Rethinking inflammation: neural circuits in the regulation of immunity

Causes and consequences of degeneration of the dorsal motor nucleus of the vagus nerve in Parkinson's disease

SIGNIFICANCE

Parkinson's disease (PD) is no longer considered merely a movement disorder caused by degeneration of dopamine neurons in the midbrain. It is now recognized as a widespread neuropathological syndrome accompanied by a variety of motor and nonmotor clinical symptoms. As such, any hypothesis concerning PD pathogenesis and pathophysiology must account for the entire spectrum of disease and not solely focus on the dopamine system.

RECENT ADVANCES

Based on its anatomy and the intrinsic properties of its neurons, the dorsal motor nucleus of the vagus nerve (DMV) is uniquely vulnerable to damage from PD. Fibers in the vagus nerve course throughout the gastrointestinal (GI) tract to and from the brainstem forming a close link between the peripheral and central nervous systems and a point of proximal contact between the environment and areas where PD pathology is believed to start. In addition, DMV neurons are under high levels of oxidative stress due to their high level of α-synuclein expression, fragile axons, and specific neuronal physiology. Moreover, several consequences of DMV damage, namely, GI dysfunction and unrestrained inflammation, may propagate a vicious cycle of injury affecting vulnerable brain regions.

CRITICAL ISSUES

Current evidence to suggest the vagal system plays a pivotal role in PD pathogenesis is circumstantial, but given the current state of the field, the time is ripe to obtain direct experimental evidence to better delineate it.

FUTURE DIRECTIONS

Better understanding of the DMV and vagus nerve may provide insight into PD pathogenesis and a neural highway with direct brain access that could be harnessed for novel therapeutic interventions.

Drosophila as a model for the two myeloid blood cell systems in vertebrates

Fish, mice, and humans rely on two coexisting myeloid blood cell systems. One is sustained by hematopoietic progenitor cells, which reside in specialized microenvironments (niches) in hematopoietic organs and give rise to cells of the monocyte lineage. The other system corresponds to the independent lineage of self-renewing tissue macrophages, which colonize organs during embryonic development and are maintained during later life by proliferation in local tissue microenvironments. However, little is known about the nature of these microenvironments and their regulation. Moreover, many vertebrate tissues contain a mix of both tissue-resident and monocyte-derived macrophages, posing a challenge to the study of lineage-specific regulatory mechanisms and function. This review highlights how research in the simple model organism Drosophila melanogaster can address many of these outstanding questions in the field. Drawing parallels between hematopoiesis in Drosophila and vertebrates, we illustrate the evolutionary conservation of the two myeloid systems across animal phyla. Much like vertebrates, Drosophila possesses a lineage of self-renewing tissue-resident macrophages, which we refer to as tissue hemocytes, as well as a "definitive" lineage of macrophages that derive from hematopoiesis in the progenitor-based lymph gland. We summarize key findings from Drosophila hematopoiesis that illustrate how local microenvironments, systemic signals, immune challenges, and nervous inputs regulate adaptive responses of tissue-resident macrophages and progenitor-based hematopoiesis to maximize fitness of the animal.

Neural control of the immune system

Neural reflexes support homeostasis by modulating the function of organ systems. Recent advances in neuroscience and immunology have revealed that neural reflexes also regulate the immune system. Activation of the vagus nerve modulates leukocyte cytokine production and alleviates experimental shock and autoimmune disease, and recent data have suggested that vagus nerve stimulation can improve symptoms in human rheumatoid arthritis. These discoveries have generated an increased interest in bioelectronic medicine, i.e., therapeutic delivery of electrical impulses that activate nerves to regulate immune system function. Here, we discuss the physiology and potential therapeutic implications of neural immune control.

Vagus nerve controls resolution and pro-resolving mediators of inflammation

Axonal guidance molecule netrin-1 promotes resolution of inflammation, with netrin-1 and resolvin D1 mutually inducing each other’s expression.

Resolution of inflammation is now recognized as a biosynthetically active process involving pro-resolving mediators. Here, we show in zymosan-initiated peritoneal inflammation that the vagus nerve regulates local expression of netrin-1, an axonal guidance molecule that activates resolution, and that vagotomy reduced local pro-resolving mediators, thereby delaying resolution. In netrin-1^+/− mice, resolvin D1 (RvD1) was less effective in reducing neutrophil influx promoting resolution of peritonitis compared with Ntn1^+/+. Netrin-1 shortened the resolution interval, decreasing exudate neutrophils, reducing proinflammatory mediators, and stimulating the production of resolvins, protectins, and lipoxins. Human monocytes incubated with netrin-1 produced proresolving mediators, including resolvins and lipoxins. Netrin-1 and RvD1 displayed bidirectional activation in that they stimulated each other’s expression and enhanced efferocytosis. These results indicate that the vagus nerve regulates both netrin-1 and pro-resolving lipid mediators, which act in a bidirectional fashion to stimulate resolution, and provide evidence for a novel mechanism for local neuronal control of resolution.

Nicotinic ACh receptors in the hippocampal circuit; functional expression and role in synaptic plasticity

Acetylcholine (ACh) can regulate neuronal excitability in the hippocampus, an important area in the brain for learning and memory, by acting on both nicotinic (nAChRs) and muscarinic ACh receptors. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca (MS-DBB), and we investigated how their activation regulated hippocampal synaptic plasticity. We found that activation of these endogenous cholinergic inputs can directly induce different forms of hippocampal synaptic plasticity with a timing precision in the millisecond range. Furthermore, we observed a prolonged enhancement of excitability both pre- and postsynaptically. Lastly we found that the presence of the α7 nAChR subtype to both pre- and postsynaptic sites appeared to be required to induce this plasticity. We propose that α7 nAChRs coordinate pre- and postsynaptic activities to induce glutamatergic synaptic plasticity, and thus provide a novel mechanism underlying physiological neuronal communication that could lead to timing-dependent synaptic plasticity in the hippocampus.

Selective activation of α7 nicotinic acetylcholine receptor (nAChRα7) inhibits muscular degeneration in mdx dystrophic mice

Amount evidence indicates that α7 nicotinic acetylcholine receptor (nAChRα7) activation reduces production of inflammatory mediators. This work aimed to verify the influence of endogenous nAChRα7 activation on the regulation of full-blown muscular inflammation in mdx mouse with Duchenne muscular dystrophy. We used mdx mice with 3 weeks-old at the height myonecrosis, and C57 nAChRα7(+/+) wild-type and nAChRα7(-/-) knockout mice with muscular injury induced with 60µL 0.5% bupivacaine (bp) in the gastrocnemius muscle. Pharmacological treatment included selective nAChRα7 agonist PNU282987 (0.3mg/kg and 1.0mg/kg) and the antagonist methyllycaconitine (MLA at 1.0mg/kg) injected intraperitoneally for 7 days. Selective nAChRα7 activation of mdx mice with PNU282987 reduced circulating levels of lactate dehydrogenase (LDH, a marker of cell death by necrosis) and the area of perivascular inflammatory infiltrate, and production of inflammatory mediators TNFα and metalloprotease MMP-9 activity. Conversely, PNU282987 treatment increased MMP-2 activity, an indication of muscular tissue remodeling associated with regeneration, in both mdx mice and WTα7 mice with bp-induced muscular lesion. Treatment with PNU282987 had no effect on α7KO, and MLA abolished the nAChRα7 agonist-induced anti-inflammatory effect in both mdx and WT. In conclusion, nAChRα7 activation inhibits muscular inflammation and activates tissue remodeling by increasing muscular regeneration. These effects were not accompanied with fibrosis and/or deposition of non-functional collagen. The nAChRα7 activation may be considered as a potential target for pharmacological strategies to reduce inflammation and activate mechanisms of muscular regeneration.

Depressive symptoms in patients with cancer: does cortisol keep cytokines from singing the blues?

Depressive symptoms are common among patients with cancer, and both psychological stress and physiological factors have been implicated in the etiology of depression. Scientific progress in this area is challenged by the changing nature of psychological and physiological processes over the course of cancer diagnosis and treatment. The article by Wu and colleagues in the current issue of Psychosomatic Medicine provides an example of thoughtful consideration of the complex longitudinal relationships between psychological stress, physiology, and depressive symptoms. These findings are put into context by discussing broader challenges in this area, with a focus on the contribution of inflammatory processes caused by cancer and/or its treatment to depressive and related sickness behavior symptoms. We outline several regulatory pathways by which cortisol, inflammatory processes, and depressive symptoms may interact in the context of cancer and highlight implications of these interactions for tumor progression. Additional research is needed to delineate these pathways and advance scientific understanding of the biobehavioral mechanisms underlying depressive symptoms in the context of cancer, with important implications for the development of effective interventions for patients undergoing initial cancer treatment, as well as for long-term survivors.

Functional expression and axonal transport of α7 nAChRs by peptidergic nociceptors of rat dorsal root ganglion

In recent pain studies on animal models, α7 nicotinic acetylcholine receptor (nAChR) agonists demonstrated analgesic, anti-hyperalgesic and anti-inflammatory effects, apparently acting through some peripheral receptors. Assuming possible involvement of α7 nAChRs on nociceptive sensory neurons, we investigated the morphological and neurochemical features of the α7 nAChR-expressing subpopulation of dorsal root ganglion (DRG) neurons and their ability to transport α7 nAChR axonally. In addition, α7 receptor activity and its putative role in pain signal neurotransmitter release were studied. Medium-sized α7 nAChR-expressing neurons prevailed, although the range covered all cell sizes. These cells accounted for one-fifth of total medium and large DRG neurons and <5% of small ones. 83.2% of α7 nAChR-expressing DRG neurons were peptidergic nociceptors (CGRP-immunopositive), one half of which had non-myelinated C-fibers and the other half had myelinated Aδ- and likely Aα/β-fibers, whereas 15.2% were non-peptidergic C-fiber nociceptors binding isolectin B4. All non-peptidergic and a third of peptidergic α7 nAChR-bearing nociceptors expressed TRPV1, a capsaicin-sensitive noxious stimulus transducer. Nerve crush experiments demonstrated that CGRPergic DRG nociceptors axonally transported α7 nAChRs both to the spinal cord and periphery. α7 nAChRs in DRG neurons were functional as their specific agonist PNU282987 evoked calcium rise enhanced by α7-selective positive allosteric modulator PNU120596. However, α7 nAChRs do not modulate neurotransmitter CGRP and glutamate release from DRG neurons since nicotinic ligands affected neither their basal nor provoked levels, showing the necessity of further studies to elucidate the true role of α7 nAChRs in those neurons.

Getting nervous about immunity

Twenty-five years ago, immunologists and neuroscientists had little science of mutual interest. This is no longer the case. Neuroscientists now know that the first formally defined cytokine, IL-1, activates a discrete population of hypothalamic neurons. This interaction leads to the release of glucocorticoids from the adrenal gland, a hormone that has a long history in immunoregulation. Immunologists have been surprised to learn that lymphoid cells synthesize acetylcholine, the first formally recognized neurotransmitter. This neurotransmitter suppresses the synthesis of TNF. These discoveries blur the distinction of neuroscience and immunology as distinct disciplines. There are now 37 formally recognized cytokines and their receptors, and at least 60 classical neurotransmitters plus over 50 neuroactive peptides. These findings explain why both immunologists and neuroscientists are getting nervous about immunity and highlight a real need to apply integrative physiological approaches in biomedical research.

New concepts of immune modulation in xenotransplantation

The shortage of human organs for transplantation has focused research on the possibility of transplanting pig organs into humans. Many factors contribute to the failure of a pig organ graft in a primate. A rapid innate immune response (natural anti-pig antibody, complement activation, and an innate cellular response; e.g., neutrophils, monocytes, macrophages, and natural killer cells) is followed by an adaptive immune response, although T-cell infiltration of the graft has rarely been reported. Other factors (e.g., coagulation dysregulation and inflammation) appear to play a significantly greater role than in allotransplantation. The immune responses to a pig xenograft cannot therefore be controlled simply by suppression of T-cell activity. Before xenotransplantation can be introduced successfully into the clinic, the problems of the innate, coagulopathic, and inflammatory responses will have to be overcome, most likely by the transplantation of organs from genetically engineered pigs. Many of the genetic manipulations aimed at protecting against these responses also reduce the adaptive response. The T-cell and elicited antibody responses can be prevented by the biological and/or pharmacologic agents currently available, in particular, by costimulation blockade-based regimens. The exogenous immunosuppressive regimen may be significantly reduced by the presence of a graft from a pig transgenic for a mutant (human) class II transactivator gene, resulting in down-regulation of swine leukocyte antigen class II expression, or from a pig with "local" vascular endothelial cell expression of an immunosuppressive gene (e.g., CTLA4-Ig). The immunomodulatory efficacy of regulatory T cells or mesenchymal stromal cells has been demonstrated in vitro but not yet in vivo.

Adrenergic β2 receptor activation stimulates anti-inflammatory properties of dendritic cells in vitro

Vagal nerve efferent activation has been shown to ameliorate the course of many inflammatory disease states. This neuro-modulatory effect has been suggested to rest on acetylcholine receptor (AChR) activation on tissue macrophages or dendritic cells (DCs). In more recent studies, vagal anti-inflammatory activity was shown involve adrenergic, splenic, pathways. Here we provide evidence that the adrenergic, rather than cholinergic, receptor activation on bone marrow derived DCs results in enhanced endocytosis uptake, enhanced IL-10 production but a decreased IL-6, IL-12p70 and IL-23 production. In antigen specific T cell stimulation assays, adrenergic β2 receptor activation on bone marrow DCs led to an enhanced potential to induce Foxp3 positive suppressive Treg cells. These effects were independent of IL10-R activation, TGFβ release, or retinoic acid (RA) secretion. Hence, adrenergic receptor β2 activation modulates DC function resulting in skewing towards anti-inflammatory T cell phenotypes.

Impact of vagal nerve stimulation on left atrial structure and function in a canine high-rate pacing model

BACKGROUND

Cervical vagal nerve stimulation (VNS) can improve left ventricular dysfunction in the setting of heart failure (HF). However, little is known about the impact of VNS on left atrial (LA) function. The aim of this study was to compare LA mechanics and histology between control and VNS-treated animals during HF development.

METHODS AND RESULTS

Fifteen mongrel dogs were randomized into control (n=7) and VNS (n=8) groups. All dogs underwent 8 weeks of high-rate ventricular pacing (at 220 beats per minute for the first 4 weeks to develop HF and another 4 weeks at 180 beats per minute to maintain HF). LA contractile function (LA negative peak strain), conduit function (LA positive peak strain), and reservoir function (LA total strain) were measured from speckle tracking in 2 groups. At the end of the terminal study, the LA appendage was obtained. Baseline LA strains were comparable in the control and VNS-treated dogs. At 4 and 8 weeks of ventricular pacing, all LA strains were decreased and LA volumes were increased in the control group compared with the VNS group (P<0.05). Histological evaluation of the left atrium revealed that percent fibrosis was significantly lower in the VNS versus the control group (8±1% versus 13±1%; P<0.001). Finally, transmitral flow showed decreased atrial contribution to left ventricular filling in the control group (P<0.05).

CONCLUSIONS

VNS improved LA function and volumes and suppressed LA fibrosis in the canine high-rate ventricular pacing model. VNS is a novel and potentially useful therapy for improving LA function during HF.

Long-term exposure to concentrated ambient PM2.5 increases mouse blood pressure through abnormal activation of the sympathetic nervous system: a role for hypothalamic inflammation

BACKGROUND

Exposure to particulate matter≤2.5 μm in diameter (PM2.5) increases blood pressure (BP) in humans and animal models. Abnormal activation of the sympathetic nervous system may have a role in the acute BP response to PM2.5 exposure. The mechanisms responsible for sympathetic nervous system activation and its role in chronic sustenance of hypertension in response to PM2.5 exposure are currently unknown.

OBJECTIVES

We investigated whether central nervous system inflammation may be implicated in chronic PM2.5 exposure-induced increases in BP and sympathetic nervous system activation.

METHODS

C57BL/6J mice were exposed to concentrated ambient PM2.5 (CAPs) for 6 months, and we analyzed BP using radioactive telemetric transmitters. We assessed sympathetic tone by measuring low-frequency BP variability (LF-BPV) and urinary norepinephrine excretion. We also tested the effects of acute pharmacologic inhibitors of the sympathetic nervous system and parasympathetic nervous system.

RESULTS

Long-term CAPs exposure significantly increased basal BP, paralleled by increases in LF-BPV and urinary norepinephrine excretion. The increased basal BP was attenuated by the centrally acting α2a agonist guanfacine, suggesting a role of increased sympathetic tone in CAPs exposure-induced hypertension. The increase in sympathetic tone was accompanied by an inflammatory response in the arcuate nucleus of the hypothalamus, evidenced by increased expression of pro-inflammatory genes and inhibitor kappaB kinase (IKK)/nuclear factor-kappaB (NF-κB) pathway activation.

CONCLUSION

Long-term CAPs exposure increases BP through sympathetic nervous system activation, which may involve hypothalamic inflammation.

The cholinergic anti-inflammatory pathway: a critical review

From a critical review of the evidence on the cholinergic anti-inflammatory pathway and its mode of action, the following conclusions were reached. (1) Both local and systemic inflammation may be suppressed by electrical stimulation of the peripheral cut end of either vagus. (2) The spleen mediates most of the systemic inflammatory response (measured by TNF-α production) to systemic endotoxin and is also the site where that response is suppressed by vagal stimulation. (3) The anti-inflammatory effect of vagal stimulation depends on the presence of noradrenaline-containing nerve terminals in the spleen. (4) There is no disynaptic connection from the vagus to the spleen via the splenic sympathetic nerve: vagal stimulation does not drive action potentials in the splenic nerve. (5) Acetylcholine-synthesizing T lymphocytes provide an essential non-neural link in the anti-inflammatory pathway from vagus to spleen. (6) Alpha-7 subunit-containing nicotinic receptors are essential for the vagal anti-inflammatory action: their critical location is uncertain, but is suggested here to be on splenic sympathetic nerve terminals. (7) The vagal anti-inflammatory pathway can be activated electrically or pharmacologically, but it is not the efferent arm of the inflammatory reflex response to endotoxemia.

Neonicotinoid clothianidin adversely affects insect immunity and promotes replication of a viral pathogen in honey bees

Large-scale losses of honey bee colonies represent a poorly understood problem of global importance. Both biotic and abiotic factors are involved in this phenomenon that is often associated with high loads of parasites and pathogens. A stronger impact of pathogens in honey bees exposed to neonicotinoid insecticides has been reported, but the causal link between insecticide exposure and the possible immune alteration of honey bees remains elusive. Here, we demonstrate that the neonicotinoid insecticide clothianidin negatively modulates NF-κB immune signaling in insects and adversely affects honey bee antiviral defenses controlled by this transcription factor. We have identified in insects a negative modulator of NF-κB activation, which is a leucine-rich repeat protein. Exposure to clothianidin, by enhancing the transcription of the gene encoding this inhibitor, reduces immune defenses and promotes the replication of the deformed wing virus in honey bees bearing covert infections. This honey bee immunosuppression is similarly induced by a different neonicotinoid, imidacloprid, but not by the organophosphate chlorpyriphos, which does not affect NF-κB signaling. The occurrence at sublethal doses of this insecticide-induced viral proliferation suggests that the studied neonicotinoids might have a negative effect at the field level. Our experiments uncover a further level of regulation of the immune response in insects and set the stage for studies on neural modulation of immunity in animals. Furthermore, this study has implications for the conservation of bees, as it will contribute to the definition of more appropriate guidelines for testing chronic or sublethal effects of pesticides used in agriculture.

Fetal cholinergic anti-inflammatory pathway and necrotizing enterocolitis: the brain-gut connection begins in utero

Necrotizing enterocolitis (NEC) is an acute neonatal inflammatory disease that affects the intestine and may result in necrosis, systemic sepsis and multisystem organ failure. NEC affects 5-10% of all infants with birth weight ≤ 1500 g or gestational age less than 30 weeks. Chorioamnionitis (CA) is the main manifestation of pathological inflammation in the fetus and is strong associated with NEC. CA affects 20% of full-term pregnancies and upto 60% of preterm pregnancies and, notably, is often an occult finding. Intrauterine exposure to inflammatory stimuli may switch innate immunity cells such as macrophages to a reactive phenotype ("priming"). Confronted with renewed inflammatory stimuli during labour or postnatally, such sensitized cells can sustain a chronic or exaggerated production of proinflammatory cytokines associated with NEC (two-hit hypothesis). Via the cholinergic anti-inflammatory pathway, a neurally mediated innate anti-inflammatory mechanism, higher levels of vagal activity are associated with lower systemic levels of proinflammatory cytokines. This effect is mediated by the α7 subunit nicotinic acetylcholine receptor (α7nAChR) on macrophages. The gut is the most extensive organ innervated by the vagus nerve; it is also the primary site of innate immunity in the newborn. Here we review the mechanisms of possible neuroimmunological brain-gut interactions involved in the induction and control of antenatal intestinal inflammatory response and priming. We propose a neuroimmunological framework to (1) study the long-term effects of perinatal intestinal response to infection and (2) to uncover new targets for preventive and therapeutic intervention.

Dysfunctional brain-bone marrow communication: a paradigm shift in the pathophysiology of hypertension

It is widely accepted that the pathophysiology of hypertension involves autonomic nervous system dysfunction, as well as a multitude of immune responses. However, the close interplay of these systems in the development and establishment of high blood pressure and its associated pathophysiology remains elusive and is the subject of extensive investigation. It has been proposed that an imbalance of the neuro-immune systems is a result of an enhancement of the "proinflammatory sympathetic" arm in conjunction with dampening of the "anti-inflammatory parasympathetic" arm of the autonomic nervous system. In addition to the neuronal modulation of the immune system, it is proposed that key inflammatory responses are relayed back to the central nervous system and alter the neuronal communication to the periphery. The overall objective of this review is to critically discuss recent advances in the understanding of autonomic immune modulation, and propose a unifying hypothesis underlying the mechanisms leading to the development and maintenance of hypertension, with particular emphasis on the bone marrow, as it is a crucial meeting point for neural, immune, and vascular networks.

Cholinergic receptors modulate immune complex-induced inflammation in vitro and in vivo

Cholinergic neural output has been shown to modulate innate immune responses to infection, injury and ischemia through stimulation of α7 nicotinic acetylcholine receptors (α7nAChR) on mononuclear phagocytes. We tested the hypothesis that cholinergic neurotransmitters, similar to those released through activation of a neural reflex, regulate responses to products of the adaptive immune system, specifically immune complex (IC)-mediated activation of effector cells. In this study, we show that stimulation of α7nAChR on human polymorphonuclear neutrophils (PMNs) and blood mononuclear phagocytes in vitro attenuates C5aR- and FcγR-triggered generation of reactive oxygen species, expression of leukocyte markers involved in cell recruitment and adhesion, and release of TNF-α and other proinflammatory cytokines. We show that this pathway is operative in vivo. Ligation of cholinergic receptors blunts IC-triggered responses in the reverse peritoneal Arthus reaction in mice. The selective 7nAChR agonist GTS21 decreased PMN accumulation and release of cytokines and chemokines at sites of IC deposition. In addition, mice lacking α7nAChR had exaggerated responses to reverse peritoneal Arthus reaction characterized by increased infiltration of PMNs and elevated of levels of TNF-α and CXCL1 in peritoneal fluid compared with wild-type mice. Taken together, these findings suggest that cholinergic output has the potential to exert tonic inhibitory activity that dampens responses to ICs and C5a and thus may be a target to minimize tissue damage in autoimmune diseases.

Brain-reactive antibodies and disease

Autoimmune diseases currently affect 5-7% of the world's population; in most diseases there are circulating autoantibodies. Brain-reactive antibodies are present in approximately 2-3% of the general population but do not usually contribute to brain pathology. These antibodies penetrate brain tissue only early in development or under pathologic conditions. This restriction on their pathogenicity and the lack of correlation between serum titers and brain pathology have, no doubt, contributed to a delayed appreciation of the contribution of autoantibodies in diseases of the central nervous system. Nonetheless, it is increasingly clear that antibodies can cause damage in the brain and likely initiate or aggravate multiple neurologic conditions; brain-reactive antibodies contribute to symptomatology in autoimmune disease, infectious disease, and malignancy.

Impact of Anesthetics on Immune Functions in a Rat Model of Vagus Nerve Stimulation

Vagus nerve stimulation (VNS) has been successfully performed in animals for the treatment of different experimental models of inflammation. The anti-inflammatory effect of VNS involves the release of acetylcholine by vagus nerve efferent fibers inhibiting pro-inflammatory cytokines (e.g. TNF-α) produced by macrophages. Moreover, it has recently been demonstrated that splenic lymphocytic populations may also be involved. As anesthetics can modulate the inflammatory response, the current study evaluated the effect of two different anesthetics, isoflurane and pentobarbital, on splenic cellular and molecular parameters in a VNS rat model. Spleens were collected for the characterization of lymphocytes sub-populations by flow cytometry and quantification of cytokines secretion after in vitro activation. Different results were observed depending on the anesthetic used. The use of isoflurane displayed a non-specific effect of VNS characterized by a decrease of most splenic lymphocytes sub-populations studied, and also led to a significantly lower TNF-α secretion by splenocytes. However, the use of pentobarbital brought to light immune modifications in non-stimulated animals that were not observed with isoflurane, and also revealed a specific effect of VNS, notably at the level of T lymphocytes’ activation. These differences between the two anesthetics could be related to the anti-inflammatory properties of isoflurane. In conclusion, pentobarbital is more adapted than isoflurane in the study of the anti-inflammatory effect of VNS on an anesthetized rat model in that it allows more accurate monitoring of subtle immunomodulatory processes.

C1 neurons: the body's EMTs

The C1 neurons reside in the rostral and intermediate portions of the ventrolateral medulla (RVLM, IVLM). They use glutamate as a fast transmitter and synthesize catecholamines plus various neuropeptides. These neurons regulate the hypothalamic pituitary axis via direct projections to the paraventricular nucleus and regulate the autonomic nervous system via projections to sympathetic and parasympathetic preganglionic neurons. The presympathetic C1 cells, located in the RVLM, are probably organized in a roughly viscerotopic manner and most of them regulate the circulation. C1 cells are variously activated by hypoglycemia, infection or inflammation, hypoxia, nociception, and hypotension and contribute to most glucoprivic responses. C1 cells also stimulate breathing and activate brain stem noradrenergic neurons including the locus coeruleus. Based on the various effects attributed to the C1 cells, their axonal projections and what is currently known of their synaptic inputs, subsets of C1 cells appear to be differentially recruited by pain, hypoxia, infection/inflammation, hemorrhage, and hypoglycemia to produce a repertoire of stereotyped autonomic, metabolic, and neuroendocrine responses that help the organism survive physical injury and its associated cohort of acute infection, hypoxia, hypotension, and blood loss. C1 cells may also contribute to glucose and cardiovascular homeostasis in the absence of such physical stresses, and C1 cell hyperactivity may contribute to the increase in sympathetic nerve activity associated with diseases such as hypertension.

Acetylcholinesterase inhibitors reduce neuroinflammation and -degeneration in the cortex and hippocampus of a surgery stress rat model

Exogenous stress like tissue damage and pathogen invasion during surgical trauma could lead to a peripheral inflammatory response and induce neuroinflammation, which can result in postoperative cognitive dysfunction (POCD). The cholinergic anti-inflammatory pathway is a neurohumoral mechanism that plays a prominent role by suppressing the inflammatory response. Treatments with acetylcholinesterase inhibitors enhance cholinergic transmission and may therefore act as a potential approach to prevent neuroinflammation. In the presence or absence of acetylcholinesterase inhibitors, adult Wistar rats underwent surgery alone or were additionally treated with lipopolysaccharide (LPS). Physostigmine, which can overcome the blood-brain barrier or neostigmine acting only peripheral, served as acetylcholinesterase inhibitors. The expression of pro- and anti-inflammatory cytokines in the cortex, hippocampus, spleen and plasma was measured after 1 h, 24 h, 3 d and 7 d using Real-Time PCR, western blot analysis or cytometric bead array (CBA). Fluoro-Jade B staining of brain slices was employed to elucidate neurodegeneration. The activity of acetylcholinesterase was estimated using a spectrofluorometric method. Surgery accompanied by LPS-treatment led to increased IL-1beta gene and protein upregulation in the cortex and hippocampus but was significantly reduced by physostigmine and neostigmine. Furthermore, surgery in combination with LPS-treatment caused increased protein expression of IL-1, TNF-alpha and IL-10 in the spleen and plasma. Physostigmine and neostigmine significantly decreased the protein expression of IL-1 and TNF-alpha. Neuronal degeneration and the activity of acetylcholinesterase were elevated after surgery with LPS-treatment and reduced by physostigmine and neostigmine. Along with LPS-treatment, acetylcholinesterase inhibitors reduce the pro-inflammatory response as well as neurodegeneration after surgery in the cortex and hippocampus. This combination may represent a tool to break the pathogenesis of POCD.

The Gut's Little Brain in Control of Intestinal Immunity

The gut immune system shares many mediators and receptors with the autonomic nervous system. Good examples thereof are the parasympathetic (vagal) and sympathetic neurotransmitters, for which many immune cell types in a gut context express receptors or enzymes required for their synthesis. For some of these the relevance for immune regulation has been recently defined. Earlier and more recent studies in neuroscience and immunology have indicated the anatomical and cellular basis for bidirectional interactions between the nervous and immune systems. Sympathetic immune modulation is well described earlier, and in the last decade the parasympathetic vagal nerve has been put forward as an integral part of an immune regulation network via its release of Ach, a system coined "the cholinergic anti-inflammatory reflex." A prototypical example is the inflammatory reflex, comprised of an afferent arm that senses inflammation and an efferent arm: the cholinergic anti-inflammatory pathway, that inhibits innate immune responses. In this paper, the current understanding of how innate mucosal immunity can be influenced by the neuronal system is summarized, and cell types and receptors involved in this interaction will be highlighted. Focus will be given on the direct neuronal regulatory mechanisms, as well as current advances regarding the role of microbes in modulating communication in the gut-brain axis.

Glutamatergic neurotransmission between the C1 neurons and the parasympathetic preganglionic neurons of the dorsal motor nucleus of the vagus

The C1 neurons are a nodal point for blood pressure control and other autonomic responses. Here we test whether these rostral ventrolateral medullary catecholaminergic (RVLM-CA) neurons use glutamate as a transmitter in the dorsal motor nucleus of the vagus (DMV). After injecting Cre-dependent adeno-associated virus (AAV2) DIO-Ef1α-channelrhodopsin2(ChR2)-mCherry (AAV2) into the RVLM of dopamine-β-hydroxylase Cre transgenic mice (DβH(Cre/0)), mCherry was detected exclusively in RVLM-CA neurons. Within the DMV >95% mCherry-immunoreactive(ir) axonal varicosities were tyrosine hydroxylase (TH)-ir and the same proportion were vesicular glutamate transporter 2 (VGLUT2)-ir. VGLUT2-mCherry colocalization was virtually absent when AAV2 was injected into the RVLM of DβH(Cre/0);VGLUT2(flox/flox) mice, into the caudal VLM (A1 noradrenergic neuron-rich region) of DβH(Cre/0) mice or into the raphe of ePet(Cre/0) mice. Following injection of AAV2 into RVLM of TH-Cre rats, phenylethanolamine N-methyl transferase and VGLUT2 immunoreactivities were highly colocalized in DMV within EYFP-positive or EYFP-negative axonal varicosities. Ultrastructurally, mCherry terminals from RVLM-CA neurons in DβH(Cre/0) mice made predominantly asymmetric synapses with choline acetyl-transferase-ir DMV neurons. Photostimulation of ChR2-positive axons in DβH(Cre/0) mouse brain slices produced EPSCs in 71% of tested DMV preganglionic neurons (PGNs) but no IPSCs. Photostimulation (20 Hz) activated PGNs up to 8 spikes/s (current-clamp). EPSCs were eliminated by tetrodotoxin, reinstated by 4-aminopyridine, and blocked by ionotropic glutamate receptor blockers. In conclusion, VGLUT2 is expressed by RVLM-CA (C1) neurons in rats and mice regardless of the presence of AAV2, the C1 neurons activate DMV parasympathetic PGNs monosynaptically and this connection uses glutamate as an ionotropic transmitter.

Vagus nerve stimulation: from epilepsy to the cholinergic anti-inflammatory pathway

BACKGROUND

The brain and the gut communicate bidirectionally through the autonomic nervous system (ANS). The vagus nerve (VN), a major component of the ANS, plays a key role in the neuro-endocrine-immune axis to maintain homeostasia through its afferents (through the activation of the hypothalamic pituitary adrenal axis and the central ANS) and through its efferents (i.e. the cholinergic anti-inflammatory pathway; CAP). The CAP has an anti-TNF effect both through the release of acetylcholine at the distal VN acting on macrophages and through the connection of the VN with the spleen through the splenic sympathetic nerve. Vagus nerve stimulation (VNS) of vagal afferents at high frequency (20-30 Hz) is used for the treatment of drug-resistant epilepsy and depression. Low-frequency (5 Hz) VNS of vagal efferents activates the CAP for an anti-inflammatory effect that is as an anti-TNF therapy in inflammatory diseases were TNF is a key cytokine as represented by experimental sepsis, postoperative ileus, burn-induced intestinal barrier injury, colitis. However, both vagal afferents and efferents are activated by VNS.

PURPOSE

The objective of this review was to explore the following: (i) the supporting evidence for the importance of VNS in epilepsy (and depression) and its mechanisms of action, (ii) the anti-inflammatory characteristics of the VN, (iii) the experimental evidence that VNS impact on inflammatory disorders focusing on the digestive tract, and (iv) how VNS could potentially be harnessed therapeutically in human inflammatory disorders such as inflammatory bowel diseases, irritable bowel syndrome, postoperative ileus, rheumatoid arthritis as an anti-inflammatory therapy.

Oxygen toxicity is reduced by acetylcholinesterase inhibition in the developing rat brain

The cholinergic anti-inflammatory pathway is a neural mechanism that suppresses the innate inflammatory response and controls inflammation employing acetylcholine as the key endogenous mediator. In this study, we investigated the effects of the cholinergic agonists, physostigmine and donepezil, on neurodegeneration, inflammation and oxidative stress during oxygen toxicity in the developing rat brain. The aim of this study was to investigate the level of neurodegeneration, expression of proinflammatory cytokines, glutathione and lipid peroxidation after hyperoxia and treatment with the acetylcholinesterase (AChE) inhibitors, physostigmine and donepezil in the brain of neonatal rats. Six-day-old Wistar rats were exposed to 80% oxygen for 12-24 h and received 100 μg/kg physostigmine or 200 μg/kg donepezil intraperitoneally. Sex-matched littermates kept in room air and injected with normal saline, physostigmine or donepezil served as controls. Treatment with both inhibitors significantly reduced hyperoxia-triggered activity of AChE, neural cell death and the upregulation of the proinflammatory cytokines IL-1β and TNF-α in the immature rat brain on the mRNA and protein level. In parallel, hyperoxia-induced oxidative stress was reduced by concomitant physostigmine and donepezil administration, as shown by an increased reduced/oxidized glutathione ratio and attenuated malondialdehyde levels, as a sign of lipid peroxidation. Our results suggest that a single treatment with AChE inhibitors at the beginning of hyperoxia attenuated the detrimental effects of oxygen toxicity in the developing brain and may pave the way for AChE inhibitors, which are currently used for the treatment of Alzheimer's disease, as potential candidates for adjunctive neuroprotective therapies to the immature brain.

Dysfunction of inflammation-resolving pathways is associated with exaggerated postoperative cognitive decline in a rat model of the metabolic syndrome

The cholinergic antiinflammatory pathway (CAP), which terminates in the spleen, attenuates postoperative cognitive decline (PCD) in rodents. Surgical patients with metabolic syndrome exhibit exaggerated and persistent PCD that is reproduced in postoperative rats selectively bred for easy fatigability and that contain all features of metabolic syndrome (low-capacity runners [LCRs]). We compared the CAP and lipoxin A(4) (LXA(4)), another inflammation-resolving pathway in LCR, with its counterpart high-capacity runner (HCR) rats. Isoflurane-anesthetized LCR and HCR rats either underwent aseptic trauma involving tibial fracture (surgery) or not (sham). At postoperative d 3 (POD3), compared with HCR, LCR rats exhibited significantly exaggerated PCD (trace fear conditioning freezing time 43% versus 57%). Separate cohorts were killed at POD3 to collect plasma for LXA4 and to isolate splenic mononuclear cells (MNCs) to analyze CAP signaling, regulatory T cells (Tregs) and M2 macrophages (M2 Mφ). Under lipopolysaccharide (LPS) stimulation, tumor necrosis factor (TNF)-α produced by splenic MNCs was 117% higher in LCR sham and 52% higher in LCR surgery compared with HCR sham and surgery rats; LPS-stimulated TNF-α production could not be inhibited by an α7 nicotinic acetylcholine receptor agonist, whereas inhibition by the β(2) adrenergic agonist, salmeterol, was significantly less (-35%) than that obtained in HCR rats. Compared to HCR, sham and surgery LCR rats had reduced β(2) adrenergic receptor-expressing T lymphocytes (59%, 44%), Tregs (47%, 54%) and M2 Mφ (45%, 39%); surgical LCR rats' hippocampal M2 Mφ was 66% reduced, and plasma LXA4 was decreased by 120%. Rats with the metabolic syndrome have ineffective inflammation-resolving mechanisms that represent plausible reasons for the exaggerated and persistent PCD.

A dialogue between the immune system and brain, spoken in the language of serotonin

Neuropsychiatric disorders have long been linked to both immune system activation and alterations in serotonin (5-HT) signaling. In the CNS, the contributions of 5-HT modulate a broad range of targets, most notably, hypothalamic, limbic and cortical circuits linked to the control of mood and mood disorders. In the periphery, many are aware of the production and actions of 5-HT in the gut but are unaware that the molecule and its receptors are also present in the immune system where evidence suggests they contribute to the both innate and adaptive responses. In addition, there is clear evidence that the immune system communicates to the brain via both humoral and neuronal mechanisms, and that CNS 5-HT neurons are a direct or indirect target for these actions. Following a brief primer on the immune system, we describe our current understanding of the synthesis, release, and actions of 5-HT in modulating immune function, including the expression of 5-HT biosynthetic enzymes, receptors, and transporters that are typically studied with respect to the roles in the CNS. We then orient our presentation to recent findings that pro-inflammatory cytokines can modulate CNS 5-HT signaling, leading to a conceptualization that among the many roles of 5-HT in the body is an integrated physiological and behavioral response to inflammatory events and pathogens. From this perspective, altered 5-HT/immune conversations are likely to contribute to risk for neurobehavioral disorders historically linked to compromised 5-HT function or ameliorated by 5-HT targeted medications, including depression and anxiety disorders, obsessive-compulsive disorder (OCD), and autism. Our review raises the question as to whether genetic variation impacting 5-HT signaling genes may contribute to maladaptive behavior as much through perturbed immune system modulation as through altered brain mechanisms. Conversely, targeting the immune system for therapeutic development may provide an important opportunity to treat mental illness.

The resolution of inflammation

In 2012, Nature Reviews Immunology organized a conference that brought together scientists and clinicians from both academia and industry to discuss one of the most pressing questions in medicine--how do we turn off rampant, undesirable inflammation? There is a growing appreciation that, similarly to the initiation of inflammation, the resolution of inflammation is an intricate and active process. Can we therefore harness the mediators involved in resolution responses to treat patients with chronic inflammatory or autoimmune diseases? Here, we ask five of the speakers from the conference to share their thoughts on this emerging field.

Neuroimmunology of the atherosclerotic plaque: a morphological approach

Atherosclerosis is a chronic inflammatory process, lasting for several decades until the onset of its clinical manifestations. The progression of the atherosclerotic lesion to a stable fibrotic plaque, narrowing the vascular lumen, or to a vulnerable plaque leading to main vascular complications, is associated to the involvement of several cell subpopulations of the innate as well as of the adaptive immunity, and to the release of chemokines and pro-inflammatory cytokines. Emerging evidence outlines that the cardiovascular risk is dependent on stress-mediators influencing cell migration and vascular remodeling. The view that atherosclerosis is initiated by monocytes and lymphocytes adhering to dysfunctional endothelial cells is substantiated by experimental and clinical observations. Macrophages, dendritic cells, T and B lymphocytes, granulocytes accumulating into the subendothelial space secrete and are stimulated by soluble factors, including peptides, proteases and cytokines acting synergistically. The final step of the disease, leading to plaque destabilization and rupture, is induced by the release, at the level of the fibrous cap, of metalloproteinases and elastases by the activated leukocytes which accumulate locally. Recruitment of specific cell subpopulations as well as the progression of atherosclerotic lesions towards a stable or an unstable phenotype, are related to the unbalance between pro-atherogenic and anti-atherogenic factors. In this connection stress hormones deserve particular attention, since their role in vascular remodeling, via vascular smooth cell proliferation, as well as in neoangiogenesis, via stimulation of endothelial cell proliferation and migration, has been already established.

Epidemiological and experimental links between air pollution and type 2 diabetes

There is increasing evidence suggesting links between exposure to environmental toxins and susceptibility to type 2 diabetes mellitus (DM). In this review, we summarize the experimental evidence to support this association that has been noted in many epidemiologic studies. Inflammation in response to particulate matter (PM(2.5)) exposure in air pollution represents a common mechanism that may interact with other pro-inflammatory influences in diet and life style to modulate susceptibility to cardiometabolic diseases. The role of innate immune cytokines released from macrophages in the lung is well known. In addition, chemokine triggers in response to air-pollution exposure may mediate a cellular response from the bone marrow/spleen through toll-like receptors (TLRs) and Nucleotide Oligomerization Domain receptors (NLRs) pathways to mediate inflammatory response in organs. Emerging data also seem to support a role for PM(2.5) exposure in endoplasmic reticulum stress-induced apoptosis and in brown adipose tissue dysfunction. Decreased expression of UCP1 in brown adipose tissue may account for reduced thermogenesis providing another link between PM(2.5) and insulin resistance. The implications of an experimental link between air-pollution exposure and type 2 DM are profound as air pollution is a pervasive risk factor throughout the world and even modest alleviation in exposure may provide substantial public health benefits.