Microglia serve as a neuroimmune substrate for stress-induced potentiation of CNS pro-inflammatory cytokine responses

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.

Role of spinal cord glia in the central processing of peripheral pain perception

BACKGROUND

The discovery that glial activation plays a critical role in the modulation of neuronal functions and affects the spinal processing of nociceptive signalling has brought new understanding on the mechanisms underlying central sensitization involved in chronic pain facilitation. Spinal glial activation is now considered an important component in the development and maintenance of allodynia and hyperalgesia in various models of chronic pain, including neuropathic pain and pain associated with peripheral inflammation. In addition, spinal glial activation is also involved in some forms of visceral hyperalgesia.

PURPOSE

We discuss the signalling pathways engaged in central glial activation, including stress pathways, and the neuron-glia bidirectional relationships involved in the modulation of synaptic activity and pain facilitation. In this expanding field of research, the characterization of the mechanisms by which glia affect spinal neuro-transmission will increase our understanding of central pain facilitation, and has the potential for the development of new therapeutic agents for common chronic pain conditions.

Interleukin-10/Ceftriaxone prevents E. coli-induced delays in sensorimotor task learning and spatial memory in neonatal and adult Sprague-Dawley rats

Intrauterine infection during pregnancy is associated with early activation of the fetal immune system and poor neurodevelopmental outcomes. Immune activation can lead to alterations in sensorimotor skills, changes in learning and memory and neural plasticity. Both interleukin-10 (IL-10) and Ceftriaxone have been shown to decrease immune system activation and increase memory capacity, respectively. Using a rodent model of intrauterine infection, we examined sensorimotor development in pups, learning and memory, via the Morris water maze, and long-term potentiation in adult rats. Pregnant rats at gestational day 17 were inoculated with 1 x 10(5) colony forming units of Escherichia coli (E. coli) or saline. Animals in the treatment group received IL-10/Ceftriaxone for 3 days following E. coli administration. Intrauterine infection delayed surface righting, negative geotaxis, startle response and eye opening. Treatment with IL-10/Ceftriaxone reduced the delay in these tests. Intrauterine infection impaired performance in the probe trial in the Morris water maze (saline 25.13+/-1.01; E. coli 20.75+/-1.01; E. coli+IL-10/Ceftriaxone 20.2+/-1.62) and reduced the induction of long-term potentiation (saline 141.5+/-4.3; E. coli 128.7+/-3.9; E. coli+IL-10/Ceftriaxone 140.0+/-10). In summary, the results of this study indicate that E. coli induced intrauterine infection delays sensorimotor and learning and memory, while IL-10/Ceftriaxone rescues some of these behaviors. These delays were also accompanied by an increase in interleukin-1beta levels, which indicates immune activation. IL-10/Ceftriaxone prevents these delays as well as decreases E. coli-induced interleukin-1beta activation and may offer a window of time in which suitable treatment could be administered.

Enduring consequences of maternal obesity for brain inflammation and behavior of offspring

Obesity is well characterized as a systemic inflammatory condition, and is also associated with cognitive disruption, suggesting a link between the two. We assessed whether peripheral inflammation in maternal obesity may be transferred to the offspring brain, in particular, the hippocampus, and thereby result in cognitive dysfunction. Rat dams were fed a high-saturated-fat diet (SFD), a high-trans-fat diet (TFD), or a low-fat diet (LFD) for 4 wk prior to mating, and remained on the diet throughout pregnancy and lactation. SFD/TFD exposure significantly increased body weight in both dams and pups compared to controls. Microglial activation markers were increased in the hippocampus of SFD/TFD pups at birth. At weaning and in adulthood, proinflammatory cytokine expression was strikingly increased in the periphery and hippocampus following a bacterial challenge [lipopolysaccharide (LPS)] in the SFD/TFD groups compared to controls. Microglial activation within the hippocampus was also increased basally in SFD rats, suggesting a chronic priming of the cells. Finally, there were marked changes in anxiety and spatial learning in SFD/TFD groups. These effects were all observed in adulthood, even after the pups were placed on standard chow at weaning, suggesting these outcomes were programmed early in life.

Distribution and expression of CD200 in the rat respiratory system under normal and endotoxin-induced pathological conditions

In vivo and in vitro studies have clearly demonstrated that signaling mediated by the interaction of CD200 and its cognate receptor, CD200R, results in an attenuation of inflammatory or autoimmune responses through multiple mechanisms. The present results have shown a differential expression of CD200 in the respiratory tract of intact rats. Along the respiratory passage, CD200 was specifically distributed at the bronchiolar epithelia with intense CD200 immunoreactivity localized at the apical surface of some ciliated epithelial cells; only a limited expression was detected on the Clara cells extending into the alveolar duct. In the alveolar septum, double immunofluorescence showed intense CD200 immunolabeling on the capillary endothelia. A moderate CD200 labeling was observed on the alveolar type II epithelial cells. It was, however, absent in the alveolar type I epithelial cells and the alveolar macrophages. Immunoelectron microscopic study has revealed a specific distribution of CD200 on the luminal front of the thin portion of alveolar endothelia. During endotoxemia, the injured lungs showed a dose- and time-dependent decline of CD200 expression accompanied by a vigorous infiltration of immune cells, some of them expressing ionized calcium binding adapter protein 1 or CD200. Ultrastructural examination further showed that the marked reduction of CD200 expression was mainly attributable to the loss of alveolar endothelial CD200. It is therefore suggested that CD200 expressed by different lung cells may play diverse roles in immune homeostasis of normal lung, in particular, the molecules on alveolar endothelia that may control regular recruitment of immune cells via CD200-CD200R interaction. Additionally, it may contribute to intense infiltration of immune cells following the loss or inefficiency of CD200 under pathological conditions.

Depression and immunity: a role for T cells?

Much attention has been paid to the potential role of the immune system in the pathophysiology of major depression in humans. While activation of innate immune responses currently dominates the research landscape, early studies in depressed patients demonstrating impairment in acquired immune responses, in particular T cell responses, may warrant further consideration. Intriguing data suggest that activated T cells may play an important neuroprotective role in the context of both stress and inflammation. For example, generation of autoreactive T cells through immunization with central nervous system (CNS) specific antigens has been shown to reverse stress-induced decreases in hippocampal neurogenesis as well as depressive-like behavior in rodents. In addition, trafficking of T cells to the brain following stress, in part related to glucocorticoids, has been found to reduce stress-induced anxiety-like behavior. Data indicate that T regulatory cells may also play a role in depression through downregulation of chronic inflammatory responses. Based on the notion that T cells may subserve neuroprotective and anti-inflammatory functions during stress and inflammation, impaired T cell function may directly contribute to the development of depression. Indeed, increased sensitivity to apoptosis as well as reduced responsiveness to glucocorticoids, may not only decrease the availability of T cells in depressed patients, but also may reduce their capacity to traffic to the brain in response to relevant neuroendocrine or immune stimuli. Further elucidation of T cell pathology may lead to new insights into immune system contributions to depression. Moreover, enhancement of T cell function may represent an alternative strategy to treat depression.

Prior exposure to glucocorticoids sensitizes the neuroinflammatory and peripheral inflammatory responses to E. coli lipopolysaccharide

Acute and chronic stress has been found to sensitize or prime the neuroinflammatory response to both peripheral and central immunologic challenges. Several studies suggest that stress-induced sensitization of neuroinflammatory processes may be mediated by the glucocorticoid (GC) response to stress. GCs, under some conditions, exhibit pro-inflammatory properties, however whether GCs are sufficient to prime neuroinflammatory responses has not been systematically investigated. In the present investigation, we tested whether acute administration of exogenous GCs would be sufficient to reproduce the stress-induced sensitization of neuroinflammatory responses under a number of different timing relationships between GC administration and immune challenge (lipopolysaccharide; LPS). We demonstrate here that GCs potentiate both the peripheral (liver) and central (hippocampus) pro-inflammatory response (e.g. TNFalpha, IL-1beta, IL-6) to a peripheral immune challenge (LPS) if GCs are administered prior (2 and 24h) to challenge. Prior exposure (24h) to GCs also potentiated the pro-inflammatory response of hippocampal microglia to LPS ex vivo. In contrast, when GCs are administered after (1h) a peripheral immune challenge, GCs suppress the pro-inflammatory response to LPS in both liver and hippocampus. GCs also up-regulated microglial activation markers including Toll-like Receptor 2. The present data suggest that the temporal relationship between GC treatment and immune challenge may be an important factor determining whether GCs exhibit pro- or anti-inflammatory properties.

The stressed CNS: when glucocorticoids aggravate inflammation

Glucocorticoids (GCs) are hormones released during the stress response that are well known for their immunosuppressive and anti-inflammatory properties; however, recent advances have uncovered situations wherein they have effects in the opposite direction. The CNS is a particularly interesting example, both because of its unique immune environment, and because GCs affect immune responses differently in different brain regions. In this minireview we discuss the contexts wherein GCs increase CNS inflammation and point out directions for future investigation.

Gene expression changes in the hypothalamus provide evidence for regionally-selective changes in IL-1 and microglial markers after acute stress

Recent work from our laboratory and others has shown that certain stressors increase expression of the pro-inflammatory cytokine interleukin-1beta (IL-1) in the hypothalamus. The first goal of the following studies was to assess the impact of acute stress on other key inflammatory factors, including both cytokines and cell surface markers for immune-derived cells resident to the CNS in adult male Sprague Dawley rats exposed to intermittent footshock (80 shocks, 90 s variable ITI, 5 s each). While scattered changes in IL-6 and GFAP were observed in the hippocampus and cortex, we found the hypothalamus to be exquisitely sensitive to the effects of footshock. At the level of the hypothalamus, mRNA for IL-1 and CD14 were significantly increased, while at the same time CD200R mRNA was significantly decreased. A subsequent experiment demonstrated that propranolol (20mg/kg i.p.) blocked the increase in IL-1 and CD14 mRNA observed in the hypothalamus, while the decrease in CD200R was unaffected by propranolol. Interestingly, inhibition of glucocorticoid synthesis via injection of metyrapone (50mg/kg s.c.) plus aminoglutethimide (100mg/kg s.c.) increased basal IL-1 mRNA and augmented IL-1 and CD14 expression provoked by footshock. Injection of minocycline, a putative microglial inhibitor, blocked the IL-1 response to footshock, while CD14 and CD200R were unaffected. Together, these gene expression changes (i) provide compelling evidence that stress may provoke neuroinflammatory changes that extend well beyond isolated changes in a single cytokine; (ii) suggest opposing roles for classic stress-responsive factors (norepinephrine and corticosterone) in the modulation of stress-related neuroinflammation; (iii) indicate microglia within the hypothalamus may be key players in stress-related neuroinflammation; and (iv) provide a potential mechanism (increased CD14) by which acute stress primes reactivity to later immune challenge.

Spinal glia modulate both adaptive and pathological processes

Recent research indicates that glial cells control complex functions within the nervous system. For example, it has been shown that glial cells contribute to the development of pathological pain, the process of long-term potentiation, and the formation of memories. These data suggest that glial cell activation exerts both adaptive and pathological effects within the CNS. To extend this line of work, the present study investigated the role of glia in spinal learning and spinal learning deficits using the spinal instrumental learning paradigm. In this paradigm rats are transected at the second thoracic vertebra (T2) and given shock to one hind limb whenever the limb is extended (controllable shock). Over time these subjects exhibit an increase in flexion duration that reduces net shock exposure. However, when spinalized rats are exposed to uncontrollable shock or inflammatory stimuli prior to testing with controllable shock, they exhibit a learning deficit. To examine the role of glial in this paradigm, spinal glial cells were pharmacologically inhibited through the use of fluorocitrate. Our results indicate that glia are involved in the acquisition, but not maintenance, of spinal learning. Furthermore, the data indicate that glial cells are involved in the development of both shock and inflammation-induced learning deficits. These findings are consistent with prior research indicating that glial cells are involved in both adaptive and pathological processes within the spinal cord.

Decreased neuronal CD200 expression in IL-4-deficient mice results in increased neuroinflammation in response to lipopolysaccharide

Maintenance of the balance between pro- and anti-inflammatory cytokines in the brain, which is affected by the activation state of microglia, is important for maintenance of neuronal function. Evidence has suggested that IL-4 plays an important neuromodulatory role and has the ability to decrease lipopolysaccharide-induced microglial activation and the production of IL-1beta. We have also demonstrated that CD200-CD200R interaction is involved in immune homeostasis in the brain. Here, we investigated the anti-inflammatory role of IL-4 and, using in vitro and in vivo analysis, established that the effect of lipopolysaccharide was more profound in IL-4(-/-), compared with wildtype, mice. Intraperitoneal injection of lipopolysaccharide exerted a greater inhibitory effect on exploratory behaviour in IL-4(-/-), compared with wildtype, mice and this was associated with evidence of microglial activation. We demonstrate that the increase in microglial activation is inversely related to CD200 expression. Furthermore, CD200 was decreased in neurons prepared from IL-4(-/-) mice, whereas stimulation with IL-4 enhanced CD200 expression. Importantly, neurons prepared from wildtype, but not from IL-4(-/-), mice attenuated the lipopolysaccharide-induced increase in pro-inflammatory cytokine production by glia. These findings suggest that the neuromodulatory effect of IL-4, and in particular its capacity to maintain microglia in a quiescent state, may result from its ability to upregulate CD200 expression on neurons.

The 5-lipoxygenase as a common pathway for pathological brain and vascular aging

Epidemiological studies indicate age as a strong risk factor for developing cardiovascular and neurodegenerative diseases. During the aging process, changes in the expression of particular genes can influence the susceptibility to these diseases. 5-Lipoxygenase (5-LO) by oxidizing fatty acids forms leukotrienes, potent mediators of oxidative and inflammatory reactions, two key pathogenic events in both clinical settings. This enzyme is widely distributed in the cardiovascular as well as in the central nervous system, where its expression levels increase with age, suggesting that it may be involved in their diseases of aging. The central theme of this article is that during aging, 5-LO acts as biologic link between different stressors and the development of cardiovascular and neurodegenerative diseases. We hypothesize that the age-dependent upregulation of 5-LO represents a "priming" factor in the vasculature as well as in the brain, where a subsequent exposure to triggering stimuli (i.e., infections) leads to an abnormal chronic inflammatory reaction, and ultimately results in increased organ vulnerability and functional deficits.

Inflammation and its discontents: the role of cytokines in the pathophysiology of major depression

Recognition that inflammation may represent a common mechanism of disease has been extended to include neuropsychiatric disorders including major depression. Patients with major depression have been found to exhibit increased peripheral blood inflammatory biomarkers, including inflammatory cytokines, which have been shown to access the brain and interact with virtually every pathophysiologic domain known to be involved in depression, including neurotransmitter metabolism, neuroendocrine function, and neural plasticity. Indeed, activation of inflammatory pathways within the brain is believed to contribute to a confluence of decreased neurotrophic support and altered glutamate release/reuptake, as well as oxidative stress, leading to excitotoxicity and loss of glial elements, consistent with neuropathologic findings that characterize depressive disorders. Further instantiating the link between inflammation and depression are data demonstrating that psychosocial stress, a well-known precipitant of mood disorders, is capable of stimulating inflammatory signaling molecules, including nuclear factor kappa B, in part, through activation of sympathetic nervous system outflow pathways. Interestingly, depressed patients with increased inflammatory biomarkers have been found to be more likely to exhibit treatment resistance, and in several studies, antidepressant therapy has been associated with decreased inflammatory responses. Finally, preliminary data from patients with inflammatory disorders, as well as medically healthy depressed patients, suggest that inhibiting proinflammatory cytokines or their signaling pathways may improve depressed mood and increase treatment response to conventional antidepressant medication. Translational implications of these findings include the unique opportunity to identify relevant patient populations, apply immune-targeted therapies, and monitor therapeutic efficacy at the level of the immune system in addition to behavior.

Role of spinal microglia in visceral hyperalgesia and NK1R up-regulation in a rat model of chronic stress

BACKGROUND & AIMS

Chronic psychological stress is associated with visceral hyperalgesia and increased expression of spinal NK1 receptors (NK1Rs). We aimed to identify the role of spinal microglia in this process.

METHODS

Male Wistar rats were exposed to water avoidance (WA) or sham stress 1 hour each day for 10 days and given daily injections of minocycline, the p38 inhibitor SB203580, or saline. Phosphorylation levels of the kinase p38 (P-p38), the microglia marker OX42, NK1R, and IkappaBalpha were assessed by immunoblotting and/or immunostaining of spinal samples collected at day 11. The visceromotor response to colorectal distention at baseline and following WA were also assayed in rats given injections of minocycline, SB203580, or vehicle. The effects of fractalkine were assessed on the visceromotor response in rats exposed to minocycline or vehicle.

RESULTS

P-p38 protein levels and immunoreactivity were increased in stressed rats and colocalized with OX42-positive cells and neurons in the dorsal horn. This increase was reversed by minocycline or SB203580 exposure. Stress-induced increased NK1R expression was blocked by minocycline but not SB203580. WA-induced decreased IkappaBalpha expression was blocked by minocycline and SB203580. WA-induced hyperalgesia was blocked by minocycline and SB203580 intrathecally. Fractalkine-induced hyperalgesia was blocked by minocycline.

CONCLUSIONS

This is the first demonstration that stress-induced activation of spinal microglia has a key role in visceral hyperalgesia and associated spinal NK1R up-regulation.

Neuroinflammatory changes increase the impact of stressors on neuronal function

In the last few years, several research groups have reported that neuroinflammation is one feature common to several neurodegenerative diseases and that similar, although perhaps less profound, neuroinflammatory changes also occur with age. Age is the greatest risk factor in many neurodegenerative diseases, and the possibility exists that the underlying age-related neuroinflammation may contribute to this increased risk. Several animal models have been used to examine this possibility, and it is now accepted that, under experimental conditions in which microglial activation is up-regulated, responses to stressors are exacerbated. In the present article, these findings are discussed and data are presented from in vitro and in vivo experiments which reveal that responses to Abeta (amyloid beta-peptide) are markedly up-regulated in the presence of LPS (lipopolysaccharide). These, and previous findings, point to a vulnerability associated with inflammation and suggest that, even though inflammation may not be the primary cause of neurodegenerative disease, its treatment may decelerate disease progression.

Interleukin-1 (IL-1): a central regulator of stress responses

Ample evidence demonstrates that the pro-inflammatory cytokine interleukin-1 (IL-1), produced following exposure to immunological and psychological challenges, plays an important role in the neuroendocrine and behavioral stress responses. Specifically, production of brain IL-1 is an important link in stress-induced activation of the hypothalamus-pituitary-adrenal axis and secretion of glucocorticoids, which mediate the effects of stress on memory functioning and neural plasticity, exerting beneficial effects at low levels and detrimental effects at high levels. Furthermore, IL-1 signaling and the resultant glucocorticoid secretion mediate the development of depressive symptoms associated with exposure to acute and chronic stressors, at least partly via suppression of hippocampal neurogenesis. These findings indicate that whereas under some physiological conditions low levels of IL-1 promote the adaptive stress responses necessary for efficient coping, under severe and chronic stress conditions blockade of IL-1 signaling can be used as a preventive and therapeutic procedure for alleviating stress-associated neuropathology and psychopathology.

Cytokines and their relationship to the symptoms and outcome of cancer

Tumours contain immune cells and a network of pro- and anti-inflammatory cytokines, which collaborate in the development and progression of cancer. Cytokine profiles might prove to be prognostic. The systemic effects of pro-inflammatory cytokines are associated with fatigue, depression and cognitive impairment, and can affect quality of life before, during and after treatment. In people with advanced cancer, pro-inflammatory cytokines are additionally associated with anorexia and cachexia, pain, toxicity of treatment and resistance to treatment. However, physical activity might modify cytokine levels and decrease fatigue in patients with cancer, and might also improve their prognosis.

Panax Notoginseng Burk attenuates impairment of learning and memory functions and increases ED1, BDNF and beta-secretase immunoreactive cells in chronic stage ischemia-reperfusion injured rats

Panax Notoginseng Burk (PN) has been reported to improve blood circulation, as well as learning and memory functions. The purpose of the present study was to investigate the effect of PN on learning and memory functions in chronic cerebral infarct rats. A cerebral infarct animal model was established by blocking the blood flow of both common carotid arteries and right middle cerebral artery for 90 min followed by reperfusion for 4 weeks. PN (0.5 g/kg) was administered orally 3 days per week for 4 weeks, whereas the control group provided bait and water only. The learning and memory functions were estimated by measuring how successful rats were able to negotiate an 8-arm radial maze test; the test was performed after operation once a week for 4 weeks. Finally, the rats were sacrificed and their brains were removed. The brains were sectioned and analyzed for ED1, glial fibrillary acid protein (GFAP), nuclear factor-kappaB, and brain derivative neurotrophin factor (BDNF) and beta-secretase by immunostaining. Cerebral infarct rats given PN were able to successfully navigate the 8-arm radial maze test four weeks after cerebral infarction. PN also increased ED1, BDNF and beta-secretase immunoreactive cells, but did not increase GFAP and NF-kappaB immunoreactive cells. PN attenuated the reduction in learning and memory functions induced by cerebral infarction in cerebral ischemia-reperfusion injured rats; it also increased the amount of activated microglia and BDNF. These data suggest that the effect of PN, at least in part, is closely related to the increase in BDNF that was generated by activated microglia. The effect that PN has on astrocytes, NF-kappaB and beta-secreatase immunoreactive cells requires further study.

Potential for early-life immune insult including developmental immunotoxicity in autism and autism spectrum disorders: focus on critical windows of immune vulnerability

Early-life immune insults (ELII) including xenobiotic-induced developmental immunotoxicity (DIT) are important factors in childhood and adult chronic diseases. However, prenatal and perinatal environmentally induced immune alterations have yet to be considered in depth in the context of autism and autism spectrum disorders (ASDs). Numerous factors produce early-life-induced immune dysfunction in offspring, including exposure to xenobiotics, maternal infections, and other prenatal-neonatal stressors. Early life sensitivity to ELII, including DIT, results from the heightened vulnerability of the developing immune system to disruption and the serious nature of the adverse outcomes arising after disruption of one-time immune maturational events. The resulting health risks extend beyond infectious diseases, cancer, allergy, and autoimmunity to include pathologies of the neurological, reproductive, and endocrine systems. Because these changes may include misregulation of resident inflammatory myelomonocytic cells in tissues such as the brain, they are a potential concern in cases of prenatal-neonatal brain pathologies and neurobehavioral deficits. Autism and ASDs are chronic developmental neurobehavioral disorders that are on the rise in the United States with prenatal and perinatal environmental factors suspected as contributors to this increase. Evidence for an association between environmentally associated childhood immune dysfunction and ASDs suggests that ELII and DIT may contribute to these conditions. However, it is not known if this linkage is directly associated with the brain pathologies or represents a separate (or secondary) outcome. This review considers the known features of ELII and DIT and how they may provide important clues to prenatal brain inflammation and the risk of autism and ASDs.

Cognitive and neuroinflammatory consequences of mild repeated stress are exacerbated in aged mice

Peripheral immune stimulation as well as certain types of psychological stress increases brain levels of inflammatory cytokines such as interleukin-1beta (IL-1beta), IL-6 and tumor necrosis factor alpha (TNFalpha). We have demonstrated that aged mice show greater increases in central inflammatory cytokines, as well as greater cognitive deficits, compared to adults in response to peripheral lipopolysaccharide (LPS) administration. Because aged mice are typically more sensitive to systemic stressors such as LPS, and certain psychological stressors induce physiological responses similar to those that follow LPS, we hypothesized that aged mice would be more sensitive to the physiological and cognitive effects of mild stress than adult mice. Here, adult (3-5 months) and aged (22-23 months) male BALB/c mice were trained in the Morris water maze for 5 days. Mice were then exposed to a mild restraint stress of 30 min before being tested in a working memory version of the water maze over a 3-day period. On day 4 mice were stressed and then killed for collection of blood and brain. In a separate group of animals, mice were killed immediately after one, two or three 30 min restraint sessions and blood was collected for peripheral corticosterone and cytokine protein measurement, and brains were dissected for central cytokine mRNA measurement. Stress disrupted spatial working memory in both adult and aged mice but to a much greater extent in the aged mice. In addition, aged mice showed an increase in stress-induced expression of hippocampal IL-1beta mRNA and MHC class II protein compared to non-stressed controls while expression in adult mice was unaffected by stress. These data show that aged mice are more sensitive to both the cognitive and inflammatory effects of mild stress than are adult mice and suggest a possible role for IL-1beta.

A link between stress and depression: shifts in the balance between the kynurenine and serotonin pathways of tryptophan metabolism and the etiology and pathophysiology of depression

Alteration of tryptophan (TRP) metabolism elicited by proinflammatory cytokines has gained attention as a new concept to explain the etiological and pathophysiological mechanisms of major depression. The kynurenine (KYN) pathway, which is initiated by indoleamine 2,3-dioxygenase (IDO), is the main TRP metabolic pathway. It shares TRP with the serotonin (5-HT) pathway. Proinflammatory cytokines induce IDO under stress, promote the KYN pathway, deprive the 5-HT pathway of TRP, and reduce 5-HT synthesis. The resultant decrease in 5-HT production may relate to the monoamine hypothesis of major depression. Furthermore, metabolites of the KYN pathway have neurotoxic/neuroprotective activities; 3-hydroxykynurenine and quinolinic acid are neurotoxic, whereas kynurenic acid is neuroprotective. The hippocampal atrophy that appears in chronic depression may be associated with imbalances in neurotoxic/neuroprotective activities. Because proinflammatory cytokines also activate the hypothalamo-pituitary-adrenal (HPA) axis, these imbalances may inhibit the hippocampal negative feedback system. Thus, changes in the TRP metabolism may also relate to the HPA axis-hyperactivity hypothesis of major depression. In this article, we review the changes in TRP metabolism by proinflammatory cytokines under stress, which is assumed to be a risk factor for major depression, and the relationship between physiological risk factors for major depression and proinflammatory cytokines.

The risky business of ageing

This commentary reflects on the work by Chen and colleagues which compares the effect of an immune challenge in hippocampus and hippocampal-dependent function young and aged mice.

Neuroendocrine-immune mechanisms of behavioral comorbidities in patients with cancer

Patients with cancer experience a host of behavioral alterations that include depression, fatigue, sleep disturbances, and cognitive dysfunction. These behavioral comorbidities are apparent throughout the process of diagnosis and treatment for cancer and can persist well into the survivorship period. There is a rich literature describing potential consequences of behavioral comorbidities in patients with cancer including impaired quality of life, reduced treatment adherence, and increased disease-related morbidity and mortality. Medical complications of cancer and its treatment such as anemia, thyroid dysfunction, and the neurotoxicity of cancer chemotherapeutic agents account in part for these behavioral changes. Nevertheless, recent advances in the neurosciences and immunology/oncology have revealed novel insights into additional pathophysiologic mechanisms that may significantly contribute to the development of cancer-related behavioral changes. Special attention has been focused on immunologic processes, specifically activation of innate immune inflammatory responses and their regulation by neuroendocrine pathways, which, in turn, influence CNS functions including neurotransmitter metabolism, neuropeptide function, sleep-wake cycles, regional brain activity, and, ultimately, behavior. Further understanding of these immunologic influences on the brain provides a novel conceptual framework for integrating the wide spectrum of behavioral alterations that occur in cancer patients and may reveal a more focused array of translational targets for therapeutic interventions and future research. Such developments warrant complementary advances in identification of cancer patients at risk as well as those currently suffering, including an increased emphasis on the status of behavior as a "sixth vital sign" to be assessed in all cancer patients throughout their disease encounter.

Neuroinflammation associated with aging sensitizes the brain to the effects of infection or stress

The aging brain is characterized by a shift from the homeostatic balance of inflammatory mediators to a proinflammatory state. This increase in neuroinflammation is marked by increased numbers of activated and primed microglia, increased steady-state levels of inflammatory cytokines and decreases in anti-inflammatory molecules. These conditions sensitize the aged brain to produce an exaggerated response to the presence of an immune stimulus in the periphery or following exposure to a stressor. In the brain, proinflammatory cytokines can have profound effects on behavioral and neural processes. As the aged brain is primed to respond to inflammatory stimuli, infection or stress may produce more severe detriments in cognitive function in the aged. Typically after an immune stimulus, aged animals display prolonged sickness behaviors, increased cytokine induction and greater cognitive impairments compared to adults. Additionally, aging can also augment the central response to stressors leading to exaggerated cytokine induction and increased decrements in learning and memory. This alteration in neuroinflammation and resultant sensitization to extrinsic and intrinsic stressors can have considerable effects upon the elderly's recovery and coping during disease and stress.

Neurobiology of depression: an integrated view of key findings

AIMS

The objectives of the present review were to summarise the key findings from the clinical literature regarding the neurobiology of major depressive disorder (MDD) and their implications for maximising treatment outcomes. Several neuroanatomical structures in the prefrontal and limbic areas of the brain are involved in affective regulation. In patients with MDD, alterations in the dynamic patterns of activity among these structures have profound implications for the pathogenesis of this illness.

DISCUSSION

The present work reviews the evidence for the progressive nature of MDD along with associated changes in neuroanatomical structure and function, especially for the hippocampus. The role of glucocorticoids, inflammatory cytokines and brain-derived growth factors are discussed as mediators of these pathological alterations. From this integrated model, the role of antidepressant therapy in restoring normative processes is examined along with additional treatment guidelines.

CONCLUSION

Major depressive disorder is an illness with significant neurobiological consequences involving structural, functional and molecular alterations in several areas of the brain. Antidepressant pharmacotherapy is associated with restoration of the underlying physiology. Clinicians are advised to intervene with MDD using an early, comprehensive treatment approach that has remission as the goal.

Effects of acute and repeated exposure to lipopolysaccharide on cytokine and corticosterone production during remyelination

Chronic exposure to the copper-chelating agent, cuprizone (CPZ), is an increasingly popular model for producing demyelination. More importantly, cessation of cuprizone exposure allows for full remyelination, which represents a window of opportunity for determining the influence of environmental factors on regenerative processes. In the present study, CPZ-treated animals were assessed for functional status of systemic and central cytokine responsiveness to LPS, as well as assessment for signs of body weight changes. Exposure of male C57BL/6J mice to 5 weeks of 0.2% CPZ in the diet was optimal in producing demyelination and microglial activation, as measured by myelin basic protein, CD11b, and CD45 immunohistochemistry. Acute challenge with LPS at the end of 5 weeks CPZ treatment did not alter IL-1beta, IL-6, nor TNFalpha responses in the spleen and corpus callosum. Similarly, repeated exposure to LPS during the remyelination phase (CPZ removal) did not influence these measures to LPS. Plasma corticosterone was unaffected following acute challenge of CPZ-pretreated animals, but after repeated LPS treatment, there was a significant augmentation of the corticosterone response in CPZ-pretreated mice. Interestingly, the basal concentration of IL-1beta in the corpus callosum of CPZ treated animals was significantly increased, which was in keeping with the increase in activated microglial cells. In conclusion, the cuprizone model of demyelination and remyelination does not appear to influence the systemic nor central IL-1, IL-6, and TNF responses to acute nor repeated LPS. This opens up the possibility for studying the contribution of systemic inflammatory processes on remyelination after cessation of CPZ treatment.

Astroglial glutamate-glutamine shuttle is involved in central sensitization of nociceptive neurons in rat medullary dorsal horn

Growing evidence suggests that astroglia are involved in pain states, but no studies have tested their possible involvement in modulating the activity of nociceptive neurons per se. This study has demonstrated that the central sensitization induced in functionally identified nociceptive neurons in trigeminal subnucleus caudalis (the medullary dorsal horn) by application of an inflammatory irritant to the rat's tooth pulp can be significantly attenuated by continuous intrathecal superfusion of methionine sulfoximine (MSO; 0.1 mM), an inhibitor of the astroglial enzyme glutamine synthetase that is involved in the glutamate-glutamine shuttle. Simultaneous superfusion of MSO and glutamine (0.25 mM) restored the irritant-induced central sensitization. In control experiments, superfusion of either MSO or glutamine alone, or vehicle, did not produce any significant changes in neuronal properties. These findings suggest that the astroglial glutamate-glutamine shuttle is essential for the initiation of inflammation-induced central sensitization but that inhibition of astroglial function may not affect normal nociceptive processing.

Molecular and cellular characterization of the age-related neuroinflammatory processes occurring in normal rat hippocampus: potential relation with the loss of somatostatin GABAergic neurons

Increased neuroinflammatory reaction is frequently observed during normal brain aging. However, a direct link between neuroinflammation and neurodegeneration during aging has not yet been clearly shown. Here, we have characterized the age-related hippocampal inflammatory processes and the potential relation with hippocampal neurodegeneration. The mRNA expression of the pro-inflammatory cytokines IL-1beta and tumor necrosis factor-alpha (TNF-alpha), and the iNOs enzyme was significantly increased in aged hippocampus. Accordingly, numerous activated microglial cells were observed in aged rats. These cells were differentially distributed along the hippocampus, being more frequently located in the hilus and the CA3 area. The mRNA expression of somatostatin, a neuropeptide expressed by some GABAergic interneurons, and the number of somatostatin-immunopositive cells decreased in aged rats. However, the number of hippocampal parvalbumin-containing GABAergic interneurons was preserved. Interestingly, in aged rats, the mRNA expression of somatostatin and IL-1beta was inversely correlated and, the decrease in the number of somatostatin-immunopositive cells was higher in the hilus of dentate gyrus than in the CA1 region. Finally, intraperitoneal chronic lipopolysaccharide (LPS) injection in young animals mimicked the age-related hippocampal inflammation as well as the decrease of somatostatin mRNA expression. Present results strongly support the neuroinflammation as a potential factor involved in the age-related degeneration of somatostatin GABAergic cells.

An inflammatory review of glucocorticoid actions in the CNS

In recent years, the classic view that glucocorticoids, the adrenal steroids secreted during stress, are universally anti-inflammatory has been challenged at a variety of levels. It was first observed that under some circumstances, acute GC exposure could have pro-inflammatory effects on the peripheral immune response. More recently, chronic exposure to GCs has been found to have pro-inflammatory effects on the specialized immune response to injury in the central nervous system. Here we review the evidence that in some cases, glucocorticoids can increase pro-inflammatory cell migration, cytokine production, and even transcription factor activity in the brain. We consider how these unexpected effects of glucocorticoids can co-exist with their well-established anti-inflammatory properties, as well as the considerable clinical implications of these findings.

Modulation of microglial pro-inflammatory and neurotoxic activity for the treatment of Parkinson's disease

Parkinson's disease (PD) is a debilitating movement disorder resulting from a progressive degeneration of the nigrostriatal dopaminergic pathway and depletion of neurotransmitter dopamine in the striatum. Molecular cloning studies have identified nearly a dozen genes or loci that are associated with small clusters of mostly early onset and genetic forms of PD. The etiology of the vast majority of PD cases remains unknown, and the precise molecular and biochemical processes governing the selective and progressive degeneration of the nigrostriatal dopaminergic pathway are poorly understood. Current drug therapies for PD are symptomatic and appear to bear little effect on the progressive neurodegenerative process. Studies of postmortem PD brains and various cellular and animal models of PD in the last 2 decades strongly suggest that the generation of pro-inflammatory and neurotoxic factors by the resident brain immune cells, microglia, plays a prominent role in mediating the progressive neurodegenerative process. This review discusses literature supporting the possibility of modulating the activity of microglia as a neuroprotective strategy for the treatment of PD.