Interactions between lung stretch and PaCO2 in modulating ventilatory activity in dogs

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Systemic leptin produces a long-lasting increase in respiratory motor output in rats

Leptin decreases food intake and increases energy expenditure. Leptin administration into the CNS of mice or rats increases alveolar ventilation and dysfunction in leptin signaling has been implicated in the hypoventilation that can accompany obesity. An increase in CO₂ chemosensitivity has been implicated in this response but it is unclear whether ventilation is augmented when PCO₂ is maintained constant. We examined the effects of intravenous leptin to test the hypothesis that systemic leptin administration in isoflurane anesthetized, mechanically ventilated and vagotomized rats would lead to a sustained increase in respiratory motor output that was independent of changes in end-tidal PCO₂, body temperature or lung inflation pressure (an indicator of overall lung and chest wall compliance). In anesthetized Sprague-Dawley rats with end-tidal PCO₂, lung compliance and rectal temperature maintained constant, injection of a bolus of leptin (0.25 mg, 0.5 mg/ml, i.v.), followed over the next 1 h by the intravenous infusion of an additional 0.25 mg, elicited a progressive increase in the peak amplitude of integrated phrenic nerve discharge lasting at least 1 h beyond the end of the infusion. The increase peaked at 90 min at 58.3 ± 5.7% above baseline. There was an associated increase in the slope of the phrenic response to increasing inspired CO₂. There was also a moderate and sustained decrease in arterial pressure 9 ± 1.3 mmHg at 120 min, with no associated change in heart rate. These data indicate that leptin elicits a sustained increase in respiratory motor output that outlasts the administration leptin via a mechanism that does not require alterations in arterial PCO₂, body temperature, or systemic afferent feedback via the vagus nerves. This stimulation may help to prevent obesity-related hypoventilation.

The apneic threshold during non-REM sleep in dogs: sensitivity of carotid body vs. central chemoreceptors

The relative importance of peripheral vs. central chemoreceptors in causing apnea/unstable breathing during sleep is unresolved. This has never been tested in an unanesthetized preparation with intact carotid bodies. We studied three unanesthetized dogs during normal sleep in a preparation in which intact carotid body chemoreceptors could be reversibly isolated from the systemic circulation and perfused. Apneic thresholds and the CO2 reserve (end-tidal Pco2 eupneic − end-tidal Pco2 apneic threshold) were determined using a pressure support ventilation technique. Dogs were studied when both central and peripheral chemoreceptors sensed transient hypocapnia induced by the pressure support ventilation and again with carotid body isolation such that only the central chemoreceptors sensed the hypocapnia. We observed that the CO2 reserve was ≅4.5 Torr when the carotid chemoreceptors sensed the transient hypocapnia but more than doubled (>9 Torr) when only the central chemoreceptors sensed hypocapnia. Furthermore, the expiratory time prolongations observed when only central chemoreceptors were exposed to hypocapnia differed from those obtained when both the central and peripheral chemoreceptors sensed the hypocapnia in that they 1) were substantially shorter for a given reduction in end-tidal Pco2, 2) showed no stimulus: response relationship with increasing hypocapnia, and 3) often occurred at a time (>45 s) beyond the latency expected for the central chemoreceptors. These findings agree with those previously obtained using an identical pressure support ventilation protocol in carotid body-denervated sleeping dogs (Nakayama H, Smith CA, Rodman JR, Skatrud JB, Dempsey JA. J Appl Physiol 94: 155–164, 2003). We conclude that hypocapnia sensed at the carotid body chemoreceptor is required for the initiation of apnea following a transient ventilatory overshoot in non-rapid eye movement sleep.

GABAergic pump cells of solitary tract nucleus innervate retrotrapezoid nucleus chemoreceptors

The retrotrapezoid nucleus (RTN) contains central respiratory chemoreceptors that are inhibited by activation of slowly adapting pulmonary stretch receptors (SARs). Here we examine whether RTN inhibition by lung inflation could be mediated by a direct projection from SAR second-order neurons (pump cells). Pump cells (n = 56 neurons, 13 rats) were recorded in the nucleus of solitary tract (NTS) of halothane-anesthetized rats with intact vagus nerves. Pump cells had discharges that coincided with lung inflation as monitored by the tracheal pressure. Their activity increased when end-expiratory pressure was raised and stopped instantly when ventilation was interrupted in expiration. Many pump cells could be antidromically activated from RTN (12/36). Nine of those were labeled with biotinamide. Of these nine cells, eight contained glutamic acid decarboxylase 67 (GAD67) mRNA and seven were found to reside in the lower half of the interstitial subnucleus of NTS (iNTS). Using the retrograde tracer cholera toxin-B, we confirmed that neurons located in or close to iNTS innervate RTN (two rats). Many such neurons contained GAD67 mRNA and a few contained glycine transporter2 (GLYT2) mRNA. Anterograde tract tracing with biotinylated dextranamide (four rats) applied to iNTS also confirmed that this region innervates RTN by a predominantly GABAergic projection. This work confirms that many rat NTS pump cells are located in and around the interstitial subnucleus at area postrema level. We demonstrate that a GABAergic subset of these pump cells innervates the RTN region. We conclude that these inhibitory neurons probably contact RTN chemoreceptors and mediate their inhibition by lung inflation.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

Parabrachial-lateral pontine neurons link nociception and breathing

We investigated the role of the parabrachial complex in cutaneous nociceptor-induced respiratory stimulation in chloralose-urethane anesthetized, vagotomized rats. Noxious stimulation (mustard oil, MO) applied topically to a forelimb or hindlimb enhanced the peak amplitude of the integrated phrenic nerve discharge and, with forelimb application, increased phrenic nerve burst frequency. Bilateral inactivation of neural activity in the parabrachial complex with injection of the GABA agonist muscimol (3nl) markedly attenuated the response to MO application. Injection of the retrograde tracer FluoroGold within the medullary ventral respiratory column labeled neurons in dorsolateral pontine regions known to receive nociceptive inputs (i.e., Kolliker-Fuse, lateral crescent, and superior lateral subnuclei of the parabrachial complex). Extracellular recordings of 65 dorsolateral parabrachial neurons revealed about 15% responded to a noxious cutaneous pinch with either an increase or a decrease in discharge and approximately 40% of these exhibited a phasic respiratory-related component to their discharge. In conclusion, parabrachial pontine neurons contribute to cutaneous nociceptor-induced increases in breathing.

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. III. Comparison of model performances during afferent nerve stimulation

The goal of the present study was to evaluate the relative plausibility of the models of the central respiratory pattern generator (CRPG) proposed in our previous paper. To test the models, we compared changes in generated patterns with the experimentally observed alterations of the respiratory pattern induced by various stimuli applied to superior laryngeal (SLN), vagus and carotid sinus (CS) nerves. In all models, short-duration SLN simulation caused phase-resetting behavior consistent with experimental data. Relatively weak sustained SLN stimulation elicited a two-phase rhythm comprising inspiration and postinspiration whereas a stronger stimulation stopped oscillations in the postinspiratory phase ("postinspiratory apnea"). In all models, sustained vagus nerve stimulation produced postinspiratory apnea. A short vagal stimulus delivered during inspiration terminated this phase. The threshold for inspiratory termination decreased during the course of the inspiratory phase. The effects of short-duration vagal stimulation applied during expiration were different in different models. In model 1, stimuli delivered in the postinspiratory phase prolonged expiration whereas the late expiratory phase was insensitive to vagal stimulation. No insensitive period was found in model 2 because vagal stimuli delivered at any time during expiration prolonged this phase. Model 3 demonstrated a short period insensitive to vagal stimulation at the very end of expiration. When phasic CS nerve stimulation was applied during inspiration or the first half of expiration, the performances of all models were similar and consistent with experimental data: stimuli delivered at the beginning inspiration shortened this phase whereas stimuli applied in the middle or at the end of inspiration prolonged it and stimuli delivered in the first half of expiration prolonged the expiratory interval. Behavior of the models were different when CS stimuli were delivered during the late expiratory phase. In model 1, these stimuli were ineffective or shortened expiration initiating the next inspiration. Alternatively, in models 2 and 3, they caused a prolongation of expiration. Although all CRPG models demonstrated a number of plausible alterations in the respiratory pattern elicited by afferent nerve stimulation, the behavior of model 1 was most consistent with experimental data. Taking into account differences in the model architectures and employed neural mechanisms, we suggest that the concept of respiratory rhythmogenesis based on the essential role of postinspiratory neurons is more plausible than the concept employing specific functional properties of decrementing expiratory (dec-E) neurons and that the ramp firing pattern of the late expiratory neuron is more likely to reflect intrinsic properties than disinhibition from the dec-E neurons.

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. II. Network models of the central respiratory pattern generator

The present paper describes several models of the central respiratory pattern generator (CRPG) developed employing experimental data and current hypotheses for respiratory rhythmogenesis. Each CRPG model includes a network of respiratory neuron types (e.g., early inspiratory; ramp inspiratory; late inspiratory; decrementing expiratory; postinspiratory; stage II expiratory; stage II constant firing expiratory; preinspiratory) and simplified models of lung and pulmonary stretch receptors (PSR), which provide feedback to the respiratory network. The used models of single respiratory neurons were developed in the Hodgkin-Huxley style as described in the previous paper. The mechanism for termination of inspiration (the inspiratory off-switch) in all models operates via late-I neuron, which is considered to be the inspiratory off-switching neuron. Several two- and three-phase CRPG models have been developed using different accepted hypotheses of the mechanism for termination of expiration. The key elements in the two-phase models are the early-I and dec-E neurons. The expiratory off-switch mechanism in these models is based on the mutual inhibitory connections between early-I and dec-E and adaptive properties of the dec-E neuron. The difference between the two-phase models concerns the mechanism for ramp firing patterns of E2 neurons resulting either from the intrinsic neuronal properties of the E2 neuron or from disinhibition from the adapting dec-E neuron. The key element of the three-phase models is the pre-I neuron, which acts as the expiratory off-switching neuron. The three-phase models differ by the mechanisms used for termination of expiration and for the ramp firing patterns of E2 neurons. Additional CRPG models were developed employing a dual switching neuron that generates two bursts per respiratory cycle to terminate both inspiration and expiration. Although distinctly different each model generates a stable respiratory rhythm and shows physiologically plausible firing patterns of respiratory neurons with and without PSR feedback. Using our models, we analyze the roles of different respiratory neuron types and their interconnections for the respiratory rhythm and pattern generation. We also investigate the possible roles of intrinsic biophysical properties of different respiratory neurons in controlling the duration of respiratory phases and timing of switching between them. We show that intrinsic membrane properties of respiratory neurons are integrated with network properties of the CRPG at three hierarchical levels: at the cellular level to provide the specific firing patterns of respiratory neurons (e.g., ramp firing patterns); at the network level to provide switching between the respiratory phases; and at the systems level to control the duration of inspiration and expiration under different conditions (e.g., lack of PSR feedback).

Phrenic nerve responses to lung inflation and hypercapnia in decerebrate dogs

The effects of changes in static airway pressure (Paw) and arterial PCO2 (PaCO2) on phrenic nerve activity were studied in unanesthetized, decerebrate dogs and compared with previous results from chloralose/urethane anesthetized dogs using the same experimental preparation (Mitchell et al. 1982; Mitchell and Selby 1987). In ten mid-collicular decerebrate dogs, the lungs were independently ventilated while the left pulmonary artery was occluded and the right vagus nerve was transected. Changes in left lung Paw, therefore, exerted effects on pulmonary stretch receptors without altering blood gases; changes in the inspired gas ventilating the right lung controlled blood gas composition, without altering lung volume feedback. Phrenic burst frequency (f) and integrated amplitude (Phr) were monitored while Paw was varied between 2 and 12 cmH2O at various constant levels of PaCO2 between 31 and 69 mmHg. The major findings of this study are: (1) hypercapnia decreases the slope of the relationship between expiratory duration (tE) and Paw in both decerebrated and anesthetized dogs; (2) hypercapnia increases the inspiratory duration (tI) in decerebrated, but not anesthetized dogs; and (3) hypercapnia decreases the slope of the relationship between f and Paw due to these effects on tE and tI. These results support previous studies indicating that vagal and suprapontine mechanisms exert independent effects on respiratory timing. It is concluded that neither suprapontine influences nor anesthesia are necessary in the mechanism underlying interactions between stretch receptors and CO2-chemoreceptors in modulating tE. Furthermore, decerebration reveals a unique effect of CO2-chemoreceptors on tI, an effect found in anesthetized dogs only after carotid denervation.

Effects of prolonged inflation on pulmonary stretch receptor discharge in turtles

The tonic and phasic discharge characteristics of single, slowly adapting pulmonary stretch receptors (SAR) were examined before and after 1 h periods of constant pressure inflation to normal resting (VLr, pressure = 0 cm H2O) and elevated (VLe, pressure = 10 cm H2O) lung volumes in turtles (Chrysemys sp.). Based on their discharge at VLr, SAR were classified as either low (n = 13) or high threshold (n = 4) receptors. Inflations were performed with both air and 5% CO2 in air. Lung gas composition and arterial PCO2 and pH were measured during the maintained inflations. In animals ventilated with air, low and high threshold receptors adapted by 57 and 30% respectively over the first 3 min at VLe. During the remainder of the 1 h period, the discharge of low threshold SAR fell an additional 20% while that of the high threshold SAR remained relatively constant. There were significant increases in both alveolar and arterial PCO2 during the maintained inflations. Ventilation with 5% CO2 reduced the static discharge levels of low and high threshold SAR by 10 and 25% respectively, suggesting that a part of the apparent adaptation of these receptors to maintained inflation for 1 h with air was due to the accumulation of metabolic CO2. Following 1 h of maintained inflation, the phasic responses to pump ventilation were decreased in low threshold SAR but remained unchanged in high threshold SAR. The static discharge associated with step inflation was unchanged in both receptor groups. The data suggest that increased SAR discharge is sustained indefinitely during increased lung volume and may account for persistent changes in breathing pattern previously observed during chronic changes in lung volume.

Effects of hypercapnia on phrenic and stretch receptor responses to lung inflation

To determine if hypercapnia and reflex bronchoconstriction attenuate lung inflation effects on ventilatory activity by indirect effects on intrapulmonary stretch receptors (PSR), phrenic nerve activity and single unit PSR were monitored at controlled levels of static airway pressure (Paw) and arterial PCO2 in 15 anesthetized dogs. Paw in a vascularly isolated lung was varied between 2 and 14 cm H2O at levels of PaCO2 between 35 and 85 mm Hg. PSR activity (n = 38) in fine strands dissected from an otherwise intact vagus nerve and the integrated phrenic neurogram were recorded. The response to Paw varied from one PSR to another, but was consistent in a given unit; PaCO2 had no consistent effect on individual responses. Selected PSR (n = 15) were averaged to yield a population response to Paw; the selection criteria were: phrenic activity responded briskly to Paw and measurements were made at three levels of PaCO2. Average PSR discharge increased linearly with Paw but was unaffected by PaCO2. On the other hand, phrenic burst frequency decreased as Paw increased and hypercapnia attenuated the slope of this relationship. These results suggest that effects on the relationship between PSR activity and Paw cannot account for attenuation of the relationship between phrenic frequency and Paw in hypercapnia. The effect of PaCO2 on the phrenic frequency vs Paw relationship probably arises from integrative mechanisms in the central nervous system.

Effects of carotid denervation on interactions between lung inflation and PaCO2 in modulating phrenic activity

Hypercapnia attenuates the effects of static airway pressure (Paw) on phrenic burst frequency (f) and the expiratory duration. We examined the role of carotid chemoreceptors in this response using an experimental preparation that allowed independent control of lung inflation and CO2 reflexes. Experiments were conducted in intact (n = 6) and carotid denervated (CBX; n = 12) chloralose/urethane anesthetized dogs. Integrated phrenic amplitude (Phr), f, and the inspiratory (TI) and expiratory durations (TE) were measured as a function of Paw (2-12 cm H2O) at levels of PaCO2 between 30 and 80 mm Hg. In intact dogs: (1) f decreased as Paw increased, and elevated PaCO2 decreased the slope of this relationship; (2) neither PaCO2 nor Paw affected TI; and (3) TE increased hyperbolically with Paw, and elevated PaCO2 attenuated this relationship. In CBX dogs: (1) f decreased as Paw increased, but this relationship was not affected by PaCO2; (2) TI increased as PaCO2 increased but was unaffected by Paw; and (3) TE increased as Paw increased but was unaffected by PaCO2. The results indicate that carotid chemoreceptors are necessary in the mechanism whereby hypercapnia attenuates the effects of Paw on f and TE. Furthermore, carotid denervation reveals an effect of hypercapnia on TI, an effect that is not evident in dogs with functional carotid chemoreceptors.

Effects of hypoxemia on phrenic nerve responses to static lung inflation in anesthetized dogs

To study interactions between hypoxemia and lung stretch in modulating ventilatory activity, an experimental preparation was used that allows independent control of static airway pressure (Paw) and arterial PO2 in anesthetized dogs. Phrenic burst frequency (f) and integrated amplitude (Phr) were monitored while Paw was varied between 2 and 12 cm H2O at levels of PaO2 between 30 and 200 mm Hg. Experiments were repeated in intact (n = 8) and carotid denervated dogs (CBX; n = 7). In intact dogs, f decreased with increasing Paw through an effect on the expiratory duration (TE). Hypoxia increased f by decreasing both the inspiratory duration (TI) and TE. Hypoxia had no effect on the slope of the f vs Paw relationship, but attenuated the effect of Paw on TE. Phr was increased by hypoxia, but Paw had little effect. After CBX, f was still inhibited by Paw, but PaO2 had no consistent effect on f, TI or TE at any level of Paw. Phr was inhibited by hypoxia after CBX, but Paw had no effect. The results indicate that Paw and PaO2 exert additive effects on f in anesthetized dogs. Hypoxia attenuates the effect of Paw on TE, which alone would attenuate the slope of the f vs Paw relationship. However, the effect of hypoxia on TI enhances the slope of the f vs Paw relationship, restoring a parallel shift. These effects are abolished by carotid denervation.