Sensory information from the upper airway: role in the control of breathing

Respiratory reflexes in response to nasal administration of halothane to anesthetized, spontaneously breathing dogs

OBJECTIVE

To characterize and determine the sensory innervation of respiratory reflexes elicited by nasal administration of halothane to dogs.

ANIMALS

10 healthy Beagles.

PROCEDURE

Dogs underwent permanent tracheostomy and, 2 to 3 weeks later, were anesthetized with thiopental and alpha-chloralose administered IV. The nasal passages were functionally isolated so that halothane could be administered to the nasal passages while dogs were breathing 100% O2 via the tracheostomy. Respiratory reflexes in response to administration of halothane at concentrations of 1.25, 1.75, and 2.5 times the minimum alveolar concentration (MAC), and 5% (administered in 100% O2 at a flow rate of 5 L/min) were recorded. Reflexes in response to administration of 5% halothane were also recorded following transection of the infraorbital nerve, transection of the caudal nasal nerve, and nasal administration of lidocaine.

RESULTS

Nasal administration of halothane induced an inhibition of breathing characterized by a dose-dependent increase in expiratory time and a resultant decrease in expired volume per unit time. Effects were noticeable immediately after the onset of halothane administration and lasted until its cessation. Reflex responses to halothane administration were attenuated by transection of the caudal nasal nerve and by nasal administration of lidocaine, but transection of the infraorbital nerve had no effect.

CONCLUSIONS AND CLINICAL RELEVANCE

Nasal administration of halothane at concentrations generally used for mask induction of anesthesia induces reflex inhibition of breathing. Afferent fibers in the caudal nasal nerve appear to play an important role in the reflex inhibition of breathing induced by halothane administration.

Protective and defensive airway reflexes evoked by nasal exposure to wood smoke in anesthetized rats

We investigated the airway responses evoked by nasal wood smoke in anesthetized Sprague-Dawley rats. Wood smoke (5 ml, 1.4 ml/s) was delivered into an isolated nasal cavity while animals breathed spontaneously. In study 1, nasal wood smoke triggered either an apneic response (n = 26) or a sniff-like response (n = 16) within 1 s after smoke exposure in 42 normal rats. Both airway responses were abolished by trigeminal nerve denervation and by nasal application of a local anesthetic or a hydroxyl radical scavenger, but they were not significantly affected by removal of smoke particulates or nasal application of a saline vehicle. In study 2, nasal wood smoke only triggered a mild apneic response in two rats neonatally treated with capsaicin and had no effect on breathing in the other six; the treatment is known to chronically ablate C fibers and some Adelta fibers. In contrast, nasal wood smoke evoked an apneic response in six rats neonatally treated with the vehicle of capsaicin and elicited a sniff-like response in the other two. These results suggest that the apneic and sniff-like responses evoked by nasal wood smoke result from the stimulation of trigeminal nasal C-fiber and Adelta-fiber afferents by the gas-phase smoke and that hydroxyl radical is the triggering chemical factor.

Similarities in reflex control of laryngeal and cardiac vagal motor neurones

We sought to test the hypothesis that laryngeal adductor and cardiac vagal motor neurones respond similarly to the activation of certain afferent inputs. Experiments were performed on a working heart-brainstem preparation of rat devoid of pulmonary stretch receptor feedback. Upper airway negative pressure receptors (UANPR), peripheral arterial chemoreceptors and receptors at the junction of the pharynx and oesophagus were stimulated selectively while recording heart rate, recurrent laryngeal, phrenic and hypoglossal motor outflows, subglottic pressure during constant translaryngeal airflow (as an index of laryngeal resistance), and single unit respiratory neurone activity. Stimulation of all three receptor types produced bradycardia, evoked discharges in the recurrent laryngeal and hypoglossal motor outflows during the post-inspiratory period and caused swallowing. Stimulation of pharyngoesophageal receptors and peripheral chemoreceptors evoked an increase in laryngeal resistance during the post-inspiratory phase indicative of laryngeal adductor motoneurone activation. Although this reflex response cannot be evaluated during UANPR stimulation, some post-inspiratory neurones were powerfully activated suggesting that UANPR probably drive laryngeal adductor muscles. Our data show that motor outflows controlling cardiac rate and laryngeal patency are concurrently activated by these sensory inputs. This may constitute the basis for a stereotyped defensive reflex response which maintains end expiratory lung volume, thus conserving oxygen in conditions of upper airway obstruction. Our observations lend further support to models of cardiorespiratory control which propose close coupling and shared central mechanisms for the regulation of the cardiovascular and respiratory systems.

Effects of perineural capsaicin treatment on compound action potentials of superior laryngeal nerve afferents in sevoflurane-anesthetized dogs

Effects of perineural capsaicin (CAPS) treatment on compound action potentials of the superior laryngeal nerve (SLN) afferents were studied in 6 sevoflurane-anesthetized dogs. Perineural CAPS (100 microg/ml) to the bilateral SLNs reduced (P<0.01) the peak and integral amplitudes of the C-wave of the compound action potential. By contrast, the perineural CAPS had no effect on the A-wave component (P>0.05). Removal of the perineural CAPS recovered the C-wave to pretreatment level. The perineural CAPS treatment selectively blocks C-wave compound action potentials of the SLN afferents, providing a useful tool for studies of laryngeal C-fibers in respiratory physiology.

Anesthesia for the superior laryngeal nerves and tartaric acid-induced cough

OBJECTIVE

The internal branch of the superior laryngeal nerve (ibSLN) conveys impulses for the laryngeal cough reflex, which protects the laryngeal aditus and prevents the development of aspiration pneumonia. The purpose of this study was to determine the effect of bilateral anesthesia of the ibSLN on the cough reflex after inhalation of a nebulized chemoirritant solution of tartaric acid.

DESIGN

Prospective, clinical investigation.

SETTING

Outpatient.

PARTICIPANTS

Nine healthy volunteers.

INTERVENTIONS

Bilateral injections of 2% lidocaine solution without epinephrine into the paraglottic space containing the ibSLN.

MAIN OUTCOME MEASURES

The tidal volume after inhalation of a nebulized 20% tartaric acid solution and forced vital capacity (FVC) were measured before and after injection. Data were analyzed using the Wilcoxon signed ranks, Mann-Whitney, and sign tests.

RESULTS

Complete anesthesia of the ibSLN abolished the laryngeal cough reflex. Postinjection tidal volumes were significantly lower than preinjection volumes (p<.01). The decrease in tidal volumes for six subjects with complete bilateral anesthesia was significantly larger than the decrease in tidal volumes for three subjects with partial anesthesia (p<.05). FVC in both the six subjects with complete bilateral anesthesia and the three subjects with partial anesthesia did not significantly change from preinjection to postinjection. None of the subjects in this study had complications or adverse respiratory sequelae.

CONCLUSION

Tartaric acid-induced cough may be useful in assessing the integrity of the laryngeal cough reflex after anesthesia or in patients with neurologic injury who are at risk of developing aspiration pneumonia. It may also be useful in making the decision whether to resume oral feeding.

Age-related changes in the ventilatory response to inspired CO2 in neonatal rats

The objective of this study was to determine whether there is an age-related ventilatory response to transient increases in inspired CO2 in unanesthetized rat pups. Using plethysmography, ventilatory responses to 30 sec of 0, 2, 4, 6, and 8% inspired CO2 were measured in 21 rat pups from two litters. Recordings were made 1, 2, 3, 5, 7, 9 and 12 days after the day of birth (day 0). On day 1 there was a significant dose-related decrease in mean ventilatory frequency in response to each of the inspired CO2 concentrations. On day 2 there was no significant change in breathing frequency in response to 2 or 4% CO2 and a significant increase in frequency in response to 6 and 8% CO2. On days 3, 5, 7, 9 and 12 there was generally a significant increase in frequency in response to each of the inspired CO2 concentrations. Tidal volume was not significantly affected by the CO2 stimuli on any of the test days. Minute ventilation exhibited a significant decrease, on day 1, in response to 6 and 8% CO2. Litter, sex or weight of the rat pups was not correlated with the ventilatory depressions observed on day 1. These results show that in neonatal rats the ventilatory response to inspired CO2 is age-related and indicates a possible link between upper airway CO2 chemoreceptors, an inhibition of breathing, and SIDS.

Effects of volatile anesthetics on the activity of laryngeal 'drive' receptors in anesthetized dogs

Effects of halothane, isoflurane and sevoflurane on laryngeal drive receptor activity were studied in the afferent activity of the superior laryngeal nerve in anesthetized spontaneously breathing dogs. Of 40 single units recorded, most of them (65%) responded to the volatile anesthetics applied to the isolated larynx at a concentration of 5%. The exposure to the anesthetics resulted in either an inspiratory increase (15%), both inspiratory and expiratory decrease (54%), or both inspiratory increase and expiratory decrease (31%) responses. The average discharge frequency of the receptors tended to be decreased on inhalation of the anesthetics, where significant decreases were observed in both respiratory phases for halothane and at expiration for isoflurane, but in neither respiratory phase for sevoflurane. These results support an advantage of sevoflurane over halothane and isoflurane for induction of anesthesia to minimize the influence of the activity of laryngeal drive receptors on the breathing pattern and airway stability.

Pontine cholinergic mechanisms enhance trigeminally evoked respiratory suppression in the anesthetized rat

In the present study, we investigated in anesthetized rats the influences of the pontine rapid-eye-movement (REM) sleep center on trigeminally induced respiratory responses. We evoked the nasotrigeminal reflex by electrical stimulation of the ethmoidal nerve (EN5) and analyzed the EN5-evoked respiratory suppression before and after injections into the pontine reticular nuclei of the cholinergic agonist carbachol. After injections of 80-100 nl of carbachol (20 mM), we observed a decrease in respiratory rate, respiratory minute volume, and blood pressure but an increase in tidal volume. In those cases in which carbachol injections alone caused these REM sleep-like autonomic responses, we also observed that the EN5-evoked respiratory suppression was significantly potentiated. Unfortunately, carbachol injections failed to depress genioglossus electromyogram (EMG) effectively, because the EMG activity was already strongly depressed by the anesthetic alpha-chloralose. We assume that pontine carbachol injections in our anesthetized rats cause autonomic effects that largely resemble REM sleep-like respiratory and vascular responses. We therefore conclude that the observed potentiation of EN5-evoked respiratory suppression after carbachol might be due to REM sleep-associated neuronal mechanisms. We speculate that activation of sensory trigeminal afferents during REM sleep might contribute to pathological REM sleep-associated respiratory failures.

Response of respiratory muscles to intravenous nicotine challenge in anaesthetized cats

The effects of an intravenous nicotine challenge on the ventilation and activity of rib cage muscles were studied in 33 pentobarbitone-chloralose anaesthetized cats. Bolus injection of nicotine (200 microg) into the right femoral vein evoked in 19 of the intact animals prompt, short-lived apnoea, or prolongation of the first expiration after the drug, the occurrence of which was significantly reduced by midcervical vagotomy (P < 0.001). In breaths that followed the apnoea, peak activity of the parasternal intercostal muscles increased from a baseline of 3.1 +/- 0.8 to 9.2 +/- 1.8 arbitrary units (P < 0.001). Nicotine produced a similar increase in peak phrenic ENG (7.0 +/- 0.5 to 14.5 +/- 1.2 arbitrary units; P < 0.001). Peak triangularis sterni muscle EMG was reduced from 8.9 +/- 1.2 to 6 +/- 1.7 arbitrary units (P < 0.05) and the onset of response was delayed to 30 s after the challenge. The changes of respiratory effectors induced by nicotine were independent of vagal integrity. The results show that post-nicotine apnoea is to large extent vagally dependent though the response of the respiratory muscles is mediated by non-vagal influences.

Nasal sensory receptors responding to capsaicin, water and tactile stimuli in sevoflurane-anesthetized dogs

Responses of nasal receptors to capsaicin and water were studied from afferent recordings of the posterior nasal nerve (PNN) in 12 anesthetized dogs. Out of 12 non-respiration-modulated nasal receptors, 7 responded only to capsaicin, 3 responded to both water and capsaicin, and 2 to neither of them. All the fibers showed a rapid adaptation to mechanical probing of the nasal mucosa. These results indicate that the presence of sensory receptors responding to capsaicin and water are involved in PNN afferents of the dog.

Larynx vs. esophagus as reflexogenic sites for acid-induced bronchoconstriction in dogs

Bronchoconstriction in asthmatic patients is frequently associated with gastroesophageal reflux. However, it is still unclear whether bronchoconstriction originates from the esophagus or from aspiration of the refluxate into the larynx and larger airway. We compared the effect of repeated esophageal and laryngeal instillations of HCl-pepsin (pH 1.0) on tracheal smooth muscle activity in eight anesthetized and artificially ventilated dogs. Saline was used as control. We used pressure in the cuff of an endotracheal tube (Pcuff) as a direct index of smooth muscle activity at the level of the larger airways controlled by vagal efferents. The Pcuff values of the first 60 s after instillations were averaged, and the difference from the baseline values was evaluated. Changes in Pcuff were significantly greater with laryngeal than with esophageal instillations (P = 0.0166). HCl-pepsin instillation into the larynx evoked greater responses than did saline (P = 0.00543), whereas no differences were detected with esophageal instillations. Repeated laryngeal exposure enhanced the responsiveness significantly (P < 0. 001). Our data indicate that the larynx is more important than the esophagus as a reflexogenic site for the elicitation of reflex bronchoconstriction in response to acidic solutions.

Assessing the laryngeal cough reflex and the risk of developing pneumonia after stroke

OBJECTIVE

To determine the effectiveness of a new reflex cough test, using nebulized tartaric acid, in the evaluation of the laryngeal cough reflex and the development of aspiration pneumonia.

STUDY DESIGN

In this two-phase study, the cough test assessed the cough reflex in 161 stroke subjects. Phase 1 was a double-blinded prospective study of 40 subjects scheduled to have both modified barium swallow and the reflex cough test. Phase 1 subjects with an abnormal cough test showed an increased pneumonia incidence, and therefore, phase 2 was not blinded. In phase 2, 121 subjects were evaluated using the cough test; 38 received a modified barium swallow. Test results were compared using the Fisher exact test.

RESULTS

A total of 131 subjects from both phases had a normal reflex cough test; none developed pneumonia (p < .01). Thirty subjects from both phases had abnormal reflex cough test results; 5 developed pneumonia. Modified barium swallow findings did not reliably indicate the risk for developing pneumonia. Specificity of a normal reflex cough test was 100%.

CONCLUSION

The reflex cough test reliably evaluated the laryngeal cough reflex and the associated risk of developing aspiration pneumonia in stroke patients. Testing the laryngeal cough reflex may significantly reduce morbidity, mortality, and costs in stroke patients.

The posterior nasal nerve plays an important role on cardiopulmonary reflexes to nasal application of capsaicin, distilled water and l-menthol in anesthetized dogs

The sensory innervation of the cardiopulmonary reflexes to nasal application of capsaicin (CAPS), distilled water (DW) and l-menthol (LM) was studied in anesthetized dogs breathing through tracheostomy. A marked cardiopulmonary reflex was observed by CAPS and DW into the nasal cavity, while a prolongation of expiration was induced by LM. All these reflexes were significantly decreased by bilateral section of the posterior nasal nerve (PNN) and completely abolished by topical nasal anesthesia with lidocaine. Responses of the whole nerve activity of the PNN to these substances corresponded to the magnitude of the reflexes. These results indicate that PNN afferents play an important role on the reflex elicitation of the noxious, water and cold stimuli from the nasal cavity.

NMDA receptor subunit NR1-immunoreactivity in the rat pons and brainstem and colocalization with Fos induced by nasal stimulation

In the present study, we examined the distribution of neurons in the parabrachial nucleus (PB), the Kölliker-Fuse nucleus (KF), the spinal trigeminal nucleus caudalis (Sp5C), the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM), which are activated by evoking the nasotrigeminal reflex and which exhibit immunoreactivity for the N-methyl-D-aspartate (NMDA) receptor subunit NR1. By stimulating the nasal mucosa with saline, we induced the expression of the immediate early gene c-fos and combined the immunocytochemical detection of the Fos protein with the detection of the NR1 subunit. Cell counts revealed that nasal stimulation, compared to anesthesia controls, resulted in highly significant increases (p < or = 0.001) of Fos-immunoreactive (-ir) neurons in the midlevel KF, the external lateral PB, and the Sp5C. In the central lateral PB, the rostral ventrolateral medulla including the Bötzinger/pre-Bötzinger complex, and in the ventrolateral and commissural NTS the increases were only moderately significant (p < or = 0.05). With respect to the numbers of NR1-/Fos-ir double-labeled neurons, significant increases were only observed in a subset of these pontomedullary nuclei. Increases were highly significant in the Sp5C (p < or = 0.001) and the midlevel KF (p < or = 0.01) and moderately significant (p < or = 0.05) in the external lateral PB, Bötzinger/pre-Bötzinger complex, and ventrolateral NTS. The present study revealed that nasotrigeminally activated neurons in mandatory and potential relay sites of the nasotrigeminal reflex circuit express the NR1 subunit. This finding strongly suggests that NMDA-type glutamate receptors are involved in the mediation of the nasotrigeminally evoked cardiovascular and respiratory responses.

Tartaric acid-induced cough and the superior laryngeal nerve evoked potential

The purpose of this study was to stimulate the laryngeal cough reflex using a nebulized, mild chemical irritant and to record an associated laryngeal evoked potential from the internal branch of the superior laryngeal nerve. The laryngeal evoked potential was obtained on ten normal subjects from the right internal branch of the superior laryngeal nerve. The electrodiagnostic setup included an active electrode placed just below the hyoid bone with a 4-cm separation and distal reference. A ground electrode was placed between the active and reference electrodes. The receptors and internal branch of the superior laryngeal nerve were stimulated by inhalation of a nebulized 20% solution of tartaric acid and normal saline. The time line was triggered by a pneumatic switch on initial inspiration of the nebulized tartaric acid. The electrodiagnostic settings were set at a sweep speed of 1 ms/division, a gain of 10 to 20 microV/division, and 20 to 2,000 filters. There were 132 variables recorded from the internal branch of the superior laryngeal nerve of the ten subjects. The mean peak distal latency was 1.66+/-0.42 ms with a 1.6 median, 1.6 mode, and 0.17 variance. The duration was 0.41 ms, and amplitude was 5.19+/-2.91 microV. In conclusion, the laryngeal evoked potential, the afferent component of the involuntary cough reflex, can be recorded from the internal branch of the superior laryngeal nerve after inhalation of tartaric acid-induced cough.

Morphology of intraepithelial corpuscular nerve endings in the nasal respiratory mucosa of the dog

Corpuscular nerve endings in the nasal respiratory mucosa of the dog were investigated by immunohistochemical staining specific for protein gene product 9.5 by light and electron microscopy. In the nasal respiratory mucosa, complex corpuscular endings, which displayed bulbous, laminar and varicose expansions, were distributed on the dorsal elevated part of the nasal septum and on the dorsal nasal concha. The endings were 300-500 microm long and 100-250 microm wide. Some axons gave rise to a single ending while others branched into 2 endings. Cryostat sections revealed that the corpuscular endings were located within the nasal respiratory epithelium. On electron microscopy, immunoreactive nerve terminals that contained organelles, including mitochondria and neurofilaments, were observed within the epithelial layer near the lumen of the nasal cavity. Some terminals contacted the goblet cell. Such terminal regions were covered by the cytoplasmic process of ciliated cells and were never exposed to the lumen of the nasal cavity. These nerve endings are probably activated by pressure changes.

Afferent receptors in the airways and cough

The roles of airway rapidly adapting receptors (RARs) and of C-fibre receptors in the induction of cough are reviewed. It is concluded that, while there is substantial evidence that irritant receptors in the laryngeal wall and RARs in the tracheobronchial mucosa can cause cough, the evidence for such a similar direct role for C-fibre receptors is tenuous. Indeed there is accumulating evidence that the C-fibre receptors may cause apnoea and rapid shallow breathing, and also reflexly inhibit cough. However the C-fibre receptors may release tachykinins when stimulated, and these in turn may cause plasma extravasation from mucosal postcapillary venules. RARs are excited by increases in interstitial liquid volume, so C-fibre receptors may indirectly enhance cough via the RARs.

Response of breathing pattern to flow and pressure in the upper airway of rats

The effects of upper airway (UAW) flows and pressures on breathing pattern and respiratory muscle activities were studied in anesthetized rats breathing through a tracheostomy. A steady flow (approximately 1000 ml/kg/min) of cold dry air, or cold wet air, or warm wet air was passed through the UAW, in the expiratory direction for approximately 20 sec (20-40 sec). In other trials positive or negative pressure was applied to the isolated UAW for a similar duration. There was a marked prolongation of the expiratory duration and decreases in peak inspiratory flow, tidal volume, and peak diaphragm electromyogram (EMG) activity in response to cold dry airflow. The responses to cold wet air were reduced but still significant. Warm wet air had no effect on breathing. These responses show that UAW cooling and drying depress breathing in the rat and that cooling itself could cause the inhibition of breathing. Negative pressure induced substantial increases in genioglossus and laryngeal inspiratory activity while positive pressure caused a decrease in genioglossus activity. Positive pressure also increased expiratory time while negative pressure increased inspiratory time. These results confirm the functional role of the UAW dilating muscles in preventing UAW from collapse in rats.

A brainstem network mediating apneic reflexes in the rat

Apnea is an important protective response to upper airway irritation, but the central mechanisms responsible for eliciting sensory-induced apnea are not well understood. Recent studies have emphasized the Kölliker-Fuse nucleus in producing apnea and proposed a trigeminoparabrachial pathway for mediating these reflexes. However, in our earlier study of apneic responses produced by glutamate stimulation in the dorsolateral pons, we found that apnea was elicited from the area just ventral to the Kölliker-Fuse nucleus, rather than within it. Because this region was not known to be involved in respiratory control, we combined chemical microstimulation with both anterograde and retrograde axonal tracing to characterize the sites in the pons that produce apneic responses. We found that apneic sites were consistently associated with the intertrigeminal region, between the principal sensory and motor trigeminal nuclei. Injections of anterograde tracer at these sites labeled terminals in the ventral respiratory group, in the ventrolateral medulla. Injection of retrograde tracer into this target region in the ventrolateral medulla disclosed a previously unrecognized population of neurons among the trigeminal motor rootlets. Injection of retrograde tracer into this intertrigeminal region demonstrated inputs from portions of the spinal trigeminal nucleus and the nucleus of the solitary tract that have been associated with producing sensory apnea. Our observations suggest that the intertrigeminal region receives a convergence of sensory inputs capable of driving apneic responses and that it may represent a common link between input from different portions of the airway and the respiratory neurons that mediate apneic reflexes.

NMDA and GABAA receptors in the rat Kolliker-Fuse area control cardiorespiratory responses evoked by trigeminal ethmoidal nerve stimulation

1. Electrical stimulation (10 s) of the ethmoidal nerve (EN5) evokes the nasotrigeminal reflex responses, including apnoea, bradycardia and rise in arterial blood pressure. In the present study, we examined the involvement of N-methyl-D-aspartate (NMDA), AMPA/kainate, (gamma-aminobutyric acidA (GABAA) and glycine receptors in the Kolliker-Fuse (KF) nucleus in the mediation of the nasotrigeminal reflex responses. 2. Unilateral injections (n = 6) of 50-100 nl of the NMDA receptor antagonist AP5 into the KF area led to a significant blockade of the EN5-evoked respiratory depression and bradycardia. Injections placed into the midlevel of the KF area were most effective (80-90 % blockade). The rise in arterial blood pressure remained unaffected. 3. Unilateral injections (n = 6) of the AMPA/kainate receptor antagonist CNQX into the KF area failed to block EN5-evoked autonomic responses significantly. 4. Unilateral injections (n = 5) of the GABAA receptor antagonist bicuculline enhanced the EN5-evoked respiratory depression and bradycardia. The effect persisted for up to 30 s after stimulation. Bicuculline injections into the midlevel of the KF area were most effective. The increase in arterial blood pressure remained unaffected. 5. Unilateral injections (n = 5) of the glycine receptor antagonist strychnine into the KF area did not produce any significant effects on EN5-evoked autonomic responses. 6. Our results suggest that the KF area represents a mandatory relay for the nasotrigeminally induced apnoea and bradycardia which are predominantly mediated by NMDA receptors in the KF. Furthermore, it appears that KF neurons are under a potent GABAergic inhibitory control. The EN5-evoked rise in arterial blood pressure was not altered by any of the drugs and, therefore, appears not to be mediated via the KF.

Neurochemical markers in the nervous plexus of the canine glottis

The structure of the nervous network and the distribution of tyrosine hydroxylase (TH)- and various neuropeptide-containing nerves were immunohistochemically studied in the glottis of the dog. The nervous network in the glottis revealed apparent regional differences in morphology. The nervous network in the cartilaginous vocal fold of the posterior glottis consisted of nerve bundles running parallel to the edge of the vocal fold. Only a small number of nerve bundles were observed in the anterior glottis, specifically in membranous vocal fold. In the subepithelial layer of the posterior glottis, a moderate number of galanin (GAL)-immunoreactive nerve fibers were observed, while only a few fibers were present in the anterior glottis. Numerous vasoactive intestinal peptide (VIP)-, GAL-, methionine-enkephalin (ENK)- and TH-immunoreactive nerve fibers were observed within and around the laryngeal submucosal seromucous gland. Many TH- and neuropeptide Y (NPY)-immunoreactive fibers were arranged around the blood vessels. In the epithelia, free nerve endings with immunoreactivity for substance P (SP) and calcitonin gene-related peptide (CGRP) was observed. Furthermore, nerve cell bodies with SP-, VIP-, GAL-, ENK-, and NPY-immunoreactivity were observed in the deep region of the submucosal layer. The results from the present study suggest that there is autonomic regulation of the glottis. Regional structural differences in the nervous network of the glottis may reflect functional differences.

Responses of laryngeal capsaicin-sensitive receptors to volatile anesthetics in anesthetized dogs

The responses of laryngeal capsaicin (CAPS)-sensitive receptors to halothane, enflurane, isoflurane and sevoflurane were evaluated in anesthetized spontaneously breathing dogs from the afferent activity of the internal branch of the superior laryngeal nerve. The CAPS-sensitive receptors were clearly distinguished from irritant receptors by their responsiveness to CAPS and their lack of responsiveness to water. All the CAPS-sensitive receptors were significantly stimulated by all volatile anesthetics in a concentration-related manner, and the activation by halothane, enflurane, and isoflurane was significantly greater than by sevoflurane. In contrast, responses of irritant receptors to the volatile anesthetics were divided into three types (stimulation, inhibition or non-response), and did not differ among anesthetics. In conclusion, the present study demonstrated that the CAPS-sensitive receptors were consistently stimulated by halogenated volatile anesthetics and especially by halothane, enflurane, and isoflurane, and that these responses were dissimilar to the variable responses of irritant receptors.

The afferent activity of the superior laryngeal nerve, and respiratory reflexes specifically responding to intralaryngeal pressure changes in anesthetized Shiba goats

This study was aimed at characterizing the superior laryngeal nerve (SLN) afferent activities under four different respiratory conditions, i.e., tracheostomy breathing (TB), upper airway breathing (UAB), tracheal occlusion (TO) and upper airway occlusion (UAO), and investigating respiratory changes in response to transmural pressures applied to the larynx in anesthetized Shiba goats. The activity recorded from the whole SLN increased at both inspiration and expiration during TB, UAB and TO, while an expiratory augmentation accompanied by an inspiratory inhibition was found during UAO. Based on recordings from 109 thin filament-preparations, 47 units were identified as 'drive' receptors, 31 as 'pressure' receptors (22 'positive' and 9 'negative' pressure receptors), and the rest 31 as 'non-modulated type' of receptors. The posterior cricoarytenoid (PCA) muscle activity showed a clear inspiratory modulation during UAB and was significantly enhanced by negative pressure applied to the isolated upper airway, where such an augmented activity was abolished by bilateral section of the SLN. No significant changes were found in the respiratory cycle during application of negative pressures to the larynx. The respiratory modulation of the SLN in Shiba goats was essentially identical to that reported for rabbits, rats and guinea pigs, but not in dogs. The reflex response of the upper airway muscles to the laryngeal pressure changes in Shiba goats were found to be less noticeable than in rabbits and dogs.

Cardiovascular reflex mechanisms by topical instillation of capsaicin and distilled water into the larynx in anesthetized dogs

Cardiovascular reflex mechanisms by topical laryngeal instillation of capsaicin (CAPS) or distilled water were evaluated in anesthetized chronic tracheostomized dogs. Both CAPS (10 micrograms/ml) and water instillation into the isolated upper airway caused a significant decrease in heart rate (P < 0.05) and a significant increase in blood pressure (P < 0.05) from the values before instillation under both spontaneous and controlled ventilation. The bradycardia was significantly reduced by atropine pretreatment (P < 0.05) and the hypertension was significantly decreased by phentolamine and propranolol pretreatments (P < 0.01). A higher concentration of CAPS (100 micrograms/ml) instillation considerably reduced the response to subsequent CAPS (100 micrograms/ml) instillation, whereas the response to water was sustained, indicating the desensitization of laryngeal CAPS-sensitive endings. All the reflex responses to CAPS and water were eliminated by topical anesthesia with lidocaine. It was concluded that the laryngeal cardiovascular reflex responses were mediated by the afferents such as the laryngeal CAPS-sensitive presumably C-fiber endings or water-responsive receptors and by both the parasympathetic and sympathetic nervous systems as efferents.

Reflex apneic response evoked by laryngeal exposure to wood smoke in rats: neural and chemical mechanisms

We investigated the neural and chemical mechanisms contributing to the immediate ventilatory responses to laryngeal exposure to wood smoke in anesthetized Sprague-Dawley rats. Five milliliters of wood smoke were delivered into a functionally isolated larynx at a constant flow rate of 1.4 ml/s while the animals breathed spontaneously. Within 1 s after exposure, laryngeal wood smoke consistently triggered an apnea in each of the 42 rats tested. The apneic duration reached 1,636.4 +/- 105.4 (SE) % (n = 42) of the baseline expiratory duration. This apneic response was not affected by denervation of recurrent laryngeal nerves (n = 6) or by removal of smoke particulates (n = 14), but it was totally eliminated by topical application of an anesthetic (n = 8; lidocaine hydrochloride, 8%) to the laryngeal mucosa or by sectioning of the superior laryngeal nerves (n = 42). Furthermore, laryngeal application of a hydroxyl radical scavenger (dimethylthiourea; 500 mg/ml; n = 8) greatly diminished or abolished the smoke-induced apneic response, but it did not affect the apneic response evoked by laryngeal exposure to air saturated with 6% ammonia. These results suggest that the immediate apneic response to laryngeal wood smoke is a reflex resulting from the stimulation of the superior laryngeal afferents by the gas phase of wood smoke and that the stimulation is mediated through a hydroxyl radical mechanism.

Innervation of taste buds in the canine larynx as revealed by immunohistochemistry for the various neurochemical markers

The distribution and innervation of the canine laryngeal taste buds were observed using immunohistochemistry with antibodies against protein gene product 9.5 (PGP 9.5) and neurofilament protein (NFP). We also observed the immunohistochemical distribution of serotonin, tyrosine hydroxylase (TH) and various neuropeptides including calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), galanin, methionine enkephalin (ENK) and neuropeptide Y (NPY). The taste buds in the canine larynx were densely distributed in the mucosa at the basal portion of the epiglottis and cuneiform process of the arytenoid cartilage. The taste cells were immunoreactive for PGP 9.5 and serotonin. The nerve fibers with immunoreactivity for PGP 9.5 in the taste buds were observed in the perigemmal region and intra- and subgemmal plexuses, and these were classified into two types based on their diameter. The thick nerve fibers corresponded to the fibers immunoreactive for NFP, while the thin nerve fibers corresponded to the fibers immunoreactive for TH and various neuropeptides. Numerous nerve fibers immunoreactive for SP and CGRP were observed in the perigemmal region, and intra- and subgemmal plexuses. A few galanin- and ENK-immunoreactive nerve fibers were also observed in the taste buds, whereas NPY-immunoreactive nerve fibers were noted beneath them. All peptide-containing fibers except for VIP-immunoreactive nerves were situated in the subgemmal regions. In conclusion, the multiple innervation to the laryngeal taste buds were documented. Thick nerve fibers are likely to be irritant receptors, while thin varicose nerve fibers seem to regulate taste buds themselves. The laryngeal taste buds may be among the important structures which are sensitive to exogeneous chemical and/or mechanical stimuli, for the protection of the airway and the regulation of the respiratory function.