Inspiratory-modulated neurons of the rostrolateral pons: effects of pulmonary afferent input

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Regulation of the chemosensory control of breathing by Kölliker-Fuse neurons

The Kölliker-Fuse region (KF) and the lateral parabrachial nucleus (LPBN) have been implicated in the maintenance of cardiorespiratory control. Here, we evaluated the involvement of the KF region and the LPBN in cardiorespiratory responses elicited by chemoreceptor activation in unanesthetized rats. Male Wistar rats (280-330 g; n = 5-9/group) with bilateral stainless-steel guide cannulas implanted in the KF region or the LPBN were used. Injection of muscimol (100 and 200 pmol/100 nl) in the KF region decreased resting ventilation (1,140 ± 68 and 978 ± 100 vs. saline: 1,436 ± 155 ml·kg(-1)·min(-1)), without changing mean arterial pressure (MAP) and heart rate (HR). Bilateral injection of the GABA-A antagonist bicuculline (1 nmol/100 nl) in the KF blocked the inhibitory effect on ventilation (1,418 ± 138 vs. muscimol: 978 ± 100 ml·kg(-1)·min(-1)) elicited by muscimol. Muscimol injection in the KF reduced the increase in ventilation produced by hypoxia (8% O2) (1,827 ± 61 vs. saline: 3,179 ± 325 ml·kg(-1)·min(-1)) or hypercapnia (7% CO2) (1,488 ± 277 vs. saline: 3,539 ± 374 ml·kg(-1)·min(-1)) in unanesthetized rats. Bilateral injection of bicuculline in the KF blocked the decrease in ventilation produced by muscimol in the KF during peripheral or central chemoreflex activation. Bilateral injection of muscimol in the LPBN did not change resting ventilation or the increase in ventilation elicited by hypoxia or hypercapnia. The results of the present study suggest that the KF region, but not the LPBN, has mechanisms to control ventilation in resting, hypoxic, or hypercapnic conditions in unanesthetized rats.

Proteomic analysis of A2780/S ovarian cancer cell response to the cytotoxic organogold(III) compound Aubipy(c)

Aubipyc is an organogold(III) compound endowed with encouraging anti-proliferative properties in vitro that is being evaluated pre-clinically as a prospective anticancer agent. A classical proteomic approach is exploited here to elucidate the mechanisms of its biological actions in A2780 human ovarian cancer cells. Based on 2-D gel electrophoresis separation and subsequent mass spectrometry identification, a considerable number of differentially expressed proteins were highlighted in A2780 cancer cells treated with Aubipyc. Bioinformatic analysis of the groups of up-regulated and down-regulated proteins pointed out that Aubipyc primarily perturbs mitochondrial processes and the glycolytic pathway. Notably, some major alterations in the glycolytic pathway were validated through Western blot and metabolic investigations.

BIOLOGICAL SIGNIFICANCE

This is the first proteomic analysis regarding Aubipyc cytotoxicity in A2780/S ovarian cancer cell line. Aubipyc is a promising gold(III) compound which manifests an appreciable cytotoxicity toward the cell line A2780, being able to overcome resistance to platinum. The proteomic study revealed for Aubipyc different cellular alterations with respect to cisplatin as well as to other gold compound such as auranofin. Remarkably, the bioinformatic analysis of proteomic data pointed out that Aubipyc treatment affected, directly or indirectly, several glycolytic enzymes. These data suggest a new mechanism of action for this gold drug and might have an impact on the use of gold-based drug in cancer treatment.

Control of breathing by interacting pontine and pulmonary feedback loops

The medullary respiratory network generates respiratory rhythm via sequential phase switching, which in turn is controlled by multiple feedbacks including those from the pons and nucleus tractus solitarii; the latter mediates pulmonary afferent feedback to the medullary circuits. It is hypothesized that both pontine and pulmonary feedback pathways operate via activation of medullary respiratory neurons that are critically involved in phase switching. Moreover, the pontine and pulmonary control loops interact, so that pulmonary afferents control the gain of pontine influence of the respiratory pattern. We used an established computational model of the respiratory network (Smith et al., 2007) and extended it by incorporating pontine circuits and pulmonary feedback. In the extended model, the pontine neurons receive phasic excitatory activation from, and provide feedback to, medullary respiratory neurons responsible for the onset and termination of inspiration. The model was used to study the effects of: (1) "vagotomy" (removal of pulmonary feedback), (2) suppression of pontine activity attenuating pontine feedback, and (3) these perturbations applied together on the respiratory pattern and durations of inspiration (T(I)) and expiration (T(E)). In our model: (a) the simulated vagotomy resulted in increases of both T(I) and T(E), (b) the suppression of pontine-medullary interactions led to the prolongation of T(I) at relatively constant, but variable T(E), and (c) these perturbations applied together resulted in "apneusis," characterized by a significantly prolonged T(I). The results of modeling were compared with, and provided a reasonable explanation for, multiple experimental data. The characteristic changes in T(I) and T(E) demonstrated with the model may represent characteristic changes in the balance between the pontine and pulmonary feedback control mechanisms that may reflect specific cardio-respiratory disorders and diseases.

Blood pressure changes alter tracheobronchial cough: computational model of the respiratory-cough network and in vivo experiments in anesthetized cats

We tested the hypothesis, motivated in part by a coordinated computational cough network model, that alterations of mean systemic arterial blood pressure (BP) influence the excitability and motor pattern of cough. Model simulations predicted suppression of coughing by stimulation of arterial baroreceptors. In vivo experiments were conducted on anesthetized spontaneously breathing cats. Cough was elicited by mechanical stimulation of the intrathoracic airways. Electromyograms (EMG) of inspiratory parasternal, expiratory abdominal, laryngeal posterior cricoarytenoid (PCA), and thyroarytenoid muscles along with esophageal pressure (EP) and BP were recorded. Transiently elevated BP significantly reduced cough number, cough-related inspiratory, and expiratory amplitudes of EP, peak parasternal and abdominal EMG, and maximum of PCA EMG during the expulsive phase of cough, and prolonged the cough inspiratory and expiratory phases as well as cough cycle duration compared with control coughs. Latencies from the beginning of stimulation to the onset of cough-related diaphragm and abdominal activities were increased. Increases in BP also elicited bradycardia and isocapnic bradypnea. Reductions in BP increased cough number; elevated inspiratory EP amplitude and parasternal, abdominal, and inspiratory PCA EMG amplitudes; decreased total cough cycle duration; shortened the durations of the cough expiratory phase and cough-related abdominal discharge; and shortened cough latency compared with control coughs. Reduced BP also produced tachycardia, tachypnea, and hypocapnic hyperventilation. These effects of BP on coughing likely originate from interactions between barosensitive and respiratory brainstem neuronal networks, particularly by modulation of respiratory neurons within multiple respiration/cough-related brainstem areas by baroreceptor input.

Respiratory and Mayer wave-related discharge patterns of raphé and pontine neurons change with vagotomy

Previous models have attributed changes in respiratory modulation of pontine neurons after vagotomy to a loss of pulmonary stretch receptor "gating" of an efference copy of inspiratory drive. Recently, our group confirmed that pontine neurons change firing patterns and become more respiratory modulated after vagotomy, although average peak and mean firing rates of the sample did not increase (Dick et al., J Physiol 586: 4265-4282, 2008). Because raphé neurons are also elements of the brain stem respiratory network, we tested the hypotheses that after vagotomy raphé neurons have increased respiratory modulation and that alterations in their firing patterns are similar to those seen for pontine neurons during withheld lung inflation. Raphé and pontine neurons were recorded simultaneously before and after vagotomy in decerebrated cats. Before vagotomy, 14% of 95 raphé neurons had increased activity during single respiratory cycles prolonged by withholding lung inflation; 13% exhibited decreased activity. After vagotomy, the average index of respiratory modulation (eta(2)) increased (0.05 +/- 0.10 to 0.12 +/- 0.18 SD; Student's paired t-test, P < 0.01). Time series and frequency domain analyses identified pontine and raphé neuron firing rate modulations with a 0.1-Hz rhythm coherent with blood pressure Mayer waves. These "Mayer wave-related oscillations" (MWROs) were coupled with central respiratory drive and became synchronized with the central respiratory rhythm after vagotomy (7 of 10 animals). Cross-correlation analysis identified functional connectivity in 52 of 360 pairs of neurons with MWROs. Collectively, the results suggest that a distributed network participates in the generation of MWROs and in the coordination of respiratory and vasomotor rhythms.

Cardiorespiratory effects of intertrigeminal area stimulation in vagotomized rats

It has been recently shown that the pontine intertrigeminal region (ITR) plays an important role in respiratory regulation, including vagally mediated apneic reflexes. Neurons of the ITR have connections with the nucleus tractus solitarius and projections to the ventrolateral medulla. However, the functional targets of these projections are not fully defined. Stimulation of ITR neurons produced respiratory effects, but cardiovascular responses have not been explored. We investigated impact of bilateral vagotomy on respiratory and cardiovascular responses to glutamate microinjections within the ITR in ketamine/xylazine anesthetized rats. Cardiorespiratory indices, including breath duration (TT), tidal volume (VT), mean cardiac intervals (RR), systolic blood pressure (SBP), pulse pressure (PP) and their coefficients of variation (CVTT, CVVT, CVSBP, CVPP, respectively) were analyzed in 30 s segments before and after injection of glutamate (10 mM, 30 L) into the ITR. This assessment was carried out both before and after bilateral vagotomy. Glutamate injection evoked apnea and increased CVTT, but these responses were not altered by bilateral vagotomy. In contrast, removing vagal pathways significantly increased volume variability (CVVT), making tidal volume more vulnerable to perturbation from the ITR. Vagotomy prolonged the increase of mean systolic blood pressure observed after glutamate injection and unmasked a delayed but sustained elevation of PP and CVPP after ITR stimulation. The present findings indicate a broad involvement of the ITR in autonomic regulation, including at least cardiovascular and respiratory effects.

Pontine respiratory-modulated activity before and after vagotomy in decerebrate cats

The dorsolateral (DL) pons modulates the respiratory pattern. With the prevention of lung inflation during central inspiratory phase (no-inflation (no-I or delayed-I) tests), DL pontine neuronal activity increased the strength and consistency of its respiratory modulation, properties measured statistically by the eta(2) value. This increase could result from enhanced respiratory-modulated drive arising from the medulla normally gated by vagal activity. We hypothesized that DL pontine activity during delayed-I tests would be comparable to that following vagotomy. Ensemble recordings of neuronal activity were obtained before and after vagotomy and during delayed-I tests in decerebrate, paralysed and ventilated cats. In general, changes in activity pattern during the delayed-I tests were similar to those after vagotomy, with the exception of firing-rate differences at the inspiratory-expiratory phase transition. Even activity that was respiratory-modulated with the vagi intact became more modulated while withholding lung inflation and following vagotomy. Furthermore, we recorded activity that was excited by lung inflation as well as changes that persisted past the stimulus cycle. Computer simulations of a recurrent inhibitory neural network model account not only for enhanced respiratory modulation with vagotomy but also the varied activities observed with the vagi intact. We conclude that (a) DL pontine neurones receive both vagal-dependent excitatory inputs and central respiratory drive; (b) even though changes in pontine activity are transient, they can persist after no-I tests whether or not changes in the respiratory pattern occur in the subsequent cycles; and (c) models of respiratory control should depict a recurrent inhibitory circuitry, which can act to maintain the stability and provide plasticity to the respiratory pattern.

GABAergic neurons in the ventrolateral subnucleus of the nucleus tractus solitarius are in contact with Kölliker-Fuse nucleus neurons projecting to the rostral ventral respiratory group and phrenic nucleus in the rat

After ipsilateral injections of biotinylated dextran amine (BDA) into the ventrolateral subnucleus of the nucleus tractus solitarius (vlNTS) and Fluoro-gold (FG) into the rostral ventral respiratory group (rVRG) region or into the phrenic nucleus (PhN) region in the rat, an overlapping distribution of BDA-labeled axon terminals and FG-labeled neurons was found in the Kölliker-Fuse (KF) nucleus ipsilateral to the injection sites. Using retrograde tracing combined with immunohistochemistry for glutamic acid decarboxylase isoform 67 (GAD67), we indicated that as many as 40% of the vlNTS neurons projecting to the KF were immunoreactive for GAD67. Using a combination of anterograde and retrograde tracing techniques, and immunohistochemistry for GAD67, we further demonstrated that the vlNTS axon terminals with GAD67 immunoreactivity established close contact to the rVRG- or PhN-projecting KF neurons. The present results suggest that GABAergic vlNTS fibers may exert inhibitory influences on the rVRG- as well as PhN-projecting KF neurons and these circuits may be involved in the respiratory reflexes such as the Hering-Breuer reflex.

The Kölliker-Fuse nucleus gates the postinspiratory phase of the respiratory cycle to control inspiratory off-switch and upper airway resistance in rat

Lesion or pharmacological manipulation of the dorsolateral pons can transform the breathing pattern to apneusis (pathological prolonged inspiration). Apneusis reflects a disturbed inspiratory off-switch mechanism (IOS) leading to a delayed phase transition from inspiration to expiration. Under intact conditions the IOS is irreversibly mediated via activation of postinspiratory (PI) neurons within the respiratory network. In parallel, populations of laryngeal premotoneurons manifest the IOS by a brief glottal constriction during the PI phase. We investigated effects of pontine excitation (glutamate injection) or temporary lesion after injection of a GABA-receptor agonist (isoguvacine) on the strength of PI-pool activity determined from respiratory motor outputs or kinesiological measurements of laryngeal resistance in a perfused brainstem preparation. Glutamate microinjections into distinct parts of the pontine Kölliker-Fuse nucleus (KF) evoked a tonic excitation of PI-motor activity or sustained laryngeal constriction accompanied by prolongation of the expiratory phase. Subsequent isoguvacine microinjections at the same loci abolished PI-motor or laryngeal constrictor activity, triggered apneusis and established a variable and decreased breathing frequency. In summary, we revealed that excitation or inhibition of defined areas within the KF activated and blocked PI activity and, consequently, IOS. Therefore, we conclude, first, that descending KF inputs are essential to gate PI activity required for a proper pattern formation and phase control within the respiratory network, at least during absence of pulmonary stretch receptor activity and, secondly, that the KF contains large numbers of laryngeal PI premotor neurons that might have a key role in the regulation of upper airway resistance during reflex control and vocalization.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

Cytoarchitecture of pneumotaxic integration of respiratory and nonrespiratory information in the rat

The "pneumotaxic center" in the Kölliker-Fuse and medial parabrachial nuclei of dorsolateral pons (dl-pons) plays an important role in respiratory phase switching, modulation of respiratory reflex, and rhythmogenesis. Recent electrophysiological and neural tracing data implicate additional pneumotaxic nuclei in (and a broader role for) the dl-pons in integrating respiratory and nonrespiratory information. Here, we examined the cytoarchitecture of the greater pneumotaxic center and its integrating function by using combined extracellular recording and juxtacellular labeling of unit respiratory rhythmic neurons in dl-pons in urethane-anesthetized, vagotomized, paralyzed, and servo-ventilated adult Sprague Dawley rats. Perievent histogram analysis identified four major types of neuronal discharge patterns: inspiratory, expiratory (with three subdivisions), inspiratory-expiratory, and expiratory-inspiratory phase spanning, sometimes with mild tonic background activity. Most recorded neurons were localized in the Kölliker-Fuse and medial parabrachial nuclei, but some were also found in lateral parabrachial nucleus, intertrigeminal nucleus, principal trigeminal sensory nucleus, and supratrigeminal nucleus. The majority of labeled neurons had large and spatially extended dendritic trees that spanned several of these dl-pons subnuclei, often with terminal dendrites ending in the ventral spinocerebellar tract. The distal sections of the primary and higher-order dendrites exhibited rich varicosities, sometimes with dendritic spines. Axons of some labeled neurons were traced all the way to the ventrolateral pons (vl-pons). These findings extend and generalize the classical definition of the pneumotaxic center to include extensive somatic-axonal-dendritic integration of complex descending and ascending respiratory information as well as nociceptive and possibly musculoskeletal and trigeminal information in multiple dl-pons and vl-pons structures in the rat.

Distribution and medullary projection of respiratory neurons in the dorsolateral pons of the rat

The dorsolateral pons around the parabrachial nucleus including the Kölliker-Fuse nucleus is closely linked with the medullary respiratory center and plays an important role in respiratory control. We aimed to elucidate the firing properties, detailed distributions, and medullary projections of pontine respiratory neurons in pentobarbitone-anesthetized, paralyzed, and artificially ventilated rats with intact vagi. A total of 235 respiratory neurons were recorded from the dorsolateral pons in and around the Kölliker-Fuse nucleus. Six types of firing patterns were identified: inspiratory, expiratory-inspiratory phase spanning, inspiratory-expiratory phase spanning, decrementing expiratory, augmenting expiratory, and whole-phase expiratory patterns. Of these, the inspiratory neurons and the expiratory-inspiratory phase spanning neurons, which constituted the largest population (61%), were characterized most carefully by changing lung inflation levels, since under some conditions both showed similar firing patterns. Many (58%) of the 133 respiratory neurons examined were antidromically activated by electrical stimulation of the medulla. They were activated from the ventrolateral medulla around the ventral respiratory group and the Bötzinger complex and from the dorsomedial medulla around the nucleus tractus solitarii and the hypoglossal nucleus. The projections to the dorsomedial medulla were bilateral in many cases, and those to the ventrolateral medulla were unilateral. Of these medullary projections, two specific projections could be characterized in detail. First, many expiratory-inspiratory phase spanning neurons projected to the hypoglossal nucleus, suggesting that these pontine neurons are important premotor neurons of the hypoglossal motoneurons. This projection explains well the hypoglossal inspiratory activity, which is often dissociated from the phrenic inspiratory activity. Second, most whole-phase expiratory neurons that were distributed medially to the KF nucleus sent their axons toward the spinal cord via the midline medulla. These findings provide a new insight into the pontine control of medullary and spinal respiratory function.

Development of adaptive behaviour of the respiratory network: implications for the pontine Kolliker-Fuse nucleus

Breathing is constantly modulated by afferent sensory inputs in order to adapt to changes in behaviour and environment. The pontine respiratory group, in particular the Kolliker-Fuse nucleus, might be a key structure for adaptive behaviours of the respiratory network. Here, we review the anatomical connectivity of the Kolliker-Fuse nucleus with primary sensory structures and with the medullary respiratory centres and focus on the importance of pontine and medullary postinspiratory neurones in the mediation of respiratory reflexes. Furthermore, we will summarise recent findings from our group regarding ontogenetic changes of respiratory reflexes (e.g., the diving response) and provide evidence that immaturity of the Kolliker-Fuse nucleus might account in neonates for a lack of plasticity in sensory evoked modulations of respiratory activity. We propose that a subpopulation of neurones within the Kolliker-Fuse nucleus represent command neurones for sensory processing which are capable of initiating adaptive behaviour in the respiratory network. Recent data from our laboratory suggest that these command neurones undergo substantial postnatal maturation.

Role in the inspiratory off-switch of vagal inputs to rostral pontine inspiratory-modulated neurons

Neurons of the pontine respiratory group (PRG) in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus are believed to play an important role in promoting the inspiratory (I) off-switch (IOS). In decerebrate gallamine-paralyzed cats ventilated with a cycle-triggered pump system (lung inflation during the neural I phase), we studied the effects of vagal (V) afferent inputs on firing of I-modulated neurons (the most numerous population in PRG) and on I duration. The predominant V effect on unit activity was inhibitory, as shown by two types of test: (a) withholding of inflation during an I phase, which produced increase of unit firing and of its respiratory modulation (58/66 neurons in 14 cats), indicating disinhibition due to removal of phasic V input; (b) delivery of afferent V stimulus trains during a no-inflation I phase, which produced decrease of unit firing and of its respiratory modulation (20 neurons). In addition, application of V input during the neural expiratory (E) phase, which lengthened E phase duration, produced no effect on the neurons' firing, suggesting that the inhibition during I was presynaptic in origin. The results may be interpreted by the hypothesis that the medullary late-I presumptive IOS neurons receive excitatory inputs from the PRG I-modulated neurons as well as from V afferents.. With relatively strong V input, this pontine excitatory output is reduced by inhibition, whereas with relatively weak V input that excitatory output is increased due to reduction of inhibition. Thus the pontine and the V influences on the IOS can operate in a complementary manner dependent on state.

Respiration-related afferents to parabrachial pontine regions

The dorsolateral pons around the parabrachial nucleus is an important participant in respiratory control. This area involves various respiration-related neurons, and their respiratory modulation is thought to arise from afferents from medullary respiratory neurons. Today, however, only a limited number of afferent sources have been identified. First, relatively well-characterized afferents to the pons are those originating from two types of the lung stretch receptors, slowly adapting and rapidly adapting receptors. That is, the majority of the second-order relay neurons of these receptors in the nucleus tractus solitarii project to the pons. Second, certain types of respiratory neurons of the medullary respiratory groups are either known to or presumed to project to the pons. For instance, major inhibitory neurons of the Botzinger complex, augmenting and decrementing expiratory neurons, send afferents to the pons. This article overviews such afferents and discusses their connectivity with pontine neurons.

Production of reflex cough by brainstem respiratory networks

Delineation of neural mechanisms involved in reflex cough is essential for understanding its many physiological and clinical complexities, and the development of more desirable antitussive agents. Brainstem networks that generate and modulate the breathing pattern are also involved in producing the motor patterns during reflex cough. Neurones of the ventrolateral medulla respiratory pattern generator mutually interact with neural networks in the pons, medulla and cerebellum to form a larger dynamic network. This paper discusses evidence from our laboratory and others supporting the involvement of the nucleus tractus solitarii, midline raphe nuclei and lateral tegmental field in the medulla, and the pontine respiratory group and cerebellum in the production of reflex cough. Gaps in our knowledge are identified to stimulate further research on this complicated issue.

Pontine respiratory group neuron discharge is altered during fictive cough in the decerebrate cat

A network of neurons in the rostral dorsal lateral pons and pons/mescencephalic junction constitute the pontine respiratory group (PRG) and is essential for reflex cough. As a next step in understanding the role of the PRG in the expression of the cough reflex, we examined neuron firing rates during fictive cough in cats. Decerebrated, thoracotomized, paralyzed, cycle-triggered ventilated adult cats were used. Extracellular activity of many single neurons and phrenic and lumbar neurograms were monitored during fictive cough produced by mechanical stimulation of the intrathoracic trachea. Neurons were tested during control periods for respiratory modulation of firing rate by cycle-triggered histograms and statistical tests. Most respiratory modulated cells were continuously active with various superimposed respiratory patterns; major categories included inspiratory decrementing (I-Dec), expiratory decrementing (E-Dec) and expiratory augmenting (E-Aug). There were alterations in the discharge patterns of respiratory, as well as, non-respiratory modulated neurons during cough. The results suggest an involvement of the PRG in the configuration of the cough motor pattern.

Role of pontile mechanisms in the neurogenesis of eupnea

We have proposed a "switching concept" for the neurogenesis of ventilatory activity. Eupnea reflects the output of a pontomedullary neuronal circuit, whereas gasping is generated by medullary pacemaker mechanisms. Pontile mechanisms, then, are hypothesized to play a fundamental role in the neurogenesis of eupnea. If pontile mechanisms do play such a critical role, several criteria must be fulfilled. First, perturbations of pontile regions must alter eupnea under all experimental conditions. Second, neuronal activities that are consistent with generating the eupneic rhythm must be recorded in pons. Finally, medullary mechanisms alone cannot fully explain the neurogenesis of eupnea. Evidence from previous studies that support the validity of these criteria is presented herein. We conclude that pontile mechanisms play a critical role in the neurogenesis of eupnea.

Commentary on eupneic breathing patterns and gasping

The term "eupneic activity pattern" is a trivial phenotypical description of a particular activity pattern in respiratory nerves as recorded under in vivo like experimental conditions. This term is, however, inadequate, because Eupnea describes a behavioral breathing performance that is trouble-free occurring without conscious effort. Obviously, the term "eupneic activity pattern" is meant to describe a neural activity that is normal and comparable with quiet breathing conditions. The various in vivo, in situ and in vitro preparations all generate their specific "normal" activity patterns, when the conditions are undisturbed. The commentary describes some of the numerous reasons why such normal activity patterns must be different in the various preparations without indicating their pathological operation. The conclusion is that special considerations are necessary for any extension of the in vitro and in situ findings into in vivo situations, because the capacity of the respiratory network is greatly reduced and thus not comparable with conditions leading to "eupneic breathing" in the fully intact animal.

High frequency oscillations in respiratory networks: functionally significant or phenomenological?

Inspiratory activities, whether recorded from medullary neurons, motoneurons or motor nerves, feature prominent oscillations in high (50-120 Hz) and medium (15-50 Hz) frequency ranges. These oscillations have been extensively characterized and are considered signatures of respiratory network activity. Their functional significance, however, if any, remains unknown. Here we review the literature describing the nature and origin of these oscillations as well as their modulation during development and by mechanoreceptive and chemoreceptive feedback, respiratory- and non-respiratory-related behaviors, temperature and anesthesia. We then consider the potential significance of these oscillations for respiratory network function by drawing on analyses of distributed motor and sensory networks of the cortex where current interest in oscillatory activity, and the synchronization of neural discharge that can result, is based on the increased efficacy with which synchronous inputs influence neuronal output, and the role that synchronous activity may play in information coding. We speculate that synchronized oscillations at the network level help coordinate activity in distributed rhythm and pattern generating systems and at the muscle level enhance force development. Data most strongly support that oscillatory synaptic inputs play an important role in controlling timing and pattern of action potential output.

Fast (3 Hz and 10 Hz) and slow (respiratory) rhythms in cervical sympathetic nerve and unit discharges of the cat

1. In seven decerebrate cats, recordings were taken from the preganglionic cervical sympathetic (CSy) nerves and from 74 individual CSy fibres. Correlation and spectral analyses showed that nerve and fibre discharges had several types of rhythm that were coherent (correlated) between population and unit activity: respiratory, '3 Hz' (2-6 Hz, usually cardiac related), and '10 Hz' (7-13 Hz). 2. Almost all units (73/74) had respiratory modulation of their discharge, either phasic (firing during only one phase) or tonic (firing during both the inspiratory (I) and expiratory (E) phases). The most common pattern consisted of tonic I-modulated firing. When the vagi were intact, lung afferent input during I greatly reduced CSy unit and nerve discharge, as evaluated by the no-inflation test. 3. The incidence of unit-nerve coherent fast rhythms (3 Hz or 10 Hz ranges) depended on unit discharge pattern: they were present in an appreciable fraction (30/58 or 52 %) of tonic units, but in only a small fraction (2/15 or 13 %) of phasic units. 4. When baroreceptor innervation (aortic depressor amd carotid sinus nerves) was intact, rhythms correlated to the cardiac cycle frequency were found in 20/34 (59 %) of units. The cardiac origin of these rhythms was confirmed by residual autospectral and partial coherence analysis and by their absence after baroreceptor denervation. 4. The 10 Hz coherent rhythm was found in 7/34 units when baroreceptor innervation was intact, where it co-existed with the cardiac-locked rhythm; after barodenervation it was found in 9/50 neurones. Where both rhythms were present, the 10 Hz component was sometimes synchronized in a 3:1 ratio to the 3 Hz (cardiac-related) frequency component. 5. The tonic and phasic CSy units seem to form distinct populations, as indicated by the differential responses to cardiac-related afferent inputs when baroreceptor innervation is intact. The high incidence of cardiac-related correlation found among tonic units suggests that they are involved in vasomotor regulation. The high incidence of respiratory modulation of discharge suggests that the CSy units may be involved in regulation of the nasal vasculature and consequent ventilation-related control of nasal airway resistance.

Premotor input to hypoglossal motoneurons from Kölliker-Fuse neurons in decerebrate cats

Experiments were performed in 18 paralysed, ventilated, decerebrate adult cats to characterise projections from the Kölliker-Fuse nucleus(KFN) to hypoglossal(HG) motoneurons. Efferent neural activity was recorded from the medial branch of both HG nerves, right recurrent laryngeal(RL) nerve, and C3 branch of the left phrenic nerve. Electrical stimulation(1 Hz, 1 msec pulse duration) of discrete areas within the KFN coordinates elicited a preferential, predominantly ipsilateral burst of HG action potentials with an average stimulus-response latency of 8 msec. Tonic stimulation(5-20 Hz) frequently produced considerable HG temporal summation and the appearance of phasic inspiratory HG activity. Injection of 40-100 nl kainic acid(6.37 mM) into the rostral pontine site with the lowest electrical threshold for HG activation elicited a prolonged tonic HG activation. Following kainate injection, the electrical stimulation threshold for HG excitation increased. Pressure injections of 5-100 nl of 2 mM N-methyl-D-aspartic acid (NMDA)and 2 mM(+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide(AMPA) into the KFN were associated with activation and/or suppression of HG motor output. The results indicate that HG motoneurons innervating protrusor tongue muscles receive a selective projection from the KFN that can be activated by glutamate receptors on KF neurons.

NMDA and GABAA receptors in the rat Kolliker-Fuse area control cardiorespiratory responses evoked by trigeminal ethmoidal nerve stimulation

1. Electrical stimulation (10 s) of the ethmoidal nerve (EN5) evokes the nasotrigeminal reflex responses, including apnoea, bradycardia and rise in arterial blood pressure. In the present study, we examined the involvement of N-methyl-D-aspartate (NMDA), AMPA/kainate, (gamma-aminobutyric acidA (GABAA) and glycine receptors in the Kolliker-Fuse (KF) nucleus in the mediation of the nasotrigeminal reflex responses. 2. Unilateral injections (n = 6) of 50-100 nl of the NMDA receptor antagonist AP5 into the KF area led to a significant blockade of the EN5-evoked respiratory depression and bradycardia. Injections placed into the midlevel of the KF area were most effective (80-90 % blockade). The rise in arterial blood pressure remained unaffected. 3. Unilateral injections (n = 6) of the AMPA/kainate receptor antagonist CNQX into the KF area failed to block EN5-evoked autonomic responses significantly. 4. Unilateral injections (n = 5) of the GABAA receptor antagonist bicuculline enhanced the EN5-evoked respiratory depression and bradycardia. The effect persisted for up to 30 s after stimulation. Bicuculline injections into the midlevel of the KF area were most effective. The increase in arterial blood pressure remained unaffected. 5. Unilateral injections (n = 5) of the glycine receptor antagonist strychnine into the KF area did not produce any significant effects on EN5-evoked autonomic responses. 6. Our results suggest that the KF area represents a mandatory relay for the nasotrigeminally induced apnoea and bradycardia which are predominantly mediated by NMDA receptors in the KF. Furthermore, it appears that KF neurons are under a potent GABAergic inhibitory control. The EN5-evoked rise in arterial blood pressure was not altered by any of the drugs and, therefore, appears not to be mediated via the KF.

Neuronal activities underlying inspiratory termination by pneumotaxic mechanisms

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

Cardiovascular and respiratory effects of stimulation of cell bodies of the parabrachial nuclei in the anaesthetized rat

1. In order to assess the importance of the parabrachial nuclei in modulating cardiorespiratory activity, electric current or microinjections of glutamate were used to stimulate discrete regions of the parabrachial nuclei in anaesthetized rats. 2. Stimulation of cell bodies in the medial region of the parabrachial nuclei and in the Kölliker-Fuse nuclei, caused an expiratory facilitatory response. This consisted mainly of a decrease in respiratory rate as measured by observing phrenic nerve activity. 3. Stimulation of cell bodies in the lateral region of the parabrachial nuclei caused an inspiratory facilitatory response. This consisted mainly of an increase in respiratory rate. 4. At the majority of sites (16 out of 20) where changes in respiratory rate were elicited by glutamate injection or electrical stimulation an increase in blood pressure was observed. The coexistence of increases in blood pressure and heart rate indicates the presence of inhibition of the heart rate component of the baroreflex and/or an increase in cardiac sympathetic drive. 5. The expiratory facilitatory response was not evoked reflexly by the rise in blood pressure since it was still present after administration of guanethidine, which abolished the rise in blood pressure. 6. The interactions between the parabrachial nuclei and the medullary respiratory complex in eliciting these changes are discussed.

Pontine respiratory neurons in anesthetized cats

The pontine respiratory neurons (PRG) in the 'pneumotaxic centre' have been hypothesized to contribute to phase-switching of neural respiratory activity, especially in terminating inspiration. To define the neural elements involved in phase-switching, we recorded respiratory neurons extra- and intracellularly in anesthetized cats with an intact central nervous system. In total, 54 neurons were recorded: 49 neurons with activity modulated by central respiratory rhythm (20 inspiratory, 17 postinspiratory and 12 expiratory) and 5 neurons with activity correlated to tracheal pressure. The recorded neurons were clustered in dorsolateral pontine tegmentum within the Kölliker-Fuse (KF) subnucleus of the parabrachial nuclei. Stable intracellular membrane potential was recorded in 11 of the 49 respiratory neurons (8 postinspiratory, 1 early inspiratory and 2 inspiratory). During continuous injection of chloride ions (n = 6), synaptic noise increased and IPSPs reversed, including a wave of IPSPs during stage-2 expiration in postinspiratory neurons. Further, relative input resistance varied through the respiratory cycle such that the least input resistance occurred during the neuron's (n = 5) quiescent period. No IPSPs nor EPSPs were evoked in pontine respiratory neurons by vagal stimulation. In conclusion, various types of respiratory neurons were recorded in the KF nucleus. Prominent excitatory and inhibitory postsynaptic activities were similar to those described for medullary neurons. These pontine respiratory neurons do not appear to receive a strong afferent input from the vagus. Rather, vagal afferent inputs seem to be directed towards non-respiratory neurons that are located more medially in the dorsal pons.

Spinal cord and trigeminal projections to the pontine parabrachial region in the rat as demonstrated with Phaseolus vulgaris leucoagglutinin

In order to determine the regions within the parabrachial nucleus that receive synaptic input from nociceptive regions of the spinal cord and medulla in the rat, we analyzed the "Golgi-like" labeling produced by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) from discrete iontophoretic injections confined to either the superficial dorsal horn of the lumbar spinal cord or to the superficial dorsal horn of the trigeminal nucleus at the level of the obex. Labeled fibers from both the spinal cord and the medulla ascended through the ventral lateral pons and coursed with the ventral spinocerebellar tract toward the parabrachial nuclei. Spinal cord injections led to labeling of fine caliber fibers and en passant and terminal enlargements in the rostral part of the contralateral lateral parabrachial nucleus (PBL), mostly in the central lateral and dorsal lateral subnuclei. Medullary injections revealed fiber and enlargement labeling primarily in the ipsilateral caudal PBL, mostly in the central lateral, external lateral, and medial subnuclei. Injections in both regions resulted in labeled terminations in the Kölliker-Fuse nucleus. These results indicate that the nociceptive regions of the spinal cord and medulla terminate in regions of the parabrachial nucleus that have been associated with autonomic functions because of their interconnections with the hypothalamus, brainstem cardiovascular and respiratory control centers, and the amygdala.