Medullary Respiratory Network Research Articles

Effects of linalool on respiratory neuron activity in the brainstem-spinal cord preparation from newborn rats.

Linalool and linalyl acetate are major components of lavender essential oil. These substances possess many biological activities, such as anti-inflammatory activity, analgesic and anxiolytic effects, and anticonvulsant properties, and they also induce modulation of neuronal activity in the autonomic nervous system. However, there are no reports of the direct effects of linalool on respiratory activity. In the present study, we analyzed the effects of linalool and linalyl acetate on central respiratory activity in the brainstem-spinal cord preparation isolated from newborn rats. Linalool dose-dependently decreased the rate of respiratory activity. This effect was reversed by bicuculline, suggesting that linalool enhanced inhibitory synaptic connections via GABAA receptors. In addition, linalool reduced the coefficient of variation of inspiratory burst intervals and thus could work to stabilize the respiratory rhythm. Linalyl acetate did not cause inhibitory effects as observed in linalool treatment. Linalool depressed burst activity of pre-inspiratory neurons in the medullary respiratory networks and increased the amplitude of inspiratory inhibitory postsynaptic potentials of pre-inspiratory neurons. We concluded that linalool caused inhibitory effects on respiratory rhythm generation mainly through activation of presynaptic GABAA receptors of pre-inspiratory neurons.

Hydrogen sulfide production in the medullary respiratory center modulates the neural circuit for respiratory pattern and rhythm generations

Hydrogen sulfide (H2S), which is synthesized in the brain, modulates the neural network. Recently, the importance of H2S in respiratory central pattern generation has been recognized, yet the function of H2S in the medullary respiratory network remains poorly understood. Here, to evaluate the functional roles of H2S in the medullary respiratory network, the Bötzinger complex (BötC), the pre-Bötzinger complex (preBötC), and the rostral ventral respiratory group (rVRG), we observed the effects of inhibition of H2S synthesis at each region on the respiratory pattern by using an in situ arterially perfused preparation of decerebrated male rats. After microinjection of an H2S synthase inhibitor, cystathionine β-synthase, into the BötC or preBötC, the amplitude of the inspiratory burst decreased and the respiratory frequency increased according to shorter expiration and inspiration, respectively. These alterations were abolished or attenuated in the presence of a blocker of excitatory synaptic transmission. On the other hand, after microinjection of the H2S synthase inhibitor into the rVRG, the amplitude of the inspiratory burst was attenuated, and the respiratory frequency decreased, which was the opposite effect to those obtained by blockade of inhibitory synaptic transmission at the rVRG. These results suggest that H2S synthesized in the BötC and preBötC functions to limit respiratory frequency by sustaining the respiratory phase and to maintain the power of inspiration. In contrast, H2S synthesized in the rVRG functions to promote respiratory frequency by modulating the interval of inspiration and to maintain the power of inspiration. The underlying mechanism might facilitate excitatory synaptic transmission and/or attenuate inhibitory synaptic transmission.

Chronic pulmonary fibrosis alters the functioning of the respiratory neural network.

Some patients with idiopathic pulmonary fibrosis present impaired ventilatory variables characterised by low forced vital capacity values associated with an increase in respiratory rate and a decrease in tidal volume which could be related to the increased pulmonary stiffness. The lung stiffness observed in pulmonary fibrosis may also have an effect on the functioning of the brainstem respiratory neural network, which could ultimately reinforce or accentuate ventilatory alterations. To this end, we sought to uncover the consequences of pulmonary fibrosis on ventilatory variables and how the modification of pulmonary rigidity could influence the functioning of the respiratory neuronal network. In a mouse model of pulmonary fibrosis obtained by 6 repeated intratracheal instillations of bleomycin (BLM), we first observed an increase in minute ventilation characterised by an increase in respiratory rate and tidal volume, a desaturation and a decrease in lung compliance. The changes in these ventilatory variables were correlated with the severity of the lung injury. The impact of lung fibrosis was also evaluated on the functioning of the medullary areas involved in the elaboration of the central respiratory drive. Thus, BLM-induced pulmonary fibrosis led to a change in the long-term activity of the medullary neuronal respiratory network, especially at the level of the nucleus of the solitary tract, the first central relay of the peripheral afferents, and the Pre-Bötzinger complex, the inspiratory rhythm generator. Our results showed that pulmonary fibrosis induced modifications not only of pulmonary architecture but also of central control of the respiratory neural network.

Efferent network of Neuromedin B neurons in the retrotrapezoid nucleus in mice

In the parafacial region of the medulla, the peptide Neuromedin B (NMB) is expressed in retrotrapezoid nucleus (RTN) neurons that are proposed to be central respiratory chemoreceptors. The efferent network of RTN Nmb neurons has been established in rats, but has not been examined in mice in detail. To map the downstream connections of RTN Nmb neurons we performed genetically-targeted tracing using novel Nmb-Cre knock-in mice. Adult Nmb-Cre mice were injected in the RTN unilaterally with a Cre-dependent AAV producing expression of red fluorescent protein (AAV-Cre ON -mScarlet). Expression of Nmb was identified by fluorescent in situ hybridization (FISH). Injections of AAV-Cre ON -mScarlet in the RTN (N=3) transduced 58 ± 11 % of Nmb-expressing neurons with a selectivity for Nmb neurons of 98 ± 1 %. Intensely labelled axons and terminals were observed in regions overlapping with the medullary and pontine respiratory network. We observe an intra-RTN network of fibers with close appositions between RTN Nmb neurons, as well as moderate projections to areas dorso-medial to the facial nucleus and medially adjacent to the locus coeruleus. There was a modest contralateral projection to the medullary respiratory network, but none to the pontine respiratory network. No labelled fibers were found rostral of the parabrachial nucleus. Anterograde tracing of RTN Nmb neurons was complemented using retrograde transport of Cre-recombinase (AAVrg-Cre) in R26-loxSTOPlox-L10-GFP mice (eGFP-reporter) mice combined with labelling of transcript for Nmb by FISH. Small volumes (3-6 nL) of AAVrg-Cre were unilaterally injected in the Pre-Bötzinger complex (PreBöt), lateral parabrachial nucleus (LPB), caudal ventral respiratory group (cVRG), and the nucleus of the solitary tract (NTS). We determined the proportion of total RTN Nmb neurons that were retro-labelled from each target (PreBöt- 75 ± 3%, n=3; LPB- 65 ± 3%, n=4; cVRG- 54 ± 6%, n=3; NTS- 38 ± 3%, n=3). This result suggests that RTN neurons have a high degree of collateralization within the medullary and pontine respiratory network. Injections of AAVrg-Cre in the PreBöt labelled 16 ± 11% of RTN Nmb neurons contralateral to the injection site. Collectively, these neuroanatomical observations show that the efferent network of RTN Nmb neurons broadly innervate and selectively the central respiratory network in alignment with prior descriptions in rat. Furthermore, we provide evidence for novel connectivity between RTN Nmb neurons. This anatomy supports the idea that RTN neurons generate a coordinated widespread excitation of the respiratory rhythm and pattern generating network in order to promote alveolar ventilation. Funding from NIH-HLBI #HL148004 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Effects of Intra‐arterial Fentanyl and Codeine on Medullary Respiratory Network Organization

The actions of mu‐opioid agonists to depress breathing are currently controversial, encompassing several proposed mechanisms that are focused on preferential effects in specific locations in the brainstem. We hypothesized that opioids have multiple actions at the respiratory network scale which include loss of synchrony and/or network dynamics. Experiments were conducted in 4 anesthetized, spontaneously breathing and 11 decerebrated, artificially ventilated, paralyzed and vagal intact cats. In spontaneously breathing animals, EMGs were recorded from diaphragm, parasternal, abdominal and upper airway muscles. In paralyzed animals, neurograms were recorded from phrenic and abdominal nerves. Fentanyl or codeine were administered via the vertebral artery (i.a.; fentanyl, 1‐10 ng/kg, codeine 1‐3000 μg/kg) while simultaneously recording from multiple medullary neurons. Analyses included assessment of autocorrelation histograms (ACH), significant features in cross correlation histograms (CCHs), and discharge identity (phenotype of firing frequency of a neuron during the breathing cycle). Qualitatively similar opioid‐induced changes in neuron firing rates, ACHs, CCHs, and discharge identity were observed during either fentanyl or codeine dose responses. Some neurons lost or gained excitability during the dose responses. ACHs of some neurons exhibited changes in spiking periodicity. Features in CCHs changed during administration of opioids including loss of some central or offset peaks. Detectability index (a metric of the magnitude of significant functional interactions) in some neuron pairs increased during opioid administration. Further, the discharge identity of many neurons changed during the dose responses with most of these changes occurring in neurons with tonic discharge during breathing. The extent of these changes strongly suggest that opioids induce significant changes in network organization including reduced synchrony and altered discharge identity of network members. These changes can occur at low doses of opioids that do not profoundly alter the breathing pattern.

In Transgenic Erythropoietin Deficient Mice, an Increase in Respiratory Response to Hypercapnia Parallels Abnormal Distribution of CO2/H+-Activated Cells in the Medulla Oblongata.

Erythropoietin (Epo) and its receptor are expressed in central respiratory areas. We hypothesized that chronic Epo deficiency alters functioning of central respiratory areas and thus the respiratory adaptation to hypercapnia. The hypercapnic ventilatory response (HcVR) was evaluated by whole body plethysmography in wild type (WT) and Epo deficient (Epo-TAgh) adult male mice under 4%CO2. Epo-TAgh mice showed a larger HcVR than WT mice because of an increase in both respiratory frequency and tidal volume, whereas WT mice only increased their tidal volume. A functional histological approach revealed changes in CO2/H+-activated cells between Epo-TAgh and WT mice. First, Epo-TAgh mice showed a smaller increase under hypercapnia in c-FOS-positive number of cells in the retrotrapezoid nucleus/parafacial respiratory group than WT, and this, independently of changes in the number of PHOX2B-expressing cells. Second, we did not observe in Epo-TAgh mice the hypercapnic increase in c-FOS-positive number of cells in the nucleus of the solitary tract present in WT mice. Finally, whereas hypercapnia did not induce an increase in the c-FOS-positive number of cells in medullary raphe nuclei in WT mice, chronic Epo deficiency leads to raphe pallidus and magnus nuclei activation by hyperacpnia, with a significant part of c-FOS positive cells displaying an immunoreactivity for serotonin in the raphe pallidus nucleus. All of these results suggest that chronic Epo-deficiency affects both the pattern of ventilatory response to hypercapnia and associated medullary respiratory network at adult stage with an increase in the sensitivity of 5-HT and non-5-HT neurons of the raphe medullary nuclei leading to stimulation of f R for moderate level of CO2.

Role of seizure activity within the parafascicular nucleus in cessation of respiratory rhythm and potentially SUDEP

Sudden unexpected death in epilepsy (SUDEP) is the most common cause of death in patients with uncontrolled epilepsy, yet the mechanisms underlying SUDEP are unknown. Recent data suggests that as seizures propagate in the brain, they can cause respiratory and/or cardiac arrest, leading to death (e.g. SUDEP). Although the precise mechanisms are unknown, seizure-induced respiratory arrest (S-IRA) is the most likely cause of SUDEP. The neural pathways propagating seizure (SZR) activity from areas in the brain where seizures initiate (cortex, hippocampus and thalamus) to autonomic respiratory centers in the brainstem, resulting in respiratory failure and SUDEP are unknown. To identify SZR propagation pathways, I am using a new mouse brainslice preparation that includes cortex, hippocampus and thalamus as well as autonomic medullary areas controlling breathing. Preliminary electrophysiological data suggest that seizure-like activity resulting in repeated activation of the parafascicular nucleus (PFn) of the thalamus, disrupts and then stops rhythmic respiratory activity generated by the medullary respiratory neural network, the pre-Bötzinger Complex (preBötC). I am testing my hypothesis that SZR propagation from the cortex to the PFn can interrupt respiratory activity, which may underlie or significantly contribute to respiratory arrest and/or SUDEP. The centromedial/parafascicular (CMn/PFn) thalamic nuclear complex is a deep brain stimulation target for control of epilepsy. However, little is known about the role the PFn may play in seizure propagation, control of breathing and/or SUDEP. The data support the hypothesis that the PFn may play a role in SUDEP, as it: 1) initiates and/or propagates SZRs; 2) innervates respiratory rhythm networks in the medulla, including the preBötC; and, 3) its activation inhibits breathing.

Probing the function of glycinergic neurons in the mouse respiratory network using optogenetics

Glycine is a primary inhibitory transmitter in the ventral medullary respiratory network, but the functional role of glycinergic neurons for breathing remains a matter of debate. We applied optogenetics to selectively modulate glycinergic neuron activity within regions of the rostral ventral respiratory column (VRC). Responses of the phrenic nerve activity to the light-driven stimulation were studied in the working heart-brainstem preparation from adult glycine transporter 2 Cre mice (GlyT2-Cre), which received a unilateral injection of a Cre-dependent AAV virus into Bötzinger and preBötzinger Complex. Sustained light stimulation from the ventral medullary surface resulted in a substantial depression of the phrenic nerve (PN) frequency, which in most cases was compensated by an increase in PN amplitude. Periodic, burst stimulation with variable intervals could alter and reset respiratory rhythm. We conclude that unilateral activation of the rostral VRC glycinergic neurons can significantly affect respiratory pattern by lengthening the expiratory interval and modulating phase transition.

Bio-Inspired Controller on an FPGA Applied to Closed-Loop Diaphragmatic Stimulation.

Cervical spinal cord injury can disrupt connections between the brain respiratory network and the respiratory muscles which can lead to partial or complete loss of ventilatory control and require ventilatory assistance. Unlike current open-loop technology, a closed-loop diaphragmatic pacing system could overcome the drawbacks of manual titration as well as respond to changing ventilation requirements. We present an original bio-inspired assistive technology for real-time ventilation assistance, implemented in a digital configurable Field Programmable Gate Array (FPGA). The bio-inspired controller, which is a spiking neural network (SNN) inspired by the medullary respiratory network, is as robust as a classic controller while having a flexible, low-power and low-cost hardware design. The system was simulated in MATLAB with FPGA-specific constraints and tested with a computational model of rat breathing; the model reproduced experimentally collected respiratory data in eupneic animals. The open-loop version of the bio-inspired controller was implemented on the FPGA. Electrical test bench characterizations confirmed the system functionality. Open and closed-loop paradigm simulations were simulated to test the FPGA system real-time behavior using the rat computational model. The closed-loop system monitors breathing and changes in respiratory demands to drive diaphragmatic stimulation. The simulated results inform future acute animal experiments and constitute the first step toward the development of a neuromorphic, adaptive, compact, low-power, implantable device. The bio-inspired hardware design optimizes the FPGA resource and time costs while harnessing the computational power of spike-based neuromorphic hardware. Its real-time feature makes it suitable for in vivo applications.

Novel Therapies for the Treatment of Central Sleep Apnea.

Neurophysiologically, central apnea is due to a temporary cessation of respiratory rhythmogenesis in medullary respiratory networks. Central apneas occur in several disorders and result in pathophysiological consequences, including arousals and desaturation. The 2 most common causes in adults are congestive heart failure and chronic use of opioids to treat pain. Under such circumstances, diagnosis and treatment of central sleep apnea may improve quality of life, morbidity, and mortality. This article discusses recent developments in the treatment of central sleep apnea in heart failure and opioids use.

Progressive Changes in a Distributed Neural Circuit Underlie Breathing Abnormalities in Mice Lacking MeCP2.

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in Methyl-CpG-binding protein 2 (MECP2). Severe breathing abnormalities are common in RTT and are reproduced in mouse models of RTT. Previously, we found that removing MeCP2 from the brainstem and spinal cord in mice caused early lethality and abnormal breathing. To determine whether loss of MeCP2 in functional components of the respiratory network causes specific breathing disorders, we used the Cre/LoxP system to differentially manipulate MeCP2 expression throughout the brainstem respiratory network, specifically within HoxA4-derived tissues, which include breathing control circuitry within the nucleus tractus solitarius and the caudal part of ventral respiratory column but do not include more rostral parts of the breathing control circuitry. To determine whether respiratory phenotypes manifested in animals with MeCP2 removed from specific pons medullary respiratory circuits, we performed whole-body plethysmography and electrophysiological recordings from in vitro brainstem slices from mice lacking MeCP2 in different circuits. Our results indicate that MeCP2 expression in the medullary respiratory network is sufficient for normal respiratory rhythm and preventing apnea. However, MeCP2 expression within components of the breathing circuitry rostral to the HoxA4 domain are neither sufficient to prevent the hyperventilation nor abnormal hypoxic ventilatory response. Surprisingly, we found that MeCP2 expression in the HoxA4 domain alone is critical for survival. Our study reveals that MeCP2 is differentially required in select respiratory components for different aspects of respiratory functions, and collectively for the integrity of this network functions to maintain proper respiration. Breathing abnormalities are a significant clinical feature in Rett syndrome and are robustly reproduced in the mouse models of this disease. Previous work has established that alterations in the function of MeCP2, the protein encoded by the gene mutated in Rett syndrome, within the hindbrain are critical for control of normal breathing. Here we show that MeCP2 function plays distinct roles in specific brainstem regions in the genesis of various aspects of abnormal breathing. This provides insight into the pathogenesis of these breathing abnormalities in Rett syndrome, which could be used to target treatments to improve these symptoms. Furthermore, it provides further knowledge about the fundamental neural circuits that control breathing.

Bimodal Respiratory-Locomotor Neurons in the Neonatal Rat Spinal Cord.

Neural networks that can generate rhythmic motor output in the absence of sensory feedback, commonly called central pattern generators (CPGs), are involved in many vital functions such as locomotion or respiration. In certain circumstances, these neural networks must interact to produce coordinated motor behavior adapted to environmental constraints and to satisfy the basic needs of an organism. In this context, we recently reported the existence of an ascending excitatory influence from lumbar locomotor CPG circuitry to the medullary respiratory networks that is able to depolarize neurons of the parafacial respiratory group during fictive locomotion and to subsequently induce an increased respiratory rhythmicity (Le Gal et al., 2014b). Here, using an isolated in vitro brainstem-spinal cord preparation from neonatal rat in which the respiratory and the locomotor networks remain intact, we show that during fictive locomotion induced either pharmacologically or by sacrocaudal afferent stimulation, the activity of both thoracolumbar expiratory motoneurons and interneurons is rhythmically modulated with the locomotor activity. Completely absent in spinal inspiratory cells, this rhythmic pattern is highly correlated with the hindlimb ipsilateral flexor activities. Furthermore, silencing brainstem neural circuits by pharmacological manipulation revealed that this locomotor-related drive to expiratory motoneurons is solely dependent on propriospinal pathways. Together these data provide the first evidence in the newborn rat spinal cord for the existence of bimodal respiratory-locomotor motoneurons and interneurons onto which both central efferent expiratory and locomotor drives converge, presumably facilitating the coordination between the rhythmogenic networks responsible for two different motor functions. Significance statement: In freely moving animals, distant regions of the brain and spinal cord controlling distinct motor acts must interact to produce the best adapted behavioral response to environmental constraints. In this context, it is well established that locomotion and respiration must to be tightly coordinated to reduce muscular interferences and facilitate breathing rate acceleration during exercise. Here, using electrophysiological recordings in an isolated in vitro brainstem-spinal cord preparation from neonatal rat, we report that the locomotor-related signal produced by the lumbar central pattern generator for locomotion selectively modulates the intracellular activity of spinal respiratory neurons engaged in expiration. Our results thus contribute to our understanding of the cellular bases for coordinating the rhythmic neural circuitry responsible for different behaviors.

Breathing Inhibited When Seizures Spread to the Amygdala and upon Amygdala Stimulation.

Sudden unexpected death in epilepsy (SUDEP) is the most common cause of death in patients with chronic refractory epilepsy. Impaired breathing during and after seizures is common and suspected to play a role in SUDEP. Understanding the cause of this peri-ictal hypoventilation may lead to preventative strategies. In epilepsy patients, we found that seizure invasion of the amygdala co-occurred with apnea and oxygen desaturation, and electrical stimulation of the amygdala reproduced these respiratory findings. Strikingly, the subjects were unaware of the apnea. These findings indicate a functional connection between the amygdala and brainstem respiratory network in humans and suggest that amygdala seizures may cause loss of spontaneous breathing of which patients are unaware-a combination that could be deadly.

Getting jittery about the mechanism of hypertension in sleep apnoea.

Sleep apnoea kills. The prevalence of the sleep-related breathing disorder is high and apparently on the rise. Growing too is the alarming list of associated co-morbidities. What is crystal clear is the causal relationship between sleep apnoea and hypertension. In obstructive sleep apnoea (OSA), recurrent airway obstruction during the night gives rise to repeated bouts of hypoxaemia, with resultant elevated sympathetic nervous activity and pressor responses. The pernicious spillover of heightened sympathetic tone persists during the day when patients do not experience obstructive airway events; thus, sympathetic overactivity is strongly implicated in the manifestation of the daytime hypertensive phenotype in OSA (Hedner et al. 1988). Mechanisms of sympathoexcitation and the subsequent development of hypertension remain elusive. It is established that presympathetic neurones of the rostral ventrolateral medulla show activity entrained to the respiratory cycle. Enhanced respiratory–sympathetic coupling is observed in several animal models of neurogenic hypertension (Moraes et al. 2012, 2014). Importantly, elevated respiratory–sympathetic coupling has been shown to present before the onset of high blood pressure, suggesting a causal influence. Of interest, hyperexcitability in pre- and postinspiratory neurones of the medullary respiratory network has been identified as a key driver of enhanced sympathetic activity in the spontaneously hypertensive rat (Moraes et al. 2014), which is very exciting indeed. Arguing against this potential unifying hypothesis, however, are data that appear to show no increase in the respiratory modulation of sympathetic vasoconstrictor drive in human patients with essential hypertension (Fatouleh & Macefield, 2011). What about OSA? Perturbation in the central control of breathing is a feature of the syndrome. Perhaps enhanced respiratory–sympathetic coupling drives aberrant sympathetic outflow in OSA patients? In this issue of Experimental Physiology, Fatouleh et al. (2014) have the nerve to suggest otherwise! Into the air; and what seemed corporal melted As breath into the wind. Would they had stay'd! So that's that? Not quite: sapere vedere. Time to catch your breath! To explore further the temporal profile of the respiratory modulation of sympathetic nervous output, correlations were constructed allowing a comprehensive analysis of the normalized timing of MSNA activity relative to the respective respiratory cycles for each test subject. This approach revealed a greater respiratory modulation of MSNA in OSA patients, with lower mean MSNA activity during inspiration and expiration, but elevated activity in the postinspiratory phase in comparison to control subjects. In short, whilst the magnitude of the respiratory modulation does not differ between OSA patients and control subjects, a temporal jitter exists such that respiratory–sympathetic coupling is de facto more pronounced in OSA patients. Of note, CPAP treatment had mixed effects on the syncopated respiratory-related rhythm of MSNA in OSA patients (though the authors contend that it was largely reversed by treatment). Of particular interest, CPAP did not completely reverse MSNA levels to control. The relevance of this finding lies in the recognized failure of CPAP to lower blood pressure in OSA patients, including those with confirmed compliant CPAP usage, such as the patients in the study by Fatouleh et al. (2014). And breathe short-winded accents of new broils To be commenced in strands afar remote Those battles lie ahead, but the pressure is rising. We will remain sympathetic to the cause. But should we continue to look for inspiration or simply get to the heart of the matter? Readers are invited to give their opinion on this article. To submit a comment, go to: http://ep.physoc.org/letters/submit/expphysiol;99/10/1283. None declared. None declared.

Medullary respiratory network drives sympathetic overactivity and hypertension in rats submitted to chronic intermittent hypoxia

We hypothesized that hypertension following chronic intermittent hypoxia (CIH) exposure is, at least in part, dependent on alterations in the respiratory network and its interaction with the sympathetic nervous system. With simultaneous recordings of respiratory and sympathetic nerves and whole cell path-clamp of respiratory and pre-sympathetic neurons of ventrolateral medulla, we evaluated the mechanisms underlining CIH-induced hypertension in juvenile rats. In CIH rats, the decrease in the frequency discharge of post-inspiratory (post-I) neurons reduced the post-I activity of cervical vagus nerve.

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 [1] and extended it by incorporating pontine circuits and pulmonary feedback (Figure ​(Figure1).1). 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) (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 (TI) and expiration (TE). Figure 1 A general schematic diagram representing the system with two interacting feedback loops. In our model: (a) the simulated vagotomy resulted in increases of both TI and TE, (b) the suppression of pontine-medullary interactions led to the prolongation of TI at relatively constant, but variable TE, and (c) these perturbations applied together resulted in apneusis, characterized by a significantly prolonged TI. The results of modeling were compared with, and provided a reasonable explanation for, multiple experimental data. The model was able to reproduce the experimentally demonstrated changes in TI and TE and phrenic pattern following vagotomy and/or pontine suppression by NMDA receptor blockers MK801and AP-5. According to the model these changes reflect the characteristic changes in the balance between the pontine and pulmonary feedback mechanisms involved in control of breathing during various cardio-respiratory disorders and diseases.

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 (TI) and expiration (TE). In our model: (a) the simulated vagotomy resulted in increases of both TI and TE, (b) the suppression of pontine-medullary interactions led to the prolongation of TI at relatively constant, but variable TE, and (c) these perturbations applied together resulted in “apneusis,” characterized by a significantly prolonged TI. The results of modeling were compared with, and provided a reasonable explanation for, multiple experimental data. The characteristic changes in TI and TE 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.

Modulation of Bulbospinal Rostral Ventral Lateral Medulla Neurons by Hypoxia/Hypercapnia but Not Medullary Respiratory Activity

Although sympathetic vasomotor discharge has respiratory modulation, the site(s) responsible for this cardiorespiratory interaction is unknown. One likely source for this coupling is the rostral ventral lateral medulla (RVLM), where presympathetic neurons originate in close apposition to respiratory neurons. The current study tested the hypothesis that RVLM bulbospinal neurons are modulated by medullary respiratory network activity using whole-cell patch-clamp electrophysiological recordings of RVLM neurons while simultaneously recording fictive respiratory bursting activity from the hypoglossal rootlet. Additionally, we examined whether challenges to cardiorespiratory function, mainly hypoxia/hypercapnia, alter the activity of bulbospinal neurons and, secondarily, whether changes in synaptic input mediate these responses. Surprisingly, our results indicate that inspiratory-related activity did not modulate glutamatergic, γ-aminobutyric acid-ergic, or glycinergic synaptic events or spontaneous action potential firing in these RVLM neurons. However, hypoxia/hypercapnia reversibly decreased the frequency of γ-aminobutyric acid and glycine inhibitory postsynaptic currents. Glycinergic inhibitory postsynaptic current frequency was depressed from the fifth through the 10th minute, whereas the depression of γ-aminobutyric acid-ergic events became significant only at the 10th minute of hypoxia/hypercapnia. On the basis of spontaneous firing activity, there were 2 populations of RVLM bulbospinal neurons. The firing frequency of low-discharging RVLM neurons was facilitated by hypoxia/hypercapnia, and this increase depended on reduced inhibitory neurotransmission. The firing frequency in RVLM neurons with high-discharge rates was inhibited, independent of synaptic input, by hypoxia/hypercapnia. This article demonstrates that sympathetic-respiratory coupling is not active in the neonatal brain stem slice, and reductions in inhibitory neurotransmission to low spontaneously active bulbospinal RVLM neurons are responsible for hypoxia/hypercapnia-elicited increases in activity.

Medullary Respiratory Network Drives Sympathetic Overactivity and Hypertension in Rats Submitted to Chronic Intermittent Hypoxia

Hypertension is a pathological condition affecting up to one third of adult population worldwide.1 Over the last few decades, the remarkable progress of the pharmacological antihypertensive therapies, such as β-adrenergic blockers, angiotensin-converting enzyme inhibitors, angiotensin II receptor type 1 antagonists, and diuretics, combined with physical activities and dietary orientations, contributed significantly to improve life quality of millions of hypertensive patients around the world by lowering their arterial pressure. However, in ≈30% of hypertensive patients the arterial blood pressure remains elevated in spite of the use of different pharmacological strategies,1–4 indicating not only that these therapies are not effective for all patients but also that the mechanisms underpinning the chronic increase of arterial pressure are not completely understood. In this context, a relevant example is the hypertension observed in patients suffering of the syndrome of obstructive sleep apnea (OSA). Over the last 2 decades, it has been demonstrated that the OSA, a condition that affects ≈20% of adult population in the United States,5 is an important risk factor for the development of arterial hypertension,6 and it is one of the major causes of secondary hypertension in patients presenting hypertension resistant to pharmacological treatment.7 Regardless of the marked correlation between OSA and arterial hypertension,8 the mechanisms contributing to the development of hypertension in OSA patients are not fully elucidated. There is evidence that hypertension in OSA patients is markedly associated with the long-term exposure to episodic hypoxemia (or intermittent hypoxia) as a consequence of recurrent obstructions of upper airways during sleep.9 The critical role of chronic intermittent hypoxia (CIH) exposure in the development of cardiovascular changes induced by OSA is supported not only by clinical studies reporting the beneficial cardiovascular effects of the treatment of OSA patients with continuous positive airway pressure10 …

Spinal and Pontine Relay Pathways Mediating Respiratory Rhythm Entrainment by Limb Proprioceptive Inputs in the Neonatal Rat

The coordination of locomotion and respiration is widespread among mammals, although the underlying neural mechanisms are still only partially understood. It was previously found in neonatal rat that cyclic electrical stimulation of spinal cervical and lumbar dorsal roots (DRs) can fully entrain (1:1 coupling) spontaneous respiratory activity expressed by the isolated brainstem/spinal cord. Here, we used a variety of preparations to determine the type of spinal sensory inputs responsible for this respiratory rhythm entrainment, and to establish the extent to which limb movement-activated feedback influences the medullary respiratory networks via direct or relayed ascending pathways. During in vivo overground locomotion, respiratory rhythm slowed and became coupled 1:1 with locomotion. In hindlimb-attached semi-isolated preparations, passive flexion-extension movements applied to a single hindlimb led to entrainment of fictive respiratory rhythmicity recorded in phrenic motoneurons, indicating that the recruitment of limb proprioceptive afferents could participate in the locomotor-respiratory coupling. Furthermore, in correspondence with the regionalization of spinal locomotor rhythm-generating circuitry, the stimulation of DRs at different segmental levels in isolated preparations revealed that cervical and lumbosacral proprioceptive inputs are more effective in this entraining influence than thoracic afferent pathways. Finally, blocking spinal synaptic transmission and using a combination of electrophysiology, calcium imaging and specific brainstem lesioning indicated that the ascending entraining signals from the cervical or lumbar limb afferents are transmitted across first-order synapses, probably monosynaptic, in the spinal cord. They are then conveyed to the brainstem respiratory centers via a brainstem pontine relay located in the parabrachial/Kölliker-Fuse nuclear complex.