Maintenance of gasping and restoration of eupnea after hypoxia is impaired following blockers of alpha1-adrenergic receptors and serotonin 5-HT2 receptors.

In severe hypoxia or ischemia, normal eupneic breathing fails and is replaced by gasping. Gasping serves as part of a process of autoresuscitation by which eupnea is reestablished. Medullary neurons, having a burster, pacemaker discharge, underlie gasping. Conductance through persistent sodium channels is essential for the burster discharge. This conductance is modulated by norepinephrine, acting on alpha 1-adrenergic receptors, and serotonin, acting on 5-HT2 receptors. We hypothesized that blockers of 5-HT2 receptors and alpha 1-adrenergic receptors would alter autoresuscitation. The in situ perfused preparation of the juvenile rat was used. Integrated phrenic discharge was switched from an incrementing pattern, akin to eupnea, to the decrementing pattern comparable to gasping in hypoxic hypercapnia. With a restoration of hyperoxic normocapnia, rhythmic, incrementing phrenic discharge returned within 10 s in most preparations. Following addition of blockers of alpha 1-adrenergic receptors (WB-4101, 0.0625-0.500 microM) and/or blockers of 5-HT2 (ketanserin, 1.25-10 microM) or multiple 5-HT receptors (methysergide, 3.0-10 microM) to the perfusate, incrementing phrenic discharge continued. Fictive gasping was still induced, although it ceased after significantly fewer decrementing bursts than in preparations than received no blockers. Moreover, the time for recovery of rhythmic activity was significantly prolonged. This prolongation was in excess of 100 s in all preparations that received both WB-4101 (above 0.125 microM) and methysergide (above 2.5 microM). We conclude that activation of adrenergic and 5-HT2 receptors is important to sustain gasping and to restore rhythmic respiratory activity after hypoxia-induced depression.     

Respiratory neural activities after caudal-to-rostral ablation of medullary regions

The purpose is to assess the importance of medullary mechanisms for the neurogenesis of eupnea. Cats that were used were decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated. Activities of the phrenic, facial, and mylohyoid nerves were monitored. Progressive caudal-to-rostral transections of the spinal cord and medulla were performed. Phrenic activity was eliminated by C1 spinal transections. Only modest changes in facial and mylohyoid activities resulted from transections as far rostral as the level of the dorsal respiratory nucleus. Rhythmic discharges ceased on transections at the pontomedullary junction. However, rhythmic mylohyoid discharges were maintained if protriptyline and strychnine were administered before and during the transection. In other studies rhythmic phrenic, facial, and mylohyoid discharges continued, albeit with an altered rhythm, after destruction of neurons in the dorsal respiratory nucleus by kainic acid. We conclude that caudal medullary mechanisms do not play an essential role in the neurogenesis of breathing movements. Rather, structures in rostral medulla and pons appear necessary for sustaining eupneic neural activities. The concept of multiple brain stem sites for ventilatory neurogenesis is discussed.     

Genesis of rhythmic respiratory activity in pons independent of medulla

We hypothesized that rhythmic respiratory-related activity could be generated in pons independent of medullary mechanisms. In decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats, we recorded efferent activities of the phrenic nerve and mylohyoid branch of the trigeminal nerve. Following transections of the brain stem at the pontomedullary junction, the phrenic and trigeminal nerves discharged with independent rhythms. Spontaneous trigeminal discharges eventually ceased but were reestablished after strychnine, doxapram, and/or protriptyline were administered. In some animals having no spontaneous trigeminal discharges after transection, these discharges appeared, with a rhythm different from the phrenic, following administration of these agents. In other cats having no transections between pons and medulla, these pharmacological agents induced trigeminal and phrenic discharges after kainic acid had been injected into the entire dorsal and ventral medullary respiratory nuclei. Phrenic and trigeminal discharges were linked, indicating survival of bulbospinal neurons or presence of pontospinal units. We conclude that rhythms, similar to respiratory rhythm, can occur by mechanisms in isolated pons. Such mechanisms are hypothesized to be within the pneumotaxic center and may underlie the neurogenesis of eupnea.     

Differing activities of medullary respiratory neurons in eupnea and gasping

Our purpose was to compare further eupneic ventilatory activity with that of gasping. Decerebrate, paralyzed, and ventilated cats were used; the vagi were sectioned within the thorax caudal to the laryngeal branches. Activities of the phrenic nerve and medullary respiratory neurons were recorded. Antidromic invasion was used to define bulbospinal, laryngeal, or not antidromically activated units. The ventilatory pattern was reversibly altered to gasping by exposure to 1% carbon monoxide in air. In eupnea, activities of inspiratory neurons commenced at various times during inspiration, and for most the discharge frequency gradually increased. In gasping, the peak discharge frequency of inspiratory neurons was unaltered. However, all commenced activities at the start of the phrenic burst and reached peak discharge almost immediately. The discharge frequencies of all groups of expiratory neurons fell in gasping, with many neurons ceasing activity entirely. These data are consistent with the hypothesis that brain stem mechanisms controlling eupnea and gasping differ fundamentally.     

Contribution of the respiratory rhythm to sinus arrhythmia in normal unanesthetized subjects during positive-pressure mechanical hyperventilation

The precise contribution of the CO2-dependent respiratory rhythm to sinus arrhythmia in eupnea is unclear. The respiratory rhythm and sinus arrhythmia were measured in 12 normal, unanesthetized subjects in normocapnia and hypocapnia during mechanical hyperventilation with positive pressure. In normocapnia (41 +/- 1 mmHg), the respiratory rhythm was always detectable from airway pressure and inspiratory electromyogram activity. The amplitude of sinus arrhythmia (138 +/- 21 ms) during mechanical hyperventilation with positive pressure was not significantly different from that in eupnea. During the same mechanical hyperventilation pattern but in hypocapnia (24 +/- 1 mmHg), the respiratory rhythm was undetectable and the amplitude of sinus arrhythmia was significantly reduced (to 40 +/- 5 ms). These results show a greater contribution to sinus arrhythmia from the respiratory rhythm during hypocapnia caused by mechanical hyperventilation than previously indicated in normal subjects during hypocapnia caused by voluntary hyperventilation. We discuss whether the respiratory rhythm provides the principal contribution to sinus arrhythmia in eupnea.     

Opioid-induced quantal slowing reveals dual networks for respiratory rhythm generation

Current consensus holds that a single medullary network generates respiratory rhythm in mammals. Pre-B?tzinger Complex inspiratory (I) neurons, isolated in transverse slices, and preinspiratory (pre-I) neurons, found only in more intact en bloc preparations and in vivo, are each proposed as necessary for rhythm generation. Opioids slow I, but not pre-I, neuronal burst periods. In slices, opioids gradually lengthened respiratory periods, whereas in more intact preparations, periods jumped nondeterministically to integer multiples of the control period (quantal slowing). These findings suggest that opioid-induced quantal slowing results from transmission failure of rhythmic drive from pre-I neurons to preB?tC I networks, depressed below threshold for spontaneous rhythmic activity. Thus, both I (in the slice), and pre-I neurons are sufficient for respiratory rhythmogenesis.     

A sodium leak current regulates pacemaker activity of adult central pattern generator neurons in Lymnaea stagnalis

The resting membrane potential of the pacemaker neurons is one of the essential mechanisms underlying rhythm generation. In this study, we described the biophysical properties of an uncharacterized channel (U-type channel) and investigated the role of the channel in the rhythmic activity of a respiratory pacemaker neuron and the respiratory behaviour in adult freshwater snail Lymnaea stagnalis. Our results show that the channel conducts an inward leak current carried by Na(+) (I(Leak-Na)). The I(Leak-Na) contributed to the resting membrane potential and was required for maintaining rhythmic action potential bursting activity of the identified pacemaker RPeD1 neurons. Partial knockdown of the U-type channel suppressed the aerial respiratory behaviour of the adult snail in vivo. These findings identified the Na(+) leak conductance via the U-type channel, likely a NALCN-like channel, as one of the fundamental mechanisms regulating rhythm activity of pacemaker neurons and respiratory behaviour in adult animals.     

Ventrolateral medullary respiratory network and a model of cough motor pattern generation

The primaryhypothesis of this study was that the cough motor pattern is produced,at least in part, by the medullary respiratory neuronal network inresponse to inputs from "cough" and pulmonary stretch receptorrelay neurons in the nucleus tractus solitarii. Computer simulations ofa distributed network model with proposed connections from the nucleustractus solitarii to ventrolateral medullary respiratory neuronsproduced coughlike inspiratory and expiratory motor patterns. Predictedresponses of various "types" of neurons (I-DRIVER, I-AUG, I-DEC,E-AUG, and E-DEC) derived from the simulations were tested in vivo.Parallel and sequential responses of functionally characterizedrespiratory-modulated neurons were monitored during fictive cough indecerebrate, paralyzed, ventilated cats. Coughlike patterns in phrenicand lumbar nerves were elicited by mechanical stimulation of theintrathoracic trachea. Altered discharge patterns were measured in mosttypes of respiratory neurons during fictive cough. The resultssupported many of the specific predictions of our cough generationmodel and suggested several revisions. The two main conclusions were asfollows: 1) TheBötzinger/rostral ventral respiratory group neurons implicated inthe generation of the eupneic pattern of breathing also participate inthe configuration of the cough motor pattern.2) This altered activity ofBötzinger/rostral ventral respiratory group neurons istransmitted to phrenic, intercostal, and abdominal motoneurons via thesame bulbospinal neurons that provide descending drive during eupnea.

     

Identification of medullary loci critical for neurogenesis of gasping

We hypothesized that a discrete medullary locus, critical for gasping neurogenesis, could be identified. In decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats, activities of phrenic, hypoglossal, and recurrent laryngeal nerves were monitored. Gasping was induced by freezing the brain stem, via a fork thermode, at the pontomedullary junction. By reversible cooling of the medulla, chemical lesions with kainic acid, and radio-frequency lesions, a critical area for gasping neurogenesis was localized bilaterally 2-3 mm rostral to obex, 2.0-2.5 mm lateral to midline, and 3-4 mm ventral to medullary surface. Electrical stimulation in this area elicited premature gasps, whereas unilateral lesions or lidocaine injections eliminated gasping activities in all nerves. These procedures did not cause similar changes during eupnea. In apneusis, however, lidocaine injections markedly altered the pattern or caused apnea. We conclude that discharge of neurons in a discrete portion of the lateral tegmental field of medulla is required for gasping neurogenesis. Our results are consistent with these neurons comprising the central pattern generator for gasping.     

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

St. John, Walter M. Medullary regions for neurogenesisof gasping: noeud vital or noeuds vitals? J. Appl.Physiol. 81(5): 1865-1877, 1996.Gasping isa critical mechanism for survival in that it serves as a mechanism forautoresuscitation when eupnea fails. Eupnea and gasping are separablepatterns of automatic ventilatory activity in all mammalian speciesfrom the day of birth. The neurogenesis of the gasp is dependent on thedischarge of neurons in the rostroventral medulla. This gasping centeroverlaps a region termed "the pre-Bötzinger complex."Neuronal activities of this complex, characterized in an in vitro brainstem spinal cord preparation of the neonatal rat, have beenhypothesized to underlie respiratory rhythm generation. Yet, therhythmic activity of this in vitro preparation is markedly differentfrom eupnea but identical with gasping in vivo. In eupnea, medullaryneuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of thepontomedullary circuit defining eupneic ventilatory activity. However,these medullary neuronal activities do not appear critical for theneurogenesis of eupnea, per se.

     

Tonic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

The perfused in situ juvenile rat preparation produces phrenic discharge patterns comparable to eupnea and gasping in vivo. These ventilatory patterns of eupnea and gasping differ in multiple aspects, including most prominently the rate of rise of inspiratory activity. Because gasping, but not eupnea, appeared similar after vagotomy in spontaneous breathing preparations, it has been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, efferent activity of the phrenic nerve was recorded during eupnea and gasping in the in situ juvenile rat preparation. Gasping was induced in hypoxic-hypercapnia or ischemia. An increase in the pressure of tonic lung inflation from 1 to 10 cmH2O caused a prolongation of the duration between phrenic bursts in both eupnea or gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation and that the brain stem circuitry generating the respiratory patterns responds to tonic activation of pulmonary stretch receptors in a similar manner in eupnea and gasping. These findings support the homology of eupnea-like phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.     

Time-frequency coherence analysis of phrenic and hypoglossal activity in the decerebrate rat during eupnea, hyperpnea, and gasping

Fast respiratory rhythms include medium- (MFO) and high-frequency oscillations (HFO), which are much faster than the fundamental breathing rhythm. According to previous studies, HFO is characterized by high coherence (Coh) in phrenic (Ph) nerve activity, thereby providing a means of distinguishing between these two types of oscillations. Changes in Coh between the Ph and hypoglossal (XII) nerves during the transition from normal eupnic breathing to gasping have not been characterized. Experiments were performed on nine unanesthetized, chemo- and barodenervated, decerebrate adult rats, in which sustained asphyxia elicited hyperpnea and gasping. A gated time-frequency Coh analysis was developed and applied to whole Ph and medial XII nerve recordings. The results showed dynamic Ph-Ph Coh during eupnea, including MFO and HFO. XII-XII Coh during eupnea was broadband and included four distinct peaks, with low-frequency Coh dominating the epochs preceding the onset of Ph activity. During gasping, only MFO-peaks were present in Ph-Ph Coh. Bilateral XII activity showed a significant reduction in Coh and a shift toward lower frequencies during gasping. In contrast, contralateral Ph-XII Coh progressively increased during state changes from eupnea to gasping, a tendency mirrored in the startup part of the Ph activity. These data suggest significant hypoxia/hypercapnia-induced alterations in synchronization between respiratory outputs during the transition from eupnea to gasping, reflecting a reconfiguration of the respiratory network and/or alterations in the circuitry associated with the motor pools, including dynamic coupling between outputs.     

Voltage-sensitive ion channels in rhythmic motor systems

Voltage-sensitive ionic currents shape both the firing properties of neurons and their synaptic integration within neural networks that drive rhythmic motor patterns. Persistent sodium currents underlie rhythmic bursting in respiratory neurons. H-type pacemaker currents can act as leak conductances in spinal motoneurons, and also control long-term modulation of synaptic release at the crayfish neuromuscular junction. Calcium currents travel in rostro-caudal waves with motoneuron activity in the spinal cord. Potassium currents control spike width and burst duration in many rhythmic motor systems. We are beginning to identify the genes that underlie these currents.     

呼吸中枢发育的基因调控研究进展

脑干呼吸中枢组成一个复杂的网络系统,产生和调控节律性呼吸。近年来,人们利用分子生物学技术研究发现,小鼠某些基因突变可影响呼吸中枢特定神经元的发育,并由此导致特殊的呼吸表型。最近,在人类某些中枢性呼吸疾病中,也发现了相应的基因突变。因此,基因水平的研究为深入认识节律性呼吸活动的产生和调控机制提供了新的研究途径。本文就目前研究较多、相对较深入的一些呼吸中枢发育调节基因的研究进展作一综述。     

Activity of medullary respiratory neurons during ventilator-induced apnea in sleep and wakefulness

Mechanical ventilation of cats in sleep andwakefulness causes apnea, often within two to three cycles of theventilator. We recorded 137 medullary respiratory neurons in four adultcats during eupnea and during apnea caused by mechanical ventilation. We hypothesized that the residual activity of respiratory neurons during apnea might reveal its cause(s). The results showed that residual activity depended on 1) theamount of nonrespiratory inputs to the cell (cells with morenonrespiratory inputs had greater amounts of residual activity);2) the cell type (expiratory cellshad more residual activity than inspiratory cells); and 3) the state of consciousness (moreresidual activity in wakefulness and rapid-eye-movement sleep than innon-rapid-eye-movement sleep). None of the cells showed an activationduring ventilation that could explain the apnea. Residual activity ofapproximately one-half of the cells was modulated in phase with theventilator. The strength of this modulation was quantified by using aneffect-size statistic and was found to be weak. The patterns ofmodulation did not support the idea that mechanoreceptors excite somerespiratory cells that, in turn, inhibit others. Indeed, most cells,inspiratory and expiratory, discharged during the deflation-inflationtransition of ventilation. Residual activity failed to reveal the causeof apnea but showed that during apnea respiratory neurons act as ifthey were disinhibited and disfacilitated.

     

Phasic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

The perfused in situ juvenile rat preparation produces patterns of phrenic discharge comparable to eupnea and gasping in vivo. These ventilatory patterns differ in multiple aspects, including most prominently the rate of rise of inspiratory activity. Although we have recently demonstrated that both eupnea and gasping are similarly modulated by a Hering-Breuer expiratory-promoting reflex to tonic pulmonary stretch, it has generally been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, we recorded eupneic and gasplike efferent activity of the phrenic nerve in the in situ juvenile rat perfused brain stem preparation, with and without phrenic-triggered phasic pulmonary inflation. We tested the hypothesis that phasic pulmonary inflation produces reflex responses in situ akin to those in vivo and that both eupnea and gasping are similarly modulated by phasic pulmonary stretch. In eupnea, we found that phasic pulmonary inflation decreases inspiratory burst duration and the period of expiration, thus increasing burst frequency of the phrenic neurogram. Phasic pulmonary inflation also decreases the duration of expiration and increases the burst frequency during gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation and that the brain stem circuitry generating the respiratory patterns respond to phasic activation of pulmonary stretch receptors in both eupnea and gasping. These findings support the homology of eupneic phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.     

Respiratory pattern generator model using Ca++-induced Ca++ release in neurons shows both pacemaker and reciprocal network properties

There are two contradictory explanations for central respiratory rhythmogenesis. One suggests that respiratory rhythm emerges from interaction between inspiratory and expiratory neural semicenters that inhibit each other and thereby provide reciprocal rhythmic activity (Brown 1914). The other uses bursting pacemaker activity of individual neurons to produce the rhythm (Feldman and Cleland 1982). Hybrid models have been developed to reconcile these two seemingly conflicting mechanisms (Smith et al. 2000; Rybak et al. 2001). Here we report computer simulations that demonstrate a unified mechanism of the two types of oscillator. In the model, we use the interaction of Ca⁺⁺-dependent K⁺ channels (Mifflin et al. 1985) with Ca⁺⁺-induced Ca⁺⁺ release from intracellular stores (McPherson and Campbell 1993), which was recently revealed in neurons (Hernandez-Cruz et al. 1997; Mitra and Slaughter 2002a,b; Scornik et al. 2001). Our computations demonstrate that uncoupled neurons with these intracellular mechanisms show conditional pacemaker properties (Butera et al. 1999) when exposed to steady excitatory inputs. Adding weak inhibitory synapses (based on increased K⁺ conductivity) between two model neural pools surprisingly synchronizes the activity of both neural pools. As inhibitory synaptic connections between the two pools increase from zero to higher values, the model produces first dissociated pacemaker activity of individual neurons, then periodic synchronous bursts of all neurons (inspiratory and expiratory), and finally reciprocal rhythmic activity of the neural pools.     

Neurogenesis, control, and functional significance of gasping

Gasps are frequently the first and last breaths of life. Gasping, which is generated by intrinsic medullary mechanisms, differs fundamentally from other automatic ventilatory patterns. A region of the lateral tegmental field of the medulla is critical for the neurogenesis of the gasp but has no role in eupnea. Neuronal mechanisms in separate brain stem regions may be responsible for the neurogenesis of different ventilatory patterns. This hypothesis is supported by the recording of independent respiratory rhythms simultaneously from isolated brain stem segments. Data from fetal and neonatal animals also support gasping and eupnea being generated by separate mechanisms. Gasping may represent the output of a simple but rugged pattern generator that functions as a backup system until the control system for eupnea is developed. Pacemaker elements are hypothesized as underlying the onset of inspiratory activity in gasping. Similar elements, in a different brain stem region, may be responsible for the onset of the eupneic inspiration with neural circuits involving the pons, the medulla, and the spinal cord serving to shape efferent respiratory-modulated neural discharges.     

Responses of bulbar respiratory neurons to stimulation of receptive fields in the air passages

Extracellular recordings were made of changes in the firing pattern of 74 respiratory neurons in 23 cats anesthetized with Nembutal evoked by blowing atmospheric air into the nose or through an isolated segment of trachea. Respiratory unit (RU) responses were compared with accompanying changes in the activity of inspiratory and expiratory neuromotor units (NMUs) and the intratracheal pressure. These procedures were accompanied by changes in the frequency, depth, and rhythm of respiration and RU and NMU activity was activated or inhibited; RUs of all types responded to these stimuli. Responding RUs were found in various structures of the medullary respiratory center. Most RUs responded differently to stimulation of the air passages and inflation of the lungs. It is concluded that afferent impulses from the nose and trachea spread to all groups of bulbar RUs responsible for generating respiratory movements. This wide extent of the afferent projections of the air passages in structures of the respiratory center could play an important role both in defensive respiratory responses and in the regulation of eupnea.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 620–630, November–December, 1971.     

Rhythmic activity of neurons in the rostral ventrolateral medulla of conscious cats: effect of removal of vestibular inputs

Although it is well established that bulbospinal neurons located in the rostral ventrolateral medulla (RVLM) play a pivotal role in regulating sympathetic nerve activity and blood pressure, virtually all neurophysiological studies of this region have been conducted in anesthetized or decerebrate animals. In the present study, we used time- and frequency-domain analyses to characterize the naturally occurring discharges of RVLM neurons in conscious cats. Specifically, we compared their activity to fluctuations in carotid artery blood flow to identify neurons with cardiac-related (CR) activity; we then considered whether neurons with CR activity also had a higher-frequency rhythmic firing pattern. In addition, we ascertained whether the surgical removal of vestibular inputs altered the rhythmic discharge properties of RVLM neurons. Less than 10% of RVLM neurons expressed CR activity, although the likelihood of observing a neuron with CR activity in the RVLM varied between recording sessions, even when tracking occurred in a very limited area and was higher after vestibular inputs were surgically removed. Either a 10-Hz or a 20- to 30-Hz rhythmic discharge pattern coexisted with the CR discharges in some of the RVLM neurons. Additionally, the firing rate of RVLM neurons, including those with CR activity, decreased after vestibular lesions. These findings raise the prospect that RVLM neurons may or may not express rhythmic firing patterns at a particular time due to a variety of influences, including descending projections from higher brain centers and sensory inputs, such as those from the vestibular system.