Exposing guinea pigs to ozone for 1 wk enhances responsiveness of rapidly adapting receptors

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.

Case report: supraventricular arrhythmia after exposure to concentrated ambient air pollution particles

CONTEXT

Exposure to air pollution can result in the onset of arrhythmias.

CASE PRESENTATION

We present a case of a 58-year-old woman who volunteered to participate in a controlled exposure to concentrated ambient particles. Twenty minutes into the exposure, telemetry revealed new onset of atrial fibrillation. The exposure was discontinued, and she reverted to normal sinus rhythm approximately 2 hr later. No abnormality was evident on the volunteer's laboratory examination or echocardiography that could explain an increased risk for supraventricular arrhythmia.

DISCUSSION

Epidemiologic evidence strongly supports a relationship between exposure to air pollutants and cardiovascular disease, but population-level data are not directly relevant to the clinical presentation of individual cases. To our knowledge, this is the only case report of an individual suffering an episode of atrial fibrillation after exposure to an air pollutant. The resolution of the arrhythmia with termination of the particle exposure further supports a causal relationship between the two.

RELEVANCE TO CLINICAL PRACTICE

Exposure to air pollution, including particulate matter, may cause supraventricular arrhythmias.

TRPA1 and sympathetic activation contribute to increased risk of triggered cardiac arrhythmias in hypertensive rats exposed to diesel exhaust

BACKGROUND

Diesel exhaust (DE), which is emitted from on- and off-road sources, is a complex mixture of toxic gaseous and particulate components that leads to triggered adverse cardiovascular effects such as arrhythmias.

OBJECTIVE

We hypothesized that increased risk of triggered arrhythmias 1 day after DE exposure is mediated by airway sensory nerves bearing transient receptor potential (TRP) channels [e.g., transient receptor potential cation channel, member A1 (TRPA1)] that, when activated by noxious chemicals, can cause a centrally mediated autonomic imbalance and heightened risk of arrhythmia.

METHODS

Spontaneously hypertensive rats implanted with radiotelemeters were whole-body exposed to either 500 μg/m³ (high) or 150 μg/m³ (low) whole DE (wDE) or filtered DE (fDE), or to filtered air (controls), for 4 hr. Arrhythmogenesis was assessed 24 hr later by continuous intravenous infusion of aconitine, an arrhythmogenic drug, while heart rate (HR) and electrocardiogram (ECG) were monitored.

RESULTS

Rats exposed to wDE or fDE had slightly higher HRs and increased low-frequency:high-frequency ratios (sympathetic modulation) than did controls; ECG showed prolonged ventricular depolarization and shortened repolarization periods. Rats exposed to wDE developed arrhythmia at lower doses of aconitine than did controls; the dose was even lower in rats exposed to fDE. Pretreatment of low wDE-exposed rats with a TRPA1 antagonist or sympathetic blockade prevented the heightened sensitivity to arrhythmia.

CONCLUSIONS

These findings suggest that a single exposure to DE increases the sensitivity of the heart to triggered arrhythmias. The gaseous components appear to play an important role in the proarrhythmic response, which may be mediated by activation of TRPA1, and subsequent sympathetic modulation. As such, toxic inhalants may partly exhibit their toxicity by lowering the threshold for secondary triggers, complicating assessment of their risk.

Ozone inhalation activates stress-responsive regions of the CNS

Ozone (O(3)), a major component of air pollution, has considerable impact on public health. Besides the well-described respiratory tract inflammation and dysfunctions, there is accumulating evidence indicating that O(3) exposure affects brain functions. However, the mechanisms through which O(3) exerts toxic effects on the brain remain poorly understood. This work aimed at precisely characterizing CNS neuronal activation after O(3) inhalation using Fos staining in adult rat. We showed that, together with lung inflammation, O(3) exposure caused a sustained time- and dose-dependent neuronal activation in the dorsolateral regions of the nucleus tractus solitarius overlapping terminal fields of lung afferents running in vagus nerves. Furthermore, we highlighted neuronal activation in interconnected central structures such as the caudal ventrolateral medulla, the parabrachial nucleus, the central nucleus of the amygdala, the bed nucleus of the stria terminalis and the paraventricular hypothalamic nucleus. In contrast, we did not detect any neuronal activation in the thoracic spinal cord where lung afferents running in spinal nerves terminate. Overall, our results demonstrate that O(3) challenge evokes a lung inflammation that induces the activation of nucleus tractus solitarius neurons through the vagus nerves and promotes neuronal activation in stress-responsive regions of the CNS.

Role of central neurotransmission and chemoreception on airway control

This review summarizes work on central neurotransmission, chemoreception and CNS control of cholinergic outflow to the airways. First, we describe the neural transmission of bronchoconstrictive signals from airway afferents to the airway-related vagal preganglionic neurons (AVPNs) via the nucleus of the solitary tract (nTS) and, second, we characterize evidence for a modulatory effect of excitatory glutamatergic, and inhibitory GABAergic, noradrenergic and serotonergic pathways on AVPN output. Excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second order neurons within the nTS, via a glutamate-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor signaling pathway. These nTS neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command through descending fibers and airway intramural ganglia to airway smooth muscle, submucosal secretory glands, and the vasculature. The strength and duration of this reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs. In addition, central noradrenergic and serotonergic inhibitory pathways appear to participate in the regulation of cholinergic drive to the tracheobronchial system. Down-regulation of these inhibitory influences results in a shift from inhibitory to excitatory drive, which may lead to increased excitability of AVPNs, heightened airway responsiveness, greater cholinergic outflow to the airways and consequently bronchoconstriction. In summary, centrally coordinated control of airway tone and respiratory drive serve to optimize gas exchange and work of breathing under normal homeostatic conditions. Greater understanding of this process should enhance our understanding of its disruption under pathophysiologic states.

Extended secondhand tobacco smoke exposure induces plasticity in nucleus tractus solitarius second-order lung afferent neurons in young guinea pigs

Infants and young children experiencing extended exposure to secondhand smoke (SHS) have an increased occurrence of asthma, as well as increased cough, wheeze, mucus production and airway hyper-reactivity. Plasticity in lung reflex pathways has been implicated in causing these symptoms, as have changes in substance P-related mechanisms. Using whole-cell voltage-clamp recordings and immunohistochemistry in brainstem slices containing anatomically identified second-order lung afferent nucleus tractus solitarius (NTS) neurons, we determined whether extended SHS exposure during the equivalent period of human childhood modified evoked or spontaneous excitatory synaptic transmission, and whether those modifications were altered by endogenous substance P. SHS exposure enhanced evoked synaptic transmission between sensory afferents and the NTS second-order neurons by eliminating synaptic depression of evoked excitatory postsynaptic currents (eEPSCs), an effect reversed by the neurokinin-1-receptor antagonist (SR140333). The recruitment of substance P in enhancing evoked synaptic transmission was further supported by an increased number of substance P-expressing lung afferent central terminals synapsing onto the second-order lung afferent neurons. SHS exposure did not change background spontaneous EPSCs. The data suggest that substance P in the NTS augments evoked synaptic transmission of lung sensory input following extended exposure to a pollutant. The mechanism may help to explain some of the exaggerated respiratory responses of children exposed to SHS.

Sensory neural targets for the treatment of cough

1. Cough is a primary defensive reflex that protects the airways from potentially harmful stimuli. 2. During many respiratory diseases, the cough reflex threshold is lowered and coughing becomes excessive. 3. Currently available therapeutics are mostly ineffective at suppressing excessive coughing. 4. In the present review, we describe the sensory neural pathways involved in cough, how these pathways may become dysfunctional in airway disease and the most recent advances that have been made in identifying future targets for cough suppression.

Early postnatal exposure to allergen and ozone leads to hyperinnervation of the pulmonary epithelium

Airway injury in infant monkeys exposed to ozone and/or house dust mite allergen (HDMA) is associated with a loss of epithelial innervation. In this study, we evaluated for persistence/recovery of the altered epithelial innervation. Thirty-day-old rhesus monkeys were exposed to repeated episodes of HDMA and/or ozone from 1 to 6 months of age and subsequently allowed to recover for 6 months in the absence of further ozone exposure and/or minimal HDMA challenge (sufficient to maintain allergen sensitization). At 1 year of age, nerve density in intrapulmonary airways was immunohistochemically evaluated using antibodies directed against protein gene product 9.5. Hyperinnervation and irregular epithelial nerve distribution was observed in both HDMA- and ozone-exposed groups; most prominent alterations were observed in animals exposed to HDMA plus ozone. Therefore, while adaptive mechanisms exist that re-establish epithelial innervation following cessation or diminution of exposure to HDMA and/or ozone, the recovery is associated with persistent proliferative mechanisms that result in hyperinnervation of the airways.

Plasticity of brainstem mechanisms of cough

The cough reflex is a brainstem reflex, consisting of specific sensory afferent nerves which trigger the reflex, by transmitting the sensory input over vagal or laryngeal nerves to a brainstem circuitry which processes and ultimately transforms the sensory input into a complex motor output to generate cough. The first synaptic target for the primary cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS). This position in the reflex pathway and intricate local circuits within the nucleus make it a strategic site where the sensory information can be modified. Plasticity at this synapse will change the nature of the output--exaggerating it, suppressing it or transforming it into some other complex pattern. This review integrates evidence implicating the NTS in exaggerated cough with proof of the concept that NTS neurons undergo plasticity to contribute to an exaggeration of cough.

Plasticity in the nucleus tractus solitarius and its influence on lung and airway reflexes

The nucleus tractus solitarius (NTS) is the first central nervous system (CNS) site for synaptic contact of the primary afferent fibers from the lungs and airways. The signal processing at these synapses will determine the output of the sensory information from the lungs and airways to all downstream synapses in the reflex pathways. The second-order NTS neurons bring to bear their own intrinsic and synaptic properties to temporally and spatially integrate the sensory information with inputs from local networks, higher brain regions, and circulating mediators, to orchestrate a coherent reflex output. There is growing evidence that NTS neurons share the rich repertoire of forms of plasticity demonstrated throughout the CNS. This review focuses on existing evidence for plasticity in the NTS, potential targets for plasticity in the NTS, and the impact of this plasticity on lung and airway reflexes.

Structural and functional localization of airway effects from episodic exposure of infant monkeys to allergen and/or ozone

Both allergen and ozone exposure increase asthma symptoms and airway responsiveness in children. Little is known about how these inhalants may differentially modify airway responsiveness in large proximal as compared to small distal airways. We evaluated whether bronchi and respiratory bronchioles from infant monkeys exposed episodically to allergen and/or ozone differentially develop intrinsic hyperresponsiveness to methacholine and whether eosinophils and/or pulmonary neuroendocrine cells play a role. Infant monkeys were exposed episodically for 5 months to: (1) filtered air, (2) aerosolized house dust mite allergen, (3) ozone 0.5 ppm, or (4) house dust mite allergen + ozone. Studying the function/structure relationship of the same lung slices, we evaluated methacholine airway responsiveness and histology of bronchi and respiratory bronchioles. In bronchi, intrinsic responsiveness was increased by allergen exposure, an effect reduced by bombesin antagonist. In respiratory bronchioles, intrinsic airway responsiveness was increased by allergen + ozone exposure. Eosinophils were increased by allergen and allergen + ozone exposure in bronchi and by allergen exposure in respiratory bronchioles. In both airways, exposure to allergen + ozone resulted in fewer tissue eosinophils than did allergen exposure alone. In bronchi, but not in respiratory bronchioles, the number of eosinophils and neuroendocrine cells correlated with airway responsiveness. We conclude that episodically exposing infant monkeys to house dust mite allergen with or without ozone increased intrinsic airway responsiveness to methacholine in bronchi differently than in respiratory bronchioles. In bronchi, eosinophils and neuroendocrine cells may play a role in the development of airway hyperresponsiveness.

CNS determinants of sleep-related worsening of airway functions: implications for nocturnal asthma

This review summarizes the recent neuroanatomical and physiological studies that form the neural basis for the state-dependent changes in airway resistance. Here, we review only the interactions between the brain regions generating quiet (non-rapid eye movement, NREM) and active (rapid eye movement, REM) sleep stages and CNS pathways controlling cholinergic outflow to the airways. During NREM and REM sleep, bronchoconstrictive responses are heightened and conductivity of the airways is lower as compared to the waking state. The decrease in conductivity of the lower airways parallels the sleep-induced decline in the discharge of brainstem monoaminergic cell groups and GABAergic neurons of the ventrolateral periaqueductal midbrain region, all of which provide inhibitory inputs to airway-related vagal preganglionic neurons (AVPNs). Withdrawal of central inhibitory influences to AVPNs results in a shift from inhibitory to excitatory transmission that leads to an increase in airway responsiveness, cholinergic outflow to the lower airways and consequently, bronchoconstriction. In healthy subjects, these changes are clinically unnoticed. However, in patients with bronchial asthma, sleep-related alterations in lung functions are troublesome, causing intensified bronchopulmonary symptoms (nocturnal asthma), frequent arousals, decreased quality of life, and increased mortality. Unquestionably, the studies revealing neural mechanisms that underlie sleep-related alterations of airway function will provide new directions in the treatment and prevention of sleep-induced worsening of airway diseases.

Brain stem excitatory and inhibitory signaling pathways regulating bronchoconstrictive responses

This review summarizes recent work on two basic processes of central nervous system (CNS) control of cholinergic outflow to the airways: 1) transmission of bronchoconstrictive signals from the airways to the airway-related vagal preganglionic neurons (AVPNs) and 2) regulation of AVPN responses to excitatory inputs by central GABAergic inhibitory pathways. In addition, the autocrine-paracrine modulation of AVPNs is briefly discussed. CNS influences on the tracheobronchopulmonary system are transmitted via AVPNs, whose discharge depends on the balance between excitatory and inhibitory impulses that they receive. Alterations in this equilibrium may lead to dramatic functional changes. Recent findings indicate that excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second-order neurons within the nucleus of the solitary tract (NTS), via a glutamate-AMPA signaling pathway. These neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command to airway effector organs: smooth muscle, submucosal secretory glands, and the vasculature, through intramural ganglionic neurons. The strength and duration of reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs and autocrine-paracrine controlling mechanisms. Downregulation of GABAergic inhibitory influences may result in a shift from inhibitory to excitatory drive that may lead to increased excitability of AVPNs, heightened airway responsiveness, and sustained narrowing of the airways. Hence a better understanding of these normal and altered central neural circuits and mechanisms could potentially improve the design of therapeutic interventions and the treatment of airway obstructive diseases.

Plasticity of central mechanisms for cough

Cough is associated with plasticity of putative cough afferent fibres, but whether plasticity in the brainstem network contributes is less well understood. A key site in the CNS network is the nucleus tractus solitarius (NTS), the first synaptic contact of the primary afferent fibres. We sought to develop a conscious guinea pig model to detect enhanced cough, to focus on the NTS as a potential site for plasticity, and to test a role for substance P in the NTS since the neuropeptide has been implicated in plasticity of the vagal afferent fibres. Guinea pigs were exposed to second-hand tobacco smoke (SHS) or filtered air (FA) from 1-6 weeks of age. At 5 weeks, cannulae were implanted in the NTS. At 6 weeks, either vehicle or a neurokinin 1 (NK-1) receptor antagonist was injected into the NTS of the conscious guinea pigs who were then exposed to citric acid aerosol. SHS exposure significantly enhanced citric acid-induced cough (56%, P<0.05), an effect attenuated by NTS NK-1 receptor blockade (P<0.05). The findings suggest that one possible mechanism for plasticity in cough is related to substance P effects in the NTS. Future studies will be required to investigate the possible mechanisms underlying the role of substance P as well as other mechanisms in generating SHS-induced cough.

Substance P released from intrinsic airway neurons contributes to ozone-enhanced airway hyperresponsiveness in ferret trachea

Exposure to ozone (O3) induces airway hyperresponsiveness mediated partly through the release of substance P (SP) from nerve terminals in the airway wall. Although substantial evidence suggests that SP is released by sensory nerves, SP is also present in neurons of airway ganglia. The purpose of this study was to investigate the role of intrinsic airway neurons in O3-enhanced airway responsiveness in ferret trachea. To remove the effects of sensory innervation, segments of ferret trachea were maintained in culture conditions for 24 h before in vitro exposure to 2 parts/million of O3 or air for 1 h. Sensory nerve depletion was confirmed by showing that capsaicin did not affect tracheal smooth muscle responsiveness to cholinergic agonist or contractility responses to electrical field stimulation (EFS). Contractions of isolated tracheal smooth muscle to EFS were significantly increased after in vitro O3 exposure, but the constrictor response to cholinergic agonist was not altered. Pretreatment with CP-99994, an antagonist of the neurokinin 1 receptor, attenuated the increased contraction to EFS after O3 exposure but had no effect in the air exposure group. The number of SP-positive neurons in longitudinal trunk ganglia, the extent of SP innervation to superficial muscular plexus nerve cell bodies, and SP nerve fiber density in tracheal smooth muscle all increased significantly after O3 exposure. The results show that release of SP from intrinsic airway neurons contributes to O3-enhanced tracheal smooth muscle responsiveness by facilitating acetylcholine release from cholinergic nerve terminals.

Neuroplasticity in nucleus tractus solitarius neurons after episodic ozone exposure in infant primates

Acute ozone exposure evokes adverse respiratory responses, particularly in children. With repeated ozone exposures, however, despite the persistent lung inflammation and increased sensory nerve excitability, the central nervous system reflex responses, i.e., rapid shallow breathing and decreased lung function, adapt, suggesting changes in central nervous system signaling. We determined whether repeated ozone exposures altered the behavior of nucleus tractus solitarius (NTS) neurons where reflex respiratory motor outputs are first coordinated. Whole cell recordings were performed on NTS neurons in brain stem slices from infant monkeys exposed to filtered air or ozone (0.5 ppm, 8 h/day for 5 days every 14 days for 11 episodes). Although episodic ozone exposure depolarized the membrane potential, increased the membrane resistance, and increased neuronal spiking responses to depolarizing current injections (P < 0.05), it decreased the excitability to vagal sensory fiber activation (P < 0.05), suggesting a diminished responsiveness to sensory transmission, despite overall increases in excitability. Substance P, implicated in lung and NTS signaling, contributed to the increased responsiveness to current injections but not to the diminished sensory transmission. The finding that NTS neurons undergo plasticity with repeated ozone exposures may help to explain the adaptation of the respiratory motor responses.

An in vivo guinea pig preparation for studying the autonomic regulation of airway smooth muscle tone

The autonomic nervous system plays a primary role in regulating airway smooth muscle tone. Here, we describe the development of an in vivo guinea pig model that permits systematic studies of the autonomic control of airway smooth muscle. The model is based on preparations previously described and utilizes measurements of isometric tension in a perfused segment of extrathoracic guinea pig trachea in situ. It has the advantage that the autonomic innervation to the tracheal segment under study can be physiologically or pharmacologically isolated and studied independently from other mechanisms regulating airway smooth muscle tone. Initial experiments were conducted to optimize model conditions. Subsequent experiments were designed to highlight the usefulness of this preparation for studying parasympathetic regulation of airway caliber. The results of the study demonstrate the utility of this model for future studies into the neural regulation of bronchomotor tone and the mechanisms of airway obstruction and hyperreactivity associated with disease.

Functional morphology and physiology of pulmonary rapidly adapting receptors (RARs)

Rapidly adapting receptors (RARs) in the airway mucosa are found from the nasopharynx to the bronchi. They have thin (Adelta) vagal afferent fibres and lie in and under the epithelium, but their morphology has not been defined. They are very sensitive to mechanical stimuli, and have a rapidly adapting irregular discharge. However, with in vitro preparations they are rather insensitive to chemical stimuli, apart from acid and nonisosmolar solutions. Their pattern of response varies with site. RARs in the nasopharynx, larynx, and trachea usually respond only during the onset of stimuli, while those in the trachea often have an off-response as well. Those in the bronchi are less rapidly adapting and more chemosensitive. Their membranes have mechanosensitive and acid-sensitive ion channels, but no vanilloid receptors. In vivo RARs are sensitive to a wide range of chemical irritants and mediators, and presumably are excited secondarily to mechanical changes in the mucosa and airway smooth muscle. In the central nervous system (CNS) they interact with other vagal afferent pathways. The reflexes they cause vary with site (inspiratory efforts from the nasopharynx, cough or expiratory efforts from the larynx and trachea, and deep breaths or tachypnoea from the bronchi). Pathways from RARs and other vagal reflexes show plasticity at the peripheral, ganglionic, and CNS levels.

Pharmacology of airway afferent nerve activity

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

Airway receptors

There are many types of afferent receptor in the airways; at least five in the larynx: pressure, drive, cold, irritant and C-fibre; and at least four in the trachea and bronchi: slowly and rapidly adapting stretch receptors (SARs and RARs), C-fibre receptors, and those in neuroepithelial bodies (NEBs). Histologically enough sensory structures have been identified to account for the various patterns of afferent activity, but most correlations are poor. For the larynx, four or more sensory structures have not definitively been identified with afferent discharges and reflex responses. For the trachea and bronchi, only SARs have been clearly identified morphologically and physiologically. The reflexes and afferent discharges from RARs and C-fibre receptors are fairly clear, some at least of the sensory terminals lie in the epithelium, but receptor complexes have not been mapped out. Nerves in NEBs have been identified, but not their local and central reflex actions.

Chronic passive cigarette smoke exposure augments bronchopulmonary C-fibre inputs to nucleus tractus solitarii neurones and reflex output in young guinea-pigs

1. Children chronically exposed to environmental tobacco smoke (passive cigarette smoke) have more wheeze, cough, bronchoconstriction, airway hyper-reactivity and mucous secretion, which may result, in part, from stimulation of the vagal bronchopulmonary C-fibre reflex. 2. Environmental tobacco smoke increases the sensitivity of bronchopulmonary C-fibre endings, but the physiological relevance of this sensitization is unknown. If this exposure augments the reflex responses via a central mechanism, then the responses of higher-order neurones in the reflex pathway and some components of the reflex output should also be augmented. 3. Guinea-pigs were chronically exposed to sidestream tobacco smoke (surrogate for environmental tobacco smoke) or filtered air for 5 days week-1 from age 1 to 6 weeks (age equivalent of human childhood) and were then anaesthetized, paralysed, ventilated and prepared with pneumothoraces. Baseline and left atrial capsaicin (0.5 and 2.0 microg kg-1)- evoked changes in the impulse activity of vagal C-fibre-activated neurones in nucleus tractus solitarii (NTS), phrenic nerve activity, tracheal pressure, arterial blood pressure and heart rate were compared in the two groups. 4. Sidestream smoke exposure significantly augmented the peak (P = 0.02) and duration (P = 0.01) of the NTS neuronal responses and the prolongation of expiratory time (P = 0.003) at the higher capsaicin dose. 5. Thus, the sensitization of the bronchopulmonary C-fibre endings by chronic exposure to sidestream tobacco smoke is transmitted to the NTS and is associated with a prolonged reflexively evoked expiratory apnoea. The findings may help to explain some related respiratory symptoms in children and be a factor in sudden infant death syndrome.