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

Doxapram stimulates respiratory activity through distinct activation of neurons in the nucleus hypoglossus and the pre-Bötzinger complex

Doxapram is a respiratory stimulant used for decades as a treatment option in apnea of prematurity refractory to methylxanthine treatment. Its mode of action, however, is still poorly understood. We investigated direct effects of doxapram on the pre-Bötzinger complex (PreBötC) and on a downstream motor output system, the hypoglossal nucleus (XII), in the transverse brainstem slice preparation. While doxapram has only a modest stimulatory effect on frequency of activity generated within the PreBötC, a much more robust increase in the amplitude of population activity in the subsequent motor output generated in the XII was observed. In whole cell patch-clamp recordings of PreBötC and XII neurons, we confirmed significantly increased firing of evoked action potentials in XII neurons in the presence of doxapram, while PreBötC neurons showed no significant alteration in firing properties. Interestingly, the amplitude of activity in the motor output was not increased in the presence of doxapram compared with control conditions during hypoxia. We conclude that part of the stimulatory effects of doxapram is caused by direct input on brainstem centers with differential effects on the rhythm generating kernel (PreBötC) and the downstream motor output (XII).

NEW & NOTEWORTHY The clinically used respiratory stimulant doxapram has distinct effects on the rhythm generating kernel (pre-Bötzinger complex) and motor output centers (nucleus hypoglossus). These effects are obliterated during hypoxia and are mediated by distinct changes in the intrinsic properties of neurons of the nucleus hypoglossus and synaptic transmission received by pre-Bötzinger complex neurons.

Advances in cellular and integrative control of oxygen homeostasis within the central nervous system

Mammals must continuously regulate the levels of O2 and CO2 , which is particularly important for the brain. Failure to maintain adequate O2 /CO2 homeostasis has been associated with numerous disorders including sleep apnoea, Rett syndrome and sudden infant death syndrome. But, O2 /CO2 homeostasis poses major regulatory challenges, even in the healthy brain. Neuronal activities change in a differentiated, spatially and temporally complex manner, which is reflected in equally complex changes in O2 demand. This raises important questions: is oxygen sensing an emergent property, locally generated within all active neuronal networks, and/or the property of specialized O2 -sensitive CNS regions? Increasing evidence suggests that the regulation of the brain's redox state involves properties that are intrinsic to many networks, but that specialized regions in the brainstem orchestrate the integrated control of respiratory and cardiovascular functions. Although the levels of O2 in arterial blood and the CNS are very different, neuro-glial interactions and purinergic signalling are critical for both peripheral and CNS chemosensation. Indeed, the specificity of neuroglial interactions seems to determine the differential responses to O2 , CO2 and the changes in pH.

Different roles for inhibition in the rhythm-generating respiratory network

Unraveling the interplay of excitation and inhibition within rhythm-generating networks remains a fundamental issue in neuroscience. We use a biophysical model to investigate the different roles of local and long-range inhibition in the respiratory network, a key component of which is the pre-Bötzinger complex inspiratory microcircuit. Increasing inhibition within the microcircuit results in a limited number of out-of-phase neurons before rhythmicity and synchrony degenerate. Thus unstructured local inhibition is destabilizing and cannot support the generation of more than one rhythm. A two-phase rhythm requires restructuring the network into two microcircuits coupled by long-range inhibition in the manner of a half-center. In this context, inhibition leads to greater stability of the two out-of-phase rhythms. We support our computational results with in vitro recordings from mouse pre-Bötzinger complex. Partial excitation block leads to increased rhythmic variability, but this recovers after blockade of inhibition. Our results support the idea that local inhibition in the pre-Bötzinger complex is present to allow for descending control of synchrony or robustness to adverse conditions like hypoxia. We conclude that the balance of inhibition and excitation determines the stability of rhythmogenesis, but with opposite roles within and between areas. These different inhibitory roles may apply to a variety of rhythmic behaviors that emerge in widespread pattern-generating circuits of the nervous system.NEW & NOTEWORTHY The roles of inhibition within the pre-Bötzinger complex (preBötC) are a matter of debate. Using a combination of modeling and experiment, we demonstrate that inhibition affects synchrony, period variability, and overall frequency of the preBötC and coupled rhythmogenic networks. This work expands our understanding of ubiquitous motor and cognitive oscillatory networks.

Environmentally induced return to juvenile-like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

KEY POINTS

The degree to which developmental programmes or environmental signals determine physiological phenotypes remains a major question in physiology. Vertebrates change environments during development, confounding interpretation of the degree to which development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) produces phenotypes. Tadpoles mainly breathe water for gas exchange and frogs may breathe water or air depending on their environment and are, therefore, exemplary models to differentiate the degree to which life-stage vs. environmental context drives developmental phenotypes associated with neural control of lung breathing. Using isolated brainstem preparations and patch clamp electrophysiology, we demonstrate that adult bullfrogs acclimatized to water-breathing conditions do not exhibit CO₂ and O₂ chemosensitivity of lung breathing, similar to water-breathing tadpoles. Our results establish that phenotypes associated with developmental stage may arise from plasticity per se and suggest that a developmental trajectory coinciding with environmental change obscures origins of stage-dependent physiological phenotypes by masking plasticity.

ABSTRACT

An unanswered question in developmental physiology is to what extent does the environment vs. a genetic programme produce phenotypes? Developing animals inhabit different environments and switch from one to another. Thus a developmental time course overlapping with environmental change confounds interpretations as to whether development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) generates phenotypes. Tadpoles of the American bullfrog, Lithobates catesbeianus, breathe water at early life-stages and minimally use lungs for gas exchange. As adults, bullfrogs rely on lungs for gas exchange, but spend months per year in ice-covered ponds without lung breathing. Aquatic submergence, therefore, removes environmental pressures requiring lung breathing and enables separation of adulthood from environmental factors associated with adulthood that necessitate control of lung ventilation. To test the hypothesis that postmetamorphic respiratory control phenotypes arise through permanent developmental changes vs. reversible environmental signals, we measured respiratory-related nerve discharge in isolated brainstem preparations and action potential firing from CO₂ -sensitive neurons in bullfrogs acclimatized to semi-terrestrial (air-breathing) and aquatic-overwintering (no air-breathing) habitats. We found that aquatic overwintering significantly reduced neuroventilatory responses to CO₂ and O₂ involved in lung breathing. Strikingly, this gas sensitivity profile reflects that of water-breathing tadpoles. We further demonstrated that aquatic overwintering reduced CO₂ -induced firing responses of chemosensitive neurons. In contrast, respiratory rhythm generating processes remained adult-like after submergence. Our results establish that phenotypes associated with life-stage can arise from phenotypic plasticity per se. This provides evidence that developmental time courses coinciding with environmental changes obscure interpretations regarding origins of stage-dependent physiological phenotypes by masking plasticity.

CO2-evoked release of PGE2 modulates sighs and inspiration as demonstrated in brainstem organotypic culture

Inflammation-induced release of prostaglandin E₂ (PGE₂) changes breathing patterns and the response to CO₂ levels. This may have fatal consequences in newborn babies and result in sudden infant death. To elucidate the underlying mechanisms, we present a novel breathing brainstem organotypic culture that generates rhythmic neural network and motor activity for 3 weeks. We show that increased CO₂ elicits a gap junction-dependent release of PGE₂. This alters neural network activity in the preBötzinger rhythm-generating complex and in the chemosensitive brainstem respiratory regions, thereby increasing sigh frequency and the depth of inspiration. We used mice lacking eicosanoid prostanoid 3 receptors (EP3R), breathing brainstem organotypic slices and optogenetic inhibition of EP3R^+/+ cells to demonstrate that the EP3R is important for the ventilatory response to hypercapnia. Our study identifies a novel pathway linking the inflammatory and respiratory systems, with implications for inspiration and sighs throughout life, and the ability to autoresuscitate when breathing fails.

DOI:http://dx.doi.org/10.7554/eLife.14170.001

Humans and other mammals breathe air to absorb oxygen into the body and to remove carbon dioxide. We know that in a part of the brain called the brainstem, several regions work together to create breaths, but it is not clear precisely how this works. These regions adjust our breathing to the demands placed on the body by different activities, such as sleeping or exercising. Sometimes, especially in newborn babies, the brainstem’s monitoring of oxygen and carbon dioxide does not work properly, which can lead to abnormal breathing and possibly death.

In the brain, cells called neurons form networks that can rapidly transfer information via electrical signals. Here, Forsberg et al. investigated the neural networks in the brainstem that generate and control breathing in mice. They used slices of mouse brainstem that had been kept alive in a dish in the laboratory. The slice contained an arrangement of neurons and supporting cells that allowed it to continue to produce patterns of electrical activity that are associated with breathing. Over a three-week period, Forsberg et al. monitored the activity of the cells and calculated how they were connected to each other. The experiments show that the neurons responsible for breathing were organized in a “small-world” network, in which the neurons are connected to each other directly or via small numbers of other neurons.

Further experiments tested how various factors affect the behavior of the network. For example, carbon dioxide triggered the release of a small molecule called prostaglandin E2 from cells. This molecule is known to play a role in inflammation and fever. However, in the carbon dioxide sensing region of the brainstem it acted as a signaling molecule that increased activity. Therefore, inflammation could interfere with the body’s normal response to carbon dioxide and lead to potentially life-threatening breathing problems. Furthermore, prostaglandin E2 induced deeper breaths known as sighs, which may be vital for newborn babies to be able to take their first deep breaths of life. Future challenges include understanding how the brainstem neural networks generate breathing and translate this knowledge to improve the treatment of breathing difficulties in babies.

DOI:http://dx.doi.org/10.7554/eLife.14170.002

When norepinephrine becomes a driver of breathing irregularities: how intermittent hypoxia fundamentally alters the modulatory response of the respiratory network

Neuronal networks are endogenously modulated by aminergic and peptidergic substances. These modulatory processes are critical for maintaining normal activity and adapting networks to changes in metabolic, behavioral, and environmental conditions. However, disturbances in neuromodulation have also been associated with pathologies. Using whole animals (in vivo) and functional brainstem slices (in vitro) from mice, we demonstrate that exposure to acute intermittent hypoxia (AIH) leads to fundamental changes in the neuromodulatory response of the respiratory network located within the preBötzinger complex (preBötC), an area critical for breathing. Norepinephrine, which normally regularizes respiratory activity, renders respiratory activity irregular after AIH. Respiratory irregularities are caused both in vitro and in vivo by AIH, which increases synaptic inhibition within the preBötC when norepinephrine is endogenously or exogenously increased. These irregularities are prevented by blocking synaptic inhibition before AIH. However, regular breathing cannot be reestablished if synaptic inhibition is blocked after AIH. We conclude that subtle changes in synaptic transmission can have dramatic consequences at the network level as endogenously released neuromodulators that are normally adaptive become the drivers of irregularity. Moreover, irregularities in the preBötC result in irregularities in the motor output in vivo and in incomplete transmission of inspiratory activity to the hypoglossus motor nucleus. Our finding has basic science implications for understanding network functions in general, and it may be clinically relevant for understanding pathological disturbances associated with hypoxic episodes such as those associated with myocardial infarcts, obstructive sleep apneas, apneas of prematurity, Rett syndrome, and sudden infant death syndrome.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Neuromodulation: purinergic signaling in respiratory control

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.

The rhythmic, transverse medullary slice preparation in respiratory neurobiology: contributions and caveats

Our understanding of the sites and mechanisms underlying rhythmic breathing as well as the neuromodulatory control of respiratory rhythm, pattern, and respiratory motoneuron excitability during perinatal development has advanced significantly over the last 20 years. A major catalyst was the development in 1991 of the rhythmically-active medullary slice preparation, which provided precise mechanical and chemical control over the network as well as enhanced physical and optical access to key brainstem regions. Insights obtained in vitro have informed multiple mechanistic hypotheses. In vivo tests of these hypotheses, performed under conditions of reduced control and precision but more obvious physiological relevance, have clearly established the significance for respiratory neurobiology of the rhythmic slice preparation. We review the contributions of this preparation to current understanding/concepts in respiratory control, and outline the limitations of this approach in the context of studying rhythm and pattern generation, homeostatic control mechanisms and murine models of human genetic disorders that feature prominent breathing disturbances.

Calmodulin and calmodulin kinase II mediate emergent bursting activity in the brainstem respiratory network (preBötzinger complex)

Emergence of persistent activity in networks can be controlled by intracellular signalling pathways but the mechanisms involved and their role are not yet fully explored. Using calcium imaging and patch-clamp we examined the rhythmic activity in the preBötzinger complex (preBötC) in the lower brainstem that generates the respiratory motor output. In functionally intact acute slices brief hypoxia, electrical stimulation and activation of AMPA receptors transiently depressed bursting activity which then recovered with augmentation. The effects were abrogated after chelation of intracellular calcium, blockade of L-type calcium channels and inhibition of calmodulin (CaM) and CaM kinase (CaMKII). Rhythmic calcium transients and synaptic drive currents in preBötC neurons in the organotypic slices showed similar CaM- and CaMKII-dependent responses. The stimuli increased the amplitude of spontaneous and miniature excitatory synaptic currents indicating postsynaptic changes at glutamatergic synapses. In the acute and organotypic slices, CaM stimulated and ADP inhibited calcium-dependent TRPM4 channels and CaMKII augmented synaptic drive currents. Experimental data and simulations show the role of ADP and CaMKII in the control of bursting activity and its relation to intracellular signalling. I propose that CaMKII-mediated facilitation of glutamatergic transmission strengthens emergent synchronous activity within preBötC that is then maintained by periodic surges of calcium during the bursts. This may find implications in restoration and consolidation of autonomous activity in the respiratory disorders.

The vesicular glutamate transporter VGLUT3 contributes to protection against neonatal hypoxic stress

Neonates respond to hypoxia initially by increasing ventilation, and then by markedly decreasing both ventilation (hypoxic ventilatory decline) and oxygen consumption (hypoxic hypometabolism). This latter process, which vanishes with age, reflects a tight coupling between ventilatory and thermogenic responses to hypoxia. The neurological substrate of hypoxic hypometabolism is unclear, but it is known to be centrally mediated, with a strong involvement of the 5-hydroxytryptamine (5-HT, serotonin) system. To clarify this issue, we investigated the possible role of VGLUT3, the third subtype of vesicular glutamate transporter. VGLUT3 contributes to glutamate signalling by 5-HT neurons, facilitates 5-HT transmission and is expressed in strategic regions for respiratory and thermogenic control. We therefore assumed that VGLUT3 might significantly contribute to the response to hypoxia. To test this possibility, we analysed this response in newborn mice lacking VGLUT3 using anatomical, biochemical, electrophysiological and integrative physiology approaches. We found that the lack of VGLUT3 did not affect the histological organization of brainstem respiratory networks or respiratory activity under basal conditions. However, it impaired respiratory responses to 5-HT and anoxia, showing a marked alteration of central respiratory control. These impairments were associated with altered 5-HT turnover at the brainstem level. Furthermore, under cold conditions, the lack of VGLUT3 disrupted the metabolic rate, body temperature, baseline breathing and the ventilatory response to hypoxia. We conclude that VGLUT3 expression is dispensable under basal conditions but is required for optimal response to hypoxic stress in neonates.

Protective action of endogenously generated H₂S on hypoxia-induced respiratory suppression and its relation to antioxidation and down-regulation of c-fos mRNA in medullary slices of neonatal rats

We previously reported that exogenous H(2)S played roles in protection of respiratory centers against hypoxic injury in medullary slices of neonatal rats. The protective action of endogenous H(2)S and its relation to antioxidation and down-regulation of c-fos mRNA were investigated in the present study. Perfusion of the slices with l-cysteine (Cys), substrate of cystathionine β-synthase (CBS, H(2)S synthase), could increase frequency of rhythmic respiratory discharge of the hypoglossal rootlets and prevent respiratory suppression induced by hypoxia, whereas perfusion with hydroxylamine (NH(2)OH, inhibitor of CBS) could postpone recovery of respiration from hypoxic inhibition. NH(2)OH also significantly enhanced hypoxia-induced increase in malondialdehyde (MDA) content of the slices. The hypoxia-induced up-regulation of c-fos mRNA could be markedly antagonized by S-adenosyl-l-methionine (SAM, activator of CBS), but greatly increased by NH(2)OH. Neither NH(2)OH, Cys nor SAM had any effect on expression of bcl-2 mRNA in hypoxic medullary slices. These results indicate that endogenously generated H(2)S was involved in protection of the medullary respiratory centers against hypoxic injury partly via antioxidation and down-regulation of c-fos.

Respiratory motoneurons and pathological conditions: lessons from hypoglossal motoneurons challenged by excitotoxic or oxidative stress

Hypoglossal motoneurons (HMs) are respiration-related brainstem neurons that command rhythmic contraction of the tongue muscles in concert with the respiratory drive. In experimental conditions, HMs can exhibit a range of rhythmic patterns that may subserve different motor outputs and functions. Neurodegenerative diseases like amyotrophic lateral sclerosis (ALS; Lou-Gehrig disease) often damage HMs with distressing symptoms like dysarthria, dysphagia and breathing difficulty related to degeneration of respiratory motoneurons. While the cause of ALS remains unclear, early diagnosis remains an important goal for potential treatment because fully blown clinical symptoms appear with degeneration of about 30% motoneurons. Using a simple in vitro model of the rat brainstem to study the consequences of excitotoxicity or oxidative stress (believed to occur during the onset of ALS) on HMs, it is possible to observe distinct electrophysiological effects associated with HM experimental pathology. In fact, excitotoxicity caused by glutamate uptake block triggers sustained bursting and enhanced synaptic transmission, whereas oxidative stress generates slow depolarization, augmented repeated firing, and decreased synaptic transmission. In either case, only a subpopulation of HMs shows abnormal functional changes. Although these two insults induce separate functional signatures, the consequences on HMs after a few hours are similar and are preceded by activation of the stress transcription factor ATF-3. The deleterious action of excitotoxicity is inhibited by early administration of riluzole, a drug currently employed for the symptomatic treatment of ALS, demonstrating that this in vitro model can be useful for testing potential neuroprotective agents.

Slow intrinsic oscillations in thick neocortical slices of hypoxia tolerant deep diving seals

Direct evidence that the mammalian neocortex is an important generator of intrinsic activity comes from isolated neocortical slices that spontaneously generate multiple rhythms including those in the beta, delta and gamma range. These oscillations are also seen in intact animals where they interact with other areas including the hippocampus, thalamus and basal ganglia. Here we show that thick isolated neocortical slices from hooded seals intrinsically generate persistent spontaneous activities, both repetitive non-rhythmic activity with activity states lasting for several minutes, and oscillating activity with rhythms that are much slower (<0.1 Hz) than the rhythms previously described in vitro. These intrinsic activities were very robust and persisted for up to 1 h even in severely hypoxic conditions. We hypothesize that the remarkable hypoxia tolerance of the hooded seal nervous system made it possible to maintain functional integrity in slices thick enough to preserve intact neuronal networks capable of generating these slow oscillations. The observed activities in seal neocortical slices support the notion that mammalian cortical networks intrinsically generate multiple states of activity that include oscillatory activity all the way down to <0.1 Hz. This intrinsic neocortical excitability is an important contributor not only to sleep but also to the default awake state of the neocortex.

Graded reductions in oxygenation evoke graded reconfiguration of the isolated respiratory network

Neurons depend on aerobic metabolism, yet are very sensitive to oxidative stress and, as a consequence, typically operate in a low O(2) environment. The balance between blood flow and metabolic activity, both of which can vary spatially and dynamically, suggests that local O(2) availability markedly influences network output. Yet the understanding of the underlying O(2)-sensing mechanisms is limited. Are network responses regulated by discrete O(2)-sensing mechanisms or, rather, are they the consequence of inherent O(2) sensitivities of mechanisms that generate the network activity? We hypothesized that a broad range of O(2) tensions progressively modulates network activity of the pre-Bötzinger complex (preBötC), a neuronal network critical to the central control of breathing. Rhythmogenesis was measured from the preBötC in transverse neonatal mouse brain stem slices that were exposed to graded reductions in O(2) between 0 and 95% O(2), producing tissue oxygenation values ranging from 20 ± 18 (mean ± SE) to 440 ± 56 Torr at the slice surface, respectively. The response of the preBötC to graded changes in O(2) is progressive for some metrics and abrupt for others, suggesting that different aspects of the respiratory network have different sensitivities to O(2).

Two types of independent bursting mechanisms in inspiratory neurons: an integrative model

The network of coupled neurons in the pre-Bötzinger complex (pBC) of the medulla generates a bursting rhythm, which underlies the inspiratory phase of respiration. In some of these neurons, bursting persists even when synaptic coupling in the network is blocked and respiratory rhythmic discharge stops. Bursting in inspiratory neurons has been extensively studied, and two classes of bursting neurons have been identified, with bursting mechanism depends on either persistent sodium current or changes in intracellular Ca(2+), respectively. Motivated by experimental evidence from these intrinsically bursting neurons, we present a two-compartment mathematical model of an isolated pBC neuron with two independent bursting mechanisms. Bursting in the somatic compartment is modeled via inactivation of a persistent sodium current, whereas bursting in the dendritic compartment relies on Ca(2+) oscillations, which are determined by the neuromodulatory tone. The model explains a number of conflicting experimental results and is able to generate a robust bursting rhythm, over a large range of parameters, with a frequency adjusted by neuromodulators.

Postnatal changes in the cardiorespiratory response and ability to autoresuscitate from hypoxic and hypothermic exposure in mammals

Most mammals are born immature and a great deal of maturational changes must occur early in the early postnatal life to prepare for life as an adult. In addition to the obvious changes such as physical and musculoskeletal growth, a myriad of physiological changes including the cardiorespiratory responses to hypoxia and hypothermia must also occur. The most intriguing developmental effect is perhaps the change in the ability to autoresuscitate, or spontaneous recovery from cardiorespiratory arrest induced by extreme hypoxia or hypothermia. For decades the ability of young animals to autoresuscitate from cardiorespiratory arrest induced by hypoxic or hypothermic exposure has been documented. In some mammalian species, including rats and humans, this ability is lost over development while others retain this ability. This review will examine the changes that occur in the cardiorespiratory response to hypoxia and hypothermia and the change to the ability to autoresuscitate from cardiorespiratory arrest over early postnatal development. Furthermore, the review will explore some of the potential neuroanatomical, neurochemical and neurophysiological changes during early postnatal development that might contribute to the altered reflex response to hypoxia and hypothermia and the ability to autoresuscitate.

P2Y1 receptor modulation of the pre-Bötzinger complex inspiratory rhythm generating network in vitro

ATP is released during hypoxia from the ventrolateral medulla (VLM) and activates purinergic P2 receptors (P2Rs) at unknown loci to offset the secondary hypoxic depression of breathing. In this study, we used rhythmically active medullary slices from neonatal rat to map, in relation to anatomical and molecular markers of the pre-Bötzinger complex (preBötC) (a proposed site of rhythm generation), the effects of ATP on respiratory rhythm and identify the P2R subtypes responsible for these actions. Unilateral microinjections of ATP in a three-dimensional grid within the VLM revealed a "hotspot" where ATP (0.1 mM) evoked a rapid 2.2 +/- 0.1-fold increase in inspiratory frequency followed by a brief reduction to 0.83 +/- 0.02 of baseline. The hotspot was identified as the preBötC based on histology, overlap of injection sites with NK1R immunolabeling, and potentiation or inhibition of respiratory frequency by SP ([Sar9-Met(O2)11]-substance P) or DAMGO ([D-Ala2,N-MePhe4,Gly-ol5]-enkephalin), respectively. The relative potency of P2R agonists [2MeSADP (2-methylthioadenosine 5'-diphosphate) approximately = 2MeSATP (2-methylthioadenosine 5'-triphosphate) approximately = ATPgammas (adenosine 5'-[gamma-thio]triphosphate tetralithium salt) approximately = ATP >> UTP approximately = alphabeta meATP (alpha,beta-methylene-adenosine 5'-triphosphate)] and attenuation of the ATP response by MRS2179 (2'-deoxy-N6-methyladenosine-3',5'-bisphosphate) (P2Y1 antagonist) indicate that the excitation is mediated by P2Y1Rs. The post-ATP inhibition, which was never observed in response to ATPgammas, is dependent on ATP hydrolysis. These data establish in neonatal rats that respiratory rhythm generating networks in the preBötC are exquisitely sensitive to P2Y1R activation, and suggest a role for P2Y1Rs in respiratory motor control, particularly in the P2R excitation of rhythm that occurs during hypoxia.

Metabotropic glutamate receptor activity induces a novel oscillatory pattern in neonatal rat hypoglossal motoneurones

Tongue muscles innervated by the hypoglossal nerves play a crucial role to ensure airway patency and milk suckling in the neonate. Using a slice preparation of the neonatal rat brain, we investigated the electrophysiological characteristics of hypoglossal motoneurones in the attempt to identify certain properties potentially capable of synchronizing motor commands to the tongue. Bath-applied DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), generated persistent, regular electrical oscillations (4-8 Hz) recorded from patch-clamped motoneurones. Under voltage clamp, oscillations were biphasic events, comprising large outward slow currents alternated with fast, repeated inward currents. Electrical oscillations had amplitude and period insensitive to cell membrane potential, and required intact glutamatergic transmission via AMPA receptors. Oscillations were mediated by subtype 1 receptors of group I mGluRs (mGluR1s), and were routinely observed during pharmacological block of glycinergic and GABAergic inhibition, although they could also be recorded in standard saline. Simultaneous recordings from pairs of motoneurones within the same hypoglossal nucleus demonstrated that oscillations were due to their strong electrical coupling and were blocked by the gap junction blocker carbenoxolone. Pacing of slow oscillations apparently depended on the operation of K(ATP) channels in view of the block by tolbutamide or glibenclamide. Under current clamp, oscillations generated more regular spike firing of motoneurones and facilitated glutamatergic excitatory inputs. These data suggest that neonatal motoneurones of the nucleus hypoglossus possess a formerly undisclosed ability to express synchronous electrical oscillations, unveiled by activation of mGluR1s.

Hyperthermia Modulates Respiratory Pacemaker Bursting Properties

Most mammals modulate respiratory frequency (RF) to dissipate heat (e.g., panting) and avoid heat stroke during hyperthermic conditions. Respiratory neural network activity recorded in an isolated brain stem-slice preparation of mice exhibits a similar RF modulation in response to hyperthermia; fictive eupneic frequency increases while inspiratory network activity amplitude and duration are significantly reduced. Here, we study the effects of hyperthermia on the activity of synaptically isolated respiratory pacemakers to examine the possibility that these changes may account for the hyperthermic RF modulation of the respiratory network. During heating, modulation of the bursting frequency of synaptically isolated pacemakers paralleled that of population bursting recorded from the intact network, whereas nonpacemaker neurons were unaffected, suggesting that pacemaker bursting may account for the temperature-enhanced RF observed at the network level. Some respiratory neurons that were tonically active at hypothermic conditions exhibited pacemaker properties at approximately the normal body temperature of eutherian mammals (36.81 ± 1.17°C; mean ± SD) and continued to burst at 40°C. At elevated temperatures (40°C), there was an enhancement of the depolarizing drive potential in synaptically isolated pacemakers, while the amplitude of integrated population activity declined. Isolated pacemaker bursting ceased at 41–42°C ( n = 5), which corresponds to temperatures at which hyperthermic-apnea typically occurs in vivo. We conclude that pacemaker properties may play an important role in the hyperthermic frequency modulation and apnea, while network effects may play important roles in generating other aspects of the hyperthermic response, such as the decreased amplitude of ventral respiratory group activity during hyperthermia.

Respiratory-Like Rhythmic Activity Can Be Produced by an Excitatory Network of Non-Pacemaker Neuron Models

It is still unclear whether the respiratory-like rhythm observed in slice preparations containing the pre-Bötzinger complex is of pacemaker or network origin. The rhythm persists in the absence of inhibition, but blocking pacemaker activity did not always result in rhythm abolition. We developed a computational model of the slice to show that respiratory-like rhythm can emerge as a network property without pacemakers or synaptic inhibition. The key currents of our model cell are the low- and high-threshold calcium currents and the calcium-dependent potassium current. Depolarization of a single unit by current steps or by raising the external potassium concentration can induce periodic bursting activity. Gaussian stimulation increased the excitability of the model without evoking oscillatory activity, as indicated by autocorrelation analysis. In response to hyperpolarizing pulses, the model produces prolonged relative refractory periods. At the network level, an increase of external potassium concentration triggers rhythmic activity that can be attributed to cellular periodic bursting, network properties, or both, depending on different parameters. Gaussian stimulation also induces rhythmic activity that depends solely on network properties. In all cases, the calcium-dependent potassium current has a central role in burst termination and interburst duration. However, when periodic inhibition is considered, the activation of this current is responsible for the characteristic amplification ramp of the emerged rhythm. Our results may explain controversial results from studies blocking pacemakers in vitro and show a shift in the role of the calcium-dependent potassium current in the presence of network inhibition.

Response of the respiratory network of mice to hyperthermia

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

Perinatal maturation of the mouse respiratory rhythm-generator: in vivo and in vitro studies

In vivo (plethysmography) and in vitro (en bloc preparations) experiments were performed from embryonic day 16 (E16) to postnatal day 9 (P9) in order to analyse the perinatal maturation of the respiratory rhythm-generator in mice. At E16, delivered foetuses did not ventilate and survive but at E18 they breathed at about 110 cycles/min with respiratory cycles of variable individual duration. From E18 to P0-P2, the respiratory cycles stabilised without changes in the breathing parameters. However, these increased several-fold during the next days. Hypoxia increased breathing frequency from E18-P5 and only significantly affected ventilation from P3 onwards. At E16, in vitro medullary preparations (pons resection) produced rhythmic phrenic bursts at a low frequency (about 5 cycles/min) with variable cycle duration. At E18, their frequency doubled but cycle duration remained variable. After birth, the frequency did not change although cycle duration stabilised. At E18 and P0-P2, the in vitro frequency decreased by around 50% under hypoxia, increased by 40-50% under noradrenaline or substance P and was permanently depressed by the pontine A5 areas. At E16 however, hypoxia had no effects, both noradrenaline and substance P drastically increased the frequency and area A5 inhibition was not expressed at this time. At E18 and P0-P2, electrical stimulation and electrolytic lesion of the rostral ventrolateral medulla affected the in vitro rhythm but failed to induce convincing effects at E16. Thus, a major maturational step in respiratory rhythmogenesis occurs between E16-E18, in agreement with the concept of multiple rhythmogenic mechanisms.

Respiratory rhythm generation: converging concepts from in vitro and in vivo approaches?

The timing and activation pattern of breathing movements are determined by the respiratory network. This network is amenable to a variety of in vivo and in vitro approaches, which offers a unique opportunity to investigate multiple organizational levels. It is only recently, however, that concepts obtained under in vivo and in vitro conditions are being integrated into a coherent model of breathing behavior. For example, the pre-Bötzinger complex as an essential site for rhythm generation was first identified in vitro, but has since been verified in vivo. Conversely, timing signals provided by other central and peripheral neuronal areas have so far been investigated in vivo, but it is now possible to address these issues with more complex in vitro preparations. Several key issues remain unresolved. For example, to what extent is the respiratory pattern controlled independently of the underlying rhythm? Answers to this and other questions require a dissection of mechanisms that is only possible through a complementary combination of experimental approaches.

Long-term modulation of respiratory network activity following anoxia in vitro

Neural networks that produce rhythmic behaviors require flexibility to respond to changes in the internal and external state of the animal. It is important to not only understand how a network responds during such perturbations but also how the network recovers. For example, the respiratory network needs to respond to and recover from temporary changes in oxygen level that can occur during sleep, exercise, and respiratory disorders. During a temporary decrease in oxygen level, there is an increase in respiratory frequency followed by a depression that can lead to complete apnea. Here we used a mouse brain stem slice preparation as a model system to examine the recovery of respiratory network activity after brief episodes of anoxia. We found the respiratory network recovers from a single anoxic episode with a transient increase in fictive respiratory frequency. Although repetitive anoxia does not elicit a greater frequency increase, it does elicit a longer lasting frequency increase persisting < or =90 min. Thus there is a centrally mediated long-lasting influence on the respiratory network elicited by decreased oxygen levels. This modulation occurs as a prolonged facilitation of fictive respiratory frequency after brief repetitive but not single anoxic exposure. These data are important to consider in the context of disorders such as sleep apnea in which brief periodic anoxic episodes are experienced.

Developmental changes in the hypoxic ventilatory response in C57BL/6 mice

C57BL/6 mice are the strain into which most null mutations for neurotransmitters or their receptors are backcrossed. A number of these transgenic mice have recently been shown to have an abnormal respiratory phenotype; however, the postnatal development of the ventilatory response to hypoxia has not been characterized in C57BL/6 mice. The effect of 8% oxygen for 5 min was examined in mice at five periods from P1 to P30 using a body plethysmograph. Neonatal and juvenile animals from P7 to P30 showed a biphasic pattern in hypoxia in which the increase in minute ventilation achieved in the first min declined towards baseline by the fifth minute and was decreased below baseline in the first minute of return to air breathing. In contrast P1-P3 C57BL/6 mice had a sustained increase in both respiratory frequency and tidal volume and their minute volume remained above baseline on return to air. The decline in oxygen consumption, measured in the fifth minute of hypoxia, was not different in P1-P3 mice compared to P8-P10. These results suggest that the earliest response to hypoxia of the respiratory system in this strain is not characterized by a time dependent depression as seen in older animals and in species whose motor systems are relatively more developed at birth.

Effect of hypoxia on the activity of respiratory and non-respiratory modulated retrotrapezoid neurons of the cat

The retrotrapezoid nucleus (RTN), a part of the rostral ventrolateral medulla, is involved in the control of breathing. A recent immunohistological study suggested a possible involvement of the RTN in hypoxic chemoreflex loop. The present electrophysiological study performed in the cat demonstrates that 23 out of 24 RTN neurons were stimulated during the biphasic respiratory response to hypoxia, which consists of a reinforcement followed by a depression of respiratory activity. This confirms the previous immunohistological study. While 15 RTN neurons might integrate either phase I (n = 7) or phase II (n = 8) O2-chemosensitive inputs, the remaining eight RTN neurons stimulated by hypoxia are susceptible to integrate both phase I and phase II O2-chemosensitive inputs. In conclusion, our results suggest that the different subsets of RTN neurons may influence respiratory output by conveying signals originating from peripheral and/or central chemoreceptors stimulated during hypoxia.

Inhibitory mechanisms in hypoxic respiratory depression studied in an in vitro preparation

A medullary-spinal cord preparation without the pons isolated from the neonatal rat was used to investigate the role of inhibitory neurotransmitters in the respiratory depression induced by hypoxia (hypoxic respiratory depression; HRD). The burst frequency (C(4)-f) and peak amplitudes of the integrated activity of the C(4) roots (integral C(4)) and of the hypoglossal nerve (integral XII) were recorded. A marked decrease in C(4)-f (to 36+/-6% of control, P<0. 05) with no change in the peak amplitudes of integral C(4) or integral XII was observed 17-21 min after superfusion with hypoxic CSF bubbled with 5% CO(2) in N(2). Antagonists of GABA(A) (bicuculline; 10 microM), GABA(B) (phaclofen; 0.2-0.5 mM), glycine (strychnine; 10 mM), adenosine (aminophylline; 100 mM) or opioid (naloxone; 1 mM) receptors were added to the bathing solution to block inhibitory synaptic transmission. Among these antagonists, only strychnine and naloxone alleviated HRD reducing the decline in C(4)-f to 57+/-11 and 53+/-6%, respectively (P<0.05). Posthypoxic neural arrest (PHNA) following resumption of oxygenation was shortened by the application of aminophylline, strychnine or naloxone (by 91+/-17, 96+/-25 and 40+/-6 s, respectively, P<0.05). These findings indicate that the reduction in the frequency component of HRD depends on glycinergic and opioid-mediated neuronal inhibition in an in vitro medullary spinal cord preparation. It was also observed that the duration of PHNA was positively correlated with the severity of the fall in C(4)-f (r=0.60, P<0.01).

Reconfiguration of the neural network controlling multiple breathing patterns: eupnea, sighs and gasps [see comment]

Are different forms of breathing derived from one or multiple neural networks? We demonstrate that brainstem slices containing the pre-Bötzinger complex generated two rhythms when normally oxygenated, with striking similarities to eupneic ('normal') respiration and sighs. Sighs were triggered by eupneic bursts under control conditions, but not in the presence of strychnine (1 microM). Although all neurons received synaptic inputs during both activities, the calcium channel blocker cadmium (4 microM) selectively abolished sighs. In anoxia, sighs ceased, and eupneic activity was reconfigured into gasping, which like eupnea was insensitive to 4 microM cadmium. This reconfiguration was accompanied by suppression of synaptic inhibition. We conclude that a single medullary network underlies multiple breathing patterns.

Mechanisms regulating hypoxic respiratory depression during fetal and postnatal life

Selected topics in the respiratory response to acute hypoxia in the fetus and newborn are reviewed. Peripheral chemoreceptors acting through ionotrophic glutamate receptors play an important role in affecting the initial augmentation phase. Whether fall off in peripheral chemoreceptor activity contributes to the secondary depressive phase remains controversial. A number of approaches including permanent electrolytic and reversible cooling lesions, Fos protein activation, and double-labeling immunohistochemistry has converged to show that an area in and around the locus ceruleus in the rostral pons affects the central depression. There is evidence that this is mediated by catecholamines acting at alpha(2)-adrenergic receptors. Tonic activity in early expiratory (postinspiratory) neurons may contribute to hypoxia-induced apneic episodes in the fetus and newborn. Desensitization of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors has been demonstrated in respiratory-related neurons both in vivo and in vitro. The role that this process might play in the depressive phase of the hypoxic ventilatory response has not been established. In vitro experiments with isolated brain stem-spinal cord preparations or transverse brain stem slices usually involve anoxia, whereas whole animal experiments use 8-15% O(2). Therefore, caution must be exercised in attempting to construct a unifying framework from these two approaches.

Pre-Bötzinger complex functions as a central hypoxia chemosensor for respiration in vivo

Recently, we identified a region located in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) in which activation of ionotropic excitatory amino acid receptors using DL-homocysteic acid (DLH) elicits a variety of excitatory responses in the phrenic neurogram, ranging from tonic firing to a rapid series of high-amplitude, rapid rate of rise, short-duration inspiratory bursts that are indistinguishable from gasps produced by severe systemic hypoxia. Therefore we hypothesized that this unique region is chemosensitive to hypoxia. To test this hypothesis, we examined the response to unilateral microinjection of sodium cyanide (NaCN) into the pre-BötC in chloralose- or chloralose/urethan-anesthetized vagotomized, paralyzed, mechanically ventilated cats. In all experiments, sites in the pre-BötC were functionally identified using DLH (10 mM, 21 nl) as we have previously described. All sites were histologically confirmed to be in the pre-BötC after completion of the experiment. Unilateral microinjection of NaCN (1 mM, 21 nl) into the pre-BötC produced excitation of phrenic nerve discharge in 49 of the 81 sites examined. This augmentation of inspiratory output exhibited one of the following changes in cycle timing and/or pattern: 1) a series of high-amplitude, short-duration bursts in the phrenic neurogram (a discharge similar to a gasp), 2) a tonic excitation of phrenic neurogram output, 3) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a gasplike burst), or 4) an increase in frequency of phrenic bursts accompanied by small increases or decreases in the amplitude of integrated phrenic nerve discharge. Our findings identify a locus in the brain stem in which focal hypoxia augments respiratory output. We propose that the respiratory rhythm generator in the pre-BötC has intrinsic hypoxic chemosensitivity that may play a role in hypoxia-induced gasping.

Development of the ventilatory response to hypoxia in Swiss CD-1 mice

We examined developmental changes in breathing pattern and the ventilatory response to hypoxia (7.4% O(2)) in unanesthetized Swiss CD-1 mice ranging in age from postnatal day 0 to 42 (P(0)-P(42)) using head-out plethysmography. The breathing pattern of P(0) mice was unstable. Apneas were frequent at P(0) (occupying 29 +/- 6% of total time) but rare by P(3) (5 +/- 2% of total time). Tidal volume increased in proportion to body mass ( approximately 10-13 ml/kg), but increases in respiratory frequency (f) (55 +/- 7, 130 +/- 13, and 207 +/- 20 cycles/min for P(0), P(3), and P(42), respectively) were responsible for developmental increases in minute ventilation (690 +/- 90, 1,530 +/- 250, and 2,170 +/- 430 ml. min(-1). kg(-1) for P(0), P(3), and P(42), respectively). Between P(0) and P(3), increases in f were mediated by reductions in apnea and inspiratory and expiratory times; beyond P(3), increases were due to reductions in expiratory time. Mice of all ages showed a biphasic hypoxic ventilatory response, which differed in two respects from the response typical of most mammals. First, the initial hyperpnea, which was greatest in mature animals, decreased developmentally from a maximum, relative to control, of 2.58 +/- 0.29 in P(0) mice to 1. 32 +/- 0.09 in P(42) mice. Second, whereas ventilation typically falls to or below control in most neonatal mammals, ventilation remained elevated relative to control throughout the hypoxic exposure in P(0) (1.73 +/- 0.31), P(3) (1.64 +/- 0.29), and P(9) (1. 34 +/- 0.17) mice but not in P(19) or P(42) mice.

Expression of 15 glutamate receptor subunits and various splice variants in tissue slices and single neurons of brainstem nuclei and potential functional implications

Brainstem nuclei serve a diverse array of functions in many of which ionotropic glutamate receptors are known to be involved. However, little detailed information is available on the expression of different glutamate receptor subunits in specific nuclei. We used RT-PCR in mice to analyze the glutamate receptor subunit composition of the pre-Bötzinger complex, the hypoglossal nucleus, the nucleus of the solitary tract, and the inferior olive. Analyzing 15 receptor subunits and five variants, we found all four alpha-amino-3-hydroxy-5-methyl-4-propionic acid (AMPA) and six NMDA receptor (NR) subunits as well as three of five kainate (KA) receptors (GluR5, GluR6, and KA1) to be expressed in all nuclei. However, some distinct differences were observed: The inferior olive preferentially expresses flop variants of AMPA receptors, GluR7 is more abundant in the pre-Bötzinger complex than in the other nuclei, and NR2C is most prominent in the nucleus of the solitary tract. In single hypoglossal motoneurons and interneurons of the pre-Bötzinger complex investigation of GluR2 editing revealed strong expression of the GluR2-R editing variant, suggesting low Ca2+ permeability of AMPA receptors. Thus, Ca2+-permeable AMPA receptors are unlikely to be the cause for the reported selective vulnerability of hypoglossal motoneurons during excitotoxic events.

Cholinergic modulation of respiratory brain-stem neurons and its function in sleep-wake state determination

1. Shifts in behavioural state are controlled by reciprocal changes in discharge of cholinergic and aminergic groups of brain-stem/pontine neurons. During rapid eye movement (REM) sleep, cholinergic neurons are most active and aminergic neurons are least active. 2. Significant changes occur in the central control of breathing during REM sleep; respiration rate increases in frequency and variability, brain-stem respiratory neuron discharge is generally enhanced and the outputs of some respiratory motor neuron pools are depressed. 3. Hypoglossal motor neurons (HM) control tongue movement and their depression during REM sleep has been implicated in obstructive sleep apnoea. The cellular basis of HM depression has been investigated in vitro and may be due to enhanced activation of cholinergic receptors or decreased activation of aminergic receptors. 4. In vitro preparations that show respiratory rhythmogenesis possess advantages for the investigation of the neurochemical basis of state-dependent changes in respiration. Cholinergic changes in respiratory modulation of HM recorded in rhythmic brain-stem slices from mice depend on the site of activation of cholinergic receptors.

Effect of ethanol upon respiratory-related hypoglossal nerve output of neonatal rat brain stem slices

The actions of ethanol (EtOH) on the respiratory output of the neonatal rat brain stem slice preparation in vitro are described. Ethanol inhibited respiratory-related hypoglossal nerve activity in a dose-dependent manner. The effect of EtOH was evident within 5 min and was reversible on EtOH washout. The actions of EtOH were qualitatively similar to those of two other alcohols, methanol and octanol. We investigated the dose-response relationship for each alcohol and determined that the order of potency was methanol < EtOH << octanol, with EC(50) values of 291 mM, 39.7 mM, and 49.2 microM respectively. Application of either strychnine (5 microM) or bicuculline (5 microM) alone, partially but not significantly, reversed the inhibition of respiratory-related hypoglossal nerve activity produced by 50 mM EtOH. Preincubation of rhythmic slices with a combination of both strychnine and bicuculline (both 5 microM) partially, but significantly, blocked the inhibitory actions of EtOH, suggesting that other mechanisms also play a role in the action of EtOH. Preincubation of the slices with 25 microM APV reduced the relative degree of inhibition caused by EtOH suggesting that N-methyl-D-aspartate (NMDA)-receptor-mediated events can be affected by EtOH. Furthermore inhibition of protein kinase C by incubation with 100 nM staurosporine also reduced the efficacy of EtOH. These results suggest that the actions of EtOH may be mediated via glycine, GABA(A), and NMDA receptors and that activation of protein kinase C is involved in the EtOH-induced inhibition of respiratory-related hypoglossal nerve activity.

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

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

L-type Ca2+ channels in inspiratory neurones of mice and their modulation by hypoxia

1. Whole-cell (ICa) and single Ca2+ channel currents were measured in inspiratory neurones of neonatal mice (4-12 days old). During whole-cell recordings, ICa slowly declined and disappeared within 10-20 min. The run-down was delayed during hypoxia, indicating ICa potentiation. 2. Ca2+ channels were recorded in cell-attached patches using pipettes which contained 110 mM Ba2+. L-type Ca2+ channels exhibited a non-ohmic I-V relationship. The slope conductance was 24 pS below and 50 pS above their null potential. The open probability of the channels increased during oxygen depletion, reaching a maximum 2 min after the onset of hypoxia. Restoration of the oxygen supply brought the channel activity back to initial levels. 3. The channel activity was enhanced by 3-30 microM S(-)Bay K 8644, an agonist of L-type Ca2+ channels. The open probability was increased about 3-fold and the activation curve was shifted by 20 mV in the hyperpolarizing direction. In the presence of the agonist, channel open time increased and long openings appeared. Agonist-modulated channels were also potentiated during oxygen depletion. The effect was due to an increase in open time and a decrease in closed time. The channels were inhibited by bath application of nifedipine (10 microM) and nitrendipine (20 microM). 4. Weak bases such as NH4Cl and TMA increased and weak acids such as sodium acetate and propionate decreased activity of the channels, indicating that they are modulated by intracellular pH. Bath application of 1 microM forskolin enhanced the channel activity, whereas 500 microM NaF suppressed it. 5. L-type Ca2+ channels were modulated by an agonist for mGluR1/5 receptors, (S)-3, 5-dihydrophenylglycine (DHPG, 5 microM). In its presence, the hypoxic facilitation of channels was abolished. 6. After blockade of L-type Ca2+ channels, the respiratory response to hypoxia was modified. The transient enhancement of the respiratory rhythm (augmentation) was no longer evident and the secondary depression occurred earlier. 7. We suggest that L-type Ca2+ channels contribute to the early hypoxic response of the respiratory centre. Glutamate release during hypoxia stimulates postsynaptic metabotropic glutamate receptors, which activate the Ca2+ channels.

Hypoxia activates ATP-dependent potassium channels in inspiratory neurones of neonatal mice

1. The respiratory centre of neonatal mice (4 to 12 days old) was isolated in 700 micro(m) thick brainstem slices. Whole-cell K+ currents and single ATP-dependent potassium (KATP) channels were analysed in inspiratory neurones. 2. In cell-attached patches, KATP channels had a conductance of 75 pS and showed inward rectification. Their gating was voltage dependent and channel activity decreased with membrane hyperpolarization. Using Ca2+-containing pipette solutions the measured conductance was lower (50 pS at 1.5 mM Ca2+), indicating tonic inhibition by extracellular Ca2+. 3. KATP channel activity was reversibly potentiated during hypoxia. Maximal effects were attained 3-4 min after oxygen removal from the bath. Hypoxic potentiation of open probability was due to an increase in channel open times and a decrease in channel closed times. 4. In inside-out patches and symmetrical K+ concentrations, channel currents reversed at about 0 mV. Channel activity was blocked by ATP (300-600 microM), glibenclamide (10-70 microM) and tolbutamide (100-300 microM). 5. In the presence of diazoxide (10-60 microM), the activity of KATP channels was increased both in inside-out, outside-out and cell-attached patches. In outside-out patches, that remained within the slice after excision, the activity of KATP channels was enhanced by hypoxia, an effect that could be mediated by a release of endogenous neuromodulators. 6. The whole-cell K+ current (IK) was inactivated at negative membrane potentials, which resembled the voltage dependence of KATP channel gating. After 3-4 min of hypoxia, K+ currents at both hyperpolarizing and depolarizing membrane potentials increased. IK was partially blocked by tolbutamide (100-300 microM) and in its presence, hypoxic potentiation of IK was abolished. 7. We conclude that KATP channels are involved in the hypoxic depression of medullary respiratory activity.

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

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