Pulmonary Stretch Receptor Afferents Research Articles

Interaction between the pulmonary stretch receptor and pontine control of expiratory duration

Medial parabrachial nucleus (mPBN) neuronal activity plays a key role in controlling expiratory (E)-duration (TE). Pulmonary stretch receptor (PSR) activity during the E-phase prolongs TE. The aims of this study were to characterize the interaction between the PSR and mPBN control of TE and underlying mechanisms. Decerebrated mechanically ventilated dogs were studied. The mPBN subregion was activated by electrical stimulation via bipolar microelectrode. PSR afferents were activated by low-level currents applied to the transected central vagus nerve. Both stimulus-frequency patterns during the E-phase were synchronized to the phrenic neurogram; TE was measured. A functional mathematical model for the control of TE and extracellular recordings from neurons in the preBötzinger/Bötzinger complex (preBC/BC) were used to understand mechanisms. Findings show that the mPBN gain-modulates, via attenuation, the PSR-mediated reflex. The model suggested functional sites for attenuation and neuronal data suggested correlates. The PSR- and PB-inputs appear to interact on E-decrementing neurons, which synaptically inhibit pre-I neurons, delaying the onset of the next I-phase.

Relevance of differential control of sympathorespiratory response magnitudes in clinical assessments.

Relevance of differential control of sympathorespiratory response magnitudes in clinical assessments.

Are ATP and glutamate released from slowly adapting pulmonary stretch receptor afferents in the NTS?

Are ATP and glutamate released from slowly adapting pulmonary stretch receptor afferents in the NTS?

Release of ATP and glutamate in the nucleus tractus solitarii mediate pulmonary stretch receptor (Breuer–Hering) reflex pathway

The Breuer–Hering inflation reflex is initiated by activation of the slowly adapting pulmonary stretch receptor afferents (SARs), which monosynaptically activate second-order relay neurones in the dorsal medullary nucleus of the solitary tract (NTS). Here we demonstrate that during lung inflation SARs release both ATP and glutamate from their central terminals to activate these NTS neurones. In anaesthetized and artificially ventilated rats, ATP- and glutamate-selective microelectrode biosensors placed in the NTS detected rhythmic release of both transmitters phase-locked to lung inflation. This release of ATP and glutamate was independent of the centrally generated respiratory rhythm and could be reversibly abolished during the blockade of the afferent transmission in the vagus nerve by topical application of local anaesthetic. Microionophoretic application of ATP increased the activity of all tested NTS second-order relay neurones which receive monosynaptic inputs from the SARs. Unilateral microinjection of ATP into the NTS site where pulmonary stretch receptor afferents terminate produced central apnoea, mimicking the effect of lung inflation. Application of P2 and glutamate receptor antagonists (pyridoxal-5′-phosphate-6-azophenyl-2′,4′-disulphonic acid, suramin and kynurenic acid) significantly decreased baseline lung inflation-induced firing of the second-order relay neurones. These data demonstrate that ATP and glutamate are released in the NTS from the central terminals of the lung stretch receptor afferents, activate the second-order relay neurones and hence mediate the key respiratory reflex — the Breuer—Hering inflation reflex.

Pulmonary stretch receptor afferents inhibit sympathetic nerve activity in early heart failure.

Pulmonary stretch receptor afferents inhibit sympathetic nerve activity in early heart failure.

Distribution and classification of respiratory neurones in the nucleus tractus solitarius of the rat medulla and their responses to central and periphereal stimuli

In the cat the dorsal respiratory group comprising the nucleus tractus solitarius (NTS) is regarded as an important respiratory centre. However the relevance of this nucleus in the control of respiration in the rat is still unclear. The present work aimed to identify and localize the various neurons involved in respiration in the nucleus tractus solitarius region of the rat medulla. Further, we intended to study the responses of these identified respiratory neurons to central and peripheral stimulation, in particular: 1)effect of stimulation of midbrain periaqueductal gray (PAG) matter and, 2)effect of stimulation of the vagus (vagal feedback). Nembutal-anaesthetised (80 mg/Kg), spontaneously-breathing rats (n = 60, 300–400 g) were used for the study. Neurons firing in relation to diaphragm electromyogram (dEMG) were located between 0.6–1.5 mm lateral to the midline, extending 0.5 mm caudal to 1.8 mm rostral to the posterior end of the area postrema (AP). Four different populations of respiratory-related neurons were recorded. They have been classified (with reference to dEMG) as: expiratory-inspiratory phase-spanning cells, early inspiratory neurons (eI-cells), cells that fire only in phase with diaphragm (I-all cells), and expiratory cells. The phase-spanning and eI-cell populations were located both in the ventrolateral and ventromedial NTS rostral to the area postrema, while the other cells were found predominantly in the more lateral regions. eI-cells were observed to start firing up to 20 ms prior to dEMG and ceased firing about one third of the way through diaphragm activity. They also showed a decrementing discharge pattern. Excitatory amino acid (DLH, 15–60 nl of 0.2 M) stimulation of PAG along its dorso-ventral axis produced dose-dependent changes in the cardiorespiratory parameters: increased respiratory frequency, decreased inspiratory and expiratory durations, increased blood pressure (BP) and heart rate. PAG stimulation elicited increases in activity in both phase-spanning and eI-neuronal populations in the NTS. The PAG sites evoking an increase in the phase-spanning cell activity were located more laterally than the PAG sites that led to increased eI-cell activity. The increase in the responses of the phase-spanning neurons was not accompanied by any changes in the respiratory frequency, whereas the PAG stimulation giving rise to an increase in eI-neuronal activity also elicited simultaneous increases in dEMG and the respiratory frequency. The central end of the cut vagus was electrically stimulated to activate pulmonary stretch-receptor afferents. Vagal stimulation caused depression of inspiratory cell activity along with bradycardia and a fall in BP. Both eI-neurons and I-all cells were inhibited. These cells exhibited a 'vagal escape' phenomenon ie, resuming activity whilst vagal stimulation continued. Phase-spanning cells and expiratory cells are yet to be tested for responses to vagal stimulation. The behaviours of these respiratory-related cells of the NTS, their temporal relationships with the dEMG, firing patterns, their responses to central and peripheral stimuli are discussed and a possible role for each in the brainstem control of respiration is proposed. Similar studies have also been carried out to elucidate the roles of ventrolateral column (VLM)/ventral respiratory group (VRG) neurons in the control of respiration in the rat. The findings of these studies are presented in an accompanying abstract.

Inspiration-promoting vagal reflex under NMDA receptor blockade in anaesthetized rabbits.

1. This study describes a novel vagal respiratory reflex in anaesthetized rabbits. In contrast to the well-known inspiratory (I) off-switching by vagal afferent excitation, this vagal reflex initiates and maintains the central I activity of phrenic nerve discharges in rabbits pre-treated with antagonists of N-methyl-D-aspartate-type excitatory amino acid receptors (NMDA-Rs). 2. Under NMDA-R blockade with either dizocilpine (0.025-0.3 mg kg-1), D-2-amino-5-phosphonopentanoic acid (AP5, 0.5-1 mg, i.c.v.) or ketamine (10 mg kg-1), vagal stimulation at low frequencies (5-40 Hz) during the I phase prevented or markedly delayed the spontaneous I termination. In contrast, stimulation of the same vagal afferent at the same intensity but at a higher frequency (100-160 Hz) during the I phase immediately terminated the I phase. 3. In non-vagotomized rabbits, maintaining the tidal volume at end-expiratory levels during the I phase prevented spontaneous I termination and maintained apneusis after NMDA-R blockade with dizocilpine. 4. Brief stimulation of vagal afferents at low frequency (5-40 Hz) during the expiratory (E) phase constantly initiated phrenic I discharge after NMDA-R block. 5. We conclude that low-frequency discharge of vagal pulmonary stretch receptor afferents, as when lung volume is near functional residual capacity, promotes central I activity under NMDA-R blockade.

Hypoxemia does not affect the strength of the inspiration-inhibiting Breuer-Hering reflex

Several studies were focused on the consequences of reduced PaO 2 (hypoxemia) on the Breuer-Hering inspiration-inhibiting vagal reflex. However, these data are often contradictory and do not allow us to decide whether hypoxemia interacts with the central integration of pulmonary vagal afferents and/or exerts peripheral influence on pulmonary stretch receptor (PSR) activity. The present study was performed in anesthetized rabbits breathing different gas mixtures containing O 2, N 2 and CO 2. Intravenous injection of bicarbonates and CO 2 addition in the inspired mixture maintained pHa and PaCO 2 within their physiological ranges. The Breuer-Hering reflex, assessed from the changes in diaphragmatic EMG activity, was elicited either by inflating the lungs at different volumes above the functional residual capacity (PSR activation) or by direct electrical stimulation of vagal afferents (central stimulation). Hypoxemia never significantly modified the strength of the reflex or its threshold. Thus, acute hypoxemia present at high altitude does not seem to modify the key role played by PSR afferents in the ventilatory control.

Pulmonary stretch receptor afferents activate excitatory amino acid receptors in the nucleus tractus solitarii in rats.

1. The goal of the present study was to identify potential neurotransmitter candidates in the Breuer-Hering (BH) reflex pathway, specifically at synapses between the primary afferents and probable second-order neurones (pump cells) within the nucleus tractus solitarii (NTS). We hypothesized that if activation of specific receptors in the NTS is required for production of the BH reflex, then (1) injection of the receptor agonist(s) would mimic the reflex response (apnoea), (2) injection of appropriate antagonists would impair the apnoea produced by either lung inflation or agonist injection, and (3) second-order neurones in the pathway would be excited by either lung inflation or agonists while antagonists would prevent the response to either. 2. Studies were carried out either in spontaneously breathing or in paralysed, thoracotomized and ventilated rats in which either diaphragm EMG or phrenic nerve activity, expired CO2 concentration and arterial pressure were continuously monitored. The BH reflex was physiologically activated by inflating the lungs. 3. Pressure injections (0.03-15 pmol) of selective excitatory amino acid (EAA) receptor agonists, quisqualic acid (Quis) and N-methyl-D-aspartic acid (NMDA) into an area of the NTS shown previously to contain neurones required for production of the BH reflex produced dose-dependent apnoeas that mimicked the response to lung inflation. Injection of substance P (0.03-4 pmol) did not alter baseline respiratory pattern. 4. Injections of the EAA antagonists, kynurenic acid (Kyn; 0.6-240 pmol), 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) or 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the BH region of the NTS reversibly impaired the apnoea produced by lung inflation. All three antagonists reduced or abolished the apnoeas resulting from injection of Quis or NMDA, and slowed baseline respiratory frequency. In contrast, injections of the highly selective NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acids (AP5), in doses sufficient to block the apnoeic response to NMDA, neither altered the reflex apnoea evoked by lung inflation nor the baseline respiratory pattern. 5. Pump cells located within the BH region were excited by pressure injections of the broad spectrum EAA agonist, DL-homocysteic acid (DLH). Kyn reversibly blocked the excitation of pump cells in response to either lung inflation or DLH injection. 6. These findings suggest that EAAs mediate primary afferent excitation of second-order neurones in the Breuer-Hering reflex pathway, primarily through the activation of non-NMDA EAA receptor subtypes.

Carotid bodies and expiratory muscle recruitment

\ e have reported previously (FASEB J 1989; 3:A401) that specific carotid body stimulation/inhibition tended to recruit/derecruit expiratory muscles in the same direction, and with similar time course, as the diaphragm in unanesthetized dogs (N = 5). One limitation of that study was the presence of pulmonary vagal afferent traffic that could have had an effect on expiratory muscle recruitment independent of the direct carotid body effects. To address this question, we blocked the pulmonary stretch receptor afferent traffic by bilateral cooling ofexteriorized (skin loops) cervical vagi in 3 awake, tracheotomized dogs. Excitatory (NaCN, 3-5 breaths N2) and inhibitory (dopamine, 3-5 breaths O ) stimuli of the carotid bodies during vagal blockade produced effects similar to those seen in nonblocked dogs; recruitment/derecruitment ofexpiratory musdes (triarigularis sterni, transversus abdominis) was in the same direction and had the same time course as changes in the crural diaphragm. We conclude that pulmonary stretch receptor afferents are not required for inspiratory or expiratory muscle recruitment/derecruitment in response to carotid body stimuli.

Carotid Bodies and Expiratory Muscle Recruitment

e have reported previously (FASEB J 1989; 3:A401) that specific carotid body stimulation/inhibition tended to recruit/derecruit expiratory muscles in the same direction, and with similar time course, as the diaphragm in unanesthetized dogs (N = 5). One limitation of that study was the presence of pulmonary vagal afferent traffic that could have had an effect on expiratory muscle recruitment independent of the direct carotid body effects. To address this question, we blocked the pulmonary stretch receptor afferent traffic by bilateral cooling ofexteriorized (skin loops) cervical vagi in 3 awake, tracheotomized dogs. Excitatory (NaCN, 3-5 breaths N2) and inhibitory (dopamine, 3-5 breaths O ) stimuli of the carotid bodies during vagal blockade produced effects similar to those seen in nonblocked dogs; recruitment/derecruitment ofexpiratory musdes (triarigularis sterni, transversus abdominis) was in the same direction and had the same time course as changes in the crural diaphragm. We conclude that pulmonary stretch receptor afferents are not required for inspiratory or expiratory muscle recruitment/derecruitment in response to carotid body stimuli.

Vagal influences on parasternal intercostal muscle inspiratory shortening during hypercapnia and airway occlusion

To determine whether increases in electromyographic activity (EMG) are necessary for respiratory muscle shortening to occur during airway occlusion, respiratory changes in parasternal intercostal musle length were measured using sonomicrometry in 11 anesthetized dogs during unoccluded (UB) and occluded (OB) breaths both before and after vagotomy. During UB the extent of parasternal intercostal inspiratory shortening was greater after than before vagotomy both during oxygen breathing and during progressive hyperoxic hypercapnia. The relation between parasternal shortening, parasternal EMG, and tidal volume was not substantially affected by vagotomy. During OB parasternal intercostal EMG increased significantly compared to UB when the vagi were intact, but airway occulusion did not significantly change EMG activity post-vagotomy. However, both before and after vagotomy the parasternal intercostal shortened during OB in all animals. Parasternal intercostal inspiratory shortening during OB as a % of that during UB was significantly greater before compared to after vagotomy during oxygen breathing and moderate hypercapnia, but vagal integrity made no significant difference at high CO 2. These result suggest that (1) pulmonary stretch receptor afferents inhibit parasternal intercostal inspiratory shortening but in proportion to their inhibitory effects on parasternal intercostal EMG and tidal volume, and (2) even when the EMG stays constant the parasternal intercostal muscle does not contract isometrically during occluded breaths.

Bilateral convergence of pulmonary stretch receptor inputs on I beta-neurons in the cat

Extracellular recordings were made from inspiratory beta- (I beta) neurons in the nucleus of the tractus solitarius in decerebrate cats. A reversible direct current block of myelinated fibers in the ipsilateral vagus nerve was used to evaluate the input from pulmonary stretch receptor afferents (PSR) of the contralateral vagus to individual I beta-neurons. This block served to remove all ipsilateral (which includes all monosynaptic) inputs from PSR to I beta-cells. The effect of withholding inflation on the firing rate and the time of onset of firing of I beta-neurons was determined before, during and after application of the direct current block. There was considerable variation in the strengths of the inputs from the ipsilateral and contralateral nerves; some cells received PSR inputs from only the ipsilateral vagus, but the majority were excited with varying magnitude from both vagi. Several neurons had powerful excitatory inputs from PSR of the contralateral vagus, with the ipsilateral (monosynaptic) contribution being of minor importance.

Respiration-related neurons in the medial nuclear complex of the solitary tract of the cat

Respiration-related (RR) neuronal activity was systematically searched for in the nuclear complex of the solitary tract (nTS) with special interest focussed on regions located medially to the ventrolateral subnucleus (vlnTS) which is widely considered to be the anatomical location of the dorsal respiratory group (DRG). The experiments were carried out on cats under pentobarbitone anaesthesia, paralyzed and artificially ventilated. RR activity was recorded consistently not only in the vlnTS, but also in more medial regions R ∝-type neurons were often encountered in ventral and medical aspects of the nTS. R β -type neurons and expiration-related neurons were found also in ventral regions of the nTS. A small number of and ‘early burst’ inspiration-related neurons was found respectively in ventral and ventromedical regions of the nTS. The activity of neurons presenting a rhythmic pattern completely determined by lung volume changes due to the respirator was consistently recorded both in ventral and medial areas of the nTS. Most of these neurons were excited by lung inflation and displayed a discharge pattern characteristic of P-neurons. A few neurons, designated as ‘inverse’ P-neurons displayed an opposite behaviour, being inhibited by lung inflation. The results are consistent with both histological and neurophysiological studies showing that pulmonary stretch receptor afferents terminate also on the mdial side of the solitary tract. It is suggested that the DRG is not restricted to the vlnTS, but comprises also neurons located in more medial regions of the nTS.

Effects of lung inflation on the excitability of dorsal respiratory group neurons

The effect of lung inflation on the excitability of inspiratory neurons of the dorsal respiratory group was studied in decerebrate, paralyzed, artificially ventilated cats. Variations in the antidromic latency (AL) were used as a measure of the changes in excitability. The antidromic responses of single cells were recorded extracellularly during electrical stimulation (20 Hz) of their spinal axons. Single-breath test inflations were delivered at the onset of inspiration (I) or expiration (E), and then maintained for the duration of that respiratory phase. In the absence of inflation during E, most of the inspiratory cells underwent progressive lengthening of the AL, indicating inhibition or disfacilitation. This effect was stronger in I β than in I α cells but there was considerable overlap. In every cell (21 I α, 17 I β), inflation during E caused a prompt AL shortening (excitation or disinhibition) that was evident in single tests. On average, I β neurons were more strongly excited by the test inflation during E, but again there was considerable overlap. The excitation was maintained for the duration of the inflation, indicating that pulmonary stretch receptor afferents (PSR) were involved. The response to slow inflations (that preferentially excite PSR) was a progressive shortening of the AL that mirrored the increase in lung volume. The results emphasize the qualitative similarity in the responses of I α and I β neurons to lung inflation and in their excitability changes during normal respiratory cycles.

Neural elements subserving pulmonary stretch receptor-mediated facilitation of phrenic motoneurons

The neural elements responsible for facilitation of phrenic nerve activity by lung inflation were investigated in cats by the simultaneous recording of individual pulmonary stretch receptor afferents, respiratory neurons of the ventrolateral nucleus of the tractus solitarius and phrenic nerve activity. Monosynaptic excitation of I beta neurons by slowly adapting pulmonary stretch receptors was demonstrated by cross-correlational analysis. It was also demonstrated that the majority of these same I beta neurons projected to the contralateral C5 phrenic motoneuron pool. Thus, this study has shown that I beta neurons can act as central neural elements to mediate the facilitatory effect of lung inflation upon phrenic nerve activity.

Bilateral reflex effects on phrenic nerve activity in response to single-shock vagal stimulation.

The bilateral reflex actions of vagus nerve afferent signals on phrenic efferent activity have been tested by unilateral graded single shock electrical stimulation. An early excitation (latency 3-5 msec) was more prominent in the phrenic nerve contralateral to the stimulated vagus. Spinal cord hemisection at C3 eliminated both contralateral and ipsilateral responses: thus, both were mediated via descending tracts in the contralateral cord. A bilaterally symmetrical early inhibition (latency 8-12 msec) followed the early excitation. The electrical thresholds for evoking the early responses and the temperature for blocking these responses during graded vagal cooling were closely similar to the threshold and blocking temperature for pulmonary stretch receptor afferents. Higher stimulus strengths evoked a strong, bilaterally similar, late excitation (latency 12-20 msec) followed by a late inhibition. At very high stimulus strengths a third excitation (latency 25-30 msec) could appear. Sometimes these responses were followed by lowered phrenic activity for the remainder of inspiration. Single shock stimulation of the intact vagus nerve or of the peripheral end of the cut recurrent laryngeal nerve provoked, by the contraction of laryngeal muscles, a strong, short latency (12 msec) inhibition of phrenic activity mediated by superior laryngeal nerve afferents. The implications of these results with respect to the reflex pathways of the different responses and their possible integration in the central respiratory control mechanisms are discussed.

Respiratory periodicity following stimulation of vagal afferents

The effects on respiratory periodicity of electrical stimulation of the cut central end of a vagus nerve were studied in anaesthetized, vagotomized, paralyzed, and ventilated rabbits. Electrical activity of a phrenic nerve was used to determine inspiratory and expiratory durations (Ti and Te). The central cut end of one vagus nerve was electrically stimulated during the entire inspiratory phase of every 10th respiratory cycle and Ti and Te of that respiratory cycle and the following 8 were measured. When only the fastest conducting afferents (pulmonary stretch receptor afferents) were stimulated, reductions in Ti and Te were restricted to the stimulated cycle. When stimulus intensity was increased, activating higher threshold, more slowly conducting afferents (including those of irritant receptors), Te but not necessarily Ti decreased, increasing the frequency of phrenic bursts (respiratory frequency). The recovery of Te following stimulation was exponential, with a time constant of 3–7 s which varied inversely with control respiratory frequency. The effects on Ti and Te on higher intensity stimulation suggest either that coupling between inspiratory and expiratory neurones in the brainstem respiratory oscillator can be "looser" than currently hypothesized or a separate population of expiratory neurones, with a short-term memory (three time constants or [Formula: see text]), mediates the observed effects.

Effects of pulmonary stretch receptor afferent stimulation on sympathetic preganglionic neuron firing.

The response of 47 sympathetic preganglionic neurons (SPNs) of the cervical nerve to activation of pulmonary stretch receptors has been studied in Nembutal-anaesthetized. paralyzed, thoracotomized, artificially ventilated cats. The discharge of 62% of the SPNs was depressed by lung inflation or by electrical stimulation of the lowest threshold afferent fibers in the vagus nerve. The SPNs which were most sensitive to these manoeuvers were those whose discharge had an inspiration-synchronous component. All the SPNs of this type tested were depressed. Their response paralleled that of phrenic motoneurons. The SPNs which were least sensitive to these manoeuvers were those whose firing pattern showed no respiratory modulation. Only 12% of the SPNs of this type tested were depressed. In some SPNs, selective suppression of inspiration-synchronous activity by hypocapnia revealed a residual firing which could not be altered by pulmonary stretch receptor afferent activation. For comparison, the reflex response of the same sample of SPNs to the systemic arterial pressure increase caused by intravenous injection of noradrenaline (5 μg/kg) was studied. The discharge of 85% of the SPNs was depressed during the pressor effect of noradrenaline. The frequency of occurrence of depressant effects was similar in respiratory-modulated and nonmodulated SPNs. Also depressed was the residual discharge in hypocapnia of respiratory-modulated SPNs. Thus, the distribution in the SPN pool of the depression evoked by a systemic arterial pressure rise contrasts with that caused by activation of pulmonary stretch receptor afferents. These results provide a direct demonstration of the depressant action of pulmonary stretch receptor afferents on SPNs, an action that had previously been hypothesized on the basis of indirect evidence. The observation that activation of these afferents depresses mainly the inspiration-synchronous component of SPN firing suggests that the effect is predominantly relayed to the SPN pool via the respiratory center.