Responses of bulbospinal and laryngeal respiratory neurons to hypercapnia and hypoxia

Carotid Bodies and the Integrated Cardiorespiratory Response to Hypoxia

Advances in our understanding of brain mechanisms for the hypoxic ventilatory response, coordinated changes in blood pressure, and the long-term consequences of chronic intermittent hypoxia as in sleep apnea, such as hypertension and heart failure, are giving impetus to the search for therapies to "erase" dysfunctional memories distributed in the carotid bodies and central nervous system. We review current network models, open questions, sex differences, and implications for translational research.

Neural Network Reconfigurations: Changes of the Respiratory Network by Hypoxia as an Example

Neural networks, including the respiratory network, can undergo a reconfiguration process by just changing the number, the connectivity or the activity of their elements. Those elements can be either brain regions or neurons, which constitute the building blocks of macrocircuits and microcircuits, respectively. The reconfiguration processes can also involve changes in the number of connections and/or the strength between the elements of the network. These changes allow neural networks to acquire different topologies to perform a variety of functions or change their responses as a consequence of physiological or pathological conditions. Thus, neural networks are not hardwired entities, but they constitute flexible circuits that can be constantly reconfigured in response to a variety of stimuli. Here, we are going to review several examples of these processes with special emphasis on the reconfiguration of the respiratory rhythm generator in response to different patterns of hypoxia, which can lead to changes in respiratory patterns or lasting changes in frequency and/or amplitude.

Phrenic motoneurons: output elements of a highly organized intraspinal network

pontomedullary respiratory network generates the respiratory pattern and relays it to bulbar and spinal respiratory motor outputs. The phrenic motor system controlling diaphragm contraction receives and processes descending commands to produce orderly, synchronous, and cycle-to-cycle-reproducible spatiotemporal firing. Multiple investigators have studied phrenic motoneurons (PhMNs) in an attempt to shed light on local mechanisms underlying phrenic pattern formation. I and colleagues (Marchenko V, Ghali MG, Rogers RF. Am J Physiol Regul Integr Comp Physiol 308: R916–R926, 2015.) recorded PhMNs in unanesthetized, decerebrate rats and related their activity to simultaneous phrenic nerve (PhN) activity by creating a time-frequency representation of PhMN-PhN power and coherence. On the basis of their temporal firing patterns and relationship to PhN activity, we categorized PhMNs into three classes, each of which emerges as a result of intrinsic biophysical and network properties and organizes the orderly contraction of diaphragm motor fibers. For example, early inspiratory diaphragmatic activation by the early coherent burst generated by high-frequency PhMNs may be necessary to prime it to overcome its initial inertia. We have also demonstrated the existence of a prominent role for local intraspinal inhibitory mechanisms in shaping phrenic pattern formation. The objective of this review is to relate and synthesize recent findings with those of previous studies with the aim of demonstrating that the phrenic nucleus is a region of active local processing, rather than a passive relay of descending inputs.

Change in network connectivity during fictive-gasping generation in hypoxia: prevention by a metabolic intermediate

The neuronal circuit in charge of generating the respiratory rhythms, localized in the pre-Bötzinger complex (preBötC), is configured to produce fictive-eupnea during normoxia and reconfigures to produce fictive-gasping during hypoxic conditions in vitro. The mechanisms involved in such reconfiguration have been extensively investigated by cell-focused studies, but the actual changes at the network level remain elusive. Since a failure to generate gasping has been linked to Sudden Infant Death Syndrome (SIDS), the study of gasping generation and pharmacological approaches to promote it may have clinical relevance. Here, we study the changes in network dynamics and circuit reconfiguration that occur during the transition to fictive-gasping generation in the brainstem slice preparation by recording the preBötC with multi-electrode arrays and assessing correlated firing among respiratory neurons or clusters of respiratory neurons (multiunits). We studied whether the respiratory network reconfiguration in hypoxia involves changes in either the number of active respiratory elements, the number of functional connections among elements, or the strength of these connections. Moreover, we tested the influence of isocitrate, a Krebs cycle intermediate that has recently been shown to promote breathing, on the configuration of the preBötC circuit during normoxia and on its reconfiguration during hypoxia. We found that, in contrast to previous suggestions based on cell-focused studies, the number and the overall activity of respiratory neurons change only slightly during hypoxia. However, hypoxia induces a reduction in the strength of functional connectivity within the circuit without reducing the number of connections. Isocitrate prevented this reduction during hypoxia while increasing the strength of network connectivity. In conclusion, we provide an overview of the configuration of the respiratory network under control conditions and how it is reconfigured during fictive-gasping. Additionally, our data support the use of isocitrate to favor respiratory rhythm generation under normoxia and to prevent some of the changes in the respiratory network under hypoxic conditions.

Hypoxia-induced short-term potentiation of respiratory-modulated facial motor output in the rat

Respiratory-modulated facial (VII) nerve discharge includes pre-inspiratory (Pre-I) and inspiratory (I) components. Tonic VII bursting is also present across the respiratory cycle. We tested the hypothesis that hypoxia-induced plasticity of VII motor activity is differentially expressed in Pre-I, I and tonic bursting. Phrenic and VII neurograms were recorded in urethane-anesthetized, vagotomized and ventilated adult rats. A 3 min isocapnic hypoxic challenge (PaO(2)=33+/-2 mmHg) was used to evoke respiratory short-term potentiation (STP). Pre-I, I and tonic VII activity increased immediately at the initial stage of hypoxia (i.e. acute response) and then progressively increased as hypoxia was maintained. Following hypoxia, I VII activity remained elevated (i.e. post-hypoxia STP) but both Pre-I and tonic activity immediately returned to baseline values. We conclude that STP following hypoxia is preferentially expressed in I compared to Pre-I and tonic VII activity.

GABAAergic and Glycinergic Inhibition in the Phrenic Nucleus Organizes and Couples Fast Oscillations in Motor Output

One of the characteristics of respiratory motor output is the presence of fast synchronous oscillations, at rates far exceeding the basic breathing rhythm, within a given functional population. However, the mechanisms responsible for organizing phrenic output into two dominant bands in vivo, medium (MFO)- and high (HFO)-frequency oscillations, have yet to be elucidated. We hypothesize that GABAAergic and glycinergic inhibition within the phrenic motor nucleus underlies the specific organization of these oscillations. To test this, the phrenic nuclei (C4) of 14 unanesthetized, decerebrate adult male Sprague-Dawley rats were microinjected unilaterally with either 4 mM strychnine ( n = 7) or GABAzine ( n = 7) to block glycine or GABAA receptors, respectively. Application of GABAzine caused an increase in overall phrenic amplitude during all three phases of respiration (inspiration, postinspiration, and expiration), while the increases caused by strychnine were most pronounced during postinspiration. Neither antagonist produced changes in inspiratory duration or respiratory rate. Power spectral analysis of inspiratory phrenic bursts showed that blockade of inhibition caused significant reduction in the relative power of MFO (GABAA and glycine receptors) and HFO (GABAA receptors only). In addition, analysis of the coherence between the firing of the ipsi- and contralateral phrenic nerves revealed that HFO coupling was significantly reduced by both antagonists and that of MFO was significantly reduced only by strychnine. We conclude that both GABAA and glycine receptors play critical roles in the organization of fast oscillations into MFO and HFO bands in the phrenic nerve, as well as in their bilateral coupling.

Influence of hypercapnic acidosis and hypoxia on abdominal expiratory nerve activity in the rat

We studied the influence of hypercapnic acidosis and hypoxia on the neural drive to abdominal muscles in anesthetized and decerebrate rats; this information is unavailable despite widespread use of the rat as an experimental model in respiratory physiology and neurobiology. To minimize confounding influences from receptors in the lungs and chest wall, the animals were vagotomized, paralyzed and mechanically ventilated, and electrical activity was recorded from abdominal muscle nerves. In anesthetized and decerebrate rats, both stimuli evoked steady, low amplitude expiratory discharge that persisted throughout the expiratory phase (E-all activity), but was inhibited during inspiration. We also observed late expiratory, high-amplitude bursts (E2 activity) superimposed on this steady activity, but only at the highest levels of respiratory drive. Hypoxia enhanced abdominal motor activity transiently, whereas hypercapnic acidosis caused a sustained increase in activity. Thus, both hypercapnic acidosis and hypoxia activate abdominal muscle motoneurons in the absence of phasic afferent inputs.

Medullary respiratory neural activity during hypoxia in NREM and REM sleep in the cat

Intact unanesthetized cats hyperventilate in response to hypocapnic hypoxia in both wakefulness and sleep. This hyperventilation is caused by increases in diaphragmatic activity during inspiration and expiration. In this study, we recorded 120 medullary respiratory neurons during sleep in hypoxia. Our goal was to understand how these neurons change their activity to increase breathing efforts and frequency in response to hypoxia. We found that the response of medullary respiratory neurons to hypoxia was variable. While the activity of a small majority of inspiratory (58%) and expiratory (56%) neurons was increased in response to hypoxia, the activity of a small majority of preinspiratory (57%) neurons was decreased. Cells that were more active in hypoxia had discharge rates that averaged 183% (inspiratory decrementing), 154% (inspiratory augmenting), 155% (inspiratory), 230% (expiratory decrementing), 191% (expiratory augmenting), and 136% (expiratory) of the rates in normoxia. The response to hypoxia was similar in non-rapid-eye-movement (NREM) and REM sleep. Additionally, changes in the profile of activity were observed in all cell types examined. These changes included advanced, prolonged, and abbreviated patterns of activity in response to hypoxia; for example, some inspiratory neurons prolonged their discharge into expiration during the postinspiratory period in hypoxia but not in normoxia. Although changes in activity of the inspiratory neurons could account for the increased breathing efforts and activity of the diaphragm observed during hypoxia, the mechanisms responsible for the change in respiratory rate were not revealed by our data.

Response of the respiratory network of mice to hyperthermia

Most mammals modulate respiratory frequency (RF) to dissipate heat (i.e., panting) and avoid heat stroke during hyperthermic conditions. During hyperthermia, the RF of intact mammals increases and then declines or ceases (apnea). It has been proposed that this RF modulation depends on the presence of higher brain structures such as the hypothalamus. However, the direct effects of hyperthermia on the respiratory neural network have not been examined. To address this issue, the respiratory neural network [i.e., ventral respiratory group (VRG)] was isolated in a brain stem preparation taken from the medulla of mice (P0 -P6). Integrated population activity, predominated by inspiratory neurons, was recorded extracellularly from VRG neurons. The bath temperature was then heated from 30 to 40 degrees C, resulting in a biphasic frequency response in VRG activity. Following an initial six- to sevenfold increase and subsequent decline, fictive RF was maintained at a frequency that was higher than baseline frequency; at 40 degrees C, the RF was maintained at about two to four times that at 30 degrees C. The inspiratory burst amplitude and duration were significantly reduced during hyperthermic conditions. An increase in RF and decrease in VRG burst amplitude and duration also occurred when heating from 37 to 40 degrees C. Fictive apnea typically occurred during cooling to the control temperature. Furthermore, changes in hypoglossal motor nucleus activity paralleled those of the VRG, suggesting that temperature modulation of the VRG is likely to have a behaviorally relevant impact on respiration. We conclude that the VRG activity itself is modulated during hyperthermia and the respiratory network is particularly sensitive to temperature changes.

Serotonin(2) receptors mediate respiratory recovery after cervical spinal cord hemisection in adult rats

The aim of the present study was to specifically investigate the involvement of serotonin [5-hydroxytryptamine (5-HT(2))] receptors in 5-HT-mediated respiratory recovery after cervical hemisection. Experiments were conducted on C(2) spinal cord-hemisected, anesthetized (chloral hydrate, 400 mg/kg ip), vagotomized, pancuronium- paralyzed, and artificially ventilated female Sprague-Dawley rats in which CO(2) levels were monitored and maintained. Twenty-four hours after spinal hemisection, the ipsilateral phrenic nerve displayed no respiratory-related activity indicative of a functionally complete hemisection. Intravenous administration of the 5-HT(2A/2C)-receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) induced respiratory-related activity in the phrenic nerve ipsilateral to hemisection under conditions in which CO(2) was maintained at constant levels and augmented the activity induced under conditions of hypercapnia. The effects of DOI were found to be dose dependent, and the recovery of activity could be maintained for up to 2 h after a single injection. DOI-induced recovery was attenuated by the 5-HT(2)-receptor antagonist ketanserin but not with the 5-HT(2C)-receptor antagonist RS-102221, suggesting that 5-HT(2A) and not necessarily 5-HT(2C) receptors may be involved in the induction of respiratory recovery after cervical spinal cord injury.

Effect of central CO(2) drive on lung inflation responses of expiratory bulbospinal neurons in dogs

The purpose of these studies is to better understand the nature of the reflex interactions that control the discharge patterns of caudal medullary, expiratory (E) bulbospinal neurons. We examined the effect of central chemodrive inputs measured as arterial CO(2) tension (Pa(CO(2))) during hyperoxia on the excitatory and inhibitory components of the lung inflation responses of these neurons in thiopental sodium-anesthetized, paralyzed dogs. Data from slow ramp inflation and deflation test patterns, which were separated by several control inflation cycles, were used to produce plots of neuronal discharge frequency (F(n)) versus transpulmonary pressure (P(t)). P(t) was used as an index of the activity arising from the slowly adapting pulmonary stretch receptors (PSRs). Changes in inspired CO(2) concentrations were used to produce Pa(CO(2)) levels that ranged from 20 to 80 mmHg. The data obtained from 41 E neurons were used to derive an empirical model that quantifies the average relationship for F(n) versus both P(t) and Pa(CO(2)). This model can be used to predict the time course and magnitude of E neuronal responses to these inputs. These data suggest that the interaction between Pa(CO(2)) and PSR-mediated excitation and inhibition of F(n) is mainly additive, but synergism between Pa(CO(2)) and excitatory inputs is also present. The implications of these findings are discussed.

Role of inspiratory pacemaker neurons in mediating the hypoxic response of the respiratory network in vitro

In severe hypoxia the breathing frequency is modulated in a biphasic manner: an initial increase (augmentation) is followed by a depression and cessation of breathing (apnea). Using a mouse slice preparation that contains the functional respiratory network, we aimed at identifying the neurons responsible for this frequency modulation. Whole-cell patch recordings revealed that expiratory neurons become tonically active during anoxia, indicating that these neurons cannot be responsible for the respiratory frequency modulation. Inspiratory neurons tended to depolarize (by 6.9 mV; n = 9), and the frequency of rhythmic activity was significantly increased during anoxia (from 0.16 to 0.4 Hz; n = 9). After the blockade of network activity with 6-cyano-7-nitroquinoxaline-2, 3-dione, most inspiratory neurons became tonically active (72%; n = 25, non-pacemaker). In anoxia, the membrane potential of these non-pacemaker neurons did not change (-0.26 mV; n = 6), and their tonic activity ceased. Only a subpopulation of inspiratory neurons remained rhythmically active in the absence of network activity (pacemaker neurons, 28%, 7 of 25 inspiratory neurons). In anoxia two subgroups of pacemaker neurons were differentiated; one group showed a transient increase in the bursting activity, followed by a decrease and cessation of rhythmic activity. These neurons tended to depolarize (by 10.3 mV) during anoxia. The second group remained rhythmic during the entire anoxic exposure and exhibited no depolarization. The time course of the frequency modulation in all pacemaker neurons resembled that of the intact network. We conclude that pacemaker neurons are primarily responsible for the frequency modulation in anoxia and that in the respiratory network there is a strict separation between rhythm- and pattern-generating mechanisms.

Effects of the serotonin synthesis inhibitor p-CPA on the expression of the crossed phrenic phenomenon 4 h following C2 spinal cord hemisection

The present study assesses the effects of para-chlorophenylalanine (p-CPA), a serotonin-depleting drug, on the recovery of respiratory-related activity in the phrenic nerve induced by asphyxia 4 h following ipsilateral C2 hemisection in young adult rats. HPLC analysis was used to quantify levels of serotonin (5-HT), dopamine (DA), norepinephrine, and the 5-HT metabolite, 5-hydroxyindoleacetic acid, in the C4 segment of the spinal cord, all of which were significantly lower in p-CPA-treated hemisected rats compared to hemisected controls receiving saline. Hemisection alone was found to significantly increase 5-HT levels and significantly decrease DA levels compared to normal controls. Eight of eight saline-injected rats expressed recovery of respiratory-related activity in the ipsilateral phrenic nerve during asphyxia 4 h following hemisection, while only 4/8 rats in the p-CPA-treated group expressed recovery in the ipsilateral nerve. Quantification of integrated phrenic nerve wave-forms indicated that the mean amplitude of respiratory-related activity in the ipsilateral phrenic nerve was significantly lower in p-CPA-treated rats than in saline controls. In addition, saline controls demonstrated significant increases in mean respiratory frequency and mean amplitude of contralateral phrenic nerve activity during asphyxia, compared to normocapnia. However, p-CPA-treated rats did not express significant differences in either mean respiratory frequency or mean amplitude of integrated respiratory wave-forms during asphyxia, compared to normocapnia. The results suggest that p-CPA treatment attenuates the recovery of respiratory-related activity in the phrenic nerve 4 h following ipsilateral C2 hemisection and attenuates asphyxia-induced increases in respiratory frequency and respiratory burst amplitude recorded from the contralateral phrenic nerve.

Differential responses of respiratory nuclei to anoxia in rhythmic brain stem slices of mice

The response of the neonatal respiratory system to hypoxia is characterized by an initial increase in ventilation, which is followed within a few minutes by a depression of ventilation below baseline levels. We used the transverse medullary slice of newborn mice as a model system for central respiratory control to investigate the effects of short-lasting periods of anoxia. Extracellular population activity was simultaneously recorded from the ventral respiratory group (VRG) and the hypoglossus (XII) nucleus (a respiration-related motor output nucleus). During anoxia, respiratory frequency was modulated in a biphasic manner and phase-locked in both the VRG and the XII. The amplitude of phasic respiratory bursts was increased only in the XII and not in the VRG. This increase in XII burst amplitude commenced approximately 1 min after the anoxic onset concomitant with a transient increase in tonic activity. The burst amplitude remained elevated throughout the entire 5 min of anoxia. Inspiratory burst amplitude in the VRG, in contrary, remained constant or even decreased during anoxia. These findings represent the first simultaneous extracellular cell population recordings of two respiratory nuclei. They provide important data indicating that rhythm generation is altered in the VRG without a concomitant alteration in the VRG burst amplitude, whereas the burst amplitude is modulated only in the XII nucleus. This has important implications because it suggests that rhythm generation and motor pattern generation are regulated separately within the respiratory network.

CO2, brainstem chemoreceptors and breathing

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.

Differential roles of ionotropic glutamate receptors in canine medullary inspiratory neurons of the ventral respiratory group

The relative roles of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in supplying excitatory drive to inspiratory (I) augmenting pattern neurons of the ventral respiratory group were studied in anesthetized, ventilated, paralyzed, and vagotomized dogs. Multibarrel micropipettes were used to record simultaneously single-unit neuronal activity and pressure microeject the NMDA antagonist, 2-amino-5-phosphonovalerate (AP5; 2 mM), the non-NMDA antagonist 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX; 0.25 mM), and an artificial cerebrospinal fluid vehicle. Ejected volume-rates were measured directly via meniscus level changes. The moving time average of phrenic nerve activity was used to determine respiratory phase durations and to synchronize cycle-triggered histograms of the discharge patterns. Both AP5 and NBQX produced dose-dependent reductions in peak spontaneous I neuronal discharge frequency (Fn). The average (+/- SE) maximum reduction in peak Fn produced by AP5 was 69.1 +/- 4.2% and by NBQX was 47.1 +/- 3.3%. Blockade of both glutamate receptor subtypes nearly silenced these neurons, suggesting that their activity is highly dependent on excitatory synaptic drive mediated by ionotropic glutamate receptors. Differential effects were found for the two glutamatergic antagonists. AP5 produced downward, parallel shifts in the augmenting pattern of discharge, whereas NBQX reduced the slope of the augmenting discharge pattern. These results suggest that time-varying excitatory input patterns to the canine I bulbospinal neurons are mediated by non-NMDA glutamate receptors and that constant or tonic input patterns to these neurons are mediated by NMDA receptors.

Rostral medullary respiratory neuronal activities of decerebrate cats in eupnea, apneusis and gasping

Eupnea is generated by mechanisms within the pons and medulla. Following removal of pons or exposure to anoxia, gasping is elicited. Eupnea and gasping are markedly different ventilatory patterns. The genesis of gasping is dependent upon rostral medullary neuronal activities. To generate the gasp, these activities should commence before the phrenic burst. In decerebrate, vagotomized, paralyzed and ventilated cats, eupnea was altered to gasping in anoxia. Rostral medullary neuronal activities had inspiratory, expiratory and phase-spanning patterns in eupnea. During gasping, some inspiratory neuronal activities commenced before the phrenic gasp; these same neurons had commenced activities after the onset of the eupneic phrenic burst. Expiratory and phase-spanning neurons did not discharge. Neuronal activities which are consonant with a role in the neurogenesis of gasping had very different discharge patterns in eupnea. Results support the concept that medullary mechanisms for gasping are incorporated in the ponto-medullary circuit responsible for the neurogenesis and expression of eupnea.

Sympathetic neuronal oscillators are capable of dynamic synchronization

In this paper we show that the discharges of sympathetic neurons innervating an identified peripheral target are driven by multiple oscillators that undergo dynamic synchronization when an entraining force, central respiratory drive (CRD), is increased. Activity was recorded from postganglionic sympathetic neurons (PGNs) innervating the caudal ventral artery of the rat tail: (1) at the population level from the ventral collector nerve (VCN); and (2) from pairs of single PGNs recorded simultaneously using a focal recording technique. Autospectral analysis of VCN activity revealed a more prominent rhythmical component in the presence of CRD than in its absence, suggesting that (1) multiple oscillators drive the discharges of PGNs and (2) these oscillators can be entrained and therefore synchronized by CRD. This interpretation was supported by analysis of the firing behavior of PGN pairs. Autocorrelation and cross-correlation analysis showed that pairs were not synchronized in the absence of CRD but showed significant synchronization when CRD was enhanced. Time-evolving spectral analysis and raster plots demonstrated that the temporal stability of PGN-to-PGN and CRD-to-PGN interactions at a given level of CRD were also dynamic in nature, with stable constant phase relationships predominating as CRD was increased. This is the first reported example of dynamic synchronization in populations of single postganglionic sympathetic neurons, and we suggest that, as in sensory processing and motor control, temporal pattern coding may also be an important feature of neuronal discharges in sympathetic pathways.

Changes in laryngeal muscle activities during hypercapnia in the cat

The larynx has three functions: phonation, airway protection, and respiration. Few studies have dealt with laryngeal respiratory function. To elucidate respiratory regulation by the larynx, we studied the changes in the activity of the intrinsic laryngeal muscles during hypercapnia in decerebrated cats. The electromyographic activities of the posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscles were recorded simultaneously with an electromyogram of the diaphragm, endotracheal pressure, and concentrations of O2 and CO2. The activity of the intrinsic laryngeal muscles during hypercapnia (end-tidal CO2, 8% to 10%) was analyzed in comparison with that during eucapnia. In hypercapnia, both the PCA and TA muscles increased their activities, and the endotracheal pressure during expiration was elevated to a higher level than that in eucapnia. TA muscle activities returned to the level during eucapnia after ligation of the common carotid arteries. These findings suggest that hypercapnia causes a further widening of the glottis during inspiration to decrease inspiratory resistance and a further narrowing of the glottis during expiration to prevent alveolar collapse. Thus it may be concluded that the larynx actively participates in respiratory regulation under the control of the brain stem through a process of peripheral inputs from the carotid receptors.

The hypoxic response of neurones within the in vitro mammalian respiratory network

1. The transverse brainstem slice preparation containing the pre-Bötzinger complex (PBC) was used in mice to study developmental changes of the response of the in vitro respiratory network to hypoxia. This preparation generates at different postnatal stages (postnatal days (P) 0-22) spontaneous rhythmic activity in hypoglossal (XII) rootlets that occur in synchrony with periodic bursts of neurones in the PBC. 2. In slices from P0-4 mice, hypoxia did not significantly affect the amplitude of rhythmic synaptic drive potentials in four of five inspiratory neurones. Hypoxia reduced, but did not suppress, the amplitude of synaptic drive potentials in only one inspiratory neurone. Spike discharge and phasic 'inspiratory' hyperpolarizations of six expiratory neurones were suppressed during hypoxia revealing a phasic 'inspiratory' depolarization. 3. The coupling between rhythmic activity in PBC neurones and XII bursts occurred under control conditions in preparations from P0-4 mice in a 1:1 manner (n = 11) and from mice older than P5 in a 3:1 manner (n = 9). During hypoxia, PBC and XII activity were linked in a 1:1 manner in all slices. 4. In six of fourteen inspiratory PBC neurones, the amplitude of synaptic drive potentials of slices from mice older than P8 was increased during the period of augmentation, reduced during the period of depression and suppressed during a hypoxic response which we refer to as central apnoea. Augmentation led to a weak-to-moderate membrane depolarization which on average was 4.8 +/- 3.7 mV. This depolarization was followed by a hyperpolarization of 6.2 +/- 4.1 mV only in four inspiratory neurones. In the majority of neurones (n = 9), however, membrane depolarization remained stable and was not followed by hyperpolarization. In expiratory neurones (n = 12) from this age group hypoxia suppressed phasic hyperpolarizations that occurred in synchrony with XII bursts. As similarly seen in inspiratory neurones, membrane potentials were depolarized by 5.1 +/- 4.1 mV during the period of hypoxic augmentation. 5. The hypoxic response of respiratory neurones within the pre-Bötzinger complex resembles the response of neurones that were previously described under in vivo conditions. Thus we conclude that the 'transverse rhythmic slice' is a good model for studying the hypoxic response of the respiratory network under in vitro conditions.

Developmental changes in the hypoxic response of the hypoglossus respiratory motor output in vitro

The transverse brain stem slice of mice containing the pre-Bötzinger complex (PBC), a region essential for respiratory rhythm generation in vitro, was used to study developmental changes of the response of the in vitro respiratory network to severe hypoxia (anoxia). This preparation generates, at different postnatal stages [postnatal day (P)0-22], spontaneous rhythmic activity in hypoglossal (XII) rootlets that are known to occur in synchrony with periodic bursts of neurons in the PBC. It is assumed that this rhythmic activity reflects respiratory rhythmic activity. At all examined stages anoxia led to a biphasic response: the frequency of rhythmic XII activity initially increased ("primary augmentation") and then decreased ("secondary depression"). In neonates (P0-7), anoxia did not significantly affect the amplitude of integrated XII bursts. Secondary depression never led to a cessation of rhythmic activity. In mice older than P7, augmentation was accompanied by a significant increase in the amplitude of XII bursts. A significant decrease of the amplitude of XII bursts occurred during secondary depression. This depression led always to cessation of rhythmic activity in XII rootlets. The anoxia-induced response of the respiratory rhythmic XII motor output is biphasic and changes during development in a similar way to the in vivo respiratory network. Whether this biphasic response is due to a biphasic response of the respiratory rhythm generator and/or to a biphasic modulation of the XII motor nucleus remains unresolved and needs further cellular analysis. We propose that the transverse slice is a useful model system for examination of the mechanisms underlying the hypoxic response.

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

Gasping is a critical mechanism for survival in that it serves as a mechanism for autoresuscitation when eupnea fails. Eupnea and gasping are separable patterns of automatic ventilatory activity in all mammalian species from the day of birth. The neurogenesis of the gasp is dependent on the discharge of neurons in the rostroventral medulla. This gasping center overlaps a region termed "the pre-Bötzinger complex." Neuronal activities of this complex, characterized in an in vitro brain stem spinal cord preparation of the neonatal rat, have been hypothesized to underlie respiratory rhythm generation. Yet, the rhythmic activity of this in vitro preparation is markedly different from eupnea but identical with gasping in vivo. In eupnea, medullary neuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of the pontomedullary circuit defining eupneic ventilatory activity. However, these medullary neuronal activities do not appear critical for the neurogenesis of eupnea, per se.

Neuronal activities underlying inspiratory termination by pneumotaxic mechanisms

The purpose was to identify and characterize the discharge patterns of pontile neurons which are responsible for the termination of inspiratory activity. Phrenic discharge is prolonged following destruction of neurons at the junction of mesencephalon and pons by neurotoxins. Neuronal activities were recorded in this region in decerebrate, vagotomized, paralyzed and ventilated cats. At normocapnia, neurons had tonic discharge patterns, most of which were linked to phasic periods of phrenic activity. Peak activities occurred in late neural inspiration or early expiration. In hypercapnia, neuronal discharge frequencies did not increase, rather activity became more concentrated during one portion of the respiratory cycle. In severe hypoxia, neuronal activities diminished in parallel with the prolongation of phrenic discharge and establishment of apneusis. During recovery, some neurons transiently acquired phasic, respiratory-modulated discharge patterns. Neuronal activities from neighboring regions did not exhibit comparable changes in hypercapnia or hypoxia. We conclude that rostral pontile neuronal activities are a primary determinant of the reversible and irreversible terminations of eupneic inspiratory activity.

Response of the medullary respiratory network of the cat to hypoxia

1. The effect of systemic hypoxia was tested in anaesthetized, immobilized, thoracotomized and artificially ventilated cats with peripheral chemoreceptor afferents either intact or cut. Extracellular recordings from different types of medullary respiratory neurones and intracellular recordings from stage 2 expiratory neurones were made to determine the hypoxia-induced changes in neuronal discharge patterns and postsynaptic activity as an index for the disturbances of synaptic interaction within the network. 2. The general effect of systemic hypoxia was an initial augmentation of respiratory activity followed by a secondary depression. In chemoreceptor-denervated animals, secondary depression led to central apnoea. 3. The effects of systemic hypoxia were comparable with those of cerebral ischaemia following occlusion of carotid and vertebral arteries. 4. In chemoreceptor-denervated animals, all types of medullary respiratory neurones ceased spontaneous action potential discharge during hypoxia. 5. Reversal of inhibitory postsynaptic potentials (IPSPs) and/or blockade of IPSPs was seen after 2-3 min of hypoxia. 6. During hypoxia, the membrane potential of stage 2 expiratory neurones showed a slight depolarization to -45 to -55 mV and then remained stable. 7. The neurone input resistance increased initially and then decreased significantly during central apnoea. 8. Rhythmogenesis of respiration was greatly disturbed. This was due to blockade of IPSPs and, in some animals, to more complex disturbances of phase switching from inspiration to expiration. 9. Central apnoea occurred while respiratory neurones were still excitable as shown by stimulus-evoked orthodromic and antidromic action potentials. 10. The results indicate that the medullary respiratory network is directly affected by energy depletion. There is indication for a neurohumoral mechanism which blocks synaptic interaction between respiratory neurones in chemoreceptor-intact animals.

Efferent phrenic nerve and respiratory neuron activities in the developing kitten: spontaneous discharges and hypoxic responses

Efferent phrenic nerve and medullary respiratory neuron discharges were examined for age-dependent changes of activities during normocapnic hyperoxia and hypoxia in anesthetized and decerebrate kittens (22-150 days old). In animals less than 39 days of age, phrenic power spectra during hyperoxia were dominated by components in the medium-frequency band (20-50 Hz), whereas spectra of animals of at least 39 days of age were dominated by components in the high-frequency band (50-100 Hz). Such high-frequency oscillations were also observed in the power spectra of some inspiratory neurons in animals of at least 43 days old. In hypoxia, the amplitude of phrenic discharge exhibited an initial facilitation followed by a diminution (i.e. biphasic response) in animals 39 days old or younger. In animals older than 39 days, however, hypoxia elicited a sustained facilitation of phrenic discharge amplitude. In contrast, no such age-dependent change in response pattern to hypoxia was observed for neuronal discharges; rather, responses of most neurons consisted of either decreases of discharge frequency, or complete abolishment of discharges.

Brain transections demonstrate the central origin of hypoxic ventilatory depression in carotid body-denervated rats

1. The characteristics of hypoxic ventilation were studied in awake adult rats after brain transections about the intercollicular level. The results were compared with studies made before transection, 17-24 h after bilateral carotid body denervation effected by carotid sinus nerve section. 2. Transection at or below the intercollicular level converted the depressive hypoxic frequency response of control studies to a stimulatory response, increased the stimulation of tidal volume by hypoxia, and so converted the dominant pattern of ventilation in hypoxia from a depression to a progressive stimulation. 3. Transection above the intercollicular level failed to reverse the hypoxic frequency response but increased the stimulation of tidal volume by hypoxia. Consequently minute ventilation progressively increased as the severity of hypoxia increased. 4. The experiments demonstrate that in the adult rat depression of respiratory frequency by hypoxia after carotid body denervation requires the integrity of a region at or immediately above the intercollicular level. In contrast, the stimulation of tidal volume by hypoxia is markedly dependent upon precollicular structures. 5. The results are discussed in relation to the hypoxic depression of fetal breathing and the biphasic hypoxic ventilatory response of the newborn.

Respiratory-modulated activities of motor units of the facial nerve

The purpose of this work was to characterize the influence of activity of vagal pulmonary receptors upon the discharge pattern of motor units of the facial nerve. Decerebrate and paralyzed cats were ventilated with a servo-respirator which produced pulmonary inflations in parallel with activity of the phrenic nerve. At normocapnia, facial units discharged phasically during neural inspiration, expiration or across both phases or discharged tonically throughout the respiratory cycle. When pulmonary inflation was withheld, the tonic discharge of some units became phasic; others changed the pattern of phasic discharge. In hypercapnia, the number of tonic fiber activities increased and, again, some phasic discharge patterns were altered. Withholding inflation caused similar alterations as in normocapnia. Activities of facial fibers in vagotomized animals differed in that no tonic activities were recorded, and no change in phasic discharge patterns was induced by hypercapnia. We conclude that afferents from pulmonary stretch receptors influence ventilatory activity throughout the entire respiratory cycle. The concept is discussed that the tonic, as well as phasic discharge of these receptors, is important for the regulation of activity of motoneurons to upper airway muscles.