Prenatal nicotinic exposure upregulates pulmonary C-fiber NK1R expression to prolong pulmonary C-fiber-mediated apneic response

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Neurokinin 1 and 2 Receptors Are Involved in PEG2- and Citric Acid-Induced Cough and Ventilatory Responses

Exposure to aerosolized citric acid (CA, 150mM) and prostaglandin E₂ (PGE₂, 0.43mM) for 10min in guinea pigs reportedly produces the distinct cough patterns (Type I vs. II) and ventilatory responses (long-lasting hyperventilation vs. brief tachypnea) even though triggering the same cough numbers. Type I and II coughs are primarily mediated by activation of TRPV1 and EP3 receptors (a PGE₂ receptor) of vagal C-fibers respectively. Substance P (SP) and neurokinin A (NKA) released by vagal pulmonary sensory fibers peripherally are capable of affecting CA-induced cough and ventilation via preferentially activating neurokinin 1 and 2 receptors (NK₁R and NK₂R) respectively. This study aimed to define the impacts of CA- and PGE₂-exposure on pulmonary SP and NKA levels and the roles of NK₁R and NK₂R in modulating CA- and PGE₂-evoked cough and ventilatory responses. In unanesthetized guinea pigs, we determined: 1) pulmonary SP and NKA contents induced by the CA- or PGE₂-exposure; 2) effects of CP-99994 and SR-48968 (a NK₁R and a NK₂R antagonist respectively) given by intraperitoneal injection (IP) or aerosol inhalation (IH) on the CA- and PGE₂-evoked cough and ventilatory responses; and 3) immunocytochemical expressions of NK₁R/NK₂R in vagal C-neurons labeled by TRPV1 or EP3 receptors. We found that CA- and PGE₂-exposure evoked Type I and II cough respectively associated with different degrees of increases in pulmonary SP and NKA. Applications of CP-99994 and SR-48968 via IP and IH efficiently suppressed the cough responses to CA with less impact on the cough response to PGE₂. These antagonists inhibited or blocked the ventilatory response to CA and caused hypoventilation in response to PGE₂. Moreover, NK₁R and NK₂R were always co-expressed in vagal C-neurons labeled by TRPV1 or EP3 receptors. These results suggest that SP and NKA endogenously released by CA- and PGE₂-exposure play important roles in generating the cough and ventilatory responses to CA and PGE₂, at least in part, via activation of NK₁R and NK₂R expressed in vagal C-neurons (pulmonary C-neurons).

Cross-effect of TRPV1 and EP3 receptor on coughs and bronchopulmonary C-neural activities

Prostaglandin E₂ (PGE₂)-induced coughs in vivo and vagal nerve depolarization in vitro are inhibited by systemic and local administration of prostaglandin EP3 receptor (L-798106) and TRPV1 antagonists (JNJ 17203212). These results indicate a modulating effect of TRPV1 on the EP3 receptor-mediated cough responses to PGE₂ likely through the vagal sensory nerve. This study aimed to determine whether 1) inhalation of aerosolized JNJ 17203212 and L-798106 affected cough responses to citric acid (CA, mainly stimulating TRPV1) and PGE₂; 2) TRPV1 and EP3 receptor morphologically are co-expressed and electrophysiologically functioned in the individual of vagal pulmonary C-neurons (cell bodies of bronchopulmonary C-fibers in the nodose/jugular ganglia); and 3) there was a cross-effect of TRPV1 and EP3 receptor on these neural excitations. To this end, aerosolized CA or PGE₂ was inhaled by unanesthetized guinea pigs pretreated without or with each antagonist given in aerosol form. Immunofluorescence was applied to identify the co-expression of TRPV1 and EP3 receptor in vagal pulmonary C-neurons (retrogradely traced by DiI). Whole-cell voltage patch clamp approach was used to detect capsaicin (CAP)- and PGE₂-induced currents in individual vagal pulmonary C-neurons and determine the effects of the TRPV1 and EP3 receptor antagonists on the evoked currents. We found that PGE₂-induced cough was attenuated by JNJ 17203212 or L-798106 and CA-evoked cough greatly suppressed only by JNJ 17203212. Approximately 1/4 of vagal pulmonary C-neurons co-expressed EP3 with a cell size < 20 μm. Both CAP- and PGE₂-induced currents could be recorded in the individuals of some vagal pulmonary C-neurons. The former was largely inhibited only by JNJ 17203212, while the latter was suppressed by JNJ 17203212 or L-798106. The similarity of the cross-effect of both antagonists on cough and vagal pulmonary C-neural activity suggests that a subgroup of vagal pulmonary C-neurons co-expressing TRPV1 and EP3 receptor is, at least in part, responsible for the cough response to PGE₂.

Prolongation of bronchopulmonary C-fiber-mediated apnea by prenatal nicotinic exposure in rat pups: role of 5-HT3 receptors

Prenatal nicotinic exposure (PNE) reportedly sensitizes bronchopulmonary C-fibers (PCFs) and prolongs PCF-mediated apnea in rat pups, contributing to the pathogenesis of sudden infant death syndrome. Serotonin, or 5-hydroxytryptamine (5-HT), induces apnea via acting on 5-HT receptor 3 (5-HT3R) in PCFs, and among the 5-HT3R subunits, 5-HT3B is responsible for shortening the decay time of 5-HT3R-mediated currents. We examined whether PNE would promote pulmonary 5-HT secretion and prolong the apnea mediated by 5-HT3Rs in PCFs via affecting the 5-HT3B subunit. To this end, the following variables were compared between the control and PNE rat pups: 1) the 5-HT content in bronchoalveolar lavage fluid, 2) the apneic response to the right atrial bolus injection of phenylbiguanide (a 5-HT3R agonist) before and after PCF inactivation, 3) 5-HT3R currents and the stimulus threshold of the action currents of vagal pulmonary C-neurons, and 4) the immunoreactivity (IR) and mRNA expression of 5-HT3A and 5-HT3B in these neurons. Our results showed that PNE up-regulated the pulmonary 5-HT concentration and strengthened the PCF 5-HT3R-mediated apnea. PNE significantly facilitated neural excitability by shortening the decay time of 5-HT3R currents, lowering the stimulus threshold, and increasing 5-HT3B IR. In summary, PNE prolongs the apnea mediated by 5-HT3Rs in PCFs, likely by increasing 5-HT3B subunits to enhance the excitability of 5-HT3 channels.-Zhao, L., Gao, X., Zhuang, J., Wallen, M., Leng, S., Xu, F. Prolongation of bronchopulmonary C-fiber-mediated apnea by prenatal nicotinic exposure in rat pups: role of 5-HT3 receptors.

α7 Nicotinic acetylcholine receptor contributes to the alleviation of lung ischemia-reperfusion injury by transient receptor potential vanilloid type 1 stimulation

BACKGROUND

Activation of transient receptor potential vanilloid type 1 (TRPV1) decreases lung ischemia-reperfusion injury (LIRI) in rabbits and rats. Stimulation of α7 nicotinic acetylcholine receptors (α7nAChRs) protects against lung injury. Here we examined whether α7nAChRs contribute to TRPV1-mediated protection against LIRI.

METHODS

Wild-type (WT) and TRPV1-knockout (KO) mice were subjected to 1-h lung ischemia by clamping left hilum, followed by 2-h reperfusion. WT or KO mice were pretreated with vehicle, TRPV1 agonist capsaicin, TRPV1 antagonist capsazepine, α7nAChR antagonist methyllycaconitine, or α7nAChR agonist PNU-282987. Arterial blood and lung tissues were obtained for blood gas, lung wet-to-dry weight ratio, interleukin (IL)1β, IL6, tumor necrosis factor-α (TNF-α), apoptosis-related proteins (caspases, Bax, Fas), and pathologic scoring.

RESULTS

Capsaicin pretreatment reduced wet-to-dry ratio, pathologic score, alveolar-arterial oxygen gradient (A-aDO₂), and IL1β, IL6, and TNFα levels in WT mice, with no effects in KO mice. This reduction was reversed by TRPV1 blockade. Furthermore, α7nAChR blockade before capsaicin exacerbated LIRI as evidenced by enhanced alveolar-arterial oxygen gradient, pathologic score, and IL1β, IL6, and TNFα levels, while α7nAChR agonist pretreatment under TRPV1 blockade showed opposite changes. Capsaicin also decreased cleaved caspase-3, caspase-3/9, and Bax protein expression, effects abolished by TRPV1 blockade. Similarly, α7nAChR blockade diminished capsaicin-induced downregulation of apoptotic proteins, and α7nAChR activation decreased expression levels even under TRPV1 blockade.

CONCLUSIONS

TRPV1 activation alleviates LIRI, partially dependent on α7nAChR activity. The α7nAChR stimulation with or without existence of TRPV1 alleviates LIRI. Thus, α7nAChR is involved in the pathway of TRPV1-mediated protection against LIRI and the specific mechanism remains to be revealed.

Roles of Bronchopulmonary C-fibers in airway Hyperresponsiveness and airway remodeling induced by house dust mite

Background

Asthma is characterized by chronic airway inflammation, airway hyperresponsiveness (AHR), and airway remodeling. While exposure of house dust mites (HDM) is a common cause of asthma, the pathogenesis of the HDM-induced asthma is not fully understood. Bronchopulmonary C-fibers (PCFs) contribute to the neurogenic inflammation, viral infection induced-persistent AHR, and ovalbumin induced collagen deposition largely via releasing neuropeptides, such as substance P (SP). However, PCF roles in the pathogenesis of the HDM-induced asthma remain unexplored. The goal of this study was to determine what role PCFs played in generating these characteristics.

Methods

We compared the following variables among the PCF-intact and -degenerated BALB/c mice with and without chronic HDM exposure (four groups): 1) AHR and pulmonary SP; 2) airway smooth muscle (ASM) mass; 3) pulmonary inflammatory cells; and 4) epithelium thickening and mucus secretion.

Results

We found that HDM evoked AHR associated with upregulation of pulmonary SP and inflammation, ASM mass increase, epithelium thickenings, and mucus hypersecretion. PCF degeneration decreased the HDM-induced changes in AHR, pulmonary SP and inflammation, and ASM mass, but failed to significantly affect the epithelium thickening and mucus hypersecretion.

Conclusion

Our data suggest an involvement of PCFs in the mechanisms by which HDM induces allergic asthma via airway inflammation, AHR, and airway remodeling.

Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation

Maternal cigarette smoke, including prenatal nicotinic exposure (PNE), is responsible for sudden infant death syndrome (SIDS). The fatal events of SIDS are characterized by severe bradycardia and life-threatening apneas. Although activation of transient receptor potential vanilloid 1 (TRPV1) of superior laryngeal C fibers (SLCFs) could induce bradycardia and apnea and has been implicated in SIDS pathogenesis, how PNE affects the SLCF-mediated cardiorespiratory responses remains unexplored. Here, we tested the hypothesis that PNE would aggravate the SLCF-mediated apnea and bradycardia via up-regulating TRPV1 expression and excitation of laryngeal C neurons in the nodose/jugular (N/J) ganglia. To this end, we compared the following outcomes between control and PNE rat pups at postnatal days 11-14: 1) the cardiorespiratory responses to intralaryngeal application of capsaicin (10 µg/ml, 50 µl), a selective stimulant for TRPV1 receptors, in anesthetized preparation; 2) immunoreactivity and mRNA of TRPV1 receptors of laryngeal sensory C neurons in the N/J ganglia retrogradely traced by 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate; and 3) TRPV1 currents and electrophysiological characteristics of these neurons by using whole-cell patch-clamp technique in vitro Our results showed that PNE markedly prolonged the apneic response and exacerbated the bradycardic response to intralaryngeal perfusion of capsaicin, which was associated with up-regulation of TRPV1 expression in laryngeal C neurons. In addition, PNE increased the TRPV1 currents, depressed the slow delayed rectifier potassium currents, and increased the resting membrane potential of these neurons. Our results suggest that PNE is capable of aggravating the SLCF-mediated apnea and bradycardia through TRPV1 sensitization and neuronal excitation, which may contribute to the pathogenesis of SIDS.-Gao, X., Zhao, L., Zhuang, J., Zang, N., Xu, F. Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation.

From the Cover: Prenatal Nicotinic Exposure Attenuates Respiratory Chemoreflexes Associated With Downregulation of Tyrosine Hydroxylase and Neurokinin 1 Receptor in Rat Pup Carotid Body

Maternal cigarette smoke is the major risk of sudden infant death syndrome (SIDS). A depressed ventilatory response to hypoxia (HVR) and hypercapnia (HCVR) is thought to be responsible for the pathogenesis of SIDS and the carotid body is critically involved in these responses. We have recently reported that prenatal nicotinic exposure (PNE) over the full gestation induces depressed HVR in rat pups. Here, we asked whether PNE (1) depressed not only HVR but also HCVR that were dependent on the carotid body, (2) affected some important receptors and neurochemicals expressed in the carotid body, such as tyrosine hydroxylase (TH), neurokinin-1 receptor (NK1R), and α7 nicotinic acetylcholine receptor (α7nAChR), and (3) blunted the ventilatory responses to activation of these receptors. To this end, HVR and HCVR in Ctrl and PNE pups were measured with plethysmography before and after carotid body ablation (Series I), mRNA expression and/or immunoreactivity (IR) of TH, NK1R, and α7nAChR in the carotid body were examined by RT-PCR and immunohistochemistry (Series II), and the ventilatory responses were tested before and after intracarotid injection of substance P (NK1R agonist) and AR-R17779 (α7nAChR agonist) (Series III). Our results showed that PNE (1) significantly depressed both HVR and HCVR and these depressions were abolished by carotid body ablation, (2) reduced the relative population of glomus cells, mRNA NK1R, and α7nAChR and IR of NK1R and TH in the carotid body, and (3) decreased ventilatory responses to intracarotid injection of substance P or AR-R17779. These results suggest that PNE acting via the carotid body could strikingly blunt HVR and HCVR, likely through downregulating TH and NK1R.

Bronchopulmonary C-fibers' IL1RI contributes to the prolonged apneic response to intra-atrial injection of capsaicin by prenatal nicotinic exposure in rat pups

Prenatal nicotinic exposure (PNE) as a SIDS model reportedly sensitizes bronchopulmonary C-fibers (PCFs), contributing to the prolonged PCF-mediated apnea in rat pups, but the relevant mechanisms are not fully understood. Pulmonary IL-1β upregulated by cigarette smoke is known to stimulate or sensitize PCFs acting via IL-1 type I receptor (IL1RI) and inhibit inspiration frequency. Because of its upregulation observed in SIDS victims, we hypothesized that PNE increased pulmonary IL-1β release and IL1RI expression in pulmonary C-neurons via action on α7 nicotinic acetylcholine receptors (α7nAChR) to induce the prolonged PCF-mediated apnea. IL-1β in BALF and IL1RI in the nodose/jugular (N/J) ganglion and vagal pulmonary C-neurons retrogradely-traced were compared between Ctrl (saline) and PNE pups and among the vehicle-treated Ctrl and PNE and methyllycaconitine (a selective α7nAChR antagonist)-treated PNE pups. The effect of IL-1RI blockade (IL-1Ra) on the PCF-mediated apnea was also compared between Ctrl and PNE pups. PNE significantly elevated IL-1β in BALF and upregulated IL1RI gene and protein expression in N/J ganglia and gene in vagal pulmonary C-neurons. All of these responses were eliminated by pretreatment with blockade of α7nAChR. In addition, the prolonged PCF-mediated apnea in PNE pups was significantly shortened by right atrial bolus injection of IL-1Ra. We conclude that PNE enhances pulmonary IL-1β release and PCF IL1RI expression acting via α7nAChR in contributing to sensitization of PCFs and prolongation of the PCF-mediated apneic response.