Neuroplasticity in nucleus tractus solitarius neurons after episodic ozone exposure in infant primates

Altered functional connectivity of the nucleus tractus solitarii in patients with chronic cough after lung surgery: an rs-fMRI study

BACKGROUND

To explore the altered functional connectivity (FC) of the nucleus tractus solitarii (NTS) in patients with chronic cough after lung surgery using resting-state functional magnetic resonance imaging (rs-fMRI), and the association between abnormal FC and clinical scale scores.

METHODS

A total of 22 patients with chronic cough after lung surgery and 22 healthy controls were included. Visual analog scale (VAS), Mandarin Chinese version of the Leicester Cough Questionnaire (LCQ-MC), and Hamilton anxiety rating scale (HAMA) scores were assessed, and rs-fMRI data were collected. The FC analysis was performed using the NTS as the seed point, and FC values with all voxels in the whole brain were calculated. A two-sample t-test was used to compare FC differences between the two groups. The FC values of brain regions with differences were extracted and correlated with clinical scale scores.

RESULTS

In comparison to healthy controls, FC values in the NTS and anterior cingulate cortex(ACC) were reduced in patients with chronic cough after lung surgery (GRF correction, p-voxel < 0.005, p-cluster < 0.05) which were positively correlated with LCQ-MC scores (r = 0.534, p = 0.011), but with VAS (r = -0.500, p = 0.018), HAMA (r = -0.713, p < 0.001) scores were negatively correlated.

CONCLUSIONS

Reduced FC of the NTS with ACC may be associated with cough hypersensitivity and may contribute to anxiety in patients with chronic cough after lung surgery.

The Solitary Nucleus Connectivity to Key Autonomic Regions in Humans MRI and Literature based Considerations

The nucleus tractus solitarius (NTS), is a key brainstem structure relaying interoceptive peripheral information to the interrelated brain centers for eliciting rapid autonomic responses and for shaping longer-term neuroendocrine and motor patterns. Structural and functional NTS' connectivity has been extensively investigated in laboratory animals. But there is limited information about NTS' connectome in humans. Using MRI, we examined diffusion and resting state data from 20 healthy participants in the Human Connectome Project. The regions within the brainstem (n=8), subcortical (n=6), cerebellar (n=2) and cortical (n=5) parts of the brain were selected via a systematic review of the literature and their white matter NTS connections were evaluated via probabilistic tractography along with functional and directional (i.e., Granger-causality) analyses. The underlying study confirms previous results from animal models and provides novel aspects on NTS integration in humans. Two key findings can be summarized: (i) the NTS predominantly processes afferent input and (ii) a lateralization towards a predominantly left-sided NTS processing. Our results lay the foundations for future investigations into the NTS' tripartite role comprised of interoreceptors' input integration, the resultant neurochemical outflow and cognitive/affective processing. The implications of these data add to the understanding of NTS' role in specific aspects of autonomic functions.

Susceptibility Variations in Air Pollution Health Effects: Incorporating Neuroendocrine Activation

Diverse host factors/phenotypes may exacerbate or diminish biological responses induced by air pollutant exposure. We lack an understanding of biological indicators of environmental exposures that culminate in a physiological response versus those that lead to adversity. Variations in response phenotype might arise centrally and/or at the local tissue level. In addition to genetic differences, the current evidence supports the roles of preexisting cardiopulmonary diseases, diabetes, diet, adverse prenatal environments, neurobehavioral disorders, childhood infections, microbiome, sex, and psychosocial stressors in modifying the susceptibility to air pollutant exposures. Animal models of human diseases, obesity, nutritional inadequacies, and neurobehavioral conditions have been compared with healthy controls to understand the causes of variations in susceptibility. Although psychosocial stressors have been associated with increased susceptibility to air pollutant effects, the contribution of neuroendocrine stress pathways in mediating these effects is just emerging. The new findings of neuroendocrine activation leading to systemic metabolic and immunological effects of air pollutants, and the potential contribution to allostatic load, emphasize the consideration of these mechanisms into susceptibility. Variations in susceptibility to air pollution health effects are likely to underlie host genetic and physiological conditions in concert with disrupted neuroendocrine circuitry that alters physiological stability under the influence of stressors.

Acute lung injury in neonatal rats causes postsynaptic depression in nucleus tractus solitarii second-order neurons

Acute Lung Injury (ALI) alters pulmonary reflex responses, in part due to changes in modulation within the lung and airway neuronal control networks. We hypothesized that synaptic efficacy of nucleus tractus solitarii (nTS) neurons, receiving input from lung, airway, and other viscerosensory afferent fibers, would decrease following ALI. Sprague Dawley neonatal rats (postnatal days 9-11) were given intratracheal installations of saline or bleomycin (a well-characterized model that reproduces the pattern of ALI) and then, one week later, in vitro slices were prepared for whole-cell and perforated whole-cell patch-clamp experiments (postnatal days 16-21). In preparations from ALI rats, 2nd-order nTS neurons had significantly decreased amplitudes of both spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs), compared to saline controls. Rise and decay times of sEPSCs were slower in whole-cell recordings from ALI animals. Similarly, the amplitude of tractus solitarii evoked EPSCs (TS-eEPSCs) were significantly lower in 2^nd-order nTS neurons from ALI rats. Overall these results suggest the presence of postsynaptic depression at TS-nTS synapses receiving lung, airway, and other viscerosensory afferent tractus solitarii input after bleomycin-induced ALI.

Airway Innervation and Plasticity in Asthma

Airway nerves represent a mechanistically and therapeutically important aspect that requires better highlighting in the context of diseases such as asthma. Altered structure and function (plasticity) of afferent and efferent airway innervation can contribute to airway diseases. We describe established anatomy, current understanding of how plasticity occurs, and contributions of plasticity to asthma, focusing on target-derived growth factors (neurotrophins). Perspectives toward novel treatment strategies and future research are provided.

Neuroendocrine Regulation of Air Pollution Health Effects: Emerging Insights

Air pollutant exposures are linked to cardiopulmonary diseases, diabetes, metabolic syndrome, neurobehavioral conditions, and reproductive abnormalities. Significant effort is invested in understanding how pollutants encountered by the lung might induce effects in distant organs. The role of circulating mediators has been predicted; however, their origin and identity have not been confirmed. New evidence has emerged which implicates the role of neuroendocrine sympathetic-adrenal-medullary (SAM) and hypothalamic-pituitary-adrenal (HPA) stress axes in mediating a wide array of systemic and pulmonary effects. Our recent studies using ozone exposure as a prototypical air pollutant demonstrate that increases in circulating adrenal-derived stress hormones (epinephrine and cortisol/corticosterone) contribute to lung injury/inflammation and metabolic effects in the liver, pancreas, adipose, and muscle tissues. When stress hormones are depleted by adrenalectomy in rats, most ozone effects including lung injury/inflammation are diminished. Animals treated with antagonists for adrenergic and glucocorticoid receptors show inhibition of the pulmonary and systemic effects of ozone, whereas treatment with agonists restore and exacerbate the ozone-induced injury/inflammation phenotype, implying the role of neuroendocrine activation. The neuroendocrine system is critical for normal homeostasis and allostatic activation; however, chronic exposure to stressors may lead to increases in allostatic load. The emerging mechanisms by which circulating mediators are released and are responsible for producing multiorgan effects of air pollutants insists upon a paradigm shift in the field of air pollution and health. Moreover, since these neuroendocrine responses are linked to both chemical and nonchemical stressors, the interactive influence of air pollutants, lifestyle, and environmental factors requires further study.

Lung-injury depresses glutamatergic synaptic transmission in the nucleus tractus solitarii via discrete age-dependent mechanisms in neonatal rats

Transition periods (TPs) are brief stages in CNS development where neural circuits can exhibit heightened vulnerability to pathologic conditions such as injury or infection. This susceptibility is due in part to specialized mechanisms of synaptic plasticity, which may become activated by inflammatory mediators released under pathologic conditions. Thus, we hypothesized that the immune response to lung injury (LI) mediated synaptic changes through plasticity-like mechanisms that depended on whether LI occurred just before or after a TP. We studied the impact of LI on brainstem 2nd-order viscerosensory neurons located in the nucleus tractus solitarii (nTS) during a TP for respiratory control spanning (postnatal day (P) 11-15). We injured the lungs of Sprague-Dawley rats by intratracheal instillation of Bleomycin (or saline) just before (P9-11) or after (P17-19) the TP. A week later, we prepared horizontal slices of the medulla and recorded spontaneous and evoked excitatory postsynaptic currents (sEPSCs/eEPSCs) in vitro from neurons in the nTS that received monosynaptic glutamatergic input from the tractus solitarii (TS). In rats injured before the TP (pre-TP), neurons exhibited blunted sEPSCs and TS-eEPSCs compared to controls. The decreased TS-eEPSCs were mediated by differences in postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid receptors (AMPAR). Specifically, compared to controls, LI rats had more Ca2+-impermeable AMPARs (CI-AMPARs) as indicated by: 1) the absence of current-rectification, 2) decreased sensitivity to polyamine, 1-Naphthyl-acetyl-spermine-trihydrochloride (NASPM) and 3) augmented immunoreactive staining for the CI-AMPAR GluA2. Thus, pre-TP-LI acts postsynaptically to blunt glutamatergic transmission. The neuroimmune response to pre-TP-LI included microglia hyper-ramification throughout the nTS. Daily intraperitoneal administration of minocycline, an inhibitor of microglial/macrophage function prevented hyper-ramification and abolished the pre-TP-LI evoked synaptic changes. In contrast, rat-pups injured after the TP (post-TP) exhibited microglia hypo-ramification in the nTS and had increased sEPSC amplitudes/frequencies, and decreased TS-eEPSC amplitudes compared to controls. These synaptic changes were not associated with changes in CI-AMPARs, and instead involved greater TS-evoked use-dependent depression (reduced paired pulse ratio), which is a hallmark of presynaptic plasticity. Thus we conclude that LI regulates the efficacy of TS → nTS synapses through discrete plasticity-like mechanisms that are immune-mediated and depend on whether the injury occurs before or after the TP for respiratory control.

Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control?

For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output-recognizable as a breath-from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity-induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea.

Acute Ozone-Induced Pulmonary and Systemic Metabolic Effects Are Diminished in Adrenalectomized Rats

Acute ozone exposure increases circulating stress hormones and induces metabolic alterations in animals. We hypothesized that the increase of adrenal-derived stress hormones is necessary for both ozone-induced metabolic effects and lung injury. Male Wistar-Kyoto rats underwent bilateral adrenal demedullation (DEMED), total bilateral adrenalectomy (ADREX), or sham surgery (SHAM). After a 4 day recovery, rats were exposed to air or ozone (1 ppm), 4 h/day for 1 or 2 days and responses assessed immediately postexposure. Circulating adrenaline levels dropped to nearly zero in DEMED and ADREX rats relative to SHAM. Corticosterone tended to be low in DEMED rats and dropped to nearly zero in ADREX rats. Adrenalectomy in air-exposed rats caused modest changes in metabolites and lung toxicity parameters. Ozone-induced hyperglycemia and glucose intolerance were markedly attenuated in DEMED rats with nearly complete reversal in ADREX rats. Ozone increased circulating epinephrine and corticosterone in SHAM but not in DEMED or ADREX rats. Free fatty acids (P = .15) and branched-chain amino acids increased after ozone exposure in SHAM but not in DEMED or ADREX rats. Lung minute volume was not affected by surgery or ozone but ozone-induced labored breathing was less pronounced in ADREX rats. Ozone-induced increases in lung protein leakage and neutrophilic inflammation were markedly reduced in DEMED and ADREX rats (ADREX > DEMED). Ozone-mediated decreases in circulating white blood cells in SHAM were not observed in DEMED and ADREX rats. We demonstrate that ozone-induced peripheral metabolic effects and lung injury/inflammation are mediated through adrenal-derived stress hormones likely via the activation of stress response pathway.

Changes in carotid body and nTS neuronal excitability following neonatal sustained and chronic intermittent hypoxia exposure

We investigated whether pre-treatment with neonatal sustained hypoxia (SH) prior to chronic intermittent hypoxia (SH+CIH) would modify in vitro carotid body (CB) chemoreceptor activity and the excitability of neurons in the caudal nucleus of the solitary tract (nTS). Sustained hypoxia followed by CIH exposure simulates an oxygen paradigm experienced by extremely premature infants who developed persistent apnea. Rat pups were treated with 5 days of SH (11% O2) from postnatal age 1 (P1) followed by 10 days of subsequent chronic intermittent hypoxia (CIH, 5% O2/5 min, 8 h/day, between P6 and P15) as described previously (Mayer et al., Respir. Physiol. Neurobiol. 187(2): 167-75, 2013). At the end of SH+CIH exposure (P16), basal firing frequency was enhanced, and the hypoxic sensory response of single unit CB chemoafferents was attenuated. Further, basal firing frequency and the amplitude of evoked excitatory post-synaptic currents (ESPC's) of nTS neurons was augmented compared to age-matched rats raised in normoxia. These effects were unique to SH+CIH exposure as neither SH or CIH alone elicited any comparable effect on chemoafferent activity or nTS function. These data indicated that pre-treatment with neonatal SH prior to CIH exposure uniquely modified mechanisms of peripheral (CB) and central (nTS) neural function in a way that would be expected to disturb the ventilatory response to acute hypoxia.

Anandamide, cannabinoid type 1 receptor, and NMDA receptor activation mediate non-Hebbian presynaptically expressed long-term depression at the first central synapse for visceral afferent fibers

Presynaptic long-term depression (LTD) of synapse efficacy generally requires coordinated activity between presynaptic and postsynaptic neurons and a retrograde signal synthesized by the postsynaptic cell in an activity-dependent manner. In this study, we examined LTD in the rat nucleus tractus solitarii (NTS), a brainstem nucleus that relays homeostatic information from the internal body to the brain. We found that coactivation of N-methyl-D-aspartate receptors (NMDARs) and type 1 cannabinoid receptors (CB1Rs) induces LTD at the first central excitatory synapse between visceral fibers and NTS neurons. This LTD is presynaptically expressed. However, neither postsynaptic activation of NMDARs nor postsynaptic calcium influx are required for its induction. Direct activation of NMDARs triggers cannabinoid-dependent LTD. In addition, LTD is unaffected by blocking 2-arachidonyl-glycerol synthesis, but its induction threshold is lowered by preventing fatty acid degradation. Altogether, our data suggest that LTD in NTS neurons may be entirely expressed at the presynaptic level by local anandamide synthesis.

Airway responsiveness to psychological processes in asthma and health

Psychosocial factors have been found to impact airway pathophysiology in respiratory disease with considerable consistency. Influences on airway mechanics have been studied particularly well. The goal of this article is to review the literature on airway responses to psychological stimulation, discuss potential pathways of influence, and present a well-established emotion-induction paradigm to study airway obstruction elicited by unpleasant stimuli. Observational studies have found systematic associations between lung function and daily mood changes. The laboratory-based paradigm of bronchoconstrictive suggestion has been used successfully to elicit airway obstruction in a substantial proportion of asthmatic individuals. Other studies have demonstrated modulation of airway responses to standard airway challenges with exercise, allergens, or pharmacological agents by psychological factors. Standardized emotion-induction techniques have consistently shown airway constriction during unpleasant stimulation, with surgery, blood, and injury stimuli being particularly powerful. Findings with various forms of stress induction have been more mixed. A number of methodological factors may account for variability across studies, such as choice of measurement technique, temporal association between stimulation and measurement, and the specific quality and intensity of the stimulus material, in particular the extent of implied action-orientation. Research has also begun to elucidate physiological processes associated with psychologically induced airway responses, with vagal excitation and ventilatory influences being the most likely candidate pathways, whereas the role of specific central nervous system pathways and inflammatory processes has been less studied. The technique of emotion-induction using films has the potential to become a standardized challenge paradigm for the further exploration of airway hyperresponsiveness mediated by central nervous system processes.

Distinct tachykinin NK(1) receptor function in primate nucleus tractus solitarius neurons is dysregulated after second-hand tobacco smoke exposure

BACKGROUND AND PURPOSE

Second-hand tobacco smoke (SHS) exposure in children increases the risk of asthma and sudden infant death syndrome. Epidemiological and experimental data have suggested SHS can alter neuroplasticity in the CNS, associated with substance P. We hypothesized that exposure to SHS in young primates changed the effect of substance P on the plasticity of neurons in the nucleus tractus solitarius (NTS), where airway sensory information is first processed in the CNS.

EXPERIMENTAL APPROACH

Thirteen-month-old rhesus monkeys were exposed to filtered air (FA, n= 5) or SHS (n= 5) for >6 months from 50 days of their fetal age. Whole-cell patch-clamp recordings were performed on NTS neurons in brainstem slices from these animals to record the intrinsic cell excitability in the absence or presence of the NK(1) receptor antagonist, SR140333 (3 µM).

KEY RESULTS

Neurons were electrophysiologically classified based on their spiking onset from a hyperpolarized membrane potential into two phenotypes: rapid-onset spiking (RS) and delayed-onset spiking (DS) types. In RS neurons, SR140333 reduced the spiking response, similarly in both FA- and SHS-exposed animals. In DS neurons, SR140333 almost abolished the spiking response in FA-exposed animals, but had no effect in SHS-exposed animals.

CONCLUSIONS AND IMPLICATIONS

The contribution of NK(1) receptors to cell excitability depended on firing phenotype of primate NTS neurons and was disrupted by SHS exposure, specifically in DS neurons. Our findings reveal a novel NK(1) receptor function in the primate brainstem and support the hypothesis that chronic exposure to SHS in children causes tachykinin-related neuroplastic changes in the CNS.

Ozone inhalation activates stress-responsive regions of the CNS

Ozone (O(3)), a major component of air pollution, has considerable impact on public health. Besides the well-described respiratory tract inflammation and dysfunctions, there is accumulating evidence indicating that O(3) exposure affects brain functions. However, the mechanisms through which O(3) exerts toxic effects on the brain remain poorly understood. This work aimed at precisely characterizing CNS neuronal activation after O(3) inhalation using Fos staining in adult rat. We showed that, together with lung inflammation, O(3) exposure caused a sustained time- and dose-dependent neuronal activation in the dorsolateral regions of the nucleus tractus solitarius overlapping terminal fields of lung afferents running in vagus nerves. Furthermore, we highlighted neuronal activation in interconnected central structures such as the caudal ventrolateral medulla, the parabrachial nucleus, the central nucleus of the amygdala, the bed nucleus of the stria terminalis and the paraventricular hypothalamic nucleus. In contrast, we did not detect any neuronal activation in the thoracic spinal cord where lung afferents running in spinal nerves terminate. Overall, our results demonstrate that O(3) challenge evokes a lung inflammation that induces the activation of nucleus tractus solitarius neurons through the vagus nerves and promotes neuronal activation in stress-responsive regions of the CNS.

Biochemical effects of ozone on asthma during postnatal development

BACKGROUND

Ozone exposure during early life has the potential to contribute to the development of asthma as well as to exacerbate underlying allergic asthma.

SCOPE OF REVIEW

Developmentally regulated aspects of sensitivity to ozone exposure and downstream biochemical and cellular responses.

MAJOR CONCLUSIONS

Developmental differences in antioxidant defense responses, respiratory physiology, and vulnerabilities to cellular injury during particular developmental stages all contribute to disparities in the health effects of ozone exposure between children and adults.

GENERAL SIGNIFICANCE

Ozone exposure has the capacity to affect multiple aspects of the "effector arc" of airway hyperresponsiveness, ranging from initial epithelial damage and neural excitation to neural reprogramming during infancy. This article is part of a Special Issue entitled: Biochemistry of Asthma.

Chronic intermittent hypoxia affects integration of sensory input by neurons in the nucleus tractus solitarii

The autonomic nervous and respiratory systems, as well as their coupling, adapt over a wide range of conditions. Chronic intermittent hypoxia (CIH) is a model for recurrent apneas and induces alterations in breathing and increases in sympathetic nerve activity which may ultimately result in hypertension if left untreated. These alterations are believed to be due to increases in the carotid body chemoreflex pathway. Here we present evidence that the nucleus tractus solitarii (nTS), the central brainstem termination site of chemoreceptor afferents, expresses a form of synaptic plasticity that increases overall nTS activity following intermittent hypoxia. Following CIH, an increase in presynaptic spontaneous neurotransmitter release occurs under baseline conditions. Furthermore, during and following afferent stimulation there is an augmentation of spontaneous transmitter release that occurs out of synchrony with sensory stimulation. On the other hand, afferent evoked synchronous transmitter release is attenuated. Overall, this shift from synchronous to asynchronous transmitter release enhances nTS cellular discharge. The role of the neurotransmitter dopamine in CIH-induced plasticity is also discussed. Dopamine attenuates synaptic transmission in nTS cells by blockade of N-type calcium channels, and this mechanism occurs tonically following normoxia and CIH. This dopaminergic pathway, however, is not altered in CIH. Taken together, alterations in nTS synaptic activity may play a role in the changes of chemoreflex function and cardiorespiratory activity in the CIH apnea model.

Influence of vagotomy on monosynaptic transmission at second-order nucleus tractus solitarius synapses

Manipulations of vagal activity are used to treat medical pathologies, but the underlying CNS changes caused by these treatments are not well understood. Furthermore, heart and lung transplant as well as treatments for many gastrointestinal disorders result in section of the vagus nerve (vagotomy). Following unilateral vagotomy under isoflurane anesthesia of Sprague-Dawley rats, electrophysiological properties were recorded with whole cell patch techniques in horizontal brain stem slices. Vagotomy significantly reduced the median amplitude of evoked excitatory postsynaptic currents (evEPSCs; -121; n = 43) in the nucleus tractus solitarius (NTS) when compared with controls (-157 pA; n = 66; P < 0.05) but had no significant effect on the passive properties or on the average amplitude or frequency of miniature EPSCs. The degree of synaptic failure exhibited during a 50-Hz train of stimuli was used to define two separate classes of synapses: "low failure" and "high failure" (HF); failure rates <5 and > or =5%, respectively. HF synapses had significantly smaller median evEPSCs (-88 vs. -184 pA; P < 0.05). After vagotomy, the percentage of HF synapses nearly doubled to 56% (n = 24/43) when compared with controls (30%; n = 20/66). Additionally, the overall percentage of failures after the second to fifth stimuli significantly increased by at least twofold. These results suggest that vagotomy causes a decrease in synaptic efficacy by both increasing the overall percentage of synaptic failures and shifting the population of NTS synapses toward more HF transmission. In addition, the alterations due to vagotomy are likely to be presynaptic in nature.

Central neuroplasticity and decreased heart rate variability after particulate matter exposure in mice

BACKGROUND

Epidemiologic studies show that exposure to fine particulate matter [aerodynamic diameter < or = 2.5 microm (PM(2.5))] increases the total daily cardiovascular mortality. Impaired cardiac autonomic function, which manifests as reduced heart rate variability (HRV), may be one of the underlying causes. However, the cellular mechanism(s) by which PM(2.5) exposure induces decreased HRV is not known.

OBJECTIVES

We tested the hypothesis that exposure to PM(2.5) impairs HRV by decreasing the excitability of the cardiac vagal neurons in the nucleus ambiguus. We also determined the effect of iron on PM-exposure-induced decrease in HRV.

METHODS

We measured 24-hr HRV in time domains from electrocardiogram telemetry recordings obtained in conscious, freely moving mice after 3 days of exposure to PM(2.5) in the form of soot only or iron-soot. In parallel studies, we determined the intrinsic properties of identified cardiac vagal neurons, retrogradely labeled with a fluorescent dye applied to the sinoatrial node.

RESULTS

Soot-only exposure decreased short-term HRV (root mean square of successive difference). With the addition of iron, all HRV parameters were significantly reduced. In nonexposed mice, vagal blockade significantly reduced all HRV parameters, suggesting that HRV is, in part, under vagal regulation in mice. Iron-soot exposure had no significant effect on resting membrane potential but decreased spiking responses of the identified cardiac vagal neurons to depolarizations (p < 0.05). The decreased spiking response was accompanied with a higher minimal depolarizing current required to evoke spikes and a lower peak discharge frequency.

CONCLUSIONS

The data suggest that PM-induced neuroplasticity of cardiac vagal neurons may be one mechanism contributing to the cardiovascular consequences associated with PM(2.5) exposure seen in humans.

Prenatal ozone exposure abolishes stress activation of Fos and tyrosine hydroxylase in the nucleus tractus solitarius of adult rat

Ozone (O3) is widely distributed in the environment, with high levels of air pollution. However, very few studies have documented the effects on postnatal development of O3 during pregnancy. The long-term effects of prenatal O3 exposure in rats (0.5 ppm 12 h/day from embryonic day E5 to E20) were evaluated in the adult nucleus tractus solitarius (NTS) regulating respiratory control. Neuronal response was assessed by Fos protein immunolabeling (Fos-IR), and catecholaminergic neuron involvement by tyrosine hydroxylase (TH) labeling (TH-IR). Adult offspring were analyzed at baseline and following immobilization stress (one hour, plus two hours' recovery); immunolabeling was observed by confocal microscopy. Prenatal O3 increased the baseline TH gray level per cell (p < 0.001). In contrast, the number of Fos-IR cells, Fos-IR/TH-IR colabeled cells and proportion of TH double-labeled with Fos remained unchanged. After stress, the TH gray level (p < 0.001), number of Fos-IR cells (p < 0.001) and of colabeled Fos-IR/TH-IR cells (p < 0.05) and percentage of colabeled Fos-IR/TH-IR neurons against TH-IR cells (p < 0.05) increased in the control group. In prenatal-O3 rats, immobilization stress abolished these increases and reduced the TH gray level (p < 0.05), indicating that prenatal O3 led to loss of adult NTS reactivity to stress. We conclude that long-lasting sequelae were detected in the offspring beyond the prenatal O3 exposure. Prenatal O3 left a print on the NTS, revealed by stress. Disruption of neuronal plasticity to new challenge might be suggested.

Plasticity in glutamatergic NTS neurotransmission

Changes in the physiological state of an animal or human can result in alterations in the cardiovascular and respiratory system in order to maintain homeostasis. Accordingly, the cardiovascular and respiratory systems are not static but readily adapt under a variety of circumstances. The same can be said for the brainstem circuits that control these systems. The nucleus tractus solitarius (NTS) is the central integration site of baroreceptor and chemoreceptor sensory afferent fibers. This central nucleus, and in particular the synapse between the sensory afferent and second-order NTS cell, possesses a remarkable degree of plasticity in response to a variety of stimuli, both acute and chronic. This brief review is intended to describe the plasticity observed in the NTS as well as the locus and mechanisms as they are currently understood. The functional consequence of NTS plasticity is also discussed.

The role of neuro-immune cross-talk in the regulation of inflammation and remodelling in asthma

Despite recent advances in the development of anti-asthmatic medication, asthma continues to be a major health problem worldwide. The symptoms of asthmatic patients include wheezing, chest tightness, cough and shortness of breath, which, together with airway hyperresponiveness, previously have been attributed to a dysfunction of airway nerves. However, research in the last two decades identified Th2-sensitization and the subsequent allergic reaction to innocuous environmental antigens as a basic immunological mechanism leading to chronic airway inflammation. Recent evidence suggests that the development of allergic asthma is influenced by events and circumstances in early childhood and even in utero. Allergen, ozone or stress exposure, as well as RSV infection in early life could be able to induce irreversible changes in the developing epithelial-mesenchymal trophic unit of the airways. The co-existence of chronic inflammation and neural dysfunction have recently drawn attention to the involvement of interaction pathways between the nervous and the immune system in the airways. Intensive basic research has accumulated morphological as well as functional evidence for the interaction between nerves and immune cells. Neuropeptides and neurotrophins have come into focus of attention as the key mediators of neuro-immune interactions, which lead to the development of several pharmacological compounds specifically targeting these molecules. This review will integrate our current knowledge on the involvement of neuro-immune pathways in asthma on the cellular and molecular level. It will summarize the results of pharmacological studies addressing the potential of neuropeptides and neurotrophins as novel therapeutic targets in asthma.

Central mechanisms II: pharmacology of brainstem pathways

Following systemic administration, centrally acting antitussive drugs are generally assumed to act in the brainstem to inhibit cough. However, recent work in humans has raised the possibility of suprapontine sites of action for cough suppressants. For drugs that may act in the brainstem, the specific locations, types of neurones affected, and receptor specificities of the compounds represent important issues regarding their cough-suppressant actions. Two medullary areas that have received the most attention regarding the actions of antitussive drugs are the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral respiratory column. Studies that have implicated these two medullary areas have employed both microinjection and in vitro recording methods to control the location of action of the antitussive drugs. Other brainstem regions contain neurones that participate in the production of cough and could represent potential sites of action of antitussive drugs. These regions include the raphe nuclei, pontine nuclei, and rostral ventrolateral medulla. Specific receptor subtypes have been associated with the suppression of cough at central sites, including 5-HT1A, opioid (mu, kappa, and delta), GABA-B, tachykinin neurokinin-1 (NK-1) and neurokinin-2, non-opioid (NOP-1), cannabinoid, dopaminergic, and sigma receptors. Aside from tachykinin NK-1 receptors in the NTS, relatively little is known regarding the receptor specificity of putative antitussive drugs in particular brainstem regions. Our understanding of the mechanisms of action of antitussive drugs would be significantly advanced by further work in this area.

Central mechanisms I: plasticity of central pathways

Cough is the most common symptom for which individuals seek medical attention and spend health-care dollars. Despite the burden induced by cough, the current treatments for cough are only partially effective. Delineating the sites and mechanisms in the cough central network for changes in the cough reflex could lead to new therapeutic strategies and drug target sites for more effective treatments. The first synaptic target in the CNS for the cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS); these neurons reorganize the primary sensory information into a coherent output. The NTS neurons have been shown to undergo neuroplasticity under a variety of conditions, such as respiratory disorders, stress, and exposures to environmental pollutants. The NTS contains a rich innervation of substance P immunoreactive nerve terminals, suggesting that substance P might be important in altered cough reflex response. This chapter summarizes our current findings on the role of substance P in enhanced cough reflex as well as the potential NTS targets for the action of substance P.

Peripheral mechanisms II: the pharmacology of peripherally active antitussive drugs

Cough is an indispensable defensive reflex. Although generally beneficial, it is also a common symptom of diseases such as asthma, chronic obstructive pulmonary disease, upper respiratory tract infections, idiopathic pulmonary fibrosis and lung cancer. Cough remains a major unmet medical need and although the centrally acting opioids have remained the antitussive of choice for decades, they have many unwanted side effects. However, new research into the behaviour of airway sensory nerves has provided greater insight into the mechanisms of cough and new avenues for the discovery of novel non-opioid antitussive drugs. In this review, the pathophysiological mechanisms of cough and the development of novel antitussive drugs are reviewed.

Adenosine induces a cholinergic tracheal reflex contraction in guinea pigs in vivo via an adenosine A1 receptor-dependent mechanism

Adenosine induces dyspnea, cough, and airways obstruction in asthma, a phenomenon that also occurs in various sensitized animal models in which a neuronal involvement has been implicated. Although adenosine has been suggested to activate cholinergic nerves, the precise mechanism has not been established. In the present study, the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) induced a cholinergic reflex, causing tracheal smooth muscle contraction that was significantly inhibited by the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 100 microg/kg) (P < 0.05) in anesthetized animals. Furthermore, the adenosine A(2) agonist 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) induced a small reflex, whereas the A(3) selective agonist N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) was without effect. The tracheal reflex induced by CPA was also inhibited by recurrent nerve ligation or muscarinic receptor blockade (P < 0.001), indicating that a cholinergic neuronal mechanism of action accounted for this response. The cholinergic reflex in response to aerosolized CPA was significantly greater in passively sensitized compared with naive guinea pigs (P < 0.01). Chronic capsaicin treatment, which inhibited sensory nerve function, failed to inhibit CPA-induced reflex tracheal contractions in passively sensitized guinea pigs, although the local anesthetic lidocaine inhibited CPA-induced tracheal contractions. The effects of CPA on the reflex response was not dependent on the release of histamine from tissue mast cells or endogenous prostaglandins as shown by the lack of effect of the histamine H(1) receptor antagonist pyrilamine (1 mg/kg) or the cyclooxygenase inhibitor meclofenamic acid (3 mg/kg), respectively. In conclusion, activation of pulmonary adenosine A(1) receptors can stimulate cholinergic reflexes, and these reflexes are increased in allergic guinea pigs.

New insights into the electrophysiology of brainstem circuits controlling blood pressure

The brainstem contains the necessary circuitry for the maintenance and regulation of arterial blood pressure. It has become increasingly clear in the past few years that the characteristics of the neurons that constitute these circuits are not static, but can be altered in the face of chronic changes in physiological state. Alterations in voltage-gated and ligand-gated ion channels have been reported in neurons located within the nucleus of the solitary tract and the nucleus ambiguus in response to hypertension and exposures to hypoxia and environmental pollutants (eg, ozone and cigarette smoke). A discussion of these neuronal adaptations, the mechanisms that might initiate and sustain the adaptations, and their potential significance is the focus of this brief review.

Air pollutants and cough

Epidemiological studies have shown that exposure to air pollution is associated with respiratory symptoms and decreases in lung function. This paper reviews recent literature showing that exposure to particulate matter, irritant gases, environmental tobacco smoke (ETS), mixed pollutants, and molds is associated with an increase in cough and wheeze. Some pollutants, like particulate matter and mixed pollutants, appear to increase cough at least as much as wheeze. Others, like irritant gases, appear to increase wheeze more than cough. For ETS, exposure during childhood is associated with cough and wheeze in adulthood, suggesting that the pollutant permanently alters some important aspect of the lungs, immune system or nervous system. We have shown in animal studies that pollutants change the neural control of airways and cough. Second hand smoke (SHS) exposure lengthened stimulated apnoea, increased the number of stimulated coughs, and augmented the degree of stimulated bronchoconstriction. The mechanisms included enhanced reactivity of the peripheral sensory neurones and second-order neurones in the nucleus tractus solitarius (NTS). NTS effects were due to a substance P mechanism at least in part. Ozone and allergen increased the intrinsic excitability of second-order neurones in the NTS. The animal studies suggest that the cough and wheeze experienced by humans exposed to pollutants may involve plasticity in the nervous system.

Early postnatal exposure to allergen and ozone leads to hyperinnervation of the pulmonary epithelium

Airway injury in infant monkeys exposed to ozone and/or house dust mite allergen (HDMA) is associated with a loss of epithelial innervation. In this study, we evaluated for persistence/recovery of the altered epithelial innervation. Thirty-day-old rhesus monkeys were exposed to repeated episodes of HDMA and/or ozone from 1 to 6 months of age and subsequently allowed to recover for 6 months in the absence of further ozone exposure and/or minimal HDMA challenge (sufficient to maintain allergen sensitization). At 1 year of age, nerve density in intrapulmonary airways was immunohistochemically evaluated using antibodies directed against protein gene product 9.5. Hyperinnervation and irregular epithelial nerve distribution was observed in both HDMA- and ozone-exposed groups; most prominent alterations were observed in animals exposed to HDMA plus ozone. Therefore, while adaptive mechanisms exist that re-establish epithelial innervation following cessation or diminution of exposure to HDMA and/or ozone, the recovery is associated with persistent proliferative mechanisms that result in hyperinnervation of the airways.

Plasticity of brainstem mechanisms of cough

The cough reflex is a brainstem reflex, consisting of specific sensory afferent nerves which trigger the reflex, by transmitting the sensory input over vagal or laryngeal nerves to a brainstem circuitry which processes and ultimately transforms the sensory input into a complex motor output to generate cough. The first synaptic target for the primary cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS). This position in the reflex pathway and intricate local circuits within the nucleus make it a strategic site where the sensory information can be modified. Plasticity at this synapse will change the nature of the output--exaggerating it, suppressing it or transforming it into some other complex pattern. This review integrates evidence implicating the NTS in exaggerated cough with proof of the concept that NTS neurons undergo plasticity to contribute to an exaggeration of cough.

Plasticity in the nucleus tractus solitarius and its influence on lung and airway reflexes

The nucleus tractus solitarius (NTS) is the first central nervous system (CNS) site for synaptic contact of the primary afferent fibers from the lungs and airways. The signal processing at these synapses will determine the output of the sensory information from the lungs and airways to all downstream synapses in the reflex pathways. The second-order NTS neurons bring to bear their own intrinsic and synaptic properties to temporally and spatially integrate the sensory information with inputs from local networks, higher brain regions, and circulating mediators, to orchestrate a coherent reflex output. There is growing evidence that NTS neurons share the rich repertoire of forms of plasticity demonstrated throughout the CNS. This review focuses on existing evidence for plasticity in the NTS, potential targets for plasticity in the NTS, and the impact of this plasticity on lung and airway reflexes.

The role of vagal afferent nerves in chronic obstructive pulmonary disease

Circumstantial evidence supports the hypothesis that the vagal nervous system is dysregulated in chronic obstructive pulmonary disease. This dysregulation can lead to an increased sensitivity of the cough reflex such that the coughing becomes, at times, "nonproductive" or inappropriate. Vagal dysregulation can also lead to an increase in the activity of the parasympathetic reflex control of the airways, which contributes to greater mucus secretion and bronchial smooth muscle contraction. Indirect evidence indicates that lung disease is accompanied by substantive changes to the entire reflex pathways, including enhanced activity of the primary afferent nerves, increases in synaptic efficacy at secondary nerves in the central nervous system, and changes in the autonomic nerve pathways. Drugs aimed at normalizing neuronal activity may, therefore, be beneficial in chronic obstructive pulmonary disease.

Structural and functional localization of airway effects from episodic exposure of infant monkeys to allergen and/or ozone

Both allergen and ozone exposure increase asthma symptoms and airway responsiveness in children. Little is known about how these inhalants may differentially modify airway responsiveness in large proximal as compared to small distal airways. We evaluated whether bronchi and respiratory bronchioles from infant monkeys exposed episodically to allergen and/or ozone differentially develop intrinsic hyperresponsiveness to methacholine and whether eosinophils and/or pulmonary neuroendocrine cells play a role. Infant monkeys were exposed episodically for 5 months to: (1) filtered air, (2) aerosolized house dust mite allergen, (3) ozone 0.5 ppm, or (4) house dust mite allergen + ozone. Studying the function/structure relationship of the same lung slices, we evaluated methacholine airway responsiveness and histology of bronchi and respiratory bronchioles. In bronchi, intrinsic responsiveness was increased by allergen exposure, an effect reduced by bombesin antagonist. In respiratory bronchioles, intrinsic airway responsiveness was increased by allergen + ozone exposure. Eosinophils were increased by allergen and allergen + ozone exposure in bronchi and by allergen exposure in respiratory bronchioles. In both airways, exposure to allergen + ozone resulted in fewer tissue eosinophils than did allergen exposure alone. In bronchi, but not in respiratory bronchioles, the number of eosinophils and neuroendocrine cells correlated with airway responsiveness. We conclude that episodically exposing infant monkeys to house dust mite allergen with or without ozone increased intrinsic airway responsiveness to methacholine in bronchi differently than in respiratory bronchioles. In bronchi, eosinophils and neuroendocrine cells may play a role in the development of airway hyperresponsiveness.

Nerve growth factor: the central hub in the development of allergic asthma?

Neurotrophins like nerve growth factor (NGF), originally described as nerve growth factors in neuronal development, have been implicated in many physiological processes in the last years. They are now regarded as important factors involved in the resolution of pathological conditions. NGF has profound effects on inflammation, repair and remodeling of tissues. However, in the lung these beneficial effects can transact into disease promoting actions, e.g., in allergic inflammation or respiratory syncytial virus (RSV) infection. Overproduction of NGF then enhances inflammation, and promotes (neuronal) airway hyperreactivity and neurogenic inflammation. We hypothesize that NGF overexpression in certain vulnerable time windows during infancy could be a major risk factor for the development of asthma symptoms.

Prolonged prenatal exposure to low-level ozone affects aggressive behaviour as well as NGF and BDNF levels in the central nervous system of CD-1 mice

The long-term effects on isolation-induced aggressive behaviour and central NGF and BDNF levels of gestational exposures to ozone (O(3)) were evaluated in adult CD-1 mice. Females were exposed to O(3), at the dose of 0.0, 0.3 or 0.6 ppm from 30 days prior the formation of breeding pairs until gestational day 17. Litters were fostered at birth to untreated dams and, at adulthood, male offspring underwent five successive daily encounters (15 min each) with a standard opponent of the same strain, sex, weight and age. The encounters on day 1, 3 and 5 were videotaped and agonistic and non-agonistic behavioural items finely scored. O(3)-exposed mice showed a significant increase in freezing and defensive postures, a decrease in nose-sniffing behaviour and reduced progressively the aggressive behavioural profile displayed on day 1. Reduced NGF levels in the hippocampus and increased BDNF in the striatum were also found upon O(3) exposure.

Genetic susceptibility to ozone-induced lung inflammation in animal models of asthma

PURPOSE OF REVIEW

Epidemiological associations between ozone exposure and allergic responsiveness are well-documented and have been corroborated in animal studies. The complex interaction between ozone and allergen has genetic and environmental components that affect atopic individuals and may increase the incidence of allergy in susceptible individuals. This review describes the advances that have been made in understanding mechanisms of genetic susceptibility to ozone-induced inflammation, and the interaction between ozone and allergen exposure in mice and a non-human primate model.

RECENT FINDINGS

Antioxidant and innate immune defense genes contribute to ozone-induced inflammation and hyperpermeability in mice and humans. Ozone exposure during the allergic challenge phase induces greater enhancement of allergic responsiveness than the sensitization stage. Ovalbumin-pulsed dendritic cells injected into naïve mice successfully sensitize the mouse to ovalbumin in the absence of adjuvant. Debate continues over the role of T helper 1-T helper 2 immune profile development in mediating the ozone-allergen interaction, and the potential confounding influence of the predominant T helper 2 system most commonly used to study these responses.

SUMMARY

The role of genetic background in susceptibility to ozone-induced lung inflammation has been confirmed, and promising candidate genes have been identified. Descriptive studies confirm that ozone exacerbates allergic responsiveness. Ozone administered during the challenge phase of ovalbumin allergen exposure induces greater responsiveness than during the sensitization phase. Allergen-induced responses enhanced by concurrent ozone exposure warrant further mechanistic research, particularly regarding the influence of susceptibility genes.

Plasticity of central mechanisms for cough

Cough is associated with plasticity of putative cough afferent fibres, but whether plasticity in the brainstem network contributes is less well understood. A key site in the CNS network is the nucleus tractus solitarius (NTS), the first synaptic contact of the primary afferent fibres. We sought to develop a conscious guinea pig model to detect enhanced cough, to focus on the NTS as a potential site for plasticity, and to test a role for substance P in the NTS since the neuropeptide has been implicated in plasticity of the vagal afferent fibres. Guinea pigs were exposed to second-hand tobacco smoke (SHS) or filtered air (FA) from 1-6 weeks of age. At 5 weeks, cannulae were implanted in the NTS. At 6 weeks, either vehicle or a neurokinin 1 (NK-1) receptor antagonist was injected into the NTS of the conscious guinea pigs who were then exposed to citric acid aerosol. SHS exposure significantly enhanced citric acid-induced cough (56%, P<0.05), an effect attenuated by NTS NK-1 receptor blockade (P<0.05). The findings suggest that one possible mechanism for plasticity in cough is related to substance P effects in the NTS. Future studies will be required to investigate the possible mechanisms underlying the role of substance P as well as other mechanisms in generating SHS-induced cough.

Smooth muscle hypertrophy in distal airways of sensitized infant rhesus monkeys exposed to house dust mite allergen

BACKGROUND

Airway smooth muscle hypertrophy is closely associated with the pathophysiology of hyper-reactive airways in allergic asthma.

OBJECTIVE

To determine whether repeated exposure to allergens during postnatal lung development promotes remodelling of airway smooth muscle.

METHODS

Infant, male rhesus monkeys (30-day-old) were sensitized to house dust mite allergen (HDMA) and then exposed to HDMA aerosol periodically over 5 months. Smooth muscle mass and bundle size and abundance in conducting airways were measured and compared with age-matched control (filtered air-exposed) monkeys.

RESULTS

Total smooth muscle mass and average bundle size were significantly greater in the conducting airways of monkeys exposed to HDMA. Smooth muscle bundle abundance was not affected by exposure to HDMA.

CONCLUSION

Repeated cycles of allergen exposure alter postnatal morphogenesis of smooth muscle, affecting both total mass and bundle size, in conducting airways of infant monkeys.

Central and peripheral changes in catecholamine biosynthesis and turnover in rats after a short period of ozone exposure

We investigated in rat the effects of ozone exposure (0.7 ppm) for 5 h on the catecholamine biosynthesis and turnover in sympathetic efferents and various brain areas. For this purpose, the activity of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, was assessed in superior cervical ganglia and in two major noradrenergic cell groups, A2 and A6 (locus coeruleus). Tyrosine hydroxylase activity was estimated in vivo by measuring the accumulation of l-dihydroxyphenylalanine after pharmacological blockade of L-aromatic acid decarboxylases by NSD-1015 (100 mg/kg i.p.). The catecholamine turnover rate was measured after inhibition of tyrosine hydroxylase by alpha-methyl-para-tyrosine (AMPT, 250 mg/kg, i.p., 2.5 h) in peripheral sympathetic target organ (heart and lungs) as well as in some brain catecholamine terminal areas (cerebral cortex, hypothalamus and striatum). Ozone caused differential effects according to the structure. Catecholamine biosynthesis was stimulated in superior cervical ganglia (+44%, P < 0.05) and caudal A2 subset (+126%, P < 0.01), whereas catecholamine turnover was increased in heart (+183%, P < 0.01) and cortex (+22%, P < 0.05). On the other hand, catecholamine turnover was inhibited in lungs (-53%, P < 0.05) and striatum (-24%, P < 0.05). A brief exposure to ozone, at a concentration chosen to mimic pollution level encountered in urban areas, can modulate catecholamine biosynthesis and utilization rate in the sympathetic and central neurones.

Postnatal remodeling of the neural components of the epithelial-mesenchymal trophic unit in the proximal airways of infant rhesus monkeys exposed to ozone and allergen

Nerves and neuroendocrine cells located within the airway epithelium are ideally situated to sample a changing airway environment, to transmit that information to the central nervous system, and to promote trophic interactions between epithelial and mesenchymal cellular and acellular components. We tested the hypothesis that the environmental stresses of ozone (O(3)) and house dust mite allergen (HDMA) in atopic infant rhesus monkeys alter the distribution of airway nerves. Midlevel bronchi and bronchioles from 6-month-old infant monkeys that inhaled filtered air (FA), house dust mite allergen HDMA, O(3), or HDMA + O(3) for 11 episodes (5 days each, 0.5 ppm O(3), 8 h/day followed by 9 days recovery) were examined using immunohistochemistry for the presence of Protein gene product 9.5 (PGP 9.5), a nonspecific neural indicator, and calcitonin gene-related peptide (CGRP). Along the axial path between the sixth and the seventh intrapulmonary airway generations, there were small significant (P < 0.05) decrements in the density of epithelial nerves in monkeys exposed to HDMA or O(3), while in monkeys exposed to HDMA + O(3) there was a greater significant (P < 0.05) reduction in epithelial innervation. In animals exposed to O(3) or HDMA + O(3) there was a significant increase in the number of PGP 9.5 positive/CGRP negative cells that were anchored to the basal lamina and emitted projections in primarily the lateral plain and often intertwined with projections and cell bodies of other similar cells. We conclude that repeated cycles of acute injury and repair associated with the episodic pattern of ozone and allergen exposure alter the normal development of neural innervation of the epithelial compartment and the appearance of a new population of undefined PGP 9.5 positive cells within the epithelium.

Effect of eosinophil adhesion on intracellular signaling in cholinergic nerve cells

Eosinophil localization to cholinergic nerves occurs in a variety of inflammatory conditions, including asthma. This localization is mediated by interactions between eosinophil integrins and neuronal vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1). Eosinophil-nerve cell interactions lead to generation of neuronal reactive oxygen species and release of eosinophil proteins. The effects of eosinophil adhesion on neuronal intracellular signaling pathways were investigated. Eosinophil adhesion to IMR32 cholinergic nerves led to a rapid and sustained activation of the nuclear transcription factors nuclear factor (NF)-kappaB and activator protein (AP)-1 in the nerve cells. Eosinophil binding to neuronal ICAM-1 led to a rapid activation of ERK1/2 in nerve cells. Inhibition of ERK1/2 prevented NF-kappaB activation. Eosinophil adhesion to VCAM-1 resulted in AP-1 activation, mediated partially by rapid activation of the p38 mitogen-activated protein kinase. These data show that adhesion of eosinophils induces mitogen-activated protein kinase-dependent activation of the transcription factors NF-kappaB and AP-1 in nerve cells, indicating that eosinophil adhesion may control nerve growth and phenotype.

Hyperbaric oxygen and chemical oxidants stimulate CO2/H+-sensitive neurons in rat brain stem slices

Hyperoxia, a model of oxidative stress, can disrupt brain stem function, presumably by an increase in O2 free radicals. Breathing hyperbaric oxygen (HBO2) initially causes hyperoxic hyperventilation, whereas extended exposure to HBO2 disrupts cardiorespiratory control. Presently, it is unknown how hyperoxia affects brain stem neurons. We have tested the hypothesis that hyperoxia increases excitability of neurons of the solitary complex neurons, which is an important region for cardiorespiratory control and central CO2/H+ chemoreception. Intracellular recordings were made in rat medullary slices during exposure to 2-3 atm of HBO2, HBO2 plus antioxidant (Trolox C), and chemical oxidants (N-chlorosuccinimide, chloramine-T). HBO2 increased input resistance and stimulated firing rate in 38% of neurons; both effects of HBO2 were blocked by antioxidant and mimicked by chemical oxidants. Hypercapnia stimulated 32 of 60 (53%) neurons. Remarkably, these CO2/H+-chemosensitive neurons were preferentially sensitive to HBO2; 90% of neurons sensitive to HBO2 and/or chemical oxidants were also CO2/H+ chemosensitive. Conversely, only 19% of HBO2-insensitive neurons were CO2/H+ chemosensitive. We conclude that hyperoxia decreases membrane conductance and stimulates firing of putative central CO2/H+-chemoreceptor neurons by an O2 free radical mechanism. These findings may explain why hyperoxia, paradoxically, stimulates ventilation.