Bilateral distribution of vagal motor and sensory nerve fibers in the rat's lungs and airways

Electrophysiological Properties and Morphology of Cardiac and Pulmonary Motoneurons within the Dorsal Motor Nucleus of the Vagus of Rats

The dorsal motor nucleus of the vagus (DMV) contains parasympathetic motoneurons that project to the heart and lungs. These motoneurons control ventricular excitability/contractility and airways secretions/blood flow, respectively. However, their electrophysiological properties, morphology and synaptic input activity remain unknown. One important ionic current described in DMV motoneurons controlling their electrophysiological behaviour is the A-type mediated by voltage-dependent K+ (Kv) channels. Thus, we compared the electrophysiological properties, synaptic activity, morphology, A-type current density, and single cell expression of Kv subunits, that contribute to macroscopic A-type currents, between DMV motoneurons projecting to either the heart or lungs of adult male rats. Using retrograde labelling, we visualized distinct DMV motoneurons projecting to the heart or lungs in acutely prepared medullary slices. Subsequently, whole cell recordings, morphological reconstruction and single motoneuron qRT-PCR studies were performed. DMV pulmonary motoneurons were more depolarized, electrically excitable, presented higher membrane resistance, broader action potentials and received greater excitatory synaptic inputs compared to cardiac DMV motoneurons. These differences were in part due to highly branched dendritic complexity and lower magnitude of A-type K+ currents. By evaluating expression of channels that mediate A-type currents from single motoneurons, we demonstrated a lower level of Kv4.2 in pulmonary versus cardiac motoneurons, whereas Kv4.3 and Kv1.4 levels were similar. Thus, with the distinct electrical, morphological, and molecular properties of DMV cardiac and pulmonary motoneurons, we surmise that these cells offer a new vista of opportunities for genetic manipulation providing improvement of parasympathetic function in cardiorespiratory diseases such heart failure and asthma.

Respiratory Abnormalities in Parkinson's Disease: What Do We Know from Studies in Humans and Animal Models?

Parkinson’s disease (PD) is the second most common progressive neurodegenerative disease characterized by movement disorders due to the progressive loss of dopaminergic neurons in the ventrolateral region of the substantia nigra pars compacta (SNpc). Apart from the cardinal motor symptoms such as rigidity and bradykinesia, non-motor symptoms including those associated with respiratory dysfunction are of increasing interest. Not only can they impair the patients’ quality of life but they also can cause aspiration pneumonia, which is the leading cause of death among PD patients. This narrative review attempts to summarize the existing literature on respiratory impairments reported in human studies, as well as what is newly known from studies in animal models of the disease. Discussed are not only respiratory muscle dysfunction, apnea, and dyspnea, but also altered central respiratory control, responses to hypercapnia and hypoxia, and how they are affected by the pharmacological treatment of PD.

Neural control of the lower airways: Role in cough and airway inflammatory disease

Airway function is under constant neurophysiological control, in order to maximize airflow and gas exchange and to protect the airways from aspiration, damage, and infection. There are multiple sensory nerve subtypes, whose disparate functions provide a wide array of sensory information into the CNS. Activation of these subtypes triggers specific reflexes, including cough and alterations in autonomic efferent control of airway smooth muscle, secretory cells, and vasculature. Importantly, every aspect of these reflex arcs can be impacted and altered by local inflammation caused by chronic lung disease such as asthma, bronchitis, and infections. Excessive and inappropriate activity in sensory and autonomic nerves within the airways is thought to contribute to the morbidity and symptoms associated with lung disease.

Heightened respiratory-parasympathetic coupling to airways in the spontaneously hypertensive rat

KEY POINTS

Carotid body (CB) chemoreceptors are hyperactive in hypertension, and their acute activation produces bronchoconstriction. We show that the respiratory-modulated bronchiolar tone, pulmonary parasympathetic efferent activity, and the firing frequency and synaptic excitation of bronchoconstrictor motoneurones in the nucleus ambiguus were all enhanced in spontaneous hypertensive (SH) rats. In SH rats, CB denervation reduced the respiratory-related parasympathetic-mediated bronchoconstrictor tone to levels seen in normotensive rats. Chemoreflex evoked bronchoconstrictor tone was heightened in SH versus normotensive rats. The intrinsic electrophysiological properties and morphology of bronchoconstrictor motoneurones were similar across rat strains. The heightened respiratory modulation of parasympathetic-mediated bronchoconstrictor tone to the airways in SH rats is caused by afferent drive from the CBs.

ABSTRACT

Much research has described heightened sympathetic activity in hypertension and diminished parasympathetic tone, especially to the heart. The carotid body (CB) chemoreceptors exhibit hyperreflexia and are hyperactive, providing excitatory drive to sympathetic networks in hypertension. Given that acute CB activation produces reflex evoked bronchoconstriction via activation of parasympathetic vagal efferents, we hypothesised that the parasympathetic bronchoconstrictor activity is enhanced in spontaneously hypertensive (SH) rats and that this is dependent on CB inputs. In situ preparations of Wistar and SH rats were used in which bronchiolar tone, the pulmonary branch of the vagus (pVN) and phrenic nerves were recorded simultaneously; whole cell patch clamp recordings of bronchoconstrictor vagal motoneurones were also made from the nucleus ambiguus. Bronchiolar tone, pVN and bronchoconstrictor motoneurones were respiratory modulated and this modulation was enhanced in SH rats. These differences were all eliminated after CB denervation. Stimulation of the CBs increased the phrenic frequency that caused a summation of the respiratory-related increases in pVN, resulting in the development of bronchoconstrictor tone. This tone was exaggerated in SH rats. The enhanced respiratory-parasympathetic coupling to airways in SH rats was not due to differences in the intrinsic electrophysiological properties of bronchoconstrictor motoneurones but reflected heightened pre-inspiratory- and inspiratory-related synaptic drive. In summary, in SH rats the phasic respiratory modulation of parasympathetic tone to the airways is elevated and the greater development of this bronchoconstrictor tone is caused by the heightened afferent drive originating from the CBs. Thus, targeting the CBs may prove effective for increasing lower airway patency.

Peripheral-to-central immune communication at the area postrema glial-barrier following bleomycin-induced sterile lung injury in adult rats

The pathways for peripheral-to-central immune communication (P → C I-comm) following sterile lung injury (SLI) are unknown. SLI evokes systemic and central inflammation, which alters central respiratory control and viscerosensory transmission in the nucleus tractus solitarii (nTS). These functional changes coincide with increased interleukin-1 beta (IL-1β) in the area postrema, a sensory circumventricular organ that connects P → C I-comm to brainstem circuits that control homeostasis. We hypothesize that IL-1β and its downstream transcriptional target, cyclooxygenase-2 (COX-2), mediate P → C I-comm in the nTS. In a rodent model of SLI induced by intratracheal bleomycin (Bleo), the sigh frequency and duration of post-sigh apnea increased in Bleo- compared to saline- treated rats one week after injury. This SLI-dependent change in respiratory control occurred concurrently with augmented IL-1β and COX-2 immunoreactivity (IR) in the funiculus separans (FS), a barrier between the AP and the brainstem. At this barrier, increases in IL-1β and COX-2 IR were confined to processes that stained for glial fibrillary acidic protein (GFAP) and that projected basolaterally to the nTS. Further, FS radial-glia did not express TNF-α or IL-6 following SLI. To test our hypothesis, we blocked central COX-1/2 activity by intracerebroventricular (ICV) infusion of Indomethacin (Ind). Continuous ICV Ind treatment prevented Bleo-dependent increases in GFAP + and IL-1β + IR, and restored characteristics of sighs that reset the rhythm. These data indicate that changes in sighs following SLI depend partially on activation of a central COX-dependent P → C I-comm via radial-glia of the FS.

Targeting the small airways with dry powder adenosine: a challenging concept

Background: Small-particle inhaled corticosteroids (ICS) provide a higher small airway deposition than large-particle ICS. However, we are still not able to identify asthma patients who will profit most from small-particle treatment.

Objective: We aimed to identify these patients by selectively challenging the small and large airways. We hypothesized that the airways could be challenged selectively using small- and large-particle adenosine, both inhaled at a high and a low flow rate.

Design: In this cross-over study 11 asthma subjects performed four dry powder adenosine tests, with either small (MMAD 2.7 µm) or large (MMAD 6.0 µm) particles, inhaled once with a low flow rate (30 l min^–1) and once with a high flow rate (60 l min^–1). Spirometry and impulse oscillometry were performed after every bronchoprovocation step. We assumed that FEV₁ reflects the large airways, and FEF_25–75%, R5-R20 and X5 reflect the small airways.

Results: The four adenosine tests were not significantly different with respect to the threshold values of FEV₁ (p = 0.12), FEF_25–75% (p = 0.37), R5-R20 (p = 0.60) or X5 (p = 0.46). Both small- and large-particle adenosine induced a response in the small airways in the majority of the tests.

Conclusions: In contrast to our hypothesis, all four adenosine tests provoked a response in the small airways and we could not identify different large- or small-airway responders. Interestingly, even the test with large particles and a high flow rate induced a small-airway response, suggesting that selective challenging of the small airways is not necessary. Future studies should investigate the relation between particle deposition and the site of an airway response.

Deficiency of CPEB2-Confined Choline Acetyltransferase Expression in the Dorsal Motor Nucleus of Vagus Causes Hyperactivated Parasympathetic Signaling-Associated Bronchoconstriction

Cytoplasmic polyadenylation element binding protein 2 (CPEB2) is an RNA-binding protein and translational regulator. To understand the physiological function of CPEB2, we generated CPEB2 knock-out (KO) mice and found that most died within 3 d after birth. CPEB2 is highly expressed in the brainstem, which controls vital functions, such as breathing. Whole-body plethysmography revealed that KO neonates had aberrant respiration with frequent apnea. Nevertheless, the morphology and function of the respiratory rhythm generator and diaphragm neuromuscular junctions appeared normal. We found that upregulated translation of choline acetyltransferase in the CPEB2 KO dorsal motor nucleus of vagus resulted in hyperactivation of parasympathetic signaling-induced bronchoconstriction, as evidenced by increased pulmonary acetylcholine and phosphorylated myosin light chain 2 in bronchial smooth muscles. Specific deletion of CPEB2 in cholinergic neurons sufficiently caused increased apnea in neonatal pups and airway hyper-reactivity in adult mice. Moreover, inhalation of an anticholinergic bronchodilator reduced apnea episodes in global and cholinergic CPEB2-KO mice. Together, the elevated airway constriction induced by cholinergic transmission in KO neonates may account for the respiratory defect and mortality.

SIGNIFICANCE STATEMENT

This study first generated and characterized cpeb2 gene-deficient mice. CPEB2-knock-out (KO) mice are born alive but most die within 3 d after birth showing no overt defects in anatomy. We found that the KO neonates showed severe apnea and altered respiratory pattern. Such respiratory defects could be recapitulated in mice with pan-neuron-specific or cholinergic neuron-specific ablation of the cpeb2 gene. Further investigation revealed that cholinergic transmission in the KO dorsal motor nucleus of vagus was overactivated because KO mice lack CPEB2-suppressed translation of the rate-limiting enzyme in the production of acetylcholine (i.e., choline acetyltransferase). Consequently, increased parasympathetic signaling leads to hyperactivated bronchoconstriction and abnormal respiration in the KO neonates.

Increased airway reactivity and hyperinsulinemia in obese mice are linked by ERK signaling in brain stem cholinergic neurons

Obesity is a major risk factor for asthma, which is characterized by airway hyperreactivity (AHR). In obesity-associated asthma, AHR may be regulated by non-TH2 mechanisms. We hypothesized that airway reactivity is regulated by insulin in the CNS, and that the high levels of insulin associated with obesity contribute to AHR. We found that intracerebroventricular (ICV)-injected insulin increases airway reactivity in wild-type, but not in vesicle acetylcholine transporter knockdown (VAChT KD(HOM-/-)), mice. Either neutralization of central insulin or inhibition of extracellular signal-regulated kinases (ERK) normalized airway reactivity in hyperinsulinemic obese mice. These effects were mediated by insulin in cholinergic nerves located at the dorsal motor nucleus of the vagus (DMV) and nucleus ambiguus (NA), which convey parasympathetic outflow to the lungs. We propose that increased insulin-induced activation of ERK in parasympathetic pre-ganglionic nerves contributes to AHR in obese mice, suggesting a drug-treatable link between obesity and asthma.

WGA-Alexa transsynaptic labeling in the phrenic motor system of adult rats: Intrapleural injection versus intradiaphragmatic injection

BACKGROUND

Intrapleural injection of CTB-Alexa 488, a retrograde tracer, provides an alternative labeling technique to the surgically invasive laparotomy required for intradiaphragmatic injection. However, CTB-Alexa 488 is incapable of crossing synapses restricting the tracer to the phrenic nuclei and the intercostal motor nuclei in the spinal cord.

NEW METHOD

Intrapleural injection of WGA-Alexa 488, a transsynaptic tracer, provides a method to label the respiratory motor pathway in both the spinal cord and medulla. Intradiaphragmatic injection of WGA-Alexa 594 and vagal nerve injections of True blue were used to confirm the phrenic nuclei and to differentiate between the rVRG and the NA in the medulla.

RESULTS

Following intrapleural injection, WGA-Alexa 488 was retrogradely transported to the phrenic nuclei and to the intercostal motor nuclei. Subsequently WGA-Alexa 488 was transsynaptically transported from the phrenic motoneurons to the pre-motor neurons in the rVRG that provide the descending drive to the phrenic neurons during inspiration. In addition WGA-Alexa 488 was identified in select cells of the NA confirmed by a dual label of both WGA-Alexa 488 and True blue.

COMPARISON WITH EXISTING METHOD

WGA-Alexa 488 demonstrates retrograde transsynaptic labeling following intrapleural injection whereas the previous method of injecting CTB-Alexa 488 only demonstrates retrograde labeling.

CONCLUSIONS

Intrapleural injection of WGA-Alexa fluor conjugates is an effective method to transsynaptically label the phrenic motor system providing an alternative for the invasive laparotomy required for intradiaphragmatic injections. Furthermore, the study provides the first anatomical evidence of a direct synaptic relationship between rVRG and select NA cells.

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.

Arrhythmia in heart and brain: KCNQ1 mutations link epilepsy and sudden unexplained death

Sudden unexplained death is a catastrophic complication of human idiopathic epilepsy, causing up to 18% of patient deaths. A molecular mechanism and an identified therapy have remained elusive. Here, we find that epilepsy occurs in mouse lines bearing dominant human LQT1 mutations for the most common form of cardiac long QT syndrome, which causes syncopy and sudden death. KCNQ1 encodes the cardiac KvLQT1 delayed rectifier channel, which has not been previously found in the brain. We have shown that, in these mice, this channel is found in forebrain neuronal networks and brainstem nuclei, regions in which a defect in the ability of neurons to repolarize after an action potential, as would be caused by this mutation, can produce seizures and dysregulate autonomic control of the heart. That long QT syndrome mutations in KCNQ1 cause epilepsy reveals the dual arrhythmogenic potential of an ion channelopathy coexpressed in heart and brain and motivates a search for genetic diagnostic strategies to improve risk prediction and prevention of early mortality in persons with seizure disorders of unknown origin.

Sonic hedgehog signalling in neurons of adult ventrolateral nucleus tractus solitarius

The transmembrane receptor Patched (Ptc) mediates the action of the diffusing factor Sonic hedgehog (Shh), which is implicated in establishing morphogenetic gradients during embryonic development. Whereas alteration of Ptc function is associated with developmental abnormalities and brain tumors, its functional activity and roles in the adult brain have yet to be elucidated. Here we describe the complementary pattern of Shh and Ptc expression in the rat dorsal vagal motor nucleus and the ventrolateral nucleus tractus solitarius (vNTS), respectively. Those two interconnected structures regulate the cardiorespiratory function during hypoxia. Bath application of a subnanomolar concentration of aminoterminal Shh protein (ShhN) to a slice preparation of the vNTS induces a rapid decrease in neuronal firing followed by a bursting activity that propagates in the neuronal network. Intracellular current injections show that bursts result from an action on the neuronal membrane electro-responsiveness. Both inhibiting and bursting effects are blocked by the monoclonal Shh antibody 5E1 and may require the Ptc binding site of ShhN. Thus, ShhN acting on specific neuronal sites controls electrophysiological properties of differentiated neurons of the vNTS. We speculate on a retrocontrol of cardiorespiratory signals in the vNTS, by Shh generated in dorsal vagal motoneurons.

New developments in tracing neural circuits with herpesviruses

Certain neurotropic viruses can invade the nervous system of their hosts and spread in chains of synaptically connected neurons. Consequently, it is possible to identify entire hierarchically connected circuits within an animal. In this review, we discuss the use of neurotropic herpesviruses as neuronal tract tracers. Although a variety of tract tracing viruses are available, each with its own unique infection characteristics, we focus on the widespread use of attenuated strains of pseudorabies virus (PRV), a swine herpesvirus with a broad host range. In particular, we focus on new applications of PRV for tract tracing including use of multiple infections by PRV reporter viruses to test for circuit convergence/divergence within the same animal. We provide examples of these combined application techniques within the context of an animal model to study the naturally occurring reversal of seasonal obesity in Siberian hamsters.

Innervation of the rat trachea by bilateral cholinergic projections from the nucleus ambiguus and direct motor fibers from the cervical spinal cord: a retrograde and anterograde tracer study

A tract-tracer method was employed to examine the innervation of the rat trachea. Cholera toxin beta subunit (CTB) was injected into the following locations in separate groups of rats: (1) ventral trachea, (2) lateral trachea, (3) ventral trachea after the excision of the nodose ganglion, and (4) ventral trachea after the transection of C1-C2 spinal nerves. CTB injection in the ventral trachea showed bilateral labeling of neurons in the nucleus ambiguus (NA), medial subnucleus of the nucleus of the solitary nucleus, dorsal motor nucleus of the vagus (DMV), and lamina IX of C1-C6. CTB injection in the lateral trachea showed significant ipsilateral predominance of neuronal labeling in the NA and lamina IX of C1-C2 segments. CTB injection in rats after the excision of the nodose ganglion revealed no labeling in the ipsilateral DMV and NA and a significant reduction of neuronal labeling in C1. CTB injection in rats after the transection of C1-C2 spinal nerves showed a significant decrease in the number of labeled neurons in ipsilateral NA, C1, and C2 and no labeling of fibers in C1-C2. The combination of retrograde fluorogold labeling and choline acetyltransferase (ChAT) immunostaining revealed that all fluorogold-labeled neurons in the NA and lamina IX of C1-C2 colocalized with ChAT. The injection of biotinylated dextran amine in NA produced labeling in axonal terminals on postganglionic neurons, but not in other regions of the trachea. Our findings indicate that the rat trachea is innervated bilaterally by cholinergic motor neurons in NA and C1-C2, while those traveling through the spinal nerves project directly to the trachea.

Right-sided Vagus Nerve Stimulation as a Treatment for Refractory Epilepsy in Humans

PURPOSE

We present three children who underwent right-sided vagus nerve stimulation (R-VNS). This treatment option for people with refractory epilepsy has not been described in children.

METHODS

We reviewed our database of >350 patients implanted with vagus nerve stimulators and now describe our experience in three patients with R-VNS for the treatment of intractable seizures. All three patients improved dramatically with left-sided vagus nerve stimulation (L-VNS), but the devices had to be removed because of infection. The patients were thought to be at high risk for nerve injury if they were reapproached for L-VNSs; therefore R-VNSs were implanted.

RESULTS

All three patients with an R-VNS had a reduction in seizures. Our first patient has had an R-VNS for 5 years; he has been seizure free for >2 years on R-VNS monotherapy. The second patient had an R-VNS for 8 months. His seizure control improved slightly, but not as dramatically as with L-VNS. The third child has had an R-VNS for >7 months and has cessation of his most disabling seizure type (generalized tonic-clonic seizures). None of the patients had cardiac side effects from therapeutic R-VNS. However, two of the three patients had respiratory events with R-VNS.

CONCLUSIONS

VNS is known to be an effective treatment in pharmacoresistant epilepsy. R-VNS should be considered if a patient has significant benefit from L-VNS but is unable to continue with L-VNS. R-VNS appears also to have antiepilepsy effects. Additionally, our case report suggests that in some patients, a differential response is found regarding seizure control with R-VNS or L-VNS, raising the question whether L-VNS failures should pursue a trial of R-VNS. Patients should be cautioned and monitored for reactive airway disease if they undergo R-VNS. More research is needed to compare the effects of right- and left-sided VNS on cardiac and pulmonary function in humans and to determine which has the best antiseizure effect.

Neurogenic mechanisms in bronchial inflammatory diseases

Neurogenic inflammation encompasses the release of neuropeptides from airway nerves leading to inflammatory effects. This neurogenic inflammatory response of the airways can be initiated by exogenous irritants such as cigarette smoke or gases and is characterized by a bi-directional linkage between airway nerves and airway inflammation. The event of neurogenic inflammation may participate in the development and progression of chronic inflammatory airway diseases such as allergic asthma or chronic obstructive pulmonary disease (COPD). The molecular mechanisms underlying neurogenic inflammation are orchestrated by a large number of neuropeptides including tachykinins such as substance P and neurokinin A, or calcitonin gene-related peptide. Also, other biologically active peptides such as neuropeptide tyrosine, vasoactive intestinal polypeptide or endogenous opioids may modulate the inflammatory response and recently, novel tachykinins such as virokinin and hemokinins were identified. Whereas the different aspects of neurogenic inflammation have been studied in detail in laboratory animal models, only little is known about the role of airway neurogenic inflammation in human diseases. However, different functional properties of airway nerves may be used as targets for future therapeutic strategies and recent clinical data indicates that novel dual receptor antagonists may be relevant new drugs for bronchial asthma or COPD.

The vagus nerve is one route of transneural invasion for intranasally inoculated influenza a virus in mice

Intranasally inoculated neurotropic influenza viruses in mice infect not only the respiratory tract but also the central nervous system (CNS), mainly the brain stem. Previous studies suggested that the route of invasion of virus into the CNS was via the peripheral nervous system, especially the vagus nerve. To evaluate the transvagal transmission of the virus, we intranasally inoculated unilaterally vagectomized mice with a virulent influenza virus (strain 24a5b) and examined the distribution of the viral protein and genome by immunohistochemistry and in situ hybridization over time. An asymmetric distribution of viral antigens was observed between vagal (nodose) ganglia: viral antigen was detected in the vagal ganglion of the vagectomized side 2 days later than in the vagal ganglion of the intact side. The virus was apparently transported from the respiratory mucosa to the CNS directly and decussately via the vagus nerve and centrifugally to the vagal ganglion of the vagectomized side. The results of this study, thus, demonstrate that neurotropic influenza virus travels to the CNS mainly via the vagus nerve.

Labeling of vagal motoneurons and central afferents after injection of cholera toxin B into the airway lumen

We tested the hypothesis that application of the subunit B of cholera toxin (CTB) to the airway mucosa would produce labeling of neuronal somata and sensory fibers in the medulla oblongata. Using (125)I-CTB as a tracer, we demonstrated first that CTB is transported across the tracheal epithelium, but once in the airway wall, it remains confined to the subepithelial space and lamina propria. Despite the rarity of intrinsic neurons in these areas, intraluminal CTB labeled approximately 10-60 neurons/rat in the nucleus ambiguus and a smaller number of neurons in the dorsal motor nucleus of the vagus. Well-defined sensory fiber terminals were also labeled in the commissural, medial, and ventrolateral subnuclei of the nucleus of the tractus solitarius. Approximately 50 and 90% of the neurons labeled by intraluminal CTB were also labeled by injections of FluoroGold into the tracheal adventitia and lung parenchyma, respectively. These findings demonstrate that a substantial number of medullary vagal motoneurons innervate targets in the vicinity of the airway epithelium. These neurons do not appear to be segregated anatomically from vagal motoneurons that project to deeper layers of the airway wall or lung parenchyma.