Mapping supramedullary pathways involved in cough using functional brain imaging: comparison with pain

Anodal transcranial direct current stimulation over the right primary somatosensory cortex increases cough reflex sensitivity: a pilot randomised controlled crossover trial

Background

The cough reflex is a protective reflex of the human body. Increases or decreases in cough reflex sensitivity may be related to chronic cough, aspiration pneumonia and other diseases. The right primary somatosensory cortex (RS1) is the main activation centre for the urge to cough. Here, we discuss the effects of transcranial direct current stimulation (tDCS) of RS1 on the cough reflex and urge to cough. In addition, we explored the role of the left dorsolateral prefrontal cortex (lDLPFC) in cough using tDCS.

Methods

24 healthy young adults completed this pilot randomised controlled crossover experiment. Each person was tested three times, receiving, in random order, anodal tDCS of RS1 or lDLPFC or sham stimulation. The current intensity was set to 2 mA, the stimulation time was 30 min and the interval between any two stimuli was ≥1 week. After each intervention, the citric acid cough challenge test was used immediately to assess the urge to cough and cough reflex sensitivity.

Results

The cough reflex thresholds, expressed as LogC2 and LogC5, were significantly reduced after RS1 anodal stimulation compared to sham stimulation, accompanied by increased urge-to-cough sensitivity (urge-to-cough log-log slope 1.19±0.40 point·L·g-1 versus 0.92±0.33 point·L·g-1, p=0.001), but the threshold for the urge to cough did not change significantly. There were no significant changes in the urge to cough and cough reflex sensitivity after tDCS anodal lDLPFC stimulation.

Conclusion

Anodal tDCS stimulation of the RS1 can increase urge-to-cough sensitivity and reduce cough reflex threshold. The effects of tDCS on cough reflex, as well as the underlying mechanisms driving those effects, should be explored further.

Unravelling vagal hypersensitivity in chronic cough: A distinct disease

Chronic cough (CC) is a common but poorly understood disease that has a negative impact on quality of life. For years, clinicians have been trying to find the underlying diagnosis and using existing disease models to describe the patients' illness. This presents a confusing picture of CC. Most patients with CC present with hypersensitivity of the cough reflex, which is characterised by laryngeal paraesthesia and an increased response to the tussive stimuli or an innocuous stimulus that would not trigger coughing in healthy people. Recently, it has been proposed that CC is a unique disease characterised by vagal hypersensitivity that projects to the central nervous system altering responsiveness. The evidence supports the hypothesis that CC is primarily a neurological disorder, consisting of different phenotypes.

Cough desensitization treatment for patients with refractory chronic cough: results of a second pilot randomized control trial

OBJECTIVE

The purpose of this study was to collect pilot efficacy data on a novel treatment for refractory chronic cough (RCC), which we call cough desensitization treatment (CDT).

DESIGN AND METHODS

In this parallel cohort, sham-controlled, randomized controlled trial, 21 adults with RCC were randomly assigned to 12 sessions of either CDT (progressive doses of aerosolized capsaicin while behaviorally suppressing cough; n = 11) or a sham treatment (repeated exposure to aerosolized saline; n = 9). The Leicester Cough Questionnaire (LCQ) was the primary outcome measure. Perceived cough severity with a visual analogue scale and cough challenge testing (for measuring cough-reflex sensitivity) were secondary outcome measures. Data were analyzed with mixed effects linear regression and follow-up contrasts.

RESULTS

Results on all measures favored CDT. Excluding one sham participant, whose baseline LCQ scores were deemed unreliable, mean change in LCQ at 3-weeks post treatment was 6.35 and 2.17 in the CDT and sham groups, respectively. There was moderate to strong evidence of a greater improvement in the CDT group in total LCQ score (p = .058) and LCQ Psychological domain (p = .026) and Physical domain (p = .045) scores. Strong evidence was found for a greater reduction in urge-to-cough during CCT in the CDT group (p = .037) and marginal for a reduction in the capsaicin cough-reflex sensitivity (p = .094). There was weak evidence of a greater reduction in cough severity in the CDT group (p = .103).

DISCUSSION

Although the study is limited due to the small sample size, the data provide additional evidence supporting further research on CDT. CDT resulted in a greater change in the primary efficacy measure (LCQ) than both pharmaceutical and behavioral treatments currently found in the literature.

TRIAL REGISTRATION

This trial (NCT05226299) was registered on Clinicaltrials.gov on 07/02/2022.

The Evolving Clinical Practice of Chronic Cough

Chronic cough, defined as a cough lasting for greater than 8 weeks, accounts for a substantial number of primary care and specialist consultations in the United States. Although cough can arise from a myriad number of serious respiratory diseases, attention has traditionally focused on diagnosing and treating gastroesophageal reflux, upper airway cough syndrome, and eosinophilic airway inflammation (asthma and nonasthmatic eosinophilic bronchitis) in patients with normal chest imaging. The newly described paradigm and entity of cough hypersensitivity syndrome (CHS) becomes useful when the etiology of cough remains elusive or when the cough remains refractory despite appropriate therapy for underlying causes. We present an update on the evolving understanding of refractory chronic cough and/or unexplained chronic cough as manifestations of laryngeal hypersensitivity and CHS. This includes a focus on understanding the pathophysiology underlying current and novel therapeutics for CHS, while also ensuring that common causes of chronic cough continue to be evaluated and treated in a systematic multidisciplinary manner.

Effects of transcranial direct current stimulation on cough reflex and urge-to-cough in healthy young adults

Background

Chronic cough is prevalent in the clinic. The existing therapies are mostly limited to medical treatment, with poor curative effects and serious side effects. Studies have suggested that the right dorsolateral prefrontal cortex (rDLPFC) may play an active role in the inhibitory pathway of cough elicitation. Thus, this study explored the effect of transcranial direct current stimulation (tDCS) on the rDLPFC activation in relation to cough reflex and urge-to-cough sensitivity.

Methods

Twenty-three healthy young adults completed the experiment. Participants randomly received tDCS anodal stimulation, cathodal stimulation, and sham stimulation, and the interval between every two stimuli was at least one week. The tDCS (2 mA, 30 min) stimulated brain rDLPFC region. After tDCS intervention, cough reflex threshold and urge-to-cough were evaluated immediately by inhalation of citric acid-saline solution.

Results

Compared with sham stimulation, the cough reflex thresholds logC₂ and logC₅ have increased under tDCS anodal stimulation (1.78 ± 0.55 g/L vs. 1.57 ± 0.57 g/L, p < 0.05; 1.92 ± 0.53 g/L vs. 1.67 ± 0.56 g/L, p < 0.05), accompanied by the increase of the urge-to-cough threshold LogC_u (0.76 ± 0.53 g/L vs. 0.47 ± 0.44 g/L, p < 0.05). In contrast, the urge-to-cough sensitivity expressed as UTC slope was not significantly changed (1.21 ± 0.86 point·L/g vs. 1.00 ± 0.37 point·L/g, p > 0.05), and there were no apparent changes in cough reflex thresholds Log C₂ and logC₅, urge-to-cough threshold LogC_u, and urge-to-cough sensitivity UTC slope under tDCS cathodal stimulation, compared with sham stimulation.

Conclusions

This study found that anodal tDCS stimulation of rDLPFC could significantly decrease cough reflex sensitivity, accompanied by the increase of urge-to-cough threshold. Further investigations targeting different brain regions using multiple central intervention techniques to explore the underlying mechanisms are warranted.

Trial registration The study protocol was registered for the clinical trial in China (registration number: ChiCTR2100045618)

Voluntary Cough Effectiveness and Airway Clearance in Neurodegenerative Disease

PURPOSE

Voluntary cough dysfunction is highly prevalent across multiple patient populations. Voluntary cough has been utilized as a screening tool for swallowing safety deficits and as a target for compensatory and exercise-based dysphagia management. However, it remains unclear whether voluntary cough dysfunction is associated with the ability to effectively clear the airway.

METHOD

Individuals with neurodegenerative disorders performed same-day voluntary cough testing and flexible endoscopic evaluations of swallowing (FEES). Participants who were cued to cough after exhibiting penetration to the vocal folds and/or aspiration with thin liquids during FEES met inclusion criteria. One-hundred and twenty-three trials were blinded, and the amount of residue before and after a cued cough on FEES was measured with a visual analog scale. Linear and binomial mixed-effects models examined the relationship between cough airflow during voluntary cough testing and the proportion of residue expelled.

RESULTS

Peak expiratory flow rate (p = .004) and cough expired volume from the entire epoch (p = .029) were significantly associated with the proportion of aspiration expelled from the subglottis. Peak expiratory flow rate values of 3.00 L/s, 3.50 L/s, and 5.30 L/s provided high predicted probabilities that ≥ 25%, ≥ 50%, and ≥ 80% aspirate was expelled. Accounting for depth of aspiration significantly improved model fit (p < .001).

CONCLUSIONS

These findings suggest that voluntary cough airflow is associated with cough effectiveness to clear aspiration from the subglottis, although aspiration amount and depth may play an important role in this relationship. These findings provide further support for the clinical utility of voluntary cough in the management of dysphagia.

Intra-Arterial, but Not Intrathecal, Baclofen and Codeine Attenuates Cough in the Cat

Centrally-acting antitussive drugs are thought to act solely in the brainstem. However, the role of the spinal cord in the mechanism of action of these drugs is unknown. The purpose of this study was to determine if antitussive drugs act in the spinal cord to reduce the magnitude of tracheobronchial (TB) cough-related expiratory activity. Experiments were conducted in anesthetized, spontaneously breathing cats (n = 22). Electromyograms (EMG) were recorded from the parasternal (PS) and transversus abdominis (TA) or rectus abdominis muscles. Mechanical stimulation of the trachea or larynx was used to elicit TB cough. Baclofen (10 and 100 μg/kg, GABA-B receptor agonist) or codeine (30 μg/kg, opioid receptor agonist) was administered into the intrathecal (i.t.) space and also into brainstem circulation via the vertebral artery. Cumulative doses of i.t. baclofen or codeine had no effect on PS, abdominal muscle EMGs or cough number during the TB cough. Subsequent intra-arterial (i.a.) administration of baclofen or codeine significantly reduced magnitude of abdominal and PS muscles during TB cough. Furthermore, TB cough number was significantly suppressed by i.a. baclofen. The influence of these drugs on other behaviors that activate abdominal motor pathways was also assessed. The abdominal EMG response to noxious pinch of the tail was suppressed by i.t. baclofen, suggesting that the doses of baclofen that were employed were sufficient to affect spinal pathways. However, the abdominal EMG response to expiratory threshold loading was unaffected by i.t. administration of either baclofen or codeine. These results indicate that neither baclofen nor codeine suppress cough via a spinal action and support the concept that the antitussive effect of these drugs is restricted to the brainstem.

Descending Modulation of Laryngeal Vagal Sensory Processing in the Brainstem Orchestrated by the Submedius Thalamic Nucleus

The nodose and jugular vagal ganglia supply sensory innervation to the airways and lungs. Jugular vagal airway sensory neurons wire into a brainstem circuit with ascending projections into the submedius thalamic nucleus (SubM) and ventrolateral orbital cortex (VLO), regions known to regulate the endogenous analgesia system. Here we investigate whether the SubM-VLO circuit exerts descending regulation over airway vagal reflexes in male and female rats using a range of neuroanatomical tracing, reflex physiology, and chemogenetic techniques. Anterograde and retrograde neuroanatomical tracing confirmed the connectivity of the SubM and VLO. Laryngeal stimulation in anesthetized rats reduced respiration, a reflex that was potently inhibited by activation of SubM. Conversely, inhibition of SubM potentiated laryngeal reflex responses, while prior lesions of VLO abolished the effects of SubM stimulation. In conscious rats, selective chemogenetic activation of SubM neurons specifically projecting to VLO significantly inhibited respiratory responses evoked by inhalation of the nociceptor stimulant capsaicin. Jugular vagal inputs to SubM via the medullary paratrigeminal nucleus were confirmed using anterograde transsynaptic conditional herpes viral tracing. Respiratory responses evoked by microinjections of capsaicin into the paratrigeminal nucleus were significantly attenuated by SubM stimulation, whereas those evoked via the nucleus of the solitary tract were unaltered. These data suggest that jugular vagal sensory pathways input to a nociceptive thalamocortical circuit capable of regulating jugular sensory processing in the medulla. This circuit organization suggests an intersection between vagal sensory pathways and the endogenous analgesia system, potentially important for understanding vagal sensory processing in health and mechanisms of hypersensitivity in disease.SIGNIFICANCE STATEMENT Jugular vagal sensory pathways are increasingly recognized for their important role in defensive respiratory responses evoked from the airways. Jugular ganglia neurons wire into a central circuit that is notable for overlapping with somatosensory processing networks in the brain rather than the viscerosensory circuits in receipt of inputs from the nodose vagal ganglia. Here we demonstrate a novel and functionally relevant example of intersection between vagal and somatosensory processing in the brain. The findings of the study offer new insights into interactions between vagal and spinal sensory processing, including the medullary targets of the endogenous analgesia system, and offer new insights into the central processes involved in airway defense in health and disease.

Behavioral and Regional Brain Responses to Inhalation of Capsaicin Modified by Painful Conditioning in Humans

BACKGROUND

Cough is a defense mechanism that protects the airways and lungs in response to airway irritation. The sensory neurons involved in detecting airway irritants and the neural pathways mediating cough share similarities with those that encode pain from the body. Painful conditioning stimuli applied to one body site are known to reduce the perception of pain at another. However, whether the neural regulation of cough is influenced by painful stimuli is not known.

RESEARCH QUESTION

What are the behavioral and neural outcomes of painful conditioning stimuli on urge-to-cough (UTC) and cough evoked by inhaled capsaicin?

STUDY DESIGN AND METHODS

Sixteen healthy participants underwent psychophysical testing and functional MRI while completing a series of capsaicin inhalations to induce UTC and cough. The responses associated with capsaicin inhalation without pain were compared with those after the application of painful conditioning stimuli.

RESULTS

Significant decreases were seen behaviorally of 18.7% ± 17.3% (P < .001) and 47.0% ± 30.8% (P < .001) in participants' UTC ratings and cough frequencies, respectively, during the application of pain. UTC ratings were reduced by 24.2% ± 36.5% (P < .005) and increased by 67% ± 40% (P < .001) for capsaicin and saline inhalation, respectively, during the scanning session. Painful conditioning stimuli were associated with widespread decreases in regional brain responses to capsaicin inhalation (P < .001). Several brain regions showed levels of reduced activation attributable to painful conditioning that correlated with related changes in behavioral responses during scanning (R² = 0.53).

INTERPRETATION

Pain-related decreases of cough and UTC are accompanied by widespread changes in brain activity during capsaicin inhalation, suggesting that pain can modify the central processing of inputs arising from the airways. A mechanistic understanding of how cough and pain processing interact within the brain may help develop more effective therapies to reduce unwanted coughing.

In Vivo Documentation of Stimulus Velocity Tuning of Mechanically Induced Reflex Cough

In order to clear airways and lungs defensive reflexes are provoked rather by the dynamic phase of mechanical stimulus. It is speculated that provocation of defensive response depends not only on stimulus duration but also on stimulus velocity. Fourteen adult rabbits were anaesthetized and tracheotomized. Mechanical stimulus was provoked by a mechanical probe introduced through the tracheotomy and rotated by a small electrical motor using a rotational velocity of 40 rpm/s and 20 rpm/s. Threshold, incidence and intensity of cough reflex (CR) were analyzed for each animal. Statistical comparisons between two velocities were performed using Friedman nonparametric test for repeated measurements. Results are median (25-75 %). The threshold of CR was significantly increased (p=0.005) from 350 ms (300-500 ms) to 550 ms (350-1150 ms) and the incidence of cough reflex was significantly reduced (p=0.002) from 50 % (19 50 %) to 0 % (0-25 %) when the rotational velocity of the mechanical probe was reduced by half. The findings of this study are of interest as they show that protective reflex cough, an important mechanism that allows clearing airways even during sleep or anesthesia, is tuned by mechanical stimulus velocity.

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.

Multiple neural circuits mediating airway sensations: Recent advances in the neurobiology of the urge-to-cough

The respiratory system is densely innervated by sensory neurons arising from the jugular (superior) and nodose (inferior) vagal ganglia. However, a distinction exists between jugular and nodose neurons as these ganglia developmentally originate from the neural crest and the epibranchial placodes, respectively. This different embryological origin underpins an important source of heterogeneity in vagal afferent biology, and may extend to include fundamentally different central neural circuits that are in receipt of jugular versus nodose afferent inputs. Indeed, recent studies using viral tract tracing and human brain imaging support the notion that airway sensors contribute inputs to multiple central circuits. Understanding the neural pathways arising from the airways and lungs may provide novel insights into aberrant sensations, such as the urge-to-cough, characteristic of respiratory disease.

Distinct brainstem and forebrain circuits receiving tracheal sensory neuron inputs revealed using a novel conditional anterograde transsynaptic viral tracing system

Sensory nerves innervating the mucosa of the airways monitor the local environment for the presence of irritant stimuli and, when activated, provide input to the nucleus of the solitary tract (Sol) and paratrigeminal nucleus (Pa5) in the medulla to drive a variety of protective behaviors. Accompanying these behaviors are perceivable sensations that, particularly for stimuli in the proximal end of the airways, can be discrete and localizable. Airway sensations likely reflect the ascending airway sensory circuitry relayed via the Sol and Pa5, which terminates broadly throughout the CNS. However, the relative contribution of the Sol and Pa5 to these ascending pathways is not known. In the present study, we developed and characterized a novel conditional anterograde transneuronal viral tracing system based on the H129 strain of herpes simplex virus 1 and used this system in rats along with conventional neuroanatomical tracing with cholera toxin B to identify subcircuits in the brainstem and forebrain that are in receipt of relayed airway sensory inputs via the Sol and Pa5. We show that both the Pa5 and proximal airways disproportionately receive afferent terminals arising from the jugular (rather than nodose) vagal ganglia and the output of the Pa5 is predominately directed toward the ventrobasal thalamus. We propose the existence of a somatosensory-like pathway from the proximal airways involving jugular ganglia afferents, the Pa5, and the somatosensory thalamus and suggest that this pathway forms the anatomical framework for sensations arising from the proximal airway mucosa.

Anatomy and neurophysiology of cough: CHEST Guideline and Expert Panel report

Bronchopulmonary C-fibers and a subset of mechanically sensitive, acid-sensitive myelinated sensory nerves play essential roles in regulating cough. These vagal sensory nerves terminate primarily in the larynx, trachea, carina, and large intrapulmonary bronchi. Other bronchopulmonary sensory nerves, sensory nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature regulate cough patterning and cough sensitivity. The responsiveness and morphology of the airway vagal sensory nerve subtypes and the extrapulmonary sensory nerves that regulate coughing are described. The brainstem and higher brain control systems that process this sensory information are complex, but our current understanding of them is considerable and increasing. The relevance of these neural systems to clinical phenomena, such as urge to cough and psychologic methods for treatment of dystussia, is high, and modern imaging methods have revealed potential neural substrates for some features of cough in the human.

A framework for understanding shared substrates of airway protection

Deficits of airway protection can have deleterious effects to health and quality of life. Effective airway protection requires a continuum of behaviors including swallowing and cough. Swallowing prevents material from entering the airway and coughing ejects endogenous material from the airway. There is significant overlap between the control mechanisms for swallowing and cough. In this review we will present the existing literature to support a novel framework for understanding shared substrates of airway protection. This framework was originally adapted from Eccles' model of cough²⁸ (2009) by Hegland, et al.⁴² (2012). It will serve to provide a basis from which to develop future studies and test specific hypotheses that advance our field and ultimately improve outcomes for people with airway protective deficits.

Afferent neural pathways mediating cough in animals and humans

The airways and lungs are densely innervated by sensory nerves, which subserve multiple roles in both the normal physiological control of respiratory functions and in pulmonary defense. These sensory nerves are therefore not homogeneous in nature, but rather have physiological, molecular and anatomical phenotypes that reflect their purpose. All sensory neuron subtypes provide input to the central nervous system and drive reflex changes in respiratory and airway functions. But less appreciated is that ascending projections from these brainstem inputs to higher brain regions can also induce behavioural changes in respiration. In this brief review we provide an overview of the current understanding of airway sensory pathways, with specific reference to those involved in reflex and behavioural cough responses following airways irritation.

Gender differences in the effect of urge-to-cough and dyspnea on perception of pain in healthy adults

Previous studies have reported that respiratory sensations, such as urge‐to‐cough and dyspnea, have an inhibitory effect on pain. Considering the existence of gender differences in both urge‐to‐cough and pain, it is conceivable that a gender difference also exists in the analgesia induced by urge‐to‐cough. In this study, we evaluated gender differences in the pain perception response to urge‐to‐cough, as well as to dyspnea. Twenty‐seven male and 26 female healthy nonsmokers were originally enrolled. Citric acid challenge was used to induce the urge‐to‐cough sensation, and dyspnea was elicited by inspiratory loaded breathing. Before and during inductions of these two respiratory sensations, perception of pain was assessed by the thermal pain threshold, and differences between men and women were compared. The thermal pain threshold in women (43.83 ± 0.17°C) was significantly lower than that in men (44.75 ± 0.28°C; P < 0.05) during the baseline period. Accompanying increases in both citric acid concentration and inspiratory resistive load, thermal pain threshold values significantly increased in both men and women. The average thermal pain threshold changes for comparable increases in the urge‐to‐cough Borg score were parallel between men and women. Furthermore, the mean value of the thermal pain threshold plotted against the dyspnea Borg score also showed no significant gender difference. These results demonstrate that although gender differences exist in respiratory sensations, that is, urge‐to‐cough and dyspnea, the inhibitory effects of these respiratory sensations on the perception of pain are not significantly different between the sexes.

The relationships between citric acid dose and urge‐to‐cough rating or thermal pain threshold in each subject grouped by sex.

Evidence for multiple sensory circuits in the brain arising from the respiratory system: an anterograde viral tract tracing study in rodents

Complex sensations accompany the activation of sensory neurons within the respiratory system, yet little is known about the organization of sensory pathways in the brain that mediate these sensations. In the present study, we employ anterograde viral neuroanatomical tract tracing with isogenic self-reporting recombinants of HSV-1 strain H129 to map the higher brain regions in receipt of vagal sensory neurons arising from the trachea versus the lungs, and single-cell PCR to characterize the phenotype of sensory neurons arising from these two divisions of the respiratory tree. The results suggest that the upper and lower airways are predominantly innervated by sensory neurons derived from the somatic jugular and visceral nodose cranial ganglia, respectively. This coincides with central circuitry that is predominately somatic-like, arising from the trachea, and visceral-like, arising from the lungs. Although some convergence of sensory pathways was noted in preautonomic cell groups, this was notably absent in thalamic and cortical regions. These data support the notion that distinct afferent subtypes, via distinct central circuits, subserve sensations arising from the upper versus lower airways. The findings may explain why sensations arising from different levels of the respiratory tree are qualitatively and quantitatively unique.

Sensorimotor circuitry involved in the higher brain control of coughing

There is an overwhelming body of evidence to support the existence of higher brain circuitries involved in the sensory detection of airways irritation and the motor control of coughing. The concept that cough is purely a reflex response to airways irritation is now superseded by the recognition that perception of an urge-to-cough and altered behavioral modification of coughing are key elements of cough disorders associated with airways disease. Understanding the pathways by which airway sensory nerves ascend into the brain and the patterns of neural activation associated with airways irritation will undoubtedly provide new insights into disordered coughing. This brief review aims to explore our current understanding of higher order cough networks by summarizing data from recent neuroanatomical and functional studies in animals and humans. We provide evidence for the existence of distinct higher order network components involved in the discrimination of signals arising from the airways and the motor control of coughing. The identification of these network components provides a blueprint for future research and the development of targeted managements for cough and the urge-to-cough.

Transneuronal tracing of airways-related sensory circuitry using herpes simplex virus 1, strain H129

Sensory input from the airways to suprapontine brain regions contributes to respiratory sensations and the regulation of respiratory function. However, relatively little is known about the central organization of this higher brain circuitry. We exploited the properties of the H129 strain of herpes simplex virus 1 (HSV-1) to perform anterograde transneuronal tracing of the central projections of airway afferent nerve pathways. The extrathoracic trachea in Sprague-Dawley rats was inoculated with HSV-1 H129, and tissues along the neuraxis were processed for HSV-1 immunoreactivity. H129 infection appeared in the vagal sensory ganglia within 24 h and the number of infected cells peaked at 72 h. Brainstem nuclei, including the nucleus of the solitary tract and trigeminal sensory nuclei were infected within 48 h, and within 96 h infected cells were evident within the pons (lateral and medial parabrachial nuclei), thalamus (ventral posteromedial, ventral posterolateral, submedius, and reticular nuclei), hypothalamus (paraventricular and lateral nuclei), subthalamus (zona incerta), and amygdala (central and anterior amygdala area). At later times H129 was detected in cortical forebrain regions including the insular, orbital, cingulate, and somatosensory cortices. Vagotomy significantly reduced the number of infected cells within vagal sensory nuclei in the brainstem, confirming the main pathway of viral transport is through the vagus nerves. Sympathetic postganglionic neurons in the stellate and superior cervical ganglia were infected by 72 h, however, there was no evidence for retrograde transynaptic movement of the virus in sympathetic pathways in the central nervous system (CNS). These data demonstrate the organization of key structures within the CNS that receive afferent projections from the extrathoracic airways that likely play a role in the perception of airway sensations.

Effect of cigarette smoking on cough reflex induced by TRPV1 and TRPA1 stimulations

BACKGROUND

Recent studies have shown that neurogenic inflammation induced by cigarette smoke is inhibited by TRPA1 antagonist, but not by TRPV1 antagonist. Since cough reflex sensitivity is known to be modified by smoking status, we investigated the effects of cigarette smoking on TRPA1- and TRPV1-induced cough and urge-to-cough in healthy males.

METHODS

Twenty-six healthy never-smokers and 30 healthy current smokers were recruited via public postings. Cough reflex thresholds and urge-to-cough were evaluated by inhalation of capsaicin, a TRPV1 agonist, and cinnamaldehyde, a TRPA1 agonist. The cough reflex thresholds were defined as the lowest concentrations of capsaicin and cinnamaldehyde that elicited two or more coughs (C(2)) and five or more coughs (C(5)), respectively. The urge-to-cough was evaluated using the modified Borg scale.

RESULTS

In capsaicin-induced cough, the cough reflex thresholds, as expressed by C(2) and C(5), in current smokers were significantly higher than those in never-smokers (p<0.01 and p<0.001, respectively). The urge-to-cough log-log slopes in current smokers were significantly lower than those of never-smokers (p<0.001). There were no significant differences in the thresholds of the urge-to-cough between never-smokers and current smokers. In cinnamaldehyde-induced cough, there were no significant differences in cough reflex thresholds in C(2) and C(5) between never-smokers and current smokers, nor were there any significant differences in urge-to-cough log-log slope between never-smokers and current smokers. There were no significant differences in the thresholds of the urge-to-cough between never-smokers and current smokers.

CONCLUSION

The study suggests that smoking has a differential effect on cough responses between TRPV1 and TRPA1 stimulations.

Co-ordination of cough and swallow in vivo and in silico

Coughing and swallowing are airway-protective behaviours. The pharyngeal phase of swallowing prevents aspiration of oral material (saliva, food and liquid) by epiglottal movement, laryngeal adduction and clearing of the mouth and pharynx. Coughing is an aspiration-response behaviour that removes material from the airway. Co-ordination of these behaviours is vital to protect the airway from further aspiration-promoting events, such as a swallowing during the inspiratory phase of coughing. The operational characteristics, primary strategies and peripheral inputs that co-ordinate coughing and swallowing are unknown. This lack of knowledge impedes understanding and treatment of deficits in airway protection, such as the co-occurrence of dystussia and dysphagia common in Parkinson's and Alzheimer's diseases, as well as stroke.

Investigation of the neural control of cough and cough suppression in humans using functional brain imaging

Excessive coughing is one of the most common reasons for seeking medical advice, yet the available therapies for treating cough disorders are inadequate. Humans can voluntarily cough, choose to suppress their cough, and are acutely aware of an irritation that is present in their airways. This indicates a significant level of behavioral and conscious control over the basic cough reflex pathway. However, very little is known about the neural basis for higher brain regulation of coughing. The aim of the present study was to use functional brain imaging in healthy humans to describe the supramedullary control of cough and cough suppression. Our data show that the brain circuitry activated during coughing in response to capsaicin-evoked airways irritation is not simply a function of voluntarily initiated coughing and the perception of airways irritation. Rather, activations in several brain regions, including the posterior insula and posterior cingulate cortex, define the unique attributes of an evoked cough. Furthermore, the active suppression of irritant-evoked coughing is also associated with a unique pattern of brain activity, including an involvement of the anterior insula, anterior mid-cingulate cortex, and inferior frontal gyrus. These data demonstrate for the first time that evoked cough is not solely a brainstem-mediated reflex response to irritation of the airways, but rather requires active facilitation by cortical regions, and is further regulated by distinct higher order inhibitory processes.

The urge-to-cough and cough motor response modulation by the central effects of nicotine

The urge-to-cough is a respiratory sensation that precedes the cough motor response. Since affective state modulates the perception of respiratory sensations such as dyspnoea, we wanted to test whether nicotine, an anxiolytic, would modulate the urge-to-cough and hence, the cough motor response. We hypothesized that withdrawal from and administration of nicotine in smoking subjects would modulate their anxiety levels, urge-to-cough and cough motor response to capsaicin stimulation. Twenty smoking (SM) adults (8F, 12M; 22+/-3 years; 2.9+/-2.0 pack years) and matched non-smoking (NS) controls (22+/-2 years) were presented with randomized concentrations of capsaicin (0-200 microM) before and after nicotine (SM only) gum and/or placebo (SM and NS) gum. Subjects rated their urge-to-cough using a Borg scale at the end of each capsaicin presentation. Cough number and cough motor pattern were determined from airflow tracings. Subjects completed State-Trait Anxiety Inventory (STAI) questionnaires before and after gum administration. SM subjects that withdrew from cigarette smoking for 12 h exhibited an increase in anxiety scores, a greater number of coughs and higher urge-to-cough ratings compared to NS subjects. Administration of nicotine gum reduced anxiety scores, cough number and urge-to-cough ratings to match the NS subjects. There was no effect of placebo gum on any of the measured parameters in the SM and NS groups. The results from this study suggest that modulation of the central neural state with nicotine withdrawal and administration in young smoking adults is associated with a change in anxiety levels which in turn modulates the perceptual and motor response to irritant cough stimulants.