CO(2)/H(+) chemoreception in the cat pre-Bötzinger complex in vivo

Contributions of carotid bodies, retrotrapezoid nucleus neurons and preBötzinger complex astrocytes to the CO2 -sensitive drive for breathing

Current models of respiratory CO2 chemosensitivity are centred around the function of a specific population of neurons residing in the medullary retrotrapezoid nucleus (RTN). However, there is significant evidence suggesting that chemosensitive neurons exist in other brainstem areas, including the rhythm-generating region of the medulla oblongata - the preBötzinger complex (preBötC). There is also evidence that astrocytes, non-neuronal brain cells, contribute to central CO2 chemosensitivity. In this study, we reevaluated the relative contributions of the RTN neurons, the preBötC astrocytes, and the carotid body chemoreceptors in mediating the respiratory responses to CO2 in experimental animals (adult laboratory rats). To block astroglial signalling via exocytotic release of transmitters, preBötC astrocytes were targeted to express the tetanus toxin light chain (TeLC). Bilateral expression of TeLC in preBötC astrocytes was associated with ∼20% and ∼30% reduction of the respiratory response to CO2 in conscious and anaesthetized animals, respectively. Carotid body denervation reduced the CO2 respiratory response by ∼25%. Bilateral inhibition of RTN neurons transduced to express Gi-coupled designer receptors exclusively activated by designer drug (DREADDGi ) by application of clozapine-N-oxide reduced the CO2 response by ∼20% and ∼40% in conscious and anaesthetized rats, respectively. Combined blockade of astroglial signalling in the preBötC, inhibition of RTN neurons and carotid body denervation reduced the CO2 -induced respiratory response by ∼70%. These data further support the hypothesis that the CO2 -sensitive drive to breathe requires inputs from the peripheral chemoreceptors and several central chemoreceptor sites. At the preBötC level, astrocytes modulate the activity of the respiratory network in response to CO2 , either by relaying chemosensory information (i.e. they act as CO2  sensors) or by enhancing the preBötC network excitability to chemosensory inputs. KEY POINTS: This study reevaluated the roles played by the carotid bodies, neurons of the retrotrapezoid nucleus (RTN) and astrocytes of the preBötC in mediating the CO2 -sensitive drive to breathe. The data obtained show that disruption of preBötC astroglial signalling, blockade of inputs from the peripheral chemoreceptors or inhibition of RTN neurons similarly reduce the respiratory response to hypercapnia. These data provide further support for the hypothesis that the CO2 -sensitive drive to breathe is mediated by the inputs from the peripheral chemoreceptors and several central chemoreceptor sites.

Does fetal growth restriction induce neuropathology within the developing brainstem?

Fetal growth restriction (FGR) is a complex obstetric issue describing a fetus that does not reach its genetic growth potential. The primary cause of FGR is placental dysfunction resulting in chronic fetal hypoxaemia, which in turn causes altered neurological, cardiovascular and respiratory development, some of which may be pathophysiological, particularly for neonatal life. The brainstem is the critical site of cardiovascular, respiratory and autonomic control, but there is little information describing how chronic hypoxaemia and the resulting FGR may affect brainstem neurodevelopment. This review provides an overview of the brainstem-specific consequences of acute and chronic hypoxia, and what is known in FGR. In addition, we discuss how brainstem structural alterations may impair functional control of the cardiovascular and respiratory systems. Finally, we highlight the clinical and translational findings of the potential roles of the brainstem in maintaining cardiorespiratory adaptation in the transition from fetal to neonatal life under normal conditions and in response to the pathological environment that arises during development in growth-restricted infants. This review emphasises the crucial role that the brainstem plays in mediating cardiovascular and respiratory responses during fetal and neonatal life. We assess whether chronic fetal hypoxaemia might alter structure and function of the brainstem, but this also serves to highlight knowledge gaps regarding FGR and brainstem development.

Intrinsic and synaptic mechanisms controlling the expiratory activity of excitatory lateral parafacial neurones of rats

Active expiration is essential for increasing pulmonary ventilation during high chemical drive (hypercapnia). The lateral parafacial (pF_L ) region, which contains expiratory neurones, drives abdominal muscles during active expiration in response to hypercapnia. However, the electrophysiological properties and synaptic mechanisms determining the activity of pF_L expiratory neurones, as well as the specific conditions for their emergence, are not fully understood. Using whole cell electrophysiology and single cell quantitative RT-PCR techniques, we describe the intrinsic electrophysiological properties, the phenotype and the respiratory-related synaptic inputs to the pF_L expiratory neurones, as well as the mechanisms for the expression of their expiratory activity under conditions of hypercapnia-induced active expiration, using in situ preparations of juvenile rats. We also evaluated whether these neurones possess intrinsic CO₂ /[H⁺ ] sensitivity and burst generating properties. GABAergic and glycinergic inhibition during inspiration and expiration suppressed the activity of glutamatergic pF_L expiratory neurones in normocapnia. In hypercapnia, these neurones escape glycinergic inhibition and generate burst discharges at the end of expiration. Evidence for the contribution of post-inhibitory rebound, Ca_V 3.2 isoform of T-type Ca²⁺ channels and intracellular [Ca²⁺ ] is presented. Neither intrinsic bursting properties, mediated by persistent Na⁺ current, nor CO₂ /[H⁺ ] sensitivity or expression of CO₂ /[H⁺ ] sensitive ion channels/receptors (TASK or GPR4) were observed. On the other hand, hyperpolarisation-activated cyclic nucleotide-gated and twik-related K⁺ leak channels were recorded. Post-synaptic disinhibition and the intrinsic electrophysiological properties of glutamatergic neurones play important roles in the generation of the expiratory oscillations in the pF_L region during hypercapnia in rats. KEY POINTS: Hypercapnia induces active expiration in rats and the recruitment of a specific population of expiratory neurones in the lateral parafacial (pF_L ) region. Post-synaptic GABAergic and glycinergic inhibition both suppress the activity of glutamatergic pF_L neurones during inspiratory and expiratory phases in normocapnia. Hypercapnia reduces glycinergic inhibition during expiration leading to burst generation by pF_L neurones; evidence for a contribution of post-inhibitory rebound, voltage-gated Ca²⁺ channels and intracellular [Ca²⁺ ] is presented. pF_L glutamatergic expiratory neurones are neither intrinsic burster neurones, nor CO₂ /[H⁺ ] sensors, and do not express CO₂ /[H⁺ ] sensitive ion channels or receptors. Post-synaptic disinhibition and the intrinsic electrophysiological properties of glutamatergic neurones both play important roles in the generation of the expiratory oscillations in the pF_L region during hypercapnia in rats.

Astrocytic contribution to glutamate-related central respiratory chemoreception in vertebrates

Central respiratory chemoreceptors play a key role in the respiratory homeostasis by sensing CO₂ and H⁺ in brain and activating the respiratory neural network. This ability of specific brain regions to respond to acidosis and hypercapnia is based on neuronal and glial mechanisms. Several decades ago, glutamatergic transmission was proposed to be involved as a main mechanism in central chemoreception. However, a complete identification of mechanism has been elusive. At the rostral medulla, chemosensitive neurons of the retrotrapezoid nucleus (RTN) are glutamatergic and they are stimulated by ATP released by RTN astrocytes in response to hypercapnia. In addition, recent findings show that caudal medullary astrocytes in brainstem can also contribute as CO₂ and H⁺ sensors that release D-serine and glutamate, both gliotransmitters able to activate the respiratory neural network. In this review, we describe the mammalian astrocytic glutamatergic contribution to the central respiratory chemoreception trying to trace in vertebrates the emergence of several components involved in this process.

Breathing regulation and blood gas homeostasis after near complete lesions of the retrotrapezoid nucleus in adult rats

KEY POINTS

The retrotrapezoid nucleus (RTN) drives breathing proportionally to brain PCO2 but its role during various states of vigilance needs clarification. Under normoxia, RTN lesions increased the arterial PCO2 set-point, lowered the PO2 set-point and reduced alveolar ventilation relative to CO₂ production. Tidal volume was reduced and breathing frequency increased to a comparable degree during wake, slow-wave sleep and REM sleep. RTN lesions did not produce apnoeas or disordered breathing during sleep. RTN lesions in rats virtually eliminated the central respiratory chemoreflex (CRC) while preserving the cardiorespiratory responses to hypoxia; the relationship between CRC and number of surviving RTN Nmb neurons was an inverse exponential. The CRC does not function without the RTN. In the quasi-complete absence of the RTN and CRC, alveolar ventilation is reduced despite an increased drive to breathe from the carotid bodies.

ABSTRACT

The retrotrapezoid nucleus (RTN) is one of several CNS nuclei that contribute, in various capacities (e.g. CO₂ detection, neuronal modulation) to the central respiratory chemoreflex (CRC). Here we test how important the RTN is to PCO₂ homeostasis and breathing during sleep or wake. RTN Nmb-positive neurons were killed with targeted microinjections of substance P-saporin conjugate in adult rats. Under normoxia, rats with large RTN lesions (92 ± 4% cell loss) had normal blood pressure and arterial pH but were hypoxic (-8 mmHg PaO₂ ) and hypercapnic (+10 mmHg ). In resting conditions, minute volume (V_E ) was normal but breathing frequency (f_R ) was elevated and tidal volume (V_T ) reduced. Resting O₂ consumption and CO₂ production were normal. The hypercapnic ventilatory reflex in 65% FiO₂ had an inverse exponential relationship with the number of surviving RTN neurons and was decreased by up to 92%. The hypoxic ventilatory reflex (HVR; FiO₂ 21-10%) persisted after RTN lesions, hypoxia-induced sighing was normal and hypoxia-induced hypotension was reduced. In rats with RTN lesions, breathing was lowest during slow-wave sleep, especially under hyperoxia, but apnoeas and sleep-disordered breathing were not observed. In conclusion, near complete RTN destruction in rats virtually eliminates the CRC but the HVR persists and sighing and the state dependence of breathing are unchanged. Under normoxia, RTN lesions cause no change in V_E but alveolar ventilation is reduced by at least 21%, probably because of increased physiological dead volume. RTN lesions do not cause sleep apnoea during slow-wave sleep, even under hyperoxia.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

Regulation of breathing and autonomic outflows by chemoreceptors

Lung ventilation fluctuates widely with behavior but arterial PCO2 remains stable. Under normal conditions, the chemoreflexes contribute to PaCO2 stability by producing small corrective cardiorespiratory adjustments mediated by lower brainstem circuits. Carotid body (CB) information reaches the respiratory pattern generator (RPG) via nucleus solitarius (NTS) glutamatergic neurons which also target rostral ventrolateral medulla (RVLM) presympathetic neurons thereby raising sympathetic nerve activity (SNA). Chemoreceptors also regulate presympathetic neurons and cardiovagal preganglionic neurons indirectly via inputs from the RPG. Secondary effects of chemoreceptors on the autonomic outflows result from changes in lung stretch afferent and baroreceptor activity. Central respiratory chemosensitivity is caused by direct effects of acid on neurons and indirect effects of CO2 via astrocytes. Central respiratory chemoreceptors are not definitively identified but the retrotrapezoid nucleus (RTN) is a particularly strong candidate. The absence of RTN likely causes severe central apneas in congenital central hypoventilation syndrome. Like other stressors, intense chemosensory stimuli produce arousal and activate circuits that are wake- or attention-promoting. Such pathways (e.g., locus coeruleus, raphe, and orexin system) modulate the chemoreflexes in a state-dependent manner and their activation by strong chemosensory stimuli intensifies these reflexes. In essential hypertension, obstructive sleep apnea and congestive heart failure, chronically elevated CB afferent activity contributes to raising SNA but breathing is unchanged or becomes periodic (severe CHF). Extreme CNS hypoxia produces a stereotyped cardiorespiratory response (gasping, increased SNA). The effects of these various pathologies on brainstem cardiorespiratory networks are discussed, special consideration being given to the interactions between central and peripheral chemoreflexes.

Strain differences in pH-sensitive K+ channel-expressing cells in chemosensory and nonchemosensory brain stem nuclei

The ventilatory CO2 chemoreflex is inherently low in inbred Brown Norway (BN) rats compared with other strains, including inbred Dahl salt-sensitive (SS) rats. Since the brain stem expression of various pH-sensitive ion channels may be determinants of the CO2 chemoreflex, we tested the hypothesis that there would be fewer pH-sensitive K(+) channel-expressing cells in BN relative to SS rats within brain stem sites associated with respiratory chemoreception, such as the nucleus tractus solitarius (NTS), but not within the pre-Bötzinger complex region, nucleus ambiguus or the hypoglossal motor nucleus. Medullary sections (25 μm) from adult male and female BN and SS rats were stained with primary antibodies targeting TASK-1, Kv1.4, or Kir2.3 K(+) channels, and the total (Nissl-stained) and K(+) channel immunoreactive (-ir) cells counted. For both male and female rats, the numbers of K(+) channel-ir cells within the NTS were reduced in the BN compared with SS rats (P < 0.05), despite equal numbers of total NTS cells. In contrast, we found few differences in the numbers of K(+) channel-ir cells among the strains within the nucleus ambiguus, hypoglossal motor nucleus, or pre-Bötzinger complex regions in both male and female rats. However, there were no predicted functional mutations in each of the K(+) channels studied comparing genomic sequences among these strains. Thus we conclude that the relatively selective reductions in pH-sensitive K(+) channel-expressing cells in the NTS of male and female BN rats may contribute to their severely blunted ventilatory CO2 chemoreflex.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

Blockade of Na+/H+ exchanger type 3 causes intracellular acidification and hyperexcitability via inhibition of pH-sensitive K+ channels in chemosensitive respiratory neurons of the dorsal vagal nucleus in rats

Extracellular pH (pHe) and intracellular pH (pHi) are important factors for the excitability of chemosensitive central respiratory neurons that play an important role in respiration and obstructive sleep apnea. It has been proposed that inhibition of central Na(+)/H(+) exchanger 3 (NHE-3), a key pHi regulator in the brainstem, decreases the pHi, leading to membrane depolarization for the maintenance of respiration. However, how intracellular pH affects the neuronal excitability of respiratory neurons remains largely unknown. In this study, we showed that NHE-3 mRNA is widely distributed in respiration-related neurons of the rat brainstem, including the dorsal vagal nucleus (DVN). Whole-cell patch clamp recordings from DVN neurons in brain slices revealed that the standing outward current (Iso) through pH-sensitive K(+) channels was inhibited in the presence of the specific NHE-3 inhibitor AVE0657 that decreased the pHi. Exposure of DVN neurons to an acidified pHe and AVE0657 (5 μmol/L) resulted in a stronger effect on firing rate and Iso than acidified pHe alone. Taken together, our results showed that intracellular acidification by blocking NHE-3 results in inhibition of a pH-sensitive K(+) current, leading to synergistic excitation of chemosensitive DVN neurons for the regulation of respiration.

Peripheral-central chemoreceptor interaction and the significance of a critical period in the development of respiratory control

Respiratory control entails coordinated activities of peripheral chemoreceptors (mainly the carotid bodies) and central chemosensors within the brain stem respiratory network. Candidates for central chemoreceptors include Phox2b-containing neurons of the retrotrapezoid nucleus, serotonergic neurons of the medullary raphé, and/or multiple sites within the brain stem. Extensive interconnections among respiratory-related nuclei enable central chemosensitive relay. Both peripheral and central respiratory centers are not mature at birth, but undergo considerable development during the first two postnatal weeks in rats. A critical period of respiratory development (∼P12-P13 in the rat) exists when abrupt neurochemical, metabolic, ventilatory, and electrophysiological changes occur. Environmental perturbations, including hypoxia, intermittent hypoxia, hypercapnia, and hyperoxia alter the development of the respiratory system. Carotid body denervation during the first two postnatal weeks in the rat profoundly affects the development and functions of central respiratory-related nuclei. Such denervation delays and prolongs the critical period, but does not eliminate it, suggesting that the critical period may be intrinsically and genetically determined.

Persistent lung oscillator response to CO2 after buccal oscillator inhibition in the adult frog

The automatic ventilatory drive in amphibians depends on two oscillators interacting with each other, the gill/buccal and lung oscillators. The lung oscillator would be homologous to the mammalian pre-Bötzinger complex and the gill/buccal oscillator homologous to the mammalian parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN). Dysfunction of the pFRG/RTN has been involved in the development of respiratory diseases associated to the loss of CO(2) chemosensitivity such as the congenital central hypoventilation syndrome. Here, on adult in vitro isolated frog brainstem, consequences of the buccal oscillator inhibition (by reducing Cl(-)) were evaluated on the respiratory rhythm developed by the lung oscillator under hypercapnic challenges. Our results show that under low Cl(-) concentration (i) the buccal oscillator is strongly inhibited and the lung burst frequency and amplitude decreased and (ii) it persists a powerful CO(2) chemosensitivity. In conclusion, in frog, the CO(2) chemosensitivity depends on cellular contingent(s) whose the functioning is independent of the concentration of Cl(-) and origin remains unknown.

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.

Respiration and autonomic regulation and orexin

Orexin, a small neuropeptide released from neurons in the hypothalamus with widespread projections throughout the central nervous system, has broad biological roles including the modulation of breathing and autonomic function. That orexin activity is fundamentally dependent on sleep-wake state, and circadian cycle requires consideration of orexin function in physiological control systems in respect to these two state-related activity patterns. Both transgenic mouse studies and focal orexin receptor antagonism support a role for orexins in respiratory chemosensitivity to CO₂ predominantly in wakefulness, with further observations limiting this role to the dark period. In addition, orexin neurons participate in the regulation of sympathetic activity, including effects on blood pressure and thermoregulation. Orexin is also essential in physiological responses to stress. Orexin-mediated processes may operate at two levels: (1) in sleep-wake and circadian states and (2) in stress, for example, the defense or "fight-or-flight" response and panic-anxiety syndrome.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

Central chemoreceptor modulation of breathing via multipath tuning in medullary ventrolateral respiratory column circuits

Ventrolateral respiratory column (VRC) circuits that modulate breathing in response to changes in central chemoreceptor drive are incompletely understood. We employed multielectrode arrays and spike train correlation methods to test predictions of the hypothesis that pre-Bötzinger complex (pre-BötC) and retrotrapezoid nucleus/parafacial (RTN-pF) circuits cooperate in chemoreceptor-evoked tuning of ventral respiratory group (VRG) inspiratory neurons. Central chemoreceptors were selectively stimulated by injections of CO(2)-saturated saline into the vertebral artery in seven decerebrate, vagotomized, neuromuscularly blocked, and artificially ventilated cats. Among sampled neurons in the Bötzinger complex (BötC)-to-VRG region, 70% (161 of 231) had a significant change in firing rate after chemoreceptor stimulation, as did 70% (101 of 144) of the RTN-pF neurons. Other responsive neurons (24 BötC-VRG; 11 RTN-pF) had a change in the depth of respiratory modulation without a significant change in average firing rate. Seventy BötC-VRG chemoresponsive neurons triggered 189 offset-feature correlograms (96 peaks; 93 troughs) with at least one responsive BötC-VRG cell. Functional input from at least one RTN-pF cell could be inferred for 45 BötC-VRG neurons (19%). Eleven RTN-pF cells were correlated with more than one BötC-VRG target neuron, providing evidence for divergent connectivity. Thirty-seven RTN-pF neurons, 24 of which were chemoresponsive, were correlated with at least one chemoresponsive BötC-VRG neuron. Correlation linkage maps and spike-triggered averages of phrenic nerve signals suggest transmission of chemoreceptor drive via a multipath network architecture: RTN-pF modulation of pre-BötC-VRG rostral-to-caudal excitatory inspiratory neuron chains is tuned by feedforward and recurrent inhibition from other inspiratory neurons and from "tonic" expiratory neurons.

Control of breathing by "nerve glue"

Long regarded as mere structural support for neurons, neuroglial cells are now considered pivotal for brain metabolism, the blood-brain barrier, cerebral hemodynamics, and neuronal function. Multitasking by glia involves numerous signaling and effector pathways that control various processes, including neurotransmitter uptake and release of gliotransmitters, such as glutamate or adenosine 5'-triphosphate (ATP). Acidosis of cerebrospinal fluid causes ATP release from astrocytic glia at the ventral brainstem surface, which excites neighboring brainstem neurons that stimulate neurons in the pre-Bötzinger complex (preBötC), which controls inspiratory breathing movements. New insights into glial regulation of complex behavior, and particularly into respiratory circuit function, are evolving from application of genetically engineered optical stimulation and Ca(2+) imaging tools, combined with other molecular and electrophysiological approaches. These advances in technology will enable direct analyses of respiratory-related neuron-glia interactions not only at the ventral brainstem surface but also within the preBötC, which generates a vital brain rhythm.

Central respiratory chemoreception

By definition central respiratory chemoreceptors (CRCs) are cells that are sensitive to changes in brain PCO(2) or pH and contribute to the stimulation of breathing elicited by hypercapnia or metabolic acidosis. CO(2) most likely works by lowering pH. The pertinent proton receptors have not been identified and may be ion channels. CRCs are probably neurons but may also include acid-sensitive glia and vascular cells that communicate with neurons via paracrine mechanisms. Retrotrapezoid nucleus (RTN) neurons are the most completely characterized CRCs. Their high sensitivity to CO(2) in vivo presumably relies on their intrinsic acid sensitivity, excitatory inputs from the carotid bodies and brain regions such as raphe and hypothalamus, and facilitating influences from neighboring astrocytes. RTN neurons are necessary for the respiratory network to respond to CO(2) during the perinatal period and under anesthesia. In conscious adults, RTN neurons contribute to an unknown degree to the pH-dependent regulation of breathing rate, inspiratory, and expiratory activity. The abnormal prenatal development of RTN neurons probably contributes to the congenital central hypoventilation syndrome. Other CRCs presumably exist, but the supportive evidence is less complete. The proposed locations of these CRCs are the medullary raphe, the nucleus tractus solitarius, the ventrolateral medulla, the fastigial nucleus, and the hypothalamus. Several wake-promoting systems (serotonergic and catecholaminergic neurons, orexinergic neurons) are also putative CRCs. Their contribution to central respiratory chemoreception may be behavior dependent or vary according to the state of vigilance.

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

Central chemoreception in wakefulness and sleep: evidence for a distributed network and a role for orexin

This minireview examines data showing the locations of central chemoreceptor sites as identified by the presence of ventilatory responses to focal, mild acidification produced in unanesthetized animals in vivo, how the site-specific responses vary by arousal state, and what the emerging role of orexin might be in this state-dependent central chemoreceptor system. We comment on the organization of this distributed central chemoreceptor system and suggest that interactions among sites are synergistic and not additive, which is an important aspect of its normal function.

TASK channels contribute to the K+-dominated leak current regulating respiratory rhythm generation in vitro

Leak channels regulate neuronal activity and excitability. Determining which leak channels exist in neurons and how they control electrophysiological behavior is fundamental. Here we investigated TASK channels, members of the two-pore domain K(+) channel family, as a component of the K(+)-dominated leak conductance that controls and modulates rhythm generation at cellular and network levels in the mammalian pre-Bötzinger complex (pre-BötC), an excitatory network of neurons in the medulla critically involved in respiratory rhythmogenesis. By voltage-clamp analyses of pre-BötC neuronal current-voltage (I-V) relations in neonatal rat medullary slices in vitro, we demonstrated that pre-BötC inspiratory neurons have a weakly outward-rectifying total leak conductance with reversal potential that was depolarized by approximately 4 mV from the K(+) equilibrium potential, indicating that background K(+) channels are dominant contributors to leak. This K(+) channel component had I-V relations described by constant field theory, and the conductance was reduced by acid and was augmented by the volatile anesthetic halothane, which are all hallmarks of TASK. We established by single-cell RT-PCR that pre-BötC inspiratory neurons express TASK-1 and in some cases also TASK-3 mRNA. Furthermore, acid depolarized and augmented bursting frequency of pre-BötC inspiratory neurons with intrinsic bursting properties. Microinfusion of acidified solutions into the rhythmically active pre-BötC network increased network bursting frequency, halothane decreased bursting frequency, and acid reversed the depressant effects of halothane, consistent with modulation of network activity by TASK channels. We conclude that TASK-like channels play a major functional role in chemosensory modulation of respiratory rhythm generation in the pre-Bötzinger complex in vitro.

Novel neuropathologic findings in the Haddad syndrome

Haddad syndrome (congenital central hypoventilation syndrome and Hirschsprung's disease) is a rare disorder for which in-depth neuropathologic analysis is lacking. We report the brain findings in a full-term male infant with Haddad syndrome who died at 27 days of life. Bilateral hypoplasia of the superior temporal lobe and gyral anomalies in the frontal cortex were present. Immunohistochemistry with an antibody to tyrosine hydroxylase (noradrenaline synthesis) demonstrated hypoplasia of the locus coeruleus (implicated in chemoreception) and A5 region. Other findings included delayed maturation of the arcuate nucleus (putative human homologue of ventral medullary neurons in animals critical for chemoreception) and aberrant fascicles in the nucleus of the solitary tract. Efforts to determine the putative gene mutation were unsuccessful. This study implicates novel brain findings in Haddad syndrome mimicking those in murine Phox2b null mutants. This case suggests that abnormalities occur in CCHS in a network of sites critical to chemoreception.

Retrotrapezoid nucleus, respiratory chemosensitivity and breathing automaticity

Breathing automaticity and CO(2) regulation are inseparable neural processes. The retrotrapezoid nucleus (RTN), a group of glutamatergic neurons that express the transcription factor Phox2b, may be a crucial nodal point through which breathing automaticity is regulated to maintain CO(2) constant. This review updates the analysis presented in prior publications. Additional evidence that RTN neurons have central respiratory chemoreceptor properties is presented, but this is only one of many factors that determine their activity. The RTN is also regulated by powerful inputs from the carotid bodies and, at least in the adult, by many other synaptic inputs. We also analyze how RTN neurons may control the activity of the downstream central respiratory pattern generator. Specifically, we review the evidence which suggests that RTN neurons (a) innervate the entire ventral respiratory column and (b) control both inspiration and expiration. Finally, we argue that the RTN neurons are the adult form of the parafacial respiratory group in neonate rats.

pH regulating transporters in neurons from various chemosensitive brainstem regions in neonatal rats

We studied the membrane transporters that mediate intracellular pH (pH(i)) recovery from acidification in brainstem neurons from chemosensitive regions of neonatal rats. Individual neurons within brainstem slices from the retrotrapezoid nucleus (RTN), the nucleus tractus solitarii (NTS), and the locus coeruleus (LC) were studied using a pH-sensitive fluorescent dye and fluorescence imaging microscopy. The rate of pH(i) recovery from an NH(4)Cl-induced acidification was measured, and the effects of inhibitors of various pH-regulating transporters determined. Hypercapnia (15% CO(2)) resulted in a maintained acidification in neurons from all three regions. Recovery in RTN neurons was nearly entirely eliminated by amiloride, an inhibitor of Na(+)/H(+) exchange (NHE). Recovery in RTN neurons was blocked approximately 50% by inhibitors of isoform 1 of NHE (NHE-1) but very little by an inhibitor of NHE-3 or by DIDS (an inhibitor of HCO(3)-dependent transport). In NTS neurons, amiloride blocked over 80% of the recovery, which was also blocked approximately 65% by inhibitors of NHE-1 and 26% blocked by an inhibitor of NHE-3. Recovery in LC neurons, in contrast, was unaffected by amiloride or blockers of NHE isoforms but was dependent on Na(+) and increased by external HCO(3)(-). On the basis of these findings, pH(i) recovery from acidification appears to be largely mediated by NHE-1 in RTN neurons, by NHE-1 and NHE-3 in NTS neurons, and by a Na- and HCO(3)-dependent transporter in LC neurons. Thus, pH(i) recovery is mediated by different pH-regulating transporters in neurons from different chemosensitive regions, but recovery is suppressed by hypercapnia in all of the neurons.

Characterization of the chemosensitive response of individual solitary complex neurons from adult rats

We studied the CO(2)/H(+)-chemosensitive responses of individual solitary complex (SC) neurons from adult rats by simultaneously measuring the intracellular pH (pH(i)) and electrical responses to hypercapnic acidosis (HA). SC neurons were recorded using the blind whole cell patch-clamp technique and loading the soma with the pH-sensitive dye pyranine through the patch pipette. We found that SC neurons from adult rats have a lower steady-state pH(i) than SC neurons from neonatal rats. In the presence of chemical and electrical synaptic blockade, adult SC neurons have firing rate responses to HA (percentage of neurons activated or inhibited and the magnitude of response as determined by the chemosensitivity index) that are similar to SC neurons from neonatal rats. They also have a typical response to isohydric hypercapnia, including decreased DeltapH(i), followed by pH(i) recovery, and increased firing rate. Thus, the chemosensitive response of SC neurons from adults is similar to the chemosensitive response of SC neurons from neonatal rats. Because our findings for adults are similar to previously reported values for neurons from neonatal rats, we conclude that intrinsic chemosensitivity is established early in development for SC neurons and is maintained throughout adulthood.

Respiratory response to systemic inhibition of the Na+/H+ exchanger type 3 in intact rats

The Na+/H+ exchangers (NHEs) are a family of antiporters involved in the maintenance of neural steady-state intracellular pH. The NHE3 seems to be the predominant subtype in central chemosensitive cells. We aimed to analyze the effect of a selective NHE3 inhibition on the respiratory pattern in spontaneously breathing rats with intact vagi. Rats were intravenously infused for 10 min with the selective NHE3 inhibitor AVE1599 (Aventis Pharma Deustchland, 0.5 and 2 mg/kg) or with phosphate buffer. Whole-body plethysmography was used to monitor breathing pattern before, during, and up to 30 min after the drug infusion. Immunohistochemistry for the c-Fos protein was performed in the animal brains and c-Fos-positive cells were counted along the brainstem. Selective NHE3 inhibition induced a significant increase in the respiratory frequency and in the number of c-Fos immunopositive cells in the lateral parabrachial nucleus, the pre-Bötzinger complex and a rostral extension of the retrotrapezoid nucleus/parapyramidal region (p<0.05, ANOVA). We conclude that systemic administration of AVE1599 increases respiratory frequency and activates ponto-medullary areas implicated in the central control of breathing and chemoreception.

Normal breathing pattern and arterial blood gases in awake and sleeping goats after near total destruction of the presumed pre-Botzinger complex and the surrounding region

Abrupt neurotoxic destruction of >70% of the pre-Bötzinger complex (preBötzC) in awake goats results in respiratory and cardiac failure (Wenninger JM, Pan LG, Klum L, Leekley T, Bastastic J, Hodges MR, Feroah TR, Davis S, Forster HV. J Appl Physiol 97: 1629-1636, 2004). However, in reduced preparations, rhythmic respiratory activity has been found in other areas of the brain stem (Huang Q, St. John WM. J Appl Physiol 64: 1405-1411, 1988; Janczewski WA, Feldman JL. J Physiol 570: 407-420, 2006; Lieske SP, Thoby-Brisson M, Telgkamo P, Ramierz JM. Nature Neurosci 3: 600-607, 2000; St. John WM, Bledsoe TA. J Appl Physiol 59: 684-690, 1985); thus we hypothesized that, when the preBötzC is destroyed incrementally over weeks, time-dependent plasticity within the respiratory network will result in a respiratory rhythm capable of maintaining normal blood gases. Microtubules were bilaterally implanted into the presumed preBötzC of seven goats. After recovery from surgery, studies were completed to establish baseline values for respiratory parameters. At weekly intervals, increasing volumes (in order 0.5, 1, 5, and 10 microl) of ibotenic acid (IA; 50 mM) were then injected into the preBötzC. All IA injections resulted in an acute tachypnea and dysrhythmia featuring augmented breaths, apneas, and increased breath-to-breath variation in breathing. In studies at night, apneas were nearly all central and occurred in the awake state. Breath-to-breath variation in breathing was greater (P < 0.05) during wakefulness than during non-rapid eye movement sleep. However, one week after the final IA injection, the breathing pattern, breath-to-breath variation, and arterial blood gases and pH were unchanged from baseline, but there was a 20% decrease in respiratory frequency (f) and CO(2) sensitivity (P < 0.05), as well as a 40% decrease in the ventilatory response to hypoxia (P < 0.001). In subsequent histological analysis of the presumed preBötzC region of lesioned goats, it was determined that there was a 90 and 92% reduction from control goats in total and neurokinin-1 receptor neurons, respectively. Therefore, it was concluded that 1) the dysrhythmic effects on breathing are state dependent; and 2) after incremental, near total destruction of the presumed preBötzC region, time-dependent plasticity within the respiratory network provides a rhythm capable of sustaining normal arterial blood gases.

Focal acidosis in the pre-Botzinger complex area of awake goats induces a mild tachypnea

There are widespread chemosensitive areas in the brain with varying effects on breathing. In the awake goat, microdialyzing (MD) 50% CO(2) at multiple sites within the medullary raphe increases pulmonary ventilation (Vi), blood pressure, heart rate, and metabolic rate (Vo(2)) (11), while MD in the rostral and caudal cerebellar fastigial nucleus has a stimulating and depressant effect, respectively, on these variables (17). In the anesthetized cat, the pre-Bötzinger complex (preBötzC), a hypothesized respiratory rhythm generator, increases phrenic nerve activity after an acetazolamide-induced acidosis (31, 32). To gain insight into the effects of focal acidosis (FA) within the preBötzC during physiological conditions, we tested the hypothesis that FA in the preBötzC during wakefulness would stimulate breathing, by increasing respiratory frequency (f). Microtubules were bilaterally implanted into the preBötzC of 10 goats. Unilateral MD of mock cerebral spinal fluid equilibrated with 6.4% CO(2) did not affect Vi, tidal volume (Vt), or f. Unilateral MD of 25 and 50% CO(2) significantly increased Vi and f by 10% (P < 0.05, n = 10, 17 trials), but Vt was unaffected. Bilateral MD of 6.4, 25, or 50% CO(2) did not significantly affect Vi, Vt, or f (P > 0.05, n = 6, 6 trials). MD of 80% CO(2) caused a 180% increase in f and severe disruptions in airflow (n = 2). MD of any level of CO(2) did not result in any significant changes in mean arterial blood pressure, heart rate, or Vo(2). Thus the data suggest that the preBötzC area is chemosensitive, but the responses to FA at this site are unique compared with other chemosensitive sites.

GFP-expressing locus ceruleus neurons from Prp57 transgenic mice exhibit CO2/H+ responses in primary cell culture

The locus ceruleus (LC) contains neurons that increase their firing rate (FR) in vitro when exposed to elevated CO(2)/H(+) and have been proposed to influence the respiratory network to make compensatory adjustments in ventilation. Prp57 transgenic mice express green fluorescent protein (GFP) in the LC and were used to isolate, culture, and target LC neurons for electrophysiological recording. We hypothesized that GFP-LC neurons would exhibit CO(2)/H(+) chemosensitivity under primary culture conditions, evidenced as a change in FR. This is the first study to quantify CO(2)/H(+) responses in LC neuron FR in cell culture. Neurons were continuously bathed with solutions containing antagonists of glutamate and GABA receptors, and the acid-base status was changed from control (5% CO(2); pH approximately 7.4) to hypercapnic acidosis (9% CO(2); pH approximately 7.2) and hypocapnic alkalosis (3% CO(2); pH approximately 7.6). FR was quantified during perforated patch current clamp recordings. Approximately 86% of GFP-LC neurons were stimulated, and approximately 14% were insensitive to changes in CO(2)/H(+). The magnitude of the response of these neurons depended on the baseline FR, ranging from 155.9 +/- 6% when FR started at 2.95 +/- 0.49 Hz to 381 +/- 55.6% when FR started at 1.32 +/- 0.31 Hz. These results demonstrate that cultured LC neurons from Prp57 transgenic mice retain functional sensing molecules necessary for CO(2)/H(+) responses. Prp57 transgenic mice will serve as a valuable model to delineate mechanisms involved in CO(2)/H(+) responsiveness in catecholaminergic neurons.

The 2008 Carl Ludwig Lecture: retrotrapezoid nucleus, CO2 homeostasis, and breathing automaticity

The retrotrapezoid nucleus (RTN) contains 2,000 glutamatergic neurons that innervate selectively the respiratory centers of the pontomedullary region. These cells are at the ventral medullary surface in a previously identified chemosensitive region. RTN neurons are highly sensitive to acid in vitro and vigorously activated by inputs from the carotid body and from the hypothalamus in vivo. Mutations of the transcription factor Phox2b cause the congenital hypoventilation syndrome (CCHS), a disease characterized by extremely reduced chemoreflexes and the loss of breathing automaticity during sleep. RTN neurons express Phox2b and develop poorly in a mouse model of CCHS, which lacks chemoreflexes. Based on these and other data, I propose that the RTN is a critical nodal point for the homeostatic regulation of arterial PCO2 and that the nucleus operates as follows. RTN always contributes a major fraction of the tonic excitatory drive to the respiratory centers. RTN neurons derive their activity from two sources: a chemosensory drive (intrinsic chemosensitivity and inputs from the carotid bodies) and synaptic inputs from higher brain centers (non-chemosensory drive). Under anesthesia or non-rapid eye movement sleep, the chemosensory drive to RTN neurons dominates, and, under these circumstances, the excitatory input from RTN to the respiratory controller is required for breathing automaticity. During waking and exercise, RTN contributes a reduced fraction of the total excitatory drive to the respiratory controller, but this fraction remains essential for CO2 homeostasis because of its exquisite chemosensitivity. The working hypothesis could explain the breathing deficits experienced by CCHS patients.

Focal CO2 dialysis in raphe obscurus does not stimulate ventilation but enhances the response to focal CO2 dialysis in the retrotrapezoid nucleus

Simultaneous inhibition of the retrotrapezoid nucleus (RTN) and raphe obscurus (ROb) decreased the systemic CO(2) response by 51%, an effect greater than inhibition of RTN (-24%) or ROb (0%) alone, suggesting that ROb modulates chemoreception by interaction with the RTN (19). We investigated this interaction further by simultaneous dialysis of artificial cerebrospinal fluid equilibrated with 25% CO(2) in two probes located in or adjacent to the RTN and ROb in conscious adult male rats. Ventilation was measured in a whole body plethysmograph at 30 degrees C. There were four groups (n = 5): 1) probes correctly placed in both RTN and ROb (RTN-ROb); 2) one probe correctly placed in RTN and one incorrectly placed in areas adjacent to ROb (RTN-peri-ROb); 3) one probe correctly placed in ROb and one probe incorrectly placed in areas adjacent to RTN (peri-RTN-ROb); and 4) neither probe correctly placed (peri-RTN-peri-ROb). Focal simultaneous acidification of RTN-ROb significantly increased ventilation (Ve) up to 22% compared with baseline, with significant increases in both breathing frequency and tidal volume. Focal acidification of RTN-peri-ROb increased Ve significantly by up to 15% compared with baseline. Focal acidification of ROb and peri-RTN had no significant effect. The simultaneous acidification of regions just outside the RTN and ROb actually decreased Ve by up to 11%. These results support a modulatory role for the ROb with respect to central chemoreception at the RTN.

Retrotrapezoid nucleus and central chemoreception

The 'distributed chemoreception theory' attributes the central chemoreflex (the stimulation of breathing by CNS acidification) to the cumulative effects of pH on multiple classes of respiratory neurons as well as on their tonic sources of drive. Opinions differ as to how many classes of pH-sensitive neurons contribute to the central chemoreflex but the number of candidates is high and growing fast. The 'specialized chemoreceptor theory', endorsed here, attributes the chemoreflex to a limited number of specialized neurons. These neurons (the central chemoreceptors) would drive a respiratory pattern generator that is not or minimally activated by acidification. In this review we first describe the properties of the retrotrapezoid nucleus (RTN) and argue that this nucleus may contain the most important central chemoreceptors. Next, we subject the assumptions that underlie the distributed chemoreception theory to a critical analysis. We propose several explanations for the apparent contradiction between the two competing theories of central chemoreception. We attribute much of the current controversy to premature extrapolations of the effects of acidification on neurons recorded in vitro (chemosensitivity) and to a semantic confusion between chemosensitivity and chemoreception (the mechanism by which CO(2) or pH activates breathing in vivo).

Anoxic persistence of lumbar respiratory bursts and block of lumbar locomotion in newborn rat brainstem spinal cords

The tolerance of breathing in neonates to oxygen depletion is reflected by persistence of inspiratory-related motor output during sustained anoxia in newborn rat brainstem preparations. It is not known whether lumbar motor networks innervating expiratory abdominal muscles are, in contrast, inhibited by anoxia similar to locomotor networks in neonatal mouse lumbar cords. To test this, we recorded inspiratory-related cervical/hypoglossal plus pre/postinspiratory lumbar/facial nerve activities and, sometimes simultaneously, locomotor rhythms in newborn rat brainstem-spinal cords. Chemical anoxia slowed 1 : 1-coupled cervical and lumbar respiratory rhythms and induced cervical burst doublets associated with depressed preinspiratory and augmented postinspiratory lumbar activities. Similarly, anoxia evoked repetitive hypoglossal bursts and shifted facial activity toward augmented postinspiratory bursting in medullas without spinal cord. Selective lumbar anoxia augmented pre/postinspiratory lumbar bursting without slowing the rhythm. This suggests a medullary origin of both anoxic inspiratory double bursts and preinspiratory depression, but a mixed medullary/lumbar origin of boosted postinspiratory lumbar activity. Lumbar respiratory rhythm is likely to be generated by the parafacial respiratory group expiratory centre as indicated by lack of normoxic and anoxic bursting following brainstem transection between the facial motonucleus and the more caudal pre-Bötzinger complex inspiratory centre. Opposed to sustained respiratory activities, anoxia reversibly abolished non-rhythmic spinal discharges and electrically or chemically evoked lumbar locomotor activities, followed by pronounced postanoxic spinal hyperexcitability. We hypothesize that (i) the anoxia tolerance of neonatal breathing includes pFRG-driven lumbar expiratory networks, (ii) the anoxic respiratory pattern transformation is due to disturbed inspiratory-expiratory centre interactions, and (iii) postanoxic lumbar hyperexcitability contributes to spasticity in cerebral palsy.

Raphe magnus nucleus is involved in ventilatory but not hypothermic response to CO2

There is evidence that serotonin [5-hydroxytryptamine (5-HT)] is involved in the physiological responses to hypercapnia. Serotonergic neurons represent the major cell type (comprising 15-20% of the neurons) in raphe magnus nucleus (RMg), which is a medullary raphe nucleus. In the present study, we tested the hypothesis 1) that RMg plays a role in the ventilatory and thermal responses to hypercapnia, and 2) that RMg serotonergic neurons are involved in these responses. To this end, we microinjected 1) ibotenic acid to promote nonspecific lesioning of neurons in the RMg, or 2) anti-SERT-SAP (an immunotoxin that utilizes a monoclonal antibody to the third extracellular domain of the serotonin reuptake transporter) to specifically kill the serotonergic neurons in the RMg. Hypercapnia caused hyperventilation and hypothermia in all groups. RMg nonspecific lesions elicited a significant reduction of the ventilatory response to hypercapnia due to lower tidal volume (Vt) and respiratory frequency. Rats submitted to specific killing of RMg serotonergic neurons showed no consistent difference in ventilation during air breathing but had a decreased ventilatory response to CO(2) due to lower Vt. The hypercapnia-induced hypothermia was not affected by specific or nonspecific lesions of RMg serotonergic neurons. These data suggest that RMg serotonergic neurons do not participate in the tonic maintenance of ventilation during air breathing but contribute to the ventilatory response to CO(2). Ultimately, this nucleus may not be involved in the thermal responses to CO(2).

Expression of Phox2b by brainstem neurons involved in chemosensory integration in the adult rat

Central congenital hypoventilation syndrome is caused by mutations of the gene that encodes the transcription factor Phox2b. The syndrome is characterized by a severe form of sleep apnea attributed to greatly compromised central and peripheral chemoreflexes. In this study, we analyze whether Phox2b expression in the brainstem respiratory network is preferentially associated with neurons involved in chemosensory integration in rats. At the very rostral end of the ventral respiratory column (VRC), Phox2b was present in many VGlut2 (vesicular glutamate transporter 2) mRNA-containing neurons. These neurons were functionally identified as the respiratory chemoreceptors of the retrotrapezoid nucleus (RTN). More caudally in the VRC, many fewer neurons expressed Phox2b. These cells were not part of the central respiratory pattern generator (CPG), because they were typically cholinergic visceral motor neurons or catecholaminergic neurons (presumed C1 neurons). Phox2b was not detected in serotonergic neurons, in the A5, A6, and A7 noradrenergic cell groups nor within the main cardiorespiratory centers of the dorsolateral pons. Phox2b was expressed by many solitary tract nucleus (NTS) neurons including those that relay peripheral chemoreceptor information to the RTN. These and previous observations by others suggest that Phox2b is expressed by an uninterrupted chain of neurons involved in the integration of peripheral and central chemoreception (carotid bodies, chemoreceptor afferents, chemoresponsive NTS neurons projecting to VRC, RTN chemoreceptors). The presence of Phox2b in this circuit and its apparent absence from the respiratory CPG could explain why Phox2b mutations disrupt breathing automaticity during sleep without causing major impairment of respiration during waking.

Central chemoreceptors and sympathetic vasomotor outflow

The present study explores how elevations in brain P(CO(2)) increase the sympathetic nerve discharge (SND). SND, phrenic nerve discharge (PND) and putative sympathoexcitatory vasomotor neurons of the rostral ventrolateral medulla (RVLM) were recorded in anaesthetized sino-aortic denervated and vagotomized rats. Hypercapnia (end-expiratory CO(2) from 5% to 10%) increased SND (97 +/- 6%) and the activity of RVLM neurons (67 +/- 4%). Injection of kynurenic acid (Kyn, ionotropic glutamate receptor antagonist) into RVLM or the retrotrapezoid nucleus (RTN) eliminated or reduced PND, respectively, but did not change the effect of CO(2) on SND. Bilateral injection of Kyn or muscimol into the rostral ventral respiratory group (rVRG-pre-Bötzinger region, also called CVLM) eliminated PND while increasing the stimulatory effect of CO(2) on SND. Muscimol injection into commissural part of the solitary tract nucleus (commNTS) had no effect on PND or SND activation by CO(2). As expected, injection of Kyn into RVLM or muscimol into commNTS virtually blocked the effect of carotid body stimulation on SND in rats with intact carotid sinus nerves. In conclusion, CO(2) increases SND by activating RVLM sympathoexcitatory neurons. The relevant central chemoreceptors are probably located within or close to RVLM and not in the NTS or in the rVRG-pre-Bötzinger/CVLM region. RVLM sympathoexcitatory neurons may be intrinsically pH-sensitive and/or receive excitatory synaptic inputs from RTN chemoreceptors. Activation of the central respiratory network reduces the overall sympathetic response to CO(2), presumably by activating barosensitive CVLM neurons and inhibiting RTN chemoreceptors.

Hypercapnic and hypoxic responses require intact neural transmission from the pre-Bötzinger complex

The central respiratory network that includes the pre-Bötzinger complex (pre-BötC), a region believed to contain rhythmogenic neurons, is capable of responding to fluctuations in CO2 and pH. However, the role of inputs from this site in mediating ventilatory responses to hypercapnia and/or hypoxia in nonsedated animals is not well established. Therefore, in the present study we tested the hypothesis that altered transmission from the pre-BötC to its target sites would decrease chemosensory responsiveness to acute hypercapnia and modulate the ventilatory response to hypoxia. Colchicine was used to block axonal transport. At 48 h after bilateral microinjections of colchicine into the pre-BötC (100 microg/uL, 100 nL/site), but not saline, the baseline frequency of breathing decreased; however, rhythmicity was not altered. In addition, there was a significant fall in the ventilatory response to hypercapnia (5 and 12% CO2) and hypoxia (8% O2). These findings indicate that, inputs from pre-BötC neurons are of critical importance in providing the normal ventilatory response to both hypercapnia and hypoxia.

Somatostatin inhibition of fictive respiration is modulated by pH

We studied the respiratory effects of the tetradecapeptide somatostatin (SST) upon fictive respiration using the in vitro brain stem spinal cord preparation from new-born mouse. We found that SST inhibits respiration, an effect that was potentiated when the chemical drive to respiration was increased. SST inhibited fictive respiration decreasing both the frequency and amplitude in a dose-dependent way. SST inhibition was not antagonized by cyclosomatostatin (cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]), a putative SST antagonist, which in contrast behaved as a partial agonist. When the chemical drive to respiration was increased, by lowering the pH of the brain stem superfusion medium from 7.4 to 7.3, the inhibitory effect of SST on respiratory frequency was potentiated. These results suggest an interaction between SST and respiratory central chemoreception in new-born mouse.

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.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

Ventilatory effects of gap junction blockade in the NTS in awake rats

We tested the hypothesis that focally perfusing carbenoxolone, which blocks gap junctions, into the nucleus tractus solitarius (NTS) would reduce the ventilatory response to CO(2). We measured minute ventilation (V(E)), tidal volume (V(T)) and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (F(I)(CO(2) = 0-8%) in rats during wakefulness. Focal perfusion of acetazolamide (10 microM) into the NTS increased V(E) and V(T) during exposure to room air. Carbenoxolone (300 microM) decreased the V(E) and V(T) response to CO(2) when perfused within, but not adjacent to the NTS in animals less than 10 weeks of age. F(R) was decreased at F(I)(CO(2) = 4% in these animals. Carbenoxolone did not decrease V(E), V(T) or F(R) in animals 10 weeks of age and older. Carbenoxolone did not decrease V(E), V(T) or F(R) when focally perfused outside the NTS at any age tested. The NTS is an important CO(2) chemosensory site at all ages, and gap junctions amplify the ventilatory response to CO(2) in animals less than 10 weeks of age.

Maturation of respiratory reflex responses in the fetus and neonate

Respiratory control in the fetus and neonate is quite immature when compared to that of adults. This immaturity involves all facets of respiration including respiratory responses to hypoxia, hypercapnia, an exaggerated apnoeic response to laryngeal stimulation and immature responses to activation of pulmonary afferents. The net result of this immaturity of breathing responses is the vulnerability of neonates and especially preterm infants to apnoea and respiratory pauses. The mechanisms behind immature control of breathing are not fully understood, but seem to originate from a predominance of inhibitory input early in life on respiratory centres. The relative contribution of up-regulation of inhibitory pathways versus down-regulation of excitatory ones is not clear. Multiple neurotransmitters have been implicated in the regulation of breathing in mammals and some of them are discussed in this chapter.

Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.