Strong expression of interleukin-1 receptor type I in the rat carotid body

P2X3 receptor antagonism attenuates the progression of heart failure

Despite advances in the treatment of heart failure, prognosis is poor, mortality high and there remains no cure. Heart failure is associated with reduced cardiac pump function, autonomic dysregulation, systemic inflammation and sleep-disordered breathing; these morbidities are exacerbated by peripheral chemoreceptor dysfunction. We reveal that in heart failure the carotid body generates spontaneous, episodic burst discharges coincident with the onset of disordered breathing in male rats. Purinergic (P2X3) receptors were upregulated two-fold in peripheral chemosensory afferents in heart failure, and when antagonized abolished these episodic discharges, normalized both peripheral chemoreceptor sensitivity and the breathing pattern, reinstated autonomic balance, improved cardiac function, and reduced both inflammation and biomarkers of cardiac failure. Aberrant ATP transmission in the carotid body triggers episodic discharges that via P2X3 receptors play a crucial role in the progression of heart failure and as such offer a distinct therapeutic angle to reverse multiple components of its pathogenesis.

The carotid body: A novel key player in neuroimmune interactions

The idea that the nervous system communicates with the immune system to regulate physiological and pathological processes is not new. However, there is still much to learn about how these interactions occur under different conditions. The carotid body (CB) is a sensory organ located in the neck, classically known as the primary sensor of the oxygen (O2) levels in the organism of mammals. When the partial pressure of O2 in the arterial blood falls, the CB alerts the brain which coordinates cardiorespiratory responses to ensure adequate O2 supply to all tissues and organs in the body. A growing body of evidence, however, has demonstrated that the CB is much more than an O2 sensor. Actually, the CB is a multimodal sensor with the extraordinary ability to detect a wide diversity of circulating molecules in the arterial blood, including inflammatory mediators. In this review, we introduce the literature supporting the role of the CB as a critical component of neuroimmune interactions. Based on ours and other studies, we propose a novel neuroimmune pathway in which the CB acts as a sensor of circulating inflammatory mediators and, in conditions of systemic inflammation, recruits a sympathetic-mediated counteracting mechanism that appears to be a protective response.

Neural control of the spleen as an effector of immune responses to inflammation: mechanisms and treatments

Immune system responses are a vital defense mechanism against pathogens. Inflammatory mediators finely regulate complex inflammatory responses from initiation to resolution. However, in certain conditions, the inflammation is initiated and amplified, but not resolved. Understanding the biological mechanisms underlying the regulation of the immune response is critical for developing therapeutic alternatives, including pharmaceuticals and bioelectronic tools. The spleen is an important immune effector organ since it orchestrates innate and adaptive immune responses such as pathogen clearance, cytokine production, and differentiation of cells, therefore playing a modulatory role that balances pro- and anti-inflammatory responses. However, modulation of splenic immune activity is a largely unexplored potential therapeutic tool that could be used for the treatment of inflammatory and life-threatening conditions. This review discusses some of the mechanisms controlling neuroimmune communication and the brain-spleen axis.

The carotid body detects circulating tumor necrosis factor-alpha to activate a sympathetic anti-inflammatory reflex

Recent evidence has suggested that the carotid bodies might act as immunological sensors, detecting pro-inflammatory mediators and signalling to the central nervous system, which, in turn, orchestrates autonomic responses. Here, we confirmed that the TNF-α receptor type I is expressed in the carotid bodies of rats. The systemic administration of TNF-α increased carotid body afferent discharge and activated glutamatergic neurons in the nucleus tractus solitarius (NTS) that project to the rostral ventrolateral medulla (RVLM), where many pre-sympathetic neurons reside. The activation of these neurons was accompanied by an increase in splanchnic sympathetic nerve activity. Carotid body ablation blunted the TNF-α-induced activation of RVLM-projecting NTS neurons and the increase in splanchnic sympathetic nerve activity. Finally, plasma and spleen levels of cytokines after TNF-α administration were higher in rats subjected to either carotid body ablation or splanchnic sympathetic denervation. Collectively, our findings indicate that the carotid body detects circulating TNF-α to activate a counteracting sympathetic anti-inflammatory mechanism.

Neurobiology of the carotid body

The carotid body (CB) is a bilateral arterial chemoreceptor located in the carotid artery bifurcation with an essential role in cardiorespiratory homeostasis. It is composed of highly perfused cell clusters, or glomeruli, innervated by sensory fibers. Glomus cells, the most abundant in each glomerulus, are neuron-like multimodal sensory elements able to detect and integrate changes in several physical and chemical parameters of the blood, in particular O₂ tension, CO₂ and pH, as well as glucose, lactate, or blood flow. Activation of glomus cells (e.g., during hypoxia or hypercapnia) stimulates the afferent fibers which impinge on brainstem neurons to elicit rapid compensatory responses (hyperventilation and sympathetic activation). This chapter presents an updated view of the structural organization of the CB and the mechanisms underlying the chemosensory responses of glomus cells, with special emphasis on the molecular processes responsible for acute O₂ sensing. The properties of the glomus cell-sensory fiber synapse as well as the organization of CB output are discussed. The chapter includes the description of recently discovered CB stem cells and progenitor cells, and their role in CB growth during acclimatization to hypoxemia. Finally, the participation of the CB in the mechanisms of disease is briefly discussed.

Carotid Body Inflammation: Role in Hypoxia and in the Anti-inflammatory Reflex

Emergent evidence indicates that the carotid body (CB) chemoreceptors may sense systemic inflammatory molecules, and is an afferent-arm of the anti-inflammatory reflex. Moreover, a pro-inflammatory milieu within the CB is involved in the enhanced CB chemosensory responsiveness to oxygen following sustained and intermittent hypoxia. In this review, we focus on the physio-pathological participation of CBs in inflammatory diseases, such as sepsis and intermittent hypoxia.

Physiological Sympathetic Activation Reduces Systemic Inflammation: Role of Baroreflex and Chemoreflex

Baroreflex and chemoreflex act through the autonomic nervous system, which is involved with the neural regulation of inflammation. The present study reports the effects of reflex physiological sympathetic activation in endotoxemic rats using bilateral carotid occlusion (BCO), a physiological approach involving the baroreflex and chemoreflex mechanisms and the influence of the baroreceptors and peripheral chemoreceptors in the cardiovascular and systemic inflammatory responses. After lipopolysaccharide (LPS) administration, the arterial pressure was recorded during 360 min in unanesthetized rats, and serial blood samples were collected to analyze the plasma cytokine levels. BCO elicited the reflex activation of the sympathetic nervous system, providing the following outcomes: (I) increased the power of the low-frequency band in the spectrum of the systolic arterial pressure during the BCO period; (II) reduced the levels of pro-inflammatory cytokines in plasma, including the tumor necrosis factor (TNF) and the interleukin (IL)-1β; (III) increased the plasma levels of anti-inflammatory cytokine IL-10, 90 min after LPS administration. Moreover, selective baroreceptor or chemoreceptor denervation deactivated mechanosensitive and chemical sensors, respectively, and decreased the release of the LPS-induced cytokine but did not alter the BCO modulatory effects. These results show, for the first time, that physiological reflex activation of the sympathetic circuit decreases the inflammatory response in endotoxemic rats and suggest a novel function for the baroreceptors as immunosensors during the systemic inflammation.

Immunity and the carotid body: implications for metabolic diseases

Neuro-immune communication has gained enormous interest in recent years due to increasing knowledge of the way in which the brain coordinates functional alterations in inflammatory and autoimmune responses, and the mechanisms of neuron-immune cell interactions in the context of metabolic diseases such as obesity and type 2 diabetes. In this review, we will explain how this relationship between the nervous and immune system impacts the pro- and anti-inflammatory pathways with specific reference to the hypothalamus-pituitary-adrenal gland axis and the vagal reflex and will explore the possible involvement of the carotid body (CB) in the neural control of inflammation. We will also highlight the mechanisms of vagal anti-inflammatory reflex control of immunity and metabolism, and the consequences of functional disarrangement of this reflex in settlement and development of metabolic diseases, with special attention to obesity and type 2 diabetes. Additionally, the role of CB in the interplay between metabolism and immune responses will be discussed, with specific reference to the different stimuli that promote CB activation and the balance between sympathetic and parasympathetic in this context. In doing so, we clarify the multivarious neuronal reflexes that coordinate tissue-specific responses (gut, pancreas, adipose tissue and liver) critical to metabolic control, and metabolic disease settlement and development. In the final section, we will summarize how electrical modulation of the carotid sinus nerve may be utilized to adjust these reflex responses and thus control inflammation and metabolic diseases, envisioning new therapeutics horizons.

Electrostimulation of the carotid sinus nerve in mice attenuates inflammation via glucocorticoid receptor on myeloid immune cells

BACKGROUND

The carotid bodies and baroreceptors are sensors capable of detecting various physiological parameters that signal to the brain via the afferent carotid sinus nerve for physiological adjustment by efferent pathways. Because receptors for inflammatory mediators are expressed by these sensors, we and others have hypothesised they could detect changes in pro-inflammatory cytokine blood levels and eventually trigger an anti-inflammatory reflex.

METHODS

To test this hypothesis, we surgically isolated the carotid sinus nerve and implanted an electrode, which could deliver an electrical stimulation package prior and following a lipopolysaccharide injection. Subsequently, 90 min later, blood was extracted, and cytokine levels were analysed.

RESULTS

Here, we found that carotid sinus nerve electrical stimulation inhibited lipopolysaccharide-induced tumour necrosis factor production in both anaesthetised and non-anaesthetised conscious mice. The anti-inflammatory effect of carotid sinus nerve electrical stimulation was so potent that it protected conscious mice from endotoxaemic shock-induced death. In contrast to the mechanisms underlying the well-described vagal anti-inflammatory reflex, this phenomenon does not depend on signalling through the autonomic nervous system. Rather, the inhibition of lipopolysaccharide-induced tumour necrosis factor production by carotid sinus nerve electrical stimulation is abolished by surgical removal of the adrenal glands, by treatment with the glucocorticoid receptor antagonist mifepristone or by genetic inactivation of the glucocorticoid gene in myeloid cells. Further, carotid sinus nerve electrical stimulation increases the spontaneous discharge activity of the hypothalamic paraventricular nucleus leading to enhanced production of corticosterone.

CONCLUSION

Carotid sinus nerve electrostimulation attenuates inflammation and protects against lipopolysaccharide-induced endotoxaemic shock via increased corticosterone acting on the glucocorticoid receptor of myeloid immune cells. These results provide a rationale for the use of carotid sinus nerve electrostimulation as a therapeutic approach for immune-mediated inflammatory diseases.

Cyclooxygenase and nitric oxide synthase pathways mediate the respiratory effects of TNF-α in rats

TNF-α is the key inflammatory cytokine. TNF-α receptors are expressed in brain stem regions involved in respiratory control and also in the carotid bodies, which are the sensory organs monitoring arterial blood O₂. We hypothesised that the circulating tumour necrosis factor (TNF)-α may affect the lung ventilation and modulate the hypoxic ventilatory response via activation of cyclooxygenase (COX) and nitric oxide synthase (NOS) pathways. The aim of the current study was to compare the respiratory effects of TNF-α before and after pretreatment with diclofenac or L-NG-nitro arginine methyl ester (L-NAME) nonspecific inhibitors of COX and NOS, respectively. The hypoxic ventilatory response was measured in anaesthetised rats using rebreathing techniques. We found that TNF-α increased the lung ventilation in normoxia but decreased the ventilatory response to hypoxia. Pretreatment with each of these inhibitors reduced respiratory effects of TNF-α. We believe that activation of COX and NOS-related pathways and also "cross-talk" between them mediates the TNF-α respiratory effects and underlies the impact of inflammation on the respiratory function.

Exploring the Mediators that Promote Carotid Body Dysfunction in Type 2 Diabetes and Obesity Related Syndromes

Carotid bodies (CBs) are peripheral chemoreceptors that sense changes in blood O₂, CO₂, and pH levels. Apart from ventilatory control, these organs are deeply involved in the homeostatic regulation of carbohydrates and lipid metabolism and inflammation. It has been described that CB dysfunction is involved in the genesis of metabolic diseases and that CB overactivation is present in animal models of metabolic disease and in prediabetes patients. Additionally, resection of the CB-sensitive nerve, the carotid sinus nerve (CSN), or CB ablation in animals prevents and reverses diet-induced insulin resistance and glucose intolerance as well as sympathoadrenal overactivity, meaning that the beneficial effects of decreasing CB activity on glucose homeostasis are modulated by target-related efferent sympathetic nerves, through a reflex initiated in the CBs. In agreement with our pre-clinical data, hyperbaric oxygen therapy, which reduces CB activity, improves glucose homeostasis in type 2 diabetes patients. Insulin, leptin, and pro-inflammatory cytokines activate the CB. In this manuscript, we review in a concise manner the putative pathways linking CB chemoreceptor deregulation with the pathogenesis of metabolic diseases and discuss and present new data that highlight the roles of hyperinsulinemia, hyperleptinemia, and chronic inflammation as major factors contributing to CB dysfunction in metabolic disorders.

Clinical and experimental aspects of breathing modulation by inflammation

Neuroinflammation is produced by local or systemic alterations and mediated mainly by glia, affecting the activity of various neural circuits including those involved in breathing rhythm generation and control. Several pathological conditions, such as sudden infant death syndrome, obstructive sleep apnea and asthma exert an inflammatory influence on breathing-related circuits. Consequently breathing (both resting and ventilatory responses to physiological challenges), is affected; e.g., responses to hypoxia and hypercapnia are compromised. Moreover, inflammation can induce long-lasting changes in breathing and affect adaptive plasticity; e.g., hypoxic acclimatization or long-term facilitation. Mediators of the influences of inflammation on breathing are most likely proinflammatory molecules such as cytokines and prostaglandins. The focus of this review is to summarize the available information concerning the modulation of the breathing function by inflammation and the cellular and molecular aspects of this process. I will consider: 1) some clinical and experimental conditions in which inflammation influences breathing; 2) the variety of experimental approaches used to understand this inflammatory modulation; 3) the likely cellular and molecular mechanisms.

Interleukin-1beta suppresses the ventilatory hypoxic response in rats via prostaglandin-dependent pathways

We investigated the effect of the major inflammatory cytokine interleukin-1beta (IL-1β) on the ventilatory response to hypoxia. The goal was to test the hypothesis that IL-1β impairs the hypoxic ventilatory response in vivo by indirectly inhibiting respiratory neurons in the brainstem via prostaglandins. Thus, IL-1β was delivered by cerebroventricular injection, and the ventilatory hypoxic response was assessed in anesthetized, spontaneously breathing rats pretreated with or without diclofenac, a nonspecific inhibitor of prostaglandin synthesis. We found that the slope of the ventilatory response to hypoxia decreased almost 2-fold from 10.4 ± 3.02 to 4.06 ± 0.86 mL·min−1·(mm Hg)−1 (–61%) 90 min after administration of IL-1β (p < 0.05). The slope of tidal volume and mean inspiratory flow also decreased from 0.074 ± 0.02 to 0.039 ± 0.01 mL·(mm Hg)−1 (–45%, p < 0.05), and from 0.36 ± 0.07 to 0.2 ± 0.04 mL·s−1·(mm Hg)−1 (–46%, p < 0.05), respectively. Pretreatment with diclofenac blocked these effects. Thus, the data indicate that IL-1β degrades the ventilatory hypoxic response by stimulating production of prostaglandin. The increase of cerebral levels of IL-1β, which is induced by the activation of immune cells in the brain, may impair respiratory chemoreflexes.

Insulin resistance: a new consequence of altered carotid body chemoreflex?

Metabolic diseases affect millions of individuals across the world and represent a group of chronic diseases of very high prevalence and relatively low therapeutic success, making them suitable candidates for pathophysiological studies. The sympathetic nervous system (SNS) contributes to the regulation of energy balance and energy expenditure both in physiological and pathological states. For instance, drugs that stimulate sympathetic activity decrease food intake, increase resting metabolic rate and increase the thermogenic response to food, while pharmacological blockade of the SNS has opposite effects. Likewise, dysmetabolic features such as insulin resistance, dyslipidaemia and obesity are characterized by a basal overactivation of the SNS. Recently, a new line of research linking the SNS to metabolic diseases has emerged with the report that the carotid bodies (CBs) are involved in the development of insulin resistance. The CBs are arterial chemoreceptors that classically sense changes in arterial blood O2 , CO2 and pH levels and whose activity is known to be increased in rodent models of insulin resistance. We have shown that selective bilateral resection of the nerve of the CB, the carotid sinus nerve (CSN), totally prevents diet-induced insulin resistance, hyperglycaemia, dyslipidaemia, hypertension and sympathoadrenal overactivity. These results imply that the beneficial effects of CSN resection on insulin action and glucoregulation are modulated by target-related efferent sympathetic nerves through a reflex that is initiated in the CBs. It also highlights modulation of CB activity as a putative future therapeutic intervention for metabolic diseases.

Focal cerebral ischemic tolerance and change in blood-brain barrier permeability after repetitive pure oxygen exposure preconditioning in a rodent model

OBJECTIVE

The goal of this study was to demonstrate that repetitive pure oxygen exposure preconditioning (O2PC) for 8 hours per day for 3 or 7 days, a practicable preconditioning for clinical use, is able to induce cerebral ischemic tolerance (IT) and further clarify the accompanying changes in the blood-brain barrier (BBB) that may be involved.

METHODS

A total of 68 adult male Sprague-Dawley rats and eight 1-day-old rat pups were used in this study. The adult rats were exposed to pure O2 (38 rats) 8 hours a day for 3 or 7 days or to room air (in an identical setup) for 8 hours a day for 7 days as controls (30 rats). Arterial O2 tension (PaO2) was measured in 6 rats exposed to O2 and 3 controls. Focal cerebral ischemia was elicited by middle cerebral artery occlusion (MCAO) in 37 rats, of which 21 had been exposed to pure O2 for 3 or 7 days and 16 to room air for 7 days as controls. Neurological behavior was scored with the Garcia score in 15 MCAO rats, of which 10 had been exposed to pure O2 for 3 or 7 days and 5 to room air for 7 days as controls, and cerebral infarct volumes were assessed with TTC (2,3,5-triphenyltetrazolium chloride) staining in 10 rats (5 from each group) after 7 days of exposure. Formamide-extraction method was used to detect leakage of Evans blue (EB) dye in 7 rats exposed to pure O2 for 7 days and 7 exposed to room air for 7 days. Fluorescence microscopy was used to analyze the leaked EB in the nonischemic areas of 4 rats exposed to pure O2 for 7 days and 4 exposed to room air for 7 days before MCAO and the brain of the rats that had not been subjected to MCAO. Astrocyte changes associated with O2PC were evaluated by means of fluorescence microscopy and electron microscopy in 14 rats that were exposed to the same O2 or control conditions as the MCAO rats but without MCAO. Astrocytes were also obtained from 8 rat pups and cultured; levels of AQP4 and VEGF were detected by Western blot and ELISA in cells with and without O2 treatment.

RESULTS

A significant increase in PaO2 was seen after O2PC. The neurological score was significantly increased in the O2PC groups (10.6 ± 0.6 in the 3-day O2PC group, p < 0.05; 12 ± 0.84 in the 7-day O2PC group, p < 0.05) compared with the control group (7 ± 0.55). The ratio of cerebral infarct volume to contralateral cerebral hemisphere volume was significantly lower in the O2PC group than in the control group (0.204 ± 0.03 vs 0.48 ± 0.05, p < 0.05). The amount of leaked EB in the ischemic cerebral hemisphere was also lower in the O2-treated rats than in controls (7.53 ± 1.4 vs 11.79 ± 3.3 μg EB/g brain weight, p < 0.05). However, fluorescence microscopy showed significantly greater BBB permeability in the nonischemic areas in the O2PC group than in controls (p < 0.05). More red fluorescence could be observed in the nonischemic areas in both the ipsilateral and contralateral sides of the ischemic brain in the O2PC animals than in the nonischemic areas in the corresponding sides of the controls. Further investigation of the effect of the O2PC itself on the BBB of rats that were not subjected to MCAO showed that there was no EB leakage in the brain parenchyma in the rats exposed to room air, but some red fluorescence patches were noticed in the normal brain from the rats in the O2PC group. Astrocytes, including those from areas around the BBB, were activated in the O2PC group. Levels of both aquaporin 4 (AQP4) and vascular endothelial growth factor (VEGF) were significantly increased in cultured astrocytes after O2PC.

CONCLUSIONS

These findings suggest that O2PC is able to induce IT, which makes it a strong candidate for clinical use. Moreover, O2PC can also promote BBB opening, which may contribute to the induction of IT as well as representing a possible strategy for promoting drug transportation into the CNS. Activated astrocytes are likely to be involved in these processes through astrocyte-derived factors, such as AQP4 and VEGF.

Interleukin-1β promotes the neurogenesis of carotid bodies by stimulating the activation of ERK1/2

The carotid body (CB) is a complex sensory organ that functions to sense homeostatic O2 in the blood. Previous studies have shown that CBs express interleukin (IL)-1 receptor type I and that the chemosensitivity of CBs is increased following stimulation with pro-inflammatory cytokines. However, the effects of pro-inflammatory cytokines, such as IL-1β, on the neurogenesis of CB are unclear. Thus, in this study, we aimed to assess the effects of IL-1β and intermittent hypobaric hypoxia (IHH) plus IL-1β on the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2, tyrosine hydroxylase (TH) and the expression of nestin, a well-established stem cell marker in the nervous system. The results showed that TH, nestin expression and ERK1/2 phosphorylation were increased in the rat CB following intraperitoneal injection of IL-1β. Moreover, IL-1β had additive effects on IHH. These results suggested that the plasticity of CB was increased following treatment with IL-1β and that ERK1/2 may be involved in neurogenic signaling in CBs.

Neural reflex regulation of systemic inflammation: potential new targets for sepsis therapy

Sepsis progresses to multiple organ dysfunction due to the uncontrolled release of inflammatory mediators, and a growing body of evidence shows that neural signals play a significant role in modulating the immune response. Thus, similar toall other physiological systems, the immune system is both connected to and regulated by the central nervous system. The efferent arc consists of the activation of the hypothalamic–pituitary–adrenal axis, sympathetic activation, the cholinergic anti-inflammatory reflex, and the local release of physiological neuromodulators. Immunosensory activity is centered on the production of pro-inflammatory cytokines, signals that are conveyed to the brain through different pathways. The activation of peripheral sensory nerves, i.e., vagal paraganglia by the vagus nerve, and carotid body (CB) chemoreceptors by the carotid/sinus nerve are broadly discussed here. Despite cytokine receptor expression in vagal afferent fibers, pro-inflammatory cytokines have no significant effect on vagus nerve activity. Thus, the CB may be the source of immunosensory inputs and incoming neural signals and, in fact, sense inflammatory mediators, playing a protective role during sepsis. Considering that CB stimulation increases sympathetic activity and adrenal glucocorticoids release, the electrical stimulation of arterial chemoreceptors may be suitable therapeutic approach for regulating systemic inflammation.

Sepsis progression to multiple organ dysfunction in carotid chemo/baro-denervated rats treated with lipopolysaccharide

Sepsis progresses to multiple organ dysfunction (MOD) due to the uncontrolled release of inflammatory mediators. Carotid chemo/baro-receptors could play a protective role during sepsis. In anesthetized male rats, we measured cardiorespiratory variables and plasma TNF-α, glucocorticoids, epinephrine, and MOD marker levels 90min after lipopolysaccharide (LPS) administration in control (SHAM surgery) and bilateral carotid chemo/baro-denervated (BCN) rats. BCN prior to LPS blunted the tachypneic response and enhanced tachycardia and hypotension. BCN-LPS rats also showed blunted plasma glucocorticoid responses, boosted epinephrine and TNF-α responses, and earlier MOD onset with a lower survival time compared with SHAM-LPS rats. Consequently, the complete absence of carotid chemo/baro-sensory function modified the neural, endocrine and inflammatory responses to sepsis. Thus, carotid chemo/baro-receptors play a protective role in sepsis.

Revisiting cAMP signaling in the carotid body

Chronic carotid body (CB) activation is now recognized as being essential in the development of hypertension and promoting insulin resistance; thus, it is imperative to characterize the chemotransduction mechanisms of this organ in order to modulate its activity and improve patient outcomes. For several years, and although controversial, cyclic adenosine monophosphate (cAMP) was considered an important player in initiating the activation of the CB. However, its relevance was partially displaced in the 90s by the emerging role of the mitochondria and molecules such as AMP-activated protein kinase and O₂-sensitive K⁺ channels. Neurotransmitters/neuromodulators binding to metabotropic receptors are essential to chemotransmission in the CB, and cAMP is central to this process. cAMP also contributes to raise intracellular Ca²⁺ levels, and is intimately related to the cellular energetic status (AMP/ATP ratio). Furthermore, cAMP signaling is a target of multiple current pharmacological agents used in clinical practice. This review (1) provides an outline on the classical view of the cAMP-signaling pathway in the CB that originally supported its role in the O₂/CO₂ sensing mechanism, (2) presents recent evidence on CB cAMP neuromodulation and (3) discusses how CB activity is affected by current clinical therapies that modify cAMP-signaling, namely dopaminergic drugs, caffeine (modulation of A_2A/A_2B receptors) and roflumilast (PDE4 inhibitors). cAMP is key to any process that involves metabotropic receptors and the intracellular pathways involved in CB disease states are likely to involve this classical second messenger. Research examining the potential modification of cAMP levels and/or interactions with molecules associated with CB hyperactivity is currently in its beginning and this review will open doors for future explorations.

Pathogenic roles of the carotid body inflammation in sleep apnea

Breathing difficulties in sleep are a hallmark of sleep-disordered breathing commonly observed in patients with sleep disorders. The pathophysiology of sleep apnea is in part due to an augmented activity of the carotid body chemoreflex. Arterial chemoreceptors in the carotid body are sensitive to inflammatory cytokines and immunogenic molecules in the circulation, because cytokine receptors are expressed in the carotid body in experimental animals and human. Intriguingly, proinflammatory cytokines are also locally produced and released in the carotid body. Also, there are significant increases in the expression of proinflammatory cytokines, cytokine receptors, and inflammatory mediators in the carotid body under hypoxic conditions, suggesting an inflammatory response of the carotid body. These upregulated cytokine signaling pathways could enhance the carotid chemoreceptor activity, leading to an overactivity of the chemoreflex adversely effecting breathing instability and autonomic imbalance. This review aims to summarize findings of the literature relevant to inflammation in the carotid body, with highlights on the pathophysiological impact in sleep apnea. It is concluded that local inflammation in the carotid body plays a pathogenic role in sleep apnea, which could potentially be a therapeutic target for the treatment of the pathophysiological consequence of sleep apnea.

Macrophage migration inhibitory factor promotes proliferation and neuronal differentiation of neural stem/precursor cells through Wnt/β-catenin signal pathway

Macrophage migration inhibitory factor (MIF) is a highly conserved and evolutionarily ancient mediator with pleiotropic effects. Recent studies demonstrated that the receptors of MIF, including CD44, CXCR2, CXCR4 and CD74, are expressed in the neural stem/progenitor cells (NSPCs). The potential regulatory effect of MIF on NSPCs proliferation and neuronal differentiation, however, is largely unknown. Here, we investigated the effect of MIF on NSPC proliferation and neuronal differentiation, and further examined the signal pathway by which MIF transduced these signal effects in mouse NSPCs in vitro. The results showed that both Ki67-positive cells and neurosphere volumes were increased in a dose-dependent manner following MIF treatment. Furthermore, the expression of nuclear β-catenin was significantly stronger in MIF-stimulated groups than that in control groups. Conversely, administration of IWR-1, the inhibitor of Wnt/β-catenin pathway, significantly inhibited the proliferative effect of MIF on NSPCs. Immunostaining and Western blot further indicated that doublecortin (DCX) and Tuj 1, two neuronal markers, were evidently increased with MIF stimulation during NSPC differentiation, and there were more Tuj1-positive cells migrated out from neurospheres in MIF-stimulated groups than those in control groups. During NSPC differentiation, MIF increased the activity of β-galactosidase that responds to Wnt/β-catenin signaling. Wnt1 and β-catenin proteins were also up-regulated with MIF stimulation. Moreover, the expression of DCX and Tuj 1 was inhibited significantly by IWR-1. Taken together, the present study indicated that MIF enhances NSPC proliferation and promotes the neuronal differentiation, by activating Wnt/β-catenin signal pathway. The interaction between MIF and Wnt/β-catenin signal pathway may play an important role in modulating NSPC renewal and fate during brain development.

Sustained exposure to cytokines and hypoxia enhances excitability of oxygen-sensitive type I cells in rat carotid body: correlation with the expression of HIF-1α protein and adrenomedullin

Recent studies in our laboratory demonstrated that chronic hypoxia (CH) induces a localized inflammatory response in rat carotid body that is characterized by macrophage invasion and increased expression of inflammatory cytokines. Moreover, CH-induced increased hypoxic sensitivity is blocked by concurrent treatment with the common anti-inflammatory drugs, ibuprofen and dexamethasone. The present study examines the hypothesis that selected cytokines enhance the excitability of oxygen-sensitive type I cells in the carotid body, and that downstream effects of cytokines involve upregulation of the transcription factor, hypoxia inducible factor-1α (HIF-1α). Cultured type I cells were exposed for 24 h to hypoxia and/or a cocktail of cytokines consisting of interleukin-1β, interleukin-6, and tumor necrosis factor-α. Subsequent evaluation of hypoxia-evoked intracellular Ca(2+)-responses showed that previous exposure to cytokines plus hypoxia resulted in a 110% (p<0.001) increase in cell excitability, whereas exposure to cytokines or hypoxia alone elicited smaller increases of 22% (not significant) and 35% (p<0.01), respectively. These changes were correlated with increased immunostaining for HIF-1α in similarly treated type I cells, where exposure to cytokines plus hypoxia promoted the nuclear translocation of the transcription factor. Moreover, treatment with cytokines and/or hypoxia elevated the expression of the HIF-1-regulated gene, adrenomedullin. These in vitro results are supported by studies which show that elevated type I cell sensitivity following in vivo CH is blocked by concurrent treatment with ibuprofen. The data suggest that CH-induced adaptation in arterial chemoreceptors may in part be mediated by cytokine/hypoxia-induced upregulation of HIF-1α, and consequent enhanced expression of specific hypoxia-sensitive genes in type I cells.

Inflammatory and immunomodulatory mechanisms in the carotid body

Evidence is available about the role of inflammatory/immunological factors in the physiology and plasticity of the carotid body, with potential clinical implications in obstructive sleep apnea syndrome and sudden infant death syndrome. In humans, lymphomonocytic aggregations (chronic carotid glomitis) have been reported in aging and opiate addiction. Glomus cells produce prostaglandin E2 and the cytokines interleukin 1β, interleukin 6 and TNF-α, with corresponding receptors. These factors modulate glomus cell excitability, catecholamine release and/or chemoreceptor discharge. The above cytokines are up-regulated in chronic sustained or intermittent hypoxia, and prevention of these changes, with ibuprofen or dexamethasone, may modulate hypoxia-induced changes in carotid body chemosensitivity. The main transcription factors considered to be involved are NF-kB and HIFs. Circulating immunogens (lipopolysaccharide) and cytokines may also affect peripheral arterial chemoreception, with the carotid body exerting an immunosensing function.

Functional role for interleukin-1 in the injured peripheral taste system

The peripheral taste system presents an excellent model for studying the consequences of neural injury, for the damaged nerve and sensory cells and the neighboring, intact neural cells. Sectioning a primary afferent nerve, the chorda tympani (CT), rapidly recruits neutrophils to both sides of the tongue. The bilateral neutrophil response induces transient functional deficits in the intact CT. Normal function is subsequently restored as macrophages respond to injury. We hypothesized that macrophages produce the proinflammatory cytokine interleukin (IL)-1, which contributes to the maintenance of normal taste function after nearby injury. We demonstrate that IL-1β protein levels are significantly increased on the injured side of the tongue at day 2 after injury. Dietary sodium deficiency, a manipulation that prevents macrophage recruitment, inhibits the elevation in IL-1β. IL-1β was expressed in several cell populations, including taste receptor cells and infiltrating neutrophils and macrophages. To test whether IL-1 modulates taste function after injury, we blocked signaling with an IL-1 receptor antagonist (IL-1 RA) and recorded taste responses from the intact CT. This treatment inhibited the bilateral macrophage response to injury and impaired taste responses in the intact CT. Cytokine actions in the taste system are largely unstudied. These results demonstrate that IL-1 has a beneficial effect on taste function after nearby injury, in contrast to its detrimental role in the injured central nervous system.

Contribution of inflammation on carotid body chemosensory potentiation induced by intermittent hypoxia

Exposure to chronic intermittent hypoxia (CIH) produces hypertension. A critical process involved in the CIH-induced hypertension is the potentiation of the carotid body (CB) chemosensory responses to acute hypoxia. The CIH-induced changes in the CB chemosensory process have been related to an enhanced reactive oxygen species (ROS) production. However, it is still a matter of debate where ROS could directly modify the CB chemosensory discharge. Recently, we found that CIH-induced increase expression of TNF-a and IL-1b within the CB. Thus, we studied the contribution of these pro-inflammatory cytokines on the enhanced CB chemosensory response to acute hypoxia in rats exposed to CIH. To study the role of TNF-a and IL-1b, male Sprague-Dawley rats were submitted to CIH (5% O(2), 12 times/hr for 8 hr/day) and received chronic ibuprofen treatment (40 mg/kg). Following 21 days of CIH, rats were anaesthetized and the CB chemosensory discharge was recorded in response to several levels FiO2 (5-100%). Exposure to CIH significantly increases the immunorreactive levels of TNF-a and IL-1b in the CB, along with an increase accumulation of the p65 NF-kb subunit. Treating rats with ibuprofen significantly prevents the CIH-induced increases in TNF-a and IL-1b in the CB chemoreceptor cells but failed to decrease the enhanced CB chemosensory reactivity to hypoxia. Our results suggest that the mechanisms underlying the potentiation of the CB chemosensory response to acute hypoxia are not linked to the increased expression of TNF-a and IL-1b within the CBs of CIH-exposed rats.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Chronic intermittent hypoxia induces local inflammation of the rat carotid body via functional upregulation of proinflammatory cytokine pathways

Maladaptive changes in the carotid body (CB) induced by chronic intermittent hypoxia (IH) account for the pathogenesis of cardiovascular morbidity in patients with sleep-disordered breathing. We postulated that the proinflammatory cytokines, namely interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α, and cytokine receptors (IL-1r1, gp130 and TNFr1) locally expressed in the rat CB play a pathophysiological role in IH-induced CB inflammation. Results showed increased levels of oxidative stress (serum 8-isoprostane and nitrotyrosine in the CB) in rats with 7-day IH treatment resembling recurrent apneic conditions when compared with the normoxic control. Local inflammation shown by the amount of ED1-containing cells (macrophage infiltration) and the gene transcripts of NADPH oxidase subunits (gp91(phox) and p22(phox)) and chemokines (MCP-1, CCR2, MIP-1α, MIP-1β and ICAM-1) in the CB were significantly more in the hypoxic group than in the control. In addition, the cytokines and receptors were expressed in the lobules of chemosensitive glomus cells containing tyrosine hydroxylase and the levels of expressions were significantly increased in the hypoxic group. Exogenous cytokines elevated the intracellular calcium ([Ca(2+)](i)) response to acute hypoxia in the dissociated glomus cells. The effect of cytokines on the [Ca(2+)](i) response was significantly greater in the hypoxic than in the normoxic group. Moreover, daily treatment of IH rats with anti-inflammatory drugs (dexamethasone or ibuprofen) attenuated the levels of oxidative stress, gp91(phox) expression and macrophage infiltration in the CB. Collectively, these results suggest that the upregulated expression of proinflammatory cytokine pathways could mediate the local inflammation and functional alteration of the CB under chronic IH conditions.

Differential expression of pro-inflammatory cytokines, endothelin-1 and nitric oxide synthases in the rat carotid body exposed to intermittent hypoxia

The enhanced carotid body (CB) chemosensory response to hypoxia induced by chronic intermittent hypoxia (CIH) has been attributed to oxidative stress, which is expected to increase the expression of chemosensory modulators including chemoexcitatory pro-inflammatory cytokines in the CB. Accordingly, we studied the time-course of the changes in the immunohistological expression of TNF-α, IL-1β, IL-6, ET-1, iNOS, eNOS and 3-nitrotyrosine in the CB, along with the progression of enhanced CB chemosensory responses to acute hypoxia in male Sprague-Dawley rats exposed to CIH (5%O₂, 12 times/h per 8h) for 7, 14 and 21 days. Exposure to CIH for 7 days resulted in a sustained potentiation of CB chemosensory responses to acute hypoxia, which persisted until 21 days of CIH. The chemosensory potentiation was paralleled by an increased 3-nitrotyrosine expression in the CB. On the contrary, CIH produced a transient 2-fold increase of ET-1 immunoreactivity at 7 days, a decrease in eNOS immunoreactivity, and a delayed but progressive increase of TNF-α, IL-1β and iNOS immunoreactivity, which was not associated with changes in systemic plasma levels or immune cell invasion within the CB. Thus, present results suggest that the local expression of chemosensory modulators and pro-inflammatory cytokines in the CB may have different temporal contribution to the CB chemosensory potentiation induced by CIH.

A chronic pain: inflammation-dependent chemoreceptor adaptation in rat carotid body

Experiments in recent years have revealed labile electrophysiological and neurochemical phenotypes in primary afferent neurons exposed to specific stimulus conditions associated with the development of chronic pain. These studies collectively demonstrate that the mechanisms responsible for functional plasticity are primarily mediated by novel neuroimmune interactions involving circulating and resident immune cells and their secretory products, which together induce hyperexcitability in the primary sensory neurons. In another peripheral sensory modality, namely the arterial chemoreceptors, sustained stimulation in the form of chronic hypoxia (CH) elicits increased chemoafferent excitability from the mammalian carotid body. Previous studies which focused on functional changes in oxygen-sensitive type I cells in this organ have only partially elucidated the molecular and cellular mechanisms which initiate and control this adaptive response. Recent studies in our laboratory indicate a unique role for the immune system in regulating the chemo-adaptive response of the carotid body to physiologically relevant levels of hypoxia.

Lipopolysaccharide signaling in the carotid chemoreceptor pathway of rats with sepsis syndrome

In addition to their role in cardiorespiratory regulation, carotid body (CB) chemoreceptors serve as sensors for inflammatory status and as a protective factor during sepsis. However, lipopolysaccharide-induced sepsis (LPS) reduces CB responsiveness to excitatory or depressant stimuli. We tested whether LPS exerts a direct effect on the carotid chemoreceptor pathway, the CB and its sensory ganglion. We determined that the rat CB and nodose-petrosal-jugular ganglion complex (NPJgc) express TLR4, TNF-α and its receptors (TNF-R1 and TNF-R2). LPS administration (15mg/kg intraperitoneally) evoked MyD88-mechanism pathway activation in CB and NPJgc, with NF-κB p65, p38 MAPK, and ERK activation. Consistently, LPS increased TNF-α and TNF-R2. Double-labeling studies showed that the aforementioned pathway occurs in TH-containing glomus cells and NPJgc neurons, components of the chemosensitive neural pathway. Thus, our results suggest that LPS acting directly through TLR4/MyD88-mechanism pathways increases TNF-α and TNF-R2 expression in the carotid chemoreceptor pathway. These results show a novel afferent pathway to the central nervous system during endotoxemia, and could be relevant in understanding sepsis pathophysiology and therapy.

Early and late administration of MnTE-2-PyP5+ in mitigation and treatment of radiation-induced lung damage

Chronic production of reactive oxygen and nitrogen species is an underlying mechanism of irradiation (IR)-induced lung injury. The purpose of this study was to determine the optimum time of delivery of an antioxidant and redox-modulating Mn porphyrin, MnTE-2-PyP(5+), to mitigate and/or treat IR-induced lung damage. Female Fischer-344 rats were irradiated to their right hemithorax (28 Gy). Irradiated animals were treated with PBS or MnTE-2-PyP(5+) (6 mg /kg/24 h) delivered for 2 weeks by sc-implanted osmotic pumps (beginning after 2, 6, 12, 24, or 72 h or 8 weeks). Animals were sacrificed 10 weeks post-IR. Endpoints were body weight, breathing frequency, histopathology, and immunohistochemistry (8-OHdG, ED-1, TGF-beta, HIF-1alpha, VEGF A). A significant radioprotective effect on functional injury, measured by breathing frequency, was observed for all animals treated with MnTE-2-PyP(5+). Treatment with MnTE-2-PyP(5+) starting 2, 6, and 12 h but not after 24 or 72 h resulted in a significant decrease in immunostaining for 8-OHdG, HIF-1alpha, TGF-beta, and VEGF A. A significant decrease in HIF-1alpha, TGF-beta, and VEGF A, as well as an overall reduction in lung damage (histopathology), was observed in animals beginning treatment at the time of fully developed lung injury (8 weeks post-IR). The catalytic manganese porphyrin antioxidant and modulator of redox-based signaling pathways MnTE-2-PyP(5+) mitigates radiation-induced lung injury when given within the first 12 h after IR. More importantly, this is the first study to demonstrate that MnTE-2-PyP(5+) can reverse overall lung damage when started at the time of established lung injury 8 weeks post-IR. The radioprotective effects are presumably mediated through its ability both to suppress oxidative stress and to decrease activation of key transcription factors and proangiogenic and profibrogenic cytokines.

Interleukin-6 increases intracellular Ca2+ concentration and induces catecholamine secretion in rat carotid body glomus cells

Although abundant evidence indicates mutual regulation between the immune and the central nervous systems, how the immune signals are transmitted to the brain is still an unresolved question. In a previous study we found strong expression of proinflammatory cytokine receptors, including interleukin (IL)-1 receptor I and IL-6 receptor alpha in the rat carotid body (CB), a well-known arterial chemoreceptor that senses a variety of chemostimuli in the arterial blood. We demonstrated that IL-1 stimulation increases intracellular calcium ([Ca(2+)](i)) in CB glomus cells, releases ATP, and increases the discharge rate in carotid sinus nerve. To explore the effect of IL-6 on CB, here we examine the effect of IL-6 on [Ca(2+)](i) and catecholamine (CA) secretion in rat CB glomus cells. Calcium imaging showed that extracellular application of IL-6 induced a rise in [Ca(2+)](i) in cultured glomus cells. Amperometry showed that local application of IL-6 evoked CA release from glomus cells. Furthermore, the CA secretory response to IL-6 was blocked by 200 microM Cd(2+), a well-known Ca(2+) channel blocker. Our experiments provide further evidence for the responsiveness of the CB to proinflammatory cytokines and indicate that the CB might play a role in inflammation sensing and transmission of such information to the brain.

Adaptation to chronic hypoxia involves immune cell invasion and increased expression of inflammatory cytokines in rat carotid body

Exposure to chronic hypoxia (CH; 3-28 days at 380 Torr) induces adaptation in mammalian carotid body such that following CH an acute hypoxic challenge elicits an abnormally large increase in carotid sinus nerve impulse activity. The current study examines the hypothesis that CH initiates an immune response in the carotid body and that chemoreceptor hyperexcitability is dependent on the expression and action of inflammatory cytokines. CH resulted in a robust invasion of ED1(+) macrophages, which peaked on day 3 of exposure. Gene expression of proinflammatory cytokines, IL-1beta, TNFalpha, and the chemokine, monocyte chemoattractant protein-1, was increased >2-fold after 1 day of hypoxia followed by a >2-fold increase in IL-6 on day 3. After 28 days of CH, IL-6 remained elevated >5-fold, whereas expression of other cytokines recovered to normal levels. Cytokine expression was not restricted to immune cells. Studies of cultured type I cells harvested following 1 day of in vivo hypoxia showed elevated transcript levels of inflammatory cytokines. In situ hybridization studies confirmed expression of IL-6 in type I cells and also showed that CH induces IL-6 expression in supporting type II cells. Concurrent treatment of CH rats with anti-inflammatory drugs (ibuprofen or dexamethasone) blocked immune cell invasion and severely reduced CH-induced cytokine expression in carotid body. Drug treatment also blocked the development of chemoreceptor hypersensitivity in CH animals. Our findings indicate that chemoreceptor adaptation involves novel neuroimmune mechanisms, which may alter the functional phenotypes of type I cells and chemoafferent neurons.

Lipopolysaccharide-induced carotid body inflammation in cats: functional manifestations, histopathology and involvement of tumour necrosis factor-alpha

In the absence of information on functional manifestations of carotid body (CB) inflammation, we studied an experimental model in which lipopolysaccharide (LPS) administration to pentobarbitone-anaesthetized cats was performed by topical application upon the CB surface or by intravenous infusion (endotoxaemia). The latter caused: (i) disorganization of CB glomoids, increased connective tissue, and rapid recruitment of polymorphonuclear cells into the vascular bed and parenchyma within 4 h; (ii) increased respiratory frequency and diminished ventilatory chemoreflex responses to brief hypoxia (breathing 100% N(2) for 10 s) and diminished ventilatory chemosensory drive (assessed by 100% O(2) tests) during normoxia and hypoxia; (iii) tachycardia, increased haematocrit and systemic hypotension in response to LPS i.v.; and (iv) increased basal frequency of carotid chemosensory discharges during normoxia, but no change in maximal chemoreceptor responses to brief hypoxic exposures. Lipopolysaccharide-induced tachypnoea was prevented by prior bilateral carotid neurotomy. Apoptosis was not observed in CBs from cats subjected to endotoxaemia. Searching for pro-inflammatory mediators, tumour necrosis factor-alpha (TNF-alpha) was localized by immunohistochemistry in glomus and endothelial cells; reverse transcriptase-polymerase chain reaction revealed that the CB expresses the mRNAs for both type-1 (TNF-R1) and type-2 TNF-alpha receptors (TNF-R2); Western blot confirmed a band of the size expected for TNF-R1; and histochemistry showed the presence of TNF-R1 in glomus cells and of TNF-R2 in endothelial cells. Experiments in vitro showed that the frequency of carotid nerve discharges recorded from CBs perfused and superfused under normoxic conditions was not significantly modified by TNF-alpha, but that the enhanced frequency of chemosensory discharges recorded along responses to hypoxic stimulation was transiently diminished in a dose-dependent manner by TNF-alpha injections. The results suggest that the CB may operate as a sensor for immune signals, that the CB exhibits histological features of acute inflammation induced by LPS, that TNF-alpha may participate in LPS-induced changes in chemosensory activity and that some pathophysiological reactions to high levels of LPS in the bloodstream may originate from changes in CB function.

Chronic hypoxia upregulates the expression and function of proinflammatory cytokines in the rat carotid body

The structure and function of the carotid body are greatly altered during chronic hypoxia. Recent studies showed the expression of interleukin (IL)-1 receptor and IL-6 receptor in the carotid body, suggesting a role of proinflammatory cytokines in the chemoreceptor function. The present study aimed to examine the hypothesis that the expression of pro-inflammatory cytokines, namely IL-1beta, IL-6 and tumor necrosis factor (TNF)alpha, plays a role in the rat carotid body in chronic hypoxia. Levels of the mRNA expression of the cytokines and their receptors IL-1r1, gp130 and TNFr1, were significantly increased in the carotid body of hypoxic rats when compared with the normoxic control. Immunohistochemistry showed that the expressions of cytokines and receptors were localized in the lobules of chemosensitive glomus cells containing tyrosine hydroxylase. There were significantly more positive-staining cells in the hypoxic groups with treatment for 3, 7 and 28 days than those of the normoxic controls. Application of exogenous cytokines (0.1 nM) elevated intracellular calcium ([Ca(2+)](i)) responses to acute hypoxia in the dissociated fura-2-loaded glomus cells. The increased [Ca(2+)](i) response in the hypoxic group was significantly greater than that of the normoxic group. Moreover, the gene transcripts of inflammatory mediator inducible nitric oxide synthase and chemokines (MCP-1, CCR2, MIP-1alpha, and ICAM-1) were increased in the carotid body of hypoxic rats. Collectively, results suggest that the increased expressions of proinflammatory cytokines play a functional role in the carotid body with local inflammation during chronic hypoxia.

Neuroimmune interaction in inflammatory diseases

The inflammatory response is modulated through interactions among the nervous, endocrine, and immune systems. Intercommunication between immune cells and the autonomic nervous system is a growing area of interest. Spatial and temporal information about inflammatory processes is relayed to the central nervous system (CNS) where neuroimmune modulation serves to control the extent and intensity of the inflammation. Over the past few decades, research has revealed various routes by which the nervous system and the immune system communicate. The CNS regulates the immune system via hormonal and neuronal pathways, including the sympathetic and parasympathetic nerves. The immune system signals the CNS through cytokines that act both centrally and peripherally. This review aims to introduce the concept of neuroimmune interaction and discuss its potential clinical application, in an attempt to broaden the awareness of this rapidly evolving area and open up new avenues that may aid in the treatment of inflammatory diseases.

Physiological and pathophysiological roles of extracellular ATP in chemosensory control of breathing

The purine nucleotide ATP mediates several distinct forms of sensory transduction in both the peripheral and central nervous systems. These processes share common mechanisms that involve the release of ATP to activate ionotropic P2X and/or metabotropic P2Y receptors. Extracellular ATP signalling plays an important role in ventilatory control, mediating both peripheral and central chemosensory transduction to changes in arterial levels of oxygen and carbon dioxide. New data also suggest that extracellular ATP may play an important role in mediating certain neurophysiological responses to systemic inflammation. Here, we propose the novel concept that both peripheral and central neurophysiological effects of ATP may contribute to alterations in ventilatory control during inflammatory pathophysiological states.

Functions of serotonin in hypoxic pulmonary vascular remodeling

In lung vasculature, reversible constriction of smooth muscle cells exists in response to acute decrease in oxygen levels (hypoxia). Progressive and irreversible structural remodeling that reduces blood vessel lumen takes place in response to chronic hypoxia and results in pulmonary hypertension. Several studies have shown a role of serotonin in regulating acute and chronic hypoxic responses. In this review the contribution of serotonin, its receptors and transporter in lung hypoxic responses is discussed. Hypoxic conditions modify plasma levels of serotonin, serotonin transporter activity, and expression of 5-HT1B and 5-HT2B receptors. These appear to be required for pulmonary vascular cell proliferation, which depends on the ratio between reactive oxygen species and nitric oxide. A heterozygous mutation was identified in the 5-HT2B receptor gene of a patient who developed pulmonary hypertension after fenfluramines anorexigen treatment. This C-terminus truncated 5-HT2B mutant receptor presents lower nitric oxide coupling, and higher cell proliferation capacity than the wild-type receptor. Under low oxygen tension, cells increase the transcription of specific genes via stabilization of the transcription factor hypoxia-inducible factor (HIF)-1. Factors such as angiotensin II or thrombin that can also control HIF-1 pathway contribute to pulmonary vascular remodeling. The 5-HT2B receptor via phosphatidylinositol-3 kinase/Akt activates nuclear factor-kappaB, which is involved in the regulation of HIF-1 expression. Acontrol of HIF- 1 by 5-HT2B receptors explains why expression of pulmonary vascular remodeling factors, such as endothelin-1 or transforming growth factor-beta, which is HIF-1-alpha regulated, is not modified in hypoxic 5-HT2B receptor mutant mice. Understanding the detailed mechanisms involved in lung hypoxic responses may provide general insight into pulmonary hypertension pathogenesis.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Up-regulation of IL-1 receptor type I and tyrosine hydroxylase in the rat carotid body following intraperitoneal injection of IL-1beta

It is well established that reciprocal modulation exists between the central nervous system and immune system. Interleukin (IL)-1beta, a proinflammatory cytokine secreted at early stage of immune challenge, has been recognized as one of the informational molecules in immune-to-brain communication. However, how this large molecule is transmitted to the brain is still unknown. In recent years it has been reported that the cranial nerves, especially the vagus, may play a pivotal role in this regard. It is proposed that IL-1beta may bind to its corresponding receptors located in the glomus cells of the vagal paraganglia and then elicit action potentials in the nerve. The existence of IL-1 receptor type I (IL-1RI) in the vagal paraganglia has been shown. The carotid body, which is the largest peripheral chemoreceptive organ, is also a paraganglion. We hypothesize that the carotid body might play a role similar to the vagal paraganglia because they are architectonically similar. Recently we verified the presence of IL-1RI in the rat carotid body and observed increase firing in the carotid sinus nerve following IL-1beta stimulation. The aim of this study was to observe the changes in expression of IL-1RI and tyrosine hydroxylase (TH), a rate-limiting enzyme for catecholamine synthesis, in the glomus cells of the rat carotid body following intraperitoneal injection of IL-1beta. The radioimmunoassay result showed that the blood IL-1beta level was increased after the intraperitoneal injection of rmIL-1beta (750 ng/kg) from 0.48+/-0.08 to 0.78+/-0.07 ng/ml (P<0.05). Immunofluorescence and Western blot analysis showed that the expression of IL-1RI and TH in the rat carotid body was increased significantly following peritoneal IL-1beta stimulation. In addition, double immunofluorescence labeling for TH and PGP9.5, a marker for glomus cells, or TH immunofluoresence with hematoxylin-eosin (HE) counterstaining revealed that a considerable number of glomus cells did not display TH immunoreactivity. These data provide morphological evidence for the response of the carotid body to proinflammatory cytokine stimulation. The results also indicate that not all of the glomus cells express detectable TH levels either in normal or in some abnormal conditions.

IL-1beta inhibits IK and increases [Ca2+]i in the carotid body glomus cells and increases carotid sinus nerve firings in the rat

Increasing evidence indicates that there exists a reciprocal communication between the immune system and the brain. Interleukin 1beta (IL-1beta), a proinflammatory cytokine produced during immune challenge, is believed to be one of the mediators of immune-to-brain communication, but how it gets into the brain is unknown because of its large molecular weight and difficulty in crossing the blood-brain barrier. Our previous work has demonstrated that IL-1 receptor type I is strongly expressed in the glomus cells of rat carotid body (CB), a well characterized polymodal chemoreceptive organ which serves not only for the detection of hypoxia, hypercapnia and acidity, but also for low temperature and blood glucose. The present study was designed to test whether IL-1beta could stimulate the CB glomus cells and alter the discharge properties in the carotid sinus nerve, the afferent nerve innervating the organ. The results from whole-cell patch-clamp recordings and calcium imaging showed that extracellular application of IL-1beta significantly decreased the outward potassium current and triggered a transient rise in [Ca(2+)](i) in the cultured glomus cells of rat CB. Furthermore, by using extracellular recordings and pharmacological intervention, it was found that IL-1beta stimulation of the CB in the anaesthetized rat in vivo significantly increased the discharge rate in the carotid sinus nerve, most probably mediated by ATP release. This experiment provides evidence that the CB responds to cytokine stimulation and proposes the possibility that the CB might play a role in immune-to-brain communication.

Developmental profile of cholinergic and purinergic traits and receptors in peripheral chemoreflex pathway in cats

This study describes the developmental profile of specific aspects of cholinergic and purinergic neurotransmission in key organs of the peripheral chemoreflex: the carotid body (CB), petrosal ganglion (PG) and superior cervical ganglion (SCG). Using real time RT-PCR and Western blot analyses, we assessed both mRNA and protein expression levels for choline-acetyl-transferase (ChAT), nicotinic receptor (subunits alpha3, alpha4, alpha7, and beta2), ATP and purinergic receptors (P2X2 and P2X3). These analyses were performed on tissue from 1- and 15-day-old, 2-month-old, and adult cats. During development, ChAT protein expression level increased slightly in CB; however, this increase was more important in PG and SCG. In CB, mRNA level for alpha4 nicotinic receptor subunit decreased during development (90% higher in 1-day-old cats than in adults). In the PG, mRNA level for beta2 nicotinic receptor subunit increased during development (80% higher in adults than in 1-day-old cats). In SCG, mRNA for alpha7 nicotinic receptor levels increased (400% higher in adults vs. 1-day-old cats). Conversely, P2X2 receptor protein level was not altered during development in CB and decreased slightly in PG; a similar pattern was observed for the P2X3 receptor. Our findings suggest that in cats, age-related changes in cholinergic and purinergic systems (such as physiological expression of receptor function) are significant within the afferent chemoreceptor pathway and likely contribute to the temporal changes of O2-chemosensitivity during development.

PC12 cells express IL-1 receptor type I and response to IL-1beta stimulation

PC12 cell line has been widely used in a diverse array of neurophysiological studies including in exploration of oxygen-sensing mechanism. In present study, we first identified with immunocytochemistry and Western blot methods that interleukin-1 receptor type I was expressed in the PC12 cells. We then demonstrated with patch clamping technique that extracellular application of IL-1beta dose-dependently inhibited the outward voltage-dependent and TEA-sensitive potassium currents (I(K)) in the PC12 cells, and pre-incubation with the interleukin-1 receptor antagonist almost completely abolished this inhibitory effect. In addition, application of IL-1beta shifted steady-state inactivation of I(K) in hyperpolarizing direction, but did not alter its steady-state activation. Furthermore, IL-1beta-induced inhibition of I(K) led to a membrane depolarization and a transient increase of [Ca(2+)](i) in PC12 cells. Taking together, the present study elucidates that PC12 cells bear interleukin-1 receptor and response to IL-1beta stimulation.

Neural-endocrine-immune complex in the central modulation of tumorigenesis: facts, assumptions, and hypotheses

For the precise coordination of systemic functions, the nervous system uses a variety of peripherally and centrally localized receptors, which transmit information from internal and external environments to the central nervous system. Tight interconnections between the immune, nervous, and endocrine systems provide a base for monitoring and consequent modulation of immune system functions by the brain and vice versa. The immune system plays an important role in tumorigenesis. On the basis of rich interconnections between the immune, nervous and endocrine systems, the possibility that the brain may be informed about tumorigenesis is discussed in this review article. Moreover, the eventual modulation of tumorigenesis by central nervous system is also considered. Prospective consequences of the interactions between tumor and brain for diagnosis and therapy of cancer are emphasized.