MDL 72222 (a selective 5-HT3 receptor antagonist) prevents stimulation of intrapulmonary C fibres by pulmonary embolization in anaesthetized rabbits

Activation of mouse bronchopulmonary C-fibres by serotonin and allergen-ovalbumin challenge

Abstract  The effect of serotonin on capsaicin-sensitive vagal C-fibre afferent nerves was evaluated in an ex vivo vagally innervated mouse lung preparation. Action potentials arising from receptive fields in the lungs were recorded with an extracellular electrode positioned in the nodose/jugular ganglion. Among the 62 capsaicin-sensitive C-fibres studied (conduction velocity ∼0.5 m s(-1)), 71% were of the nodose phenotype and 29% of the jugular phenotype. The nodose C-fibres responded strongly to serotonin and this effect was blocked with the 5-HT3-receptor antagonist ondansetron. Using single cell RT-PCR, we noted that the vast majority of nodose neurons retrogradely labelled from the lung, expressed 5-HT3 receptor mRNA. The jugular C-fibres also responded strongly to serotonin with action potential discharge, but this effect was not inhibited by ondansetron. Lung-specific jugular neurons did not express 5-HT3 receptor mRNA but frequently expressed 5-HT1 or 5-HT4 receptor mRNA. Mast cells are the major source of serotonin in healthy murine airways. Ovalbumin-induced mast cell activation in actively sensitized lungs caused action potential discharge in jugular but not nodose C-fibres. The data show that vagal C-fibres in the respiratory tract of the mouse are strongly activated by serotonin. Depending on the C-fibre subtype both 5-HT3 and non-5-HT3 mechanisms are involved.

Cough sensors. V. Pharmacological modulation of cough sensors

Several airway afferent nerve subtypes have been implicated in coughing. These include bronchopulmonary C-fibers, rapidly adapting airway mechanoreceptors and touch-sensitive tracheal Adelta-fibers (also called cough receptors). Although the last two afferent nerve subtypes are primarily sensitive to mechanical stimuli, all can be acted upon by one or more different chemical stimuli. In this review we catalogue the chemical agents that stimulate and/or modulate the activity of the airway afferent nerves involved in cough, and describe the specific mechanisms involved in these effects. In addition, we describe the mechanisms of action of a number of chemical inhibitors of these afferent nerve subtypes, and attempt to relate this information to the regulation of coughing in health and disease.

Airway nerves and dyspnea associated with inflammatory airway disease

The neurobiology of dyspnea is varied and complex, but there is little doubt that vagal nerves within the airways are capable of causing or modulating some dyspneic sensations, especially those associated with inflammatory airway diseases. A major contributor to the dyspnea associated with inflammatory airway disease is explained by airway narrowing and increases in the resistance to airflow. The autonomic (parasympathetic) airway nerves directly contribute to this by regulating bronchial smooth muscle tone and mucus secretion. In addition, a component of the information reaching the brainstem via airway mechanosensing and nociceptive afferent nerves likely contributes to the overall sensations of breathing. The airway narrowing can lead to activation of low threshold mechanosensitive stretch receptors, and vagal and spinal C-fibers as well as some rapidly adapting stretch receptor in the airways that are directly activated by various aspects of the inflammatory response. Inflammatory mediators can induce long lasting changes in afferent nerve activity by modulating the expression of key genes. The net effect of the increase in afferent traffic to the brainstem modulates synaptic efficacy at the second-order neurons via various mechanisms collectively referred to as central sensitization. Many studies have shown that stimuli that activate bronchopulmonary afferent nerves can lead to dyspnea in healthy subjects. A logical extension of the basic research on inflammation and sensory nerve function is that the role of vagal sensory nerve in causing or shaping dyspneic sensations will be exaggerated in those suffering from inflammatory airway disease.

Reflex regulation of airway smooth muscle tone

Autonomic nerves in most mammalian species mediate both contractions and relaxations of airway smooth muscle. Cholinergic-parasympathetic nerves mediate contractions, whereas adrenergic-sympathetic and/or noncholinergic parasympathetic nerves mediate relaxations. Sympathetic-adrenergic innervation of human airway smooth muscle is sparse or nonexistent based on histological analyses and plays little or no role in regulating airway caliber. Rather, in humans and in many other species, postganglionic noncholinergic parasympathetic nerves provide the only relaxant innervation of airway smooth muscle. These noncholinergic nerves are anatomically and physiologically distinct from the postganglionic cholinergic parasympathetic nerves and differentially regulated by reflexes. Although bronchopulmonary vagal afferent nerves provide the primary afferent input regulating airway autonomic nerve activity, extrapulmonary afferent nerves, both vagal and nonvagal, can also reflexively regulate autonomic tone in airway smooth muscle. Reflexes result in either an enhanced activity in one or more of the autonomic efferent pathways, or a withdrawal of baseline cholinergic tone. These parallel excitatory and inhibitory afferent and efferent pathways add complexity to autonomic control of airway caliber. Dysfunction or dysregulation of these afferent and efferent nerves likely contributes to the pathogenesis of obstructive airways diseases and may account for the pulmonary symptoms associated with extrapulmonary disorders, including gastroesophageal reflux disease, cardiovascular disease, and rhinosinusitis.

Sensory transduction in cough-associated nerves

Before a tussive stimulus in the airways can evoke a cough reflex it must first cause action potential discharge in cough-associated vagal sensory nerves. This is initiated by the stimulus first interacting with the receptors and ion channels in the terminal membrane of the sensory fiber in a manner that leads to membrane depolarization. If the stimulus-induced membrane depolarization, referred to as a generator potential, is of sufficient magnitude, action potentials are elicited that are then conducted to the central nervous system. If the action potentials are of sufficient number and frequency, a cough is evoked. The most common tussive stimuli include mechanical perturbations, anosmotic solutions, acidic solutions, and various chemical agents. The mechanisms underlying the transduction of most of these tussive stimuli into a generator potential are only partially understood. In general terms, chemical stimuli interact directly with receptors that are classified as either ligand gated ion channels or metabotropic receptors (e.g. G-protein coupled receptors). Ligand gated receptors are those in which the receptor protein also serves as the ion channel. The metabotropic receptors indirectly modulate the ion channels activity via various signal transduction schemes. Mechanical stimuli are thought to interact with mechanically gated ion channels, and acid can interact with acid sensing ion channels in addition to the capsaicin receptor TRPV1. In this overview some of the specific receptors and ion channels involved in the tussive stimulus-induced generator potentials in vagal afferent nerve terminals are discussed.

5HT2 and 5HT3 receptors' contribution to modeling of post-serotonin respiratory pattern in cats

Cardio-respiratory reflex effects of an exogenous serotonin challenge are suggested to be modulated by activation of the peripheral 5HT2 and 5HT3 receptors. In the present experiments the blocking effects of serotoninergic active drugs: ketanserin and tropanserin (MDL 72222) were studied in six pentobarbitone-chloralose anaesthetized cats. Bolus injection of serotonin (0.05 mg.kg(-1)) into the right femoral vein evoked prompt apnea, hypotension followed by tachypnoeic breathing. Pre-treatment with ketanserin (0.1 mg.kg(-1)), 5HT2 receptor antagonist, shortened the duration of post-serotonin apnea (P < 0.05), but had no effect on the pattern of post-apnoeic breathing. 5HT3 receptor blockade with the selective antagonist MDL 72222 (0.2 mg.kg(-1)) totally eliminated respiratory response to serotonin. In breaths that followed post-serotonin apnea, peak amplitude of the integrated phrenic signal was reduced (P < 0.001), unbiased by ketanserin blockade, and remained at the baseline level in MDL treated rats. Serotonin-induced hypotension was unaffected by the blockade of 5HT2 receptors. Inactivation of 5HT3 receptors with MDL attenuated the fall in blood pressure (P < 0.05). This data suggests that the squeal of serotonin-induced pulmonary chemoreflex, i.e. respiratory arrest, post-apnoeic pattern of breathing, bradycardia, and partially hypotension are mediated by 5HT3 receptors.

Supranodose vagotomy precludes reflex respiratory responses to serotonin in cats

Mediation of the respiratory reflex effects of an exogenous serotonin challenge goes beyond the lung vagi and is suggested to involve the nodose ganglia. In the present experiments the effects of an intravenous serotonin challenge on breathing pattern were studied in 8 pentobarbitone-chloralose anaesthetised cats. Bolus injection of serotonin oxalate (50 microg/kg) into the right femoral vein evoked prompt apnoea of 19.2 (+/- 2.4)-second duration in all 8 cats while intact; the apnoea was much shorter after midcervical vagal section (8.1 +/- 0.9 s, p < 0.001), and was abolished by supranodose vagotomy. In post-apnoeic breaths, the tidal volume was reduced from a baseline of 34.1 +/- 4.0 to 13.5 +/- 1.1 ml (p < 0.001) prior to, and from a baseline of 43.9 +/- 5.4 to 33.8 +/- 6.6 ml (p < 0.01) after midcervical vagotomy; the serotonin challenge did not affect tidal volume following supranodose vagal section (p = 0.4). The increase in respiratory rate found in intact (p < 0.001) and midcervically vagotomised cats (p < 0.01) was eliminated by the neurotomy above the nodose ganglia. Supranodose vagotomy altered cardiovascular response to serotonin by replacing the fall in blood pressure with an increase. These data suggest that the sequelae of serotonin-induced pulmonary chemoreflex, i.e. respiratory arrest, cardiovascular changes and post-apnoeic pattern of breathing require intact nodose ganglia.

The effect of serotonin on airway transepithelial sodium ion pathways

The action of serotonin on ionic transport processes in the airways was revealed in a study of the isolated rabbit tracheal wall mounted in a Ussing apparatus. It was shown that the transepithelial electrical potential difference of the tissue was hyperpolarized transiently after mechanical stimulation by means of gentle rinsing of its mucosal side. This hyperpolarization was greatly influenced by the addition of serotonin and/or ambroxol to the stimulation (rinsing) fluid. By means of selective blocking of ion transport transepithelial pathways with amiloride and/or bumetanide, the effects of serotonin and/or ambroxol on the hyperpolarization after mechanical stimulation could be explained as changes of sodium ion currents. The importance of stimulated ionic currents for airway clearance and for the efficacy of drugs acting on airways is postulated.

Nonvagal tachypnea following alpha2-adrenoceptor stimulation in awake goats

To assess the influence of vagal afferent feedback in the development of respiratory instabilities induced by alpha2-adrenoceptor (alpha2-AR) stimulation in the goat, we examined the ventilatory effects of clonidine, an alpha2-AR agonist, in awake tracheostomized goats before and after bilateral mid-cervical vagotomy. Prior to vagal section, systemic administration of clonidine (0.5-3.0 microg kg(-1)) induced a highly dysrhythmic pattern of breathing in all animals that was characterized by alternating episodes of tachypnea and slow irregular breathing patterns including prolonged and variable expiratory time (TE) intervals. Periods of apnea were commonly observed. Bilateral vagotomy resulted in a slower deeper breathing pattern and abolished the tachypnea evoked by intravenous administration of phenylbiguanide (PBG; 20-50 microg kg(-1)), a selective serotonin type 3 (5-HT3) receptor agonist. However, respiratory disturbances associated with alpha2-AR stimulation (including tachypnea) persisted after vagal section and were qualitatively and quantitatively similar to pre-vagotomy data demonstrating that vagal afferent feedback is not necessary for the development of respiratory disturbances induced by clonidine. The results suggest that respiratory dysrhythmias caused by alpha2-AR agonists in the goat are mediated by alpha2-ARs in the CNS.