Airway-related vagal preganglionic neurons express brain-derived neurotrophic factor and TrkB receptors: implications for neuronal plasticity

Neurotrophic factors and their receptors in lung development and implications in lung diseases

Although lung innervation has been described by many studies in humans and rodents, the regulation of the respiratory system induced by neurotrophins is not fully understood. Here, we review current knowledge on the role of neurotrophins and the expression and function of their receptors in neurogenesis, vasculogenesis and during the embryonic development of the respiratory tree and highlight key implications relevant to respiratory diseases.

Neuroimmune System as a Driving Force for Plasticity Following CNS Injury

Following an injury to the central nervous system (CNS), spontaneous plasticity is observed throughout the neuraxis and affects multiple key circuits. Much of this spontaneous plasticity can elicit beneficial and deleterious functional outcomes, depending on the context of plasticity and circuit affected. Injury-induced activation of the neuroimmune system has been proposed to be a major factor in driving this plasticity, as neuroimmune and inflammatory factors have been shown to influence cellular, synaptic, structural, and anatomical plasticity. Here, we will review the mechanisms through which the neuroimmune system mediates plasticity after CNS injury. Understanding the role of specific neuroimmune factors in driving adaptive and maladaptive plasticity may offer valuable therapeutic insight into how to promote adaptive plasticity and/or diminish maladaptive plasticity, respectively.

Asthmatic Airway Vagal Hypertonia Involves Chloride Dyshomeostasis of Preganglionic Neurons in Rats

Airway vagal hypertonia is closely related to the severity of asthma; however, the mechanisms of its genesis are unclear. This study aims to prove that asthmatic airway vagal hypertonia involves neuronal Cl^– dyshomeostasis. The experimental airway allergy model was prepared with ovalbumin in male adult Sprague-Dawley rats. Plethysmography was used to evaluate airway vagal response to intracisternally injected γ-aminobutyric acid (GABA). Immunofluorescent staining and Western-blot assay were used to examine the expression of microglia-specific proteins, Na⁺-K⁺-2Cl^– co-transporter 1 (NKCC1), K⁺-Cl^– co-transporter 2 (KCC2) and brain-derived nerve growth factor (BDNF) in airway vagal centers. Pulmonary inflammatory changes were examined with hematoxylin and eosin staining of lung sections and ELISA assay of ovalbumin-specific IgE in bronchoalveolar lavage fluid (BALF). The results showed that histochemically, experimental airway allergy activated microglia, upregulated NKCC1, downregulated KCC2, and increased the content of BDNF in airway vagal centers. Functionally, experimental airway allergy augmented the excitatory airway vagal response to intracisternally injected GABA, which was attenuated by intracisternally pre-injected NKCC1 inhibitor bumetanide. All of the changes induced by experimental airway allergy were prevented or mitigated by chronic intracerebroventricular or intraperitoneal injection of minocycline, an inhibitor of microglia activation. These results demonstrate that experimental airway allergy augments the excitatory response of airway vagal centers to GABA, which might be the result of neuronal Cl^– dyshomeostasis subsequent to microglia activation, increased BDNF release and altered expression of Cl^– transporters. Cl^– dyshomeostasis in airway vagal centers might contribute to the genesis of airway vagal hypertonia in asthma.

Respiratory function after selective respiratory motor neuron death from intrapleural CTB-saporin injections

Amyotrophic lateral sclerosis (ALS) causes progressive motor neuron degeneration, paralysis and death by ventilatory failure. In rodent ALS models: 1) breathing capacity is preserved until late in disease progression despite major respiratory motor neuron death, suggesting unknown forms of compensatory respiratory plasticity; and 2) spinal microglia become activated in association with motor neuron cell death. Here, we report a novel experimental model to study the impact of respiratory motor neuron death on compensatory responses without many complications attendant to spontaneous motor neuron disease. In specific, we used intrapleural injections of cholera toxin B fragment conjugated to saporin (CTB-SAP) to selectively kill motor neurons with access to the pleural space. Motor neuron survival, CD11b labeling (microglia), ventilatory capacity and phrenic motor output were assessed in rats 3-28days after intrapleural injections of: 1) CTB-SAP (25 and 50μg), or 2) unconjugated CTB and SAP (i.e. control; (CTB+SAP). CTB-SAP elicited dose-dependent phrenic and intercostal motor neuron death; 7days post-25μg CTB-SAP, motor neuron survival approximated that in end-stage ALS rats (phrenic: 36±7%; intercostal: 56±10% of controls; n=9; p<0.05). CTB-SAP caused minimal cell death in other brainstem or spinal cord regions.

CTB-SAP

1) increased CD11b fractional area in the phrenic motor nucleus, indicating microglial activation; 2) decreased breathing during maximal chemoreceptor stimulation; and 3) diminished phrenic motor output in anesthetized rats (7days post-25μg,

CTB-SAP

0.3±0.07V; CTB+SAP: 1.5±0.3; n=9; p<0.05). Intrapleural CTB-SAP represents a novel, inducible model of respiratory motor neuron death and provides an opportunity to study compensation for respiratory motor neuron loss.

Brain-derived neurotrophic factor in the airways

In addition to their well-known roles in the nervous system, there is increasing recognition that neurotrophins such as brain derived neurotrophic factor (BDNF) as well as their receptors are expressed in peripheral tissues including the lung, and can thus potentially contribute to both normal physiology and pathophysiology of several diseases. The relevance of this family of growth factors lies in emerging clinical data indicating altered neurotrophin levels and function in a range of diseases including neonatal and adult asthma, sinusitis, influenza, and lung cancer. The current review focuses on 1) the importance of BDNF expression and signaling mechanisms in early airway and lung development, critical to both normal neonatal lung function and also its disruption in prematurity and insults such as inflammation and infection; 2) how BDNF, potentially derived from airway nerves modulate neurogenic control of airway tone, a key aspect of airway reflexes as well as dysfunctional responses to allergic inflammation; 3) the emerging idea that local BDNF production by resident airway cells such as epithelium and airway smooth muscle can contribute to normal airway structure and function, and to airway hyperreactivity and remodeling in diseases such as asthma. Furthermore, given its pleiotropic effects in the airway, BDNF may be a novel and appealing therapeutic target.

Evidence that BDNF regulates heart rate by a mechanism involving increased brainstem parasympathetic neuron excitability

Autonomic control of heart rate is mediated by cardioinhibitory parasympathetic cholinergic neurons located in the brainstem and stimulatory sympathetic noradrenergic neurons. During embryonic development the survival and cholinergic phenotype of brainstem autonomic neurons is promoted by brain-derived neurotrophic factor (BDNF). We now provide evidence that BDNF regulates heart rate by a mechanism involving increased brainstem cardioinhibitory parasympathetic activity. Mice with a BDNF haploinsufficiency exhibit elevated resting heart rate, and infusion of BDNF intracerebroventricularly reduces heart rate in both wild-type and BDNF+/- mice. The atropine-induced elevation of heart rate is diminished in BDNF+/- mice and is restored by BDNF infusion, whereas the atenolol-induced decrease in heart rate is unaffected by BDNF levels, suggesting that BDNF signaling enhances parasympathetic tone which is diminished with BDNF haploinsufficiency. Whole-cell recordings from pre-motor cholinergic cardioinhibitory vagal neurons in the nucleus ambiguus indicate that BDNF haploinsufficiency reduces cardioinhibitory vagal neuron activity by increased inhibitory GABAergic and diminished excitatory glutamatergic neurotransmission to these neurons. Our findings reveal a previously unknown role for BDNF in the control of heart rate by a mechanism involving increased activation of brainstem cholinergic parasympathetic neurons.

Neurokinin-neurotrophin interactions in airway smooth muscle

Neurally derived tachykinins such as substance P (SP) play a key role in modulating airway contractility (especially with inflammation). Separately, the neurotrophin brain-derived neurotrophic factor (BDNF; potentially derived from nerves as well as airway smooth muscle; ASM) and its tropomyosin-related kinase receptor, TrkB, are involved in enhanced airway contractility. In this study, we hypothesized that neurokinins and neurotrophins are linked in enhancing intracellular Ca(2+) concentration ([Ca(2+)](i)) regulation in ASM. In rat ASM cells, 24 h exposure to 10 nM SP significantly increased BDNF and TrkB expression (P < 0.05). Furthermore, [Ca(2+)](i) responses to 1 μM ACh as well as BDNF (30 min) effects on [Ca(2+)](i) regulation were enhanced by prior SP exposure, largely via increased Ca(2+) influx (P < 0.05). The enhancing effect of SP on BDNF signaling was blunted by the neurokinin-2 receptor antagonist MEN-10376 (1 μM, P < 0.05) to a greater extent than the neurokinin-1 receptor antagonist RP-67580 (5 nM). Chelation of extracellular BDNF (chimeric TrkB-F(c); 1 μg/ml), as well as tyrosine kinase inhibition (100 nM K252a), substantially blunted SP effects (P < 0.05). Overnight (24 h) exposure of ASM cells to 50% oxygen increased BDNF and TrkB expression and potentiated both SP- and BDNF-induced enhancement of [Ca(2+)](i) (P < 0.05). These results suggest a novel interaction between SP and BDNF in regulating agonist-induced [Ca(2+)](i) regulation in ASM. The autocrine mechanism we present here represents a new area in the development of bronchoconstrictive reflex response and airway hyperreactive disorders.

Neurotrophins in lung health and disease

Neurotrophins (NTs) are a family of growth factors that are well-known in the nervous system. There is increasing recognition that NTs (nerve growth factor, brain-derived neurotrophic factor and NT3) and their receptors (high-affinity TrkA, TrkB and TrkC, and low-affinity p75NTR) are expressed in lung components including the nasal and bronchial epithelium, smooth muscle, nerves and immune cells. NT signaling may be important in normal lung development, developmental lung disease, allergy and inflammation (e.g., rhinitis, asthma), lung fibrosis and even lung cancer. In this review, we describe the current status of our understanding of NT signaling in the lung, with hopes of using aspects of the NT signaling pathway in the diagnosis and therapy of lung diseases.

Role of central neurotransmission and chemoreception on airway control

This review summarizes work on central neurotransmission, chemoreception and CNS control of cholinergic outflow to the airways. First, we describe the neural transmission of bronchoconstrictive signals from airway afferents to the airway-related vagal preganglionic neurons (AVPNs) via the nucleus of the solitary tract (nTS) and, second, we characterize evidence for a modulatory effect of excitatory glutamatergic, and inhibitory GABAergic, noradrenergic and serotonergic pathways on AVPN output. Excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second order neurons within the nTS, via a glutamate-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor signaling pathway. These nTS neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command through descending fibers and airway intramural ganglia to airway smooth muscle, submucosal secretory glands, and the vasculature. The strength and duration of this reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs. In addition, central noradrenergic and serotonergic inhibitory pathways appear to participate in the regulation of cholinergic drive to the tracheobronchial system. Down-regulation of these inhibitory influences results in a shift from inhibitory to excitatory drive, which may lead to increased excitability of AVPNs, heightened airway responsiveness, greater cholinergic outflow to the airways and consequently bronchoconstriction. In summary, centrally coordinated control of airway tone and respiratory drive serve to optimize gas exchange and work of breathing under normal homeostatic conditions. Greater understanding of this process should enhance our understanding of its disruption under pathophysiologic states.

Adverse effects of salmeterol in asthma: a neuronal perspective

BACKGROUND

Regular use of inhaled beta(2)-agonists has been associated with a paradoxical loss of asthma control and a deterioration of airway hyper-responsiveness, but the underlying mechanism is unknown. The neurotrophin brain-derived neurotrophic factor (BDNF) has recently been identified as a mediator of airway hyper-responsiveness in asthma.

METHODS

Eighteen patients with mild allergic asthma who did not use any regular antiasthmatic therapy inhaled the long-acting beta(2)-agonist salmeterol for 2 weeks followed by 2 weeks of combination therapy with salmeterol and the corticosteroid fluticasone. Airway responsiveness to histamine and BDNF concentrations in blood were assessed prior to entry, after 14 days of salmeterol therapy and after 14 days of combination therapy. In a separate experiment, salmeterol effects on BDNF release by human peripheral blood mononuclear cells were assessed.

RESULTS

Monotherapy with salmeterol significantly increased BDNF concentrations in serum and platelets. This increase was abolished by the addition of fluticasone to the treatment. The findings were confirmed in vitro: salmeterol increased the release of BDNF by mononuclear cells, and this was inhibited by co-incubation with fluticasone. Increased BDNF concentrations in serum and platelets correlated with the deterioration of airway hyper-responsiveness following salmeterol monotherapy. In contrast, there was no association between beta(2)-receptor polymorphisms and changes in airway responsiveness.

CONCLUSION

Increased BDNF concentrations may underly the adverse effects of salmeterol monotherapy on airway responsiveness in asthma.

TRIAL REGISTRATION NUMBER

NCT00736801.

Role of brain-derived neurotrophic factor in hyperoxia-induced enhancement of contractility and impairment of relaxation in lung parenchyma

Prolonged hyperoxic exposure contributes to neonatal lung injury, and airway hyperreactivity is characterized by enhanced contraction and impaired relaxation of airway smooth muscle. Our previous data demonstrate that hyperoxia in rat pups upregulates expression of brain-derived neurotrophic factor (BDNF) mRNA and protein, disrupts NO-cGMP signaling, and impairs cAMP production in airway smooth muscle. We hypothesized that BDNF-tyrosine kinase B (TrkB) signaling plays a functional role in airway hyperreactivity via upregulation of cholinergic mechanisms in hyperoxia-exposed lungs. Five-day-old rat pups were exposed to >or=95% oxygen or room air for 7 days and administered daily tyrosine kinase inhibitor K-252a (50 microg x kg(-1) x day(-1) i.p.) to block BDNF-TrkB signaling or vehicle. Lungs were removed for HPLC measurement of ACh or for in vitro force measurement of lung parenchymal strips. ACh content doubled in hyperoxic compared with room air-exposed lungs. K-252a treatment of hyperoxic pups restored ACh content to room air levels. Hyperoxia increased contraction and impaired relaxation of lung strips in response to incremental electrical field stimulation. K-252a administration to hyperoxic pups reversed this increase in contraction and decrease in relaxation. K-252a or TrkB-Fc was used to block the effect of exogenous BDNF in vitro. Both K-252a and TrkB-Fc blocked the effects of exogenous BDNF. Hyperoxia decreased cAMP and cGMP levels in lung strips, and blockade of BDNF-TrkB signaling restored cAMP but not cGMP to control levels. Therefore, hyperoxia-induced increase in activity of BDNF-TrkB receptor signaling appears to play a critical role in enhancing cholinergically mediated contractile responses of lung parenchyma.

Neurotrophic factors in autonomic nervous system plasticity and dysfunction

During development, neurotrophic factors are known to play important roles in regulating the survival of neurons in the autonomic nervous system (ANS) and the formation of their synaptic connectivity with their peripheral targets in the cardiovascular, digestive, and other organ systems. Emerging findings suggest that neurotrophic factors may also affect the functionality of the ANS during adult life and may, in part, mediate the effects of environmental factors such as exercise and dietary energy intake on ANS neurons and target cells. In this article, we describe the evidence that ANS neurons express receptors for multiple neurotrophic factors, and data suggesting that activation of those receptors can modify plasticity in the ANS. Neurotrophic factors that may regulate ANS function include brain-derived neurotrophic factor, nerve growth factor, insulin-like growth factors, and ciliary neurotrophic factor. The possibility that perturbed neurotrophic factor signaling is involved in the pathogenesis of ANS dysfunction in some neurological disorders is considered, together with implications for neurotrophic factor-based therapeutic interventions.

5-HT(2) receptor subtypes mediate different long-term changes in GABAergic activity to parasympathetic cardiac vagal neurons in the nucleus ambiguus

Serotonin (5-HT), and in particular 5-HT(2) receptors, play an important role in cardiorespiratory function within the brainstem. In addition, abnormalities in the 5-HT system have been implicated in many cardiorespiratory disorders, including sudden infant death syndrome. However, little is known about the mechanisms of action of 5-HT(2) receptors in altering the activity of parasympathetic cardiac neurons in the brainstem. In this study we examined the effects of activation of different subtypes of 5-HT(2) receptors on spontaneous and respiratory-evoked GABAergic neurotransmission to cardioinhibitory vagal neurons within the nucleus ambiguus as well as rhythmic fictive inspiratory-related activity in rats. A single application of alpha-Me-5-hydroxytryptamine maleate (alpha-Me-5-HT), a 5-HT(2) receptor agonist, did not significantly alter the frequency of spontaneous or respiratory-evoked GABAergic inhibitory postsynaptic currents (IPSCs) in cardiac vagal neurons. However, repetitive successive applications of alpha-Me-5-HT elicited a long-lasting (>/=1 h) decrease in the frequency of spontaneous as well as inspiratory-related GABAergic IPSCs to cardiac vagal neurons. This study demonstrates multiple, but not single applications of the 5-HT(2) receptor agonist alpha-Me-5-HT caused a long-lasting inhibition of both spontaneous and fictive inspiratory-related GABAergic neurotransmission to CVNs, which can be prevented by the 5-HT(2B) receptor antagonist SB204741, but persisted with the 5-HT(2A/2C) receptor antagonist ketanserin. The 5-HT(2) receptor agonist alpha-Me-5-HT also reversibly and transiently excited central fictive inspiratory activity, which was abolished by ketanserin, but was unaffected by the 5-HT(2B) receptor antagonist SB204741.

Platelet and plasma BDNF in lower respiratory tract infections of the adult

Enhanced bronchial responsiveness during and following lower respiratory tract infections is a major clinical problem, but its pathogenesis is poorly understood. Brain-derived neurotrophic factor (BDNF), which can be released by platelets and leukocytes, has been identified as a mediator of bronchial hyperresponsiveness. It is unknown whether the release of BDNF is altered during lower respiratory tract infections of the adult. In this clinical pilot study, 16 patients (35-80 years old) with the diagnosis of an acute bacterial lower respiratory tract infection and elevated serum concentrations of c-reactive protein (>100 microg/ml) and procalcitonin (>0.1 ng/ml) were examined on admission to the hospital and 1 week after antibiotic treatment. Sixteen age- and sex-matched controls were examined in the same time period. BDNF concentrations in serum and platelets, but not in plasma, were markedly reduced in patients on the day of admission (median <25% of the controls). Analysis of the platelet marker serotonin (5-HT) suggested that the decrease of platelet BDNF is part of a non-specific release of platelet-derived mediators in this condition. Clinical improvement was accompanied by a restoration of serum and platelet BDNF concentrations which returned to control levels after 1 week of treatment. Cell culture experiments revealed that bacterial lipopolysaccharide (LPS) enhanced the release of BDNF by peripheral blood mononuclear cells of the patients at both time points. In conclusion, these data suggest that lower respiratory tract infections might be associated with an augmented release of BDNF by platelets and mononuclear cells.

Motor and sensory re-innervation of the lung and heart after re-anastomosis of the cervical vagus nerve in rats

There is no study in the literature dealing with re-innervation of the cardiopulmonary vagus nerve after its transection followed by re-anastomosis. In the present study, we explored the bronchomotor, heart rate and respiratory responses in rats at 2, 3 and 6 months after re-anastomosis of one cervical vagus trunk. The conduction velocity of A, B and C waves was calculated in the compound vagal action potential. We searched for afferent vagal activities in phase with pulmonary inflation to assess the persistence of pulmonary stretch receptor (PSR) discharge in re-innervated lungs. In each animal, data from the stimulation or recording of one re-anastomosed vagus nerve were compared with those obtained in the contra-lateral intact one. Two and three months after surgery, the conduction velocities of A and B waves decreased, but recovery of conduction velocity was complete at 6 months. By contrast, the conduction velocity of the C wave did not change until 6 months, when it was doubled. The PSR activity was present in 50% of re-anastomosed vagus nerves at 2 and 3 months and in 75% at 6 months. Respiratory inhibition evoked by vagal stimulation was significantly weaker from the re-anastomosed than intact nerve at 2 but not 3 months. Vagal stimulation did not elicit cardiac slowing or bronchoconstriction 6 months after re-anastomosis. Our study demonstrates the capacity of pulmonary vagal sensory neurones to regenerate after axotomy followed by re-anastomosis, and the failure of the vagal efferents to re-innervate both the lungs and heart.

Epilepsy, electroacupuncture and the nucleus of the solitary tract

Vagal nerve stimulation and electroacupuncture have some promise as neuroprotective therapies for patients with poorly controlled epilepsy. It has been demonstrated that stimulation of acupuncture points on the extremities results in stimulation of the vagus nerve. It is possible that the antiepileptic effects of these two applications might be targeting the same centre in the brain. The nucleus of the solitary tract, which is a primary site at which vagal afferents terminate, is also the site for afferent pathways of facial, scalp and auricular acupuncture via trigeminal, cervical spinal and glossopharyngeal nerves. Taken together with laboratory findings, the neuroprotective pathways of electroacupuncture in epileptic models may stem from the collaboration of its anti-inflammatory and neurotrophic actions through the nucleus of the solitary tract via vagus nerve stimulation.

Regulation by nicotine of Gpr51 and Ntrk2 expression in various rat brain regions

Our previous genetic studies demonstrated that variants of the gamma-Aminobutyric acid B receptor subunit 2 (GPR51) and neurotrophic tyrosine kinase receptor type 2 (NTRK2) genes are significantly associated with nicotine dependence (ND) in smokers. However, whether such genetic associations lead to changes in the expression of the two genes in response to nicotine remains undetermined. In this study, we investigated the regulatory effect of nicotine on the expression of Gpr51 and Ntrk2 in seven rat brain regions during the administration of nicotine in a daily dose of 3.15 mg/kg for 7 days. With quantitative real-time RT-PCR, we found that nicotine increased the mRNA of Gpr51 by 70, 78, and 32% in the amygdala, striatum, and prefrontal cortex (PFC), respectively, but decreased by 54% in the nucleus accumbens (NA). The Gpr51 protein was upregulated by nicotine in the amygdala (26%), striatum (73%), PFC (28%), and medial basal hypothalamus (MBH; 19%) but downregulated in the NA (-72%). Similarly, the mRNA level of Ntrk2 was enhanced by nicotine in the striatum (86%) and PFC (38%), but decreased in the NA (-46%) and ventral tegmental area (VTA; -49%). A significant change in protein expression was also obtained for Ntrk2 in the PFC (24%), MBH (33%), NA (-33%), and VTA (-70%). Interestingly, these two genes showed a closely coordinated expression pattern in response to nicotine in most of the brain regions examined. In summary, our results demonstrate that the expression of Gpr51 and Ntrk2 is significantly regulated by nicotine at both the mRNA and protein levels in various brain regions, which provides further evidence that these two genes are involved in the etiology of ND, as reported in our previous genetic association studies in humans.

Neonatal lung and airway injury: a role for neurotrophins

Maintenance of patency in distal airways is essential for gas exchange in neonatal life, and its disruption may have long-lasting effects on respiratory function. However, neural mechanisms that regulate caliber of intrapulmonary airways during early postnatal life, and their disruption by hyperoxic exposure, have not been well characterized. We have previously shown that cholinergically mediated airway contractile responses in rat pups are upregulated after hyperoxic exposure, and that increased expression of neuropeptides, such as substance P, may be contributory. More recently, we have documented impairment of neurally mediated airway relaxation in response to hyperoxic stress associated with loss of nitric oxide and prostaglandin-induced airway relaxation as well as inhibition of long chain myosin phosphatase. Our most recent data demonstrate significantly enhanced expression of the neurotrophin, brain-derived neurotrophic factor (BDNF) and its high affinity specific tyrosine kinase B (TrkB) receptor in hyperoxia-exposed airway smooth muscle. The existence of a BDNF-TrkB receptor autocrine and paracrine loops in the airways provides a basis for understanding local regulatory mechanisms of airway homeostasis. A mechanistic role for BDNF-TrkB signaling in hyperoxia-induced airway hyperreactivity in early postnatal life could serve to modulate both afferent and efferent neural pathways that result in enhanced contractile responses of immature airways exposed to hyperoxic stress. Greater insight into these neural pathways may lead to future preventive strategies for preterm infants surviving neonatal intensive care and developing chronic lung disease.

Airway-related vagal preganglionic neurons express multiple nicotinic acetylcholine receptor subunits

Nicotine acting centrally increases bronchomotor tone and airway secretion, suggesting that airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) express nicotinic acetylcholine receptors (nAChRs). In the present study, we examined the three main functionally characterized subtypes of nAChRs in the CNS, the alpha7 homomeric and alpha4beta2 heteromeric receptors. First, we characterized the expression of these subunits at the message (mRNA) and protein levels in brain tissues taken from the rNA region, the site where AVPNs are located. In addition, double labeling fluorescent immunohistochemistry and confocal laser microscopy were used to define the presence of alpha7, alpha4, and beta2 nAChRs on AVPNs that were retrogradely labeled with cholera toxin beta subunit (CTb), injected into the upper lung lobe (n=4) or extrathoracic trachea (n=4). Our results revealed expression of all three studied subunits at mRNA and protein levels within the rNA region. Furthermore, virtually all identified AVPNs innervating intrapulmonary airways express alpha7 and alpha4 nAChR subunits. Similarly, a majority of labeled AVPNs projecting to extrathoracic trachea contain alpha7 and beta2 subunits, but less than half of them show detectable alpha4 nAChR traits. These results suggest that AVPNs express three major nAChR subunits (alpha7, alpha4, and beta2) that could assemble into functional homologous or heterologous pentameric receptors, mediating fast and sustained nicotinic effects on cholinergic outflow to the airways.

Adenosine A2A receptors are expressed by GABAergic neurons of medulla oblongata in developing rat

During early development, adenosine contributes to the occurrence of respiratory depression and recurrent apneas. Recent physiological studies indicate that GABAergic mechanisms may be involved in this inhibitory action of adenosine, via their A(2A) receptors. In the present study, in situ hybridization with ribonucleotide probes for A(2A) receptor (A(2A)R) mRNA was combined with the immunolabeling technique for parvalbumin and transneuronal retrograde tracing method using green fluorescent protein expressing pseudorabies virus (GFP-PRV) to (1) characterize age-dependent changes in the expression of adenosine A(2A)Rs mRNA in brain stem regions where GABAergic neurons are located; (2) determine whether GABA-containing neurons express A(2A)R mRNA traits, and (3) identify whether bulbospinal GABAergic neurons projecting to phrenic nuclei contain A(2A)R mRNA. Results revealed expression of A(2A) receptors in regions of medulla oblongata containing GABAergic neurons, namely in the ventral aspect of the medulla, within the Bötzinger region and caudal to it, the gigantocellular reticular nucleus, midline neurons and the caudal ventrolateral medulla oblongata. Furthermore, a subpopulation of identified GABAergic neurons, projecting to the phrenic motor nuclei, possess A(2A)R mRNA. It is concluded that adenosine A(2A)Rs expressed by GABAergic neurons are likely to play a role in mediating adenosine-induced respiratory depression.

CNS determinants of sleep-related worsening of airway functions: implications for nocturnal asthma

This review summarizes the recent neuroanatomical and physiological studies that form the neural basis for the state-dependent changes in airway resistance. Here, we review only the interactions between the brain regions generating quiet (non-rapid eye movement, NREM) and active (rapid eye movement, REM) sleep stages and CNS pathways controlling cholinergic outflow to the airways. During NREM and REM sleep, bronchoconstrictive responses are heightened and conductivity of the airways is lower as compared to the waking state. The decrease in conductivity of the lower airways parallels the sleep-induced decline in the discharge of brainstem monoaminergic cell groups and GABAergic neurons of the ventrolateral periaqueductal midbrain region, all of which provide inhibitory inputs to airway-related vagal preganglionic neurons (AVPNs). Withdrawal of central inhibitory influences to AVPNs results in a shift from inhibitory to excitatory transmission that leads to an increase in airway responsiveness, cholinergic outflow to the lower airways and consequently, bronchoconstriction. In healthy subjects, these changes are clinically unnoticed. However, in patients with bronchial asthma, sleep-related alterations in lung functions are troublesome, causing intensified bronchopulmonary symptoms (nocturnal asthma), frequent arousals, decreased quality of life, and increased mortality. Unquestionably, the studies revealing neural mechanisms that underlie sleep-related alterations of airway function will provide new directions in the treatment and prevention of sleep-induced worsening of airway diseases.

Ablation of vagal preganglionic neurons innervating the extra-thoracic trachea affects ventilatory responses to hypercapnia and hypoxia

This study tested the hypothesis that during hypercapnia or hypoxia, airway-related vagal preganglionic neurons (AVPNs) of the nucleus ambiguus (NA) release acetylcholine (ACh), which in a paracrine fashion, activates ACh receptors expressed by inspiratory rhythm generating cells. AVPNs in the NA were ablated by injecting a saporin- (SA) cholera toxin b subunit (CTb-SA) conjugate into the extra-thoracic trachea (n=6). Control animals were injected with free CTb (n=6). In CTb treated rats, baseline ventilation and ventilatory responses to hypercapnia (5 and 12% CO(2) in O(2)) or hypoxia (8% O(2) in N(2)) were similar (p>0.05) prior to and 5 days after injection. CTb-SA injected rats maintained rhythmic breathing patterns 5 days post injection, however, tachypneic responses to hypercapnia or hypoxia were significantly reduced. The number of choline acetyltransferase (ChAT) immunoreactive cells in the NA was much lower (p<0.05) in CTb-SA rats as compared to animals receiving CTb only. These results suggest that AVPNs participate in the respiratory frequency response to hypercapnia or hypoxia.