Effect of stimulation of pulmonary C-fiber receptors on canine respiratory muscles

Pharmacologically evoked apnoeas. Receptors and nervous pathways involved

This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT_1A serotonin receptors in overcoming opioid-induced respiratory inhibition.

Nodose ganglia-modulatory effects on respiration

The key role of the vagus nerves in the reflex control of breathing is generally accepted. Cardiopulmonary vagal receptors and their afferent connection with the medullary respiratory centers secures the proper regulatory feedback. Section of the vagi at the midcervical level interrupts primary vagal reflexes and those due to activation of lung afferents by neuroactive substances. In this context the present review focuses on the reflex contribution of the inferior (nodose) vagal ganglia to the respiratory pattern, considering that this structure contains perikarya of vagal afferent neurons which house neurotransmitters, neuropeptides and neurochemical substances. In experimental animals with removed sensory input from the lungs (midcervical vagotomy) the following evidence was reported. Transient respiratory suppression in the form of apnoea, occurring after systemic injection of serotonin, adenosine triphosphate and anandamide (N-arachidonoyl-ethanolamine-endogenous cannabinoid neurotransmitter), which was abrogated by nodose ganglionectomy. Preserved nodose-NTS connection conditioned respiratory depression affecting the timing component of the breathing pattern evoked by N-6-cyclopentyl-adenosine (CPA) and inhibition of both respiratory constituents induced by NPY. Stimulatory effect of NPY13-36 on tidal volume required nodosal connection. The cardiovascular effects of majority of the tested substances occurred beyond the nodose ganglia (with exclusion of serotonin and anandamide).

Midcervical vagotomy precludes respiratory response to novel anti-inflammatory and anti-tumour drug arvanil in rats

Arvanil is a metabolically stable hybrid between anandamide and capsaicin. The present study was designed to test the role of the vagal pathway in post-arvanil respiratory and blood pressure responses. Respiratory and pressure changes evoked by an intravenous injection of arvanil were investigated in 21 urethane-chloralose anaesthetised and spontaneously breathing rats. In control neurally intact rats the effects of arvanil were checked to establish the appropriate dose of the drug. In the experimental group rats were challenged with arvanil while intact, following bilateral midcervical vagotomy and after subsequent supranodose vagotomy. In all neurally intact animals bolus injection of 0.8 mg/kg of arvanil into the right femoral vein induced a significant increase of tidal volume (+1+/-0.11 ml; P<0.01) and diaphragm activity (+1.72+/-0.1 arbitrary units; P<0.01) as well as hypertension (+31.9+/-2.9 mm Hg; P<0.001) and a fall in respiratory rate (-24.7+/-0.4 breath/min; P<0.001). Bilateral midcervical vagotomy precluded the alteration of respiratory parameters but did not eliminate blood pressure response. Arvanil-induced increase in mean arterial blood pressure still persisted after supranodose vagotomy. Results indicated that the respiratory effects evoked by arvanil administered via the peripheral circulation require intact midcervical vagi. Supranodose vagotomy failed to eliminate the hypertension evoked by arvanil.

Uncoupling of upper airway motor activity from phrenic bursting by positive end-expired pressure in the rat

Phasic bursting in the hypoglossal nerve can be uncoupled from phrenic bursting by application of positive end-expired pressure (PEEP). We wished to determine whether similar uncoupling can also be induced in other respiratory-modulated upper airway (UAW) motor outputs. Discharge of the facial, hypoglossal, superior laryngeal, recurrent laryngeal, and phrenic nerves was recorded in anesthetized, ventilated rats during stepwise changes in PEEP with a normocapnic, hyperoxic background. Application of 3- to 6-cmH2O PEEP caused the onset inspiratory (I) UAW nerve bursting to precede the phrenic burst but did not uncouple bursting. In contrast, application of 9- to 12-cmH2O PEEP uncoupled UAW neurograms such that rhythmic bursting occurred during periods of phrenic quiescence. Single-fiber recording experiments were conducted to determine whether a specific population of UAW motoneurons is recruited during uncoupled bursting. The data indicate that expiratory-inspiratory (EI) motoneurons remained active, while I motoneurons did not fire during uncoupled UAW bursting. Finally, we examined the relationship between motoneuron discharge rate and PEEP during coupled UAW and phrenic bursting. EI discharge rate was linearly related to PEEP during preinspiration, but showed no relationship to PEEP during inspiration. Our results demonstrate that multiple UAW motor outputs can be uncoupled from phrenic bursting, and this response is associated with bursting of EI nerve fibers. The relationship between PEEP and EI motoneuron discharge rate differs during preinspiratory and I periods; this may indicate that bursting during these phases of the respiratory cycle is controlled by distinct neuronal outputs.

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

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

Effects of pulse lung inflation on chest wall expiratory motor activity

The effects of pulse lung inflation (LI) on expiratory muscle activity and phase duration (Te) were determined in anesthetized, spontaneously breathing dogs (n = 20). A volume syringe was used to inflate the lungs at various times during the expiratory phase. The magnitude of lung volume was assessed by the corresponding change in airway pressure (Paw; range 2-20 cmH(2)O). Electromyographic (EMG) activities were recorded from both thoracic and abdominal muscles. Parasternal muscle EMG was used to record inspiratory activity. Expiratory activity was assessed from the triangularis sterni (TS), internal intercostal (IIC), and transversus abdominis (TA) muscles. Lung inflations <7 cmH(2)O consistently inhibited TS activity but had variable effects on TA and IIC activity and expiratory duration. Lung inflations resulting in Paw values >7 cmH(2)O, however, inhibited expiratory EMG activity of each of the expiratory muscles and lengthened Te in all animals. The responses of expiratory EMG and Te were directly related to the magnitude of the lung inflation. The inhibition of expiratory motor activity was independent of the timing of pulse lung inflation during the expiratory phase. The inhibitory effects of lung inflation were eliminated by bilateral vagotomy and could be reproduced by electrical stimulation of the vagus nerve. We conclude that pulse lung inflation resulting in Paw between 7 and 20 cmH(2)O produces a vagally mediated inhibition of expiratory muscle activity that is directly related to the magnitude of the inflation. Lower inflation pressures produce variable effects that are muscle specific.

Modulation of upper airway muscle activities by bronchopulmonary afferents

Here we review the influence of bronchopulmonary receptors (slowly and rapidly adapting pulmonary stretch receptors, and pulmonary/bronchial C-fiber receptors) on respiratory-related motor output to upper airway muscles acting on the larynx, tongue, and hyoid arch. Review of the literature shows that all muscles in all three regions are profoundly inhibited by lung inflation, which excites slowly adapting pulmonary stretch receptors. This widespread coactivation includes the recruitment of muscles that have opposing mechanical actions, suggesting that the stiffness of upper airway muscles is highly regulated. A profound lack of information on the modulation of upper airway muscles by rapidly adapting receptors and bronchopulmonary C-fiber receptors prohibits formulation of a conclusive opinion as to their actions and underscores an urgent need for new studies in this area. The preponderance of the data support the view that discharge arising in slowly adapting pulmonary stretch receptors plays an important role in the initiation of the widespread and highly coordinated recruitment of laryngeal, tongue, and hyoid muscles during airway obstruction.

Afferent properties and reflex functions of bronchopulmonary C-fibers

Bronchopulmonary C-fiber afferents are characterized by their distinct sensitivity to chemical stimuli in the airways or pulmonary circulation. Responses evoked by activating these afferents are mediated by both central reflex pathways and by local or axon reflexes involving the release of tachykinins from sensory endings. Bronchopulmonary C-fiber stimulation reflexly reduces tidal volume and increases respiratory rate, constricts the airways, increases mucus secretion in the airways, and is associated with coughing. Cardiovascular effects include bradycardia, a fall in cardiac output, and bronchial vasodilation that increases airway blood flow despite systemic hypotension. In animals, C-fiber stimulation inhibits skeletal muscle activity, and in humans, is accompanied by burning and choking sensations in the throat and upper chest. Recent studies have identified additional physiologic and pharmacologic stimuli to these afferents, such as hydrogen ions, adenosine, reactive oxygen species, and hyperosmotic solutions. Furthermore, increasing evidence indicates that the excitability of these afferents is enhanced by the local release of certain autocoids (e.g. PGE2) during airway inflammation. These findings further indicate that vagal C-fiber endings in the lungs and airways play an important role in regulating the cardiopulmonary functions under both normal and abnormal physiologic conditions.

Recurrent hypoxic exposure and reflex responses during development in the piglet

The effects of recurrent hypoxia on cardiorespiratory reflexes were characterized in anesthetized piglets at 2-10 d (n=15), 2-3 weeks (n=11) and 8-10 weeks (n=8). Responses of phrenic and hypoglossal electroneurograms (ENG(phr) and ENG (hyp)) to hypoxia (8% 0(2), bal N(2), 5 min), hypercapnia (7% CO(2) bal O(2), 5 min) and intravenous capsaicin were tested before and after recurrent exposure to 11 episodes of hypoxia (8% O(2) bal N(2), 5 min). In piglets 2-10 d, ENG(phr) response to hypoxia declined in proportion to the number of hypoxic exposures; however, ENG (hyp) response to hypoxia was unchanged. In piglets at 2-10 d, intracisternal injection of bicuculline (GABA(A) receptor antagonist) reversed effects of recurrent hypoxia on ENG(phr) hypoxic response, eliminated apnea during hypoxia, as well as the delay in appearance of ENG(phr) after hypoxia. The ENG(phr) response to 7% CO(2) inhalation also decreased after recurrent hypoxia; however, the ENG(phr) response to C-fiber stimulation by capsaicin was unaltered. Piglets at 2-3 and 8-10 weeks were resistant to the depressive effects of recurrent hypoxia on respiratory reflex responses. We conclude that the response of the anesthetized newborn piglet to recurrent hypoxia is dominated by increasing inhibition of phrenic neuroelectrical output during successive hypoxic exposures. Central GABAergic inhibition may contribute significantly to the cumulative effects of repeated hypoxia in the newborn piglet experimental model.

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.

Effect of cardiopulmonary C fibre activation on the firing activity of ventral respiratory group neurones in the rat

1. Cardiopulmonary C fibre receptor stimulation elicits apnoea and rapid shallow breathing, but the effects on the firing activity of central respiratory neurones are not well understood. This study examined the responses of ventral respiratory group neurones: decrementing expiratory (Edec), augmenting expiratory (Eaug), and inspiratory (I) neurones during cardiopulmonary C fibre receptor-evoked apnoea and rapid shallow breathing. 2. Extracellular neuronal activity, phrenic nerve activity and arterial pressure were recorded in urethane-anaesthetized rats. Cardiopulmonary C fibre receptors were stimulated by right atrial injections of phenylbiguanide. Neurones were tested for antidromic activation from the contra- and ipsilateral ventral respiratory group (VRG), spinal cord and cervical vagus nerve. 3. Edec neurones discharged tonically during cardiopulmonary C fibre-evoked apnoea and rapid shallow breathing, displaying increased burst durations, number of impulses per burst, and mean impulse frequencies. Edec neurones recovered either with the phrenic nerve activity (25 s) or much later (3 min). 4. By contrast, the firing activity of Eaug and most I neurones was decreased, featuring decreased burst durations and number of impulses per burst and increased interburst intervals. Eaug activity recovered in approximately 3 min and inspiratory activity in approximately 1 min. 5. The results indicate that cardiopulmonary C fibre receptor stimulation causes tonic firing of Edec neurones and decreases in Eaug and I neuronal activity coincident with apnoea or rapid shallow breathing.

Abdominal muscle length during respiratory defensive reflexes

The purpose of the present study was to assess the mechanical behavior of an expiratory muscle during defensive reflexes. Transversus abdominis muscle length changes were measured using sonomicrometry in anesthetized dogs. The abdominal muscle lengthened during the inspiratory phase and shortened at a rapid velocity during the expiratory portion of coughs and sneezes. The mean extent of muscle shortening was not different during coughing compared to breathing (P greater than 0.20) but was approximately double in magnitude during sneezing compared to breathing (P less than 0.005). On the other hand, the peak velocity of muscle shortening was approximately 5-fold greater during coughing (P less than 0.002) and 10-fold greater during sneezing (P less than 0.05) than during breathing. During the largest coughs and sneezes in each animal, peak velocity of muscle shortening averaged 77 +/- 9 and 179 +/- 65% of end-inspiratory length per sec, respectively. Muscle end-inspiratory length during coughs and sneezes differed from values during breathing (range +/- 8%), although for the group of animals the mean changes were small (+/- 1%). Despite these changes in end-inspiratory length, the abdominal muscle continued to operate at lengths both above and below its resting length. These results suggest that during defensive reflexes, greater increases occur in the velocity than in the extent of transversus abdominis muscle shortening relative to during breathing. In addition the transversus abdominis muscle appears to play an active respiratory role during defensive reflexes.

Contractile and endurance properties of feline triangularis sterni muscle

The triangularis sterni muscle has recently been found to play an active role in the modulation of airflow during expiration in several species; furthermore its electrical activity is influenced by many chemical and mechanical stimuli which influence breathing. To determine feline triangularis sterni contractile and endurance properties, strips of triangularis sterni and costal diaphragm muscle were removed from anesthetized ventilated cats, and studied in vitro. The isometric contractile kinetics of the two muscles were similar; contraction times were 37 +/- 2 and 36 +/- 1 ms for the triangularis sterni and diaphragm, respectively. However, the twitch to tetanic tension ratio of the triangularis sterni was lower than that of the diaphragm (0.19 +/- 0.01 versus 0.37 +/- 0.03; P less than 0.001), and the force frequency relationship of the triangularis sterni was located to the right of that of the diaphragm. Repetitive stimulation (40 Hz trains, duty cycle 0.33) produced a greater decline in force for the diaphragm than the triangularis sterni. The fatigue index (ratio of force at 2 min to initial force) was significantly higher for the triangularis sterni (0.31 +/- 0.04) than for the diaphragm (0.18 +/- 0.02; P less than 0.01). These data indicate that the contractile and endurance properties of the feline triangularis sterni are different in some but not all respects from those of the diaphragm, which may reflect adaptations to patterns of use during breathing.