Pulmonary rapidly adapting receptors reflexly increase airway secretion in dogs

We attempted to determine whether stimulation of pulmonary rapidly adapting receptors (RARs) increase tracheal submucosal gland secretion in anesthetized open-chest dogs. Electroneurographic studies of pulmonary afferents established that RARs but not lung C-fibers were stimulated by intermittent lung collapse during deflation, collapse being produced by removing positive end-expiratory pressure (PEEP, 4 cmH2O) or by applying negative end-expiratory pressure (NEEP, -4 cmH2O). We measured tracheal secretion by the "hillocks" method. Removing PEEP or applying NEEP for 1 min increased secretion from a base line of 6.0 +/- 1.1 to 11.8 +/- 1.7 and 22.0 +/- 2.8 hillocks.cm-2.min-1, respectively (P less than 0.005). After PEEP was restored, dynamic lung compliance (Cdyn) was 37% below control, and secretion remained elevated (P less than 0.05). A decrease in Cdyn stimulates RARs but not other pulmonary afferents. Hyperinflation, which restored Cdyn and RAR activity to control, returned secretion rate to base line. Secretory responses to lung collapse were abolished by vagal cooling (6 degrees C), by pulmonary vagal section, or by atropine. We conclude that RAR stimulation reflexly increases airway secretion. We cannot exclude the possibility that reduced input from slowly adapting stretch receptors contributed to the secretory response.     

Influence of ventrolateral surface of medulla on tracheal gland secretion.

Airway secretion can be modified reflexly as well as locally. Previous studies indicate that neurons in a circumscribed region near the ventral surface of the medulla (VMS) can substantially modify airway tone and reflex responses to vagal inputs. In the present studies we assessed the importance of these neurons on tracheal gland secretion. We examined the changes in the number of hillocks of secretion appearing from submucosal glands in an exposed field of tracheal epithelium (1.2 cm2) coated with tantalum dust before and after interventions on the VMS. Experiments were performed in alpha-chloralose-anesthetized dogs paralyzed and ventilated with 40% O2. Stimulation of nicotinergic receptors by application of a pledget containing nicotine in 11 dogs caused a significant elevation in tracheal gland secretion in the subsequent 60 s, compared with a control period in which buffered saline was applied. Prior application of lidocaine or hexamethonium bromide to the VMS blocked the effect of topically applied nicotine. The central effects of nicotine were diminished by atropine methylnitrate given intravenously. In addition, lidocaine application to the VMS or focal cooling of intermediate areas to between 20 and 15 degrees C significantly decreased secretion rates reflexly produced by capsaicin-induced stimulation of pulmonary C-fiber receptors and by mechanical stimulation of the carina and larynx. These findings suggest that the ventral medulla contains cells near its surface that influence tracheal fluid secretion and modulate reflex responses of airway submucosal glands, probably by altering the level of general excitation within the central respiratory integrating circuits.     

Cigarette smoke in lungs evokes reflex increase in tracheal submucosal gland secretion in dogs.

Stimulation of pulmonary C-fibers (PCs) by capsaicin and of rapidly adapting receptors (RARs) by reduced lung compliance reflexly increases airway submucosal gland secretion in dogs. Because both PCs and RARs are stimulated by cigarette smoke (nicotine being the primary stimulus), we performed experiments in anesthetized open-chest artificially ventilated dogs (with aortic nerves cut) to determine whether cigarette smoke reflexly stimulates airway secretion. We measured submucosal gland secretion by counting the hillocks in a 1.2-cm2 field of tracheal epithelium coated with tantalum dust. Secretion was stimulated by delivery of 40-320 ml smoke from high-nicotine cigarettes to the lower trachea, secretion rate increasing from 7.4 +/- 1.3 to 48.1 +/- 5.1 hillocks.cm-2.min-1. Results of cutting the pulmonary vagal branches or carotid sinus nerves or both indicated that the secretory response was initiated by stimulation of lower respiratory vagal afferents and augmented several seconds later by stimulation of carotid chemoreceptors. Results of cooling the cervical vagus nerves to 7 and 0 degrees C indicated that most of the vagally mediated increase in secretion was due to stimulation of afferent lung C-fibers.     

Pulmonary C-fibers reflexly increase secretion by tracheal submucosal glands in dogs

Stimulation of bronchial C-fibers evokes a reflex increase in secretion by tracheal submucosal glands, but the influence of pulmonary C-fibers on tracheal gland secretion is uncertain. In anesthetized dogs with open chests, we sprayed powdered tantalum on the exposed mucosa of a segment of the upper trachea to measure the rate of secretion by submucosal glands. Secretions from the gland ducts caused elevations (hillocks) in the tantalum layer. We counted hillocks at 10-s intervals for 60 s before and 60 s after we injected capsaicin (10-20 micrograms/kg) into the right atrium to stimulate pulmonary C-fiber endings. Right atrial injection of capsaicin increased the rate of hillock formation fourfold, but left atrial injection had no significant effect. The response was abolished by cutting the vagus nerves or cooling them to 0 degree C. We conclude that the reflex increase in tracheal submucosal gland secretion evoked by right atrial injection of capsaicin was initiated as capsaicin passed through the pulmonary vascular bed, and hence that pulmonary C-fibers, like bronchial C-fibers, reflexly increase airway secretion.     

Possible sensory receptor of nonadrenergic inhibitory nervous system

To determine the sensory receptor of the nonadrenergic inhibitory nervous system (NAIS), 22 cats were anesthetized and serotonin was continuously administered (50-250 micrograms.kg-1.min-1 iv) to increase pulmonary resistance (RL) to 377 +/- 57% (SE) of the control value. We then 1) mechanically irritated the trachea, 2) intravenously administered capsaicin (5 micrograms/kg), or 3) induced hypoxia (arterial PO2 30-40 Torr) to stimulate irritant and bronchial C-fiber receptors, pulmonary C-fiber receptors, or the carotid body (chemoreceptors), respectively. After treatment with atropine (3 mg/kg iv) and propranolol (2 mg/kg iv), the serotonin-induced change in RL was reduced by 58.6 +/- 14.3% by mechanical irritation and 63.3 +/- 12.1% by intravenous capsaicin. However, hypoxia produced no dilatation of the airways. In further experiments, we employed capsaicin inhalation to stimulate bronchial C-fiber receptors. Inhaled capsaicin (0.1%, for 5 breaths) also reduced RL by 79.2 +/- 9.2% of the elevated value, after atropine and propranolol. Treatment with a ganglionic blocking agent, hexamethonium (2 mg/kg iv), abolished bronchodilator responses, implying that a reflex pathway through vagal nerves is involved in this phenomenon. These results suggest that pulmonary and bronchial C-fiber receptors may be involved as sensory receptors in NAIS reflex bronchodilatation.     

Acute effects of acrolein on breathing: role of vagal bronchopulmonary afferents.

Spontaneous inhalation of acrolein vapor (350 ppm, 1 ml/100 g body wt) elicited an immediate and transient inhibitory effect on breathing in anesthetized rats, characterized by a prolongation of expiratory duration and accompanied by a bradycardia; ventilation was reduced by 47 +/- 6%, which returned to baseline after three to seven breaths. When both vagi were cooled to 6.6 +/- 0.1 degrees C, the reflex apneic response to lung inflation was completely abolished but the bradypneic response to acrolein was not affected. After perineural capsaicin treatment of both cervical vagi to selectively block the capsaicin-sensitive C-fiber afferents, acrolein no longer evoked an inhibitory effect on breathing; conversely, an augmented inspiration was consistently elicited with the first breath of acrolein inhalation, which was subsequently abolished by cooling both vagi to 6.5 degrees C. The inhibitory effect of inhaling acrolein at a lower concentration (200 ppm) was not detectable, whereas that of a higher concentration (600 ppm) was more intense and prolonged. All these responses were completely eliminated by bilateral vagotomy. These results suggest that inhaled acrolein activated both vagal C-fiber endings and rapidly adapting irritant receptors in the airways, but the acrolein-induced inhibitory effect on breathing was elicited primarily by the C-fiber afferent stimulation.     

Salivary secretion induced by substance P

Secretion of fluid, ions, and amylase from parotid and submaxillary glands of rat, induced by intravenous injection of substance P (SP), was examined. The action of SP on salivary glands, like physalaemin, resembled that of cholinergic stimulation. While SP-evoked salivary flow from both glands was blocked by atropine, atropine did not modify composition of SP-evoked saliva. The present study suggests that salivary secretion and secretion of ions and amylase evoked by SP are mediated via SP-sensitive cholinergic receptors and specific SP receptors, respectively.     

Aquaporin-5 dependent fluid secretion in airway submucosal glands

Fluid and macromolecule secretion by submucosal glands in mammalian airways is believed to play an important role in airway defense and surface liquid homeostasis and in the pathogenesis of cystic fibrosis. Immunocytochemistry revealed strong expression of aquaporin water channel AQP5 at the luminal membrane of serous epithelial cells in submucosal glands throughout the mouse nasopharynx and upper airways and AQP4 at the contralateral basolateral membrane in some glands. Novel methods were applied to measure secretion rates and composition of gland fluid in wild type mice and knockout mice lacking AQP4 or AQP5. In mice breathing through a tracheotomy, total gland fluid output was measured from the dilution of a volume marker present in the fluid-filled nasopharynx and upper trachea. Pilocarpine-stimulated fluid secretion was 4.3 +/- 0.4 microl/min in wild type mice, 4.9 +/- 0.9 microl/min in AQP4 null mice, and 1.9 +/- 0.3 microl/min in AQP5 null mice (p < 0.001). Similar results were obtained when secreted fluid was collected in the oil-filled nasopharyngeal cavity. Real-time video imaging of fluid droplets secreted from individual submucosal glands near the larynx in living mice showed a 57 +/- 4% reduced fluid secretion rate in AQP5 null mice. Analysis of secreted fluid showed a 2.3 +/- 0.2-fold increase in total protein in AQP5 null mice and a smaller increase in [Cl(-)], suggesting intact protein and salt secretion across a relatively water impermeable epithelial barrier. Submucosal gland morphology and density did not differ significantly in wild type versus AQP5 null mice. These results indicate that AQP5 facilitates fluid secretion in submucosal glands and that the luminal membrane of gland epithelial cells is the rate-limiting barrier to water movement. Modulation of gland AQP5 expression or function might provide a novel approach to treat hyperviscous gland secretions in cystic fibrosis and excessive fluid secretions in infectious or allergic bronchitis/rhinitis.     

Role of bronchopulmonary C-fiber afferents in the apneic response to cigarette smoke

The role of vagal bronchopulmonary C-fiber afferents in eliciting the immediate changes in breathing pattern after acute inhalation of cigarette smoke was assessed with a selective blockade of myelinated vagal afferents (innervating both stretch and irritant receptors) utilizing the method of differential cooling. In 15 of 17 chloralose-anesthetized dogs tested, spontaneous inhalation of cigarette smoke (19.7% avg conc, 500-700 ml vol) reproducibly caused the following immediate responses: apnea, bradycardia, and hypotension. These responses occurred within 1 to 2 breaths of smoke inhalation and were followed by a delayed hyperpnea. The apneic duration reached 326 +/- 33% (SE) (n = 15) of the mean base-line expiratory duration. Differential cold block of both vagi (coolant temperature 8.4 +/- 0.3 degrees C) abolished the reflex apnea induced by a positive-pressure (7-10 cmH2O) lung inflation but did not affect the apneic response to smoke inhalation (345 +/- 35%). The smoke-induced apnea was completely abolished by lowering the coolant temperature to -1.3 +/- 0.2 degrees C (n = 10) or by bilateral vagotomy (n = 5) and returned to the control level after both vagi were rewarmed. Based on these results, we suggest that the immediate apneic response to inhaled cigarette smoke is elicited by a stimulation of vagal C-fiber afferents in the lungs and airways.     

Maturation of respiratory reflex responses in the piglet

Stimulation of chemo-, irritant, and pulmonary C-fiber receptors reflexly constricts airway smooth muscle and alters ventilation in mature animals. These reflex responses of airway smooth muscle have, however, not been clearly characterized during early development. In this study we compared the maturation of reflex pathways regulating airway smooth muscle tone and ventilation in anesthetized, paralyzed, and artificially ventilated 2- to 3- and 10-wk-old piglets. Tracheal smooth muscle tension was measured from an open tracheal segment by use of a force transducer, and phrenic nerve activity was measured from a proximal cut end of the phrenic nerve. Inhalation of 7% CO2 caused a transient increase in tracheal tension in both age groups, whereas hypoxia caused no airway smooth muscle response in either group. The phrenic responses to 7% CO2 and 12% O2 were comparable in both age groups. Lung deflation and capsaicin (20 micrograms/kg iv) administration did not alter tracheal tension in the younger piglets but caused tracheal tension to increase by 87 +/- 28 and 31 +/- 10%, respectively, in the older animals (both P less than 0.05). In contrast, phrenic response to both stimuli was comparable between ages: deflation increased phrenic activity while capsaicin induced neural apnea. Laryngeal stimulation did not increase tracheal tension but induced neural apnea in both age groups. These data demonstrate that between 2 and 10 wk of life, piglets exhibit developmental changes in the reflex responses of airway smooth muscle situated in the larger airways in response to irritant and C-fiber but not chemoreceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ghrelin acts on the dorsal vagal complex to stimulate pancreatic protein secretion

Ghrelin receptors are present in the central nervous system. We hypothesized that ghrelin released from the stomach acts as an endocrine substance and stimulates brain stem vagovagal circuitry to evoke pancreatic secretion. In an in vivo anesthetized rat model, an intravenous infusion of ghrelin at doses of 5, 10, and 25 nmol increased pancreatic protein secretion from a basal level of 125 +/- 6 to 186 +/- 8, 295 +/- 12, and 356 +/- 11 mg/h, respectively. Pretreatment with atropine or hexamethonium or an acute vagotomy, but not a perivagal application of capsaicin, completely abolished pancreatic protein secretion responses to ghrelin. In conscious rats, an intravenous infusion of ghrelin at a dose of 10 nmol resulted in a 2.2-fold increase in pancreatic protein secretion over basal volume. Selective ablation of the area postrema abolished pancreatic protein secretion stimulated by intravenous infusion of ghrelin but did not alter the increase in pancreatic protein secretion evoked by diversion of bile-pancreatic juice. Immunohistochemical staining showed a marked increase in the number of c-Fos-expressing neurons in the area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus after an intravenous infusion of ghrelin in sham-lesioned rats; selective ablation of the area postrema eliminated this increase. In conclusion, ghrelin stimulates pancreatic secretion via a vagal cholinergic efferent pathway. Circulating ghrelin gains access to the brain stem vagovagal circuitry via the area postrema, which represents the primary target on which peripheral ghrelin may act as an endocrine substance to stimulate pancreatic secretion.     

Depletion of neuropeptides in rat parotid glands and declining atropine-resistant salivary secretion upon continuous parasympathetic nerve stimulation

In rats the parasympathetic auriculo-temporal nerve on one side was continuously stimulated at 40 Hz for 20-80 min in the presence of adrenergic blockers (dihydroergotamine and propranolol) +/- atropine. During the first 10 min this gave rise to a flow of saliva from the parotid gland that in the atropinized rats amounted to 35% of that found in rats not treated with atropine, while the protein and amylase outputs were 75% of those in non-atropinized rats. The atropine-resistant secretion of fluid and proteins declined to 5-10% of the initial value within 40 min but did not cease completely even after 80 min. The marked reduction in secretory responses was not due to desensitization or exhaustion of the gland cells. The nerve stimulation reduced the parotid gland content of vasoactive intestinal peptide (VIP) and substance P (SP) to approximately 60 and 25% of that of contralateral glands after 20 and 60 min, respectively. The probable explanation for the decline in secretory response seems to be depletion of non-adrenergic, non-cholinergic transmitter(s). The present results suggest that neuropeptides are involved in the regulation of salivary secretion but provide no direct evidence that either VIP or SP is responsible for the atropine-resistant salivary secretion.     

Consequences of capsaicin treatment on pulmonary vagal reflexes and chemoreceptor activity in lambs.

The aim of this study was to test the hypothesis that capsaicin treatment in lambs selectively inhibits bronchopulmonary C-fiber function but does not alter other vagal pulmonary receptor functions or peripheral and central chemoreceptor functions. Eleven lambs were randomized to receive a subcutaneous injection of either 25 mg/kg capsaicin (6 lambs) or solvent (5 lambs) under general anesthesia. Capsaicin-treated lambs did not demonstrate the classical ventilatory response consistently observed in response to capsaicin bolus intravenous injection in control lambs. Moreover, the ventilatory responses to stimulation of the rapidly adapting pulmonary stretch receptors (intratracheal water instillation) and slowly adapting pulmonary stretch receptors (Hering-Breuer inflation reflex) were similar in both groups of lambs. Finally, the ventilatory responses to various stimuli and depressants of carotid body activity and to central chemoreceptor stimulation (CO(2) rebreathing) were identical in control and capsaicin-treated lambs. We conclude that 25 mg/kg capsaicin treatment in lambs selectively inhibits bronchopulmonary C-fiber function without significantly affecting the other vagal pulmonary receptor functions or that of peripheral and central chemoreceptors.     

Neural regulation of lysozyme secretion from tracheal submucosal glands of ferrets in vivo.

To investigate how central and peripheral nerves affect lysozyme secretion from tracheal submucosal glands in ferrets we injected substance P (20 nmol/kg in 200 microliters) intracisternally or intravenously into anesthetized artificially ventilated ferrets. We collected 3-ml samples from a perfused (3 ml/5 min) segment of trachea in situ during 15 min before and 45 min after injection of substance P. Content of lysozyme, a specific marker of tracheal submucosal gland serous cell secretion in ferrets, was measured spectrophotometrically in each sample. Intracisternal substance P increased peak lysozyme output threefold compared with baseline. This increase was abolished completely by cutting both superior laryngeal nerves (SLN) and was partially inhibited by atropine, phentolamine, or propranolol. Intravenous substance P increased peak lysozyme output 10-fold compared with baseline. This increase was partly abolished by cutting both SLN. We concluded that intracisternal substance P stimulated the central nervous system (CNS) and activated cholinergic, adrenergic, and nonadrenergic noncholinergic secretomotor nerves to tracheal glands and that intravenous substance P increased lysozyme secretion both by acting directly on tracheal glands and indirectly on the CNS to activate secretomotor nerves.     

Central nervous system control of airway tone in guinea pigs: the role of histamine

The central nervous system (CNS) plays an important role in the reflex control of bronchomotor tone, but the relevant neurotransmitters and neuromodulators have not been identified. In this study we have investigated the effect of histamine. Anesthetized male guinea pigs were prepared with a chronically implanted intracerebroventricular (icv) cannula and instrumented for the measurement of pulmonary resistance (RL), dynamic lung compliance (Cdyn), tidal volume (VT), respiratory rate (f), blood pressure (BP), and heart rate (HR). Administration of histamine (2-30 micrograms) icv caused a significant (P less than 0.05) reduction of Cdyn with no change in RL, VT, and f. At a dose of 100 micrograms icv, histamine caused an increase in RL (202 +/- 78%), a reduction of Cdyn (77 +/- 9%), an increase in f (181 +/- 64%), and a reduction of VT (53 +/- 18%). There were no changes in BP and HR after 100 micrograms of icv histamine. In contrast, intravenous administration of histamine (0.1-2 micrograms/kg) caused a dose-dependent decrease in Cdyn and increase in RL that was associated with tachypnea at each bronchoconstrictor dose. Intravenous histamine (2 micrograms/kg) produced a fall in BP and an increase in HR. The bronchoconstrictor responses to icv histamine were completely blocked by vagotomy and significantly reduced by atropine (0.1 mg/kg iv), whereas vagotomy and atropine did not block the bronchospasm due to intravenous histamine. Additional studies indicated that the pulmonary responses due to icv histamine (100 micrograms) were blocked by pretreatment with the H1-antagonist chlorpheniramine (1 and 10 micrograms, icv). These data indicate that histamine may serve a CNS neurotransmitter function in reflex bronchoconstriction in guinea pigs.     

Effect of substance P on mucus secretion of isolated submucosal gland from feline trachea

To elucidate how substance P (SP) produces submucosal gland secretion, we examined the effects of SP on the glandular contractile response and 3H-labeled glycoconjugate release in isolated submucosal glands from feline tracheae. SP (10(-12) to 10(-4) M) produced dose-dependent increases in the contractile response, and the maximal tension induced by SP was approximately 70% of the response to methacholine. SP-induced contraction is blocked completely by atropine and augmented by neostigmine. Pretreatment with hemicholinium 3, an acetylcholine synthesis inhibitor, inhibited the contractile response to SP. Pretreatment with tetrodotoxin did not inhibit the contractile response to SP. Capsaicin induced tension of a magnitude similar to that of SP. SP (10(-7) M) produced a significant increase (74% above control) in radiolabeled glycoconjugate release from isolated glands, whereas SP had no significant effects on glycoconjugate release from tracheal explants, probably because of epithelial suppression. Atropine abolished SP-evoked glycoconjugate release in isolated glands. Our findings indicate that 1) SP induces glandular contraction, which is related to the squeezing of mucus in the ducts and secretory tubules, 2) SP stimulates radiolabeled glycoconjugate release in isolated submucosal gland, probably involving mucus synthesis and/or cellular secretion, and 3) these two actions are mediated by a peripheral cholinergic mechanism.     

Tachykinin receptors mediating airway macromolecular secretion.

Three tachykinin receptor types, termed NK1, NK2, and NK3, can be distinguished by the relative potency of various peptides in eliciting tissue responses. Airway macromolecular secretion is stimulated by the tachykinin substance P (SP). The purposes of this study were to determine the tachykinin receptor subtype responsible for this stimulation, and to examine the possible involvement of other neurotransmitters in mediating this effect. Ferret tracheal explants maintained in organ culture were labeled with 3H-glucosamine, a precursor of high molecular weight glycoconjugates (HMWG) which are released by airway secretory cells. Secretion of labeled HMWG then was determined in the absence and presence of the tachykinins SP, neurokinin A (NKA), neurokinin B (NKB), physalaemin (PHY), and eledoisin (ELE). All the tachykinins tested stimulated HMWG release to an approximately equal degree. Stimulation was concentration-related, with log concentrations giving half-maximal effects (EC50) as follows: SP -9.47, NKA -7.37, NKB -5.98, PHY -8.08, and ELE -7.68. This rank order of potency (SP greater than PHY greater than or equal to ELE greater than or equal to NKA greater than NKB) is most consistent with NK1 receptors. To evaluate the possible contribution of other mediators, tachykinin stimulation was examined in the presence of several receptor blockers. The potency of SP was not diminished by pretreatment with atropine, propranolol, or chlorpheniramine, and atropine actually increased the magnitude of the secretory response. The SP receptor antagonist [D-Arg1,D-Phe5, D-Trp7,9, Leu11]-SP blocked SP-induced secretion. These findings indicate that SP is a potent stimulus of airway macromolecular secretion. This effect occurs through the action of NK1 receptors, and is not dependent upon cholinergic, beta-adrenergic, or H-1 histamine receptors. The facilitation by atropine of SP stimulation suggests the existence of a mechanism of cholinergic inhibition of SP-induced stimulation.     

Substance P evokes bradycardia by stimulation of postganglionic cholinergic neurons.

Substance P (SP) evokes bradycardia that is mediated by cholinergic neurons in experiments with isolated guinea pig hearts. This project investigates the negative chronotropic action of SP in vivo. Guinea pigs were anesthetized with urethane, vagotomized and artificially respired. Using this model, IV injection of SP (32 nmol/kg/50 microl saline) caused a brief decrease in heart rate (-30+/-3 beats/min from a baseline of 256+/-4 beats/min, n = 27) and a long-lasting decrease in blood pressure (-28+/-2 mmHg from baseline of 51+/-5 mmHg, n = 27). The negative chronotropic response to SP was attenuated by muscarinic receptor blockade with atropine (-29 +/- 9 beats/min before vs -8 +/- 2 beats/min after treatment, P = 0.0204, n = 5) and augmented by inhibition of cholinesterases with physostigmine (-23 +/- 6 beats/min before versus -74 +/- 20 beats/min after treatment, P = 0.0250, n = 5). Ganglion blockade with chlorisondamine did not diminish the negative chronotropic response to SP. In another series of experiments, animals were anesthetized with sodium pentobarbital or urethane and studied with or without vagotomy. Neither anesthetic nor vagotomy had a significant effect on the negative chronotropic response to SP (F3,24 = 1.97, P = 0.2198). Comparison of responses to 640 nmol/kg nitroprusside and 32 nmol/kg SP demonstrated that the bradycardic effect of SP occurs independent of vasodilation. These results suggest that SP can evoke bradycardia in vivo through stimulation of postganglionic cholinergic neurons.     

Distribution and function of the cannabinoid-1 receptor in the modulation of ion transport in the guinea pig ileum: relationship to capsaicin-sensitive nerves

We investigated the distribution and function of cannabinoid (CB)(1) receptors in the submucosal plexus of the guinea pig ileum. CB(1) receptors were found on both types of submucosal secretomotor neurons, colocalizing with VIP and neuropeptide Y (NPY), the noncholinergic and cholinergic secretomotor neurons, respectively. CB(1) receptors colocalized with transient receptor potential vanilloid-1 receptors on paravascular nerves and fibers in the submucosal plexus. In the submucosal ganglia, these nerves were preferentially localized at the periphery of the ganglia. In denervated ileal segments, CB(1) receptor immunoreactivity in submucosal neurons was not modified, but paravascular and intraganglionic fiber staining was absent. Short-circuit current (I(sc)) was measured as an indicator of net electrogenic ion transport in Ussing chambers. In the ion-transport studies, I(sc) responses to capsaicin, which activates extrinsic primary afferents, and to electrical field stimulation (EFS) were reduced by pretreatment with the muscarinic antagonist atropine, abolished by tetrodotoxin, but were unaffected by VIP receptor desensitization, hexamethonium, alpha-amino-3-hydroxy-5-methlisoxazole-4-proprionic acid, or N-methyl-d-aspartate glutamate receptor antagonists. The responses to capsaicin and EFS were reduced by 47 +/- 12 and 30 +/- 14%, respectively, by the CB(1) receptor agonist WIN 55,212-2. This inhibitory effect was blocked by the CB(1) receptor antagonist, SR 141716A. I(sc) responses to forskolin or carbachol, which act directly on the epithelium, were not affected by WIN 55,212-2. The inhibitory effect of WIN 55,212-2 on EFS-evoked secretion was not observed in extrinsically denervated segments of ileum. Taken together, these data show cannabinoids act at CB(1) receptors on extrinsic primary afferent nerves, inhibiting the release of transmitters that act on cholinergic secretomotor pathways.     

Effects of neurokinin A and calcitonin gene-related peptide on mucociliary activity in rabbit maxillary sinus

Substance P (SP) released from sensory C-fibers in the airways increases the mucociliary (m.c.) activity in the rabbit maxillary sinus. The purpose of the present study was to investigate the m.c. effects of two other neuropeptides, coexisting with SP in sensory neurones, neurokinin A (NKA) and calcitonin gene-related peptide (CGRP). NKA increased the m.c. activity dose-dependently (dose range 0.1-10.0 micrograms/kg, 88 pmol to 8.8 nmol/kg), the maximum increase being 41.9 +/- 2.6%. The effect was inhibited by pretreatment with the tachykinin antagonist (D-Pro2,D-Trp7,9)SP, but not with atropine or hexamethonium. Thus NKA released from sensory C-fibers may contribute to the non-cholinergic increase of m.c. activity observed after C-fiber stimulation. In contrast CGRP did not influence the m.c. activity. Neither did it influence the responses to NKA or SP. It is concluded that CGRP is unlikely to be involved in the control of m.c. function.