Secondhand tobacco smoke exposure differentially alters nucleus tractus solitarius neurons at two different ages in developing non-human primates

Global Physiology and Pathophysiology of Cough: Part 2. Demographic and Clinical Considerations: CHEST Expert Panel Report

BACKGROUND

Cough characteristics vary between patients, and this can impact clinical diagnosis and care. The purpose of part two of this state-of-the-art review is to update the American College of Chest Physicians (CHEST) 2006 guideline on global physiology and pathophysiology of cough.

STUDY DESIGN AND METHODS

A review of the literature was conducted using PubMed and MEDLINE databases from 1951 to 2019 using prespecified search terms.

RESULTS

We describe the demographics of typical patients with cough in the clinical setting, including how cough characteristics change across age. We summarize the effect of common clinical conditions impacting cough mechanics and the physical properties of mucus on airway clearance.

INTERPRETATION

This is the second of a two-part update to the 2006 CHEST cough guideline; it complements part one on basic phenomenology of cough by providing an extended clinical picture of cough along with the factors that alter cough mechanics and efficiency in patients. A greater understanding of the physiology and pathophysiology of cough will improve clinical management.

Glial activation and inflammation in the NTS in a rat model after exposure to diesel exhaust particles

Airway pollution can affect the central nervous system, but whether this causes glial activation and inflammation in the nucleus of solitary tract (NTS) remains unclear. We used a rat model with exposure to diesel exhaust particulate matter (DEP) at 200 μg/m3 (low exposure) and 1000 μg/m3 (high exposure) for 14 days. Activation of microglia and astrocytes in the NTS was assessed using Iba-1 and glial fibrillary acidic protein (GFAP) staining. The expression of neurotrophic factors including brain-derived neurotrophic factor (BDNF), glial-derived neurotrophic factor (GDNF), and nerve growth factor (NGF) in the NTS were evaluated by immunofluorescence. Changes in the intracellular structure of NTS neurons were observed via electron microscopy. Inflammatory cytokines and oxidant stress levels in the medulla were also measured. Exposure to DEP can cause NTS inflammation as well as airway inflammation, especially in the H-exposure group. We showed that the numbers of microglia and astrocytes in the NTS, as well as NGF expression in the NTS, were significantly higher in both exposure groups than in controls, but BDNF or GDNF expression was not detected. Exposure to DEP induced ultrastructural changes in NTS neurons as reflected by endoplasmic reticulum dilation, ribosomal loss, mitochondrial vacuolization, and a sparse myelin sheath. Medulla inflammation and an imbalance of oxidants and antioxidants also resulted from exposure to DEP. The H-exposure group showed an imbalance of oxidants and antioxidants with decreased levels of SOD and GSH and increased levels of MDA and ROS compared to the control group (both p < 0.01) in the medulla. Inflammatory cytokines (IL-1β, IL-6, and TNF-α) were also significantly increased in the H-exposure group. Fourteen days of exposure to DEP can affect the NTS neurons in rat. Glial activation and inflammation may play important roles in the response of the NTS to DEP.

Seasonal Timing of Infant Bronchiolitis, Apnea and Sudden Unexplained Infant Death

Rates of Sudden Unexplained Infant Death (SUID), bronchiolitis, and central apnea increase in winter in temperate climates. Though associations between these three conditions are suggested, more work is required to establish if there is a causal pathway linking bronchiolitis to SUID through inducing central apnea. Utilizing a large population-based cohort of infants studied over a 20-year period (n = 834,595, from birth years 1989–2009)), we analyzed ecological associations between timing of SUID cases, bronchiolitis, and apnea healthcare visits. Data were analyzed between 2013 and 2015. We used a Cox Proportional Hazards model to analyze possible interactions between maternal smoking and maternal asthma with infant bronchiolitis on time to SUID. SUID and bronchiolitis both occurred more frequently in winter. An increase in bronchiolitis clinical visits occurred within a few days prior to apnea visits. We found a temporal relationship between infant bronchiolitis and apnea. In contrast, no peak in SUID cases was seen during peaks of bronchiolitis. Among those without any bronchiolitis visits, maternal smoking was associated with an increased risk of SUID: Hazard Ratio (HR) of 2.38 (95% CI: 2.11, 2.67, p-value <0.001). Maternal asthma was associated with an increased risk of SUID among infants with at least one bronchiolitis visit: HR of 2.40 (95% CI: 1.04, 5.54, p-value = 0.04). Consistent trends between bronchiolitis, apnea, and SUID were not established due to small numbers of SUID cases. However, interaction analysis revealed potential differential associations of bronchiolitis and SUID by maternal smoking, maternal asthma status.

Protection of signal processing at low temperature in baroreceptive neurons in the nucleus tractus solitarius of Syrian hamsters, a hibernating species

We previously described synaptic currents between baroreceptor fibers and second-order neurons in the nucleus tractus solitarius (NTS) that were larger in Syrian hamsters than in rats. This suggested that although electrical activity throughout the hamster brain decreased as brain temperature declined, the greater synaptic input to its NTS would support continued operation of cardiorespiratory reflexes at low body temperatures. Here, we focused on properties that would protect these neurons against potential damage from the larger synaptic inputs, testing the hypotheses that hamster NTS neurons exhibit: 1) intrinsic N-methyl-D-aspartate receptor (NMDAR) properties that limit Ca(2+) influx to a greater degree than do rat NTS neurons and 2) properties that reduce gating signals to NMDARs to a greater degree than in rat NTS neurons. Whole cell patch-clamp recordings on anatomically identified second-order NTS baroreceptive neurons showed that NMDAR-mediated synaptic currents between sensory fibers and second-order NTS neurons were larger in hamsters than in rats at 33°C and 15°C, with no difference in their permeability to Ca(2+). However, at 15°C, but not at 33°C, non-NMDAR currents evoked by glutamate released from baroreceptor fibers had significantly shorter durations in hamsters than in rats. Thus, hamster NMDARs did not exhibit lower Ca(2+) influx than did rats (negating hypothesis 1), but they did exhibit significant differences in non-NMDAR neuronal properties at low temperature (consistent with hypothesis 2). The latter (shorter duration of non-NMDAR currents) would likely limit NMDAR coincidence gating and may help protect hamster NTS neurons, enabling them to contribute to signal processing at low body temperatures.

Realignment of signal processing within a sensory brainstem nucleus as brain temperature declines in the Syrian hamster, a hibernating species

Crucial for survival, the central nervous system must reliably process sensory information over all stages of a hibernation bout to ensure homeostatic regulation is maintained and well-matched to dramatically altered behavioral states. Comparing neural responses in the nucleus tractus solitarius of rats and euthermic Syrian hamsters, we tested the hypothesis that hamster neurons have adaptations sustaining signal processing while conserving energy. Using patch-clamp techniques, we classified second-order neurons in the nucleus as rapid-onset or delayed-onset spiking phenotypes based on their spiking onset to a depolarizing pulse (following a -80 mV prepulse). As temperature decreased from 33 to 15°C, the excitability of all neurons decreased. However, hamster rapid-onset spiking neurons had the highest spiking response and shortest action potential width at every temperature, while hamster delayed-onset spiking neurons had the most negative resting membrane potential. The frequency of spontaneous excitatory postsynaptic currents in both phenotypes decreased as temperature decreased, yet the amplitudes of tractus solitarius stimulation-evoked currents were greater in hamsters than in rats regardless of phenotype and temperature. Changes were significant (P < 0.05), supporting our hypothesis by showing that, as temperature falls, rapid-onset neurons contribute more to signal processing but less to energy conservation than do delayed-onset neurons.

Effects of cigarette smoke exposure on nicotinic acetylcholine receptor subunits α7 and β2 in the sudden infant death syndrome (SIDS) brainstem

It is postulated that nicotine, as the main neurotoxic constituent of cigarette smoke, influences SIDS risk through effects on nicotinic acetylcholine receptors (nAChRs) in brainstem nuclei that control respiration and arousal. This study compared α7 and β2 nAChR subunit expression in eight nuclei of the caudal and rostral medulla and seven nuclei of the pons between SIDS (n=46) and non-SIDS infants (n=14). Evaluation for associations with known SIDS risk factors included comparison according to whether infants had a history of exposure to cigarette smoke in the home, and stratification for sleep position and gender. Compared to non-SIDS infants, SIDS infants had significantly decreased α7 in the caudal nucleus of the solitary tract (cNTS), gracile and cuneate nuclei, with decreased β2 in the cNTS and increased β2 in the facial. When considering only the SIDS cohort: 1-cigarette smoke exposure was associated with increased α7 in the vestibular nucleus and increased β2 in the rostral dorsal motor nucleus of the vagus, rNTS and Cuneate, 2-there was a gender interaction for α7 in the gracile and cuneate, and β2 in the cNTS and rostral arcuate nucleus, and 3-there was no effect of sleep position on α7, but prone sleep was associated with decreased β2 in three nuclei of the pons. In conclusion, SIDS infants demonstrate differences in expression of α7 and β2 nAChRs within brainstem nuclei that control respiration and arousal, which is independent on prior history of cigarette smoke exposure, especially for the NTS, with additional differences for smoke exposure (β2), gender (α7 and β2) and sleep position (β2) evident.

Distinct tachykinin NK(1) receptor function in primate nucleus tractus solitarius neurons is dysregulated after second-hand tobacco smoke exposure

BACKGROUND AND PURPOSE

Second-hand tobacco smoke (SHS) exposure in children increases the risk of asthma and sudden infant death syndrome. Epidemiological and experimental data have suggested SHS can alter neuroplasticity in the CNS, associated with substance P. We hypothesized that exposure to SHS in young primates changed the effect of substance P on the plasticity of neurons in the nucleus tractus solitarius (NTS), where airway sensory information is first processed in the CNS.

EXPERIMENTAL APPROACH

Thirteen-month-old rhesus monkeys were exposed to filtered air (FA, n= 5) or SHS (n= 5) for >6 months from 50 days of their fetal age. Whole-cell patch-clamp recordings were performed on NTS neurons in brainstem slices from these animals to record the intrinsic cell excitability in the absence or presence of the NK(1) receptor antagonist, SR140333 (3 µM).

KEY RESULTS

Neurons were electrophysiologically classified based on their spiking onset from a hyperpolarized membrane potential into two phenotypes: rapid-onset spiking (RS) and delayed-onset spiking (DS) types. In RS neurons, SR140333 reduced the spiking response, similarly in both FA- and SHS-exposed animals. In DS neurons, SR140333 almost abolished the spiking response in FA-exposed animals, but had no effect in SHS-exposed animals.

CONCLUSIONS AND IMPLICATIONS

The contribution of NK(1) receptors to cell excitability depended on firing phenotype of primate NTS neurons and was disrupted by SHS exposure, specifically in DS neurons. Our findings reveal a novel NK(1) receptor function in the primate brainstem and support the hypothesis that chronic exposure to SHS in children causes tachykinin-related neuroplastic changes in the CNS.

Pathophysiological impact of cigarette smoke exposure on the cerebrovascular system with a focus on the blood-brain barrier: expanding the awareness of smoking toxicity in an underappreciated area

Recent evidence has indicated that active and passive cigarette smoking are associated, in a dose-dependent manner, with dysfunction of normal endothelial physiology. Tobacco smoke (TS) may predispose individuals to atherogenic and thrombotic problems, significantly increasing the risk for ischemic manifestations such as acute coronary syndrome and stroke. Despite the strong evidence for an association between smoking and vascular impairment, the impact of TS exposure on the blood-brain barrier (BBB) has only been marginally addressed. This is a major problem given that the BBB is crucial in the maintenance of brain homeostasis. Recent data have also shown that chronic smokers have a higher incidence of small vessel ischemic disease (SVID), a pathological condition characterized by leaky brain microvessels and loss of BBB integrity. In the brain TS increases the risk of silent cerebral infarction (SCI) and stroke owing to the pro-coagulant and atherogenic effects of smoking. In this article we provide a detailed review and analysis of current knowledge of the pathophysiology of tobacco smoke toxicity at the cerebrovascular levels. We also discuss the potential toxicity of recently marketed “potential-reduced exposure products”.

Secondhand smoke exposure alters K+ channel function and intrinsic cell excitability in a subset of second-order airway neurons in the nucleus tractus solitarius of young guinea pigs

Extended exposure to secondhand smoke (SHS) in infants and young children increases the incidence of cough, wheeze, airway hyper-reactivity and the prevalence and earlier onset of asthma. The adverse effects may result from environmentally-induced plasticity in the neural network regulating cough and airway function. Using whole-cell patch-clamp recordings in brainstem slices containing anatomically identified second-order lung afferent neurons in the nucleus tractus solitarius (NTS), we determined the effects of extended SHS exposure in young guinea pigs for a duration equivalent to human childhood on the intrinsic excitability of NTS neurons. SHS exposure resulted in marked decreases in the intrinsic excitability of a subset of lung afferent second-order NTS neurons. The neurons exhibited a decreased spiking capacity, prolonged action potential duration, reduced afterhyperpolarization, decrease in peak and steady-state outward currents, and membrane depolarization. SHS exposure effects were mimicked by low concentrations of the K+ channel blockers 4-aminopyridine and/or tetraethyl ammonium. The data suggest that SHS exposure downregulates K+ channel function in a subset of NTS neurons, resulting in reduced cell excitability. The changes may help to explain the exaggerated neural reflex responses in children exposed to SHS.