The subpopulation of microglia sensitive to neurotransmitters/neurohormones is modulated by stimulation with LPS, interferon-γ, and IL-4

Effects of hypernatremia on the microglia

Signs and symptoms of hypernatremia largely indicate central nervous system dysfunction. Acute hypernatremia can cause demyelinating lesions similar to that observed in osmotic demyelination syndrome (ODS). We have previously demonstrated that microglia accumulate in ODS lesions and minocycline protects against ODS by inhibiting microglial activation. However, the direct effect of rapid rise in the sodium concentrations on microglia is largely unknown. In addition, the effect of chronic hypernatremia on microglia also remains elusive. Here, we investigated the effects of acute (6 or 24 h) and chronic (the extracellular sodium concentration was increased gradually for at least 7 days) high sodium concentrations on microglia using the microglial cell line, BV-2. We found that both acute and chronic high sodium concentrations increase NOS2 expression and nitric oxide (NO) production. We also demonstrated that the expression of nuclear factor of activated T-cells-5 (NFAT5) is increased by high sodium concentrations. Furthermore, NFAT5 knockdown suppressed NOS2 expression and NO production. We also demonstrated that high sodium concentrations decreased intracellular Ca2+ concentration and an inhibitor of Na+/Ca2+ exchanger, NCX, suppressed a decrease in intracellular Ca2+ concentrations and NOS2 expression and NO production induced by high sodium concentrations. Furthermore, minocycline inhibited NOS2 expression and NO production induced by high sodium concentrations. These in vitro data suggest that microglial activity in response to high sodium concentrations is regulated by NFAT5 and Ca2+ efflux through NCX and is suppressed by minocycline.

A programmable microfluidic platform to monitor calcium dynamics in microglia during inflammation

Neuroinflammation is characterized by the elevation of cytokines and adenosine triphosphate (ATP), which in turn activates microglia. These immunoregulatory molecules typically form gradients in vivo, which significantly influence microglial behaviors such as increasing calcium signaling, migration, phagocytosis, and cytokine secretion. Quantifying microglial calcium signaling in the context of inflammation holds the potential for developing precise therapeutic strategies for neurological diseases. However, the current calcium imaging systems are technically challenging to operate, necessitate large volumes of expensive reagents and cells, and model immunoregulatory molecules as uniform concentrations, failing to accurately replicate the in vivo microenvironment. In this study, we introduce a novel calcium monitoring micro-total analysis system (CAM-μTAS) designed to quantify calcium dynamics in microglia (BV2 cells) within defined cytokine gradients. Leveraging programmable pneumatically actuated lifting gate microvalve arrays and a Quake valve, CAM-μTAS delivers cytokine gradients to microglia, mimicking neuroinflammation. Our device automates sample handling and cell culture, enabling rapid media changes in just 1.5 s, thus streamlining the experimental workflow. By analyzing BV2 calcium transient latency to peak, we demonstrate location-dependent microglial activation patterns based on cytokine and ATP gradients, offering insights contrasting those of non-gradient-based perfusion systems. By harnessing advancements in microsystem technology to quantify calcium dynamics, we can construct simplified human models of neurological disorders, unravel the intricate mechanisms of cell-cell signaling, and conduct robust evaluations of novel therapeutics.

A One-Stone-Two-Birds Strategy of Targeting Microbubbles with "Dual" Anti-Inflammatory and Blood-Brain Barrier "Switch" Function for Ischemic Stroke Treatment

Inflammation is considered to be the main target of the development of new stroke therapies. There are three key issues in the treatment of stroke inflammation: the first one is how to overcome the blood-brain barrier (BBB) to achieve drug delivery, the second one is how to select drugs to treat stroke inflammation, and the third one is how to achieve targeted drug delivery. In this study, we constructed hydrocortisone-phosphatidylserine microbubbles and combined them with ultrasound (US)-targeted microbubble destruction technology to successfully open the BBB to achieve targeted drug delivery. Phosphatidylserine on the microbubbles was used for its "eat me" effect to increase the targeting of the microvesicles. In addition, we found that hydrocortisone can accelerate the closure of the BBB, achieving efficient drug delivery while reducing the entry of peripheral toxins into the brain. In the treatment of stroke inflammation, it was found that hydrocortisone itself has anti-inflammatory effects and can also change the polarization of microglia from the harmful pro-inflammatory M1 phenotype to the beneficial anti-inflammatory M2 phenotype, thus achieving dual anti-inflammatory effects and enhancing the anti-inflammatory effects in ischemic areas after stroke, well reducing the cerebellar infarction volume by inhibiting the inflammatory response after cerebral ischemia. A confocal microendoscope was used to directly observe the polarization of microglial cells in living animal models for dynamic microscopic visualization detection showing the advantage of being closer to clinical work. Taken together, this study constructed a multifunctional targeted US contrast agent with the function of "one-stone-two-birds", which can not only "on-off" the BBB but also have "two" anti-inflammatory functions, providing a new strategy of integrated anti-inflammatory targeted delivery and imaging monitoring for ischemic stroke treatment.

Microglia heterogeneity in health and disease

Microglia, the resident immune cells of the central nervous system (CNS), have received significant attention due to their critical roles in maintaining brain homeostasis and mediating cerebral immune responses. Understanding the origin of microglia has been a subject of great interest, and emerging evidence suggests that microglia consist of multiple subpopulations with unique molecular and functional characteristics. These subpopulations of microglia may exhibit specialized roles in response to different environmental cues as in disease conditions. The newfound understanding of microglial heterogeneity has significant implications for elucidating their roles in both physiological and pathological conditions. In the context of disease, microglia have been studied rigorously as they play a very important role in neuroinflammation. Dysregulated microglial activation and function contribute to chronic inflammation. Further exploration of microglial heterogeneity and their interactions with other cell types in the CNS will undoubtedly pave the way to novel therapeutic strategies targeting microglia-mediated pathologies. In this review, we discuss the latest advances in the field of microglia research, focusing specifically on the origin and subpopulations of microglia, the populations of microglia types in the brains of patients with neurodegenerative diseases, and how microglia are regulated in the healthy CNS.

Receptors on Microglia

Microglial cells are the most receptive cells in the central nervous system (CNS), expressing several classes of receptors reflecting their immune heritage and newly acquired neural specialisation. Microglia possess, depending on the particular context, receptors to neurotransmitters and neuromodulators as well as immunocompetent receptors. This rich complement allows microglial cells to monitor the functional status of the nervous system, contribute actively to the regulation of neural activity and plasticity and homeostasis, and guard against pathogens as well as other challenges to the CNS's integrity and function.

Microglia at the Tripartite Synapse during Postnatal Development: Implications for Autism Spectrum Disorders and Schizophrenia

Synapses are the fundamental structures of neural circuits that control brain functions and behavioral and cognitive processes. Synapses undergo formation, maturation, and elimination mainly during postnatal development via a complex interplay with neighboring astrocytes and microglia that, by shaping neural connectivity, may have a crucial role in the strengthening and weakening of synaptic functions, that is, the functional plasticity of synapses. Indeed, an increasing number of studies have unveiled the roles of microglia and astrocytes in synapse formation, maturation, and elimination as well as in regulating synaptic function. Over the past 15 years, the mechanisms underlying the microglia- and astrocytes-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of these glial cells in early postnatal development may underlie the cause of synaptic dysfunction that leads to neurodevelopmental disorders such as autism spectrum disorder (ASD) and schizophrenia.

Dopaminergic regulation of inflammation and immunity in Parkinson's disease: friend or foe?

Parkinson's disease (PD) is a neurodegenerative disease affecting 7-10 million people worldwide. Currently, there is no treatment available to prevent or delay PD progression, partially due to the limited understanding of the pathological events which lead to the death of dopaminergic neurons in the substantia nigra in the brain, which is known to be the cause of PD symptoms. The current available treatments aim at compensating dopamine (DA) deficiency in the brain using its precursor levodopa, dopaminergic agonists and some indirect dopaminergic agents. The immune system is emerging as a critical player in PD. Therefore, immune-based approaches have recently been proposed to be used as potential antiparkinsonian agents. It has been well-known that dopaminergic pathways play a significant role in regulating immune responses in the brain. Although dopaminergic agents are the primary antiparkinsonian treatments, their immune regulatory effect has yet to be fully understood. The present review summarises the current available evidence of the immune regulatory effects of DA and its mimics and discusses dopaminergic agents as antiparkinsonian drugs. Based on the current understanding of their involvement in the regulation of neuroinflammation in PD, we propose that targeting immune pathways involved in PD pathology could offer a better treatment outcome for PD patients.

Failure of Alzheimer’s Mice Brain Resident Neural Precursor Cells in Supporting Microglia-Mediated Amyloid β Clearance

The failure of brain microglia to clear excess amyloid β (Aβ) is considered a leading cause of the progression of Alzheimer’s disease pathology. Resident brain neural precursor cells (NPCs) possess immune-modulatory and neuro-protective properties, which are thought to maintain brain homeostasis. We have recently showed that resident mouse brain NPCs exhibit an acquired decline in their trophic properties in the Alzheimer’s disease brain environment. Therefore, we hypothesized that functional NPCs may support microglial phagocytic activity, and that NPCs derived from the adult AD mouse brain may fail to support the clearance of Aβ by microglia. We first identified in the AD brain, in vivo and ex vivo, a subpopulation of microglia that express high Aβ phagocytic activity. Time-lapse microscopy showed that co-culturing newborn NPCs with microglia induced a significant increase in the fraction of microglia with high Aβ phagocytic activity. Freshly isolated NPCs from adult wild type, but not AD, mouse brain, induced an increase in the fraction of microglia with high Aβ phagocytic activity. Finally, we showed that NPCs also possess the ability to promote Aβ degradation within the microglia with high Aβ phagocytic activity. Thus, resident brain NPCs support microglial function to clear Aβ, but NPCs derived from the AD environment fail to do so. We suggest that the failure of AD brain NPCs to support Aβ clearance from the brain by microglia may accelerate disease pathology.

Microglia form satellites with different neuronal subtypes in the adult murine central nervous system

Microglia are the innate immune cells of the central nervous system (CNS). In the adult uncompromised CNS, they have a highly ramified morphology and continuously extend and retract their processes. A subpopulation of microglial cells forms close soma-to-soma contacts with neurons and have been termed satellite microglia, yet the role of such interaction is largely unknown. Here, we analyzed the distribution of satellite microglia in different areas of the CNS of adult male mice applying transgenic- and immunolabeling of neuronal subtypes and microglia followed by three-dimensional imaging analysis. We quantified satellite microglia associated with GABAergic and glutamatergic neurons in the somatosensory cortex, striatum, and thalamus; with dopaminergic and serotonergic neurons in the basal forebrain and raphe nucleus, respectively; and with cerebellar Purkinje cell neurons. Satellite microglia in the retina were assessed qualitatively. Microglia form satellites with all neuronal subtypes studied, whereas a preference for a specific neuron subtype was not found. The occurrence and frequency of satellite microglia is determined by the histo-architectural organization of the brain area and the densities of neuronal somata therein.

Histamine and Microglia

Microglia, a category of glial cells in the central nervous system (CNS), have attracted much attention because of their important role in neuroinflammation. Many translational studies are currently ongoing to discover novel drugs targeting microglia for the treatment of various CNS disorders, such as Alzheimer's disease, Parkinson's disease (PD), and depression. Recent studies have shown that brain histamine, a neurotransmitter essential for the regulation of diverse brain functions, controls glial cells and neurons. In vitro studies using primary microglia and microglial cell lines have reported that histamine receptors are expressed in microglia and control microglial functions, including chemotaxis, migration, cytokine secretion, and autophagy. In vivo studies have demonstrated that histamine-related reagents could ameliorate abnormal symptoms in animal models of human diseases, such as amyotrophic lateral sclerosis (ALS), PD, and brain ischemia. Several human studies have revealed alterations in histamine receptor levels in ALS and PD, emphasizing the importance of the CNS histamine system, including histamine-dependent microglial modulation, as a therapeutic target for these disorders. In this review article, we summarize histamine-related research focusing on microglial functions.

Microglia sense neuronal activity via GABA in the early postnatal hippocampus

Microglia, the resident macrophages in the central nervous system, express receptors for classical neurotransmitters, such as γ-aminobutyric acid (GABA) and glutamate, suggesting that they sense synaptic activity. To detect microglial Ca²⁺ responses to neuronal activity, we generate transgenic mouse lines expressing the fluorescent Ca²⁺ indicator GCaMP6m, specifically in microglia and demonstrate that electrical stimulation of the Schaffer collateral pathway results in microglial Ca²⁺ responses in early postnatal but not adult hippocampus. Preceding the microglial responses, we also observe similar Ca²⁺ responses in astrocytes, and both are sensitive to tetrodotoxin. Blocking astrocytic glutamate uptake or GABA transport abolishes stimulation-induced microglial responses as well as antagonizing the microglial GABA_B receptor. Our data, therefore, suggest that the neuronal activity-induced glutamate uptake and the release of GABA by astrocytes trigger the activation of GABA_B receptors in microglia. This neuron, astrocyte, and microglia communication pathway might modulate microglial activity in developing neuronal networks.

Age and sex differences in primary microglia culture: A comparative study

BACKGROUND

Microglia play a central role in neuroinflammation in various CNS diseases.Neonatal microglial culture has been extensively used to in vitro study microglial activation; however, as many neuroinflammatory diseases occur in the elderly, the neonatal microglial culture may not fully replicate the aged microglial activity seen in these diseases.

NEW METHOD

Primary microglia from both 18-24-month-old and P0-P4 C57BL/6 mice were cultured simultaneously. Morphology and activation profiles of the two age groups of microglia were examined following ischemic stimulation, by ELISA, RT-PCR, live microscopy, immunocytochemistry, and Western blotting.

RESULTS

We showed that aged microglia had larger cell bodies, more cytoplasmic inclusions, and enhanced phagocytosis than neonatal microglia. Cytokine production in these cells exhibited heterogeneity either after or before ischemic stimulation. The baseline expression of microglial marker CD11b was significantly higher in aged vs. neonatal cells; ischemic stimulation increased the expression in neonatal vs. aged microglia only in males but not in females.

COMPARISON WITH EXISTING METHODS

Previous primary microglia cultures have been limited to using neonatal/adult cells. This method is complementary to exiting methods and works for aged microglia, and does not suffer from potential limitations due to filtering artifacts. The protocol renders microglial culture no need for meningeal/hippocampal removal prior to brain tissue dissociation, and compares microglia between males vs. females, and between the aged vs. neonates.

CONCLUSIONS

We concluded that neonatal microglial culture is not appropriate for those in vitro studies that mimic the neuroinflammatory central nervous system disorders occurring in the elderly, in which case the aged microglial culture should be applied, and sex differences should be considered.

Understanding microglial diversity and implications for neuronal function in health and disease

Genetic data implicate microglia as central players in brain health and disease, urging the need to better understand what microglia do in the brain. Microglia are critical partners in neuronal wiring and function during development and disease. Emerging literature suggests that microglia have diverse functional roles, raising the intriguing question of which functions of microglia become impaired in disease to undermine proper neuronal function. It is also becoming increasingly clear that microglia exist in heterogeneous cell states. Microglial cell states appear context-dependent, that is, age, sex, location, and health of their microenvironment; these are further influenced by external signaling factors including gut microbiota and lipid metabolites. These data altogether suggest that microglia exist in functional clusters that impact, and are impacted by, surrounding neuronal microenvironment. However, we still lack understanding of how we can translate microglia cell states into function. Here, we summarize the state-of-the-art on the diverse functions of microglia in relation to neuronal health. Then, we discuss heterogeneity during developing, healthy adult and diseased brains, and whether this may be predetermined by origin and/or regulated by local milieu. Finally, we propose that it is critical to gain high-resolution functional discernment into microglia-neuron interactions while preserving the spatial architecture of the tissue. Such insight will reveal specific targets for biomarker and therapeutic development toward microglia-neuron crosstalk in disease.

Hypothalamic neuropeptides and neurocircuitries in Prader Willi syndrome

Prader-Willi Syndrome (PWS) is a rare and incurable congenital neurodevelopmental disorder, resulting from the absence of expression of a group of genes on the paternally acquired chromosome 15q11-q13. Phenotypical characteristics of PWS include infantile hypotonia, short stature, incomplete pubertal development, hyperphagia and morbid obesity. Hypothalamic dysfunction in controlling body weight and food intake is a hallmark of PWS. Neuroimaging studies have demonstrated that PWS subjects have abnormal neurocircuitry engaged in the hedonic and physiological control of feeding behavior. This is translated into diminished production of hypothalamic effector peptides which are responsible for the coordination of energy homeostasis and satiety. So far, studies with animal models for PWS and with human post-mortem hypothalamic specimens demonstrated changes particularly in the infundibular and the paraventricular nuclei of the hypothalamus, both in orexigenic and anorexigenic neural populations. Moreover, many PWS patients have a severe endocrine dysfunction, e.g. central hypogonadism and/or growth hormone deficiency, which may contribute to the development of increased fat mass, especially if left untreated. Additionally, the role of non-neuronal cells, such as astrocytes and microglia in the hypothalamic dysregulation in PWS is yet to be determined. Notably, microglial activation is persistently present in non-genetic obesity. To what extent microglia, and other glial cells, are affected in PWS is poorly understood. The elucidation of the hypothalamic dysfunction in PWS could prove to be a key feature of rational therapeutic management in this syndrome. This review aims to examine the evidence for hypothalamic dysfunction, both at the neuropeptidergic and circuitry levels, and its correlation with the pathophysiology of PWS.

Histamine triggers microglial responses indirectly via astrocytes and purinergic signaling

Histamine is a monoaminergic neurotransmitter which is released within the entire brain from ascending axons originating in the tuberomammillary nucleus in a sleep state-dependent fashion. Besides the modulation of neuronal firing patterns, brain histamine levels are also thought to modulate functions of glial cells. Microglia are the innate immune cells and professional phagocytes of the central nervous system, and histamine was previously shown to have multiple effects on microglial functions in health and disease. Isolated microglia respond only to agonists of the Hrh2 subtype of histamine receptors (Hrh), and the expression of that isoform is confirmed by a metadata analysis of microglia transcriptomes. When we studied the effect of the histamine receptor isoforms in cortical and thalamic microglia by in situ live cell Ca²⁺ imaging using a novel, microglia-specific indicator mouse line, microglial cells respond to external histamine application mainly in a Hrh1-, and to a lower extent also in a Hrh2-dependent manner. The Hrh1 response was sensitive to blockers of purinergic P2ry12 receptors, and since Hrh1 expression was predominantly found in astrocytes, we suggest that the Hrh1 response in microglia is mediated by astrocyte ATP release and activation of P2ry12 receptors in microglia. Histamine also stimulates microglial phagocytic activity via Hrh1- and P2ry12-mediated signaling. Taken together, we provide evidence that histamine acts indirectly on microglial Ca²⁺ levels and phagocytic activity via astrocyte histamine receptor-controlled purinergic signaling.

Phosphorylation of Microglial IRF5 and IRF4 by IRAK4 Regulates Inflammatory Responses to Ischemia

Background: Interferon Regulatory Factor (IRF) 5 and 4 play a determinant role in regulating microglial pro- and anti-inflammatory responses to cerebral ischemia. How microglial IRF5 and IRF4 signaling are activated has been elusive. We hypothesized that interleukin-1 receptor associated kinase 4 (IRAK4) phosphorylates and activates IRF5 and IRF4 in ischemic microglia. We aimed to explore the upstream signals of the two IRFs, and to determine how the IRAK4-IRF signaling regulates the expression of inflammatory mediators, and impacts neuropathology. Methods: Spontaneously Immortalized Murine (SIM)-A9 microglial cell line, primary microglia and neurons from C57BL/6 WT mice were cultured and exposed to oxygen-glucose deprivation (OGD), followed by stimulation with LPS or IL-4. An IRAK4 inhibitor (ND2158) was used to examine IRAK4′s effects on the phosphorylation of IRF5/IRF4 and the impacts on neuronal morphology by co-immunoprecipitation (Co-IP)/Western blot, ELISA, and immunofluorescence assays. Results: We confirmed that IRAK4 formed a Myddosome with MyD88/IRF5/IRF4, and phosphorylated both IRFs, which subsequently translocated into the nucleus. Inhibition of IRAK4 phosphorylation quenched microglial pro-inflammatory response primarily, and increased neuronal viability and neurite lengths after ischemia. Conclusions: IRAK4 signaling is critical for microglial inflammatory responses and a potential therapeutic target for neuroinflammatory diseases including cerebral ischemia.

Histamine 2/3 receptor agonists alleviate perioperative neurocognitive disorders by inhibiting microglia activation through the PI3K/AKT/FoxO1 pathway in aged rats

BACKGROUND

Microglia, the principal sentinel immune cells of the central nervous system (CNS), play an extensively vital role in neuroinflammation and perioperative neurocognitive disorders (PND). Histamine, a potent mediator of inflammation, can both promote and prevent microglia-related neuroinflammation by activating different histamine receptors. Rat microglia express four histamine receptors (H1R, H2R, H3R, and H4R), among which the histamine 1 and 4 receptors can promote microglia activation, whereas the role and cellular mechanism of the histamine 2 and 3 receptors have not been elucidated. Therefore, we evaluated the effects and potential cellular mechanisms of histamine 2/3 receptors in microglia-mediated inflammation and PND.

METHODS

This study investigated the role of histamine 2/3 receptors in microglia-induced inflammation and PND both in vivo and in vitro. In the in vivo experiments, rats were injected with histamine 2/3 receptor agonists in the right lateral ventricle and were then subjected to exploratory laparotomy. In the in vitro experiments, primary microglia were pretreated with histamine 2/3 receptor agonists before stimulation with lipopolysaccharide (LPS). Cognitive function, microglia activation, proinflammatory cytokine production, NF-κb expression, M1/M2 phenotypes, cell migration, and Toll-like receptor-4 (TLR4) expression were assessed.

RESULTS

In our study, the histamine 2/3 receptor agonists inhibited exploratory laparotomy- or LPS-induced cognitive decline, microglia activation, proinflammatory cytokine production, NF-κb expression, M1/M2 phenotype transformation, cell migration, and TLR4 expression through the PI3K/AKT/FoxO1 pathway.

CONCLUSION

Based on our findings, we conclude that histamine 2/3 receptors ameliorate PND by inhibiting microglia activation through the PI3K/AKT/FoxO1 pathway. Our results highlight histamine 2/3 receptors as potential therapeutic targets to treat neurological conditions associated with PND.

Priming of microglia with IFN-γ impairs adult hippocampal neurogenesis and leads to depression-like behaviors and cognitive defects

Neuroinflammation driven by interferon-gamma (IFN-γ) and microglial activation has been linked to neurological disease. However, the effects of IFN-γ-activated microglia on hippocampal neurogenesis and behavior are unclear. In the present study, IFN-γ was administered to mice via intracerebroventricular injection. Mice received intraperitoneal injection of ruxolitinib to inhibit the JAK/STAT1 pathway or injection of minocycline to inhibit microglial activation. During a 7-day period, mice were assessed for depressive-like behaviors and cognitive impairment based on a series of behavioral analyses. Effects of the activated microglia on neural stem/precursor cells (NSPCs) were examined, as was pro-inflammatory cytokine expression by activated microglia. We showed that IFN-γ-injected animals showed long-term adult hippocampal neurogenesis reduction, behavior despair, anhedonia, and cognitive impairment. Chronic activation with IFN-γ induces reactive phenotypes in microglia associated with morphological changes, population expansion, MHC II and CD68 up-regulation, and pro-inflammatory cytokine (IL-1β, TNF-α, IL-6) and nitric oxide (NO) release. Microglia isolated from the hippocampus of IFN-γ-injected mice suppressed NSPCs proliferation and stimulated apoptosis of immature neurons. Inhibiting of the JAK/STAT1 pathway in IFN-γ-injected animals to block microglial activation suppressed microglia-mediated neuroinflammation and neurogenic injury, and alleviated depressive-like behaviors and cognitive impairment. Collectively, these findings suggested that priming of microglia with IFN-γ impairs adult hippocampal neurogenesis and leads to depression-like behaviors and cognitive defects. Targeting microglia by modulating levels of IFN-γ the brain may be a therapeutic strategy for neurodegenerative diseases and psychiatric disorders.

Neuron-glia interaction through Serotonin-BDNF-NGFR axis enables regenerative neurogenesis in Alzheimer's model of adult zebrafish brain

It was recently suggested that supplying the brain with new neurons could counteract Alzheimer’s disease (AD). This provocative idea requires further testing in experimental models in which the molecular basis of disease-induced neuronal regeneration could be investigated. We previously found that zebrafish stimulates neural stem cell (NSC) plasticity and neurogenesis in AD and could help to understand the mechanisms to be harnessed for developing new neurons in diseased mammalian brains. Here, by performing single-cell transcriptomics, we found that amyloid toxicity-induced interleukin-4 (IL4) promotes NSC proliferation and neurogenesis by suppressing the tryptophan metabolism and reducing the production of serotonin. NSC proliferation was suppressed by serotonin via down-regulation of brain-derived neurotrophic factor (BDNF)-expression in serotonin-responsive periventricular neurons. BDNF enhances NSC plasticity and neurogenesis via nerve growth factor receptor A (NGFRA)/ nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NFkB) signaling in zebrafish but not in rodents. Collectively, our results suggest a complex neuron-glia interaction that regulates regenerative neurogenesis after AD conditions in zebrafish.

Can regeneration of lost neurons counteract neurodegenerative disease? This study shows that serotonergic neurons alter neural stem cell proliferation and neurogenesis via a complex neuron-glia interaction involving interleukin-4, BDNF and NGF receptor in a zebrafish model of Alzheimer's disease.

PACAP-PAC1 Receptor Activation Is Necessary for the Sympathetic Response to Acute Intermittent Hypoxia

Repetitive hypoxia is a key feature of obstructive sleep apnoea (OSA), a condition characterized by intermittent airways obstruction. Patients with OSA present with persistent increases in sympathetic activity and commonly develop hypertension. The objectives of this study were to determine if the persistent increases in sympathetic nerve activity, known to be induced by acute intermittent hypoxia (AIH), are mediated through activation of the pituitary adenylate cyclase activating polypeptide (PACAP) signaling system. Here, we show that the excitatory neuropeptide PACAP, acting in the spinal cord, is important for generating the sympathetic response seen following AIH. Using PACAP receptor knockout mice, and pharmacological agents in Sprague Dawley rats, we measured blood pressure, heart rate, pH, PaCO₂, and splanchnic sympathetic nerve activity, under anaesthesia, to demonstrate that the sympathetic response to AIH is mediated via the PAC1 receptor, in a cAMP-dependent manner. We also report that both intermittent microinjection of glutamate into the rostroventrolateral medulla (RVLM) and intermittent infusion of a sub-threshold dose of PACAP into the subarachnoid space can mimic the sympathetic response to AIH. All the sympathetic responses are independent of blood pressure, pH or PaCO₂ changes. Our results show that in AIH, PACAP signaling in the spinal cord helps drive persistent increases in sympathetic nerve activity. This mechanism may be a precursor to the development of hypertension in conditions of chronic intermittent hypoxia, such as OSA.

Improvement in inflammation is associated with the protective effect of Gly-Pro-Glu and cycloprolylglycine against Aβ-induced depletion of the hippocampal somatostatinergic system

Glycine-proline-glutamate (GPE) is a cleaved tripeptide of IGF-I that can be processed to cycloprolylglycine (cPG) in the brain. IGF-I protects the hippocampal somatostatinergic system from β-amyloid (Aβ) insult and although neither IGF-I-derived peptides bind to IGF-I receptors, they exert protective actions in several neurological disorders. As their effects on the hippocampal somatostatinergic system remain unknown, the objective of this study was to evaluate if cPG and/or GPE prevent the deleterious effects of Aβ_25-35 infusion on this system and whether changes in intracellular-related signaling and interleukin (IL) content are involved in their protective effect. We also determined the effect of cPG or GPE co-administration with Aβ_25-35 on IL secretion in glial cultures and the influence of these ILs on signaling activation and somatostatin synthesis in neuronal cultures. cPG or GPE co-administration reduced Aβ-induced cell death and pro-inflammatory ILs, increased IL-4 and partially avoided the reduction of components of the somatostatinergic system affected by Aβ_25-35. GPE increased activation of Akt and CREB and reduced GSK3β activation and astrogliosis, whereas cPG increased phosphorylation of extracellular signal-regulated kinases. Both peptides converged in the activation of mTOR and S6 kinase. Co-administration of these peptides with Aβ_25-35 to glial cultures increased IL-4 and reduced IL-1β; this release of IL-4 could be responsible for activation of Akt and increased somatostatin in neuronal cultures. Our findings suggest that cPG and GPE exert protective effects against Aβ on the somatostatinergic system by a reduction of the inflammatory environment that may activate different pro-survival pathways in these neurons.

Differential Microglial Morphological Response, TNFα, and Viral Load in Sedentary-like and Active Murine Models After Systemic Non-neurotropic Dengue Virus Infection

Peripheral inflammatory stimuli increase proinflammatory cytokines in the bloodstream and central nervous system and activate microglial cells. Here we tested the hypothesis that contrasting environments mimicking sedentary and active lives would be associated with differential microglial morphological responses, inflammatory cytokines concentration, and virus load in the peripheral blood. For this, mice were maintained either in standard (standard environment) or enriched cages (enriched environment) and then subjected to a single (DENV1) serotype infection. Blood samples from infected animals showed higher viral loads and higher tumor necrosis factor-α (TNFα) mRNA concentrations than control subjects. Using an unbiased stereological sampling approach, we selected 544 microglia from lateral septum for microscopic 3D reconstruction. Morphological complexity contributed most to cluster formation. Infected groups exhibited significant increase in the microglia morphological complexity and number, despite the absence of dengue virus antigens in the brain. Two microglial phenotypes (type I with lower and type II with higher morphological complexity) were found in both infected and control groups. However, microglia from infected mice maintained in enriched environment showed only one morphological phenotype. Two-way ANOVA revealed that environmental changes and infection influenced type-I and II microglial morphologies and number. Environmental enrichment and infection interactions may contribute to microglial morphological change to a point that type-I and II morphological phenotypes could no longer be distinguished in infected mice from enriched environment. Significant linear correlation was found between morphological complexity and TNFα peripheral blood. Our findings demonstrated that sedentary-like and active murine models exhibited differential microglial responses and peripheral inflammation to systemic non-neurotropic infections with DENV1 virus.

Unravelling the glial response in the pathogenesis of Alzheimer's disease

Alzheimer's disease is a progressive, incurable neurodegenerative disease targeting specific neuronal populations within the brain while neighboring neurons appear unaffected. The focus for defining mechanisms has therefore been on the pathogenesis in affected neuronal populations and developing intervention strategies to prevent their cell death. However, there is growing recognition of the importance of glial cells in the development of pathology. Determining exactly how glial cells are involved in the disease process and the susceptibility of the aging brain provides unprecedented challenges. The present review examines recent studies attempting to unravel the glial response during the course of disease and how this action may dictate the outcome of neurodegeneration. The importance of regional heterogeneity of glial cells within the CNS during healthy aging and disease is examined to understand how the glial cells may contribute to neuronal susceptibility or resilience during the neurodegenerative process.-Alibhai, J. D., Diack, A. B., Manson, J. C. Unravelling the glial response in the pathogenesis of Alzheimer's disease.

Mitochondria, Oxytocin, and Vasopressin: Unfolding the Inflammatory Protein Response

Neuroendocrine and immune signaling pathways are activated following insults such as stress, injury, and infection, in a systemic response aimed at restoring homeostasis. Mitochondrial metabolism and function have been implicated in the control of immune responses. Commonly studied along with mitochondrial function, reactive oxygen species (ROS) are closely linked to cellular inflammatory responses. It is also accepted that cells experiencing mitochondrial or endoplasmic reticulum (ER) stress induce response pathways in order to cope with protein-folding dysregulation, in homeostatic responses referred to as the unfolded protein responses (UPRs). Recent reports indicate that the UPRs may play an important role in immune responses. Notably, the homeostasis-regulating hormones oxytocin (OXT) and vasopressin (AVP) are also associated with the regulation of inflammatory responses and immune function. Intriguingly, OXT and AVP have been linked with ER unfolded protein responses (UPR^ER), and can impact ROS production and mitochondrial function. Here, we will review the evidence for interactions between these various factors and how these neuropeptides might influence mitochondrial processes.

HIV-1 Tat regulation of dopamine transmission and microglial reactivity is brain region specific

The transactivator of transcription protein, HIV-1 Tat, is linked to neuroAIDS, where degeneration of dopamine neurons occurs. Using a mouse model expressing GFAP-driven Tat protein under doxycycline (Dox) regulation, we investigated microglial-neuronal interactions in the rostral substantia nigra pars compacta (SNc). Immunohistochemistry for microglia and tyrosine hydroxylase (TH) showed that the ratio of microglia to dopamine neurons is smaller in the SNc than in the ventral tegmental area (VTA) and that this difference is maintained following 7-day Dox exposure in wild type animals. Administration of Dox to wild types had no effect on microglial densities. In addressing the sensitivity of neurons to potentially adverse effects of HIV-1 Tat, we found that HIV-1 Tat exposure in vivo selectively decreased TH immunoreactivity in the SNc but not in the VTA, while levels of TH mRNA in the SNc remained unchanged. HIV-1 Tat induction in vivo did not alter the total number of neurons in these brain regions. Application of Tat (5 ng) into dopamine neurons with whole-cell patch pipette decreased spontaneous firing activity. Tat induction also produced a decline in microglial cell numbers, but no microglial activation. Thus, disappearance of dopaminergic phenotype is due to a loss of TH immunoreactivity rather than to neuronal death, which would have triggered microglial activation. We conclude that adverse effects of HIV-1 Tat produce a hypodopamine state by decreasing TH immunoreactivity and firing activity of dopamine neurons. Reduced microglial numbers after Tat exposure in vivo suggest impaired microglial functions and altered bidirectional interactions between dopamine neurons and microglia.

The role of dopaminergic immune cell signalling in poststroke inflammation

Upon ischaemic stroke, brain-resident and peripheral immune cells accumulate in the central nervous system (CNS). Interestingly, these cells express pattern specific to neurotransmitter receptors and, therefore, seem to be susceptible to neurotransmitter stimulation, potentially modulating their properties and functions. One of the principal neurotransmitters in the CNS, dopamine, is involved in the regulation of processes of brain development, motor control and higher brain functions. It is constantly released in the brain and there is experimental and clinical evidence that dopaminergic signalling is involved in recovery of lost neurological function after stroke. Independent studies have revealed specific but different patterns of dopamine receptor subtypes on different populations of immune cells. Those patterns are dependent on the activation status of cells. Generally, exposure to dopamine or dopamine receptor agonists decreases detrimental actions of immune cells. In contrast, a reduction of dopaminergic inputs perpetuates a pro-inflammatory state associated with increased release of pro-inflammatory molecules. In addition, subsets of immune cells have been identified to synthesize and release dopamine, suggesting autoregulatory mechanisms. Evidence supports that inflammatory processes activated following ischaemic stroke are modulated by dopaminergic signalling.

Interleukin 4 modulates microglia homeostasis and attenuates the early slowly progressive phase of amyotrophic lateral sclerosis

Microglia activation is a commonly pathological hallmark of neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), a devastating disorder characterized by a selective motor neurons degeneration. Whether such activation might represent a causal event rather than a secondary epiphenomenon remains elusive. Here, we show that CNS-delivery of IL-4—via a lentiviral-mediated gene therapy strategy—skews microglia to proliferate, inducing these cells to adopt the phenotype of slowly proliferating cells. Transcriptome analysis revealed that IL-4-treated microglia express a broad number of genes normally encoded by embryonic microglia. Since embryonic microglia sustain CNS development, we then hypothesized that turning adult microglia to acquire such phenotype via IL-4 might be an efficient in vivo strategy to sustain motor neuron survival in ALS. IL-4 gene therapy in SOD1^G93A mice resulted in a general amelioration of clinical outcomes during the early slowly progressive phase of the disease. However, such approach did not revert neurodegenerative processes occurring in the late and fast progressing phase of the disease.

New extracellular factors in glioblastoma multiforme development: neurotensin, growth differentiation factor-15, sphingosine-1-phosphate and cytomegalovirus infection

Recent years have seen considerable progress in understanding the biochemistry of cancer. For example, more significance is now assigned to the tumor microenvironment, especially with regard to intercellular signaling in the tumor niche which depends on many factors secreted by tumor cells. In addition, great progress has been made in understanding the influence of factors such as neurotensin, growth differentiation factor-15 (GDF-15), sphingosine-1-phosphate (S1P), and infection with cytomegalovirus (CMV) on the 'hallmarks of cancer' in glioblastoma multiforme. Therefore, in the present work we describe the influence of these factors on the proliferation and apoptosis of neoplastic cells, cancer stem cells, angiogenesis, migration and invasion, and cancer immune evasion in a glioblastoma multiforme tumor. In particular, we discuss the effect of neurotensin, GDF-15, S1P (including the drug FTY720), and infection with CMV on tumor-associated macrophages (TAM), microglial cells, neutrophil and regulatory T cells (T_reg), on the tumor microenvironment. In order to better understand the role of the aforementioned factors in tumoral processes, we outline the latest models of intratumoral heterogeneity in glioblastoma multiforme. Based on the most recent reports, we discuss the problems of multi-drug therapy in treating glioblastoma multiforme.

Usage of Multiparameter Flow Cytometry to Study Microglia and Macrophage Heterogeneity in the Central Nervous System During Neuroinflammation and Neurodegeneration

The resident macrophages of the central nervous system (CNS), also known as microglia, and blood-derived macrophages play an important role in the functional activity of the normal CNS, as well as in the development of neuroinflammation during various neurodegenerative disorders. Microglia and macrophages represent heterogeneous populations, which can modulate CNS environment and have different effects on neuronal regeneration. In this chapter, the main features of microglial and macrophage subsets and current methods for investigation of their heterogeneity will be discussed.

The Role of Microglia and Macrophages in CNS Homeostasis, Autoimmunity, and Cancer

Macrophages are major cell types of the immune system, and they comprise both tissue-resident populations and circulating monocyte-derived subsets. Here, we discuss microglia, the resident macrophage within the central nervous system (CNS), and CNS-infiltrating macrophages. Under steady state, microglia play important roles in the regulation of CNS homeostasis through the removal of damaged or unnecessary neurons and synapses. In the face of inflammatory or pathological insults, microglia and CNS-infiltrating macrophages not only constitute the first line of defense against pathogens by regulating components of innate immunity, but they also regulate the adaptive arms of immune responses. Dysregulation of these responses contributes to many CNS disorders. In this overview, we summarize the current knowledge regarding the highly diverse and complex function of microglia and macrophages during CNS autoimmunity—multiple sclerosis and cancer—malignant glioma. We emphasize how the crosstalk between natural killer (NK) cells or glioma cells or glioma stem cells and CNS macrophages impacts on the pathological processes. Given the essential role of CNS microglia and macrophages in the regulation of all types of CNS disorders, agents targeting these subsets are currently applied in preclinical and clinical trials. We believe that a better understanding of the biology of these macrophage subsets offers new exciting paths for therapeutic intervention.

Immunoregulatory effect of mast cells influenced by microbes in neurodegenerative diseases

When related to central nervous system (CNS) health and disease, brain mast cells (MCs) can be a source of either beneficial or deleterious signals acting on neural cells. We review the current state of knowledge about molecular interactions between MCs and glia in neurodegenerative diseases such as Multiple Sclerosis, Alzheimer's disease, Amyotrophic Lateral Sclerosis, Parkinson's disease, Epilepsy. We also discuss the influence on MC actions evoked by the host microbiota, which has a profound effect on the host immune system, inducing important consequences in neurodegenerative disorders. Gut dysbiosis, reduced intestinal motility and increased intestinal permeability, that allow bacterial products to circulate and pass through the blood-brain barrier, are associated with neurodegenerative disease. There are differences between the microbiota of neurologic patients and healthy controls. Distinguishing between cause and effect is a challenging task, and the molecular mechanisms whereby remote gut microbiota can alter the brain have not been fully elucidated. Nevertheless, modulation of the microbiota and MC activation have been shown to promote neuroprotection. We review this new information contributing to a greater understanding of MC-microbiota-neural cells interactions modulating the brain, behavior and neurodegenerative processes.

Dopaminergic Regulation of Innate Immunity: a Review

Dopamine (DA) is a neurotransmitter in the central nervous system as well as in peripheral tissues. Emerging evidence however points to DA also as a key transmitter between the nervous system and the immune system as well as a mediator produced and released by immune cells themselves. Dopaminergic pathways have received so far extensive attention in the adaptive branch of the immune system, where they play a role in health and disease such as multiple sclerosis, rheumatoid arthritis, cancer, and Parkinson's disease. Comparatively little is known about DA and the innate immune response, although DA may affect innate immune system cells such as dendritic cells, macrophages, microglia, and neutrophils. The present review aims at providing a complete and exhaustive summary of currently available evidence about DA and innate immunity, and to become a reference for anyone potentially interested in the fields of immunology, neurosciences and pharmacology. A wide array of dopaminergic drugs is used in therapeutics for non-immune indications, such as Parkinson's disease, hyperprolactinemia, shock, hypertension, with a usually favorable therapeutic index, and they might be relatively easily repurposed for immune-mediated disease, thus leading to innovative treatments at low price, with benefit for patients as well as for the healthcare systems.

Recent Advances in the Study of Bipolar/Rod-Shaped Microglia and their Roles in Neurodegeneration

Microglia are the resident immune cells of the central nervous system (CNS) and they contribute to primary inflammatory responses following CNS injuries. The morphology of microglia is closely associated with their functional activities. Most previous research efforts have attempted to delineate the role of ramified and amoeboid microglia in the pathogenesis of neurodegenerative diseases. In addition to ramified and amoeboid microglia, bipolar/rod-shaped microglia were first described by Franz Nissl in 1899 and their presence in the brain was closely associated with the pathology of infectious diseases and sleeping disorders. However, studies relating to bipolar/rod-shaped microglia are very limited, largely due to the lack of appropriate in vitro and in vivo experimental models. Recent studies have reported the formation of bipolar/rod-shaped microglia trains in in vivo models of CNS injury, including diffuse brain injury, focal transient ischemia, optic nerve transection and laser-induced ocular hypertension (OHT). These bipolar/rod-shaped microglia formed end-to-end alignments in close proximity to the adjacent injured axons, but they showed no interactions with blood vessels or other types of glial cell. Recent studies have also reported on a highly reproducible in vitro culture model system to enrich bipolar/rod-shaped microglia that acts as a powerful tool with which to characterize this form of microglia. The molecular aspects of bipolar/rod-shaped microglia are of great interest in the field of CNS repair. This review article focuses on studies relating to the morphology and transformation of microglia into the bipolar/rod-shaped form, along with the differential gene expression and spatial distribution of bipolar/rod-shaped microglia in normal and pathological CNSs. The spatial arrangement of bipolar/rod-shaped microglia is crucial in the reorganization and remodeling of neuronal and synaptic circuitry following CNS injuries. Finally, we discuss the potential neuroprotective roles of bipolar/rod-shaped microglia, and the possibility of transforming ramified/amoeboid microglia into bipolar/rod-shaped microglia. This will be of considerable clinical benefit in the development of novel therapeutic strategies for treating various neurodegenerative diseases and promoting CNS repair after injury.

Regulation of microglial activation in stroke

When ischemic stroke occurs, oxygen and energy depletion triggers a cascade of events, including inflammatory responses, glutamate excitotoxicity, oxidative stress, and apoptosis that result in a profound brain injury. The inflammatory response contributes to secondary neuronal damage, which exerts a substantial impact on both acute ischemic injury and the chronic recovery of the brain function. Microglia are the resident immune cells in the brain that constantly monitor brain microenvironment under normal conditions. Once ischemia occurs, microglia are activated to produce both detrimental and neuroprotective mediators, and the balance of the two counteracting mediators determines the fate of injured neurons. The activation of microglia is defined as either classic (M1) or alternative (M2): M1 microglia secrete pro-inflammatory cytokines (TNFα, IL-23, IL-1β, IL-12, etc) and exacerbate neuronal injury, whereas the M2 phenotype promotes anti-inflammatory responses that are reparative. It has important translational value to regulate M1/M2 microglial activation to minimize the detrimental effects and/or maximize the protective role. Here, we discuss various regulators of microglia/macrophage activation and the interaction between microglia and neurons in the context of ischemic stroke.

Dopamine D1 Receptor Immunoreactivity on Fine Processes of GFAP-Positive Astrocytes in the Substantia Nigra Pars Reticulata of Adult Mouse

Substantia nigra pars reticulata (SNr), the major output nucleus of the basal ganglia, receives dopamine from dendrites extending from dopaminergic neurons of the adjacent nucleus pars compacta (SNc), which is known for its selective degeneration in Parkinson's disease. As a recipient for dendritically released dopamine, the dopamine D1 receptor (D1R) is a primary candidate due to its very dense immunoreactivity in the SNr. However, the precise location of D1R remains unclear at the cellular level in the SNr except for that reported on axons/axon terminals of presumably striatal GABAergic neurons. To address this, we used D1R promotor-controlled, mVenus-expressing transgenic mice. When cells were acutely dissociated from SNr of mouse brain, prominent mVenus fluorescence was detected in fine processes of glia-like cells, but no such fluorescence was detected from neurons in the same preparation, except for the synaptic bouton-like structure on the neurons. Double immunolabeling of SNr cells dissociated from adult wild-type mice brain further revealed marked D1R immunoreactivity in the processes of glial fibrillary acidic protein (GFAP)-positive astrocytes. Such D1R imunoreactivity was significantly stronger in the SNr astrocytes than that in those of the visual cortex in the same preparation. Interestingly, GFAP-positive astrocytes dissociated from the striatum demonstrated D1R immunoreactivity, either remarkable or minimal, similarly to that shown in neurons in this nucleus. In contrast, in the SNr and visual cortex, only weak D1R immunoreactivity was detected in the neurons tested. These results suggest that the SNr astrocyte may be a candidate recipient for dendritically released dopamine. Further study is required to fully elucidate the physiological roles of divergent dopamine receptor immunoreactivity profiles in GFAP-positive astrocytes.

Neuropeptides and Microglial Activation in Inflammation, Pain, and Neurodegenerative Diseases

Microglial cells are responsible for immune surveillance within the CNS. They respond to noxious stimuli by releasing inflammatory mediators and mounting an effective inflammatory response. This is followed by release of anti-inflammatory mediators and resolution of the inflammatory response. Alterations to this delicate process may lead to tissue damage, neuroinflammation, and neurodegeneration. Chronic pain, such as inflammatory or neuropathic pain, is accompanied by neuroimmune activation, and the role of glial cells in the initiation and maintenance of chronic pain has been the subject of increasing research over the last two decades. Neuropeptides are small amino acidic molecules with the ability to regulate neuronal activity and thereby affect various functions such as thermoregulation, reproductive behavior, food and water intake, and circadian rhythms. Neuropeptides can also affect inflammatory responses and pain sensitivity by modulating the activity of glial cells. The last decade has witnessed growing interest in the study of microglial activation and its modulation by neuropeptides in the hope of developing new therapeutics for treating neurodegenerative diseases and chronic pain. This review summarizes the current literature on the way in which several neuropeptides modulate microglial activity and response to tissue damage and how this modulation may affect pain sensitivity.

Dual role of histamine on microglia-induced neurodegeneration

Several hypotheses have been raised about the dual role of histamine in neurological disorders, and evidences have shown its crucial involvement in the modulation of microglia-mediated neuroinflammation. Previously, we reported that the administration of histamine induces a deleterious effect by promoting a pro-inflammatory phenotype on microglia that in turn compromises dopaminergic neuronal survival. Contrary, under lipopolysaccharide challenge, histamine inhibits the injurious effect of microglia-mediated inflammation, protecting dopaminergic neurons, suggesting that the modulation of microglial activity is dependent on the environmental context. Thus, histamine and/or histamine receptor agonists may serve to develop new therapeutic approaches to overcome neurodegenerative disorders.

Microglia in Physiology and Disease

As the immune-competent cells of the brain, microglia play an increasingly important role in maintaining normal brain function. They invade the brain early in development, transform into a highly ramified phenotype, and constantly screen their environment. Microglia are activated by any type of pathologic event or change in brain homeostasis. This activation process is highly diverse and depends on the context and type of the stressor or pathology. Microglia can strongly influence the pathologic outcome or response to a stressor due to the release of a plethora of substances, including cytokines, chemokines, and growth factors. They are the professional phagocytes of the brain and help orchestrate the immunological response by interacting with infiltrating immune cells. We describe here the diversity of microglia phenotypes and their responses in health, aging, and disease. We also review the current literature about the impact of lifestyle on microglia responses and discuss treatment options that modulate microglial phenotypes.

Histamine regulation of microglia: Gene-environment interaction in the regulation of central nervous system inflammation

Microglia mediate neuroinflammation and regulate brain development and homeostasis. Microglial abnormalities are implicated in a range of neuropsychiatric pathology, including Tourette syndrome (TS) and autism. Histamine (HA) is both a neurotransmitter and an immune modulator. HA deficiency has been implicated as a rare cause of TS and may contribute to other neuropsychiatric conditions. In vitro studies suggest that HA can regulate microglia, but this has never been explored in vivo. We used immunohistochemistry to examine the effects of HA deficiency in histidine decarboxylase (Hdc) knockout mice and of HA receptor stimulation in wild-type animals. We find HA to regulate microglia in vivo, via the H4 receptor. Chronic HA deficiency in Hdc knockout mice reduces ramifications of microglia in the striatum and (at trend level) in the hypothalamus, but not elsewhere in the brain. Depletion of histaminergic neurons in the hypothalamus has a similar effect. Microglia expressing IGF-1 are particularly reduced, However, the microglial response to challenge with lipopolysacchariade (LPS) is potentiated in Hdc knockout mice. Genetic abnormalities in histaminergic signaling may produce a vulnerability to inflammatory challenge, setting the state for pathogenically dysregulated neuroimmune responses.

Neurotensin stimulates sortilin and mTOR in human microglia inhibitable by methoxyluteolin, a potential therapeutic target for autism

We had reported elevated serum levels of the peptide neurotensin (NT) in children with autism spectrum disorders (ASD). Here, we show that NT stimulates primary human microglia, the resident immune cells of the brain, and the immortalized cell line of human microglia-SV40. NT (10 nM) increases the gene expression and release (P < 0.001) of the proinflammatory cytokine IL-1β and chemokine (C-X-C motif) ligand 8 (CXCL8), chemokine (C-C motif) ligand 2 (CCL2), and CCL5 from human microglia. NT also stimulates proliferation (P < 0.05) of microglia-SV40. Microglia express only the receptor 3 (NTR3)/sortilin and not the NTR1 or NTR2. The use of siRNA to target sortilin reduces (P < 0.001) the NT-stimulated cytokine and chemokine gene expression and release from human microglia. Stimulation with NT (10 nM) increases the gene expression of sortilin (P < 0.0001) and causes the receptor to be translocated from the cytoplasm to the cell surface, and to be secreted extracellularly. Our findings also show increased levels of sortilin (P < 0.0001) in the serum from children with ASD (n = 36), compared with healthy controls (n = 20). NT stimulation of microglia-SV40 causes activation of the mammalian target of rapamycin (mTOR) signaling kinase, as shown by phosphorylation of its substrates and inhibition of these responses by drugs that prevent mTOR activation. NT-stimulated responses are inhibited by the flavonoid methoxyluteolin (0.1-1 μM). The data provide a link between sortilin and the pathological findings of microglia and inflammation of the brain in ASD. Thus, inhibition of this pathway using methoxyluteolin could provide an effective treatment of ASD.

Brain-Gut-Bone Marrow Axis: Implications for Hypertension and Related Therapeutics

Hypertension is the most prevalent modifiable risk factor for cardiovascular disease and disorders directly influencing cardiovascular disease morbidity and mortality, such as diabetes mellitus, chronic kidney disease, obstructive sleep apnea, etc. Despite aggressive attempts to influence lifestyle modifications and advances in pharmacotherapeutics, a large percentage of patients still do not achieve recommended blood pressure control worldwide. Thus, we think that mechanism-based novel strategies should be considered to significantly improve control and management of hypertension. The overall objective of this review is to summarize implications of peripheral- and neuroinflammation as well as the autonomic nervous system-bone marrow communication in hematopoietic cell homeostasis and their impact on hypertension pathophysiology. In addition, we discuss the novel and emerging field of intestinal microbiota and roles of gut permeability and dysbiosis in cardiovascular disease and hypertension. Finally, we propose a brain-gut-bone marrow triangular interaction hypothesis and discuss its potential in the development of novel therapies for hypertension.

A critical time window for the analgesic effect of central histamine in the partial sciatic ligation model of neuropathic pain

Background

It is known that histamine participates in pain modulation. However, the effect of central histamine on neuropathic pain is not fully understood. Here, we report a critical time window for the analgesic effect of central histamine in the partial sciatic nerve ligation model of neuropathic pain.

Methods

Neuropathic pain was induced by partial sciatic nerve ligation (PSL) in rats, wild-type (C57BL/6J) mice and HDC^−/− (histidine decarboxylase gene knockout) and IL-1R^−/− (interleukin-1 receptor gene knockout) mice. Histidine, a precursor of histamine that can increase the central histamine levels, was administered intraperitoneally (i.p.). Histidine decarboxylase (HDC) enzyme inhibitor α-fluoromethylhistidine was administered intracerebroventricularly (i.c.v.). Histamine H₁ receptor antagonist mepyramine and H₂ receptor antagonist cimetidine were given intrathecally (i.t.) and intracisternally (i.c.). Withdrawal thresholds to tactile and heat stimuli were measured with a set of von Frey hairs and infrared laser, respectively. Immunohistochemistry and Western blot were carried out to evaluate the morphology of microglia and IL-1β production, respectively.

Results

Histidine (100 mg/kg, i.p.) administered throughout days 0–3, 0–7, or 0–14 postoperatively (PO) alleviated mechanical allodynia and thermal hyperalgesia in the hindpaw following PSL in rats. Intrathecal histamine reversed PSL-induced thermal hyperalgesia in a dose-dependent manner and intracisternal histamine alleviated both mechanical allodynia and thermal hyperalgesia. Moreover, α-fluoromethylhistidine (i.c.v.) abrogated the analgesic effect of histidine. However, histidine treatment initiated later than the first postoperative day (treatment periods included days 2–3, 4–7, and 8–14 PO) did not show an analgesic effect. In addition, histidine treatment initiated immediately, but not 3 days after PSL, inhibited microglial activation and IL-1β upregulation in the lumbar spinal cord, in parallel with its effects on behavioral hypersensitivity. Moreover, the inhibitory effects on pain hypersensitivity and spinal microglial activation were absent in HDC^−/− mice and IL-1R^−/− mice. H₁ receptor antagonist mepyramine (200 ng/rat i.t. or i.c.), but not H₂ receptor antagonist cimetidine (200, 500 ng/rat i.t. or 500 ng/rat i.c.), blocked the effects of histidine on pain behavior and spinal microglia.

Conclusions

These results demonstrate that central histamine is analgesic within a critical time window in the PSL model of neuropathic pain via histamine H₁ receptors. This effect may partly relate to the inhibition of microglial activation and IL-1β production in the spinal cord following nerve injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0637-0) contains supplementary material, which is available to authorized users.

De novo expression of dopamine D2 receptors on microglia after stroke

Dopamine is the predominant catecholamine in the brain and functions as a neurotransmitter. Dopamine is also a potent immune modulator. In this study, we have characterized the expression of dopamine receptors on murine microglia. We found that cultured primary microglia express dopamine D1, D2, D3, D4, and D5 receptors. We specifically focused on the D2 receptor (D2R), a major target of antipsychotic drugs. Whereas D2Rs were strongly expressed on striatal neurons in vivo, we did not detect any D2R expression on resident microglia in the healthy brains of wild-type mice or transgenic mice expressing the green fluorescent protein (GFP) under the control of the Drd2 promoter. However, cerebral ischemia induced the expression of D2R on Iba1-immunoreactive inflammatory cells in the infarct core and penumbra. Notably, D2R expression was confined to CD45(hi) cells, and GFP BM chimeras revealed that D2R was expressed on activated resident microglia as well as on peripherally derived macrophages in the ischemic brain. Importantly, the D2/3R agonist, pramipexole, enhanced the secretion of nitrite by cultured microglia in response to proinflammatory stimuli. Thus, dopamine may serve as a modulator of microglia function during neuroinflammation.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Axon initial segment-associated microglia

Microglia are the brain's resident immune cells and function as the main defense against pathogens or injury. However, in the absence of disease, microglia have other functions in the normal brain. For example, previous studies showed that microglia contribute to circuit refinement and synaptic plasticity in the developing and adult brain, respectively. Thus, microglia actively participate in regulating neuronal excitability and function. Here, we report that in the cortex, but not other brain regions, a subset of microglia extend a single process that specifically associates and overlaps with the axon initial segment (AIS), the site where action potentials are generated. Similar associations were not observed with dendrites or distal axons. Microglia-AIS interactions appear early in development, persist throughout adulthood, and are conserved across species including mice, rats, and primates. However, these interactions are lost after microglial activation following brain injury, suggesting that such interactions may be part of healthy brain function. Loss of microglial CX3CR1 receptors, or the specialized extracellular matrix surrounding the AIS, did not disrupt the interaction. However, loss of AIS proteins by the neuron-specific deletion of the master AIS scaffold AnkyrinG disrupted microglia-AIS interactions. These results reveal a unique population of microglia that specifically interact with the AIS in the adult cortex.