Rapid shallow breathing evoked by selective stimulation of airway C fibres in dogs

Potential pharmaceuticals targeting neuroimmune interactions in treating acute lung injury

BACKGROUND AND MAIN BODY

Although interactions between the nervous and immune systems have been recognized decades ago, it has become increasingly appreciated that neuroimmune crosstalk is among the driving factors of multiple pulmonary inflammatory diseases including acute lung injury (ALI). Here, we review the current understanding of nerve innervations towards the lung and summarize how the neural regulation of immunity and inflammation participates in the onset and progression of several lung diseases, especially ALI. We then present advancements in the development of potential drugs for ALI targeting neuroimmune interactions, including cholinergic anti-inflammatory pathway, sympathetic-immune pathway, purinergic signalling, neuropeptides and renin-angiotensin system at different stages from preclinical investigation to clinical trials, including the traditional Chinese medicine.

CONCLUSION

This review highlights the importance of considering the therapeutic strategy of inflammatory diseases within a conceptual framework that integrates classical inflammatory cascade and neuroimmune circuits, so as to deepen the understanding of immune modulation and develop more sophisticated approaches to treat lung diseases represented by ALI.

KEY POINTS

The lungs present abundant nerve innervations. Neuroimmune interactions exert a modulatory effect in the onset and progression of lung inflammatory diseases, especially acute lung injury. The advancements of potential drugs for ALI targeting neuroimmune crosstalk at different stages from preclinical investigation to clinical trials are elaborated. Point out the direction for the development of neuroimmune pharmacology in the future.

Regulation of breathing by cardiopulmonary afferents

This chapter broadly reviews cardiopulmonary sympathetic and vagal sensors and their reflex functions during physiologic and pathophysiologic processes. Mechanosensory operating mechanisms, including their central projections, are described under multiple sensor theory. In addition, ways to interpret evidence surrounding several controversial issues are provided, with detailed reasoning on how conclusions are derived. Cardiopulmonary sensory roles in breathing control and the development of symptoms and signs and pathophysiologic processes in cardiopulmonary diseases (such as cough and neuroimmune interaction) also are discussed.

Identification of lung innervating sensory neurons and their target specificity

Known as the gas exchange organ, the lung is also critical for responding to the aerosol environment in part through interaction with the nervous system. The diversity and specificity of lung innervating neurons remain poorly understood. Here, we interrogated the cell body location and molecular signature and projection pattern of lung innervating sensory neurons. Retrograde tracing from the lung coupled with whole tissue clearing highlighted neurons primarily in the vagal ganglia. Centrally, they project specifically to the nucleus of the solitary tract in the brainstem. Peripherally, they enter the lung alongside branching airways. Labeling of nociceptor Trpv1+ versus peptidergic Tac1+ vagal neurons showed shared and distinct terminal morphology and targeting to airway smooth muscles, vasculature including lymphatics, and alveoli. Notably, a small population of vagal neurons that are Calb1+ preferentially innervate pulmonary neuroendocrine cells, a demonstrated airway sensor population. This atlas of lung innervating neurons serves as a foundation for understanding their function in lung.

Molecular/Ionic Basis of Vagal Bronchopulmonary C-Fiber Activation by Inflammatory Mediators

Stimulation of bronchopulmonary vagal afferent C fibers by inflammatory mediators can lead to coughing, chest tightness, and changes in breathing pattern, as well as reflex bronchoconstriction and secretions. These responses serve a defensive function in healthy lungs but likely contribute to many of the signs and symptoms of inflammatory airway diseases. A better understanding of the mechanisms underlying the activation of bronchopulmonary C-fiber terminals may lead to novel therapeutics that would work in an additive or synergic manner with existing anti-inflammatory strategies.

Neural Sensing of Organ Volume

Many internal organs change volume periodically. For example, the stomach accommodates ingested food and drink, the bladder stores urine, the heart fills with blood, and the lungs expand with every breath. Specialized peripheral sensory neurons function as mechanoreceptors that detect tissue stretch to infer changes in organ volume and then relay this information to the brain. Central neural circuits process this information and evoke perceptions (satiety, nausea), control physiology (breathing, heart rate), and impact behavior (feeding, micturition). Yet, basic questions remain about how neurons sense organ distension and whether common sensory motifs are involved across organs. Here, we review candidate mechanosensory receptors, cell types, and neural circuits, focusing on the stomach, bladder, and airways. Understanding mechanisms of organ stretch sensation may provide new ways to treat autonomic dysfunction.

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.

Characteristic plethysmographic findings in a guinea pig model of COPD

AIM OF THE STUDY

Long-term exposure to cigarette smoke generates chronic obstructive pulmonary disease (COPD) in guinea pigs, but a comprehensive evaluation of changes in lung function, as assessed by barometric whole body plethysmography (WBP), is lacking.

MATERIALS AND METHODS

Female guinea pigs were exposed to the smoke of 20 cigarettes/day, 5 days/week, during 10 weeks (COPD group, n = 8), and were compared with unexposed female guinea pigs of the same age (control group, n = 8). WBP was performed in both groups, followed by lung histology.

RESULTS

At the end of the exposure period, guinea pigs in the COPD group had higher respiratory frequency, while duty cycle (Ti/Ttot) was unaffected. There was a trend toward minute ventilation (MV) and expiratory flow at the mid-tidal volume (EF50) to be higher in the COPD group. Enhanced pause (Penh) was lower, while time of braking (TB) and time to PEF relative to Te (Rpef) were higher in the COPD group. All guinea pigs exposed to tobacco smoke developed emphysematous lesions in their lungs and gained less body weight than controls.

CONCLUSIONS

In this COPD model, exposure to cigarette smoke produced changes in WBP characterized by a shallow breathing pattern with decreased Penh and a trend toward increasing EF50 (probably due to decreased elastic recoil), increased TB (suggesting dynamic laryngeal narrowing), and a trend of increasing MV (probably due to a higher metabolic rate). Many of these functional changes resemble those observed in patients with COPD and corroborate the suitability of this guinea pig model for the study of COPD.

Restrictive Lung Function Is Related to Sympathetic Hyperactivity in Patients With Heart Failure

BACKGROUND

Sympathoexcitation and impaired lung function are common in patients with severe heart failure (HF). However, the association between impaired lung function and sympathoexcitation remains unknown.

METHODS AND RESULTS

Muscle sympathetic nerve activity (MSNA) and clinical variables were determined in 83 HF patients with left ventricular ejection fraction (LVEF) <0.45. Restrictive and obstructive changes on spirometry were defined as reduced forced vital capacity (FVC) of <80% of predicted and a ratio of forced expiratory volume in the first second to FVC of <70%, respectively. Restrictive and obstructive changes were identified in 17 and 21 patients, respectively. MSNA was higher in patients with restrictive changes than in those without restrictive changes (84 vs 66 bursts per 100 beats; P < .01), but was similar in those with and without obstructive changes. Univariate analyses showed that FVC, estimated glomerular filtration rate (eGFR), specific activity scale, B-type natriuretic peptide level, LVEF, age, and use of aldosterone receptor blockers were significant predictors of MSNA burst incidence. Multivariate analysis revealed that FVC, LVEF, and eGFR were independent factors for increased burst incidence. Changes in FVC during follow-up negatively correlated with changes in burst rate (n = 11; P < .01).

CONCLUSION

Restrictive lung function was associated with increased sympathetic nerve activity independently from HF severity.

Vagal Sensory Neuron Subtypes that Differentially Control Breathing

Breathing is essential for survival and under precise neural control. The vagus nerve is a major conduit between lung and brain required for normal respiration. Here, we identify two populations of mouse vagus nerve afferents (P2ry1, Npy2r), each a few hundred neurons, that exert powerful and opposing effects on breathing. Genetically guided anatomical mapping revealed that these neurons densely innervate the lung and send long-range projections to different brainstem targets. Npy2r neurons are largely slow-conducting C fibers, while P2ry1 neurons are largely fast-conducting A fibers that contact pulmonary endocrine cells (neuroepithelial bodies). Optogenetic stimulation of P2ry1 neurons acutely silences respiration, trapping animals in exhalation, while stimulating Npy2r neurons causes rapid, shallow breathing. Activating P2ry1 neurons did not impact heart rate or gastric pressure, other autonomic functions under vagal control. Thus, the vagus nerve contains intermingled sensory neurons constituting genetically definable labeled lines with different anatomical connections and physiological roles.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Effects of oleic acid-induced lung injury on oxygen transport and aerobic capacity

We tested the hypothesis that oleic-acid (OA) infusion impairs gas exchange, decreases total cardiopulmonary O2 delivery and lowers maximal aerobic capacity ( [Formula: see text] ). We infused 0.05ml OAkg(-1) (∼3ml) and ∼563ml saline into the right atria of four goats [59.1±14.0 (SD) kg] prior to running them on a treadmill at [Formula: see text] 2-h and 1-d following OA-induced acute lung injury, and with no lung injury. Acute lung injury decreased [Formula: see text] , O2 delivery, arterial O2 concentration and arterial O2 partial pressure compared to no lung injury. The [Formula: see text] positively correlated with O2 delivery and inversely correlated with alveolar-arterial O2 partial pressure difference, suggesting that impaired pulmonary gas exchange decreased O2 delivery and uptake. Results indicate OA infusion may be a useful model for acutely impairing pulmonary gas exchange for exercise studies. Seven OA infusions induced smaller chronic gas exchange and arterial O2 partial pressure changes than acute infusion.

Increased parasympathetic tone as the underlying cause of asthma: a hypothesis

Asthma is a chronic inflammatory disease of airways that is characterized by increased responsiveness of the tracheo-bronchial tree to multiple number of stimuli. Immunological theory does not explain all features in asthma, for example hyper-reactivity of the airways. Neurogenic theory also fails to explain the pathogenesis of asthma comprehensively. Higher parasympathetic tone has been reported in asthmatics but has never been suggested as a major underlying cause of asthma. This article attempts to explain the occurrence of hyper-responsiveness, inflammatory/allergic reactions and broncho-constriction in asthma on a common basis of inherent higher parasympathetic tone in asthmatics. The higher background parasympathetic firing leads to increased nitric oxide (NO) production owing to its co-localization with acetylcholine (ACh) in inhibitory non-adrenergic and non-cholinergic (i-NANC) nerves. NO is a neurotransmitter of i-NANC system and it mediates bronchodilation. Increased NO release has been found to be responsible for hyper-responsiveness and increased inflammation in the airways. The authors suggest that an inherently higher background parasympathetic tone in concert with inflammation or a specific genetic background could modify the effects of NO on lung homeostasis in humans leading to increased susceptibility to an asthmatic state.

Cough sensors. I. Physiological and pharmacological properties of the afferent nerves regulating cough

The afferent nerves regulating cough have been reasonably well defined. The selective effects of general anesthesia on C-fiber-dependent cough and the opposing effects of C-fiber subtypes in cough have led to some uncertainty about their regulation of this defensive reflex. But a role for C-fibers in cough seems almost certain, given the unique pharmacological properties of these unmyelinated vagal afferent nerves and the ability of many C-fiber-selective stimulants to evoke cough. The role of myelinated laryngeal, tracheal, and bronchial afferent nerve subtypes that can be activated by punctate mechanical stimuli, inhaled particulates, accumulated secretions, and acid has also been demonstrated. These "cough receptors" are distinct from the slowly and rapidly adapting intrapulmonary stretch receptors responding to lung inflation. Indeed, intrapulmonary rapidly and slowly adapting receptors and pulmonary C-fibers may play no role or a nonessential role in cough, or might even actively inhibit cough upon activation. A critical review of the studies of the afferent nerve subtypes most often implicated in cough is provided.

Bradykinin B2 receptors mediate pulmonary sympathetic afferents induced reflexes in rabbits

Endogenous bradykinin (BK) is an established mediator of pulmonary inflammation, yet its role in lung disease is unclear. In the rabbit, injecting BK into the lung parenchyma elicits reflex hyperpnea, tachypnea, hypotension, and bradycardia by stimulating pulmonary sympathetic afferents. To further explore bradykinin effects, breathing pattern (phrenic nerve and abdominal muscle activities) and hemodynamics (blood pressure and heart rate) were examined in anesthetized, open-chest, and mechanically ventilated rabbits. Three receptor agonists [bradykinin, selective B(1) (des-Arg(9)-BK), and selective B(2) (Tyr(8)-BK)], as well as three B(2) receptor antagonists, B6029 (N alpha-Adamantaneacetyl)-Bradykinin, B(1)650 (D-Arg-[Hyp(3), Thi(5,8), D-Phe(7)]-Bradykinin, or Hoe-140 (D-Arg-[Hyp(3), Thi(5), D-Tic(7), Oic(8)] bradykinin), were used to identify the responsible receptor subtype. In both intact and vagotomized rabbits, injecting BK or a selective B(2) agonist into the lung elicited similar cardiopulmonary responses. These reflex responses were greatly attenuated or blocked by pre-injecting B(2) antagonists into the right atrium or into the lung parenchyma. In contrast, the B(1) agonist elicited fewer cardiopulmonary effects in intact rabbits and had no effect in vagotomized rabbits. We conclude that BK stimulates pulmonary sympathetic afferents [Soukhova, G., Wang, Y., Ahmed, M., Walker, J., Yu, J., 2003. Bradykinin stimulates respiratory drive by activating pulmonary sympathetic afferents in the rabbit. J. Appl. Physiol. 95, 241-249.; Wang, Y., Soukhova, G., Proctor, M., Walker, J., Yu, J., 2003. Bradykinin causes hypotension by activating pulmonary sympathetic afferents in the rabbit. J. Appl. Physiol. 95, 233-240.], eliciting a characteristic cardiopulmonary reflex via B(2) receptors.

Bradykinin causes hypotension by activating pulmonary sympathetic afferents in the rabbit

Bradykinin (BK) activates sympathetic afferents in the heart, intestine, and kidney, and it alters hemodynamics. However, we know little about the influence of pulmonary sympathetic afferents on circulation. Activation of pulmonary afferents by directly injecting stimulants into the lung parenchyma permits examination of reflexes that originate in the lung without confounding effects from the systemic circulation. In the present study, we tested the hypothesis that pulmonary sympathetic afferents exert a significant influence on hemodynamics. We examined reflex effects of injecting BK (1 microg/kg in 0.1 ml) into the lung parenchyma on circulation in anesthetized, open-chest, artificially ventilated rabbits. BK significantly decreased mean arterial blood pressure (BP) (27 +/- 3 mmHg) and heart rate (19 +/- 4 beats/min). Both effects remained after bilateral vagotomy. To rule out possible direct systemic vasodilation by BK, we examined renal sympathetic nerve activity (RSNA) in response to BK injection and examined BP responses to injection of ACh (0.1 ml of 10-4 M). BK suppressed the RSNA before and after vagotomy. ACh did not change BP when injected into the lung parenchyma, but it decreased BP (31 +/- 3 mmHg) when injected into the right atrium. Our data indicate that activating pulmonary sympathetic afferents reflexly suppresses hemodynamics.

Bradykinin stimulates respiratory drive by activating pulmonary sympathetic afferents in the rabbit

We recently identified a vagally mediated excitatory lung reflex by injecting hypertonic saline into the lung parenchyma (Yu J, Zhang JF, and Fletcher EC. J Appl Physiol 85: 1485-1492, 1998). This reflex increased amplitude and burst rate of phrenic (inspiratory) nerve activity and suppressed external oblique abdominal (expiratory) muscle activity. In the present study, we tested the hypothesis that bradykinin may activate extravagal pathways to stimulate breathing by assessing its reflex effects on respiratory drive. Bradykinin (1 microg/kg in 0.1 ml) was injected into the lung parenchyma of anesthetized, open-chest and artificially ventilated rabbits. In most cases, bradykinin increased phrenic amplitude, phrenic burst rate, and expiratory muscle activity. However, a variety of breathing patterns resulted, ranging from hyperpnea and tachypnea to rapid shallow breathing and apnea. Bradykinin acts like hypertonic saline in producing hyperpnea and tachypnea, yet the two agents clearly differ. Bradykinin produced a higher ratio of phrenic amplitude to inspiratory time and had longer latency than hypertonic saline. Although attenuated, bradykinin-induced respiratory responses persisted after vagotomy. We conclude that bradykinin activates multiple afferent pathways in the lung; portions of its respiratory reflexes are extravagal and arise from sympathetic afferents.

Influence of bronchial C fibre receptors on respiration in cats: possible role in humans

In order to find out whether the bronchial C fibre receptors influence respiration in cats, as the J receptors do, two drugs known to stimulate them consistently, i.e. phenyl diguanide (PDG) and capsaicin were injected into the left atrium (LA). The influence of these drugs, in suprathreshold doses, was seen on the intrapleural pressure (IPP), frequency of respiration (f(R)), systemic blood pressure (BP) and cardiac frequency (f(H)). PDG LA produced an inhibitory influence on respiration of a majority of cats, accompanied by a fall in BP and f(H). With capsaicin, a stimulation of respiration and a notable increase in BP (sometimes preceded by a fall also of f(H)) was seen. After blocking the epicardial/cardiac receptors with xylocaine, a local anaesthetic, the effects of PDG LA on the above variables were blocked, whereas, those of capsaicin survived--which they did, also after a bilateral vagotomy. Similar responses were obtained from injecting the drugs into the distal part of the aorta i.e. a notable stimulation of respiration by capsaicin, which survived vagotomy and hardly any effect, by PDG. It is concluded that activation of bronchial C fibre receptors does not influence respiration or BP in cats.

5-HT(4) receptors in nucleus tractus solitarii attenuate cardiopulmonary reflex in anesthetized rats

We determined whether the cAMP-protein kinase A (PKA) pathway modulation of the cardiopulmonary reflex was caused by activation of 5-HT(4) receptors at the level of the nucleus tractus solitarii (NTS) of the anesthetized rat. NTS microinjection of 5-methoxytryptamine (5-MeOT, 2.25 pmol, n = 13), a 5-HT-receptor agonist, attenuated the cardiopulmonary reflex-evoked bradycardia and tachypnea. Microinjection of RS-39604 (4.5 pmol, n = 6), a selective 5-HT(4)-receptor antagonist, blocked the attenuating effect of 5-MeOT. NTS microinjection of 8-bromoadenosine 3', 5'-cyclic monophosphate (8-BrcAMP, 9 nmol, 45 nl, n = 10), a membrane-permeant analog of cAMP, significantly attenuated the reflex bradycardia and tachypnea. Rp-adenosine 3',5'-cyclic monophosphorothioate (4.5 nmol, n = 6), a cAMP-dependent PKA inhibitor, had no effect on the cardiopulmonary reflex when microinjected into the NTS alone but when given before a microinjection of either 8-BrcAMP (n = 6) or 5-MeOT (n = 6) blocked the attenuating effect on the reflex-evoked bradycardia. Thus stimulation of 5-HT(4) receptors within the NTS depresses the reflex bradycardia components of the cardiopulmonary reflex via a cAMP-dependent PKA pathway.

Shock after blast wave injury is caused by a vagally mediated reflex

OBJECTIVE

Bomb blast survivors occasionally suffer from profound shock and hypoxemia without signs of external injury. We hypothesize that a vagally mediated reflex such as the pulmonary defensive reflex is the cause of shock from blast wave injury. This study was a prospectively randomized, controlled animal study.

METHODS

By using a previously described model of blast wave injury, we randomized rats to one of four groups: control, blast-only, bilateral cervical vagotomy plus atropine 200 microg/kg i.p. only, and bilateral cervical vagotomy plus atropine 200 microg/kg i.p. before blast injury. Cardiopulmonary parameters were recorded for 90 minutes after the blast or until death.

RESULTS

Bradycardia, hypotension, and absence of compensatory peripheral vasoconstriction, typically seen in animals subjected to a blast pressure injury, were prevented by bilateral cervical vagotomy and intraperitoneal injection of atropine methyl-bromide. Hypoxia and lung injury were not statistically significant between the blasted groups, suggesting equivalent injury.

CONCLUSION

Our data implicate a vagally mediated reflex such as the pulmonary defensive reflex as the cause of shock seen immediately after a blast pressure wave injury.

Inhibition of sodium metabisulphite induced bronchoconstriction by frusemide in asthma: role of cyclooxygenase products

BACKGROUND

Inhaled frusemide inhibits airway responses to sodium metabisulphite and other indirect bronchial challenges in asthma by undetermined mechanisms which may relate to its ability to stimulate prostaglandin release. Inhalation of sodium metabisulphite provokes indirect bronchoconstriction, possibly by activating sensory nerves. To investigate the role of cyclooxygenase products in the airway actions of frusemide and sodium metabisulphite, the effects of a potent cyclooxygenase inhibitor, flurbiprofen, alone and in combination with frusemide were investigated against airway responsiveness to sodium metabisulphite.

METHODS

In a double blind double placebo controlled study, 12 mild asthmatic subjects attended on four occasions to undergo three inhalation challenges with sodium metabisulphite. A baseline challenge was performed one hour before oral intake of flurbiprofen 200 mg or matched placebo, and two hours before inhalation of frusemide 40 mg or matched placebo. A second challenge was performed immediately after inhalation of frusemide (two hours after flurbiprofen) with a further challenge three hours later. The log concentration provoking a 20% fall in FEV1 (log PC20) was used to assess airway responsiveness to sodium metabisulphite.

RESULTS

Frusemide caused an immediate 1.9 doubling dose protection and a lesser 0.7 doubling dose protection at three hours. This protection was enhanced by flurbiprofen at both time points to 2.7 (early) and 1.9 (late) doubling doses. In addition, flurbiprofen alone significantly reduced airway responsiveness to sodium metabisulphite by 1.1 doubling doses at both two and five hours.

CONCLUSIONS

The generation of bronchoprotective prostaglandins is unlikely to underlie the inhibitory action of frusemide against airway responsiveness to sodium metabisulphite. Endogenous contractile prostaglandins within the airways may be involved in the bronchoconstrictor response to sodium metabisulphite.

Effect of bradykinin on respiratory rate in anaesthetized rabbits; role of rapidly adapting receptors

1. This study was performed in anaesthetized, spontaneously breathing rabbits: (a) to determine the effect of bradykinin administered into the right atrium on the respiratory rate, and (b) to elucidate the potential role of rapidly adapting receptors (RARs) in mediating this effect. The role of RARs was established by graded cooling of the cervical vagi. The respiratory rate was measured from an intrapleural pressure tracing. 2. Dose-response curves relating right atrial injections of bradykinin (0.25, 0.5, 1.0 and 1.5 micrograms/kg) to the respiratory rate were established in the control state (i.e. vagi at 37 degrees C). The respiratory rate increased significantly (P < 0.01, ANOVA) from a control value of 51.3 +/- 6.8 breaths/min by 12 +/- 3, 25 +/- 5, 43 +/- 7 and 58 +/- 11% respectively. At doses of 1.0 and 1.5 micrograms/kg I.V., the increase in rate was preceded by apnoea. 3. The dose-response curves were repeated with bolus injections of bradykinin (0.25, 0.5, 1.0 and 1.5 micrograms/kg) after cooling the cervical vagi to 8-9 degrees C. The increase in respiratory rate was attenuated significantly (P < 0.01 ANOVA). The rate increased from a control value of 27.2 +/- 2.1 breaths/min by 5 +/- 2, 6 +/- 2, 16 +/- 5 and 21 +/- 8% respectively. With vagi cooled, apnoea was increased in duration and occurred at lower doses. On rewarming vagi, the original responses were reestablished. 4. When the study was repeated after bilateral vagotomy, apnoea was abolished but there was a small residual increase in rate. This increase was similar to that seen after cooling the vagi (P > 0.05). 5. RAR (n = 5) activity was recorded from the cervical vagus. Right atrial injections of bradykinin (0.25-1.0 micrograms/kg) stimulated RARs. On cooling the vagi to 8-9 degrees C caudal to the recording site, the increase in activity was blocked. 6. These data support the proposition that bradykinin increases the respiratory rate in rabbits and that this response is, in part, a reflex mediated by RARs. In addition, bradykinin has other secondary effects on respiration: an aponea which is mediated by non-myelinated vagal afferents and a small stimulatory effect on respiration which persists after bilateral vagotomy.

Capsaicin and sensory neuropeptide stimulation of goblet cell secretion in guinea-pig trachea

1. We studied the effect of capsaicin and sensory neuropeptides on tracheal goblet cell secretion in anaesthetized guinea-pigs using a semi-quantitative morphometric technique whereby the magnitude of discharge of stained intracellular mucus, expressed as a mucus score (MS), was related inversely to discharge. 2. Capsaicin (i.v.) induced goblet cell secretion: a decrease of 50% in MS below control (indicative of increased secretion) was maximal at 3.3 x 10(-9) mol/kg. 3. Capsaicin-induced secretion was unaffected either by prior vagus nerve section or by pre-treatment with atropine, propranolol and phentolamine which suggests that local axon reflexes with release of sensory neuropeptides are involved in the response. 4. Intravenous substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and calcitonin gene-related peptide (CGRP) produced dose-related increases in goblet cell secretion, with SP the most potent. Doses (mol/kg) causing a 50% decrease in MS from control were 3.5 x 10(-12) for SP; 72 x 10(-10) for NKA; 1.6 x 10(-9) for NKB; and 1.2 x 10(-8) for CGRP. The maximal increase in goblet cell secretion was 75% of control and occurred with SP at 10(-10) mol/kg. 5. SP-induced mucus discharge was not inhibited by atropine or the histamine receptor antagonists mepyramine or cimetidine. 6. We conclude that in guinea-pig trachea, goblet cell secretion is under the control of capsaicin-sensitive sensory nerves and release of neuropeptides from these nerves may induce mucus discharge via tachykinin receptors of the NK-1 subtype (indicated by an order of potency of SP greater than NKA greater than NKB).

Diaphragmatic vasodilation elicited by pulmonary C-fiber stimulation

This study was performed to assess the effect of pulmonary C-fiber stimulation with capsaicin on vascular resistance in the diaphragm. Nine dogs were anesthetized with pentobarbital sodium and were instrumented with right and left ventricular catheters. The left phrenic artery was isolated and perfused from an extracorporeal reservoir. Right ventricular capsaicin injections (5-20 micrograms/kg) caused significant decreases in phrenic perfusion pressure (-16%), systemic arterial pressure (-32%), and heart rate (-19%). Injection of identical doses of capsaicin into the left ventricle led to no significant changes in phrenic arterial perfusion pressure or systemic arterial pressure but a 10% decrease in heart rate. Bilateral cervical vagotomy eliminated the response to both right and left ventricular injection of capsaicin, as did administration of the autonomic ganglion blocker hexamethonium bromide. The results indicate that pulmonary C-fiber stimulation reflexly vasodilates vessels in the diaphragm.

Nedocromil sodium and sensory nerves in the dog lung

The effect of nedocromil sodium on the main sensory nerve types in the dog lung has been studied. Nedocromil sodium (0.1, 1.0 and 10 mg/kg i.v.) did not stimulate or inhibit the discharge pattern of pulmonary stretch receptors, rapidly adapting irritant receptors or pulmonary C-fibre endings. Nedocromil sodium 5 micrograms/kg given into the aortic arch did, however, stimulate bronchial C-fibre endings. These endings were also stimulated when the drug was given by aerosol. The possibility that nedocromil sodium suppresses cough in the dog by stimulation of bronchial C-fibre endings is discussed.

Effect of pulmonary venous congestion on respiratory rate in dogs

1. The effect of pulmonary venous congestion on the respiratory rate was examined in dogs anaesthetized with alpha-chloralose. The study was done on both spontaneously breathing and artificially ventilated animals. Pulmonary venous congestion was produced by partial obstruction of the mitral valve sufficient to raise the left atrial pressure by 5 mmHg. 2. In artificially ventilated dogs, pulmonary venous congestion increased significantly the activity in phrenic nerves. Both the number of bursts/min and the total number of impulses/min increased. However, there was no significant change in the number of impulses/burst. 3. In spontaneously breathing dogs, pulmonary venous congestion produced a significant increase in the frequency of breathing with a significant shortening of the inspiratory and expiratory durations. 4. Cooling of the cervical vagi to 8-9 degrees C abolished both the above responses. 5. Pulmonary venous congestion (left atrial pressure +5 mmHg) stimulated the rapidly adapting receptors of the airways. This effect was abolished by cooling the ipsilateral vagus proximally to 8-9 degrees C. 6. It is concluded that pulmonary venous congestion increases the respiratory rate reflexly in dogs. The afferent pathway for this reflex response resides in the vagus and the rapidly adapting receptors are likely to be the receptors involved in this response.

Lung C-fibre receptor activation and defensive reflexes in anaesthetized cats

1. With pentobarbitone-anaesthetized cats we have elicited cough reflexes from the tracheobronchial tree and the larynx, and the aspiration and sneeze reflexes from the nasopharynx and the nose respectively. The reflexes were induced by mechanical stimulation of the mucosa, before and during activation of pulmonary C-fibre receptors by intravenous injections of capsaicin or phenylbiguanide. 2. During the 20-30 s apnoea due to C-fibre stimulation, the cough reflex from both sites and the sneeze reflex were completely abolished, whereas the aspiration reflex response was approximately halved. Reflex contractions of genioglossus muscle still occurred at this time, but were far weaker than in the control state. 3. During the rapid shallow breathing that immediately followed apnoea due to C-fibre receptor stimulation, the defensive reflexes recovered: the aspiration and sneeze reflexes fully and the cough reflexes to about half of the control response. 4. Acute hypotension due to haemorrhage, of a size considerably greater than that due to stimulation of the pulmonary C-fibre receptors, caused no significant inhibition of the cough reflex from the tracheobronchial tree. 5. We conclude that the pulmonary C-fibre reflex powerfully inhibits airway defensive reflexes, and that its activation is unlikely to contribute positively to coughing induced by aerosols of capsaicin and similar agents.

Reciprocal action of pulmonary vagal afferents on tracheal smooth muscle tension in dogs

Tracheal smooth muscle usually relaxes when the lungs are transiently inflated, an effect attributed to inhibitory input from pulmonary stretch receptors (PSRs). Relaxation is often followed by contraction, however, and occasionally contraction is the sole response. We attempted to identify the afferents responsible for this reflex contraction. In anesthetized, artificially ventilated dogs with open chest we recorded transverse tension in an upper tracheal segment innervated only by the superior laryngeal nerves and periodically hyperinflated the lungs as the cervical vagus nerves were cooled. Hyperinflation usually evoked tracheal relaxation when vagal temperature was 37 degrees C, but contraction became more frequent as temperature decreased and was the sole response below 8 degrees C. We hypothesise that above 6 degrees C contraction was triggered by rapidly adapting receptors and lung C fibers, whereas below 6 degrees C only C fibers were involved. Contraction, which appeared to represent the bronchomotor counterpart of Head's paradoxical reflex, was abolished below 2 degrees C. Cooling alone without periodic hyperinflation increased baseline tracheal tension to a maximum at 7-8 degrees C; further cooling often decreased tension, sometimes to control levels. Cutting the pulmonary vagal branches abolished these effects. Our results indicate that PSRs and C fibers act reciprocally, one causing bronchodilation, the other bronchoconstriction, and that background activity in C fibers may contribute to bronchomotor tone, an effect unmasked by selectively blocking A fibers.

The differential organization of medullary post-inspiratory activities

Membrane potential trajectories of 68 bulbar respiratory neurones from the peri-solitary and peri-ambigual areas of the brain-stem were recorded in anaesthetized cats to explore the synaptic influences of post-inspiratory neurones upon the medullary inspiratory network. A declining wave of inhibitory postsynaptic potentials resembling the discharge of post-inspiratory neurones was seen in both bulbospinal and non-bulbospinal inspiratory neurones, including alpha- and beta-inspiratory, early-inspiratory, late-inspiratory and ramp-inspiratory neurones. Activation of laryngeal and high-threshold pulmonary receptor afferents excited bulbar post-inspiratory neurones, whilst in the case of inspiratory neurones such stimulation produced enhanced postsynaptic inhibition during the same period of the cycle. Activation of post-inspiratory neurones and enhanced post-inspiratory inhibition of inspiratory bulbospinal neurones was accompanied by suppression of the after-discharge of phrenic motoneurones. These results suggest that a population of post-inspiratory neurones exerts a widespread inhibitory function at the lower brain-stem level. Implications of such an inhibitory function for the organization of the respiratory network are discussed in relation to the generation of the respiratory rhythm.

Retrograde tracing shows that CGRP-immunoreactive nerves of rat trachea and lung originate from vagal and dorsal root ganglia

The origins of sensory innervation of the lower respiratory tract are thought to be principally the nodose and jugular ganglia of the vagus nerve. It has been suggested and partially demonstrated that there is also a component arising from dorsal root ganglia, but the segmental levels involved are not known precisely. We have therefore investigated the origins of sensory nerves within the rat respiratory tract, particularly those containing calcitonin gene-related peptide (CGRP), using the technique of retrograde axonal tracing combined with immunohistochemistry. Injections of True blue were made into extra-thoracic trachea (n = 4 rats) and percutaneously into the right and left lung (n = 4 each). Retrogradely labelled neuronal perikarya were detected in vagal and dorsal root ganglia, and sympathetic chain ganglia. CGRP-immunoreactive cells were seen only in vagal and dorsal root ganglia. Tracheal innervation arose bilaterally in the vagal sensory ganglia but those on the right side represented the principal source; the majority of CGRP-containing neurons occurred in the jugular ganglion. A very small component of labelling occurred in spinal ganglia at levels C2-C6. The sensory innervation of the lungs was seen to arise predominantly from the ipsilateral dorsal root ganglia (45% of cells CGRP-immunoreactive) at levels T1-T6. In contrast to the trachea, the contribution of vagal sensory neurones to the lungs appeared to be less than that of the spinal ganglia. These results show that the sensory innervation of the rat lungs has a major origin in the dorsal root ganglia, in which almost half of the involved neurons contain CGRP, and confirm that most CGRP-immunoreactive nerves in the trachea arise in the right jugular ganglion.

Effects of end-expired pressure on phrenic output in servo-ventilated dogs

The pattern of breathing induced by increases in end-expired lung volume (EEVL) was determined in 9 anesthetized dogs. The pulmonary and systemic circulations were separately pump-perfused and the lungs were ventilated with a servo-ventilator actuated from the phrenic neurogram. EEVL was increased as a continuous ramp by slowly raising end-expired transpulmonary pressure from 1.5 to 12 cm H2O. Tidal volume (VT), inspiratory time (TI), and expiratory time (TE) were measured at vagal temperatures of 39 degrees C and 7 degrees C and following vagotomy. At a vagal temperature of 39 degrees C, increasing EEVL produced significant reductions in VT and TI while greatly prolonging TE. Vagal cooling to 7 degrees C, substantially altered the reflex response to increased EEVL. At 7 degrees C, VT decreased as EEVL increased, but the reduction was not so pronounced as at 39 degrees C. In addition, both TI and TE shortened. Increasing EEVL following vagotomy had no consistent effects on breathing pattern. We conclude that increasing EEVL stimulates tachypneic promoting pulmonary afferent nerves, most likely pulmonary C-fibers, but at normal vagal temperature their effect is masked by the stronger reflex inhibition of slowly adapting pulmonary stretch receptors.

Ventilatory effects of inhaled capsaicin in man

Nebulised capsaicin (10(-7) M) was inhaled by 8 normal subjects to study its effects on the pattern of breathing. When compared to the diluent alone capsaicin increased mean inspiratory flow, a reflection of central inspiratory drive (mean increase: 25 +/- 6%, SEM, p less than 0.01), with a trend to increasing ventilation through more rapid but not more shallow breathing. If capsaicin selectively stimulates non-myelinated fibres in the lung in man as it does in dogs, these results suggest that such stimulation in man can alter the pattern of breathing.

Background activity in pulmonary vagal C-fibers and its effects on breathing

Vagal cooling experiments suggest that the deep slow breathing observed after vagotomy results not only from loss of pulmonary stretch receptor feedback, but also from loss of some unidentified vagal input. To investigate this possibility we cooled the vagus nerves in anesthetized dogs. In dogs breathing spontaneously, the Hering-Breuer reflex was abolished at 7 degrees C, but average expiratory time was unchanged and lengthened only on cooling below 3 degrees C. In artificially ventilated dogs the pulmonary vagus nerves were cooled in the chest and phrenic activity was recorded. Entrainment of phrenic bursts to the ventilator cycle ceased at 7 degrees C, and expiratory pauses shortened; they lengthened again on cooling below 3 degrees C. Cervical vagotomy did not change breathing pattern after the pulmonary vagus nerves were cut. Recording of afferent impulses during cooling showed that at 5 degrees C or less pulmonary vagal input was confined largely to nonmyelinated fibers; at 3 degrees C, background activity in pulmonary C-fibers was still 78% of control whereas myelinated afferents were virtually silent. We suggest that in eupnea low frequency, background activity in pulmonary afferent C-fibers shortens expiratory time.

Tachypnoeic response to amyl nitrite inhalation in the rabbit

Intratracheal inhalation of amyl nitrite, a non-specific smooth muscle relaxant, in the pentobarbitone/urethane anaesthetized rabbit caused reductions in tidal volume and both inspiratory and expiratory times, without a preceding apnoea, that were independent of the associated hypotension and of reflex influences from the carotid sinus region but dependent on supra-abdominal vagal integrity. In artificially ventilated, paralyzed rabbits amyl nitrite caused a pronounced sensitization of pulmonary stretch receptors (PSRs) during the inflation phase, typically with a reduction in the level of activity during the deflation phase. The time course of the change in the pattern of PSR activity paralleled that of the tachypnoeic response. The sensitization of a small sample of rapidly adapting 'irritant' receptors was of a significantly shorter duration. A unitary analysis of non-myelinated vagal afferents was not attempted. The sensitization of these vagal afferents cannot be attributed to the smooth muscle relaxant properties of amyl nitrite since other relaxants (sodium nitrite, sodium nitroprusside, phentolamine) did not possess this capacity, and such properties would be expected to diminish their activities.

Capsaicin inhalation in man and the effects of sodium cromoglycate

The inhalation of capsaicin for 1 min, delivered as an aerosol by nebulising solutions of capsaicin at concentrations of 2-65 mumol 1(-1), caused dose-dependent coughing in normal volunteers and subjects with mild asthma. Capsaicin did not cause a feeling of breathlessness, and had no effect on forced expiratory volume in 1 s (FEV1) measured at the 1st, 5th and 9th min after the challenge was completed. Coughing started within seconds of applying the face mask, continued throughout the minute of capsaicin inhalation, and stopped within seconds of the mask being removed. In any one subject the number of coughs was reproducible when repeated on the same day or after an interval of several days. Experiments using local anaesthesia applied to the buccal mucosa or larynx indicated that the cough was caused by the stimulation of capsaicin-sensitive nerve terminals situated in the larynx. Cough response was not altered by the prior inhalation of sodium cromoglycate.