Vagal and mediator mechanisms underlying the tachypnea caused by pulmonary air embolism in dogs.

We investigated the vagal and mediator mechanisms underlying the tachypnea caused by pulmonary air embolism (PAE) in anesthetized and spontaneously breathing dogs. PAE was induced by infusion of air into the right atrium (0.2 ml. kg(-1). min(-1) for 10 min). The first PAE induction caused an increase in respiratory frequency accompanied by a decrease in tidal volume in each of the 30 animals studied. Subsequently, animals were evenly divided into five groups, and a second PAE induction was repeated after various experimental interventions. The tachypneic response to PAE was not significantly altered by pretreatment with a saline vehicle but was largely attenuated by either perivagal capsaicin treatment (a technique that selectively blocks the conduction of unmyelinated C fibers), pretreatment with ibuprofen (a cyclooxygenase inhibitor), or pretreatment with dimethylthiourea (a hydroxyl radical scavenger). Ultimately, the tachypneic response was nearly abolished by a bilateral cervical vagotomy. These results suggest that 1) lung vagal unmyelinated C-fiber afferents play a predominant role in evoking the reflex tachypneic response to PAE and 2) both cyclooxygenase products and hydroxyl radical are important in eliciting this vagally mediated response.     

Laryngeal C-fiber afferents are not involved in the apneic response to laryngeal wood smoke in anesthetized rats

Laryngeal exposure to wood smoke in rats evokes a reflex apnea which is mediated through superior laryngeal afferents (J. Appl. Physiol. 83: 723-730, 1997). To study the role of laryngeal C-fiber afferents in eliciting this response, capsaicin aerosol (0.05 - 0.2 microg/ml) and 5 ml of wood smoke were delivered separately into a functionally isolated larynx of anesthetized Sprague-Dawley rats at a constant flow rate of 1.4 ml/s, while animals breathed spontaneously. Studies were repeated after either an intravenous injection of ruthenium red (2 mg/kg; n = 8), a perineural capsaicin treatment (200 microg/ml for 5 min; n = 8) of the superior laryngeal nerves, or a perineural sham treatment (n = 8); Ruthenium red inhibits the stimulation of afferent C-fiber nerve endings by capsaicin, whereas perineural capsaicin treatment selective blocks the conduction of C-fiber afferents. Either ruthenium red or perineural capsaicin treatment abolished the apneic response to laryngeal capsaicin, but did not significantly affect the apneic response to laryngeal wood smoke. Furthermore, the apneic responses to both types of irritants were not significantly altered by perineural sham treatment, yet were completely eliminated by a subsequent denervation of superior laryngeal nerves. Our results suggest that superior laryngeal C-fiber afferents are not involved in eliciting the reflex apneic response to laryngeal wood smoke in anesthetized rats. It is speculated that this response may result mainly from the stimulation of myelinated afferents, possibly laryngeal irritant receptors.     

Two subgroups of lung vagal C-fibers with different vulnerabilities to blockades by perivagal capsaicin and vagal cooling in dogs

Perivagal capsaicin treatment and vagal cooling are two techniques that have been widely used to study the respiratory reflexes mediated by lung vagal C-fibers because they can block the neural conduction of unmyelinated fibers. We hypothesized that there are two subgroups of lung vagal C-fibers which have different vulnerabilities to blockades by these two techniques. To test this hypothesis, afferent activity arising from lung vagal C-fibers was recorded in 29 anesthetized, paralyzed, and artificially ventilated dogs. Afferent C-fiber activity was recorded before and after various concentrations of perivagal capsaicin treatment or before and during various temperatures of vagal cooling. Of the 89 lung vagal C-fibers studied, 73 fibers were classified as the group of "low resistance" to capsaicin, while the other 16 were classified as the group of "high resistance". The former group differed from the latter due to their afferent activity being blocked at relatively low concentrations of perivagal capsaicin and at relatively low temperatures of vagal cooling. Our results suggest that lung vagal C-fibers can be categorized into two subgroups, based upon their different blocking thresholds for perivagal capsaicin and vagal cooling. Our data may provide information for researchers to further differentiate the respiratory reflexes originating from these two subgroups of lung vagal C-fibers.     

Effects of human eosinophil granule-derived cationic proteins on C-fiber afferents in the rat lung.

Experiments were performed to test the hypothesis that human eosinophil granule-derived cationic proteins stimulate vagal C-fiber afferents in the lungs and elicit pulmonary chemoreflex responses in anesthetized Sprague-Dawley rats. Intratracheal instillation of eosinophil cationic protein (ECP; 1-2 mg/ml, 0.1 ml) consistently induced an irregular breathing pattern, characterized by tachypnea (change in breathing frequency of 44.7%) and small unstable tidal volume (VT). The tachypnea, accompanied by decreased heart rate and arterial blood pressure, started within 30 s after the delivery of ECP and lasted for >30 min. These ECP-induced cardiorespiratory responses were completely prevented by perineural capsaicin treatment of both cervical vagi, which selectively blocked C-fiber conduction, suggesting the involvement of these afferents. Indeed, direct recording of single-unit activities of pulmonary C-fibers further demonstrated that the same dose of ECP evoked a pronounced and sustained (>30-min) stimulatory effect on pulmonary C-fibers. Furthermore, the sensitivity of these afferents to lung inflation was also markedly elevated after the ECP instillation, whereas the vehicle of ECP administered in the same manner had no effect. Other types of eosinophil granule cationic proteins, such as major basic protein and eosinophil peroxidase, induced very similar respiratory and cardiovascular reflex responses. In conclusion, these results show that eosinophil granule-derived cationic proteins induce a distinct stimulatory effect on vagal pulmonary C-fiber endings, which may play an important role in the airway hyperresponsiveness associated with eosinophil infiltration in the airways.     

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.     

Role of vagal C-fiber afferents in the bronchomotor response to lactic acid in the newborn dog.

We addressed the hypothesis that vagal C-fiber afferents and cyclooxygenase products are the mechanisms responsible for lactic acid (LA)-induced bronchoconstriction in the newborn dog. Perineural capsaicin and indomethacin were used to block conduction of vagal C fibers and production of cyclooxygenase products, respectively. Perineural capsaicin eliminated (85%) the increase in lung resistance (RL; 45 +/- 5.6%) due to capsaicin (25 microg/kg), whereas the increase in RL (54 +/- 6.9%) due to LA (0.4 mmol/kg) was only inhibited by 37 +/- 4.7% (P < 0.05). Atropine reduced LA-induced bronchoconstriction (42 +/- 2.1%) by an amount similar to that obtained with perineural capsaicin. However, inhibition was significantly increased when atropine was combined with indomethacin (61 +/- 2.7%; P < 0.05), implicating cyclooxygenase products in the LA-induced bronchoconstrictor response. We conclude that the mechanisms responsible for LA-induced bronchoconstriction in the newborn are 1) activation of vagal C-fibers, which, through projections to medullary respiratory centers, leads to activation of vagal cholinergic efferents; 2) production of cyclooxygenase products, which cause bronchoconstriction independent of medullary involvement; and 3) an unknown bronchoconstrictor mechanism, putatively tachykinin mediated. On the basis of our data, pharmaceutical targeting of pulmonary afferents would prevent multiple downstream mechanisms that lead to airway narrowing due to inflammatory lung disease.     

Vagal modulation of intestinal afferent sensitivity to systemic LPS in the rat

The central nervous system modulates inflammation in the gastrointestinal tract via efferent vagal pathways. We hypothesized that these vagal efferents receive synaptic input from vagal afferents, representing an autonomic feedback mechanism. The consequence of this vagovagal reflex for afferent signal generation in response to LPS was examined in the present study. Different modifications of the vagal innervation or sham procedures were performed in anesthetized rats. Extracellular mesenteric afferent nerve discharge and systemic blood pressure were recorded in vivo before and after systemic administration of LPS (6 mg/kg iv). Mesenteric afferent nerve discharge increased dramatically following LPS, which was unchanged when vagal efferent traffic was eliminated by acute vagotomy. In chronically vagotomized animals, to eliminate both vagal afferent and efferent traffic, the increase in afferent firing 3.5 min after LPS was reduced to 3.2 +/- 2.5 impulses/s above baseline compared with 42.2 +/- 2.0 impulses/s in controls (P < 0.001). A similar effect was observed following perivagal capsaicin, which was used to eliminate vagal afferent traffic only. LPS also caused a transient hypotension (<10 min), a partial recovery, and then persistent hypertension that was exacerbated by all three procedures. Mechanosensitivity was increased 15 min following LPS but had recovered at 30 min in all subgroups except for the chronic vagotomy group. In conclusion, discharge in capsaicin-sensitive mesenteric vagal afferents is augmented following systemic LPS. This activity, through a vagovagal pathway, helps to attenuate the effects of septic shock. The persistent hypersensitivity to mechanical stimulation after chronic vagal denervation suggests that the vagus exerts a regulatory influence on spinal afferent sensitization following LPS.     

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.     

Role of vagal fibers in the hypoxia-induced increases in end-expiratory lung volume and diaphragmatic activity

Bonora, M., and M. Vizek. Role of vagalfibers in the hypoxia-induced increases in end-expiratory lung volumeand diaphragmatic activity. J. Appl.Physiol. 83(3): 700-706, 1997.The possible role of pulmonary C fibers in thehypoxia-induced concomitant increases in end-expiratory lung volume(EELV) and in the activity of the diaphragm at the end of expiration(DE) were evaluated bymeasuring the effects of hypoxia (10%O₂) on ventilation, EELV, andDE in eight chloralose-urethananesthetized rats. Recordings were made before and after blocking vagalC fibers and after bilateral vagotomy. C-fiber conduction was blockedby applying capsaicin perineurally to the cervical vagi. The efficiencyof C-fiber blockade was tested with intravenous capsaicin and itsselectivity by the Hering-Breuer reflex. Perineural capsaicin abolishedthe reflex apnea induced by intravenous capsaicin and transientlyreduced Hering-Breuer reflex. Perineural capsaicin affected neitherventilation, DE, and EELV in airnor the hypoxia-induced increases in these parameters. Vagotomy causedthe typical changes of breathing pattern in air, but the ventilatoryresponse to hypoxia was unchanged. Vagotomy performed during hypoxiaresulted in large decreases inDE and EELV. Hypoxia increasedDE and EELV in vagotomized rats but less than in intact rats. We conclude that the hypoxia-induced increases in EELV and diaphragmatic activity are probably not mediatedby vagal C fibers and that vagal afferents are involved but not fullyresponsible for this phenomenon.

     

Acute effects of cigarette smoke on breathing in rats: vagal and nonvagal mechanisms

The acute ventilatory response to inhalation of cigarette smoke was studied in anesthetized Sprague-Dawley rats. Cigarette smoke (6 ml, 50%) generated by a machine was inhaled spontaneously via a tracheal cannula. Within the first two breaths of smoke inhalation, a slowing of respiration resulting from a prolonged expiratory duration (173 +/- 6% of the base line; n = 32) was elicited in 88% of the rats studied. This initial inhibitory effect on breathing was not affected either by an increase (410%) in the nicotine content of the cigarette smoke or by pretreatment with hexamethonium (33 mg/kg iv). However, bilateral vagotomy completely eliminated the initial ventilatory inhibition. Cooling both vagi to 5.1 degrees C blocked the reflex apneic response to lung inflation, but it did not abolish the inhibitory effect of smoke. After the initial response, a rapid shallow breathing pattern developed and reached its peak 5-12 breaths after inhalation of high-nicotine cigarette smoke; this delayed response could not be prevented by vagotomy and was undetectable after inhalation of low-nicotine smoke. We conclude that the initial inhibitory effect of smoke on breathing is mediated by vagal bronchopulmonary C-fiber afferents, which are stimulated by smoke constituents other than nicotine, whereas the delayed tachypneic response to smoke is caused by the absorbed nicotine.     

Capsaicin application to central or peripheral vagal fibers attenuates CCK satiety

Capsaicin treatment destroys small primary sensory neurons including a subpopulation of vagal afferents. Intraperitoneal, fourth ventricular or perivagal application of capsaicin attenuated or abolished cholecystokinin (CCK)-induced suppression of food intake. Capsaicin applied to the thoracolumbar spinal cord or to the pyloric region of the stomach did not alter CCK-induced reductions of food intake. Intraperitoneal capsaicin treatment reduced substance P-like immunoreactivity (SPLI) in the spinal dorsal horn and parts of the dorsal hindbrain. SPLI depletion, therefore, served as a histochemical indicator of the spread of capsaicin from its site of application. Capsaicin applied directly to the vagal trunks did not reduce SPLI in the spinal cord or hindbrain. Intraventricular capsaicin reduced SPLI in the hindbrain but not in the spinal cord. These data indicate that localized capsaicin application attenuates CCK-induced suppression of food intake by impairing the function of either central or peripheral portions of vagal afferent neurons. The data also support the conclusion that intraperitoneal capsaicin attenuates CCK-induced suppression of feeding by impairing vagal sensory function.     

Vagal afferents are essential for maximal resection-induced intestinal adaptive growth in orally fed rats

Small bowel resection stimulates intestinal adaptive growth by a neuroendocrine process thought to involve both sympathetic and parasympathetic innervation and enterotrophic hormones such as glucagon-like peptide-2 (GLP-2). We investigated whether capsaicin-sensitive vagal afferent neurons are essential for maximal resection-induced intestinal growth. Rats received systemic or perivagal capsaicin or ganglionectomy before 70% midjejunoileal resection or transection and were fed orally or by total parenteral nutrition (TPN) for 7 days after surgery. Growth of residual bowel was assessed by changes in mucosal mass, protein, DNA, and histology. Both systemic and perivagal capsaicin significantly attenuated by 48-100% resection-induced increases in ileal mucosal mass, protein, and DNA in rats fed orally. Villus height was significantly reduced in resected rats given capsaicin compared with vehicle. Sucrase specific activity in jejunal mucosa was not significantly different; ileal mucosal sucrase specific activity was significantly increased by resection in capsaicin-treated rats. Capsaicin did not alter the 57% increase in ileal proglucagon mRNA or the 150% increase in plasma concentration of bioactive GLP-2 resulting from resection in orally fed rats. Ablation of spinal/splanchnic innervation by ganglionectomy failed to attenuate resection-induced adaptive growth. In TPN rats, capsaicin did not attenuate resection-induced mucosal growth. We conclude that vagal afferents are not essential for GLP-2 secretion when the ileum has direct contact with luminal nutrients after resection. In summary, vagal afferent neurons are essential for maximal resection-induced intestinal adaptation through a mechanism that appears to involve stimulation by luminal nutrients.     

Alveolar hypercapnia augments pulmonary C-fiber responses to chemical stimulants: role of hydrogen ion.

To determine whether the excitabilities of pulmonary C fibers to chemical and mechanical stimuli are altered by CO(2)-induced acidosis, single-unit pulmonary C-fiber activity was recorded in anesthetized, open-chest rats. Transient alveolar hypercapnia (HPC) was induced by administering CO(2)-enriched gas mixture (15% CO(2), balance air) via the respirator inlet for 30 s, which rapidly lowered the arterial blood pH from a baseline of 7.40 +/- 0.01 to 7.17 +/- 0.02. Alveolar HPC markedly increased the responses of these C-fiber afferents to several chemical stimulants. For example, the C-fiber response to right atrial injection of the same dose of capsaicin (0.25-1.0 microg/kg) was significantly increased from 3.07 +/- 0.70 impulses/s at control to 8.48 +/- 1.52 impulses/s during HPC (n = 27; P < 0.05), and this enhanced response returned to control within approximately 10 min after termination of HPC. Similarly, alveolar HPC also induced significant increases in the C-fiber responses to right atrial injections of phenylbiguanide (4-8 microg/kg) and adenosine (0.2 mg/kg). In contrast, HPC did not change the response of pulmonary C fibers to lung inflation. Furthermore, the peak response of these C fibers to capsaicin during HPC was greatly attenuated when the HPC-induced acidosis was buffered by infusion of bicarbonate (1.36-1.82 mmol. kg(-1). min(-1) for 35 s). In conclusion, alveolar HPC augments the responses of these afferents to various chemical stimulants, and this potentiating effect of CO(2) is mediated through the action of hydrogen ions on the C-fiber sensory terminals.     

Phenotypic characterization of gastric sensory neurons in mice

Recent studies suggest that the capsaicin receptor [transient receptor potential vanilloid (TRPV)1] may play a role in visceral mechanosensation. To address the potential role of TRPV1 in vagal sensory neurons, we developed a new in vitro technique allowing us to determine TRPV1 expression directly in physiologically characterized gastric sensory neurons. Stomach, esophagus, and intact vagus nerve up to the central terminations were carefully dissected and placed in a perfusion chamber. Intracellular recordings were made from the soma of nodose neurons during mechanical stimulation of the stomach. Physiologically characterized neurons were labeled iontophoretically with neurobiotin and processed for immunohistochemical experiments. As shown by action potential responses triggered by stimulation of the upper thoracic vagus with a suction electrode, essentially all abdominal vagal afferents in mice conduct in the C-fiber range. Mechanosensitive gastric afferents encode stimulus intensities over a wide range without apparent saturation when punctate stimuli are used. Nine of 37 mechanosensitive vagal afferents expressed TRPV1 immunoreactivity, with 8 of the TRPV1-positive cells responding to stretch. A small number of mechanosensitive gastric vagal afferents express neurofilament heavy chains and did not respond to stretch. By maintaining the structural and functional integrity of vagal afferents up to the nodose ganglion, physiological and immunohistochemical properties of mechanosensory gastric sensory neurons can be studied in vitro. Using this novel technique, we identified TRPV1 immunoreactivity in only one-fourth of gastric mechanosensitive neurons, arguing against a major role of this ion channel in sensation of mechanical stimuli under physiological conditions.     

Afferent vagal pathways mediating respiratory reflexes evoked by ROS in the lungs of anesthetized rats.

We investigated the afferent vagal pathways mediating respiratory reflexes evoked by reactive oxygen species (ROS) in the lungs of anesthetized rats. Spontaneous inhalation of 0.2% aerosolized H(2)O(2) acutely evoked initial bradypnea followed by delayed tachypnea, which was frequently mixed with delayed augmented inspiration. The initial response was abolished after perivagal capsaicin treatment (PCT), but was prolonged during vagal cooling (VC) to 7 degrees C; PCT and VC are known to differentially block the conduction of unmyelinated C and myelinated fibers, respectively. The delayed responses were eliminated during VC but emerged earlier after PCT. Vagotomy, catalase (an antioxidant for H(2)O(2)), dimethylthiourea (an antioxidant for. OH), or deferoxamine (an antioxidant for. OH) largely or totally suppressed these reflexive responses, whereas sham nerve treatment, heat-inactivated catalase, saline vehicle, or iron-saturated deferoxamine failed to do so. These results suggest that 1) the H(2)O(2)-evoked initial and delayed airway reflexes are antagonistic and may result from stimulation of lung C fibers and rapidly adapting receptors, respectively, and 2) the reflex effects of H(2)O(2) are, in part, due to the action of. OH on these afferents.     

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.     

Pulmonary C-fiber activation enhances respiratory-related activities of the recurrent laryngeal nerve in rats

The purpose of the current study was to characterize the response of the recurrent laryngeal nerve (RLN) to pulmonary C-fiber activation. Male rats of Wistar strain were anesthetized by urethane (1.2 g/kg, i.p.). Tracheostomy was performed. Catheter was inserted into the femoral artery and vein. Additional catheter was placed near the entrance of the right atrium via the right jugular vein. The animal was then paralyzed with gallamine triethiodide, ventilated and maintained at normocapnia in hyperoxia. Activities of the phrenic (PNA) and recurrent laryngeal nerves (RLNA) were monitored simultaneously. Two experimental protocols were completed. In the first experiment, various doses of capsaicin were delivered into the right atrium to activate pulmonary C-fibers with vagal intact. Low dose of capsaicin (1.25 microg/kg) produced apnea, a decrease in amplitude of PNA, an enhancement of RLNA during apnea and recovery from apnea, hypotension, and bradycardia. High dose of capsaicin (5 and 20 microg/kg) evoked the same tendency of response for both nerves and biphasic changes in blood pressure. Dose dependency was only seen in the period of apnea but not observable in nerve amplitudes. After bilateral vagotomy, low dose of capsaicin produced an increase in PNA without apnea, no significant change in RLNA, and hypertension. These results suggest that activation of vagal and nonvagal C-fibers could produce different reflex effects on cardiopulmonary functions. The reflex responses evoked by these two types of afferents might play defensive and protective roles in the airways and lungs.     

Pulmonary chemoreflexes elicited by intravenous injection of lactic acid in anesthetized rats

Lee, Lu-Yuan, Robert F. Morton, and Jan M. Lundberg.Pulmonary chemoreflexes elicited by intravenous injection oflactic acid in anesthetized rats. J. Appl.Physiol. 81(6): 2349-2357, 1996.Experiments werecarried out to characterize the cardiorespiratory reflex responses tointravenous injection of lactic acid and to determine the involvementof vagal bronchopulmonary C-fiber afferents in eliciting theseresponses in anesthetized rats. Bolus injection of lactic acid (0.2 mmol/kg iv) immediately elicited apnea, bradycardia, and hypotension,which were then followed by a sustained hyperpnea. The immediate apneicand bradycardiac responses to lactic acid were completely abolished bybilateral vagotomy and were absent when the same dose of lactic acidwas injected into the left ventricle. The subsequent hyperpneicresponse was substantially attenuated by denervation of carotid bodychemoreceptors. After a perineural capsaicin treatment of both vagusnerves to block the conduction of C fibers, lactic acid no longerevoked the immediate apnea and bradycardia, whereas the hyperpneicresponse became more pronounced and sustained, presumably because ofthe removal of the inhibitory effect on breathing mediated by pulmonaryC-fiber activation. Single-unit electrophysiological recording showedthat intravenous injection of lactic acid consistently evoked an abruptand intense burst of discharge from the vagal C-fiber afferent endingsin the lungs. In conclusion, the cardiorespiratory depressor responses induced by lactic acid are predominantly elicited by activation ofvagal pulmonary C fibers.

     

Secretin activates vagal primary afferent neurons in the rat: evidence from electrophysiological and immunohistochemical studies

In this study, we evaluated the vagal afferent response to secretin at physiological concentrations and localized the site of secretin's action on vagal afferent pathways in the rat. The discharge of sensory neurons supplying the gastrointestinal tract was recorded from nodose ganglia. Of 91 neurons activated by electrical vagal stimulation, 19 neurons showed an increase in firing rate in response to intestinal perfusion of 5-HT (from 1.5 +/- 0.2 to 25 +/- 4 impulses/20 s) but no response to intestinal distension. A close intra-arterial injection of secretin (2.5 and 5.0 pmol) elicited responses in 15 of these 19 neurons (from 1.5 +/- 0.2 impulses/20 s at basal to 21 +/- 4 and 43 +/- 5 impulses/20 s, respectively). Subdiaphragmatic vagotomy and perivagal application of capsaicin, but not supranodose vagotomy, completely abolished the secretin-elicited vagal nodose neuronal response. In a separate study, 9 tension receptor afferents among 91 neurons responded positively to intestinal distension but failed to respond to luminal 5-HT. These nine neurons also showed no response to administration of secretin. As expected, immunohistochemical studies showed that secretin administration significantly increased the number of Fos-positive neurons in vagal nodose ganglia. In conclusion, we demonstrated for the first time that vagal sensory neurons are activated by secretin at physiological concentrations. A subpopulation of secretin-sensitive vagal afferent fibers is located in the intestinal mucosa, many of which are responsive to luminal 5-HT.     

Selective ablation of spinal afferent neurons containing CGRP attenuates gastric hyperemic response to acid.

The gastric mucosa, in particular submucosal blood vessels, are innervated by afferent neurons containing neuropeptides such as calcitonin gene-related peptide. Stimulation of sensory neurons innervating the gastric mucosa increases submucosal blood flow. Since sensory neurons supplying the stomach are of dual origin from nodose and dorsal root ganglia, we examined the effect of selective ablation of either the vagal or spinal sensory innervation to the upper gastrointestinal tract on the increase in gastric mucosal blood flow in response to acid back diffusion into the gastric mucosa. Perineural application of capsaicin to the celiac/superior mesenteric ganglia, but not to the vagus nerves, significantly inhibited by 53% the hyperemic response to acid back diffusion. Tissue levels of immunoreactive calcitonin gene-related peptide in the gastric corpus were significantly reduced (by 73%) by periceliac capsaicin treatment, but unaffected by perivagal capsaicin treatment. These data suggest that spinal capsaicin-sensitive afferents containing calcitonin gene-related peptide immunoreactivity are involved in mediating increases in gastric mucosal blood flow. This increase in gastric mucosal blood flow mediated by sensory neurons may act as a protective mechanism against mucosal injury, similar to responses seen in other tissues such as skin.