Evidence that antidromically stimulated vagal afferents activate inhibitory neurones innervating guinea-pig trachealis

Molecular identity, anatomy, gene expression and function of neural crest vs. placode-derived nociceptors in the lower airways

The lower airways (larynx to alveoli) are protected by a complex array of neural networks that regulate respiration and airway function. Harmful stimuli trigger defensive responses such as apnea, cough and bronchospasm by activating a subpopulation of sensory afferent nerves (termed nociceptors) which are found throughout the airways. Airway nociceptive fibers are projected from the nodose vagal ganglia, the jugular vagal ganglia and the dorsal root ganglia, which are derived from distinct embryological sources: the former from the epibranchial placodes, the latter two from the neural crest. Embryological source determines nociceptive gene expression of receptors and neurotransmitters and recent evidence suggests that placode- and neural crest-derived nociceptors have distinct stimuli sensitivity, innervation patterns and functions. Improved understanding of the function of each subset in specific reflexes has substantial implications for therapeutic targeting of the neuronal components of airway disease such as asthma, viral infections and chronic obstructive pulmonary disease.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Autonomic neural control of intrathoracic airways

Autonomic neural control of the intrathoracic airways aids in optimizing air flow and gas exchange. In addition, and perhaps more importantly, the autonomic nervous system contributes to host defense of the respiratory tract. These functions are accomplished by tightly regulating airway caliber, blood flow, and secretions. Although both the sympathetic and parasympathetic branches of the autonomic nervous system innervate the airways, it is the later that dominates, especially with respect to control of airway smooth muscle and secretions. Parasympathetic tone in the airways is regulated by reflex activity often initiated by activation of airway stretch receptors and polymodal nociceptors. This review discusses the preganglionic, ganglionic, and postganglionic mechanisms of airway autonomic innervation. Additionally, it provides a brief overview of how dysregulation of the airway autonomic nervous system may contribute to respiratory diseases.

Calcium channel subtypes for cholinergic and nonadrenergic noncholinergic neurotransmission in isolated guinea pig trachea

The Ca(2+) channel subtypes in the neurotransmission of isolated guinea pig trachea were elucidated by monitoring the effects of specific Ca(2+) channel blockers on cholinergic contractions and nonadrenergic noncholinergic (NANC) relaxation elicited by electrical field stimulation (EFS). In isolated guinea pig trachea, cholinergic contractile responses to low- and high-frequency EFS were inhibited by the selective N-type calcium channel blocker, ω-conotoxin MVIIA. ω-Agatoxin IVA (a selective P-type blocker), ω-conotoxin MVIIC (a nonselective N-, Q-, and P-type blocker), and nifedipine (a selective L-type blocker) were ineffective, whereas Ni(2+) (a T- and R-type blocker) facilitated cholinergic contractions and produced a late contracture when its concentration exceeded 30 μM. The more the concentration of Ni(2+) increased, the greater the number of incidences and the late contracture areas which occurred. Late contracture did not seem to be due to the effects of acetylcholine, tachykinins, or other polypeptides, but disappeared in the absence of indomethacin. The NANC relaxant responses elicited by the low- and high-frequency EFS were inhibited by ω-conotoxin MVIIA or Ni(2+), but unaffected by ω-Agatoxin IVA, ω-conotoxin MVIIC, and nifedipine. In the absence of indomethacin, Ni(2+) did not alter the ω-conotoxin MVIIA (100 nM)-resistant component of cholinergic contraction, but significantly further inhibited that of NANC relaxation. These results suggest that in isolated guinea pig trachea, cholinergic contraction is regulated by N-type calcium channels which may mask T- and R-type calcium channels and may be co-modulated by both, while NANC relaxation is mainly and independently controlled by N-, T-, and R-type calcium channels.

Modeling the dynamics of airway constriction: effects of agonist transport and binding

Recent advances have revealed that during exogenous airway challenge, airway diameters cannot be adequately predicted by their initial diameters. Furthermore, airway diameters can also vary greatly in time on scales shorter than a breath. To better understand these phenomena, we developed a multiscale model that allowed us to simulate aerosol challenge in the airways during ventilation. The model incorporates agonist-receptor binding kinetics to govern the temporal response of airway smooth muscle contraction on individual airway segments, which, together with airway wall mechanics, determines local airway caliber. Global agonist transport and deposition are coupled with pressure-driven flow, linking local airway constrictions with global flow dynamics. During the course of challenge, airway constriction alters the flow pattern, redistributing the agonist to less constricted regions. This results in a negative feedback that may be a protective property of the normal lung. As a consequence, repetitive challenge can cause spatial constriction patterns to evolve in time, resulting in a loss of predictability of airway diameters. Additionally, the model offers new insights into several phenomena including the intra- and interbreath dynamics of airway constriction throughout the tree structure.

Neurokinin3 receptor regulation of the airways

Neurokinin(3) (NK(3)) receptors may regulate the airways primarily through actions on the nerves. In the periphery, airway parasympathetic ganglia neurons are depolarized following NK(3) receptor activation resulting subsequently in the facilitation of synaptic transmission. Such an effect may account for the excessive parasympathetic reflex effects (e.g. airway smooth muscle contraction, vascular engorgement, mucus secretion) associated with asthma and chronic obstructive pulmonary disease (COPD). In the central nervous system (CNS), NK(3) receptor activation may regulate airway vagal afferent relay neurons, rendering them hyperresponsive to parallel inputs from glutamate containing afferent nerves. This process is analogous to the process of central sensitization regulating hyperalgesia and pain in somatic tissues. In both the CNS and in the airways, NK(3) receptors are likely activated by either substance P and/or neurokinin A (NKA), both of which are full agonists at NK(3) receptors, as there is little evidence that airway nerves express neurokinin B (NKB). Evidence for other potential sites of regulation by NK(3) receptors in the airways (e.g. vasculature, airway smooth muscle, epithelium, mucus glands) is either inconclusive or conflicting.

Effects of capsaicin on visceral smooth muscle: a valuable tool for sensory neurotransmitter identification

Studying the visceral effects of the sensory stimulant capsaicin is a useful and relatively simple tool of neurotransmitter identification and has been used for this purpose for approximately 25 years in the authors' and other laboratories. We believe that conclusions drawn from experiments on visceral preparations may have an impact on studies dealing with the central endings of primary afferent neurons, i.e. research on nociception at the spinal level. The present review concentrates on the effects of capsaicin--through the transient receptor potential vanilloid receptor type 1 (TRPV1) receptor--on innervated gastrointestinal, respiratory and genitourinary smooth muscle preparations. Tachykinins and calcitonin gene-related peptide (CGRP) are the most widely accepted transmitters to mediate "local efferent" effects of capsaicin-sensitive nerves in tissues taken from animals. Studies more and more frequently indicate a supra-additive interaction of various types of tachykinin receptors (tachykinin NK(1), NK(2), NK(3) receptors) in the excitatory effects of capsaicin. There is also evidence for a mediating role of ATP, acting on P(2) purinoceptors. Non-specific inhibitory actions of capsaicin-like drugs have to be taken into consideration while designing experiments with these drugs. Results obtained on human tissues may be sharply different from those of animal preparations. Capsaicin potently inhibits tone and movements of human intestinal preparations, an effect mediated by nitric oxide (NO) and/or vasoactive intestinal polypeptide.

A Novel Role for Tachykinin Neurokinin-3 Receptors in Regulation of Human Bronchial Ganglia Neurons

The neuropeptide tachykinins and their receptors have been implicated in the pathogenesis of lung disease, although the role of the tachykinin neurokinin-3 receptor has not been elucidated. Using confocal microscopy, we identified tachykinin neurokinin-3 receptors on human bronchial parasympathetic ganglion neurons. Electrophysiologic recordings demonstrated that activation of sensory nerve fibers, either by antidromic stimulation or capsaicin, depolarized these neurons. This response was mimicked by exogenously applied tachykinin neurokinin-3 receptor-selective agonist, senktide analogue, but not significantly by tachykinin neurokinin-1 or neurokinin-2 receptor-selective agonists. Responses to endogenous tachykinins or exogenous selective tachykinin neurokinin-3 receptor activation with senktide analogue were inhibited by the selective tachykinin neurokinin-3 receptor antagonists, SB 223412 or SB 235375. We provide the first evidence that tachykinin neurokinin-3 receptors regulate human bronchial parasympathetic ganglion neurotransmission by activation of a peripheral reflex. This pathway may play a significant role in controlling bronchomotor tone and air flow to the lung.

Pharmacology of vagal afferent nerve activity in guinea pig airways

The excitability and activity of vagal afferent nerves innervating the airways can be pharmacologically increased and decreased. Autacoids released as a result of airway inflammation can lead to substantial increases in afferent nerve activity, consequently altering pulmonary reflex physiology. In a manner analogous to hyperalgesia associated with inflammation in the somato-sensory system, increases in vagal afferent nerve activity in inflamed airways may lead to a heightened cough reflex, and increases in autonomic activity in the airways. These effects may contribute to many of the symptoms of inflammatory airway disease. Here we provide a brief overview of some of the mechanisms by which the afferent activity in airway nerves can be pharmacologically modified.

Peripheral tachykinin receptors as potential therapeutic targets in visceral diseases

More than 10 years of intensive preclinical investigation of selective tachykinin (TK) receptor antagonists has provided a rationale to the speculation that peripheral neurokinin (NK)-1, -2 and -3 receptors may be involved in the pathophysiology of various human diseases at the visceral level. In the airways, despite promising effects in animal models of asthma, pilot clinical trials with selective NK-1 or -2 receptor antagonists in asthmatics have been ambiguous, whereas the potential antitussive effects of NK-1, -2 or -3 antagonists have not yet been verified in humans. In the gastrointestinal (GI) tract, irritable bowel syndrome (IBS) and pancreatitis are appealing targets for peripherally-acting NK-1 and -2 antagonists, respectively. In the genito-urinary tract, NK-1 receptor antagonists could offer some protection against nephrotoxicity and cytotoxicity induced by chemotherapeutic agents, whereas NK-2 receptor antagonists appear to be promising new agents for the treatment of neurogenic bladder hyperreflexia. Finally, there is preclinical evidence for hypothesising an effect of NK-3 receptor antagonists on the cardiovascular disturbance that characterises pre-eclampsia. Other more speculative applications are also mentioned.

Physiology and plasticity of putative cough fibres in the Guinea pig

Cough is initiated by activation of afferent nerve fibers with rapidly adapting receptors (RAR) that conduct action potentials in the Adelta range. In addition, various stimuli that activate airway unmylenated C-fibres evoke cough reflexes. We have used a vagally innervated, larynx-trachea-bronchus preparation, isolated from guinea pigs, to study the pharmacology of RARs and C-fibres in vitro. In this preparation afferent fibres with the RAR phenotype are exquisitely sensitive to mechanical perturbation of their receptive fields, but are unaffected by a variety of mediators (e.g. prostaglandins, histamine, bradykinin, serotonin) and by capsaicin. By contrast, C-fibres are much less sensitive to mechanical stimulation, but can be activated by capsaicin and bradykinin. Preliminary evidence supports the hypothesis that bradykinin activate C-fibre by stimulating the capsaicin (vanilloid) receptor VR1. Acids activate both C-fibres and RARs. Acids stimulate RAR fibres by a mechanism that is rapidly inactivated. C-fibres are stimulated by both a rapidly inactivating mechanism, as well as a slowly inactivating mechanism. Drugs that block VR1 inhibit the latter mechanism. Airway inflammation substantially increases the mechanical sensitivity of RAR fibres without affecting their adaptive properties. Airway inflammation also causes a phenotypic switch in neuropeptide innervation of the airways that RAR neurons begin to synthesis neurokinins and calcitonin gene related peptide. In non-inflamed animals these peptides are expressed only in C-fibre neurons. Thus, airway inflammation may not only increase the sensitivity of cough fibres, but may also qualitatively change the role played by sensory neuropeptides in cough reflexes.

Transmission in autonomic ganglia

The activity of airway smooth muscle, glands and vasculature is under tonic control by the autonomic nervous system. Information regarding the function and state of the airway (e.g. blood flow, temperature, oxygen levels, movement, irritants, inflammation, etc.) is relayed to the central nervous system (CNS) in the form of action potentials carried by sensory nerves. This input is integrated at many levels in the CNS and this information is ultimately transformed into coded action potentials carried by various preganglionic nerve pathways from the CNS to peripheral clusters of neurons referred to as autonomic ganglia. In the autonomic ganglia the CNS-derived action potentials cause the release of neurotransmitter(s) at a synapse between the preganglionic nerve terminal and the principal ganglion neuron. The fact that synaptic transmission exists makes the ganglion neuron the final site of integration in this complex reflex pathway. Whether this transmission of information from the CNS occurs, by activating the autonomic ganglion neuron and consequently the effector organ, depends on neurochemical, anatomical, and electrophysiological factors within the ganglion that is the subject of this review.

Neural integration and allergic disease

Changes in neural activity play a key role in many symptoms of allergic disease, including sneezing, coughing, itching, and ocular irritation, among others. The mechanisms underlying allergen-induced changes in neural activity (reflexes) are largely unknown and under active investigation. Allergic inflammation can affect neural activity on a variety of levels, including at the primary afferent sensory nerve, integrative centers of the central nervous system, autonomic ganglia, and autonomic neuroeffector junction. At the level of the afferent sensory nerve, mediators released after allergen exposure either directly or indirectly increase neuronal firing. At the level of sensory ganglia, which contain cell bodies that innervate a variety of organs, changes in neuronal excitability may lead to a generalization of allergic symptoms. In the central nervous system, where afferent inputs from throughout the body converge, allergic inflammation may be associated with central sensitization, leading to the modulation of the neural reflexes. Finally, at the autonomic ganglia and neuroeffector junction, allergic inflammation appears to be associated with enhanced ganglionic transmission and neurotransmitter release, respectively. Mechanisms by which allergen challenge affects neuronal activity at various levels of the nervous system are reviewed, with a primary emphasis on studies of airway physiologic factors.

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

This study combined single and transneuronal labeling to define the origin of midline-crossing vagal fibers projecting to the rat's lungs. Injections of the beta-subunit of cholera toxin (CT-beta) into the lungs labeled similar numbers of neuronal somata in the nucleus ambiguus and dorsal motor nucleus of the vagus on both sides of the medulla, even though vagal stimulation increased lung resistance 50% less in the contralateral than in the ipsilateral lung. Unilateral cervical vagotomy prevented CT-beta labeling of ipsilateral neuronal somata and sensory fibers, indicating that lung-bound vagal fibers undergo decussation only inside the thorax. Injections of CT-beta and FluoroGold into opposite main stem bronchi double labeled 30% and 11% of all neuronal somata immunoreactive for CT-beta and FluoroGold, respectively, showing that one single vagal motoneuron can innervate airways on both sides. Injections of pseudorabies virus into the right lung revealed a bilateral network of infected neurons, even after unilateral vagotomy. The latter did not prevent infection of the ipsilateral vagal nuclei. These findings demonstrate that vagal motoneurons that project to the lungs receive contralateral inputs from the airway premotor network and vagal bronchomotor centers.

Role of nitric oxide and septide-insensitive NK(1) receptors in bronchoconstriction induced by aerosolised neurokinin A in guinea-pigs

The tachykinin, neurokinin A (NKA), contracts guinea-pig airways both in vitro and in vivo, preferentially activating smooth muscle NK(2) receptors, although smooth muscle NK(1) receptors may also contribute. In vitro evidence suggests that NKA activates epithelial NK(1) receptors, inducing the release of nitric oxide (NO) and subsequent smooth muscle relaxation. A number of selective NK(1) receptor agonists have been reported to activate both smooth muscle and epithelial NK(1) receptors, however septide appears only to activate smooth muscle NK(1) receptors. The aim of the present study was to investigate whether NKA-induced bronchoconstriction in guinea-pigs in vivo may be limited by NO release via NK(1) receptor activation, and whether selective NK(1) receptor agonists may activate this mechanism differently. Aerosolized NKA caused an increase in total pulmonary resistance (RL) that was markedly reduced by the NK(2) receptor antagonist, SR 48968, and abolished by the combination of SR 48968 and the NK(1) receptor antagonist, CP-99, 994. The increase in RL evoked by NKA was potentiated by pretreatment with the NO synthase (NOs) inhibitor, L-NAME, but not by the inactive enantiomer D-NAME. Potentiation by L-NAME of NKA-induced increase in RL was reversed by L-Arginine, but not by D-Arginine. Pretreatment with L-NAME did not affect the increase in RL induced by the selective NK(2) receptor agonist, [beta-Ala(8)]NKA(4-10), and by the selective NK(1) receptor agonist, septide, whereas it markedly potentiated the increase in RL caused by a different NK(1) selective agonist, [Sar(9),Met(O(2))(11)]SP. Dose-response curves showed that septide was a more potent bronchoconstrictor than [Sar(9),Met(O(2))(11)]SP to cause bronchoconstriction. Pretreatment with the NK(1) receptor antagonist, CP-96,994, abolished the ability of L-NAME to increase bronchoconstriction to aerosolized NKA. Bronchoconstriction to aerosolized NKA was increased by L-NAME, after pretreatment with the NK(3) receptor antagonist, SR 142801. The present study shows that in vivo bronchoconstriction in response to the aerosolized naturally occurring tachykinin, NKA, is limited by its own ability to release relaxant NO via NK(1) receptor activation. This receptor is apparently insensitive to septide, thus justifying, at least in part, the high potency of septide to cause bronchoconstriction in guinea-pigs.

Characterization of non-adrenergic, non-cholinergic inhibitory responses of the isolated guinea-pig trachea: differences between pre- and post-ganglionic nerve stimulation

1 Differences in the mechanism of non-adrenergic, non-cholinergic (NANC) inhibitory responses to preganglionic- and post-ganglionic nerve stimulation were investigated in the guinea-pig isolated trachea. 2 Stimulation of the vagus nerve at frequencies above 4 Hz elicited NANC relaxation of the trachealis muscle. Responses to low frequencies of stimulation (4-8 Hz) were abolished by the nitric oxide (NO) synthase inhibitor L-NOARG (10 microM), while a L-NOARG resistant component was observed at higher stimulus frequencies. The L-NOARG-resistant component of NANC inhibitory responses to higher frequencies of vagus nerve stimulation were significantly attenuated by the proteinase alpha-chymotrypsin (2 U/ml), suggesting that a neuropeptide such as VIP may contribute to NANC responses. 3 When postganglionic nerves were stimulated by electrical field stimulation (EFS), responses were readily elicited at frequencies below 4 Hz. Like responses to vagus nerve stimulation, responses to low frequency (<4 Hz) EFS were abolished by L-NOARG while a L-NOARG-resistant component was apparent at higher stimulus frequencies. 4 The L-NOARG-resistant component of NANC inhibitory responses to EFS was sensitive to alpha-chymotrypsin only if stimuli were delivered in either long trains at a low frequency (4 Hz for 10-30 s) or short trains of high frequency (16 Hz for 2.5-7.5 s). 5 Responses to preganglionic nerve stimulation were approximately 35% of the amplitude of responses to EFS in the same preparations. 6 In conclusion, responses to preganglionic and postganglionic NANC inhibitory nerve stimulation in the guinea-pig trachea differ in maximum amplitude, frequency-response characteristics and the contributions of cotransmitters. We suggest that these differences may be explained by filtering of preganglionic input to postganglionic NANC neurons. These results have implications in all studies where EFS is considered to be representative of physiological stimulation of post-ganglionic nerve stimulation.

Regulation of baseline cholinergic tone in guinea-pig airway smooth muscle

1. We quantified baseline cholinergic tone in the trachealis of mechanically ventilated guinea-pigs and determined the influence of vagal afferent nerve activity on this parasympathetic tone. 2. There was a substantial amount of baseline cholinergic tone in the guinea-pig trachea, eliciting contractions of the trachealis that averaged 24.6 +/- 3.5 % (mean +/- s.e.m.) of the maximum attainable contraction. This tone was essentially abolished by vagotomy or ganglionic blockade, suggesting that it was dependent upon on-going pre-ganglionic input arising from the central nervous system. 3. Cholinergic tone in the trachealis could be markedly and rapidly altered (either increased or decreased) by changes in ventilation (e. g. cessation of ventilation; hyperpnoea; slow, deep breathing) and by lung distention (via positive end-expiratory pressure). These effects were not accompanied by marked alterations in blood gases and were abolished by vagotomy or atropine. By contrast, tachykinin receptor antagonists, which abolished capsaicin-induced bronchospasm, were without effect on baseline cholinergic tone. This and other evidence suggests that capsaicin-sensitive nerves have little if any influence on baseline parasympathetic tone. Likewise, while activation of afferent nerves innervating the larynx can alter airway parasympathetic nerve activity, transection of the superior laryngeal nerves was without effect on baseline cholinergic tone. 4. Cutting the vagus nerves caudal to the recurrent laryngeal nerves, thus leaving the preganglionic parasympathetic innervation of the trachealis intact but disrupting all afferent nerves innervating the lungs and intrapulmonary airways, abolished baseline cholinergic tone in the trachea. Sham vagotomy or cutting the vagi caudal to the lungs did not reduce baseline cholinergic tone. 5. The results indicate that baseline airway cholinergic nerve activity is necessarily dependent upon afferent nerve activity arising from the intrapulmonary airways and lungs. More specifically, the data are consistent with the hypothesis that on-going activity arising from the nerve terminals of intrapulmonary rapidly adapting receptors determines the level of baseline airway cholinergic tone.

Naloxone blocks endomorphin-1 but not endomorphin-2 induced inhibition of tachykinergic contractions of guinea-pig isolated bronchus

The recently identified endogenous agonists on the mu-opioid-receptor (mu OR), endomorphin-1 (EM-1) and endomorphin-2 (EM-2), induce a concentration dependent inhibition of electrical field stimulation (EFS)-induced tachykinin-mediated contractions of the guinea-pig bronchus (ED50s < 10 nM for both compounds). Surprisingly, only endomorphin-1 effects could be blocked by naloxone (10 microM), whereas endomorphin-2 effects were not affected by specific antagonists for the mu-, kappa-, and delta-opioid-receptor.

Pro-inflammatory effects induced by bradykinin in a murine model of pleurisy

Bradykinin caused a dose-related increase in cell influx 4 h after its administration into the mouse pleural cavity (ED50 = 3.2 nmol/cav., 95% confidence limits = 0.6-15.5). Cell influx peaked at 4 h and remained elevated for up to 72 h, whereas exudation was detected between 2 and 6 h after bradykinin administration. Both HOE 140 (D-Arg-[Hyp3,Thi5,D-Tic7, Oic8]bradykinin) and NPC 17731 (D-Arg0-[Hyp3 D-HypE(transpropyl7)Oic8]bradykinin) inhibited bradykinin-induced cell influx (ID50 0.028 (0.05-0.16) and 0.4 (0.3-0.7) pmol/cav., respectively). Des-Arg9-[Leu8]bradykinin (0.1 and 3.0 nmol/cav., 30 min before) did not inhibit the effects of bradykinin. Pre-treatment of animals with either indomethacin, terfenadine, dexamethasone, N(omega)-nitro-L-arginine benzyl ester, cromolyn, theophylline, salbutamol, FK 888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-propyl]N-met hyl-N-phenyl-methyl-3-(2-naphthyl)-L-alaninamide) or SR 142801 ((N)-(1-[3-[1-benzoyl-3-(3,4-dichloro-phenyl)-piperidin-3-yl]pr opy l]-4-phenyl-piperidin-4-yl)-N-methyl-acetamide) significantly inhibited cell migration (P < 0.01). These results indicate that bradykinin had a significant pro-inflammatory effect on the pleural cavity of the mice. This effect seems to be primarily mediated via activation bradykinin B2 receptors which trigger the release of other mediators.

Nitric oxide regulatory role in sensitized guinea pig trachea

Nitric oxide (NO) has been cited to play an important regulatory role in airway function. Moreover, the NO synthase expression in models of inflammation is documented. The aim of this study was to investigate, in vitro, the NO modulation of cholinergic responses in sham-sensitized and ovalbumin-sensitized guinea pig trachea by using L-arginine (L-ARG), a precursor of NO synthesis, and L-Ng-nitro-arginine-methyl-ester (L-NAME), an inhibitor of NO synthase. Our results showed that NO's ability to modulate cholinergic responses in ovalbumin-sensitized guinea pig trachea is lost. Indeed L-ARG and L-NAME modify acetylcholine sensitivity in sham-sensitized guinea pig but not in ovalbumin-sensitized guinea pig.

Anatomical and electrophysiological comparison of the sensory innervation of bronchial and tracheal parasympathetic ganglion neurons

Anatomical and electrophysiological techniques were used to examine differences in the level of sensory nerve terminals localized in parasympathetic ganglia in guinea pig trachea and bronchus. We quantified substance-P-immunoreactive nerve terminal profiles within the ganglia and the amplitude of capsaicin-sensitive depolarization (slow excitatory postsynaptic potential or sEPSP) evoked by vagus (bronchial) or recurrent (tracheal) nerve-stimulation. Substance-P-immunoreactive axon profiles per intrinsic tracheal neuron were fewer in number than per bronchial ganglion neuron. The average amplitude and duration of the capsaicin-sensitive sEPSPs were similar in tracheal and bronchial ganglion neurons. Furthermore, capsaicin evoked a nearly equal depolarization of bronchial and tracheal ganglion neurons. The sEPSPs were reduced in both areas by a selective neurokinin-3 receptor antagonist. SR142801. These results demonstrate that although the number of sensory nerve fibers in tracheal ganglia are significantly less than those in the bronchus, this did not translate to an obvious physiological difference in sEPSP amplitude.