenge, the normal aged neuroinflammatory response is potentiated. • Sensitized microglia appears to be the source of this amplified response. • Elevated glucocorticoids may ...play a role in sensitizing microglia. • Potentiated neuroinflammation in the hippocampus leads to memory impairments. • Anti-inflammatory (e.g., exercise) interventions are effective treatments.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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Classically, the central nervous system (CNS) and the immune system are thought to operate independently of each other. This simplistic view has been corrected in recent years, first with the ...recognition that the brain dynamically modulates the immune system, and later with the reverse; that is, that the immune system modulates the CNS as well. The evidence that the immune system regulates CNS functions is first reviewed. This immune‐to‐brain communication pathway triggers the production of a constellation of CNS‐mediated phenomena, collectively referred to as ‘sickness responses’. These sickness responses are created by immune‐to‐brain signals activating CNS glia to release glial proinflammatory cytokines. The most recently recognized member of this constellation of changes is enhanced pain responsivity. The hypothesis is then developed that pathological, chronic pain may result from ‘tapping into’ this ancient survival‐oriented circuitry, including the activation of immune and glial cells and the release of immune/glial proinflammatory cytokines. This can occur at the level of peripheral nerves, dorsal root ganglia, spinal cord, and likely at higher brain areas. The implications of this model for human chronic pain syndromes and clinical resolution of these chronic pain states are then discussed.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract Glutamate neurotransmission is highly regulated, largely by glutamate transporters. In the spinal cord, the glutamate transporter GLT-1 is primarily responsible for glutamate clearance. ...Downregulation of GLT-1 can occur in activated astrocytes, and is associated with increased extracellular glutamate and neuroexcitation. Among other conditions, astrocyte activation occurs following repeated opioids and in models of chronic pain. If GLT-1 downregulation occurs in these states, GLT-1 could be a pharmacological target for improving opioid efficacy and controlling chronic pain. The present studies explored whether daily intrathecal treatment of rats with ceftriaxone, a β-lactam antibiotic that upregulates GLT-1 expression, could prevent development of hyperalgesia and allodynia following repeated morphine, reverse pain arising from central or peripheral neuropathy, and reduce glial activation in these models. Ceftriaxone pre-treatment attenuated the development of hyperalgesia and allodynia in response to repeated morphine, and prevented associated astrocyte activation. In a model of multiple sclerosis (experimental autoimmune encephalomyelitis; EAE), ceftriaxone reversed tactile allodynia and halted the progression of motor weakness and paralysis. Similarly, ceftriaxone reversed tactile allodynia induced by chronic constriction nerve injury (CCI). EAE and CCI each significantly reduced the expression of membrane-bound, dimerized GLT-1 protein in lumbar spinal cord, an effect normalized by ceftriaxone. Lastly, ceftriaxone normalized CCI- and EAE-induced astrocyte activation in lumbar spinal cord. Together, these data indicate that increasing spinal GLT-1 expression attenuates opioid-induced paradoxical pain, alleviates neuropathic pain, and suppresses associated glial activation. GLT-1 therefore may be a therapeutic target that could improve available treatment options for patients with chronic pain.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Neuroimmune diseases have diverse symptoms and etiologies but all involve pathological inflammation that affects normal central nervous system signaling. Critically, many neuroimmune diseases also ...involve insufficient signaling/bioavailability of interleukin-10 (IL-10). IL-10 is a potent anti-inflammatory cytokine released by immune cells and glia, which drives the regulation of a variety of anti-inflammatory processes. This review will focus on the signaling pathways and function of IL-10, the current evidence for insufficiencies in IL-10 signaling/bioavailability in neuroimmune diseases, as well as the implications for IL-10-based therapies to treating such problems. We will review in detail four pathologies as examples of the common etiologies of such disease states, namely neuropathic pain (nerve trauma), osteoarthritis (peripheral inflammation), Parkinson's disease (neurodegeneration), and multiple sclerosis (autoimmune). A number of methods to increase IL-10 have been developed (e.g. protein administration, viral vectors, naked plasmid DNA, plasmid DNA packaged in polymers to enhance their uptake into target cells, and adenosine 2A agonists), which will also be discussed. In general, IL-10-based therapies have been effective at treating both the symptoms and pathology associated with various neuroimmune diseases, with more sophisticated gene therapy-based methods producing sustained therapeutic effects lasting for several months following a single injection. These exciting results have resulted in IL-10-targeted therapeutics being positioned for upcoming clinical trials for treating neuroimmune diseases, including neuropathic pain. Although further research is necessary to determine the full range of effects associated with IL-10-based therapy, evidence suggests IL-10 may be an invaluable target for the treatment of neuroimmune disease.
This article is part of a Special Issue entitled ‘Neuroimmunology and Synaptic Function’.
•Many neuroimmune diseases involve insufficient IL-10 signaling/bioavailability.•IL-10 is a potent anti-inflammatory cytokine released by immune cells and glia.•A variety of methods to increase IL-10 have been developed.•IL-10 therapy has been effective to treat neuroimmune diseases.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UM, UPCLJ, UPUK
Opioid action was thought to exert reinforcing effects solely via the initial agonism of opioid receptors. Here, we present evidence for an additional novel contributor to opioid reward: the innate ...immune pattern-recognition receptor, toll-like receptor 4 (TLR4), and its MyD88-dependent signaling. Blockade of TLR4/MD2 by administration of the nonopioid, unnatural isomer of naloxone, (+)-naloxone (rats), or two independent genetic knock-outs of MyD88-TLR4-dependent signaling (mice), suppressed opioid-induced conditioned place preference. (+)-Naloxone also reduced opioid (remifentanil) self-administration (rats), another commonly used behavioral measure of drug reward. Moreover, pharmacological blockade of morphine-TLR4/MD2 activity potently reduced morphine-induced elevations of extracellular dopamine in rat nucleus accumbens, a region critical for opioid reinforcement. Importantly, opioid-TLR4 actions are not a unidirectional influence on opioid pharmacodynamics, since TLR4(-/-) mice had reduced oxycodone-induced p38 and JNK phosphorylation, while displaying potentiated analgesia. Similar to our recent reports of morphine-TLR4/MD2 binding, here we provide a combination of in silico and biophysical data to support (+)-naloxone and remifentanil binding to TLR4/MD2. Collectively, these data indicate that the actions of opioids at classical opioid receptors, together with their newly identified TLR4/MD2 actions, affect the mesolimbic dopamine system that amplifies opioid-induced elevations in extracellular dopamine levels, therefore possibly explaining altered opioid reward behaviors. Thus, the discovery of TLR4/MD2 recognition of opioids as foreign xenobiotic substances adds to the existing hypothesized neuronal reinforcement mechanisms, identifies a new drug target in TLR4/MD2 for the treatment of addictions, and provides further evidence supporting a role for central proinflammatory immune signaling in drug reward.
The initial reinforcing properties of drugs of abuse, such as cocaine, are largely attributed to their ability to activate the mesolimbic dopamine system. Resulting increases in extracellular ...dopamine in the nucleus accumbens (NAc) are traditionally thought to result from cocaine's ability to block dopamine transporters (DATs). Here we demonstrate that cocaine also interacts with the immunosurveillance receptor complex, Toll-like receptor 4 (TLR4), on microglial cells to initiate central innate immune signaling. Disruption of cocaine signaling at TLR4 suppresses cocaine-induced extracellular dopamine in the NAc, as well as cocaine conditioned place preference and cocaine self-administration. These results provide a novel understanding of the neurobiological mechanisms underlying cocaine reward/reinforcement that includes a critical role for central immune signaling, and offer a new target for medication development for cocaine abuse treatment.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The term ‘learned helplessness’ refers to a constellation of behavioral changes that follow exposure to stressors that are not controllable by means of behavioral responses, but that fail to occur if ...the stressor is controllable. This paper discusses the nature of learned helplessness, as well as the role of the dorsal raphe nucleus, serotonin, and corticotropin-releasing hormone in mediating the behavioral effects of uncontrollable stressors. Recent research indicates that (a) uncontrollable stressors sensitize serotonergic neurons in the dorsal raphe, and that a corticotropin-releasing factor-related ligand, acting at the Type II receptor, is essential to this sensitization process, and (b) the consequent exaggerated release of serotonin in response to subsequent input is at least in part responsible for the behavioral changes that occur. Finally, implications for the general role of corticotropin-releasing hormone in stress-related phenomena and for the learned helplessness paradigm as an animal model of either depression or anxiety are discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Department of Psychology and the Center for
Neuroscience, University of Colorado at Boulder, Boulder,
Colorado
Watkins, Linda R. and
Steven F. Maier.
Beyond Neurons: Evidence That Immune and Glial
...Cells Contribute to Pathological Pain States. Physiol. Rev. 82: 981-1011, 2002. Chronic pain can occur after peripheral
nerve injury, infection, or inflammation. Under such neuropathic pain
conditions, sensory processing in the affected body region becomes
grossly abnormal. Despite decades of research, currently available
drugs largely fail to control such pain. This review explores the
possibility that the reason for this failure lies in the fact that such
drugs were designed to target neurons rather than immune or glial
cells. It describes how immune cells are a natural and inextricable
part of skin, peripheral nerves, dorsal root ganglia, and spinal cord. It then examines how immune and glial activation may participate in the
etiology and symptomatology of diverse pathological pain states in both
humans and laboratory animals. Of the variety of substances released by
activated immune and glial cells, proinflammatory cytokines (tumor
necrosis factor, interleukin-1, interleukin-6) appear to be of special
importance in the creation of peripheral nerve and neuronal
hyperexcitability. Although this review focuses on immune modulation of
pain, the implications are pervasive. Indeed, all nerves and neurons
regardless of modality or function are likely affected by immune and
glial activation in the ways described for pain.
Abstract The degree of behavioral control that an organism has over a stressor determines the behavioral and neurochemical sequelae of the stressor, with the presence of control preventing the ...typical outcomes that occur when the stressor is uncontrollable (e.g. failure to learn, exaggerated fear, dorsal raphe nucleus (DRN) 5-HT activation). Furthermore, an experience with a controllable stressor blocks the consequences of later uncontrollable stressors (“immunization”). These effects of control have been argued to be mediated by control-induced activation of ventral medial prefrontal cortex (mPFCv) output to the DRN. The experiments that have led to this interpretation have all involved the inactivation of the mPFCv with muscimol, showing that inactivation during the stressor eliminates the stressor-resistance produced by control, with the controllable stressor now acting as if it were uncontrollable. The present experiments in rats employed the opposite strategy, activating the mPFCv during the stressor. mPFCv microinjection of picrotoxin during the stressor eliminated the DRN 5-HT activation that normally occurs during the uncontrollable stressor, as well as the escape learning deficit and exaggerated fear that normally follows uncontrollable stress. Furthermore, mPFCv activation during an initial exposure to an uncontrollable stressor led the uncontrollable stressor to produce behavioral and neurochemical immunization when the subjects were later exposed to an uncontrollable stressor. That is, the conjoint activation of the mPFCv and exposure to an uncontrollable stressor led the uncontrollable stressor to act as if it were controllable. These results provide strong support for the argument that behavioral control produced stress-resistance by activating the mPFCv.
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