Functions of the retrofacial nucleus in chemosensitivity and ventilatory neurogenesis

Proton detection and breathing regulation by the retrotrapezoid nucleus

We discuss recent evidence which suggests that the principal central respiratory chemoreceptors are located within the retrotrapezoid nucleus (RTN) and that RTN neurons are directly sensitive to [H(+) ]. RTN neurons are glutamatergic. In vitro, their activation by [H(+) ] requires expression of a proton-activated G protein-coupled receptor (GPR4) and a proton-modulated potassium channel (TASK-2) whose transcripts are undetectable in astrocytes and the rest of the lower brainstem respiratory network. The pH response of RTN neurons is modulated by surrounding astrocytes but genetic deletion of RTN neurons or deletion of both GPR4 and TASK-2 virtually eliminates the central respiratory chemoreflex. Thus, although this reflex is regulated by innumerable brain pathways, it seems to operate predominantly by modulating the discharge rate of RTN neurons, and the activation of RTN neurons by hypercapnia may ultimately derive from their intrinsic pH sensitivity. RTN neurons increase lung ventilation by stimulating multiple aspects of breathing simultaneously. They stimulate breathing about equally during quiet wake and non-rapid eye movement (REM) sleep, and to a lesser degree during REM sleep. The activity of RTN neurons is regulated by inhibitory feedback and by excitatory inputs, notably from the carotid bodies. The latter input operates during normo- or hypercapnia but fails to activate RTN neurons under hypocapnic conditions. RTN inhibition probably limits the degree of hyperventilation produced by hypocapnic hypoxia. RTN neurons are also activated by inputs from serotonergic neurons and hypothalamic neurons. The absence of RTN neurons probably underlies the sleep apnoea and lack of chemoreflex that characterize congenital central hypoventilation syndrome.

Proposal for mechanisms of protection of supine sleep against sudden infant death syndrome: an integrated mechanism review

Supine sleep decreases sudden infant death syndrome (SIDS) incidence, however the mechanisms for this are unclear. The triple risk model for SIDS requires that one or more underlying abnormalities of breathing or autonomic control are present; these are rare, but brainstem defects are found in most SIDS cases. Supine sleep increases sympathetic nervous system tone, and level of state organization, and may therefore act as a stressor. This is evidenced by physiological arousal, and by delayed neurodevelopment in supine compared to prone sleepers. It is argued here that prone sleep position is the biological normative standard in healthy infants, supporting autonomic regulation. During rapid eye movement (REM) sleep (and other circumstances), a parasympathetic-mediated adverse autonomic event (AAE) may be spontaneously triggered. In healthy infants, gasping initiates autoresuscitation and recovery.

HYPOTHESIS

The underlying vulnerability to SIDS is specific to autoresuscitation from an AAE, the initial serotonin-dependent gasp is commonly compromised. Serotonin metabolism defects also influence sleep architecture, increasing the likelihood of AAE. The mechanism whereby supine sleep decreases SIDS may therefore be a stressor effect, disturbing sleep architecture to decrease REM and AAEs, and increasing sympathetic tone, which may prevent and counteract the purely parasympathetic-mediated AAE, thereby decreasing the risk of SIDS.

Computational models and emergent properties of respiratory neural networks

Computational models of the neural control system for breathing in mammals provide a theoretical and computational framework bringing together experimental data obtained from different animal preparations under various experimental conditions. Many of these models were developed in parallel and iteratively with experimental studies and provided predictions guiding new experiments. This data-driven modeling approach has advanced our understanding of respiratory network architecture and neural mechanisms underlying generation of the respiratory rhythm and pattern, including their functional reorganization under different physiological conditions. Models reviewed here vary in neurobiological details and computational complexity and span multiple spatiotemporal scales of respiratory control mechanisms. Recent models describe interacting populations of respiratory neurons spatially distributed within the Bötzinger and pre-Bötzinger complexes and rostral ventrolateral medulla that contain core circuits of the respiratory central pattern generator (CPG). Network interactions within these circuits along with intrinsic rhythmogenic properties of neurons form a hierarchy of multiple rhythm generation mechanisms. The functional expression of these mechanisms is controlled by input drives from other brainstem components,including the retrotrapezoid nucleus and pons, which regulate the dynamic behavior of the core circuitry. The emerging view is that the brainstem respiratory network has rhythmogenic capabilities at multiple levels of circuit organization. This allows flexible, state-dependent expression of different neural pattern-generation mechanisms under various physiological conditions,enabling a wide repertoire of respiratory behaviors. Some models consider control of the respiratory CPG by pulmonary feedback and network reconfiguration during defensive behaviors such as cough. Future directions in modeling of the respiratory CPG are considered.

Ventrolateral medullary functional connectivity and the respiratory and central chemoreceptor-evoked modulation of retrotrapezoid-parafacial neurons

The medullary ventral respiratory column (VRC) of neurons is essential for respiratory motor pattern generation; however, the functional connections among these cells are not well understood. A rostral extension of the VRC, including the retrotrapezoid nucleus/parafacial region (RTN-pF), contains neurons responsive to local perturbations of CO(2)/pH. We addressed the hypothesis that both local RTN-pF interactions and functional connections from more caudal VRC compartments--extending from the Bötzinger and pre-Bötzinger complexes to the ventral respiratory group (Böt-VRG)--influence the respiratory modulation of RTN-pF neurons and their responses to central chemoreceptor and baroreflex activation. Spike trains from 294 RTN-pF and 490 Böt-VRG neurons were monitored with multielectrode arrays along with phrenic nerve activity in 14 decerebrate, vagotomized cats. Overall, 214 RTN-pF and 398 Böt-VRG neurons were respiratory modulated; 124 and 95, respectively, were cardiac modulated. Subsets of these neurons were tested with sequential, selective, transient stimulation of central chemoreceptors and arterial baroreceptors; each cell's response was evaluated and categorized according to the change in firing rate (if any) following the stimulus. Cross-correlation analysis was applied to 2,884 RTN-pF↔RTN-pF and 8,490 Böt-VRG↔RTN-pF neuron pairs. In total, 174 RTN-pF neurons (59.5%) had significant features in short-time scale correlations with other RTN-pF neurons. Of these, 49 neurons triggered cross-correlograms with offset peaks or troughs (n = 99) indicative of paucisynaptic excitation or inhibition of the target. Forty-nine Böt-VRG neurons (10.0%) were triggers in 74 Böt-VRG→RTN-pF correlograms with offset features, suggesting that Böt-VRG trigger neurons influence RTN-pF target neurons. The results support the hypothesis that local RTN-pF neuron interactions and inputs from Böt-VRG neurons jointly contribute to respiratory modulation of RTN-pF neuronal discharge patterns and promotion or limitation of their responses to central chemoreceptor and baroreceptor stimulation.

Respiratory responses evoked by blockades of ionotropic glutamate receptors within the Bötzinger complex and the pre-Bötzinger complex of the rabbit

The respiratory role of excitatory amino acid (EAA) receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (pre-BötC) was investigated in alpha-chloralose-urethane anaesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30-50 nL) of EAA receptor antagonists. Blockade of both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors by 50 mM kynurenic acid (KYN) within the BötC induced a pattern of breathing characterized by low-amplitude, high-frequency irregular oscillations superimposed on tonic phrenic activity and successively the disappearance of respiratory rhythmicity in the presence of intense tonic inspiratory discharges (tonic apnea). KYN microinjections into the pre-BötC caused similar respiratory responses that, however, never led to tonic apnea. Blockade of NMDA receptors by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 1, 10 and 20 mM) within the BötC induced increases in respiratory frequency and decreases in peak phrenic amplitude; the highest concentrations caused tonic apnea insensitive to chemical stimuli. Blockade of non-NMDA receptors by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 10 and 20 mM) within the BötC produced only less pronounced increases in respiratory frequency. Responses to D-AP5 in the pre-BötC were similar, although less pronounced than those elicited in the BötC and never characterized by tonic apnea. In the same region, CNQX provoked increases in respiratory frequency similar to those elicited in the BötC, associated with slight reductions in peak phrenic activity. The results show that EAA receptors within the investigated medullary subregions mediate a potent control on both the intensity and frequency of inspiratory activity, with a major role played by NMDA receptors.

Effects of lignocaine blockades and kainic acid lesions in the Bötzinger complex on spontaneous expiratory activity and cough reflex responses in the rabbit

We investigated the role played by Bötzinger complex (Böt. c.) region in the genesis of the cough reflex and expiratory drive to expiratory neurons of the caudal ventral respiratory group (cVRG) in pentobarbitone-anesthetized spontaneously breathing rabbits. Phrenic nerve and abdominal muscle activities were monitored. Microinjections (30-50 nl) of 4% lignocaine or 4.7 mM kainic acid in the Böt. c. region suppressed spontaneous rhythmic expiratory activity as well as the inspiratory and expiratory components of the cough reflex evoked by mechanical stimulation of the tracheobronchial tree. These results support the view that neurons located in the Böt. c. have an important role not only in the genesis of the synaptic drive to cVRG expiratory neurons, but also in determining the overall characteristics of the cough motor pattern.

Respiratory changes induced by kainic acid lesions in rostral ventral respiratory group of rabbits

The role played by the Bötzinger complex (BötC), the pre-Bötzinger complex (pre-BötC), and the more rostral extent of the inspiratory portion of the ventral respiratory group (iVRG) in the genesis of the eupneic pattern of breathing was investigated in anesthetized, vagotomized, paralyzed, and artificially ventilated rabbits by means of kainic acid (KA, 4.7 mM) microinjections (20-30 nl). Unilateral KA microinjections into all of the investigated VRG subregions caused increases in respiratory frequency associated with moderate decreases in peak phrenic amplitude in the BötC and pre-BötC regions. Bilateral KA microinjections into either the BötC or pre-BötC transiently eliminated respiratory rhythmicity and caused the appearance of tonic phrenic activity ("tonic apnea"), whereas injections into the rostral iVRG completely suppressed inspiratory activity. Rhythmic activity resumed as low-amplitude, high-frequency oscillations and displayed a progressive, although incomplete, recovery. Combined bilateral KA microinjections (BötC and pre-BötC) caused persistent (>3 h) tonic apnea. Results show that all of the investigated VRG subregions exert a potent control on both the intensity and frequency of inspiratory activity, thus suggesting that these areas play a major role in the genesis of the eupneic pattern of breathing.

Role of the Bötzinger complex in fastigial nucleus-mediated respiratory responses

We have reported that the phrenic neurogram (PN) is modulated by stimulation of the fastigial nucleus (FN) of the cerebellum. The present study was undertaken to search for brainstem site(s) involved in the FN efferent pathway to modulate phrenic nerve activities. Experiments were performed on 35 anesthetized, paralyzed, and ventilated cats, using the PN as the index of the respiratory motor output. Results showed that bilateral electrolytic lesions of the red nucleus (RN), the paramedian reticular nucleus (PRN), or the pontine respiratory group (PRG) had little effect on the ability of FN stimulation to modulate the respiratory output. However, the modulation was abolished by bilateral electrolytic lesions of the Bötzinger complex (BötC). Further studies showed that bilateral chemical inactivation of BötC neurons produced by topical microinjection of kainic acid or cobalt chloride failed to abolish the modulation. We concluded that fibers of passage, not synapses or cell bodies in the BötC, were involved in the modulatory effect of FN stimulation on the PN. The RN, PRN, and PRG appear not to be important in the neural circuitry responsible for the FN modulation of the phrenic activity.

The involvement of rostral ventromedullary neuronal structures in regulating the mechanism of formation of the respiratory rhythm in rats

The role of neuronal structures in the rostral parts of the ventral surface of the medulla oblongata of the rat in regulating the central inspiratory activity of the respiratory center was analyzed. It is suggested that neuronal structures of the subretrofascial area, located close to the ventral surface of the medulla oblongata have direct associations with the mechanisms generating and regulating the respiratory rhythm. These have excitatory effects on neurons of the respiratory center which generate inspiratory activity.

Respiratory effects of kynurenic acid microinjected into the ventromedullary surface of the rat

Several studies demonstrate that, within the ventral medullary surface (VMS), excitatory amino acids are necessary components of the neural circuits involved in the tonic and reflex control of respiration and circulation. In the present study we investigated the cardiorespiratory effects of unilateral microinjections of the broad spectrum glutamate antagonist kynurenic acid (2 nmol/200 nl) along the VMS of urethane-anesthetized rats. Within the VMS only one region was responsive to this drug. This area includes most of the intermediate respiratory area, partially overlapping the rostral ventrolateral medulla (IA/RVL). When microinjected into the IA/RVL, kynurenic acid produced a respiratory depression, without changes in mean arterial pressure or heart rate. The respiratory depression observed was characterized by a decrease in ventilation, tidal volume and mean inspiratory flow and an increase in respiratory frequency. Therefore, the observed respiratory depression was entirely due to a reduction in the inspiratory drive. Microinjections of vehicle (200 nl of saline) into this area produced no significant changes in breathing pattern, blood pressure or heart rate. Respiratory depression in response to the blockade of glutamatergic receptors inside the rostral VMS suggests that neurons at this site have an endogenous glutamatergic input controlling the respiratory cycle duration and the inspiratory drive transmission.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

PreBötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation

Identification of the sites and mechanisms underlying the generation of respiratory rhythm is of longstanding interest to physiologists and neurobiologists. Recently, with the development of novel experimental preparations, especially in vitro en bloc and slice preparations of rodent brainstem, progress has been made In particular, a site in the ventrolateral medulla, the preBötzinger Complex, is hypothesized to contain neuronal circuits generating respiratory rhythm. Lesions or disruption of synaptic transmission within the preBötzinger Complex, either in vivo or in vitro, can abolish respiratory activity. Furthermore, the persistence of respiratory rhythm following interference with postsynaptic inhibition and the subsequent discovery of neurons with endogenous bursting properties within the preBötzinger Complex have led to the hypothesis that rhythmogenesis results from synchronized activity of pacemaker or group-pacemaker neurons.

Medullary loci critical for expression of gasping in adult rats

1. Our purpose was to define whether a region of medulla could be identified that is critical for the expression of gasping. 2. Decerebrate, vagotomized, paralysed and ventilated adult rats were used. The pattern of phrenic activity was reversibly altered from eupnoea to gasping by exposure to hypoxia or anoxia. 3. Gasping was irreversibly eliminated following unilateral electrolytic lesions of the lateral tegmental field of the medulla. The eupnoeic rhythm continued after these lesions. 4. Injections of kainic acid into the lateral tegmental field also eliminated gasping. Phrenic activity in eupnoea was not altered. 5. Lesions outside the lateral tegmental field caused marked changes in the eupnoeic rhythm, including expiratory apnoea. Upon exposure to hypoxia or anoxia, gasping was still induced. 6. This region for the neurogenesis of gasping in rats is identical to the region that serves a comparable function in cats. Moreover, it overlaps with the 'pre-Bötzinger' complex which has been described for the in vitro brainstem preparation of the neonatal rat. Our results raise doubts that this complex plays a role in the neurogenesis of eupnoea.

Brain areas essential or non-essential for emesis

This study was undertaken to better delineate those brain regions that are either essential or non-essential for vomiting. Fictive vomiting, identified by a characteristic pattern of respiratory nerve discharge, was induced by a combination of emetic drugs and electrical stimulation of abdominal vagal afferents in decerebrate, paralyzed cats. Regions non-essential for coordinating vomiting included the entire cerebellum, structures rostral to the medullary retrofacial nucleus, and spinal cord. Fictive coughing was also elicited following cerebellar removal but was not studied after other procedures. The respiratory-related components of fictive vomiting were abolished by large lesions or kainic acid injections in the lateral medulla at the level of the retrofacial nucleus, where respiratory pre-motor and motor neurons are known to exist. Electrical stimulation of this region of the brainstem failed to evoke vomiting. The results of the present study are consistent with our previous electrical stimulation [Brain Res., 270 (1983) 154-158] and c-fos [J. Neurosci., 14 (1994) 871-888] studies and the hypothesis that emesis is coordinated not by a unique, well-defined 'vomiting center' but rather by a distributed control system located in the medulla between the levels of the obex and the retrofacial nucleus.

Characteristics of slow bursting activities recorded in cervical ventral roots in the in vitro brainstem-spinal cord preparation of the neonatal rat

The aim of the present work was to disclose, through pharmacological activation of an isolated central nervous system maintained in vitro, spinal locomotor and respiratory-like activities inferred from an in vivo rabbit preparation. In a brainstem-spinal cord preparation in neonatal rats (0-3 days old), medullary respiratory activity occurred spontaneously in the cervical ventral roots. During 5-hydroxytryptophan (5-HTP) superfusion (0.2 mM), a slower rhythm with longer burst duration developed in the same ventral roots, with the pre-existing long-lasting slow bursting (LLSB) activity. At the same time, locomotor bursts were recorded from lumbar ventral roots. The LLSB activity was mainly recorded in cervical ventral roots, but they could also be encountered at the lumbar level, where they were eliminated after thoracic transection. The LLSB activity and the locomotor bursting were maintained after a C1 or C2 spinal transection, whereas medullary activity disappeared. Bilateral recording of the three types of rhythmic activity demonstrated that the LLSB activity and the medullary respiratory bursting typically displayed a synchronous bilateral coupling, whereas at caudal levels an alternate bilateral pattern was the rule for locomotor activity. Lactic acid could reinduce LLSB activity if introduced after it had just disappeared during the washout phase following 5-HTP superfusion. These results strongly suggest that the LLSB activity that originates from cervical generators belongs to the respiratory system, and not to locomotor activity. Finally, similar results in an in vivo rabbit preparation have been obtained through pharmacological activation. This preparation appears to be a suitable model for the analysis of this cervical burst generator and for the study of interactions among the different pattern generators.

Responses of respiratory modulated and tonic units in the retrotrapezoid nucleus to CO2

We hypothesized that the retrotrapezoid nucleus (RTN) contains both respiratory modulated (RM) and non-respiratory modulated (NRM) neurons which participate in the ventilatory response to increased CO2. We made extracellular recordings of the activity of 46 single units in the RTN of 9 decerebrate, paralyzed, ventilated cats (5 intact; 4 with carotid body and sinus ablation) under eucapnic (PCO2 = 34.2 +/- 3.5 mmHg; mean +/- SD) and hypercapnic (PCO2 = 47.4 +/- 3.4 conditions. To define a RM unit, we used the eta 2 statistic which is the ratio of the variance of the unit firing rate within respiratory cycles to that across respiratory cycles. We classified the units as RM (N = 17) if the eta 2 values in eucapnia or hypercapnia were > or = 0.25 and as NRM (N = 29) if the values were < 0.25. Overall, 19/46 units (41%) increased their firing rate with increased CO2, 5 decreased their firing rate, and 22 had no significant change in firing rate. Of 17 RM units, 8 (47%) increased their mean firing rate with hypercapnia from 7.6 +/- 3.9 to 23.2 +/- 6.8 spikes/sec. These included 5 inspiratory units, 2 inspiratory units that had an onset of firing in late expiration (Pre-I/I), and 1 expiratory unit. Seven of these also changed their discharge pattern (eucapnic eta 2 = 0.02 to 0.12; hypercapnic eta 2 = 0.34 to 0.79) Of 29 NRM units, 11 (38%) showed a significant increase in mean firing rate with CO2 stimulation from 19.8 +/- 7.2 to 31.3 +/- 8.2 spikes/sec. The RTN has RM units which change their discharge pattern and firing rate in response to increased CO2, as do units within the medulla and pons, and it has NRM units which are also responsive to increased CO2. These data indicate that some neurons of the RTN are involved in the central chemoreceptor response but they provide no direct evidence that chemoreception resides within the RTN.

Expiratory neurons of the Bötzinger Complex in the rat: a morphological study following intracellular labeling with biocytin

The term "Bötzinger Complex" (BOT) refers to a distinct group of neurons, located near the rostral portion of the nucleus ambiguus, which are known to play an important role in the control of respiratory movements. Previous studies conducted in cats have demonstrated that most of these neurons are active during expiration, exerting a monosynaptic inhibitory action on several subpopulations of inspiratory neurons in the medulla and spinal cord. The aim of this study was to examine morphological properties and possible synaptic targets of BOT neurons in the rat. Forty-one expiratory neurons were labeled intracellularly with biocytin; 12 were interneurons (BOT neurons) and 29 were motoneurons. The latter could not be antidromically activated following stimulation of the superior laryngeal or vagal nerves. BOT neurons showed extensive axonal arborisations in the ipsilateral medulla, with some projections to the contralateral side. Bouton-like axon varicosities mainly clustered in two areas: near the parent cell bodies, and in the area corresponding to the rostral part of the ventral respiratory group (VRG). In five pairs of labeled neurons, each consisting of one BOT neuron and one inspiratory neuron in the rostral VRG, no appositions were identified at the light microscopic level between axons of BOT neurons and dendrites or cell bodies of inspiratory neurons. These results demonstrate that some features of BOT expiratory neurons in the rat are similar to those previously described in cats. The differences include their more ventral location in relation to the compact formation of nucleus ambiguus (retrofacial nucleus), and the relative paucity in the rat of neurons displaying an augmenting pattern of activity and of neurons with spinally projecting axons. In addition, we were unable to find morphological evidence for contacts between labeled BOT neurons and ipsilateral inspiratory neurons near the obex level, a finding not consistent with previous electrophysiological studies in the cat in which such synaptic connections have been identified.

Modulation of respiratory reflexes by an excitatory amino acid mechanism in the ventrolateral medulla

Results from several studies suggest that the ventrolateral medulla (VLM) is involved in modulating the respiratory response to central and/or peripheral chemoreceptor stimulation. Furthermore, the excitatory amino acid (EAA) glutamate has been shown to have marked effects on respiration when administered to VLM sites. The purpose of this study was to determine if an excitatory amino acid mechanism in the VLM modulates the respiratory responses to hypoxia or hypercapnia in anesthetized rats. Exposure to hypoxic or hypercapnic gas under control conditions elicited increases in respiratory activity (diaphragmatic EMG activity and breathing frequency). Bilateral injection of kynurenic acid (KYN), an EAA antagonist, into rostral VLM sites evoked significant increases in breathing frequency; injections more caudal in the VLM typically caused apnea. Significantly larger increases in respiratory output were elicited by both hypoxia and hypercapnia after rostral VLM microinjections of KYN. The accentuated responses returned to control levels after a recovery of approximately 100 min. Microinjection of xanthurenic acid (XAN), an inactive analog of kynurenic acid, into the VLM prior to KYN had only slight effects on resting respiratory activity and no effects on the responses to hypoxia or hypercapnia. These results suggest two separate VLM sites which modulate respiration by EAA mechanisms. A more rostral site tonically inhibits respiratory activity and the respiratory responses to chemoreceptor stimulation and more caudal VLM sites may be required for the maintenance of respiratory activity.