Pulmonary C-fiber Research Articles

Effect of increasing temperature on TRPV1-mediated responses in isolated rat pulmonary sensory neurons

Hyperthermia has been shown to sensitize vagal pulmonary C-fibers in anesthetized rats. However, it was not clear whether the effect was due to a direct action of hyperthermia on these sensory neurons. To answer this question, we carried out this study to determine the effect of increasing temperature on the responses to various chemical stimuli in isolated nodose and jugular ganglion neurons innervating the rat lungs. In the whole cell perforated patch-clamp study, when the temperature was increased from normal (approximately 36 degrees C) to hyperthermic (approximately 40.6 degrees C) level of the rat body temperature, the inward currents evoked by capsaicin, a selective activator of the transient receptor potential vanilloid type 1 (TRPV1), and 2-aminoethoxydiphenyl borate (2-APB), a nonselective activator of TRPV1-3 receptors, were both significantly increased. This potentiating effect was clearly present even at a moderate level of hyperthermia (approximately 39 degrees C). However, only the slow, sustained component of acid-evoked current mediated through the TRPV1 receptor was potentiated by hyperthermia, whereas the rapid, transient component was inhibited. In contrast, the currents evoked by adenosine 5'-triphosphate and acetylcholine, neither of which is known to activate the TRPV1 channel, did not increase when the same temperature elevation was applied. Furthermore, the hyperthermia-induced potentiation of the cell response to 2-APB was significantly attenuated by either capsazepine or AMG 9810, selective TRPV1 antagonists. In conclusion, increasing temperature within the physiological range exerts a potentiating effect on the response to TRPV1 activators in these neurons, which is probably mediated through a positive interaction between hyperthermia and these chemical activators at the TRPV1 channel.

Pulmonary C-fiber receptor activation abolishes uncoupled facial nerve activity from phrenic bursting during positive end-expired pressure in the rat

Phasic respiratory bursting in the facial nerve (FN) can be uncoupled from phrenic bursting by application of 9 cmH(2)O positive end-expired pressure (PEEP). This response reflects excitation of expiratory-inspiratory (EI) and preinspiratory (Pre-I) facial neurons during the Pre-I period and inhibition of EI neurons during inspiration (I). Because activation of pulmonary C-fiber (PCF) receptors can inhibit the discharge of EI and Pre-I neurons, we hypothesized that PCF receptor activation via capsaicin would attenuate or abolish uncoupled FN bursting with an increase from 3 cmH(2)O (baseline) to 9 cmH(2)O PEEP. Neurograms were recorded in the FN and phrenic nerve in anesthetized, ventilated, vagally intact adult Wistar rats. Increasing PEEP to 9 cmH(2)O resulted in a persistent rhythmic discharge in the FN during phrenic quiescence (i.e., uncoupled bursting). Combination of PEEP with intrajugular capsaicin injection severely attenuated or eliminated uncoupled bursting in the FN (P < 0.05). Additional experiments examined the pattern of facial motoneuron (vs. neurogram) bursting during PEEP application and capsaicin treatment. These single-fiber recordings confirmed that Pre-I and EI (but not I) neurons continued to burst during PEEP-induced phrenic apnea. Capsaicin treatment during PEEP substantially inhibited Pre-I and EI neuron discharge. Finally, analyses of FN and motoneuron bursting across the respiratory cycle indicated that the inhibitory effects of capsaicin were more pronounced during the Pre-I period. We conclude that activation of PCF receptors can inhibit FN bursting during PEEP-induced phrenic apnea by inhibiting EI and I facial motoneuron discharge.

Potentiation of pulmonary reflex response to capsaicin 24h following whole-body acrolein exposure is mediated by TRPV1

Pulmonary C-fibers are stimulated by irritant air pollutants producing apnea, bronchospasm, and decrease in HR. Chemoreflex responses resulting from C-fiber activation are sometimes mediated by TRPV1 and release of substance P. While acrolein has been shown to stimulate C-fibers, the persistence of acrolein effects and the role of C-fibers in these responses are unknown. These experiments were designed to determine the effects of whole-body acrolein exposure and pulmonary chemoreflex response post-acrolein. Rats were exposed to either air or 3 ppm acrolein for 3 h while ventilatory function and HR were measured; 1-day later response to capsaicin challenge was measured in anesthetized rats. Rats experienced apnea and decrease in HR upon exposure to acrolein, which was not affected by either TRPV1 antagonist or NK(1)R antagonist pretreatment. Twenty-four hours later, capsaicin caused apnea and bronchoconstriction in control rats, which was potentiated in rats exposed to acrolein. Pretreatment with TRPV1 antagonist or NK(1)R antagonist prevented potentiation of apneic response and bronchoconstriction 24h post-exposure. These data suggest that although potentiation of pulmonary chemoreflex response 24h post-acrolein is mediated by TRPV1 and release of substance P, cardiopulmonary inhibition during whole-body acrolein exposure is mediated through other mechanisms.

Vitamin A deficiency increases airway resistance following C-fiber stimulation

Vitamin A deficiency (VAD) remains an important problem in the developing world where environmental air pollution is increasing. Because the coincidence of these factors could increase the prevalence of asthma in these regions, airway reactivity to methacholine (MCh), capsaicin, and sulfur dioxide was studied in VAD rats. The impedance to a small volume oscillation was analyzed to determine airway resistance (R(aw)) and lung elastance (H). VAD rats demonstrated larger increases in R(aw) and H after the administration of methacholine, and in R(aw) after administration of capsaicin or sulfur dioxide. The muscarinic receptor-2 (M2R) selective antagonist AFDX-116 enhanced the effect of capsaicin in vitamin A sufficient (VAS) but not VAD rats and retinoic acid-administration partially restored this enhancement. These data indicate that diminished auto-inhibitory muscarinic receptor-2 function contributed to this increased airway responsiveness to pulmonary C-fiber stimulation in VAD rats. If children with VAD also have diminished M2R function, they may be more prone to develop asthma, particularly in the presence of environmental co-factors such as sulfur dioxide.

Acrolein exposure augments pulmonary C fiber chemoreflex response

Pulmonary C fibers are stimulated by irritant air pollutants such as acrolein (Lee et al. 1992) producing apnea, bronchospasm, and decreases in heart rate (HR). Lung C fiber-induced chemoreflex activation is mediated by TRPV1 vanilloid receptors and release of substance P (SP). While acrolein has been shown to stimulate C-fibers the persistence of acrolein effects (24hrs) and the role of C-fibers in these responses are unknown. These experiments were designed to determine whether pulmonary C-fiber hyperresponsiveness post-acrolein is mediated by TRPV1 activation and release of SP. Rats were exposed to either air or 3ppm acrolein for 3 hours; 1 day later capsaicin (cap)-induced ventilatory and HR responses were measured in anesthetized rats. Cap (1-8 μg/kg, iv) caused apnea and bronchoconstriction in control rats, that was potentiated in rats exposed to acrolein. Pretreatment with TRPV1-antagonist SB 366791 (2mg/kg, ip) prevented potentiation of this acrolein apneic response but not bronchoconstriction, but pretreatment with NK1R antagonist RP 67580 (3mg/kg, ip) prevented both. Drugs had no effect on controls nor were there any changes in HR. These data suggest that sensitization of pulmonary C-fibers by acrolein involves activation of TRPV1 and release of SP, but does not impact cardiac function. Supported by Curriculum in Toxicology/UNC Grant T32-ES07126. (This abstract does not reflect EPA policy.)

Inhibition of respiratory‐related activity of the facial nerve by pulmonary vagal C‐fiber activation in the rat

We have previously reported that respiratory-related facial nerve activity can be modulated by hypercapnia, lung inflation, and positive end-expired pressure (PEEP). Whether respiratory-related rhythmic activities of the facial nerve could be modulated by pulmonary C-fiber activation is unclear. Activity of the facial and phrenic nerve was recorded simultaneously in the anesthetized and ventilated adult rat. Activation pulmonary C-fiber by intra-jugular injection of capsaicin produced a dose-dependent reduction of amplitude and onset of the facial nerve activity. Furthermore, uncoupled facial activity (persistent rhythmic activity during phrenic cessation) evoked by increasing PEEP could be also suppressed by capsaicin injection. Single fiber recording from the facial nerve revealed that capsaicin treatment produced inhibition on activity of expiratory-inspiratory (EI) motoneuron during expiration such that became inspiratory motoneurons and of pre-inspiratory motoneurons. In addition, capsaicin injection could attenuate the uncoupled activity of EI from the phrenic burst caused by PEEP manipulation. These results implied that capsaicin might differentially inhibit pre-inspiratory than inspiratory facial activity.

Enhanced excitability of rat vagal pulmonary sensory neurons during hyperthermia

Tissue hyperthermia can occur in the lungs and airways under pathophysiological conditions and/or heavy exercises. However, the effect of hyperthermia on the activation of respiratory sensory afferent is not clear. A recent study in our lab showed that hyperthermia exerts both stimulatory and sensitizing effect on vagal pulmonary C-fibers (J. Physiol. 565:, 2005). However, whether the sensitizing effect of hyperthermia is due to its direct action on these neurons is not known. To answer this question, we carried out this experiment to determine the effect of hyperthermia on the responses to various chemical stimuli of the nodose and jugular ganglion neurons innervating the lungs and airways of adult SD rats, and the possible involvement of TRPV channels. In the whole-cell perforated patch clamp study, when the temperature was increased from 36 – 40 °C, the inward currents evoked by capsaicin (0.3 μM), proton (pH 5.5 or 6) and 2APB (0.3 mM) was increased by 190 ± 44% (p < 0.01; n = 7), 123 ± 51% (p < 0.01; n = 9) and 76 ± 11% (p < 0.0001; n = 8), respectively; all three chemical agents activate transient receptor potential vanilloid receptor 1 (TRPV1). These results suggest that hyperthermia can exert a direct sensitizing effect on isolated vagal pulmonary sensory neurons, which may explain the hyperthermia-induced hypersensitivity of vagal pulmonary nerve activity in anesthetized rats. Furthermore, this potentiating effect probably results from a positive interaction between hyperthermia and the activator of the TRPV1. Supported by NIH grant HL 67379

Sensitization of isolated rat vagal pulmonary sensory neurons by human eosinophil granule‐derived cationic proteins

We have previously demonstrated that airway exposure to human eosinophil granule-derived cationic proteins stimulates vagal pulmonary C-fibers and markedly potentiated their responses to lung inflation in anesthetized rats (J Appl Physiol 91: , 2001). However, whether the effects result from a direct action of these proteins on the sensory endings was not known. The present study was therefore carried out to determine the effect of these proteins on isolated rat vagal pulmonary sensory neurons. Using perforated patch-clamp technique, our results showed that: in voltage-clamp mode, pretreatment with human eosinophil granule-derived major basic protein (MBP) significantly increased the capsaicin-evoked inward current; this effect peaked around 10 min after MBP and lasted for more than 60 min; in current-clamp mode, MBP substantially increased the number of action potentials evoked by either current injection, capsaicin or acid; the sensitizing effect of MBP was completely abolished by low molecular weight heparin; pretreatment with another eosinophil granule-derived protein, eosinophil cationic protein, induced a similar but less potent sensitizing effect on these neurons. We conclude that these cationic proteins directly sensitize vagal pulmonary sensory neurons, and the effects are dependent on the cationic charges carried by these proteins. (NIH HL465737 and PBF Fellowship)

Prostaglandin E2 enhances the sensitizing effect of hyperthermia on pulmonary C-fibers in rats

This study was carried out to investigate whether the pulmonary C-fiber hypersensitivity induced by hyperthermia is altered by prostaglandin E(2) (PGE(2)). Single-unit afferent activities of pulmonary C-fibers were recorded in anesthetized, artificially ventilated rats when the intrathoracic temperature (T(it)) was maintained at normal (N; approximately 36 degrees C) and hyperthermia levels (H; approximately 41 degrees C) by perfusion of heated saline into the thoracic chamber for 3min. After approximately 20min of recovery, the fiber activities were recorded again during infusion of PGE(2) at both N and H levels of T(it). Our study showed: (1) The baseline fiber activity and responses to lung inflation, right-atrial injection of capsaicin and adenosine were all increased by increasing T(it) from N to H, and these hyperthermia-induced increases in sensitivities were also significantly augmented by PGE(2). (2) These enhanced sensitivities induced by PGE(2) were abolished by pretreatment with AH6809 and AH23848, selective antagonists of EP(2) and EP(4) prostanoid receptors, respectively. In conclusion, the hyperthermia-induced hypersensitivity of vagal pulmonary C-fibers is potentiated by PGE(2), and this effect is mediated through activation of EP(2) and EP(4) prostanoid receptors.

Neural drive to tongue protrudor and retractor muscles following pulmonary C-fiber activation

Hypoglossal (XII) nerve recordings indicate that pulmonary C-fiber (PCF) receptor activation reduces inspiratory bursting and triggers tonic discharge. We tested three hypotheses related to this observation: 1) PCF receptor activation inhibits inspiratory activity in XII branches innervating both tongue protrudor muscles (medial branch; XIImed) and retractor muscles (lateral branch; XIIlat); 2) reduced XII neurogram amplitude reflects decreased XII motoneuron discharge rate; and 3) tonic XII activity reflects recruitment of previously silent motoneurons. Phrenic, XIImed, and XIIlat neurograms were recorded in anesthetized, paralyzed, and ventilated rats. Capsaicin delivered to the jugular vein reduced phrenic bursting at doses of 0.625 and 1.25 mug/kg but augmented bursting at 5 mug/kg. All doses reduced inspiratory amplitude in XIImed and XIIlat (P < 0.05), and these effects were eliminated following bilateral vagotomy. Single-fiber recordings indicated that capsaicin causes individual XII motoneurons to either decrease discharge rate (n = 101/153) or become silent (n = 39/153). Capsaicin also altered temporal characteristics such that both XIImed and XIIlat inspiratory burst onset occurred after the phrenic burst (P < 0.05). Increases in tonic discharge after capsaicin were greater in XIImed vs. XIIlat (P < 0.05); single-fiber recordings indicated that tonic discharge reflected recruitment of previously silent motoneurons. We conclude that PCF receptor activation reduces inspiratory XII motoneuron discharge and transiently attenuates neural drive to both tongue protrudor and retractor muscles. However, tonic discharge appears to be selectively enhanced in tongue protrudor muscles. Accordingly, reductions in upper airway stiffness associated with reduced XII burst amplitude may be offset by enhanced tonic activity in tongue protrudor muscles.

Thermal sensitivity of isolated vagal pulmonary sensory neurons: role of transient receptor potential vanilloid receptors

A recent study has demonstrated that increasing the intrathoracic temperature from 36 degrees C to 41 degrees C induced a distinct stimulatory and sensitizing effect on vagal pulmonary C-fiber afferents in anesthetized rats (J Physiol 565: 295-308, 2005). We postulated that these responses are mediated through a direct activation of the temperature-sensitive transient receptor potential vanilloid (TRPV) receptors by hyperthermia. To test this hypothesis, we studied the effect of increasing temperature on pulmonary sensory neurons that were isolated from adult rat nodose/jugular ganglion and identified by retrograde labeling, using the whole cell perforated patch-clamping technique. Our results showed that increasing temperature from 23 degrees C (or 35 degrees C) to 41 degrees C in a ramp pattern evoked an inward current, which began to emerge after exceeding a threshold of approximately 34.4 degrees C and then increased sharply in amplitude as the temperature was further increased, reaching a peak current of 173 +/- 27 pA (n = 75) at 41 degrees C. The temperature coefficient, Q10, was 29.5 +/- 6.4 over the range of 35-41 degrees C. The peak inward current was only partially blocked by pretreatment with capsazepine (Delta I = 48.1 +/- 4.7%, n = 11) or AMG 9810 (Delta I = 59.2 +/- 7.8%, n = 8), selective antagonists of the TRPV1 channel, but almost completely abolished (Delta I = 96.3 +/- 2.3%) by ruthenium red, an effective blocker of TRPV1-4 channels. Furthermore, positive expressions of TRPV1-4 transcripts and proteins in these neurons were demonstrated by RT-PCR and immunohistochemistry experiments, respectively. On the basis of these results, we conclude that increasing temperature within the normal physiological range can exert a direct stimulatory effect on pulmonary sensory neurons, and this effect is mediated through the activation of TRPV1, as well as other subtypes of TRPV channels.

Calcium transient evoked by nicotine in isolated rat vagal pulmonary sensory neurons

It has been shown that inhaled cigarette smoke activates vagal pulmonary C fibers and rapidly adapting receptors (RARs) in the airways and that nicotine contained in the smoke is primarily responsible. This study was carried out to determine whether nicotine alone can activate pulmonary sensory neurons isolated from rat vagal ganglia; the response of these neurons was determined by fura-2-based ratiometric Ca(2+) imaging. The results showed: 1) Nicotine (10(-4) M, 20 s) evoked a transient increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in 175 of the 522 neurons tested (Delta[Ca(2+)](i) = 142.2 +/- 12.3 nM); the response was reproducible, with a small reduction in peak amplitude in the same neurons when the challenge was repeated 20 min later. 2) A majority (59.7%) of these nicotine-sensitive neurons were also activated by capsaicin (10(-7) M). 3) 1,1-Dimethyl-4-phenylpiperazinium iodide (DMPP; 10(-4) M, 20 s), a selective agonist of the neuronal nicotinic acetylcholine receptors (NnAChRs), evoked a pattern of response similar to that of nicotine. 4) The responses to nicotine and DMPP were either totally abrogated or markedly attenuated by hexamethonium (10(-4) M). 5) In anesthetized rats, right atrial bolus injection of nicotine (75-200 mug/kg) evoked an immediate (latency <1-2 s) and intense burst of discharge in 47.8% of the pulmonary C-fiber endings and 28.6% of the RARs tested. In conclusion, nicotine exerts a direct stimulatory effect on vagal pulmonary sensory nerves, and the effect is probably mediated through an activation of the NnAChRs expressed on the membrane of these neurons.

Effect of PGE 2 on pulmonary C‐fiber response to hyperthermia in rats

Body temperature frequently increases to ~41°C during strenuous exercise or severe fever. Local tissue hyperthermia is known to occur during inflammatory reaction, and an increase in airway temperature has been reported in asthmatic patients (Am J Respir Crit Care Med. 165:181, 2002). A recent study in our lab (J Physiol. 565:295, 2005) showed intrathoracic hyperthermia induces the hypersensitivity of pulmonary C-fibers. In this study, we investigated whether the response of pulmonary C-fibers to hyperthermia is altered by prostaglandin E2 (PGE2), one of the major autacoids released in the lung during airway inflammation. Single-unit afferent activities of pulmonary C-fibers were recorded in anesthetized, artificially ventilated rats when the intrathoracic temperature (Tit) was maintained at normal (~36 °C; N) and hyperthermia levels (~41 °C; H) by the perfusion of heated saline into the thoracic chamber for 3 min. After 20 min of recovery, the fiber activities were recorded again during the perfusion of PGE2 (3 μg/kg/min, 3 min) at these two Tit, N and H. Our studies show: 1) During PGE2 perfusion, baseline fiber activities were higher than the control at both N and H levels of Tit. Furthermore, the hyperthermia-induced increase in baseline activity was also significantly augmented by PGE2 (P < 0.01). 2) The responses of pulmonary C-fibers to both lung inflation (15 and 30 cmH2O) and right atrial injection of capsaicin (1 μg/kg) during the perfusion of PGE2 increased in a similar pattern as that of the baseline. In conclusion, PGE2 markedly potentiates the hyperthermia-induced hypersensitivity of vagal pulmonary C-fibers to lung inflation and capsaicin. (Supported by NIH HL 67379)

Stimulatory effect of CO2 on vagal bronchopulmonary C-fiber afferents during airway inflammation

This study investigated 1) whether pulmonary C fibers are activated by a transient increase in the CO2 concentration of alveolar gas; and 2) if the CO2 sensitivity of these afferents is altered during airway inflammation. Single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering a CO2-enriched gas mixture (25-30% CO2, 21% O2, balance N2) for five to eight breaths, which increased alveolar CO2 concentration progressively to near or above 13% for 3-5 s and lowered the arterial pH transiently to 7.10 +/- 0.05. Our results showed the following. 1) HPC evoked only a mild stimulation in a small fraction (4/47) of pulmonary C fibers, and there was no significant change in fiber activity (change in fiber activity = 0.22 +/- 0.16 imp/s; P > 0.1, n = 47). 2) In sharp contrast, after airway exposure to poly-L-lysine, a cationic protein known to induce mucosal injury, the same challenge of transient HPC activated 87.5% of the pulmonary C fibers tested and evoked a distinct stimulatory effect on these afferents (change in fiber activity = 6.59 +/- 1.78 imp/s; P < 0. 01, n = 8). 3) Similar potentiation of the C-fiber response to HPC was also observed after acute exposure to ozone (n = 6) and during a constant infusion of inflammatory mediators such as adenosine (n = 15) or prostaglandin E2 (n = 12). 4) The enhanced C-fiber sensitivity to CO2 after poly-L-lysine was completely abrogated by infusion of NaHCO3 (1.82 mmol.kg(-1).min(-1)) that prevented the reduction in pH during HPC (n = 6). In conclusion, only a small percentage (<10%) of the bronchopulmonary C fibers exhibit CO2 sensitivity under control conditions, but alveolar HPC exerts a consistent and pronounced stimulatory effect on the C-fiber endings during airway inflammation. This effect of CO2 is probably mediated through the action of hydrogen ions.

THE CARDIOPULMONARY EFFECTS OF A CANNABINOID 1 RECEPTOR (CB1) AGONIST IS MEDIATED BY THE TRANSIENT RECEPTOR POTENTIAL VANILLOID 1 (TRPV1) CHANNEL

PURPOSE: Anandamide (AEA), a cannabinoid 1 receptor (CB1) agonist, has been suggested to be involved in the pathogenesis of hypertension (Lake et al, 1997) and cardiogenic shock (Wagner et al, 2001). AEA also binds to transient receptor potential vanilloid 1 (TRPV1) channels (Zygmunt et al, 1999), which are expressed on pulmonary C-fibers (Michael & Priestley, 1999). The latter are believed to be involved in the sensation of inflammation during lung disease (Lee & Widdicombe, 2001), and dyspnea during exercise (Paintal, 1973). In rats, intravenous injection of AEA elicits the pulmonary chemoreflex (apnea, bradycardia and hypotension; Lin & Lee, 2002), which is a C-fiber mediated reflex (Coleridge & Coleridge, 1984). However, the role of the transient receptor potential vanilloid 1 (TRPV1) receptor in this cardiopulmonary response remains unknown.

Effect of olvanil and anandamide on vagal C‐fiber subtypes in guinea pig lung

Certain fatty acid amides such as anandamide (AEA) and olvanil are agonists for the transient receptor potential, vanilloid-1 (TRPV1) receptor, but have been found to activate TRPV1-containing C-fibers in some tissues but not others. We used extracellular recording and whole-cell patch clamp techniques to investigate the effect of olvanil and AEA on different types of vagal C-fibers innervating the same tissue, namely jugular and nodose vagal C-fibers in guinea pig lungs. A 30 s exposure to AEA and olvanil caused action potential discharge in all nodose C-fiber innervating lung but failed to activate jugular C-fibers innervating lung and airways. The activation of nodose C-fibers was blocked by the TRPV1 antagonist iodo-resiniferatoxin. In whole-cell patch clamp recordings of dissociated nodose and jugular capsaicin-sensitive neurons labeled from lungs and airways, olvanil induced large TRPV1-dependent inward currents in cell bodies of both nodose and jugular ganglion neurons. Prolonged exposure (up to 5 min) to olvanil caused action potential discharge in jugular C-fiber innervating lung but the onset latency was four times longer in jugular than in nodose C-fibers. The onsets of capsaicin response in nodose and jugular C-fibers were not different. Decreasing the tissue temperature to 25 degrees C increased the onset latency of olvanil-induced activation of nodose C-fibers 2-3-fold, but did not effect the latency of the capsaicin response. Capsaicin, olvanil, and AEA stimulate jugular C-fibers leading to tachykinergic contractions of isolated bronchi. The time to reach half-maximum is more than four times longer for olvanil and AEA, as compared to capsaicin in evoking contractions. We conclude that brief exposure to certain fatty acid amides, such as AEA and olvanil activate nodose but not jugular C-fiber terminals in the lungs. We hypothesize that this is because the nodose C-fiber terminals are equipped with a temperature-dependent mechanism for effectively and rapidly transporting the TRPV1 agonists so that they gain access to the intracellular binding sites on TRPV1. This transport mechanism may be differently expressed in two distinct subtypes of pulmonary C-fiber terminals innervating the same tissue.

Sensitization of pulmonary chemosensitive neurons by bombesin-like peptides in rats

Small cell lung cancer (SCLC) patients suffer from pulmonary stresses such as dyspnea and chest pain, and the pathogenic mechanisms are not known. SCLC cells secrete a variety of bioactive neuropeptides, including bombesin-like peptides. We hypothesize that these peptides may enhance the sensitivity of the pulmonary chemosensitive nerve endings, contributing to the development of these pulmonary stresses in SCLC patients. This study was therefore carried out to determine the effects of bombesin and gastrin-releasing peptide (GRP), a major bombesin-like peptide, on the sensitivities of pulmonary chemoreflex and isolated pulmonary vagal chemosensitive neurons. In anesthetized, spontaneously breathing rats, intravenous infusion of bombesin or GRP significantly amplified the pulmonary chemoreflex responses to chemical stimulants such as capsaicin and ATP. The enhanced responses were completely abolished by perineural capsaicin treatment of both cervical vagi, suggesting the involvement of pulmonary C-fiber afferents. In isolated pulmonary vagal chemosensitive neurons, pretreatment with bombesin or GRP potentiated the capsaicin-induced Ca(2+) transient. This sensitizing effect was further demonstrated in patch-clamp recording studies; the sensitivities of these neurons to both chemical (capsaicin and ATP) and electrical stimuli were significantly enhanced by the presence of either bombesin or GRP. In summary, our results have demonstrated that bombesin and GRP upregulate the pulmonary chemoreflex sensitivity in vivo and the excitability of isolated pulmonary chemosensitive neurons in vitro.

Hypersensitivity of pulmonary chemoreflex induced by poly-l-lysine: Role of cationic charge

This study was carried out to investigate the role of cationic charge in the hypersensitivity of pulmonary C-fibers induced by airway exposure to synthetic cationic protein poly-L-lysine (PLL) in anesthetized rats. Inhalation of PLL aerosol induced a distinctly irregular breathing pattern, and significantly enhanced the pulmonary chemoreflex responses to capsaicin. However, after the cationic charges were completely removed from PLL by succinylation, the succinylated PLL no longer produced any change in either the baseline breathing pattern or the reflex responses to capsaicin. In addition, the effects of PLL were also abolished after premixing it with a polyanion, poly-L-glutamic or poly-L-aspartic acid, before delivery. In sharp contrast, when delivered within 5 min after the PLL aerosol, these two polyanions were completely ineffective in reversing the effects of PLL. Electrophysiological recording of the afferent activity of single pulmonary C-fibers further supported our conclusion that the cationic charge carried by this protein is primarily responsible for generating the stimulatory and sensitizing effects of PLL on these afferents.

Depressed ventilatory reflexes after capsaicin challenge in streptozotocin-treated rats

Diabetic sensory neuropathy is an affliction that decreases sensory perception in a number of organ systems. Although little is known of its pulmonary effects certain diabetic patients have reduced airway reactivity to cold air and elevated cough threshold to irritant inhalation, reflexes reported to be mediated by pulmonary C-fibers. Therefore we studied the effects the selective C-fiber activator capsaicin (0.01% aerosol, 30 s) on variables of ventilation using a whole-body plethysmograph in age-matched rats treated with streptozotocin (STZ) or its vehicle at 6 and 12 weeks after treatment. Body weight increased and plasma glucose and glycosylated hemoglobin were stable in vehicle-treated rats. In STZ-treated rats body weight decreased and plasma glucose and glycosylated hemoglobin increased. Capsaicin challenge decreased tidal volume, respiratory rate and therefore minute ventilation in non-treated and vehicle-treated rats. However capsaicin challenge increased tidal volume thereby altering minute ventilation in STZ-treated rats. Specific airway resistance increased in both groups after capsaicin challenge. Changes in ventilation in response to capsaicin challenge in STZ-treated rats may involve C-fiber sensory neuropathy.

Response of Respiratory-Related Hypoglossal Nerve Activity to Capsaicin-Induced Pulmonary C-Fiber Activation in Rats

This study was designed to examine respiratory-related hypoglossal nerve activity in response to activation of pulmonary C-fibers by capsaicin. Rats were anesthetized with urethane (1.2 g/kg, i.p.). Tracheostomy was performed. Catheters were introduced into the femoral vein and artery. Another catheter was placed near the entrance of the right atrium via the right jugular vein. Rats were paralyzed with gallamine triethiodide (5 mg/kg, i.v.), and ventilated artificially. Activities of the phrenic nerve (PNA) and the hypoglossal nerve (HNA) were recorded simultaneously. Varied doses of capsaicin (0.625, 1.25, and 5 µg/kg) were delivered into the right atrium to activate pulmonary C-fibers. Before bilateral vagotomy, apnea, decreases in PNA and HNA were observed in response to pulmonary C-fiber activation by the low and moderate doses of capsaicin. The high dose of capsaicin evoked an increase in PNA, an immediate tonic discharge of the hypoglossal nerve, and a decrease in phasic HNA. The onset time of HNA preceding PNA was abolished and replaced by a time lagged pattern as pulmonary C-fibers were activated. Raising CO₂ concentration did not attenuate the inhibitory effect of pulmonary C-fiber activation upon PNA and HNA. After bilateral sectioning of the vagi, administration of the moderate dose of capsaicin to activate non-vagal C-fibers produced increases in PNA and HNA. These results suggest that pulmonary vagal C-fiber activation may narrow the diameter at the oropharyngeal level by a decrease in phasic HNA, which may be disadvantageous for the maintenance of a patent upper airway.