α7-cholinergic receptor mediates vagal induction of splenic norepinephrine

Neuroendocrine regulation of inflammation

The interaction between the sympathetic nervous system and the immune system has been documented over the last several decades. In this review, the neuroanatomical, cellular, and molecular evidence for neuroimmune regulation in the maintenance of immune homeostasis will be discussed, as well as the potential impact of neuroimmune dysregulation in health and disease.

Nicotinic acetylcholine receptors in glucose homeostasis: the acute hyperglycemic and chronic insulin-sensitive effects of nicotine suggest dual opposing roles of the receptors in male mice

Cigarette smoking causes insulin resistance. However, nicotine induces anti-inflammation and improves glucose tolerance in insulin-resistant animal models. Here, we determined the effects of nicotine on glucose metabolism in insulin-sensitive C57BL/J6 mice. Acute nicotine administration (30 min) caused fasting hyperglycemia and lowered insulin sensitivity acutely, which depended on the activation of nicotinic-acetylcholine receptors (nAChRs) and correlated with increased catecholamine secretion, nitric oxide (NO) production, and glycogenolysis. Chlorisondamine, an inhibitor of nAChRs, reduced acute nicotine-induced hyperglycemia. qRT-PCR analysis revealed that the liver and muscle express predominantly β4 > α10 > α3 > α7 and β4 > α10 > β1 > α1 mRNA for nAChR subunits respectively, whereas the adrenal gland expresses β4 > α3 > α7 > α10 mRNA. Chronic nicotine treatment significantly suppressed expression of α3-nAChR (predominant peripheral α-subunit) in liver. Whereas acute nicotine treatment raised plasma norepinephrine (NE) and epinephrine (Epi) levels, chronic nicotine exposure raised only Epi. Acute nicotine treatment raised both basal and glucose-stimulated insulin secretion (GSIS). After chronic nicotine treatment, basal insulin level was elevated, but GSIS after acute saline or nicotine treatment was blunted. Chronic nicotine exposure caused an increased buildup of NO in plasma and liver, leading to decreased glycogen storage, along with a concomitant suppression of Pepck and G6Pase mRNA, thus preventing hyperglycemia. The insulin-sensitizing effect of chronic nicotine was independent of weight loss. Chronic nicotine treatment enhanced PI-3-kinase activities and increased Akt and glycogen synthase kinase (GSK)-3β phosphorylation in an nAChR-dependent manner coupled with decreased cAMP response element-binding protein (CREB) phosphorylation. The latter effects caused suppression of Pepck and G6Pase gene expression. Thus, nicotine causes both insulin resistance and insulin sensitivity depending on the duration of the treatment.

Reflex control of inflammation by the splanchnic anti-inflammatory pathway is sustained and independent of anesthesia

Following an immune challenge, there is two-way communication between the nervous and immune systems. It is proposed that a neural reflex--the inflammatory reflex--regulates the plasma levels of the key proinflammatory cytokine TNF-α, and that its efferent pathway is in the splanchnic sympathetic nerves. The evidence for this reflex is based on experiments on anesthetized animals, but anesthesia itself suppresses inflammation, confounding interpretation. Here, we show that previous section of the splanchnic nerves strongly enhances the levels of plasma TNF-α in conscious rats 90 min after they received intravenous LPS (60 μg/kg). The same reflex mechanism, therefore, applies in conscious as in anesthetized animals. In anesthetized rats, we then determined the longer-term effects of splanchnic nerve section on responses to LPS (60 μg/kg iv). We confirmed that prior splanchnic nerve section enhanced the early (90 min) peak in plasma TNF-α and found that it reduced the 90-min peak of the anti-inflammatory cytokine IL-10; both subsequently fell to low levels in all animals. Splanchnic nerve section also enhanced the delayed rise in two key proinflammatory cytokines IL-6 and interferon γ. That enhancement was undiminished after 6 h, when other measured cytokines had subsided. Finally, LPS treatment caused hypotensive shock in rats with cut splanchnic nerves but not in sham-operated animals. These findings demonstrate that reflex activation of the splanchnic anti-inflammatory pathway has a powerful and sustained restraining influence on inflammatory processes.

Nicotinic receptor Alpha7 expression during mouse adrenal gland development

The nicotinic acetylcholine receptor alpha 7 (α7) is a ligand-activated ion channel that contributes to a diversity of cellular processes involved in development, neurotransmission and inflammation. In this report the expression of α7 was examined in the mouse developing and adult adrenal gland that expresses a green fluorescent protein (GFP) reporter as a bi-cistronic extension of the endogenous α7 transcript (α7(G)). At embryonic day 12.5 (E12.5) α7(G) expression was associated with the suprarenal ganglion and precursor cells of the adrenal gland. The α7(G) cells are catecholaminergic chromaffin cells as reflected by their progressive increase in the co-expression of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) that is complete by E18.5. In the adult, α7(G) expression is limited to a subset of chromaffin cells in the adrenal medulla that cluster near the border with the adrenal cortex. These chromaffin cells co-express α7(G), TH and DBH, but they lack phenylethanolamine N-methyltransferase (PNMT) consistent with only norepinephrine (NE) synthesis. These cell groups appear to be preferentially innervated by pre-ganglionic afferents identified by the neurotrophin receptor p75. No afferents identified by beta-III tubulin, neurofilament proteins or p75 co-expressed α7(G). Occasional α7(G) cells in the pre-E14.5 embryos express neuronal markers consistent with intrinsic ganglion cells and in the adult some α7(G) cells co-express glutamic acid decarboxylase. The transient expression of α7 during adrenal gland development and its prominent co-expression by a subset of NE chromaffin cells in the adult suggests that the α7 receptor contributes to multiple aspects of adrenal gland development and function that persist into adulthood.

Endogenous vagal activation dampens intestinal inflammation independently of splenic innervation in postoperative ileus

Postoperative ileus is encountered by patients undergoing open abdominal surgery and is characterized by intestinal inflammation associated with impaired gastrointestinal motility. We recently showed that inflammation of the gut muscularis triggered activation of the vagal efferent pathway mainly targeting the inflamed zone. In the present study we investigate further the modulatory role of endogenous activation of the vagal motor pathway on the innate immune response. Intestinal or splenic denervation was performed two weeks prior to intestinal manipulation (IM) or laparotomy (L). Twenty-four hour post-surgery, the gastrointestinal transit, immune cell influx, and pro-inflammatory cytokine levels were measured in the gut muscularis. Manipulation of the small intestine led to a delay in intestinal transit, an influx of leukocytes and increased pro-inflammatory cytokine expression. Surgical lesion of the vagal branch that selectively innervates the small intestine did not further delay the intestinal transit but significantly enhanced the expression levels of the pro-inflammatory cytokines IL-1β and IL-6 in the gut muscularis. Splenic denervation did not affect intestinal inflammation or gastrointestinal transit after intestinal manipulation. Our study demonstrates that selective vagotomy, leaving the splenic innervation intact, increases surgery-induced intestinal inflammation. These data suggest that endogenous activation of the vagal efferent pathway by intestinal inflammation directly dampens the local immune response triggered by intestinal manipulation independently of the spleen.

Vagus nerve through α7 nAChR modulates lung infection and inflammation: models, cells, and signals

Cholinergic anti-inflammatory pathway (CAP) bridges immune and nervous systems and plays pleiotropic roles in modulating inflammation in animal models by targeting different immune, proinflammatory, epithelial, endothelial, stem, and progenitor cells and signaling pathways. Acute lung injury (ALI) is a devastating inflammatory disease. It is pathogenically heterogeneous and involves many cells and signaling pathways. Here, we emphasized the research regarding the modulatory effects of CAP on animal models, cell population, and signaling pathways that involved in the pathogenesis of ALI. By comparing the differential effects of CAP on systemic and pulmonary inflammation, we postulated that a pulmonary parasympathetic inflammatory reflex is formed to sense and respond to pathogens in the lung. Work targeting the formation and function of pulmonary parasympathetic inflammatory reflex would extend our understanding of how vagus nerve senses, recognizes, and fights with pathogens and inflammatory responses.

The spleen responds to intestinal manipulation but does not participate in the inflammatory response in a mouse model of postoperative ileus

Background

Postoperative ileus is characterized by a transient impairment of the gastrointestinal motility after abdominal surgery. The intestinal inflammation, triggered by handling of the intestine, is the main factor responsible for the prolonged dysmotility of the gastrointestinal tract. Secondary lymphoid organs of the intestine were identified as essential components in the dissemination of inflammation to the entire gastrointestinal tract also called field effect. The involvement of the spleen, however, remains unclear.

Aim

In this study, we investigated whether the spleen responds to manipulation of the intestine and participates in the intestinal inflammation underlying postoperative ileus.

Methods

Mice underwent Laparotomy (L) or Laparotomy followed by Intestinal Manipulation (IM). Twenty-four hours later, intestinal and colonic inflammation was assessed by QPCR and measurement of the intestinal transit was performed. Analysis of homeostatic chemokines in the spleen was performed by QPCR and splenic cell populations analysed by Flow Cytometry. Blockade of the egress of cells from the spleen was performed by administration of the Sphingosine-1-phosphate receptor 1 (S1P₁) agonist CYM-5442 10 h after L/IM.

Results

A significant decrease in splenic weight and cellularity was observed in IM mice 24 h post-surgery, a phenomenon associated with a decreased splenic expression level of the homeostatic chemokine CCL19. Splenic denervation restored the expression of CCL19 and partially prevented the reduction of splenocytes in IM mice. Treatment with CYM-5442 prevented the egress of splenocytes but did not ameliorate the intestinal inflammation underlying postoperative ileus.

Conclusions

Intestinal manipulation results in two distinct phenomena: local intestinal inflammation and a decrease in splenic cellularity. The splenic response relies on an alteration of cell trafficking in the spleen and is partially regulated by the splenic nerve. The spleen however does not participate in the intestinal inflammation during POI.

A distinct vagal anti-inflammatory pathway modulates intestinal muscularis resident macrophages independent of the spleen

The cholinergic anti-inflammatory pathway (CAIP) has been proposed as a key mechanism by which the brain, through the vagus nerve, modulates the immune system in the spleen. Vagus nerve stimulation (VNS) reduces intestinal inflammation and improves postoperative ileus. We investigated the neural pathway involved and the cells mediating the anti-inflammatory effect of VNS in the gut. The effect of VNS on intestinal inflammation and transit was investigated in wild-type, splenic denervated and Rag-1 knockout mice. To define the possible role of α7 nicotinic acetylcholine receptor (α7nAChR), we used knockout and bone marrow chimaera mice. Anterograde tracing of vagal efferents, cell sorting and Ca(2+) imaging were used to reveal the intestinal cells targeted by the vagus nerve. VNS attenuates surgery-induced intestinal inflammation and improves postoperative intestinal transit in wild-type, splenic denervated and T-cell-deficient mice. In contrast, VNS is ineffective in α7nAChR knockout mice and α7nAChR-deficient bone marrow chimaera mice. Anterograde labelling fails to detect vagal efferents contacting resident macrophages, but shows close contacts between cholinergic myenteric neurons and resident macrophages expressing α7nAChR. Finally, α7nAChR activation modulates ATP-induced Ca(2+) response in small intestine resident macrophages. We show that the anti-inflammatory effect of the VNS in the intestine is independent of the spleen and T cells. Instead, the vagus nerve interacts with cholinergic myenteric neurons in close contact with the muscularis macrophages. Our data suggest that intestinal muscularis resident macrophages expressing α7nAChR are most likely the ultimate target of the gastrointestinal CAIP.

Dopamine mediates vagal modulation of the immune system by electroacupuncture

Previous anti-inflammatory strategies against sepsis, a leading cause of death in hospitals, had limited efficacy in clinical trials, in part because they targeted single cytokines and the experimental models failed to mimic clinical settings. Neuronal networks represent physiological mechanisms, selected by evolution to control inflammation, that can be exploited for the treatment of inflammatory and infectious disorders. Here, we report that sciatic nerve activation with electroacupuncture controls systemic inflammation and rescues mice from polymicrobial peritonitis. Electroacupuncture at the sciatic nerve controls systemic inflammation by inducing vagal activation of aromatic L-amino acid decarboxylase, leading to the production of dopamine in the adrenal medulla. Experimental models with adrenolectomized mice mimic clinical adrenal insufficiency, increase the susceptibility to sepsis and prevent the anti-inflammatory effects of electroacupuncture. Dopamine inhibits cytokine production via dopamine type 1 (D1) receptors. D1 receptor agonists suppress systemic inflammation and rescue mice with adrenal insufficiency from polymicrobial peritonitis. Our results suggest a new anti-inflammatory mechanism mediated by the sciatic and vagus nerves that modulates the production of catecholamines in the adrenal glands. From a pharmacological perspective, the effects of selective dopamine agonists mimic the anti-inflammatory effects of electroacupuncture and can provide therapeutic advantages to control inflammation in infectious and inflammatory disorders.

Autonomic regulation of cellular immune function

The nervous system and the immune system (IS) are two integrative systems that work together to detect threats and provide host defense, and to maintain/restore homeostasis. Cross-talk between the nervous system and the IS is vital for health and well-being. One of the major neural pathways responsible for regulating host defense against injury and foreign antigens and pathogens is the sympathetic nervous system (SNS). Stimulation of adrenergic receptors (ARs) on immune cells regulates immune cell development, survival, proliferative capacity, circulation, trafficking for immune surveillance and recruitment, and directs the cell surface expression of molecules and cytokine production important for cell-to-cell interactions necessary for a coordinated immune response. Finally, AR stimulation of effector immune cells regulates the activational state of immune cells and modulates their functional capacity. This review focuses on our current understanding of the role of the SNS in regulating host defense and immune homeostasis. SNS regulation of IS functioning is a critical link to the development and exacerbation of chronic immune-mediated diseases. However, there are many mechanisms that need to be further unraveled in order to develop sound treatment strategies that act on neural-immune interaction to resolve or prevent chronic inflammatory diseases, and to improve health and quality of life.

Neuro-anatomical evidence indicating indirect modulation of macrophages by vagal efferents in the intestine but not in the spleen

Background

Electrical stimulation of the vagus nerve suppresses intestinal inflammation and normalizes gut motility in a mouse model of postoperative ileus. The exact anatomical interaction between the vagus nerve and the intestinal immune system remains however a matter of debate. In the present study, we provide additional evidence on the direct and indirect vagal innervation of the spleen and analyzed the anatomical evidence for neuroimmune modulation of macrophages by vagal preganglionic and enteric postganglionic nerve fibers within the intestine.

Methods

Dextran conjugates were used to label vagal preganglionic (motor) fibers projecting to the small intestine and spleen. Moreover, identification of the neurochemical phenotype of the vagal efferent fibers and enteric neurons was performed by immunofluorescent labeling. F4/80 antibody was used to label resident macrophages.

Results

Our anterograde tracing experiments did not reveal dextran-labeled vagal fibers or terminals in the mesenteric ganglion or spleen. Vagal efferent fibers were confined within the myenteric plexus region of the small intestine and mainly endings around nNOS, VIP and ChAT positive enteric neurons. nNOS, VIP and ChAT positive fibers were found in close proximity of intestinal resident macrophages carrying α7 nicotinic receptors. Of note, VIP receptors were found on resident macrophages located in close proximity of VIP positive nerve fibers.

Conclusion

In the present study, we show that the vagus nerve does not directly interact with resident macrophages in the gut or spleen. Instead, the vagus nerve preferentially interacts with nNOS, VIP and ChAT enteric neurons located within the gut muscularis with nerve endings in close proximity of the resident macrophages.

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.

Reflex control of inflammation by sympathetic nerves, not the vagus

We investigated a neural reflex that controls the strength of inflammatory responses to immune challenge - the inflammatory reflex. In anaesthetized rats challenged with intravenous lipopolysaccharide (LPS, 60 μg kg(-1)), we found strong increases in plasma levels of the key inflammatory mediator tumour necrosis factor α (TNFα) 90 min later. Those levels were unaffected by previous bilateral cervical vagotomy, but were enhanced approximately 5-fold if the greater splanchnic sympathetic nerves had been cut. Sham surgery had no effect, and plasma corticosterone levels were unaffected by nerve sections, so could not explain this result. Electrophysiological recordings demonstrated that efferent neural activity in the splanchnic nerve and its splenic branch was strongly increased by LPS treatment. Splenic nerve activity was dependent on inputs from the splanchnic nerves: vagotomy had no effect on the activity in either nerve. Together, these data demonstrate that immune challenge with this dose of LPS activates a neural reflex that is powerful enough to cause an 80% suppression of the acute systemic inflammatory response. The efferent arm of this reflex is in the splanchnic sympathetic nerves, not the vagi as previously proposed. As with other physiological responses to immune challenge, the afferent pathway is presumptively humoral: the present data show that vagal afferents play no measurable part. Because inflammation sits at the gateway to immune responses, this reflex could play an important role in immune function as well as inflammatory diseases.

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.

Neural networks in intestinal immunoregulation

Key physiological functions of the intestine are governed by nerves and neurotransmitters. This complex control relies on two neuronal systems: an extrinsic innervation supplied by the two branches of the autonomic nervous system and an intrinsic innervation provided by the enteric nervous system. As a result of constant exposure to commensal and pathogenic microflora, the intestine developed a tightly regulated immune system. In this review, we cover the current knowledge on the interactions between the gut innervation and the intestinal immune system. The relations between extrinsic and intrinsic neuronal inputs are highlighted with regards to the intestinal immune response. Moreover, we discuss the latest findings on mechanisms underlying inflammatory neural reflexes and examine their relevance in the context of the intestinal inflammation. Finally, we discuss some of the recent data on the identification of the gut microbiota as an emerging player influencing the brain function.

Parasympathetic stimulation via the vagus nerve prevents systemic organ dysfunction by abrogating gut injury and lymph toxicity in trauma and hemorrhagic shock

We tested if vagus nerve stimulation (VNS) would prevent gut injury, mesenteric lymph toxicity, and systemic multiple organ dysfunction syndrome following trauma-hemorrhagic shock (T/HS). Four groups of experiments were performed. The first tested whether VNS (5 V for 10 min) would protect against T/HS-induced increases in gut and lung permeability as well as neutrophil priming. In the second experiment, mesenteric lymph was collected from rats subjected to T/HS or trauma-sham shock with or without VNS and then injected into naive mice to assess its biologic activity. Lung permeability, neutrophil priming, and red blood cell deformability were measured. Next, the role of the spleen in VNS-mediated protection was tested by measuring gut and lung injury in splenectomized rats subjected to sham or actual VNS. Lastly, the ability of nicotine to replicate the gut-protective effect of VNS was tested. Vagus nerve stimulation protected against T/HS-induced gut injury, lung injury, and neutrophil priming (P < 0.05). Not only did VNS limit organ injury after T/HS, but in contrast to the mesenteric lymph collected from the sham-VNS T/HS rats, the mesenteric lymph from the VNS T/HS rats did not cause lung injury, neutrophil priming, or loss of red blood cell deformability (P < 0.05) when injected into naive mice. Removal of the spleen did not prevent the protective effects of VNS on gut or lung injury after T/HS. Similar to VNS, the administration of nicotine also protected the gut from injury after T/HS. Vagus nerve stimulation prevents T/HS-induced gut injury, lung injury, neutrophil priming, and the production of biologically active mesenteric lymph. This protective effect of VNS was not dependent on the spleen but appeared to involve a cholinergic nicotinic receptor, because its beneficial effects could be replicated with nicotine.

Noradrenergic neurons regulate monocyte trafficking and mortality during gram-negative peritonitis in mice

Effective host defense requires a robust, yet self-limited response to pathogens. A poorly calibrated response can lead to either bacterial dissemination due to insufficient inflammation or organ injury due to excessive inflammation. Recent evidence suggests that the cholinergic anti-inflammatory reflex helps calibrate the immune response. However, the influence of peripheral noradrenergic neurons, which are primarily sympathetic neurons, in regulating immunity remains incompletely characterized. Using a model of 6-hydroxydopamine-mediated noradrenergic nerve ablation, we show that elimination of noradrenergic neurons improves survival during Klebsiella pneumoniae peritonitis (67 versus 23%, p < 0.005) in mice. The survival benefit results from enhanced MCP-1-dependent monocyte recruitment and a subsequent decrease in bacterial loads. Splenectomy eliminated both the survival benefit of 6-hydroxydopamine and monocyte recruitment, suggesting that monocytes recruited to the peritoneum originate in the spleen. These results suggest that noradrenergic neurons regulate the immune response through two pathways. First, sympathetic nerve-derived norepinephrine directly restrains MCP-1 production by peritoneal macrophages during infection. Second, norepinephrine derived from the vagally innervated splenic nerve regulates splenic monocyte egress. Removal of these two modulators of the immune response enhances antibacterial immunity and improves survival. These results may have implications for how states of catecholamine excess influence the host response to bacterial infections.

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.

Stress induced neuroendocrine-immune plasticity: A role for the spleen in peripheral inflammatory disease and inflammaging?

Research over the past decade has revealed close interaction between the nervous and immune systems in regulation of peripheral inflammation linking psychosocial stress with chronic somatic disease and aging. Moreover emerging data suggests that chronic inflammations lead to a pro-inflammatory status underlying premature aging called inflammaging. In this context, the spleen can be seen as a switch board monitoring peripherally derived neuroendocrine-immune mediators in the blood and keeping up a close communication with the central stress response via its mainly sympathetic innervation. The effect aims at balanced and well-timed stress axis activation and immune adaptation in acute peripheral inflammatory events. Constant adjustment to the needs generated by environmental and endogenous challenges is provided by neuroendocrine-immune plasticity. However, maladaptive plasticity induced e.g., by chronic stress-axis activation and excessive non-neuronal derived neuroendocrine mediators may be at the heart of the observed stress sensitivity promote inflammaging under chronic inflammatory conditions. We here review the role of neurotransmitters, neuropeptides and neurotrophins as stress mediators modulating the immune response in the spleen and their potential role in inflammaging.

Rivastigmine alleviates experimentally induced colitis in mice and rats by acting at central and peripheral sites to modulate immune responses

The cholinergic anti-inflammatory system and α7 nicotinic receptors in macrophages have been proposed to play a role in neuroimmunomodulation and in the etiology of ulcerative colitis. We investigated the ability of a cholinesterase (ChE) inhibitor rivastigmine, to improve the pathology of ulcerative colitis by increasing the concentration of extracellular acetylcholine in the brain and periphery. In combination with carbachol (10 µM), rivastigmine (1 µM) significantly decreased the release of nitric oxide, TNF-α, IL-1β and IL-6 from lipopolysaccharide-activated RAW 264.7 macrophages and this effect was abolished by α7 nicotinic receptor blockade by bungarotoxin. Rivastigmine (1 mg/kg) but not (0.5 mg/kg), injected subcutaneously once daily in BALB/c mice with colitis induced by 4% dextran sodium sulphate (DSS), reduced the disease activity index (DAI) by 60% and damage to colon structure. Rivastigmine (1 mg/kg) also reduced myeloperoxidase activity and IL-6 by >60%, and the infiltration of CD11b expressing cells by 80%. These effects were accompanied by significantly greater ChE inhibition in cortex, brain stem, plasma and colon than that after 0.5 mg/kg. Co-administration of rivastigmine (1 mg/kg) with the muscarinic antagonist scopolamine significantly increased the number of CD11b expressing cells in the colon but did not change DAI compared to those treated with rivastigmine alone. Rivastigmine 1 and 2 mg given rectally to rats with colitis induced by rectal administration of 30 mg dintrobezene sulfonic acid (DNBS) also caused a dose related reduction in ChE activity in blood and colon, the number of ulcers and area of ulceration, levels of TNF-α and in MPO activity. The study revealed that the ChE inhibitor rivastigmine is able to reduce gastro-intestinal inflammation by actions at various sites at which it preserves ACh. These include ACh released from vagal nerve endings that activates alpha7 nicotinic receptors on circulating macrophages and in brainstem neurons.

Uncovering the neuroenteric-pulmonary axis: vagal nerve stimulation prevents acute lung injury following hemorrhagic shock

AIMS

Trauma/hemorrhagic shock (T/HS) induced gut injury is known to initiate a systemic inflammatory response which can lead to secondary lung injury. We have shown that vagal nerve stimulation (VNS) protects intestinal epithelial integrity after a severe burn insult. We hypothesize that VNS will protect the lung from injury following T/HS by preventing intestinal barrier failure.

MAIN METHODS

Male Balb/c mice were subjected to a T/HS model with and without cervical VNS. Intestinal injury was evaluated by measuring changes in gut barrier function and tight junction protein localization. Lung injury was evaluated using histology and markers of lung inflammation. Using NF-kB-luciferase (NF-kB-luc) transgenic mice, NF-kb-DNA binding was measured by photon emission analysis at 4 after injury.

KEY FINDINGS

T/HS is associated gut injury characterized by histologic injury, increased epithelial permeability, and altered localization of gut tight junction proteins. Cervical VNS prevented the T/HS-induced changes in gut barrier integrity. Gut injury after T/HS was associated with acute lung injury at 24 h characterized by histologic injury, increased number of MPO positive stained cells and MPO enzymatic activity, and increased ICAM-1 expression in lung endothelium. VNS decreased T/HS-induced lung injury with a marked decrease in lung inflammation compared to T/HS alone. Lungs harvested from NF-kB-luc mice at 4h post VNS+T/HS demonstrated decreased DNA binding of NF-kB compared to T/HS alone as measured by changes in bioluminescence.

SIGNIFICANCE

VNS is effective in protecting against acute lung injury caused by hemorrhagic shock through its ability to prevent gut barrier dysfunction.

NDP-MSH inhibits neutrophil migration through nicotinic and adrenergic receptors in experimental peritonitis

Melanocortin is a potent anti-inflammatory molecule. However, little is known about the effect of melanocortin on acute inflammatory processes such as neutrophil migration. In the present study, we investigated the ability of [Nle4, D-Phe7]-melanocyte-stimulating hormone (NDP-MSH), a semisynthetic melanocortin compound, in the inhibition of neutrophil migration in carrageenin-induced peritonitis model. Herein, subcutaneous pretreatment with NDP-MSH decreased neutrophil trafficking in the peritoneal cavity in a dose-dependent manner. NDP-MSH inhibited vascular leakage, leukocyte rolling, and adhesion and reduced peritoneal macrophage inflammatory protein 2, but not TNF-alpha, IL-1beta, IL-10, and keratinocyte-derived chemokine production. In addition, the effect on neutrophil migration was reverted by the pretreatment with both propranolol (a nonselective beta-adrenergic antagonist) and mecamylamine (a nonselective nicotinic antagonist) but not by splenectomy surgery. Moreover, NDP-MSH intracerebroventricular administration inhibited neutrophil migration, indicating participation of the central nervous system. Our results propose that the NDP-MSH effect may be due to a spleen-independent neuro-immune pathway that efficiently regulates excessive neutrophil recruitment to tissues.

The application of PET imaging in psychoneuroimmunology research

Positron emission tomography (PET) imaging is a research tool that allows in vivo measurements of brain metabolism and specific target molecules. PET imaging can be used to measure these brain variables in a variety of species, including human and non-human primates, and rodents. PET imaging can therefore be combined with various experimental and clinical model systems that are commonly used in psychoneuroimmunology research.

Rethinking inflammation: neural circuits in the regulation of immunity

Neural reflex circuits regulate cytokine release to prevent potentially damaging inflammation and maintain homeostasis. In the inflammatory reflex, sensory input elicited by infection or injury travels through the afferent vagus nerve to integrative regions in the brainstem, and efferent nerves carry outbound signals that terminate in the spleen and other tissues. Neurotransmitters from peripheral autonomic nerves subsequently promote acetylcholine-release from a subset of CD4(+) T cells that relay the neural signal to other immune cells, e.g. through activation of α7 nicotinic acetylcholine receptors on macrophages. Here, we review recent progress in the understanding of the inflammatory reflex and discuss potential therapeutic implications of current findings in this evolving field.

Mechanisms of action of adrenocorticotropic hormone and other melanocortins relevant to the clinical management of patients with multiple sclerosis

The therapeutic benefits of adrenocorticotropic hormone in multiple sclerosis are usually ascribed to its corticotropic actions. Evidence is presented that adrenocorticotropic hormone, approved for multiple sclerosis relapses, acts via corticosteroid-independent melanocortin pathways to engender down-modulating actions on immune-system cells and the cytokines they synthesize. Immune response-dampening effects are also brought about by agent-induced neurotransmitters that inhibit immunocytes. The likelihood that adrenocorticotropic hormone promotes microglial quiescence and counteracts glucocorticoid-mediated bone resorption is discussed.

Downregulation of alpha7 nicotinic acetylcholine receptor in two-kidney one-clip hypertensive rats

Background

Inflammation processes are important participants in the pathophysiology of hypertension and cardiovascular diseases. The role of the alpha7 nicotinic acetylcholine receptor (α7nAChR) in inflammation has recently been identified. Our previous study has demonstrated that the α7nAChR-mediated cholinergic anti-inflammatory pathway is impaired systemically in the genetic model of hypertension. In this work, we investigated the changes of α7nAChR expression in a model of secondary hypertension.

Methods

The 2-kidney 1-clip (2K1C) hypertensive rat model was used. Blood pressure, vagus nerve function, serum tumor necrosis factor-α (TNF-α) and both the mRNA and protein levels of α7nAChR in tissues from heart, kidney and aorta were measured at 4, 8 and 20 weeks after surgery.

Results

Compared with age-matched control, it was found that vagus nerve function was significantly decreased in 2K1C rats with the development of hypertension. Serum levels of TNF-α were greater in 2K1C rats than in age-matched control at 4, 8 and 20 weeks. α7nAChR mRNA in the heart was not altered in 2K1C rats. In the kidney of 2K1C rats, α7nAChR expression was significantly decreased at 8 and 20 weeks, but markedly increased at 4 weeks. α7nAChR mRNA was less in aorta of 2K1C rats than in age-matched control at 4, 8 and 20 weeks. These findings were confirmed at the protein levels of α7nAChR.

Conclusions

Our results suggested that secondary hypertension may induce α7nAChR downregulation, and the decreased expression of α7nAChR may contribute to inflammation in 2K1C hypertension.

Targeting α-7 nicotinic acetylcholine receptor in the enteric nervous system: a cholinergic agonist prevents gut barrier failure after severe burn injury

We have previously shown that vagal nerve stimulation prevents intestinal barrier loss in a model of severe burn injury in which injury was associated with decreased expression and altered localization of intestinal tight junction proteins. α-7 Nicotinic acetylcholine receptor (α-7 nAchR) has been shown to be necessary for the vagus nerve to modulate the systemic inflammatory response, but the role of α-7 nAchR in mediating gut protection remained unknown. We hypothesized that α-7 nAchR would be present in the gastrointestinal tract and that treatment with a pharmacological agonist of α-7 nAchR would protect against burn-induced gut barrier injury. The effects of a pharmacological cholinergic agonist on gut barrier integrity were studied using an intraperitoneal injection of nicotine 30 minutes after injury. Intestinal barrier integrity was examined by measuring permeability to 4-kDa fluorescein isothiocyanate-dextran and by examining changes in expression and localization of the intestinal tight junction proteins occludin and ZO-1. Nicotine injection after injury prevented burn-induced intestinal permeability and limited histological gut injury. Treatment with nicotine prevented decreased expression and altered localization of occludin and ZO-1, as seen in animals undergoing burn alone. Defining the interactions among the vagus nerve, the enteric nervous system, and the intestinal epithelium may lead to development of targeted therapeutics aimed at reducing gut barrier failure and intestinal inflammation after severe injury.

Neural reflexes in inflammation and immunity

The mammalian immune system and the nervous system coevolved under the influence of infection and sterile injury. Knowledge of homeostatic mechanisms by which the nervous system controls organ function was originally applied to the cardiovascular, gastrointestinal, musculoskeletal, and other body systems. Development of advanced neurophysiological and immunological techniques recently enabled the study of reflex neural circuits that maintain immunological homeostasis, and are essential for health in mammals. Such reflexes are evolutionarily ancient, dating back to invertebrate nematode worms that possess primitive immune and nervous systems. Failure of these reflex mechanisms in mammals contributes to nonresolving inflammation and disease. It is also possible to target these neural pathways using electrical nerve stimulators and pharmacological agents to hasten the resolution of inflammation and provide therapeutic benefit.

Stroke-induced activation of the α7 nicotinic receptor increases Pseudomonas aeruginosa lung injury

Infectious complications, predominantly pneumonia, are the most common cause of death in the postacute phase of stroke, although the mechanisms underlying the corresponding immunosuppression are not fully understood. We tested the hypothesis that activation of the α7 nicotinic acetylcholine receptor (α7nAChR) pathway is important in the stroke-induced increase in lung injury caused by Pseudomonas aeruginosa pneumonia in mice. Prior stroke increased lung vascular permeability caused by P. aeruginosa pneumonia and was associated with decreased lung neutrophil recruitment and bacterial clearance in mice. Pharmacologic inhibition (methyllycaconitine IC(50): 0.2-0.6 nM) or genetic deletion of the α7nAChR significantly (P<0.05) attenuates the effect of prior stroke on lung injury and mortality caused by P. aeruginosa pneumonia in mice. Finally, pretreatment with PNU-282987, a pharmacologic activator of the α7nAChR (EC(50): 0.2 μM), significantly (P<0.05) increased lung injury caused by P. aeruginosa pneumonia, significantly (P<0.05) decreased the release of KC, a major neutrophil chemokine, and significantly (P<0.05) decreased intracellular bacterial killing by a mouse alveolar macrophage cell line and primary mouse neutrophils. In summary, the α7 nicotinic cholinergic pathway plays an important role in mediating the systemic immunosuppression observed after stroke and directly contributes to more severe lung damage induced by P. aeruginosa.

Neural regulation of inflammation: no neural connection from the vagus to splenic sympathetic neurons

The 'inflammatory reflex' acts through efferent neural connections from the central nervous system to lymphoid organs, particularly the spleen, that suppress the production of inflammatory cytokines. Stimulation of the efferent vagus has been shown to suppress inflammation in a manner dependent on the spleen and splenic nerves. The vagus does not innervate the spleen, so a synaptic connection from vagal preganglionic neurons to splenic sympathetic postganglionic neurons was suggested. We tested this idea in rats. In a preparatory operation, the anterograde tracer DiI was injected bilaterally into the dorsal motor nucleus of vagus and the retrograde tracer Fast Blue was injected into the spleen. On histological analysis 7-9 weeks later, 883 neurons were retrogradely labelled from the spleen with Fast Blue as follows: 89% in the suprarenal ganglia (65% left, 24% right); 11% in the left coeliac ganglion; but none in the right coeliac or either of the superior mesenteric ganglia. Vagal terminals anterogradely labelled with DiI were common in the coeliac but sparse in the suprarenal ganglia, and confocal analysis revealed no putative synaptic connection with any Fast Blue-labelled cell in either ganglion. Electrophysiological experiments in anaesthetized rats revealed no effect of vagal efferent stimulation on splenic nerve activity or on that of 15 single splenic-projecting neurons recorded in the suprarenal ganglion. Together, these findings indicate that vagal efferent neurons in the rat neither synapse with splenic sympathetic neurons nor drive their ongoing activity.

Nicotinic acetylcholine receptor expression and susceptibility to cholinergic immunomodulation in human monocytes of smoking individuals

OBJECTIVE

Smoking is generally accepted as a factor that affects the disease course in inflammatory bowel disease patients. Whether these effects can be contributed to the immunomodulatory effects of nicotine via nicotinic acetylcholine receptor (nAChR) activation is unclear. As previous data suggest that the α7 nicotinic acetylcholine receptor (CHRNA7) and its duplicated variant CHRFAM7A may specifically participate in the inflammatory response of monocytes, we evaluated whether repeated nicotine exposure or smoking affects monocyte CHRNA7 and CHRFAM7A expression and cholinergic immunomodulation.

METHODS

The human monocyte cell line THP-I was incubated with nicotine for different time points before endotoxin exposure. In a pilot volunteer study using smoking (n = 4) and nonsmoking (n = 7) individuals, vagal output was stimulated by olive oil administration after which monocytes were analyzed for nicotinic receptor expression. Serum tumor necrosis factor (TNF) levels were determined using ELISA and expression levels of the nAChR subunits CHRNA7, CHRNB2 or CHRFAM7A were analyzed using QPCR.

RESULTS

Repeated nicotine exposure upregulated CHRNA7 expression on THP-I monocytes and led to an enhanced potential of α7 nAChR agonist GSK1345038A to reduce TNF levels. Furthermore, CHRNA7 was only detectable in isolated blood monocytes of smokers. On the other hand, the expression of CHRFAM7A and CHRNB2 was not affected by nicotine exposure. Lipopolysaccharides-induced TNF secretion was inhibited by nicotinic receptor activation in THP-I monocytes, but this response was not consistently seen in blood monocytes from smoking individuals.

CONCLUSIONS

We conclude that CHRNA7 expression on blood monocytes is upregulated in smoking individuals, which may contribute to cholinergic immunomodulation.

Role of α7 nicotinic acetylcholine receptors in regulating tumor necrosis factor-α (TNF-α) as revealed by subtype selective agonists

Immunological responses to protect against excessive inflammation can be regulated by the central nervous system through the cholinergic anti-inflammatory pathway wherein acetylcholine released from vagus nerves can inhibit inflammatory cytokines. Although a role for the α7 nicotinic acetylcholine receptor (α7 nAChR) in mediating this pathway has been suggested, pharmacological modulation of the pathway by selective agonists remains to be further elucidated. In this study, the role of α7 nAChRs in the regulation of TNF-α release was investigated using high affinity and selective α7 nAChR agonists in mouse peritoneal macrophage and human whole blood in vitro, and in mouse serum in vivo. In mouse peritoneal macrophages, LPS-induced TNF-α release in vitro was inhibited by a selective α7 nAChR agonist, A-833834 (5-[6-(5-Methyl-hexahydro-pyrrolo[3,4-c]pyrrol-2-yl)-pyridazin-3-yl]-1H-indole), and that effect was attenuated by α7 nAChR antagonist methyllycaconitine. The inhibitory effect of A-833834 on LPS-induced TNF-α release was also observed in human whole blood in vitro. I.v. LPS-induced TNF-α release in mouse serum was attenuated following i.p. administration of A-833834. Similarly, i.v. LPS-induced TNF-α release in mouse serum was also attenuated following i.p. administration of A-585539, another α7 nAChR agonist with limited brain penetration, suggesting that these effects are mediated by peripheral α7 nAChRs. A-833834 was also efficacious in suppressing TNF-α release in mouse serum following oral administration in zymosan-induced peritonitis. These studies collectively demonstrate that selectively targeting α7 nAChRs could offer a novel therapeutic modality to treat acute and chronic inflammatory disease states.

β2-Adrenoreceptors of regulatory lymphocytes are essential for vagal neuromodulation of the innate immune system

The nervous system is classically organized into sympathetic and parasympathetic systems acting in opposition to maintain physiological homeostasis. Here, we report that both systems converge in the activation of β2-adrenoceptors of splenic regulatory lymphocytes to control systemic inflammation. Vagus nerve stimulation fails to control serum TNF levels in either β2-knockout or lymphocyte-deficient nude mice. Unlike typical suppressor CD25(+) cells, the transfer of CD4(+)CD25(-) regulatory lymphocytes reestablishes the anti-inflammatory potential of the vagus nerve and β2-agonists to control inflammation in both β2-knockout and nude mice. β2-Agonists inhibit cytokine production in splenocytes (IC(50)≈ 1 μM) and prevent systemic inflammation in wild-type but not in β2-knockout mice. β2-Agonists rescue wild-type mice from established polymicrobial peritonitis in a clinically relevant time frame. Regulatory lymphocytes reestablish the anti-inflammatory potential of β2-agonists to control systemic inflammation, organ damage, and lethal endotoxic shock in β2-knockout mice. These results indicate that β2-adrenoceptors in regulatory lymphocytes are critical for the anti-inflammatory potential of the parasympathetic vagus nerve, and they represent a potential pharmacological target for sepsis.

Anti-inflammatory effects of nicotine in obesity and ulcerative colitis

Cigarette smoke is a major risk factor for a number of diseases including lung cancer and respiratory infections. Paradoxically, it also contains nicotine, an anti-inflammatory alkaloid. There is increasing evidence that smokers have a lower incidence of some inflammatory diseases, including ulcerative colitis, and the protective effect involves the activation of a cholinergic anti-inflammatory pathway that requires the α7 nicotinic acetylcholine receptor (α7nAChR) on immune cells. Obesity is characterized by chronic low-grade inflammation, which contributes to insulin resistance. Nicotine significantly improves glucose homeostasis and insulin sensitivity in genetically obese and diet-induced obese mice, which is associated with suppressed adipose tissue inflammation. Inflammation that results in disruption of the epithelial barrier is a hallmark of inflammatory bowel disease, and nicotine is protective in ulcerative colitis. This article summarizes current evidence for the anti-inflammatory effects of nicotine in obesity and ulcerative colitis. Selective agonists for the α7nAChR could represent a promising pharmacological strategy for the treatment of inflammation in obesity and ulcerative colitis. Nevertheless, we should keep in mind that the anti-inflammatory effects of nicotine could be mediated via the expression of several nAChRs on a particular target cell.