Acetylcholine and the alpha 7 nicotinic receptor: a potential therapeutic target for the treatment of periodontal disease?

Objectives

The aim of this review is to examine the evidence for a functional cholinergic system operating within the periodontium and determine the evidence for its role in periodontal immunity.

Introduction

Acetylcholine can influence the immune system via the ‘cholinergic anti-inflammatory pathway’. This pathway is mediated by the vagus nerve which releases acetylcholine to interact with the α7 subunit of the nicotinic acetylcholine receptor (α7nAChR) on proximate immuno-regulatory cells. Activation of the α7nAChR on these cells leads to down-regulated expression of pro-inflammatory mediators and thus regulates localised inflammatory responses. The role of the vagus nerve in periodontal pathophysiology is currently unknown. However, non-neuronal cells can also release acetylcholine and express the α7nAChR; these include keratinocytes, fibroblasts, T cells, B cells and macrophages. Therefore, by both autocrine and paracrine methods non-neuronal acetylcholine can also be hypothesised to modulate the localised immune response.

Methods

A Pubmed database search was performed for studies providing evidence for a functional cholinergic system operating in the periodontium. In addition, literature on the role of the ‘cholinergic anti-inflammatory pathway’ in modulating the immune response was extrapolated to hypothesise that similar mechanisms of immune regulation occur within the periodontium.

Conclusion

The evidence suggests a functional non-neuronal ‘cholinergic anti-inflammatory pathway’ may operate in the periodontium and that this may be targeted therapeutically to treat periodontal disease.     

Cell Specific Synovial Expression of Nicotinic Alpha 7 Acetylcholine Receptor in Rheumatoid Arthritis and Psoriatic Arthritis

Neuroimmune interactions are known to influence several chronic inflammatory and rheumatic diseases, but the underlying mechanisms have been insufficiently elucidated. The cholinergic anti-inflammatory pathway is characterized by neural regulation of systemic inflammation, mediated by the vagus nerve and specific cholinergic stimulation of the nicotinic α-7 acetylcholine receptor (α7nAChR) on immune cells. Moreover, α7nAChR has been shown important for immune regulation also in the absence of nerves, but little is known about these mechanisms in chronic joint inflammation. The expression and localization of α7nAChR in synovial biopsies from patients with rheumatoid arthritis and psoriatic arthritis was investigated by immunohistochemistry using monoclonal antibody against α7nAChR. Surface staining of α7nAChR was observed in synovial tissue of all arthritis patients investigated and could also to a lesser extent be detected in the synovium of healthy individuals. α7nAChR positive cells were detected in mainly synovial lining cells and vessels. The α7nAChR positively stained cells were by double immunofluorescence identified as primarily macrophages and fibroblasts, with the majority of these cells expressing the receptor. These results indicate the importance of α7nAChR and cholinergic mechanisms in arthritis pathogenesis and implicate specific cholinergic modulation as a potential anti-inflammatory therapeutic strategy in joint inflammation.     

Vagal Modulation of the Inflammatory Response in Sepsis

The vagus nerve can sense peripheral inflammation and transmit action potentials from the periphery to the brainstem. Vagal afferent signaling is integrated in the brainstem, and efferent vagus nerves carry outbound signals that terminate in spleen and other organs. Stimulation of efferent vagus nerve leads to the release of acetylcholine in these organs. In turn, acetylcholine interacts with members of the nicotinic acetylcholine receptor (nAChR) family, particularly with the alpha7 nicotinic acetylcholine receptor (α7nAChR), which is expressed by macrophages and other cytokine-producing cells. Ultimately, the production of proinflammatory cytokines is markedly inhibited. This neuroimmune communication is termed “the inflammatory reflex”. The uncontrolled inflammation as a result from sepsis can lead to multiple organ failure, and even death. Experimental data show that regulation of the inflammatory reflex appears to be a useful interventional strategy for septic response. Herein, we review recent advances in the understanding of the inflammatory reflex and discuss potential therapeutics that vagal modulation of the immune system for the treatment of severe sepsis and septic shock.     

Cholinergic agonists may produce preservation of myocardial ischaemia/reperfusion injury

The best treatment for myocardial infarction is to restore blood flow in the ischaemic region, though it will bring new myocardial damage known as myocardial ischaemia/reperfusion (I/R) injury. Both the ischaemia preconditioning and the ischaemia postcondioning have been shown to reduce the myocardial I/R injury, but their deficits restrict wide clinical availability. It has been demonstrated that inflammation plays a critical role in the I/R injury process. Also plasma levels of cytokines and inflammation response can be regulated by specifically augmenting cholinergic signaling via the efferent vagus nerve and α7 subunit-containing nicotinic acetylcholine receptor (α7nAChR). Because cholinergic modalities, acting through vagus nerve- and/or α7nAChR-mediated mechanism, have been confirmed to suppress excessive inflammation during the I/R injury in kidney, liver, lung and intestine, therefore, we hypothesize that cholinergic agonists may also provide a protection for the myocardial I/R injury.     

可乐定通过激活胆碱能抗炎通路抑制肺损伤小鼠炎性反应

目的:探讨可乐定对肺损伤小鼠炎性反应的影响及其作用机制。方法:以脂多糖(lipopolysac charide,LPS)诱导的肺损伤小鼠为模型,静脉注射可乐定,取左肺上叶组织,检测肺湿干重比(W/D)和总肺含水量(TLW),ELISA试剂盒检测炎性因子IL-1β、IL-6和TNF-α的含量,RT-PCR和Western blot检测α7烟碱型乙酰胆碱受体(α7nAChR)、高迁移率族蛋白B1(HMGB1)表达的变化;鼻内吸入法将α7nAChR基因RNA干扰慢病毒载体导入肺损伤模型小鼠肺内,或者静脉注射HMGB1外源性蛋白,检测炎性因子和HMGB1的变化。结果:可乐定可减轻LPS诱导的小鼠肺损伤,并通过降低细胞因子IL-6、IL-10和TNF-α水平从而抑制LPS诱导的炎性反应;可乐定促进α7nAChR表达从而激活胆碱能抗炎通路,并抑制HMGB1的表达。结论:可乐定对肺损伤小鼠具有抗炎作用,这可能与激活胆碱能抗炎通路,抑制HMGB1的表达有关。     

Nicotinic Cholinergic Signaling in Adipose Tissue and Pancreatic Islets Biology: Revisited Function and Therapeutic Perspectives

Nicotinic acetylcholine receptors (nAChRs) are membrane ligand-gated cation channels whose activation is triggered by the binding of the endogenous neurotransmitter acetylcholine or other biologic compounds including nicotine. Their roles in synaptic transmission in the central and peripheral nervous system as well as in the neuromuscular junction have been extensively studied. Recent implications of nAChRs in intracellular signaling and their detection in peripheral nonneural cells (including epithelial cells and immune cells) have renewed the interest for this class of ionotropic receptors. In the present review, we focus our attention on the potential use of nicotinic cholinergic signaling in the treatment of metabolic diseases (such as obesity and diabetes) in browsing functions of nAChRs in adipose tissue and pancreatic islet biology. In fact, different nAChR subunits can be detected in these metabolic tissues, as well as in immune cells interacting with them. Various rodent models of obesity and diabetes benefit from stimulation of the nicotinic cholinergic pathway, whereas mice deficient for some nAChRs, in particular the α7 nAChR subunit, harbor a worsened metabolic phenotype. In contrast to potential therapeutic applications in metabolic diseases, an overstimulation of this signaling pathway during the early stage of development (typically through nicotine exposure during fetal life) presents deleterious consequences on ontogeny and functionality of adipose tissue and the endocrine pancreas which persist throughout life.     

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.     

Stimulation of alpha7 nicotinic acetylcholine receptor by nicotine attenuates inflammatory response in macrophages and improves survival in experimental model of sepsis through heme oxygenase-1 induction

Activation of nicotinic acetylcholine receptor alpha7 subunit (α7nAChR) by nicotine leads to the improved survival rate in experimental model of sepsis. Previously, we demonstrated that heme oxygenase (HO)-1 inducers or carbon monoxide significantly increased survival of lipopolysaccharide (LPS)-induced and cecal ligation and puncture-induced septic mice by reduction of high mobility group box 1 release, a late mediator of sepsis. However, that activation of α7nAChR by nicotine provides anti-inflammatory action through HO-1 upregulation has not been elucidated. Here we show that HO-1-inducible effect by nicotine was mediated through sequential event-Ca(2+) influx, classical protein kinase C activation, and reactive oxygen species production-which activates phosphoinositol-3-kinase/Akt/Nrf-2 pathway. In addition, HO-1 is required for nicotine-mediated suppression of tumor necrosis factor-α, inducible nitric oxide synthase, and high mobility group box 1 expression induced by LPS in macrophages, as evidenced by the fact that nicotine failed to inhibit production of these mediators when HO-1 was suppressed. Importantly, nicotine-induced survival rate was reduced by inhibition of HO-1 in LPS- and cecal ligation and puncture-treated septic mice. Collectively, these data suggest that activation of α7nAChR by nicotine is critical in the regulation of anti-inflammatory process, which could be mediated through HO-1 expression. Thus, we conclude that activation of α7nAChR by nicotine provides anti-inflammatory action through HO-1 upregulation.     

The effect of the cholinergic anti-inflammatory pathway on experimental colitis

Inflammatory bowel diseases (IBD) are characterized by proinflammatory cytokines, tissue damage and loss of neuron in inflamed mucosa, which implies the cholinergic anti-inflammatory pathway may be destroyed during the process of inflammatory response. In the study, we identified the effect of cholinergic agonist as anabaseine (AN) and nicotinic receptor antagonist as chlorisondamine diiodide (CHD) on trinitrobenzene sulfonic acid (TNBS)-induced colitis, to investigate the potential therapeutic effect of the cholinergic anti-inflammatory pathway on IBD. Experimental colitis was induced by TNBS at day 1, 10 mug AN or 1.5 mug CHD was injected i.p. to mouse right after the induction of colitis, and repeated on interval day till the mice were sacrificed at day 8. Colonic inflammation was examined by histological analysis, myeloperoxidase (MPO) activity, and the production of tumour necrosis factor (TNF)-alpha in tissue. Lamina propria mononuclear cells (LPMC) were isolated, and NF-kappaB activation was detected by western blot. The mice with colitis treated by AN showed less tissue damage, less MPO activity, less TNF-alpha production in colon, and inhibited NF-kappaB activation in LPMC, compared with those mice with colitis untreated, whereas the mice with colitis treated by CHD showed the worst tissue damage, the highest MPO activity, the highest TNF-alpha level, and enlarged NF-kappaB activation in LPMC. Agonist of the cholinergic anti-inflammatory pathway inhibits colonic inflammatory response by downregulating the production of TNF-alpha, and inhibiting NF-kappaB activation, which suggests that modulating the cholinergic anti-inflammatory pathway may be a new potential management for IBD.     

From CNI-1493 to the immunological homunculus: physiology of the inflammatory reflex

The inflammatory reflex is a neurophysiological mechanism that regulates the immune system. The efferent branch of the reflex the cholinergic anti-inflammatory pathway, which inhibits inflammation by suppressing cytokine synthesis via release of acetylcholine in organs of the reticuloendothelial system, including the spleen, liver, and gastrointestinal tract. Acetylcholine binds to alpha7 nicotinic acetylcholine receptors expressed by macrophages and other cytokine-producing cells. Receptor-ligand engagement suppresses proinflammatory cytokines and prevents tissue damage. Herein is a review of some of the experimental studies that define the inflammatory reflex and its anatomic and physiologic components.     

胆碱能抗炎通路的机制及其在脓毒症的应用

胆碱能抗炎通路是近年来发现的对全身性炎症反应有调节作用的通路,可通过迷走神经抑制促炎细胞因子的合成从而抑制机体炎症反应.核因子-κB和Janus激酶/信号转导与转录激活子是该通路细胞内信号转导中最重要的两条信号通路.脓毒症以全身炎症反应为特点,是危重症患者的常见死因.根据胆碱能抗炎通路作用迅速有效的特点,推测其可应用于...     

Postconditioning with α7nAChR Agonist Attenuates Systemic Inflammatory Response to Myocardial Ischemia–Reperfusion Injury in Rats

The inflammatory response plays a major role in ischemia-reperfusion injury (IRI). Considering that cholinergic stimulation can inhibit inflammatory response through the cholinergic anti-inflammatory pathway (CAP) and the α subunit-containing nicotinic acetylcholine receptor(α7nAChR) expressed by immune cells is an important component of CAP, we assessed the effect of postconditioning with α7nAChR agonist on systemic inflammatory response during the myocardial ischemia-reperfusion process in an in vivo rat model. Thirty Sprague Dawley rats were randomly divided into three groups: sham group, control group, and postconditioning with α7nAChR agonist group (PP group). In the groups other than the sham group, the left anterior descending coronary artery was ligated for 30 min followed by a 180-min reperfusion. At the end of the experiment, the serum levels of troponin I, tumor necrosis factor α, interleukin-6, and high-mobility group box 1 were assayed, and the infarct size was assessed. The results showed that postconditioning with α7nAChR agonist significantly attenuated the systemic inflammatory response to myocardial IRI, as evidenced by decreased serum levels of tumor necrosis factor α and high-mobility group box 1. Also, this treatment protected against myocardial IRI, as shown by reduced infarct size and serum troponin I level.     

Dexmedetomidine Controls Systemic Cytokine Levels through the Cholinergic Anti-inflammatory Pathway

Previous studies have shown that dexmedetomidine exerted anti-inflammatory effect on several animal models with inflammation, but the mechanism is not clear. This study intends to elucidate the anti-inflammatory mechanism of dexmedetomidine through the cholinergic anti-inflammatory pathway. To investigate this therapeutic potential of dexmedetomidine, a murine model of endotoxemia was established induced by lipopolysaccharide (LPS). Animals were assigned to one of four protocols. Protocol one: animals were randomly assigned to control group, dexmedetomidine group, and sterile saline group (n?=?20 each), and these animals were used for survival analysis. The survival rate was assessed up to 120 h after endotoxin injection. Protocol two: animals were randomly assigned to one of four groups (n?=?16 each): group 1 (group Saline), treated with sterile saline 15 min prior to endotoxin treatment (10 mg kg^?1 over 2 min); group 2 (group Dex), treated with dexmedetomidine 15 min prior to endotoxin treatment; group 3 (group αBGT?+?Dex), treated with alpha-7 nicotinic acetylcholine receptors (α7nAChR) antagonist alpha-bungarotoxin (αBGT, 1 μg/kg) 15 min prior to dexmedetomidine treatment; group 4 (group saline?+?Dex), treated with equivalent sterile saline 15 min prior to dexmedetomidine treatment. Protocol three: animals were randomly assigned to one of two groups (n?=?16 each): vagotomy group (group VNX?+?Dex), right cervical vagus nerve was exposed and transected; sham-operated group (group SHAM?+?Dex), the cervical vagus nerve was visualized, but was neither isolated from the surrounding tissues nor transected. Protocol four: animals were treated with dexmedetomidine (40 μg/kg) and sterile saline to observe the discharge activity of cervical vagus nerves by using BL-420F data acquisition and analysis system (n?=?16 each). In the survival analysis groups, the survival rate of dexmedetomidine group was significantly higher than that of the endotoxemia group (65 versus 25 %, P?P?α-bungarotoxin group and vagotomy group (TNF-alpha, IL-1beta, IL-6, P?>?0.05, respectively). Furthermore, preemptive administration of dexmedetomidine significantly increased the discharge frequency of cervical vagus nerves in comparison with sterile saline treatment (P?nAChR antagonist pretreatment mice failed to suppress TNF levels, indicating that the vagus nerve and α7nAChR-mediated cholinergic anti-inflammatory pathway is required for the anti-inflammatory effect of dexmedetomidine. These findings show that central alpha-2 agonist dexmedetomidine suppresses systemic inflammation through vagal- and α7nAChR-dependent mechanism.     

Unphosphorylated STAT3 modulates alpha7 nicotinic receptor signaling and cytokine production in sepsis

The role of STAT3 in infectious diseases remains undetermined, in part because unphosphorylated STAT3 has been considered an inactive protein. Here, we report that unphosphorylated STAT3 contributes to cholinergic anti‐inflammation, prevents systemic inflammation, and improves survival in sepsis. Bacterial endotoxin induced STAT3 tyrosine phosphorylation in macrophages. Both alpha7 nicotinic receptor (alpha7nAChR) activation and inhibition of JAK2 blunt STAT3 phosphorylation. Inhibition of STAT3 phosphorylation mimicked the alpha7nAChR signaling, inhibiting NF‐κB and cytokine production in macrophages. Transfection of macrophages with the dominant‐negative mutant STAT3F, to prevent its tyrosine phosphorylation, reduced TNF production but did not prevent the alpha7nAChR signaling. However, inhibition of STAT3 protein expression enhanced cytokine production and abrogated alpha7nAChR signaling. Alpha7nAChR controls TNF production in macrophages through a mechanism that requires STAT3 protein expression, but not its tyrosine phosphorylation. In vivo, inhibition of STAT3 tyrosine phosphorylation by stattic prevented systemic inflammation and improved survival in experimental sepsis. Stattic also prevented the production of late mediators of sepsis and improved survival in established sepsis. These results reveal the immunological implications of tyrosine‐unphosphorylated STAT3 in infectious diseases.     

The Vagus Nerve and Nicotinic Receptors Involve Inhibition of HMGB1 Release and Early Pro-inflammatory Cytokines Function in Collagen-Induced Arthritis

Objectives  

The cholinergic anti-inflammatory pathway, a vagus nerve-dependent mechanism, inhibits cytokine releases in models of acute inflammatory disease. We investigated the efficacy and elucidated the possible mechanism of the cholinergic anti-inflammatory pathway on collagen-induced arthritis (CIA) in mice.     

成熟DCs固有胆碱能系统及其调控

目的：探讨成熟树突状细胞（dendritic cells, DCs）是否存在非神经元性胆碱能系统及其调控机制。 方法：分离正常小鼠骨髓细胞，体外用细胞因子（IL-4, GM-CSF）诱导、分化为幼稚DCs，细菌脂多糖(LPS)刺激其成熟。光镜观察其形态变化、流式细胞术进行分子鉴定。免疫细胞化学方法、免疫荧光抗体技术、流式细胞术和RT-PCR方法检测DCs烟碱样乙酰胆碱受体亚单位α7（nAChR α 7）、胆碱乙酰转移酶（ChAT）和乙酰胆碱酯酶（AChE）。同时，流式细胞术检测美加明（MEC）作用于成熟DCs 12 h前后 nAChR α 7表达。 结果： 成熟DCs表达nAChR α 7、ChAT、AChE蛋白和mRNA；nAChR α 7蛋白主要分布于细胞膜，ChAT和AChE分布于细胞浆。其中AChE蛋白表达强于ChAT(P<0.05)，与nAChR α 7比较有上升趋势；MEC 作用组nAChR α 7蛋白表达明显低于无MEC作用组(P<0.05)。 结论:成熟DC存在固有胆碱能系统，外源性因素（美加明等）可调节其效应。DCs可能通过胆碱能系统而发挥其神经免疫调节作用。     

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.     

Nicotinic acetylcholine receptor structure and function in the efferent auditory system

This article reviews and presents new data regarding the nicotinic acetylcholine receptor subunits alpha9 and alpha10. Although phylogentically ancient, these subunits have only recently been identified as critical components of the efferent auditory system and medial olivocochlear pathway. This pathway is important in auditory processing by modulating outer hair cell function to broadly tune the cochlea and improve signal detection in noise. Pharmacologic properties of the functionally expressed alpha9alpha10 receptor closely resemble the cholinergic response of outer hair cells. Molecular, immunohistochemical, and knockout mice studies have added further weight to the role this receptor plays in mediating the efferent auditory response. Alternate and complementary mechanisms of outer hair cell efferent activity might also be mediated through the nAChR alpha9alpha10, either through secondary calcium stores, second messengers, or direct protein-protein interactions. We investigated protein-protein interactions using a yeast-two-hybrid screen of the nAChR alpha10 intracellular loop against a rat cochlear cDNA library. Among the identified proteins was prosaposin, a precursor of saposins, which have been shown to act as neurotrophic factors in culture, can bind to a putative G0-coupled cell surface receptor, and may be involved in the prevention of cell death. This study and review suggest that nAChR alpha9alpha10 may represent a potential therapeutic target for a variety of ear disorders, including preventing or treating noise-induced hearing loss, or such debilitating disorders as vertigo or tinnitus.     

The mouse autonomic nervous system modulates inflammation and epithelial renewal after corneal abrasion through the activation of distinct local macrophages

Inflammation and reepithelialization after corneal abrasion are critical for the rapid restoration of vision and the prevention of microbial infections. However, the endogenous regulatory mechanisms are not completely understood. Here we report that the manipulation of autonomic nervous system (ANS) regulates the inflammation and healing processes. The activation of sympathetic nerves inhibited reepithelialization after corneal abrasion but increased the influx of neutrophils and the release of inflammatory cytokines. Conversely, the activation of parasympathetic nerves promoted reepithelialization and inhibited the influx of neutrophils and the release of inflammatory cytokines. Furthermore, we observed that CD64⁺CCR2⁺ macrophages in the cornea preferentially expressed the β-2 adrenergic receptor (AR), whereas CD64⁺CCR2⁻ macrophages preferentially expressed the α-7 nicotinic acetylcholine receptor (α7nAChR). After abrasion, the topical administration of a β2AR agonist further enhanced the expression of the proinflammatory genes in the CD64⁺CCR2⁺ cell subset sorted from injured corneas. In contrast, the topical administration of an α7nAChR agonist further enhanced the expression of the anti-inflammatory genes in the CD64⁺CCR2⁻ subset. Thus crosstalk between the ANS and local macrophage populations is necessary for the progress of corneal wound repair. Manipulation of ANS inputs to the wounded cornea may represent an alternative approach to the treatment of impaired wound healing.