Acute sustained hypoxia suppresses the cough reflex in healthy subjects

Cough as a neurological sign: What a clinician should know

Cough is a common respiratory complaint driving patients to seek medical advice. Besides being a fundamental respiratory sign, it is also a crucial neurological sign. There are three main types of coughs: Reflex cough (type I), voluntary cough (type II), and evoked cough (type III). Cough is a reflex predominantly mediated by control centers in the respiratory areas of the brainstem, modulated by the cerebral cortex. Cough reflex sensitivity could be increased in many neurological disorders such as brainstem space-occupying lesions, medullary lesions secondary to Chiari type I malformations, tics disorders such as Tourette's syndrome, somatic cough, cerebellar neurodegenerative diseases, and chronic vagal neuropathy due to allergic and non-allergic conditions. Meanwhile, cough sensitivity decreases in multiple sclerosis, brain hypoxia, cerebral hemispheric stroke with a brainstem shock, Parkinson's disease, dementia due to Lewy body disease, amyotrophic lateral sclerosis, and peripheral neuropathy as diabetic neuropathy, hereditary sensory and autonomic neuropathy type IV, vitamin B12, and folate deficiency. Arnold's nerve ear-cough reflex, syncopal cough, cough headache, opioids-associated cough, and cough-anal reflex are signs that could help diagnose underlying neurological conditions. Cough reflex testing is a quick, easy, and cheap test performed during the cranial nerve examination. In this article, we reviewed the role of cough in various neurological disorders that increase or decrease cough sensitivity.

Intermittent hypercapnic hypoxia during sleep does not induce ventilatory long-term facilitation in healthy males

Intermittent hypoxia-induced ventilatory neuroplasticity is likely important in obstructive sleep apnea pathophysiology. Although concomitant CO2levels and arousal state critically influence neuroplastic effects of intermittent hypoxia, no studies have investigated intermittent hypercapnic hypoxia effects during sleep in humans. Thus the purpose of this study was to investigate if intermittent hypercapnic hypoxia during sleep induces neuroplasticity (ventilatory long-term facilitation and increased chemoreflex responsiveness) in humans. Twelve healthy males were exposed to intermittent hypercapnic hypoxia (24 × 30 s episodes of 3% CO2and 3.0 ± 0.2% O2) and intermittent medical air during sleep after 2 wk washout period in a randomized crossover study design. Minute ventilation, end-tidal CO2, O2saturation, breath timing, upper airway resistance, and genioglossal and diaphragm electromyograms were examined during 10 min of stable stage 2 sleep preceding gas exposure, during gas and intervening room air periods, and throughout 1 h of room air recovery. There were no significant differences between conditions across time to indicate long-term facilitation of ventilation, genioglossal or diaphragm electromyogram activity, and no change in ventilatory response from the first to last gas exposure to suggest any change in chemoreflex responsiveness. These findings contrast with previous intermittent hypoxia studies without intermittent hypercapnia and suggest that the more relevant gas disturbance stimulus of concomitant intermittent hypercapnia frequently occurring in sleep apnea influences acute neuroplastic effects of intermittent hypoxia. These findings highlight the need for further studies of intermittent hypercapnic hypoxia during sleep to clarify the role of ventilatory neuroplasticity in the pathophysiology of sleep apnea.

NEW & NOTEWORTHY Both arousal state and concomitant CO2levels are known modulators of the effects of intermittent hypoxia on ventilatory neuroplasticity. This is the first study to investigate the effects of combined intermittent hypercapnic hypoxia during sleep in humans. The lack of neuroplastic effects suggests a need for further studies more closely replicating obstructive sleep apnea to determine the pathophysiological relevance of intermittent hypoxia-induced ventilatory neuroplasticity.

Down-Regulation of Cough during Exercise Is Less Frequent in Healthy Children than Adults. Role of the Development and/or Atopy?

Cough is typically associated with physical activity in children with asthma, but the characteristics of the relationship between cough and exercise has not been established under physiological conditions. The aim of the study was to describe the effect of exercise on the reflex cough response elicited by a single breath of capsaicin in non-asthmatic children. A group of non-asthmatic adults was studied as reference. Thirty children and 29 adults were recruited. The cough reflex sensitivity to capsaicin was first determined to establish the dose that provokes 5 cough efforts (C5). The number of coughs elicited by C5 (NC5) was then compared at baseline and during a standardized submaximal treadmill exercise. Data are expressed as median (interquartile range). Children and adults showed a significant decrease in NC5 (respectively from 5.0 (4.0–6.0) to 2.5 (2.0–4.0), p < 0.0005 and from 6.0 (5.0–7.0) to 2.0 (0.0–3.0, p < 0.0005). During exercise, NC5 was observed to decrease in all adult subjects, but in only 24/30 children (80%, p = 0.02). A trend for a higher incidence of personal and familial atopy was observed in children that lacked cough down-regulation during exercise compared with other children. It is concluded that the cough reflex response to capsaicin is down regulated by exercise in both children and adults. The effect however is less consistently observed in the former. The difference may reflect maturation of descending inhibitory pathways of the cough reflex, but may also be associated to atopy. The data stress the importance of assessing the time relationship of cough and exercise in questionnaire studies of asthma.

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.

Asthma Control and Its Relationship with Obstructive Sleep Apnea (OSA) in Older Adults

Background/Objectives. Asthma in older individuals is poorly understood. We aimed to characterize the older asthma phenotype and test its association with obstructive sleep apnea (OSA). Design. Cross-sectional. Setting. Pulmonary and Asthma/Allergy clinics. Participants. 659 asthma subjects aged 18–59 years (younger) and 154 aged 60–75 (older). Measurements. Sleep Apnea scale of Sleep Disorders Questionnaire (SA-SDQ), asthma severity step (1–4, severe if step 3 or 4), established OSA diagnosis, continuous positive airway pressure (CPAP) use, and comorbidities. Results. Older versus younger had worse control, as assessed by asthma step, lung function, and inhaled corticosteroid use. Among older subjects, after controlling for known asthma aggravators, OSA diagnosis was the only factor robustly associated with severe asthma: on average, OSA was associated with nearly 7 times greater likelihood of severe asthma in an older individual (OR = 6.67). This relationship was of greater magnitude than in younger subjects (OR = 2.16). CPAP use attenuated the likelihood of severe asthma in older subjects by 91% (P = 0.005), much more than in the younger asthmatics. Conclusion. Diagnosed OSA increases the risk for worse asthma control in older patients, while CPAP therapy may have greater impact on asthma outcomes. Unrecognized OSA may be a reason for poor asthma control, particularly among older patients.

Coordination of breathing with nonrespiratory activities

Many articles in this section of Comprehensive Physiology are concerned with the development and function of a central pattern generator (CPG) for the control of breathing in vertebrate animals. The action of the respiratory CPG is extensively modified by cortical and other descending influences as well as by feedback from peripheral sensory systems. The central nervous system also incorporates other CPGs, which orchestrate a wide variety of discrete and repetitive, voluntary and involuntary movements. The coordination of breathing with these other activities requires interaction and coordination between the respiratory CPG and those governing the nonrespiratory activities. Most of these interactions are complex and poorly understood. They seem to involve both conventional synaptic crosstalk between groups of neurons and fluid identity of neurons as belonging to one CPG or another: neurons that normally participate in breathing may be temporarily borrowed or hijacked by a competing or interrupting activity. This review explores the control of breathing as it is influenced by many activities that are generally considered to be nonrespiratory. The mechanistic detail varies greatly among topics, reflecting the wide variety of pertinent experiments.

Integrating the overlap of obstructive lung disease and obstructive sleep apnoea: OLDOSA syndrome

Obstructive lung diseases (OLD) such as asthma and chronic obstructive pulmonary disease (COPD) are very prevalent conditions. Disease phenotypes (e.g. chronic bronchitis, emphysema, etc.) often overlap, and significant confusion exists about their optimal nosologic characterization. Obstructive sleep apnoea (OSA) is also a common condition that features bidirectional interactions with OLD. OSA appears to be more commonly seen in patients with OLD, perhaps as a result of shared risk factors, for example obesity, smoking, increased airway resistance, local and systemic inflammation, anti-inflammatory therapy. Conversely, OSA is associated with worse clinical outcomes in patients with OLD, and continuous positive airway pressure therapy has potential beneficial effects on this vicious pathophysiological interaction. Possible shared mechanistic links include increased parasympathetic tone, hypoxaemia-related reflex bronchoconstriction/vasoconstriction, irritation of upper airway neural receptors, altered nocturnal neurohormonal secretion, pro-inflammatory mediators, within and inter-breath interactions between upper and lower airways, lung volume-airway dependence, etc. While the term overlap syndrome has been defined as the comorbid association of COPD and OSA, the interaction between asthma and OSA has not been integrated yet nosologically; in this review, the latter will be called alternative overlap syndrome. In an effort to bolster further investigations in this area, an integrated, lumping nomenclature for OSA in the setting of OLD is proposed here--OLDOSA (obstructive lung disease and obstructive sleep apnoea) syndrome.

Dyspnea perception in cystic fibrosis patients

We evaluated dyspnea perception in cystic fibrosis patients compared with normal subjects, during an inspiratory resistive loading test and 6-min walk test. We also evaluated the correlation between dyspnea scores induced by resistive loads and by the 6-min walk test. In this prospective, cross-sectional study, 31 patients with cystic fibrosis (≥15 years of age) and 31 age-, gender-, and ethnicity-matched healthy volunteers (20 females and 11 males per group) underwent inspiratory resistive loading, spirometry, and the 6-min walk test. As the magnitude of the inspiratory loads increased, dyspnea scores increased (P<0.001), but there was no difference between groups in dyspnea score (P=0.654). Twenty-six (84%) normal subjects completed all the resistive loads, compared with only 12 (39%) cystic fibrosis patients (P<0.001). Dyspnea scores were higher after the 6-min walk test than at rest (P<0.001), but did not differ between groups (P=0.080). Post-6-min walk test dyspnea scores correlated significantly with dyspnea scores induced by resistive loads. We conclude that dyspnea perception induced in cystic fibrosis patients by inspiratory resistive loading and by 6-min walk test did not differ from that induced in normal subjects. However, cystic fibrosis patients discontinued inspiratory resistive loading more frequently. There were significant correlations between dyspnea perception scores induced by inspiratory resistance loading and by the 6-min walk test. This study should alert clinicians to the fact that some cystic fibrosis patients fail to discriminate dyspnea perception and could be at risk for delay in seeking medical care.

Association of obstructive sleep apnea risk or diagnosis with daytime asthma in adults

OBJECTIVE

Obstructive sleep apnea (OSA) worsens nocturnal asthma, but its potential impact on daytime asthma remains largely unassessed. We investigated whether the sleep disorder is associated with daytime, in addition to nighttime, asthma symptoms.

METHODS

Asthma patients at tertiary-care centers completed the Sleep Apnea scale of the Sleep Disorders Questionnaire (SA-SDQ), and an asthma control questionnaire. SA-SDQ scores ≥36 for males and ≥32 for females defined high OSA risk. Medical records were reviewed for established diagnosis of OSA and continuous positive airway pressure (CPAP) use.

RESULTS

Among 752 asthma patients, high OSA risk was associated similarly with persistent daytime and nighttime asthma symptoms (p < .0001 for each). A diagnosis of OSA was robustly associated with persistent daytime (p < .0001) in addition to nighttime (p = .0008) asthma symptoms. In regression models that included obesity and other known asthma aggravators, high OSA risk retained associations with persistent daytime (odds ratio [OR] = 1.96 [95% confidence interval [CI] = 1.31-2.94]) and nighttime (1.97 [1.32-2.94]) asthma symptoms. Diagnosed OSA retained an association with persistent daytime (2.08 [1.13-3.82]) but not with nighttime (1.48 [0.82-2.69]) asthma symptoms. CPAP use was associated with lower likelihood of persistent daytime symptoms (0.46 [0.23-0.94]).

CONCLUSIONS

Questionnaire-defined OSA risk and historical diagnosis were each associated with persistent daytime asthma symptoms, to an extent that matched or exceeded associations with nighttime asthma symptoms. Unrecognized OSA may be a reason for persistent asthma symptoms during the day as well as the night.

Association of obstructive sleep apnea risk with asthma control in adults

BACKGROUND

Unrecognized obstructive sleep apnea (OSA) may lead to poor asthma control despite optimal therapy. Our objective was to evaluate the relationship between OSA risk and asthma control in adults.

METHODS

Patients with asthma seen routinely at tertiary-care clinic visits completed the validated Sleep Apnea Scale of the Sleep Disorders Questionnaire (SA-SDQ) and Asthma Control Questionnaire (ACQ). An ACQ score of >or= 1.5 defined not-well-controlled asthma, and an SA-SDQ score of >or= 36 for men and >or= 32 for women defined high OSA risk. Logistic regression was used to model associations of high OSA risk with not-well-controlled asthma (ACQ full version and short versions).

RESULTS

Among 472 subjects with asthma, the mean +/- SD ACQ (full version) score was 0.87 +/- 0.90, and 80 (17%) subjects were not well controlled. Mean SA-SDQ score was 27 +/- 7, and 109 (23%) subjects met the definition of high OSA risk. High OSA risk was associated, on average, with 2.87-times higher odds for not-well-controlled asthma (ACQ full version) (95% CI, 1.54-5.32; P = .0009) after adjusting for obesity and other factors known to worsen asthma control. Similar independent associations were seen when using the short ACQ versions.

CONCLUSIONS

High OSA risk is significantly associated with not-well-controlled asthma independent of known asthma aggravators and regardless of the ACQ version used. Patients who have difficulty achieving adequate asthma control should be screened for OSA.

Abdominal compression increases upper airway collapsibility during sleep in obese male obstructive sleep apnea patients

STUDY OBJECTIVES

Abdominal obesity, particularly common in centrally obese males, may have a negative impact on upper airway (UA) function during sleep. For example, cranial displacement of the diaphragm with raised intra-abdominal pressure may reduce axial tension exerted on the UA by intrathoracic structures and increase UA collapsibility during sleep.

DESIGN

This study aimed to examine the effect of abdominal compression on UA function during sleep in obese male obstructive sleep apnea patients.

SETTING

Participants slept in a sound-insulated room with physiologic measurements controlled from an adjacent room.

PARTICIPANTS

Fifteen obese (body mass index: 34.5 +/- 1.1 kg/m2) male obstructive sleep apnea patients (apnea-hypopnea index: 58.1 +/- 6.8 events/h) aged 50 +/- 2.6 years participated.

INTERVENTIONS

Gastric (PGA) and transdiaphragmatic pressures (P(DI)), UA closing pressure (UACP), UA airflow resistance (R(UA)), and changes in end-expiratory lung volume (EELV) were determined during stable stage 2 sleep with and without abdominal compression, achieved via inflation of a pneumatic cuff placed around the abdomen. UACP was assessed during brief mask occlusions.

MEASUREMENTS AND RESULTS

Abdominal compression significantly decreased EELV by 0.53 +/- 0.24 L (P=0.045) and increased PGA (16.2 +/- 0.8 versus 10.8 +/- 0.7 cm H2O, P < 0.001), P(DI) (11.7 +/- 0.9 versus 7.6 +/- 1.2 cm H2O, P < 0.001) and UACP (1.4 +/- 0.8 versus 0.9 +/- 0.9 cm H2O, P = 0.039) but not R(UA)(6.5 +/- 1.4 versus 6.9 +/- 1.4 cm H2O x L/s, P=0.585).

CONCLUSIONS

Abdominal compression negatively impacts on UA collapsibility during sleep and this effect may help explain strong associations between central obesity and obstructive sleep apnea.

Cough reflex is additively potentiated by inputs from the laryngeal and tracheobronchial [corrected] receptors and enhanced by stimulation of the central respiratory neurons

The cough is an essential airway defense reflex. In this study we investigated the coordination of inputs from the laryngeal and tracheobronchial receptors in the cough reflex. In 15 beagle dogs (7-9 kg) lightly anesthetized with intravenous profobol (20-30 mg/kg/h), the cough response was elicited with mechanical stimulation of either the vocal chord or tracheal bifurcation. Simultaneous stimulation of both sites increased all the parameters of cough strength, that is, mean pleural pressure (P (pl)), mean expiratory flow, number of cough bouts, and cough duration, in comparison with stimulation of the sites individually. The increases in mean P (pl) and cough duration reached statistical significance (13.3 vs. 18.4 cmH(2)O and 13.3 vs. 18.2 s, respectively). When the anesthetic level became deeper, the prolongation of cough duration almost disappeared, but the augmentation of mean P (pl) was much less affected. During stimulation of the central respiratory neurons by intravenous dimorphoramine or acute hyperoxic hypercapnia, the cough strength increased significantly. We concluded that inputs from the laryngeal and tracheobonchial cough receptors acted in concert and potentiated the cough reflex. Furthermore, stimulation of the central respiratory neurons may increase the intensity of a cough response.

Modulating effects of hypoxia and hypercarbia on glottic closing force

OBJECTIVES

Aspiration has been identified as one of the independent risk factors for development of respiratory tract infections, the incidence of which varies from 10% to 65% in patients in intensive care units. The primary defense mechanism for protection of the lower airway is the glottic closure reflex (GCR), elicited by stimulation of the internal branch of the superior laryngeal nerve. This reflex, once considered highly stable, is now considered vulnerable to a growing number of clinical factors. This study was designed to explore the biomechanical effects of hypoxia and hypercarbia, common occurrences among critically ill patients, on the GCR.

METHODS

Five adult male Yorkshire pigs were used in the study. Both internal superior laryngeal nerves were simultaneously stimulated with bipolar platinum-iridium electrodes. The glottic closing force (GCF) was then measured by placing a pressure transducer between the adducting vocal folds under 3 different protocols: protocol 1 (control), protocol 2 (hypoxia: partial pressure of arterial oxygen [PaO2] levels of 90, 70, and 50 mm Hg), and protocol 3 (hypercarbia: partial pressure of arterial carbon dioxide [PaCO2] levels of 60 and 70 mm Hg). Six readings were recorded under each experimental paradigm, and Student's t-test was applied to calculate the statistical significance against the control.

RESULTS

Hypoxia reduced the GCF to 75%, 40%, and 29% of control for PaO2 levels of 90, 70, and 50 mm Hg, respectively, and hypercarbia reduced the GCF to 40% and 27% of control for PaCO2 levels of 60 and 70 mm Hg, respectively.

CONCLUSIONS

This is the first study that highlights the biomechanical impact of hypoxia and hypercarbia on the GCR, providing a unified explanation for the increased incidence of life-threatening aspiration in critically ill patients with such alterations.

Effects of hypoxia on genioglossus and scalene reflex responses to brief pulses of negative upper-airway pressure during wakefulness and sleep in healthy men

Hypoxia can depress ventilation, respiratory load sensation, and the cough reflex, and potentially other protective respiratory reflexes such as respiratory muscle responses to increased respiratory load. In sleep-disordered breathing, increased respiratory load and hypoxia frequently coexist. This study aimed to examine the effects of hypoxia on the reflex responses of 1) the genioglossus (the largest upper airway dilator muscle) and 2) the scalene muscle (an obligatory inspiratory muscle) to negative-pressure pulse stimuli during wakefulness and sleep. We hypothesized that hypoxia would impair these reflex responses. Fourteen healthy men, 19-42 yr old, were studied on two separate occasions, approximately 1 wk apart. Bipolar fine-wire electrodes were inserted orally into the genioglossus muscle, and surface electrodes were placed overlying the left scalene muscle to record EMG activity. In random order, participants were exposed to mild overnight hypoxia (arterial oxygen saturation approximately 85%) or medical air. Respiratory muscle reflex responses were elicited via negative-pressure pulse stimuli (approximately -10 cmH(2)O at the mask, 250-ms duration) delivered in early inspiration during wakefulness and sleep. Negative-pressure pulse stimuli resulted in a short-latency activation followed by a suppression of the genioglossus EMG that did not alter with hypoxia. Conversely, the predominant response of the scalene EMG to negative-pressure pulse stimuli was suppression followed by activation with more pronounced suppression during hypoxia compared with normoxia (mean +/- SE suppression duration 64 +/- 6 vs. 38 +/- 6 ms, P = 0.006). These results indicate differential sensitivity to the depressive effects of hypoxia in the reflex responsiveness to sudden respiratory loads to breathing between these two respiratory muscles.

Cough reflex sensitivity is decreased in female obese patients with obstructive sleep apnea

Obstructive sleep apnea (OSA) is associated with impaired airway reflexes. Cough is the main airway defense mechanism but the effect of OSA on cough is unknown. Thirty-two female obese patients scheduled to undergo bariatric surgery were studied. They were classified as presenting OSA (20 patients) when the apnea-hypopnea index (AHI) was greater than 5h. Cough sensitivity was measured with citric acid. Increasing concentrations of nebulized citric acid were delivered until cough was elicited. The concentrations eliciting one (C1) and two coughs (C2) were recorded and log transformed (log C1 and log C2). log C1 and log C2 (median (interquartile)) were 1.90 mg/mL (0.90) and 2.2mg/mL (0.30) in OSA patients and 1.60 mg/mL (0.45) and 1.60 mg/mL (0.45) in non-OSA patients, respectively (comparison between groups: p=0.0372 for log C1 and p=0.0227 for log C2). A significant relationship was observed between AHI and log C1 and log C2. Cough sensitivity is therefore, decreased in female obese OSA patients and this decreased sensitivity is positively correlated with disease severity.

Effects of hypoxia and hypercapnia on nonnutritive swallowing in newborn lambs

The aim of the present study was to investigate the effect of hypercapnia and hypoxia on apnea and nonnutritive swallowing (NNS) frequency, as well as on the coordination between NNS and phases of the respiratory cycle in newborn lambs, while taking into account the potential effects of states of alertness. Six lambs were chronically instrumented for recording electroencephalogram, eye movements, diaphragm and thyroarytenoid muscle (a glottal adductor) activity, nasal airflow, and electrocardiogram. Polysomnographic recordings were performed in nonsedated lambs exposed to air (control), 10% O(2), and 5% CO(2) in a random order at 3, 4, and 5 days of age. Although hypercapnia decreased apnea frequency in wakefulness and active sleep (P = 0.002 vs. air and hypoxia), hypoxia had no significant effect on apnea. In addition, although hypercapnia increased NNS frequency during wakefulness and quiet sleep (P < 0.005 vs. air and hypoxia), hypoxia tended to decrease NNS frequency. Finally, only hypercapnia altered NNS-breathing coordination by increasing NNS at the transition from inspiration to expiration (ie-type NNS; P < 0.001 vs. air and hypoxia). In conclusion, whereas hypercapnia increases overall NNS frequency by specifically increasing ie-type NNS, hypoxia has the inverse tendency. Results were identical in all three states of alertness.

Microinjection of DLH into the region of the caudal ventral respiratory column in the cat: evidence for an endogenous cough-suppressant mechanism

The caudal ventral respiratory column (cVRC) contains premotor expiratory neurons that play an important role in cough-related expiratory activity of chest wall and abdominal muscles. Microinjection of d,l-homocysteic acid (DLH) was used to test the hypothesis that local activation of cVRC neurons can suppress the cough reflex. DLH (20-50 mM, 10-30 nl) was injected into the region of cVRC in nine anesthetized spontaneously breathing cats. Repetitive coughing was elicited by mechanical stimulation of the intrathoracic airways. Electromyograms (EMG) were recorded bilaterally from inspiratory parasternal and expiratory transversus abdominis (ABD) and unilaterally from laryngeal posterior cricoarytenoid and thyroarytenoid muscles. Unilateral microinjection of DLH (1-1.5 nmol) elicited bilateral increases in tonic and phasic respiratory ABD EMG activity, and it altered the respiratory pattern and laryngeal motor activities. However, DLH also decreased cough frequency by 51 +/- 7% compared with control (P < 0.001) and the amplitude of the contralateral (-35 +/- 3%; P < 0.001) and ipsilateral (-34 +/- 5%; P < 0.001) ABD EMGs during postinjection coughs compared with control. The cough alterations were much less pronounced after microinjection of a lower dose of DLH (0.34-0.8 nmol). No cough depression was observed after microinjections of vehicle. These results suggest that an endogenous cough suppressant neuronal network in the region of the cVRC may exist, and this network may be involved in the control of cough reflex excitability.