Research Opportunities in Autonomic Neural Mechanisms of Cardiopulmonary Regulation: A Report From the National Heart, Lung, and Blood Institute and the National Institutes of Health Office of the Director Workshop

This virtual workshop was convened by the National Heart, Lung, and Blood Institute, in partnership with the Office of Strategic Coordination of the Office of the National Institutes of Health Director, and held September 2 to 3, 2020. The intent was to assemble a multidisciplinary group of experts in basic, translational, and clinical research in neuroscience and cardiopulmonary disorders to identify knowledge gaps, guide future research efforts, and foster multidisciplinary collaborations pertaining to autonomic neural mechanisms of cardiopulmonary regulation. The group critically evaluated the current state of knowledge of the roles that the autonomic nervous system plays in regulation of cardiopulmonary function in health and in pathophysiology of arrhythmias, heart failure, sleep and circadian dysfunction, and breathing disorders. Opportunities to leverage the Common Fund's SPARC (Stimulating Peripheral Activity to Relieve Conditions) program were characterized as related to nonpharmacologic neuromodulation and device-based therapies. Common themes discussed include knowledge gaps, research priorities, and approaches to develop novel predictive markers of autonomic dysfunction. Approaches to precisely target neural pathophysiological mechanisms to herald new therapies for arrhythmias, heart failure, sleep and circadian rhythm physiology, and breathing disorders were also detailed.

Different role of TTX-sensitive voltage-gated sodium channel (NaV 1) subtypes in action potential initiation and conduction in vagal airway nociceptors

KEY POINTS

The action potential initiation in the nerve terminals and its subsequent conduction along the axons of afferent nerves are not necessarily dependent on the same voltage-gated sodium channel (Na_V 1) subunits. The action potential initiation in jugular C-fibres within airway tissues is not blocked by TTX; nonetheless, conduction of action potentials along the vagal axons of these nerves is often dependent on TTX-sensitive channels. This is not the case for nodose airway Aδ-fibres and C-fibres, where both action potential initiation and conduction is abolished by TTX or selective Na_V 1.7 blockers. The difference between the initiation of action potentials within the airways vs. conduction along the axons should be considered when developing Na_V 1 blocking drugs for topical application to the respiratory tract.

ABSTRACT

The action potential (AP) initiation in the nerve terminals and its subsequent AP conduction along the axons do not necessarily depend on the same subtypes of voltage-gated sodium channels (Na_V 1s). We evaluated the role of TTX-sensitive and TTX-resistant Na_V 1s in vagal afferent nociceptor nerves derived from jugular and nodose ganglia innervating the respiratory system. Single cell RT-PCR was performed on vagal afferent neurons retrogradely labelled from the guinea pig trachea. Almost all of the jugular neurons expressed the TTX-sensitive channel Na_V 1.7 along with TTX-resistant Na_V 1.8 and Na_V 1.9. Tracheal nodose neurons also expressed Na_V 1.7 but, less frequently, Na_V 1.8 and Na_V 1.9. Na_V 1.6 were expressed in ∼40% of the jugular and 25% of nodose tracheal neurons. Other Na_V 1 α subunits were only rarely expressed. Single fibre recordings were made from the vagal nodose and jugular nerve fibres innervating the trachea or lung in the isolated perfused vagally-innervated preparations that allowed for selective drug delivery to the nerve terminal compartment (AP initiation) or to the desheathed vagus nerve (AP conduction). AP initiation in jugular C-fibres was unaffected by TTX, although it was inhibited by Na_V 1.8 blocker (PF-01247324) and abolished by combination of TTX and PF-01247324. However, AP conduction in the majority of jugular C-fibres was abolished by TTX. By contrast, both AP initiation and conduction in nodose nociceptors was abolished by TTX or selective Na_V 1.7 blockers. Distinction between the effect of a drug with respect to inhibiting AP in the nerve terminals within the airways vs. at conduction sites along the vagus nerve is relevant to therapeutic strategies involving inhaled Na_V 1 blocking drugs.

Sphingosine-1-phosphate selectively activates vagal afferent C-fiber subtype in guinea pig esophagus

BACKGROUND

Activation and sensitization of visceral afferent nerves by inflammatory mediators play important roles in visceral nociception. Sphingosine-1-phosphate (S1P) is a lipid with intracellular and extracellular functions. Extracellularly, it can act as an autacoid via interactions with S1P receptors. The present study aims to determine the effect of S1P on esophageal vagal afferent nerve functions.

METHODS

Extracellular single-unit recordings were performed in ex vivo guinea pig esophageal-vagal preparations. The action potentials (APs) evoked by mechanical distension and chemical perfusions applied to the vagal afferent nerve endings in the esophagus were recorded at their intact neuronal cell bodies in either nodose or jugular ganglia. The effects of S1P and its receptor subtype agonists on vagal afferents were recorded and compared. The expression of S1P receptors (S1PR1-3) in esophageal-labeled vagal nodose and jugular neurons was studied by single-cell RT-PCR.

KEY RESULTS

Sphingosine-1-phosphate evoked AP discharges in almost all esophageal jugular but not nodose C-fibers without changing their responses to esophageal distension. Esophageal-labeled vagal nodose and jugular neurons highly expressed transcripts of S1PR1 and S1PR3. Agonists of S1PR1 and S1PR3 each partially mimicked S1P-induced effect in jugular C-fibers, suggesting that these receptors may contribute partially to S1P-induced activation effect on esophageal jugular C-fiber subtype.

CONCLUSIONS & INFERENCES

These data, for the first time, demonstrated a selective activation effect of S1P on vagal afferent nerve subtype in the gastrointestinal tract. This may help to better understand its role in visceral inflammatory nociception.

Mechanisms of pruritogen-induced activation of itch nerves in isolated mouse skin

KEY POINTS

Chloroquine (CQ) stimulates itch nerves and causes intense scratching in mice by activating the G-protein coupled receptor (GPCR) MrgprA3; it is not known how stimulation of MrgprA3 (or other GPCRs) leads to activation of the itch nerve terminals in the skin, but previous studies have found that transient receptor potential A1 (TRPA1) gene deletion blocks CQ-induced scratching. In the present study we used a novel dorsal skin-nerve preparation to evaluate mechanisms underlying CQ- and histamine-induced action potential discharge in itch nerve terminals. We found that CQ activation of the nerves requires the beta3 isoform of phospholipase C, but TRPA1 or other TRP channel are not required. Evidence is provided for a role for calcium-activated chloride channels such as TMEM16a in GPCR-activation of itch nerve terminals. The mechanism by which TRP channels participate in pruritogen-induced scratching may involve sites of action other than the primary afferent terminals.

ABSTRACT

Chloroquine (CQ) and histamine are pruritogens commonly used to study itch in the mouse. A novel skin-nerve preparation was used to evaluate chloroquine (CQ)- and histamine-induced activation of afferent nerves in the dorsal thoracic skin of the mouse. All CQ sensitive nerves were C-fibres, and were also sensitive to histamine. The response to CQ, but not histamine, was largely absent in mrgpr-cluster Δ^-/- mice, supporting the hypothesis that CQ evokes itch largely via stimulation of MrgprA3 receptors. The CQ-induced action potential discharge was largely absent in phospholipase Cβ3 knockout animals. The CQ and histamine responses were not influenced by removal of TRPA1, TRPV1, TRPC3 or TRPC6, nor by the TRP channel blocker Ruthenium Red. The bouts of scratching in response to CQ were not different between wild-type and TRPA1-deficient mice. A selective inhibitor of the calcium-activated chloride channel TMEM16A, N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA), inhibited CQ-induced action potential discharge at itch nerve terminals and bouts of scratching by about 50%. Although TRPA1 and TRPV1 channels may be involved in the scratching responses to intradermal pruritogens, this is unlikely to be due to an effect at the nerve terminals, where chloride channels may play a more important role.

Basic mechanisms of itch

Chronic itch represents a burdensome clinical problem that can originate from a variety of aetiologies. Pruriceptive itch originates following the activation of peripheral sensory nerve endings following damage or exposure to inflammatory mediators and ascends to the brain through the spinal thalamic tract. Much insight has been gained into the understanding of the mechanisms underlying pruriceptive itch through studies using humans and experimental animals. More than one sensory nerve subtype is thought to subserve pruriceptive itch which includes both unmyelinated C-fibres and thinly myelinated Aδ nerve fibres. There are a myriad of mediators capable of stimulating these afferent nerves leading to itch, including biogenic amines, proteases, cytokines, and peptides. Some of these mediators can also evoke sensations of pain and the sensory processing underlying both sensations overlaps in complex ways. Studies have demonstrated that both peripheral and central sensitization to pruritogenic stimuli occur during chronic itch.

Transgene expression and effective gene silencing in vagal afferent neurons in vivo using recombinant adeno-associated virus vectors

Vagal afferent fibres innervating thoracic structures such as the respiratory tract and oesophagus are diverse, comprising several subtypes of functionally distinct C-fibres and A-fibres. Both morphological and functional studies of these nerve subtypes would be advanced by selective, effective and long-term transduction of vagal afferent neurons with viral vectors. Here we addressed the hypothesis that vagal sensory neurons can be transduced with adeno-associated virus (AAV) vectors in vivo, in a manner that would be useful for morphological assessment of nerve terminals, using enhanced green fluorescent protein (eGFP), as well as for the selective knock-down of specific genes of interest in a tissue-selective manner. We found that a direct microinjection of AAV vectors into the vagal nodose ganglia in vivo leads to selective, effective and long-lasting transduction of the vast majority of primary sensory vagal neurons without transduction of parasympathetic efferent neurons. The transduction of vagal neurons by pseudoserotype AAV2/8 vectors in vivo is sufficiently efficient such that it can be used to functionally silence TRPV1 gene expression using short hairpin RNA (shRNA). The eGFP encoded by AAV vectors is robustly transported to both the central and peripheral terminals of transduced vagal afferent neurons allowing for bright imaging of the nerve endings in living tissues and suitable for structure-function studies of vagal afferent nerve endings. Finally, the AAV2/8 vectors are efficiently taken up by the vagal nerve terminals in the visceral tissue and retrogradely transported to the cell body, allowing for tissue-specific transduction.

Characterization of mast cell subtypes, distribution, and antigen-induced activation in the guinea pig esophagus

The number of esophageal mucosa mast cells (MCs) increases in allergic and inflammation conditions in the esophagus, but their role in these conditions is less clear. MCs are derived from bone marrow, migrate and mature in the peripheral tissues. Two subsets of MCs have been characterized as mucosal MC (MMC) and connective tissue MC (CTMC) defined by anatomic location, granule contents, and functions. Whether esophageal MCs share typical features with either MMC or CTMC has yet to be determined. This study characterized esophageal MCs subtypes, distribution, antigen-induced sensitization, and degranulation as measured by MC staining and histamine release assay. Immunofluorescent double staining of MC tryptase and chymase were performed in the esophagus, intestine, and skin from normal and ovalbumin (OVA) actively sensitized guinea pigs. Histamine release was measured in the esophagus from OVA-sensitized guinea pigs following in vitro antigen challenge. Similar to the MCs in the intestine and skin, esophageal MCs contained three subtypes, which included 62% MCtc (tryptase+/chymase+), 17% MCc (chymase+/tryptase-), and 21% MCt (tryptase+/chymase-). In contrast to the ileal MCs, which were distributed all over the mucosa, submucosa, and serosa, MCs in the esophagus almost all (more than 98%) lined along the lamina propria. OVA active sensitization significantly increased the esophageal MC subtype MCtc. OVA in vitro challenge of the esophagus from sensitized guinea pig significantly decreased tryptase-positive MC subtypes MCtc and MCt, and released a significant amount of tissue histamine content. In conclusion, MCs in the guinea pig esophagus have unique features in immunophenotypes, distribution, and degranulation response to OVA challenge with the release of significant amounts of proteases and histamine into the tissue. These characteristics may indicate that OVA in vitro challenge in OVA-sensitized guinea pig esophagus could be a good model to study the role of esophageal MCs in allergic and inflammation conditions.

Cough sensors. V. Pharmacological modulation of cough sensors

Several airway afferent nerve subtypes have been implicated in coughing. These include bronchopulmonary C-fibers, rapidly adapting airway mechanoreceptors and touch-sensitive tracheal Adelta-fibers (also called cough receptors). Although the last two afferent nerve subtypes are primarily sensitive to mechanical stimuli, all can be acted upon by one or more different chemical stimuli. In this review we catalogue the chemical agents that stimulate and/or modulate the activity of the airway afferent nerves involved in cough, and describe the specific mechanisms involved in these effects. In addition, we describe the mechanisms of action of a number of chemical inhibitors of these afferent nerve subtypes, and attempt to relate this information to the regulation of coughing in health and disease.

Leukotriene D4 increases the excitability of capsaicin-sensitive nasal sensory nerves to electrical and chemical stimuli

BACKGROUND AND PURPOSE

Clinical studies have demonstrated significant reductions in allergen-induced nasal symptoms of atopic rhinitis subjects by CysLT1 antagonists, including neuronally mediated symptoms such as sneeze, itch and reflex hypersecretion. Here, we test the hypothesis that cysteinyl leukotrienes activate and/or alter the activity of nasal nociceptive (capsaicin-sensitive) sensory neurones.

EXPERIMENTAL APPROACH

Using retrograde tracer (DiI), we labelled guinea-pig trigeminal sensory neurones that projected fibres to the nasal mucosa. Single-neurone reverse transcriptase (RT)-PCR was used to evaluate CysLT receptor gene expression. The effect of cysteinyl leukotrienes on individual nasal sensory nerve activity was assessed in Ca2+ assays and whole-cell gramicidin-perforated patch-clamp studies.

KEY RESULTS

Nasal C-fibre neurones express CysLT1 but not CysLT2 mRNA. LTD4 and LTC4 increased intracellular [Ca2+]free in a population of capsaicin-sensitive trigeminal nerves, an effect blocked by the CysLT1 antagonist ICI198615. In current clamp mode, LTD4 had no effect on resting membrane potential. However, LTD4 significantly increased electrical excitability (action potential discharge during current pulses) threefold in capsaicin-sensitive nasal neurones, which was inhibited by CysLT1 antagonists ICI198615 and montelukast. LTD4 had no effect on electrical excitability in capsaicin-insensitive neurones. Finally, LTD4 significantly augmented histamine-induced responses in capsaicin-sensitive neurones as measured by increased action potential discharge, peak frequency and membrane depolarization.

CONCLUSIONS AND IMPLICATIONS

LTD4, likely through CysLT1 receptors, directly increases the excitability of capsaicin-sensitive guinea-pig nasal trigeminal neurones, demonstrating a novel mechanism for the actions of cysteinyl leukotrienes and potentially explains the effectiveness of CysLT1 antagonists in treating nasal allergen-induced neuronal symptoms.

Relative contributions of TRPA1 and TRPV1 channels in the activation of vagal bronchopulmonary C-fibres by the endogenous autacoid 4-oxononenal

Transient receptor potential (TRP) A1 channels are cation channels found preferentially on nociceptive sensory neurones, including capsaicin-sensitive TRPV1-expressing vagal bronchopulmonary C-fibres, and are activated by electrophilic compounds such as mustard oil and cinnamaldehyde. Oxidative stress, a pathological feature of many respiratory diseases, causes the endogenous formation of a number of reactive electrophilic alkenals via lipid peroxidation. One such alkenal, 4-hydroxynonenal (4HNE), activates TRPA1 in cultured sensory neurones. However, our data demonstrate that 100 microm 4HNE was unable to evoke significant action potential discharge or tachykinin release from bronchopulmonary C-fibre terminals. Instead, another endogenously produced alkenal, 4-oxononenal (4ONE, 10 microm), which is far more electrophilic than 4HNE, caused substantial action potential discharge and tachykinin release from bronchopulmonary C-fibre terminals. The activation of mouse bronchopulmonary C-fibre terminals by 4ONE (10-100 microm) was mediated entirely by TRPA1 channels, based on the absence of responses in C-fibre terminals from TRPA1 knockout mice. Interestingly, although the robust increases in calcium caused by 4ONE (0.1-10 microm) in dissociated vagal neurones were essentially abolished in TRPA1 knockout mice, at 100 microm 4ONE caused a large TRPV1-dependent response. Furthermore, 4ONE (100 microm) was shown to activate TRPV1 channel-expressing HEK cells. In conclusion, the data support the hypothesis that 4-ONE is a relevant endogenous activator of vagal C-fibres via an interaction with TRPA1, and at less relevant concentrations, it may activate nerves via TRPV1.

Subtypes of vagal afferent C-fibres in guinea-pig lungs

An ex vivo, vagally innervated, lung preparation was used to address the hypothesis that vagal C-fibres comprise at least two distinct phenotypes. Histological and extracellular electrophysiological experiments revealed that vagal C-fibres innervating the pulmonary system are derived from cell bodies situated in two distinct vagal sensory ganglia. The jugular (superior) ganglion neurones project C-fibres to both the extrapulmonary airways (larynx, trachea and bronchus) and the lung parenchymal tissue. By contrast, C-fibres from nodose (inferior) neurones innervate primarily structures within the lungs. Histologically, nodose neurones projecting lung C-fibres were different from the jugular neurones in that they were significantly less likely to express neurokinins. The nerve terminals within the lungs of both nodose and jugular C-fibres responded with action potential discharge to capsaicin and bradykinin application, but only the nodose C-fibre population responded with action potential discharge to the P2X selective receptor agonist alpha,beta-methylene-ATP. Whole cell patch clamp recording of capsaicin-sensitive nodose and jugular ganglion neurones retrogradely labelled from the lung tissue revealed that, like the nerve terminals, lung specific nodose C-fibre neurones express functional P2X receptors, whereas lung specific jugular C-fibres do not. The data support the hypothesis that both neural crest-derived neurones (jugular ganglia) and placode-derived neurones (nodose ganglia) project C-fibres in the vagus, and that these two C-fibre populations represent distinct phenotypes.

Activation of bronchopulmonary vagal afferent nerves with bradykinin, acid and vanilloid receptor agonists in wild-type and TRPV1-/- mice

The vanilloid receptor TRPV1 (formerly VR1) has been implicated in the activation of nociceptive sensory nerves by capsaicin, noxious heat, protons, bradykinin, cannabinoids such as anandamide, and certain metabolites of arachidonic acid. Using TRPV1 knockout mouse (TRPV1-/-) we address the question of whether TRPV1 is obligatory for action potential discharge in vagal C-fibre terminals evoked by capsaicin, anandamide, acid and bradykinin. The response of a defined subtype of the vagal afferent bronchopulmonary C-fibres (conduction velocity < 0.7 ms(-1)) to the putative TRPV1 activators was studied in vitro in the mouse isolated/perfused lung-nerve preparation. Capsaicin (1 microm) evoked action potential discharge of approximately 90% (28/31) of C-fibres in the TRPV1+/+ mice, but failed to activate bronchopulmonary C-fibres in TRPV1-/- animals (n = 10). Anandamide (3-100 microm) induced concentration-dependent activation of capsaicin-sensitive TRPV1+/+ C-fibres with a threshold of 3-10 microm, but failed to evoke substantive discharge in TRPV1-/- C-fibres. In the TRPV1+/+ mice, the B2 receptor-mediated activation by bradykinin (1 microm) was restricted to the capsaicin-sensitive C-fibres. Bradykinin was effective in evoking B2 receptor-mediated action potential discharge in TRPV1-/- C-fibres, but the response was significantly (P < 0.05) less persistent than in TRPV1+/+ C-fibres. Exposing the tissue to acid (pH = 5) excited both TRPV1+/+ and TRPV1-/- C-fibres. We conclude that TRPV1 is obligatory for vagal C-fibre activation by capsaicin and anandamide. By contrast, whereas TRPV1 may have a modulatory role in bradykinin and acid-induced activation of bronchopulmonary C-fibres, it is not required for action potential discharge evoked by these stimuli.

Pharmacology of airway afferent nerve activity

Afferent nerves in the airways serve to regulate breathing pattern, cough, and airway autonomic neural tone. Pharmacologic agents that influence afferent nerve activity can be subclassified into compounds that modulate activity by indirect means (e.g. bronchial smooth muscle spasmogens) and those that act directly on the nerves. Directly acting agents affect afferent nerve activity by interacting with various ion channels and receptors within the membrane of the afferent terminals. Whether by direct or indirect means, most compounds that enter the airspace will modify afferent nerve activity, and through this action alter airway physiology.

Inhibition of voltage-gated K(+) currents by endothelin-1 in human pulmonary arterial myocytes

Recent studies demonstrate that endothelin-1 (ET-1) constricts human pulmonary arteries (PA). In this study, we examined possible mechanisms by which ET-1 might constrict human PA. In smooth muscle cells freshly isolated from these arteries, whole cell patch-clamp techniques were used to examine voltage-gated K(+) (K(V)) currents. K(V) currents were isolated by addition of 100 nM charybdotoxin and were identified by current characteristics and inhibition by 4-aminopyridine (10 mM). ET-1 (10(-8) M) caused significant inhibition of K(V) current. Staurosporine (1 nM), a protein kinase C (PKC) inhibitor, abolished the effect of ET-1. Rings of human intrapulmonary arteries (0.8-2 mm OD) were suspended in tissue baths for isometric tension recording. ET-1-induced contraction was maximal at 10(-8) M, equal to that induced by K(V) channel inhibition with 4-aminopyridine, and attenuated by PKC inhibitors. These data suggest that ET-1 constricts human PA, possibly because of myocyte depolarization via PKC-dependent inhibition of K(V). Our results are consistent with data we reported previously in the rat, suggesting similar mechanisms may be operative in both species.

Inflammation-induced plasticity of the afferent innervation of the airways

The activation of primary afferent neurons that innervate the airways leads to homeostatic and defensive reflexes. The anatomic and physiologic characteristics of these afferent fibers do not appear to be static properties but rather appear to change rapidly in response to inflammation. The threshold for activation of airway afferent neurons to various stimuli, for example, is not fixed; these fibers can be become sensitized during inflammation. A subset of nociceptive-like (C-fibers) airway afferent neurons not only participates in centrally mediated reflexes but is also thought to release neuropeptides at their peripheral terminals, leading to neurogenic inflammation. An increase in the content of tachykinins is commonly seen in inflamed tissues, and there is accumulating evidence that irritation and inflammation of the airways is associated with the induction of tachykinin synthesis in non-nociceptive airway afferent fibers that under normal conditions do not contain neuropeptides. The release of neurokinins from the peripheral terminals in the airways and their central terminals in the brain stem may contribute to the symptoms of inflammatory airway diseases. Elevated release of neurokinins from peripheral terminals may promote local inflammatory responses, and the release of neurokinins in the brainstem, together with inflammation-induced increases in the excitability of afferent fibers, may culminate in altered visceral autonomic reflex activity, changes in breathing pattern, and cough.

Ion channels in airway afferent neurons

Action potentials initiated at the peripheral terminal of an afferent nerve are conducted to the central nervous system therein causing release of neurotransmitters that excite secondary neurons in the brain stem or spinal cord. Various chemicals, extremes in osmolarity and pH as well as mechanical stimuli are sensed by primary afferent nerves that innervate the airways. The processes leading to excitation of afferent nerve endings, conduction of action potentials along axons, transmitter secretion, and neuronal excitability are regulated by ions flowing through channels in the nerve membrane. Voltage-gated ion channels selective for K+ and Na+ ions allow the generation and conduction of action potentials and along with families of ion channels selective for other ions such as Ca2+ or Cl- are thought to play distinctive roles in regulating neuronal excitability and transmitter secretion. Here we discuss, in general terms, the roles played by various classes of ion channels in the activation, neurotransmitter secretion and excitability of primary afferent neurons.

Neurotrophins and asthma

The neurotrophins are a family of peptides that promote survival, growth, and differentiation of neurons. Neurotrophins may also influence the function of nonneuronal cell types, including immune cells. The development and maintenance of asthma is thought to involve the nervous system and the immune system, but the role that neurotrophins play in asthma is unknown. The cellular sources of the neurotrophins include mast cells, lymphocytes, macrophages, epithelial cells, smooth muscle cells, and eosinophils. The activation of neurotrophin receptors in immune cells and neurons involves ligand-induced homodimerization, which leads to activation of intrinsic Trk receptor kinase. The exact consequences of activating these receptors on immune cells is unknown, but rather than having unique actions on immune cells, the neurotrophins appear to act in concert with known immune regulating factors to modulate the maturation, accumulation, proliferation, and activation of immune cells. Neurotrophins can modulate afferent nerve function by stimulating the production of neuropeptides within airway afferent neurons. These neuropeptides may be released from the central terminals of airway afferent neurons, which leads to heightened autonomic reflex activity, and increased reactivity in the airways.

Trypsin-induced, neurokinin-mediated contraction of guinea pig bronchus

Proteases may act as cell signaling molecules via protease-activated receptors (PARs). PAR1, PAR3, and PAR4, but not PAR2, are activated by thrombin, whereas trypsin can activate PAR2 and PAR4. In this study, trypsin (3-100 nM) evoked concentration-dependent contractions of guinea pig isolated bronchus, however, thrombin (3-300 nM) was a weak spasmogen. Neither the PAR2-activating peptide SLIGRL (100 microM) nor mast cell tryptase (100 nM), a trypsin-like protease known to activate PAR2, evoked contraction. A role for neurokinins in trypsin-induced contraction is suggested by our observation that contractions to trypsin were markedly attenuated in the presence of neurokinin receptor antagonists. Depletion of neurokinins in sensory nerves with capsaicin also markedly reduced the ability of trypsin to evoke contraction. In electrophysiological studies, trypsin did not evoke action potentials in C-fiber afferents whose receptive fields were located in the trachea or main bronchi. The results from this study support the hypothesis that trypsin activates a mechanism allowing for local release of sensory neurokinins from afferent C-fibers and that this release occurs independently of the sensory function of these nerves.

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.

Dexfenfluramine-induced contraction of human and rat isolated pulmonary arteries

Mechanisms of dexfenfluramine-induced vasoconstriction were studied in isolated pulmonary arteries suspended in organ baths for isometric tension recording. Dexfenfluramine (10(-7)-10(-4) M) caused concentration-dependent contractions in rat and human pulmonary arteries with and without endothelium. In pulmonary arteries of the rat, the response to dexfenfluramine was nearly abolished by treatment with the alpha-adrenoceptor antagonists, phentolamine (10(-6) M) or prazosin (10(-7) M). In human pulmonary arteries, the concentration-response curve to dexfenfluramine was unaltered by the presence of phentolamine (10(-6) M), prazosin (10(-7) M), ketanserin (10(-6) M), or indomethacin (3x10(-6) M). The results suggest that dexfenfluramine causes contraction of pulmonary vascular smooth muscle by multiple mechanisms, one of which involves activation of alpha-adrenoceptors within the blood vessel wall. The mechanisms by which dexfenfluramine causes pulmonary vasoconstriction may differ between rat and human pulmonary arteries.

Nociceptin effects in the airways

The opioid-like heptadecapeptide nociceptin (NC) has the following effects in the airways (investigated in isolated tracheae and bronchi from guinea pig or rat): the electric field stimulation (EFS)-induces release of acetylcholine (ACh), the tachykinin substance P (SP) and calcitonin gene-related peptide (CGRP) is reduced after pretreatment with NC, and EFS-induced tachykinergic nonadrenergic-noncholinergic (NANC) bronchoconstriction is inhibited by NC. Both the NC-mediated inhibition of neurotransmission and of smooth muscle contraction occurred in a concentration-dependent manner. Because these effects were naloxone-insensitive, were blocked by the NC receptor antagonist [F/G]NC(1-13)NH(2), and could be mimicked by the NC analogs, NCNH(2) and NC(1-13)NH(2), it is thought that they are distinct from the classic opioid receptors. That these pharmacological actions of NC are of relevance for airway physiology is highly probable given the presence of NC-immunoreactivity in the nerve fibers of the airways and of opioid-like receptor (ORL-1) transcripts in the jugular ganglia, from where the tachykinin-containing afferents arise.

Nerve growth factor-induced phenotypic switch in guinea pig airway sensory neurons

Immunohistochemistry was combined with retrograde tracing techniques to characterize the effect of nerve growth factor (NGF) on substance P (SP) producing vagal neurons innervating the guinea pig trachea. Fast blue dye instilled into the trachea retrogradely labeled nerve cell bodies located in the nodose and jugular ganglia. In untreated guinea pigs > 99% of the SP-containing neurons labeled with fast blue were located in the jugular ganglia. The SP-positive neurons were small in diameter (23 +/- 1 microm) and were negative for neurofilament immunoreactivity. The fast-blue-positive neurons in the nodose ganglia, by contrast, were large in diameter (40 +/- 3 microm) and were negative for SP immunoreactivity and positive for neurofilament immunoreactivity. After NGF-beta injections into the tracheal wall, approximately 10% of the large-diameter nodose neurofilament-positive neurons projecting fibers to the trachea became SP-positive (p < 0.05). We previously demonstrated that nodose nerve endings supplying the trachea are exquisitely mechanically sensitive, but capsaicin- and bradykinin-insensitive. These results suggest that NGF not only increases SP expression in airway neurons, but changes the neuronal phenotype such that large, capsaicin-insensitive nodose neurons with fast-conducting "Adelta" fibers provide a component of the tachykinergic innervation.

Potassium channel blockade induces action potential generation in guinea-pig airway vagal afferent neurones

Electrophysiological studies of vagal sensory nerves with cell bodies in the nodose ganglion and mechanically sensitive receptive fields in the guinea-pig trachea/bronchus, were performed. Exposure of the mechanically sensitive receptive fields to 4-aminopyridine (100 microM-1 mM) caused pronounced action potential discharge in all fibres studied. Action potential generation was also produced by alpha-dendrotoxin, and in a subset of fibres, by barium. By contrast, neither iberiotoxin, tetraethyl ammonium, glybenclamide, BDS-II, nor apamin caused action potential generation in the vagal afferent nerve fibres. Tetramethylrhodamine dextran was instilled into the trachea to retrogradely label cell bodies within the nodose ganglion. In these cells, 4-aminopyridine caused a large depolarization of the resting membrane potential, concomitant with an increase in input impedance. The data suggest 4-aminopyridine- and alpha-dendrotoxin-sensitive ion channels within the airway afferent nerve membrane hold the resting membrane potential below the threshold for action potential generation. Mechanisms that lead to an inhibition of these channels will likely lead to an increase in excitability of the airway afferent neurones.

Pharmacological examination of the neurokinin-1 receptor mediating relaxation of human intralobar pulmonary artery

The effect of selective tachykinin receptor agonists and antagonists on human isolated intralobar pulmonary arterial rings was investigated. Neither Substance P (SP) nor neurokinin A (NKA) contracted the arteries. Both of these agonists, however, were potent and efficacious at relaxing the arteries that were precontracted with phenylephrine. The negative log (M) EC(50) values for SP and NKA were 9.0 and 8.3, respectively. The neurokinin (NK)-3 selective agonist, senktide-analog, and the NK-2 receptor selective agonist, [beta-Ala(8)]NKA(4-10), caused neither contractions nor relaxations of the arteries, whereas the NK-1 receptor agonist Ac-[Arg6, Sar9, Met(O2)11]SP(6-11) (ASM-SP) relaxed the tissue with a potency similar to SP. The relaxations to SP, NKA, and ASM-SP were competitively antagonized by the selective NK-1 receptor antagonist CP 99994, with a pK(b) in the nanomolar range. Antagonism of the ASM-SP-induced relaxations was also noted with FK 888, RP 67580, and L 732,138, although these antagonists were much less potent than CP 99994 in this regard. Another NK-1 receptor selective antagonist, SR 140333, caused an insurmountable antagonism of the SP-induced relaxations. The NK-1 receptor-mediated relaxations could be blocked by removing the endothelium, or by a combination of N-nitro-L-arginine and indomethacin. Measurement of prostanoid generation revealed that in endothelial-intact but not endothelial-denuded tissue, ASM-SP caused a selective increase in the production of 6-keto-PGF1alpha, the stable metabolite of prostacyclin. The results indicate that stimulation of NK-1 receptors leads to relaxation of human intralobar pulmonary arteries, which is mediated largely by nitric oxide and prostacyclin released from the endothelium of these vessels.

Antigen inhalation unmasks NK-2 tachykinin receptor-mediated responses in vagal afferents

The majority of airway sensory innervation originates from afferent neurons whose somata reside in vagal (nodose and jugular) ganglia. Using guinea pigs immunized with chick ovalbumin, we have discovered that airway inflammation provokes phenotypic changes in the tachykinin responsiveness of nodose neurons. Bath application of substance P (SP; 0.1 to 10 microM) to nodose neurons isolated from guinea pigs with normal uninflamed airways did not elicit measurable changes in resting electrophysiological properties. In sharp contrast, 80% of nodose neurons isolated 24 h after in vivo aerosolized antigen challenge of the airway were depolarized by 100 nM SP. Inhalation of a nonantigenic protein did not evoke the expression of SP responses. Pharmacological analysis revealed that SP responses unmasked by airway inflammation were mediated by neurokinin-2 (NK-2) tachykinin receptors. There are several potential mechanisms for transduction of an "unmasking signal" from the inflamed airway to vagal afferent somata. The vagus nerve may relay the signal, either through anterograde transport and/or nerve impulse activity. Alternatively, a signal generated by airway inflammation may be carried by the circulation to the nodose ganglia. Unilateral vagotomy significantly reduced the percentage of SP-responsive neurons compared with intact controls, suggesting that the vagus nerve is required for unmasking of NK-2 responses.

Adaptation of guinea-pig vagal airway afferent neurones to mechanical stimulation

1. Intracellular and extracellular electrophysiological recording techniques were employed to examine the mechanisms involved in adaptation of guinea-pig airway sensory neurones to suprathreshold mechanical stimulation in vitro. Extracellular recordings performed using an in vitro airway preparation revealed two unambiguously distinct subsets of mechanically sensitive nerve endings in the trachea/bronchus. In one group of fibres, the mechanical stimulus caused a brief burst of action potentials, after which the nerve rapidly adapted. In the other group of fibres, repetitive action potentials were evoked as long as the stimulus was maintained above threshold. 2. The adaptation response strictly correlated with ganglionic origin of the soma. Those fibres derived from the nodose ganglion adapted rapidly, whereas those derived from the jugular ganglion were slowly or non-adapting. 3. Intracellular recordings from airway-identified neurones in isolated intact ganglia revealed that the majority of neurones within either the nodose or jugular ganglion adapted rapidly to prolonged suprathreshold depolarizing current injections. 4. The electrophysiological adaptation of nodose ganglion-derived neurones following prolonged suprathreshold current steps was greatly reduced by 4-aminopyridine. However, 4-aminopyridine did not affect the adaptation of rapidly adapting nodose ganglion-derived nerve endings in response to mechanical stimuli. 5. The data suggest that ganglionic origin dictates adaptive characteristics of guinea-pig tracheal and mainstem bronchial afferent neurones in response to mechanical stimulation. Also, the rapid adaptation of nodose nerve endings in the trachea observed during a mechanical stimulus does not appear to be related to the adaptation observed at the soma during prolonged suprathreshold depolarizing current injections.

Identification and substance P content of vagal afferent neurons innervating the epithelium of the guinea pig trachea

Both the nodose and jugular vagal ganglia provide sensory innervation to the airways. The purpose of this study was to localize and characterize the substance P (SP) content of vagal afferent neurons that project specifically to the tracheal epithelium. A retrograde neuronal tracer, fast blue dye or rhodamine-labeled latex microspheres, was instilled into the guinea pig trachea. After 7 d, the nodose and jugular ganglia were removed, sectioned, and prepared for immunocytochemistry. Sections of tracheal mucosa demonstrated that fast blue dye diffused throughout the airway wall, whereas the rhodamine-labeled microspheres, as expected, did not penetrate the basement membrane and were thus localized to the epithelium. When the diffusible fast blue dye was used, approximately 60% of the labeled neurons were found in the nodose ganglia and 40% in the jugular ganglia. By contrast, when the beads were used to label only epithelial nerve fibers, 97 +/- 1% of the tracheal neurons taking up the dye were derived from jugular neurons, 60 +/- 6% of which contained SP immunoreactivity. These studies demonstrate that, in contrast to the submucosa, nerve fibers innervating the epithelium of the trachea are derived nearly exclusively from neurons with cell bodies in the jugular ganglia.

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.

Characterization of vagal afferent subtypes stimulated by bradykinin in guinea pig trachea

In vitro electrophysiological techniques were used to examine the effect of bradykinin on guinea pig trachea and bronchus afferent nerve endings arising from the nodose or jugular ganglia. The data reveal that bradykinin activates nerve terminals of jugular C and Adelta fibers. Although the fibers were too few in number to study rigorously, bradykinin also stimulated nodose C fibers innervating the trachea and bronchus. In contrast, Adelta fibers arising from the nodose ganglion were unresponsive to bradykinin challenge. The responses in both jugular C and Adelta fiber types were blocked by a selective bradykinin B2 receptor antagonist and were not dependent on the efferent release of sensory neuropeptides. These data indicate that the sensitivity of guinea pig airway afferent fibers to bradykinin is dependent more on the ganglionic origin of the cell body than on the conduction velocity of its axon.

Allergen-induced sensory neuroplasticity in airways

This study investigates the influence of allergic inflammation in airway sensory innervation. We conclude that allergic inflammation in the guinea pig leads to both an increase in excitability, as manifested by an increase in the mechanical sensitivity of the airway nerve endings, and an induction of substance P production in airway sensory neurons. The data are consistent with the hypothesis that the induction of substance P occurs in fast conducting nodose sensory neurons that were previously devoid of this neuropeptide. Thus, allergen challenge is associated with a phenotypic change in the airway tachykinergic innervation. We also provide evidence that nerve growth factor is a potentially important mediator for these effects, and that it is elevated in the bronchoalveolar lavage of asthmatic subjects.

Evidence for an esophageal origin of VIP-IR and NO synthase-IR nerves innervating the guinea pig trachealis: a retrograde neuronal tracing and immunohistochemical analysis

Nonadrenergic noncholinergic (NANC) relaxations of the guinea pig trachea are thought to be mediated by vasoactive intestinal peptide (VIP) and nitric oxide (NO). Physiological studies have indicated that the parasympathetic ganglion neurons mediating NANC relaxations of the guinea pig trachea but not the ganglion neurons mediating cholinergic contractions are in some way associated with the adjacent esophagus. In the present study, we attempted to locate precisely the noncholinergic parasympathetic ganglia innervating the trachealis. Two days after injection of the retrograde neuronal tracer DiI into the trachealis of organotypic cultures of the guinea pig trachea and esophagus, neurons within the myenteric plexus of the esophagus or closely associated with the outer striated longitudinal muscle layers of the esophagus were labeled. Subsequent immunohistochemical analyses revealed that a majority of the retrogradely labeled neurons possessed VIP immunoreactivity (IR) or NO synthase (NOS)-IR or had VIP-IR nerve fibers associated with their cell bodies. By contrast, no labeling of esophageal neurons was seen when the tissue between the trachea and esophagus had been disrupted by blunt dissection prior to tracer injection or when the cultures were treated with the axonal transport inhibitor colchicine. The results of these experiments provide the first direct evidence that VIP-IR and NOS-IR neurons intrinsic to the guinea pig esophagus project axons to the adjacent trachealis. Based on their location and phenotype and the results of our previous studies, it is likely that these neurons are the postganglionic parasympathetic neurons mediating NANC relaxations of the trachealis.

Inhibition of 5-lipoxygenase diminishes neurally evoked tachykinergic contraction of guinea pig isolated airway

The role of endogenous 5-lipoxygenase products in modulating tachykinergic neurotransmission in guinea pig isolated trachea was investigated. Tachykinin-containing afferent nerve fibers were stimulated with either electrical field stimulation or antidromic stimulation of the right vagus nerve. This resulted in contractions of the isolated caudal trachea and bronchus that could be blocked with either tetrodotoxin or a combination of neurokinin-1 and neurokinin-2 receptor antagonists. The 5-lipoxygenase inhibitor ZD 2138 (1 microM) significantly inhibited these neurally mediated tachykinergic contractions, by approximately 50%, yet had no effect on the contractions evoked by stimulating tachykinergic fibers in an action potential-independent fashion with capsaicin or by exogenously applied neurokinin A. The effect of ZD 2138 on action potential-driven tachykinergic contractions was mimicked by pobilukast, pranlukast, montelukast and zafirlukast, four structurally unrelated antagonists of the cysteinyl leukotriene 1 receptor subtype. Pobilukast had no effect on the tachykinergic contraction in tissues pretreated with ZD 2138. Likewise, ZD 2138 had no effect on the tachykinergic contractions in tissues pretreated with pobilukast. Intracellular electrophysiological recording of the membrane properties of jugular ganglion neurons, the source of tachykinins in the guinea pig trachea/bronchus, demonstrated that leukotriene D4 caused a membrane depolarization of vagal afferent C-fiber neurons and an increase in input impedance, both of which were abolished by zafirlukast. Taken together, these data indicate that in the resting guinea pig isolated trachea/bronchus, endogenous 5-lipoxygenase activity leads to the production of cysteinyl leukotrienes that amplify action potential-dependent release of tachykinins from airway afferent nerve fibers.

Nociceptin-induced inhibition of tachykinergic neurotransmission in guinea pig bronchus

Nociceptin is a novel neuropeptide of the opioid peptide family recently identified as the endogenous ligand of the opioid receptor-like "orphan" receptor. Unlike other opioids, nociceptin has hyperalgesic effects in vivo. In the present study, nociceptin was found to inhibit electrical field stimulation-induced tachykinergic contractions of the guinea pig isolated bronchus preparation. The threshold effect was about 1 nM, and at 0.1 microM, nociceptin inhibited contractions evoked by 5-Hz stimulation by more than 50%. This inhibitory effect was found to be mediated by a prejunctional mechanism involving none of the classical (mu, delta and kappa) opioid receptors. Although the hypothesis that the effect of nociceptin was secondary to opioid receptor-like stimulation cannot be pharmacologically addressed, opioid receptor-like-receptor-mRNA was found to be expressed in the upper vagal sensory ganglion, where the cell bodies of the tachykinin-containing sensory neurons are located. Nociceptin immunoreactive nerve fibers in the airway wall, distinct from the tachykinin-containing fibers, were identified as an endogenous source of nociceptin. These data indicate that nociceptin may influence airway physiology by modulating tachykinergic neurotransmission.

Selective stimulation of jugular ganglion afferent neurons in guinea pig airways by hypertonic saline

We evaluated the ability of hyperosmolar stimuli to activate afferent nerves in the guinea pig trachea and main bronchi and investigated the neural pathways involved. By using electrophysiological techniques, studies in vitro examined the effect of hyperosmolar solutions of sodium chloride (hypertonic saline) on guinea pig airway afferent nerve endings arising from either vagal nodose or jugular ganglia. The data reveal a differential sensitivity of airway afferent neurons to activation with hypertonic saline. Afferent fibers (both A delta and C fibers) with cell bodies located in jugular ganglia were much more sensitive to stimulation with hypertonic saline, compared with afferent neurons with cell bodies located in nodose ganglia. Additional studies in vivo demonstrated that inhalation of aerosols of hypertonic saline induced plasma extravasation in guinea pig trachea that was mediated via tachykinin NK1 receptors. Identification of a differential sensitivity of guinea pig airway afferent nerves to hypertonic saline leads to the speculation that airway responses to hyperosmolar stimuli may result from activation of afferent neurons originating predominantly from the jugular ganglion.

Pharmacological analysis of the tachykinin receptors that mediate activation of nonadrenergic, noncholinergic relaxant nerves that innervate guinea pig trachealis

Previous studies indicated that antidromic stimulation of capsaicin-sensitive vagal afferent fibers activated, via peripheral release of tachykinins, nonadrenergic, noncholinergic parasympathetic ganglion neurons that mediate relaxations of guinea pig trachealis. On the basis of the effects of selective agonists and inhibition with a nonselective receptor antagonist (SR 48968), we speculated that tachykinin-mediated activation of neurokinin3 (NK3) receptors might be involved. Using the recently developed NK3-selective receptor antagonist SR 142801, we further assessed the role of NK3 receptors in these relaxant responses. Relaxations of the guinea pig trachea elicited by antidromic stimulation of capsaicin-sensitive vagal afferent nerves were markedly inhibited by 0.3 microM SR 142801 and were abolished by a combination of SR 142801 and either of the NK1-selective receptor antagonists SR 140333 and CP 99994 (0.3 microM each). The NK3 receptor antagonist had similar effects on the relaxant responses elicited by capsaicin and substance P, but it had no effect on relaxations of the trachealis elicited by electrical field stimulation of the postganglionic nerves that innervate the trachealis or by stimulation of the preganglionic parasympathetic vagal nerves that innervate the trachea. These results and the observation that the ganglion neurons that mediate these responses are densely innervated by substance P-containing nerve fibers lead us conclude that stimulation of capsaicin-sensitive visceral afferent fibers activates, upon peripheral release of tachykinins, nonadrenergic, noncholinergic inhibitory neurons innervating guinea pig trachealis via activation of both NK3 and NK1 receptors.

Enhancement of tachykinin-induced contractions of guinea pig isolated bronchus by corticotropin-releasing factor

The effect of corticotropin-releasing factor (CRF) on contractions of guinea pig isolated airways in response to electrical or chemical stimulation of tachykinergic nerve fibers was studied. CRF (1 microgram/ml) caused a 70% enhancement of the peak magnitude of the response to electrical field stimulation (EFS). CRF had a similar effect on contractions of the isolated bronchus evoked by capsaicin. CRF also potentiated contractions evoked by exogenously applied substance P. This effect was selective, as CRF has no effect on contractions evoked by neurokinin A or the substance P analog ASMSP. The potentiation of the substance P-induced contractions of airway smooth muscle was blocked by the CRF receptor antagonist alpha helical (9-41) CRF. These data support the hypothesis that CRF enhances the airway smooth muscle response to stimulation of tachykinin-containing nerve fibers and that this effect is due to a post-junctional mechanism of action.

Cysteinyl leukotrienes induce P-selectin expression in human endothelial cells via a non-CysLT1 receptor-mediated mechanism

Cysteinyl leukotrienes are bioactive lipid mediators known to possess potent proinflammatory actions. Included in these are effects on vascular endothelium to promote surface expression of the adhesion molecule P-selectin. In the present study we were interested in investigating the receptor mechanism(s) involved in cysteinyl leukotriene-induced endothelial P-selectin expression. As such we examined the effect of several potent and selective cysteinyl leukotriene receptor antagonists on this response. Incubation of cultured human umbilical vein endothelial cells (HUVEC) with the cysteinyl leukotrienes leukotriene C4 (LTC4) or leukotriene D4 (LTD4) induced surface expression of P-selectin which was concentration dependent and rapid in onset. Expression of endothelial P-selectin induced by either LTC4 or LTD4 was not blocked however by pretreatment of HUVEC with the selective cysteinyl leukotriene-1 (CysLT1) receptor antagonists SKF 104353, pranlukast or zafirlukast before agonist exposure. In contrast, SKF 104353 effectively antagonized the LTC4-induced contractions in isolated human bronchial smooth muscle preparations, shifting the agonist dose-response curve to the right by some 3 log-fold in this tissue. The present results suggest that cysteinyl leukotrienes induce surface expression of endothelial P-selectin via a mechanism independent of the CysLT1 receptor.

Tachykinin-independent effects of capsaicin on smooth muscle in human isolated bronchi

Contractile and relaxant responses to capsaicin and resiniferatoxin were examined in human isolated bronchus (5-12 mm o.d.). Bronchi isolated from 10 of 16 lungs contracted in response to capsaicin. The contractions averaged 20% of maximal contraction at 1 microM and averaged > 40% maximal contraction at 300 microM (the highest concentration studied). The capsaicin-induced contractions were mimicked by resiniferatoxin (0.1-10 microM) and inhibited by the putative capsaicin receptor antagonist, capsazepine (10 microM). The contractile response to capsaicin was not affected by the potent NK-2 selective antagonist SR 48968 (0.3 microM), whereas responses to concentrations of neurokinin A (10 nM), neurokinin B (0.1 microM), substance P (1 microM), neuropeptide gamma (10 nM), and neuropeptide K (10 nM) which produced similar-size contractions were almost abolished by 0.1 microM SR 48968. The bronchi isolated from 8 of 16 lungs also exhibited relaxations in response to capsaicin. Capsaicin-induced relaxations were not inhibited by the nitric oxide synthase inhibitor L-nitro-n-arginine (10 microM). In whole-cell patch-clamp experiments on human cultured airway smooth muscle cells, capsaicin was found to enhance outward currents due to the activation of charybdotoxin-sensitive large conductance Ca2+-activated K+ channels. Neither the capsaicin-induced contractions nor the relaxations were mimicked by angiotensin II, bombesin, or calcitonin gene-related peptide at concentrations up to 1 microM. These results suggest that capsaicin and resiniferatoxin can alter smooth muscle tone, but this response does not appear to involve substance P or related neurokinins. Relaxations to capsaicin may, however, involve the activation of large conductance Ca2+-activated K+ channels.

Protective effect of substance P on permeability of airway epithelial cells in culture

Although substance P (SP) has been shown to mediate microvascular leakage in response to various stimuli, some data suggest that, in contrast, SP may play a protective role in the maintenance of airway epithelial integrity. To investigate the effect of SP on epithelial barrier function, we measured paracellular mannitol flux and the transepithelial potential difference (PD) of human bronchial epithelial (HBE) and canine bronchial epithelial (CBE) cells. Incubation of confluent cell cultures with SP had no effect on baseline flux. However, pretreatment inhibited the flux-enhancing effects of 0.5 ppm ozone by 50% in HBE cells and 40% in CBE cells and inhibited the ozone-induced decrease in PD in CBE cells by 54%. SP-afforded protection was reduced by the neurokinin (NK)-1 receptor antagonist CP-96,345.NK1 and NK3 receptor agonists also inhibited ozone-induced permeability, whereas an NK2 receptor agonist was without significant effect. These data indicate that SP exerts a protective effect on bronchial epithelial barrier function under conditions of challenge, which appears to be mediated in large part through NK1 receptors.

Effect of nedocromil sodium on neurogenic mechanisms in vitro

We have developed a guinea pig model that allows monitoring of single afferent nerve fiber activity after stimulation of tracheal nerve endings with various stimuli. Action potentials from nodose or jugular neurons are recorded extracellularly. Previous experiments have shown that the excitability of the nerve endings to, for example, mechanical stimuli can be increased by antigen challenge of presensitized guinea pigs. Nedocromil sodium 10(-4) mol/L significantly reduced by more than 50% the generation of action potentials in C fibers in response to stimulation by capsaicin. The precise mechanism is not known, although the membrane depolarization induced by capsaicin was reduced in the presence of nedocromil sodium. This agent did not have a generalized inhibitory effect on afferent excitability because it had no effect on sensitivity to electrical or mechanical stimuli. Nedocromil sodium did not affect the generation of action potentials in A delta and C fibers in the presence of hypertonic saline solution.

Effects of organotypic culture on parasympathetic innervation of guinea pig trachealis

Nonadrenergic, noncholinergic (NANC) relaxations of airway smooth muscle are thought to be mediated by vasoactive intestinal peptide (VIP) and nitric oxide (NO). Previous studies of the parasympathetic innervation of guinea pig trachealis suggest that the ganglion neurons mediating NANC relaxations but not cholinergic contractions are associated with the esophagus. In this study, the location of the neurons mediating these responses and their neurochemical phenotype was further assessed. Guinea pig tracheas maintained in organotypic culture for 2 days with the adjacent esophagus intact displayed cholinergic contractions and NANC relaxations to electrical field stimulation (EFS) as well as VIP and NO synthase (NOS) nerve fiber densities that were similar to those of control tracheas. By contrast, in tracheas cultured without the esophagus, NANC relaxations to EFS were not observed, and VIP and NOS nerve fiber densities were reduced > 80%. EFS-induced cholinergic contractions were unaffected by esophagus removal. These results provide further evidence that NANC relaxations are mediated by VIP and NO coreleased from noncholinergic parasympathetic nerve endings derived from neurons intrinsic to the esophagus.

Interganglionic segregation of distinct vagal afferent fibre phenotypes in guinea-pig airways

1. The present study addressed the hypothesis that jugular and nodose vagal ganglia contain the somata of functionally and anatomically distinct airway afferent fibres. 2. Anatomical investigations were performed by injecting guinea-pig airways with the neuronal tracer Fast Blue. The animals were killed 7 days later, and the ganglia were removed and immunostained with antisera against substance P (SP) and neurofilament protein (NF). In the nodose ganglion, NF-immunoreactive neurones accounted for about 98% of the Fast Blue-labelled cells while in the jugular ganglion they accounted for approximately 48%. SP and NF immunoreactivity was never (n = 100) observed in the same cell suggesting that the antisera labelled distinct populations. 3. Electrophysiological investigations were performed using an in vitro guinea-pig tracheal and bronchial preparation with intact afferent vagal pathways, including nodose and jugular ganglia. Action potentials arriving from single airway afferent nerve endings were monitored extracellularly using a glass microelectrode positioned near neuronal cell bodies in either ganglion. 4. The nodose ganglion contained the somata of mainly fast-conducting tracheal A delta fibres whereas the jugular ganglion contained equal numbers of C fibre and A delta fibre tracheal afferent somata. The nodose A delta neurones adapted rapidly to mechanical stimulation, had relatively low mechanical thresholds, were not activated by capsaicin and adapted rapidly to a hyperosmotic stimulus. By contrast, jugular A delta and C fibres adapted slowly to mechanical stimulation, were often activated by capsaicin, had higher mechanical thresholds and displayed a slow adaptation to a hyperosmotic stimulus. 5. The anatomical, physiological and pharmacological data provide evidence to support the contention that the vagal ganglionic source of the fibre supplying the airways ultimately dictates its neurochemical and physiological phenotype.

Inhibition by zinc protoporphyrin-IX of vasoactive intestinal peptide-induced relaxations of guinea pig isolated trachea

Carbon monoxide, formed as a product of heme oxygenase activity, has been postulated to act as an intra- and intercellular messenger molecule. We addressed the hypothesis that heme oxygenase is involved in the relaxation of the guinea pig trachealis elicited by vasoactive intestinal peptide (VIP) or by electrical field stimulation. Immunohistochemical studies revealed the presence of heme oxygenase-II in airway smooth muscle and epithelium. Zinc protoporphyrin-IX (ZnPPn), an inhibitor of heme oxygenase, effectively inhibited VIP-induced relaxations of tracheal smooth muscle. Surprisingly, the potency of ZnPPn was increased if the drug was preincubated with the VIP solution before addition to the tissue bath. The relaxant responses to 3-morpholinosydnonimine were unaffected by ZnPPn. Zinc deuteroporphyrin-IX 2,4 bisglycol, a more potent inhibitor of heme oxygenase than ZnPPn, did not affect the VIP responses. ZnPPn (300 microM) had no effect on nonadrenergic, noncholinergic relaxations of the guinea pig trachea. These data indicate that although ZnPPn is an efficacous inhibitor of VIP-induced relaxations of the guinea pig trachealis, it is unlikely that heme oxygenase plays an important role in this response. Rather, the data are consistent with the hypothesis that ZnPPn inhibits the VIP response via an interaction with the VIP molecules themselves. Although the results demonstrate the existence of heme oxygenase-II in the guinea pig trachealis, they do not support the hypothesis that it plays a role in electrical field stimulation-induced nonadrenergic, noncholinergic relaxations.

Muscarinic receptor regulation of synaptic transmission in airway parasympathetic ganglia

Muscarinic receptor regulation of synaptic transmission in guinea pig bronchial parasympathetic ganglia was evaluated with the use of intracellular recording of the intrinsic ganglion neurons. Methacholine (1 microM) decreased the amplitude of vagus nerve-stimulated fast excitatory postsynaptic potentials (fEPSP) by 33 and 46% (at 0.8 and 8.0 Hz, respectively) but had no effect on the amplitude of the depolarizations evoked by a bath-applied nicotinic receptor agonist. Methoctramine (1 microM) inhibited methacholine's effect on fEPSP but alone did not influence the magnitude of the fEPSP evoked by vagus nerve stimulation. Methacholine (10 microM) depolarized a subpopulation of neurons (approximately 4 mV), which was blocked by pirenzepine (0.1 microM). In other neurons, either no effect or a small (1-5 mV) hyperpolarization was noted. Cholinergic contractions of bronchial smooth muscle elicited by electrical field stimulation were potentiated by methoctramine to the same extent as those evoked by vagus nerve (preganglionic) stimulation. The data indicate that M2 receptor activation can lead to inhibition of presynaptic acetylcholine release and consequently a significant inhibition of synaptic transmission in bronchial parasympathetic ganglia. The physiological role of this neuromodulatory effect appears limited, however, when studied in the in vitro setting.

Immunomodulation of afferent neurons in guinea-pig isolated airway

1. The trachea, larynx and main bronchi with the right vagus nerve and nodose ganglion were isolated from guinea-pigs passively immunized 24 h previously with serum containing anti-ovalbumin antibody. 2. The airways were placed in one compartment of a Perspex chamber for recording of isometric tension while the nodose ganglion and attached vagus nerve were pulled into another compartment. Action potentials arriving from single airway afferent nerve endings were monitored extracellularly using a glass microelectrode positioned near neuronal cell bodies in the ganglion. Mechanosensitivity of the nerve endings was quantified using calibrated von Frey filaments immediately before and after exposure to antigen (10 micrograms ml-1 ovalbumin). 3. Ten endings responded to the force exerted by the lowest filament (0.078 mN) and were not further investigated. In airways from thirteen immunized guinea-pigs, the mechanical sensitivity of A delta afferent fibres (conduction velocity = 4.3 +/- 0.6 m s-1) was enhanced 4.1 +/- 0.9-fold following airway exposure to antigen (P < 0.005). Mechanical sensitivities of afferent fibres (conduction velocity = 4.3 +/- 0.6 m s-1) from non-immunized control guinea-pig airways were unaffected by antigen (n = 13). 4. Antigen did not overtly cause action potential generation except in one instance when the receptive field was located over the smooth muscle. This ending also responded to methacholine suggesting that spatial changes in the receptive field, induced by muscle contraction, were responsible for the activation. 5. The mediators responsible for these effects are unknown, although histamine, prostaglandins, leukotrienes and tachykinins do not appear to be essential. The increase in mechanical responsiveness was not associated with the smooth muscle contraction since leukotriene C4, histamine and tachykinins, which all caused a similar contraction to antigen, did not affect mechanical thresholds. Moreover, the antigen-induced increases in excitability persisted beyond the duration of the smooth muscle contraction. 6. These results demonstrate that antigen-antibody-mediated inflammatory processes may enhance the excitability of vagal afferent nerve terminals projecting from the airway and thus may contribute to the pathophysiology of allergic airway diseases.

Pharmacological characterization of a new class of nonpeptide neurokinin A antagonists that demonstrate species selectivity

We examined the pharmacology of ZM253,270 and two representative examples of the pyrrolopyrimidines, a new class of nonpeptide, NK-2 receptor (NK-2R) antagonists. ZM253,270 competitively inhibited [3H]NKA binding to native or cloned NK-2R from hamster urinary bladder (Ki = 2 nM), but was a weaker (48-fold) inhibitor of [3H]NKA binding to cloned human NK-2R. A similar species selectivity was observed with less potent analogs of ZM253,270. The pyrrolopyrimidines demonstrated only marginal inhibition of [3H]SP binding to NK-1R in guinea pig lung membranes (Ki > 2 microM). In hamster trachea, ZM253,270 competitively antagonized the contractile response evoked by neurokinin A (NKA, -logKB = 7.5). In human bronchus, ZM253,270 was about 90-fold less potent as a competitive antagonist of NKA. The data from ligand binding assays in cloned receptors combined with functional receptor assays in airway smooth muscles, demonstrate that the nonpeptide antagonist ZM253,270 is selective for the NK2 receptor species that are prevalent in hamster, compared with those found in human tissues.

Cysteinyl leukotrienes in asthma: old mediators up to new tricks

The cysteinyl leukotrienes have long been suspected to play a role in the pathogenesis of asthma. This speculation was based largely on their release in human lung following antigen challenge as well as their potent bronchoconstrictor activity. However, there is increasing evidence that the cysteinyl leukotrienes also produce several pro-inflammatory effects and alter the activity of neuronal pathways in the airways. Douglas Hay, Theodore Torphy and Bradley Undem review these recent data and discuss the therapeutic possibilities of cysteinyl leukotriene receptor antagonists and 5-lipoxygenase inhibitors.