Phox2b-expressing neurons of the parafacial region regulate breathing rate, inspiration, and expiration in conscious rats

Interactions between Arousal State and CO2 Determine the Activity of Central Chemoreceptor Neurons That Drive Breathing

The homeostatic regulation of pulmonary ventilation, and ultimately arterial PCO2, depends on interactions between respiratory chemoreflexes and arousal state. The ventilatory response to CO2 is triggered by neurons in the retrotrapezoid nucleus (RTN) that function as sensors of central pH, which can be identified in adulthood by the expression of Phox2b and neuromedin B. Here, we examine the dynamic response of genetically defined RTN neurons to hypercapnia and arousal state in freely behaving adult male and female mice using the calcium indicator jGCaMP7 and fiber photometry. We found that hypercapnia vigorously activates RTN neurons with a low CO2 recruitment threshold and with response kinetics that match respiratory activity whereas hypoxia had little effect. RTN activity increased transiently during wakefulness and respiratory-related arousals and rose persistently during rapid eye movement sleep, and their CO2 response persisted under anesthesia. Complementary studies using inhibitory optogenetics show that RTN activity supports eupneic breathing under anesthesia as well as during states of high arousal, but their activity is redundant for voluntary breathing patterns. Collectively, this study demonstrates that CO2-activated RTN neurons are exquisitely sensitive to the arousal state, which determines their contribution to alveolar ventilation in relation to arterial PCO2.

Respiratory Depression Associated with Opioids: A Narrative Review

OPINION

All opioids have a risk of causing respiratory depression and reduced cerebral circulation. Fentanyl has the greatest risk of causing both. This is particularly a concern when combined with illicit opioids such as diamorphine (also known as heroin). Fentanyl should not be used as a frontline potent opioid due its significant risks. Buprenorphine, a schedule III opioid, morphine, or hydromorphone is preferred, followed by oxycodone, which has a significant risk of abuse relative to buprenorphine and morphine. Although all opioids were equally effective in producing analgesia, the relative safety of each opioid is no longer a secondary concern when prescribing. In the face of an international opioid epidemic, clinicians need to choose opioid analgesics safely, wisely, and carefully.

Mapping responses to focal injections of bicuculline in the lateral parafacial region identifies core regions for maximal generation of active expiration

The lateral parafacial area (pFL) is a crucial region involved in respiratory control, particularly in generating active expiration through an expiratory oscillatory network. Active expiration involves rhythmic abdominal (ABD) muscle contractions during late-expiration, increasing ventilation during elevated respiratory demands. The precise anatomical location of the expiratory oscillator within the ventral medulla's rostro-caudal axis is debated. While some studies point to the caudal tip of the facial nucleus (VIIc) as the oscillator's core, others suggest more rostral areas. Our study employed bicuculline (a γ-aminobutyric acid type A [GABA-A] receptor antagonist) injections at various pFL sites (-0.2 mm to +0.8 mm from VIIc) to investigate the impact of GABAergic disinhibition on respiration. These injections consistently elicited ABD recruitment, but the response strength varied along the rostro-caudal zone. Remarkably, the most robust and enduring changes in tidal volume, minute ventilation, and combined respiratory responses occurred at more rostral pFL locations (+0.6/+0.8 mm from VIIc). Multivariate analysis of the respiratory cycle further differentiated between locations, revealing the core site for active expiration generation with this experimental approach. Our study advances our understanding of neural mechanisms governing active expiration and emphasizes the significance of investigating the rostral pFL region.

Genetic identification of medullary neurons underlying congenital hypoventilation

Mutations in the transcription factors encoded by PHOX2B or LBX1 correlate with congenital central hypoventilation disorders. These conditions are typically characterized by pronounced hypoventilation, central apnea, and diminished chemoreflexes, particularly to abnormally high levels of arterial PCO2. The dysfunctional neurons causing these respiratory disorders are largely unknown. Here, we show that distinct, and previously undescribed, sets of medullary neurons coexpressing both transcription factors (dB2 neurons) account for specific respiratory functions and phenotypes seen in congenital hypoventilation. By combining intersectional chemogenetics, intersectional labeling, lineage tracing, and conditional mutagenesis, we uncovered subgroups of dB2 neurons with key functions in (i) respiratory tidal volumes, (ii) the hypercarbic reflex, (iii) neonatal respiratory stability, and (iv) neonatal survival. These data provide functional evidence for the critical role of distinct medullary dB2 neurons in neonatal respiratory physiology. In summary, our work identifies distinct subgroups of dB2 neurons regulating breathing homeostasis, dysfunction of which causes respiratory phenotypes associated with congenital hypoventilation.

Knockdown of PHOX2B in the retrotrapezoid nucleus reduces the central CO2 chemoreflex in rats

PHOX2B is a transcription factor essential for the development of different classes of neurons in the central and peripheral nervous system. Heterozygous mutations in the PHOX2B coding region are responsible for the occurrence of Congenital Central Hypoventilation Syndrome (CCHS), a rare neurological disorder characterised by inadequate chemosensitivity and life-threatening sleep-related hypoventilation. Animal studies suggest that chemoreflex defects are caused in part by the improper development or function of PHOX2B expressing neurons in the retrotrapezoid nucleus (RTN), a central hub for CO2 chemosensitivity. Although the function of PHOX2B in rodents during development is well established, its role in the adult respiratory network remains unknown. In this study, we investigated whether reduction in PHOX2B expression in chemosensitive neuromedin-B (NMB) expressing neurons in the RTN altered respiratory function. Four weeks following local RTN injection of a lentiviral vector expressing the short hairpin RNA (shRNA) targeting Phox2b mRNA, a reduction of PHOX2B expression was observed in Nmb neurons compared to both naive rats and rats injected with the non-target shRNA. PHOX2B knockdown did not affect breathing in room air or under hypoxia, but ventilation was significantly impaired during hypercapnia. PHOX2B knockdown did not alter Nmb expression but it was associated with reduced expression of both Task2 and Gpr4, two CO2/pH sensors in the RTN. We conclude that PHOX2B in the adult brain has an important role in CO2 chemoreception and reduced PHOX2B expression in CCHS beyond the developmental period may contribute to the impaired central chemoreflex function.

Etonogestrel promotes respiratory recovery in an in vivo rat model of central chemoreflex impairment

AIM

The central CO2 chemoreflex is a vital component of respiratory control networks, providing excitatory drive during resting conditions and challenges to blood gas homeostasis. The retrotrapezoid nucleus is a crucial hub for CO2 chemosensitivity; its ablation or inhibition attenuates CO2 chemoreflexes and diminishes restful breathing. Similar phenotypes characterize certain hypoventilation syndromes, suggesting underlying retrotrapezoid nucleus impairment in these disorders. Progesterone stimulates restful breathing and CO2 chemoreflexes. However, its mechanisms and sites of actions remain unknown and the experimental use of synthetic progestins in patients and animal models have been met with mixed respiratory outcomes.

METHODS

We investigated whether acute or chronic administration of the progestinic drug, etonogestrel, could rescue respiratory chemoreflexes following selective lesion of the retrotrapezoid nucleus with saporin toxin. Adult female Sprague Dawley rats were grouped based on lesion size determined by the number of surviving chemosensitive neurons, and ventilatory responses were measured by whole body plethysmography.

RESULTS

Ventilatory responses to hypercapnia (but not hypoxia) were compromised in a lesion-dependent manner. Chronic etonogestrel treatment improved CO2 chemosensitivity selectively in rats with moderate lesion, suggesting that a residual number of chemosensitive neurons are required for etonogestrel-induced CO2 chemoreflex recovery.

CONCLUSION

This study provides new evidence for the use of progestins as respiratory stimulants under conditions of central hypoventilation and provides a new testable model for assessing the mechanism of action of progestins in the respiratory network.

Kir4.1 channels contribute to astrocyte CO2/H+-sensitivity and the drive to breathe

Astrocytes in the retrotrapezoid nucleus (RTN) stimulate breathing in response to CO2/H+, however, it is not clear how these cells detect changes in CO2/H+. Considering Kir4.1/5.1 channels are CO2/H+-sensitive and important for several astrocyte-dependent processes, we consider Kir4.1/5.1 a leading candidate CO2/H+ sensor in RTN astrocytes. To address this, we show that RTN astrocytes express Kir4.1 and Kir5.1 transcripts. We also characterized respiratory function in astrocyte-specific inducible Kir4.1 knockout mice (Kir4.1 cKO); these mice breathe normally under room air conditions but show a blunted ventilatory response to high levels of CO2, which could be partly rescued by viral mediated re-expression of Kir4.1 in RTN astrocytes. At the cellular level, astrocytes in slices from astrocyte-specific inducible Kir4.1 knockout mice are less responsive to CO2/H+ and show a diminished capacity for paracrine modulation of respiratory neurons. These results suggest Kir4.1/5.1 channels in RTN astrocytes contribute to respiratory behavior.

The Coding Logic of Interoception

Interoception, the ability to precisely and timely sense internal body signals, is critical for life. The interoceptive system monitors a large variety of mechanical, chemical, hormonal, and pathological cues using specialized organ cells, organ innervating neurons, and brain sensory neurons. It is important for maintaining body homeostasis, providing motivational drives, and regulating autonomic, cognitive, and behavioral functions. However, compared to external sensory systems, our knowledge about how diverse body signals are coded at a system level is quite limited. In this review, we focus on the unique features of interoceptive signals and the organization of the interoceptive system, with the goal of better understanding the coding logic of interoception.

Whole-brain analysis of CO2 chemosensitive regions and identification of the retrotrapezoid and medullary raphé nuclei in the common marmoset (Callithrix jacchus)

Respiratory chemosensitivity is an important mechanism by which the brain senses changes in blood partial pressure of CO2 (PCO2). It is proposed that special neurons (and astrocytes) in various brainstem regions play key roles as CO2 central respiratory chemosensors in rodents. Although common marmosets (Callithrix jacchus), New-World non-human primates, show similar respiratory responses to elevated inspired CO2 as rodents, the chemosensitive regions in marmoset brain have not been defined yet. Here, we used c-fos immunostainings to identify brain-wide CO2-activated brain regions in common marmosets. In addition, we mapped the location of the retrotrapezoid nucleus (RTN) and raphé nuclei in the marmoset brainstem based on colocalization of CO2-induced c-fos immunoreactivity with Phox2b, and TPH immunostaining, respectively. Our data also indicated that, similar to rodents, marmoset RTN astrocytes express Phox2b and have complex processes that create a meshwork structure at the ventral surface of medulla. Our data highlight some cellular and structural regional similarities in brainstem of the common marmosets and rodents.

Criteria for central respiratory chemoreceptors: experimental evidence supporting current candidate cell groups

An interoceptive homeostatic system monitors levels of CO2/H+ and provides a proportionate drive to respiratory control networks that adjust lung ventilation to maintain physiologically appropriate levels of CO2 and rapidly regulate tissue acid-base balance. It has long been suspected that the sensory cells responsible for the major CNS contribution to this so-called respiratory CO2/H+ chemoreception are located in the brainstem-but there is still substantial debate in the field as to which specific cells subserve the sensory function. Indeed, at the present time, several cell types have been championed as potential respiratory chemoreceptors, including neurons and astrocytes. In this review, we advance a set of criteria that are necessary and sufficient for definitive acceptance of any cell type as a respiratory chemoreceptor. We examine the extant evidence supporting consideration of the different putative chemoreceptor candidate cell types in the context of these criteria and also note for each where the criteria have not yet been fulfilled. By enumerating these specific criteria we hope to provide a useful heuristic that can be employed both to evaluate the various existing respiratory chemoreceptor candidates, and also to focus effort on specific experimental tests that can satisfy the remaining requirements for definitive acceptance.

Neuromedin B-Expressing Neurons in the Retrotrapezoid Nucleus Regulate Respiratory Homeostasis and Promote Stable Breathing in Adult Mice

Respiratory chemoreceptor activity encoding arterial Pco2 and Po2 is a critical determinant of ventilation. Currently, the relative importance of several putative chemoreceptor mechanisms for maintaining eupneic breathing and respiratory homeostasis is debated. Transcriptomic and anatomic evidence suggests that bombesin-related peptide Neuromedin-B (Nmb) expression identifies chemoreceptor neurons in the retrotrapezoid nucleus (RTN) that mediate the hypercapnic ventilatory response, but functional support is missing. In this study, we generated a transgenic Nmb-Cre mouse and used Cre-dependent cell ablation and optogenetics to test the hypothesis that RTN Nmb neurons are necessary for the CO2-dependent drive to breathe in adult male and female mice. Selective ablation of ∼95% of RTN Nmb neurons causes compensated respiratory acidosis because of alveolar hypoventilation, as well as profound breathing instability and respiratory-related sleep disruption. Following RTN Nmb lesion, mice were hypoxemic at rest and were prone to severe apneas during hyperoxia, suggesting that oxygen-sensitive mechanisms, presumably the peripheral chemoreceptors, compensate for the loss of RTN Nmb neurons. Interestingly, ventilation following RTN Nmb -lesion was unresponsive to hypercapnia, but behavioral responses to CO2 (freezing and avoidance) and the hypoxia ventilatory response were preserved. Neuroanatomical mapping shows that RTN Nmb neurons are highly collateralized and innervate the respiratory-related centers in the pons and medulla with a strong ipsilateral preference. Together, this evidence suggests that RTN Nmb neurons are dedicated to the respiratory effects of arterial Pco2/pH and maintain respiratory homeostasis in intact conditions and suggest that malfunction of these neurons could underlie the etiology of certain forms of sleep-disordered breathing in humans.SIGNIFICANCE STATEMENT Respiratory chemoreceptors stimulate neural respiratory motor output to regulate arterial Pco2 and Po2, thereby maintaining optimal gas exchange. Neurons in the retrotrapezoid nucleus (RTN) that express the bombesin-related peptide Neuromedin-B are proposed to be important in this process, but functional evidence has not been established. Here, we developed a transgenic mouse model and demonstrated that RTN neurons are fundamental for respiratory homeostasis and mediate the stimulatory effects of CO2 on breathing. Our functional and anatomic data indicate that Nmb-expressing RTN neurons are an integral component of the neural mechanisms that mediate CO2-dependent drive to breathe and maintain alveolar ventilation. This work highlights the importance of the interdependent and dynamic integration of CO2- and O2-sensing mechanisms in respiratory homeostasis of mammals.

Upregulation of breathing rate during running exercise by central locomotor circuits in mice

While respiratory adaptation to exercise is compulsory to cope with the increased metabolic demand, the neural signals at stake remain poorly identified. Using neural circuit tracing and activity interference strategies in mice, we uncover here two systems by which the central locomotor network can enable respiratory augmentation in relation to running activity. One originates in the mesencephalic locomotor region (MLR), a conserved locomotor controller. Through direct projections onto the neurons of the preBötzinger complex that generate the inspiratory rhythm, the MLR can trigger a moderate increase of respiratory frequency, prior to, or even in the absence of, locomotion. The other is the lumbar enlargement of the spinal cord containing the hindlimb motor circuits. When activated, and through projections onto the retrotrapezoid nucleus (RTN), it also potently upregulates breathing rate. On top of identifying critical underpinnings for respiratory hyperpnea, these data also expand the functional implication of cell types and pathways that are typically regarded as "locomotor" or "respiratory" related.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Neural bases for the genesis and CO2 therapy of periodic Cheyne-Stokes breathing in neonatal male connexin-36 knockout mice

Periodic Cheyne-Stokes breathing (CSB) oscillating between apnea and crescendo-decrescendo hyperpnea is the most common central apnea. Currently, there is no proven therapy for CSB, probably because the fundamental pathophysiological question of how the respiratory center generates this form of breathing instability is still unresolved. Therefore, we aimed to determine the respiratory motor pattern of CSB resulting from the interaction of inspiratory and expiratory oscillators and identify the neural mechanism responsible for breathing regularization induced by the supplemental CO₂ administration. Analysis of the inspiratory and expiratory motor pattern in a transgenic mouse model lacking connexin-36 electrical synapses, the neonatal (P14) Cx36 knockout male mouse, with a persistent CSB, revealed that the reconfigurations recurrent between apnea and hyperpnea and vice versa result from cyclical turn on/off of active expiration driven by the expiratory oscillator, which acts as a master pacemaker of respiration and entrains the inspiratory oscillator to restore ventilation. The results also showed that the suppression of CSB by supplemental 12% CO₂ in inhaled air is due to the stabilization of coupling between expiratory and inspiratory oscillators, which causes the regularization of respiration. CSB rebooted after washout of CO₂ excess when the inspiratory activity depressed again profoundly, indicating that the disability of the inspiratory oscillator to sustain ventilation is the triggering factor of CSB. Under these circumstances, the expiratory oscillator activated by the cyclic increase of CO₂ behaves as an "anti-apnea" center generating the crescendo-decrescendo hyperpnea and periodic breathing. The neurogenic mechanism of CSB identified highlights the plasticity of the two-oscillator system in the neural control of respiration and provides a rationale base for CO₂ therapy.

Cardiorespiratory alterations following intermittent photostimulation of RVLM C1 neurons: Implications for long-term blood pressure, breathing and sleep regulation in freely moving rats

AIM

Sympathoexcitation and sleep-disordered breathing are common contributors for disease progression. Catecholaminergic neurons from the rostral ventrolateral medulla (RVLM-C1) modulate sympathetic outflow and have anatomical projections to respiratory neurons; however, the contribution of highly selective activation of RVLM-C1 neurons on long-term autonomic and breathing (dys)regulation remains to be understood.

METHODS

To explore this relationship, a lentiviral vector carrying the light-sensitive cation channel channelrhodopsin-2 (LVV-PRSX8-ChR2-YFP) was unilaterally injected into the RVLM of healthy rats. On the contralateral side, LVV-PRSX8-ChR2-YFP was co-injected with a specific immunotoxin (DβH-SAP) targeted to eliminate C1 neurons.

RESULTS

Intermittent photostimulation of RVLM-C1 in vivo, in unrestrained freely moving rats, elicited long-term facilitation of the sympathetic drive, a rise in blood pressure and sympatho-respiratory coupling. In addition, photoactivation of RVLM-C1 induced long-lasting ventilatory instability, characterized by oscillations in tidal volume and increased breathing variability, but only during non-rapid eye movement sleep. These effects were not observed when photostimulation of the RVLM was performed in the presence of DβH-SAP toxin.

CONCLUSIONS

The finding that intermittent activation of RVLM-C1 neurons induces autonomic and breathing dysfunction suggest that episodic stimulation of RVLM-C1 may serve as a pathological substrate for the long-term development of cardiorespiratory disorders.

Regulation of blood vessels by ATP in the ventral medullary surface in a rat model of Parkinson's disease

Parkinson's disease (PD) patients often experience impairment of autonomic and respiratory functions. These include conditions such as orthostatic hypotension and sleep apnea, which are highly correlated with dysfunctional central chemoreception. Blood flow is a fundamental determinant of tissue CO₂/H⁺, yet the extent to which blood flow regulation within chemoreceptor regions contributes to respiratory behavior during neurological disease remains unknown. Here, we tested the hypothesis that 6-hydroxydopamine injection to inducing a known model of PD results in dysfunctional vascular homeostasis, biochemical dysregulation, and glial morphology of the ventral medullary surface (VMS). We show that hypercapnia (FiCO₂ = 10%) induced elevated VMS pial vessel constriction in PD animals through a P2-receptor dependent mechanism. Similarly, we found a greater CO₂-induced vascular constriction after ARL67156 (an ectonucleotidase inhibitor) in control and PD-induced animals. In addition, we also report that weighted gene correlational network analysis of the proteomic data showed a protein expression module differentially represented between both groups. This module showed that gene ontology enrichment for components of the ATP machinery were reduced in our PD-model compared to control animals. Altogether, our data indicate that dysfunction in purinergic signaling, potentially through altered ATP bioavailability in the VMS region, may compromise the RTN neuroglial vascular unit in a PD animal model.

Respiratory alkalosis provokes spike-wave discharges in seizure-prone rats

Hyperventilation reliably provokes seizures in patients diagnosed with absence epilepsy. Despite this predictable patient response, the mechanisms that enable hyperventilation to powerfully activate absence seizure-generating circuits remain entirely unknown. By utilizing gas exchange manipulations and optogenetics in the WAG/Rij rat, an established rodent model of absence epilepsy, we demonstrate that absence seizures are highly sensitive to arterial carbon dioxide, suggesting that seizure-generating circuits are sensitive to pH. Moreover, hyperventilation consistently activated neurons within the intralaminar nuclei of the thalamus, a structure implicated in seizure generation. We show that intralaminar thalamus also contains pH-sensitive neurons. Collectively, these observations suggest that hyperventilation activates pH-sensitive neurons of the intralaminar nuclei to provoke absence seizures.

Respiratory rhythm and pattern generation: Brainstem cellular and circuit mechanisms

Breathing movements in mammals are driven by rhythmic neural activity automatically generated within spatially and functionally organized brainstem neural circuits comprising the respiratory central pattern generator (CPG). This chapter reviews up-to-date experimental information and theoretical studies of the cellular and circuit mechanisms of respiratory rhythm and pattern generation operating within critical components of this CPG in the lower brainstem. Over the past several decades, there have been substantial advances in delineating the spatial architecture of essential medullary regions and their regional cellular and circuit properties required to understand rhythm and pattern generation mechanisms. A fundamental concept is that the circuits in these regions have rhythm-generating capabilities at multiple cellular and circuit organization levels. The regional cellular properties, circuit organization, and control mechanisms allow flexible expression of neural activity patterns for a repertoire of respiratory behaviors under various physiologic conditions that are dictated by requirements for homeostatic regulation and behavioral integration. Many mechanistic insights have been provided by computational modeling studies driven by experimental results and have advanced understanding in the field. These conceptual and theoretical developments are discussed.

Central respiratory chemoreception

Brain PCO2 is sensed primarily via changes in [H+]. Small pH changes are detected in the medulla oblongata and trigger breathing adjustments that help maintain arterial PCO2 constant. Larger perturbations of brain CO2/H+, possibly also sensed elsewhere in the CNS, elicit arousal, dyspnea, and stress, and cause additional breathing modifications. The retrotrapezoid nucleus (RTN), a rostral medullary cluster of glutamatergic neurons identified by coexpression of Phoxb and Nmb transcripts, is the lynchpin of the central respiratory chemoreflex. RTN regulates breathing frequency, inspiratory amplitude, and active expiration. It is exquisitely responsive to acidosis in vivo and maintains breathing autorhythmicity during quiet waking, slow-wave sleep, and anesthesia. The RTN response to [H+] is partly an intrinsic neuronal property mediated by proton sensors TASK-2 and GPR4 and partly a paracrine effect mediated by astrocytes and the vasculature. The RTN also receives myriad excitatory or inhibitory synaptic inputs including from [H+]-responsive neurons (e.g., serotonergic). RTN is silenced by moderate hypoxia. RTN inactivity (periodic or sustained) contributes to periodic breathing and, likely, to central sleep apnea. RTN development relies on transcription factors Egr2, Phox2b, Lbx1, and Atoh1. PHOX2B mutations cause congenital central hypoventilation syndrome; they impair RTN development and consequently the central respiratory chemoreflex.

Disordered breathing in a Pitt-Hopkins syndrome model involves Phox2b-expressing parafacial neurons and aberrant Nav1.8 expression

Pitt-Hopkins syndrome (PTHS) is a rare autism spectrum-like disorder characterized by intellectual disability, developmental delays, and breathing problems involving episodes of hyperventilation followed by apnea. PTHS is caused by functional haploinsufficiency of the gene encoding transcription factor 4 (Tcf4). Despite the severity of this disease, mechanisms contributing to PTHS behavioral abnormalities are not well understood. Here, we show that a Tcf4 truncation (Tcf4^tr/+) mouse model of PTHS exhibits breathing problems similar to PTHS patients. This behavioral deficit is associated with selective loss of putative expiratory parafacial neurons and compromised function of neurons in the retrotrapezoid nucleus that regulate breathing in response to tissue CO₂/H⁺. We also show that central Nav1.8 channels can be targeted pharmacologically to improve respiratory function at the cellular and behavioral levels in Tcf4^tr/+ mice, thus establishing Nav1.8 as a high priority target with therapeutic potential in PTHS.

Disordered breathing is a hallmark of Pitt-Hopkins syndrome (PTHS), yet little is known regarding how loss of Tcf4 (gene associated with PTHS) affects development and function of respiratory neurons. Here, the authors show that parafacial respiratory neurons are selectively disrupted in a mouse model of PTHS, and central Nav1.8 channels can be targeted to improve PTHS-associated behavior abnormalities.

Acute intermittent hypoxia evokes ventilatory long-term facilitation and active expiration in unanesthetized rats

Acute intermittent hypoxia (AIH) modifies the functioning of the respiratory network, causing respiratory motor facilitation in anesthetized animals and a compensatory increase in pulmonary ventilation in freely behaving animals. However, it is still unclear whether the ventilatory facilitation induced by AIH in unanesthetized animals is associated with changes in the respiratory pattern. We found that Holtzman male rats (80-150 g) exposed to AIH (10 × 6% O₂ for 30-40 s every 5 min, n = 9) exhibited a prolonged (30 min) increase in baseline minute ventilation (P < 0.05) compared to control animals (n = 13), combined with the occurrence of late expiratory peak flow events, suggesting the presence of active expiration. The increase in ventilation after AIH was also accompanied by reductions in arterial CO₂ and body temperature (n = 5-6, P < 0.05). The systemic treatment with ketanserin (a 5-HT₂ receptor antagonist) before AIH prevented the changes in ventilation and active expiration (n = 11) but potentiated the hypothermic response (n = 5, P < 0.05) when compared to appropriate control rats (n = 13). Our findings indicate that the ventilatory long-term facilitation elicited by AIH exposure in unanesthetized rats is linked to the generation of active expiration by mechanisms that may depend on the activation of serotonin receptors. In contrast, the decrease in body temperature induced by AIH may not require 5-HT₂ receptor activation.

The lamprey respiratory network: Some evolutionary aspects

Breathing is a complex behaviour that involves rhythm generating networks. In this review, we examine the main characteristics of respiratory rhythm generation in vertebrates and, in particular, we describe the main results of our studies on the role of neural mechanisms involved in the neuromodulation of the lamprey respiration. The lamprey respiratory rhythm generator is located in the paratrigeminal respiratory group (pTRG) and shows similarities with the mammalian preBötzinger complex. In fact, within the pTRG a major role is played by glutamate, but also GABA and glycine display important contributions. In addition, neuromodulatory influences are exerted by opioids, substance P, acetylcholine and serotonin. Both structures respond to exogenous ATP with a biphasic response and astrocytes there located strongly contribute to the modulation of the respiratory pattern. The results emphasize that some important characteristics of the respiratory rhythm generating network are, to a great extent, maintained throughout evolution.

Neural mechanisms underlying respiratory regulation within the preBötzinger complex of the rabbit

The preBötzinger complex (preBötC) is a medullary area essential for normal breathing and widely recognized as necessary and sufficient to generate the inspiratory phase of respiration. It has been studied mainly in rodents. Here we report the main results of our studies revealing the characteristics of the rabbit preBötC identified by means of neuronal recordings, D,L-homocysteic acid microinjections and histological controls. A crucial role in the respiratory rhythmogenesis within this neural substrate is played by excitatory amino acids, but also GABA and glycine display important contributions. Increases in respiratory frequency are induced by microinjections of neurokinins, somatostatin as well by serotonin (5-HT) through an action on 5-HT_1A and 5-HT₃ receptors or the disinhibition of a GABAergic circuit. Respiratory depression is observed in response to microinjections of the μ-opioid receptor agonist DAMGO. Our results show similarities and differences with the rodent preBötC and emphasize the importance of comparative studies on the mechanisms underlying respiratory rhythmogenesis in different animal species.

Excitatory and inhibitory modulation of parafacial respiratory neurons in the control of active expiration

In order to increase ventilation, the respiratory system engages active expiration through recruitment of abdominal muscles. Here, we reviewed the new advances in the modulation of parafacial respiratory (pF) region to trigger active expiration. In addition, we also made a comprehensive discussion of experiments indicating that the lateral aspect of the pF (pFL) is anatomically and functionally distinct from the adjacent and partially overlapping chemosensitive neurons of the ventral aspect of the pF (pFV) also named the retrotrapezoid nucleus. Recent evidence suggest a complex network responsible for the generation of active expiration and neuromodulatory systems that influence its activity. The activity of the pFL is tonically inhibited by inhibitory inputs and also receives excitatory inputs from chemoreceptors (central x peripheral) as well as from catecholaminergic C1 neurons. Therefore, the modulatory inputs and the physiological conditions under which these mechanisms are used to recruit active expiration and increase ventilation need further investigation.

Chemoreceptor mechanisms regulating CO2 -induced arousal from sleep

Arousal from sleep in response to CO₂ is a life-preserving reflex that enhances ventilatory drive and facilitates behavioural adaptations to restore eupnoeic breathing. Recurrent activation of the CO₂ -arousal reflex is associated with sleep disruption in obstructive sleep apnoea. In this review we examine the role of chemoreceptors in the carotid bodies, the retrotrapezoid nucleus and serotonergic neurons in the dorsal raphe in the CO₂ -arousal reflex. We also provide an overview of the supra-medullary structures that mediate CO₂ -induced arousal. We propose a framework for the CO₂ -arousal reflex in which the activity of the chemoreceptors converges in the parabrachial nucleus to trigger cortical arousal.

Differential Contribution of the Retrotrapezoid Nucleus and C1 Neurons to Active Expiration and Arousal in Rats

Collectively, the retrotrapezoid nucleus (RTN) and adjacent C1 neurons regulate breathing, circulation and the state of vigilance, but previous methods to manipulate the activity of these neurons have been insufficiently selective to parse out their relative roles. We hypothesize that RTN and C1 neurons regulate distinct aspects of breathing (e.g., frequency, amplitude, active expiration, sighing) and differ in their ability to produce arousal from sleep. Here we use optogenetics and a combination of viral vectors in adult male and female Th-Cre rats to transduce selectively RTN (Phox2b⁺ /Nmb ⁺) or C1 neurons (Phox2b⁺/Th ⁺) with Channelrhodopsin-2. RTN photostimulation modestly increased the probability of arousal. RTN stimulation robustly increased breathing frequency and amplitude; it also triggered strong active expiration but not sighs. Consistent with these responses, RTN innervates the entire pontomedullary respiratory network, including expiratory premotor neurons in the caudal ventral respiratory group, but RTN has very limited projections to brainstem regions that regulate arousal (locus ceruleus, CGRP⁺ parabrachial neurons). C1 neuron stimulation produced robust arousals and similar increases in breathing frequency and amplitude compared with RTN stimulation, but sighs were elicited and active expiration was absent. Unlike RTN, C1 neurons innervate the locus ceruleus, CGRP⁺ processes within the parabrachial complex, and lack projections to caudal ventral respiratory group. In sum, stimulating C1 or RTN activates breathing robustly, but only RTN neuron stimulation produces active expiration, consistent with their role as central respiratory chemoreceptors. Conversely, C1 stimulation strongly stimulates ascending arousal systems and sighs, consistent with their postulated role in acute stress responses.SIGNIFICANCE STATEMENT The C1 neurons and the retrotrapezoid nucleus (RTN) reside in the rostral ventrolateral medulla. Both regulate breathing and the cardiovascular system but in ways that are unclear because of technical limitations (anesthesia, nonselective neuronal actuators). Using optogenetics in unanesthetized rats, we found that selective stimulation of either RTN or C1 neurons activates breathing. However, only RTN triggers active expiration, presumably because RTN, unlike C1, has direct excitatory projections to abdominal premotor neurons. The arousal potential of the C1 neurons is far greater than that of the RTN, however, consistent with C1's projections to brainstem wake-promoting structures. In short, C1 neurons orchestrate cardiorespiratory and arousal responses to somatic stresses, whereas RTN selectively controls lung ventilation and arterial Pco₂ stability.

Chemogenetic modulation of the parafacial respiratory group influences the recruitment of abdominal activity during REM sleep

Current theories on respiratory control postulate that the respiratory rhythm is generated by oscillatory networks in the medulla: preBötzinger complex (preBötC) is the master oscillator responsible for generating inspiration, while parafacial respiratory group (pFRG) drives active expiration through recruitment of expiratory abdominal (ABD) muscle activity. Research addressing the role of pFRG in ventilation and rhythm generation across sleep states is limited. We recently reported the occurrence of ABD recruitment occurring despite the induction of muscle paralysis during REM sleep. This ABD recruitment was associated with increased tidal volume and regularization of the respiratory period in rats. As pFRG generates active expiration through the engagement of ABD muscles, we hypothesized that the expiratory oscillator is also responsible for the ABD recruitment observed during REM sleep. To test this hypothesis, we inhibited and activated pFRG using chemogenetics (i.e. designer receptors exclusively activated by designer drugs) while recording EEG and respiratory muscle EMG activities across sleep-wake cycles in male Sprague-Dawley rats. Our results suggest that inhibition of pFRG reduced the number of REM events expressing ABD recruitment, in addition to the intensity and prevalence of these events. Conversely, activation of pFRG resulted in an increase in the number of REM events in which ABD recruitment was observed, as well as the intensity and prevalence of ABD recruitment. Interestingly, modulation of pFRG activity did not affect ABD recruitment during NREM sleep or wakefulness. These results suggest that the occurrence of ABD recruitment during sleep is dependent on pFRG activity and is state dependent.

Rostral ventrolateral medullary catecholaminergic neurones mediate irregular breathing pattern in volume overload heart failure rats

KEY POINTS

A strong association between disordered breathing patterns, elevated sympathetic activity, and enhanced central chemoreflex drive has been shown in experimental and human heart failure (HF). The aim of this study was to determine the contribution of catecholaminergic rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) to both haemodynamic and respiratory alterations in HF. Apnoea/hypopnoea incidence (AHI), breathing variability, respiratory-cardiovascular coupling, cardiac autonomic control and cardiac function were analysed in HF rats with or without selective ablation of RVLM-C1 neurones. Partial lesion (∼65%) of RVLM-C1 neurones reduces AHI, respiratory variability, and respiratory-cardiovascular coupling in HF rats. In addition, the deleterious effects of central chemoreflex activation on cardiac autonomic balance and cardiac function in HF rats was abolished by ablation of RVLM-C1 neurones. Our findings suggest that RVLM-C1 neurones play a pivotal role in breathing irregularities in volume overload HF, and mediate the sympathetic responses induced by acute central chemoreflex activation.

ABSTRACT

Rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) modulate sympathetic outflow and breathing under normal conditions. Heart failure (HF) is characterized by chronic RVLM-C1 activation, increased sympathetic activity and irregular breathing patterns. Despite studies showing a relationship between RVLM-C1 and sympathetic activity in HF, no studies have addressed a potential contribution of RVLM-C1 neurones to irregular breathing in this context. Thus, the aim of this study was to determine the contribution of RVLM-C1 neurones to irregular breathing patterns in HF. Sprague-Dawley rats underwent surgery to induce volume overload HF. Anti-dopamine β-hydroxylase-saporin toxin (DβH-SAP) was used to selectively lesion RVLM-C1 neurones. At 8 weeks post-HF induction, breathing pattern, blood pressures (BP), respiratory-cardiovascular coupling (RCC), central chemoreflex function, cardiac autonomic control and cardiac function were studied. Reduction (∼65%) of RVLM-C1 neurones resulted in attenuation of irregular breathing, decreased apnoea-hypopnoea incidence (11.1 ± 2.9 vs. 6.5 ± 2.5 events h-1 ; HF+Veh vs. HF+DβH-SAP; P < 0.05) and improved cardiac autonomic control in HF rats. Pathological RCC was observed in HF rats (peak coherence >0.5 between breathing and cardiovascular signals) and was attenuated by DβH-SAP treatment (coherence: 0.74 ± 0.12 vs. 0.54 ± 0.10, HF+Veh vs. HF+DβH-SAP rats; P < 0.05). Central chemoreflex activation had deleterious effects on cardiac function and cardiac autonomic control in HF rats that were abolished by lesion of RVLM-C1 neurones. Our findings reveal that RVLM-C1 neurones play a major role in irregular breathing patterns observed in volume overload HF and highlight their contribution to cardiac dysautonomia and deterioration of cardiac function during chemoreflex activation.

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

KEY POINTS

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

ABSTRACT

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

The Retrotrapezoid Nucleus: Central Chemoreceptor and Regulator of Breathing Automaticity

The ventral surface of the rostral medulla oblongata has been suspected since the 1960s to harbor central respiratory chemoreceptors [i.e., acid-activated neurons that regulate breathing to maintain a constant arterial PCO₂ (PaCO₂)]. The key neurons, a.k.a. the retrotrapezoid nucleus (RTN), have now been identified. In this review we describe their transcriptome, developmental lineage, and anatomical projections. We also review their contribution to CO₂ homeostasis and to the regulation of breathing automaticity during sleep and wake. Finally, we discuss several mechanisms that contribute to the activation of RTN neurons by CO₂in vivo: cell-autonomous effects of protons; paracrine effects of pH mediated by surrounding astrocytes and blood vessels; and excitatory inputs from other CO₂-responsive CNS neurons.

Episodic stimulation of central chemoreceptor neurons elicits disordered breathing and autonomic dysfunction in volume overload heart failure

Enhanced central chemoreflex (CC) gain is observed in volume overload heart failure (HF) and is correlated with autonomic dysfunction and breathing disorders. The aim of this study was to determine the role of the CC in the development of respiratory and autonomic dysfunction in HF. Volume overload was surgically created to induce HF in male Sprague-Dawley rats. Radiotelemetry transmitters were implanted for continuous monitoring of blood pressure and heart rate. After recovering from surgery, conscious unrestrained rats were exposed to episodic hypercapnic stimulation [EHS; 10 cycles/5 min, inspiratory fraction of carbon dioxide (FICO2) 7%] in a whole body plethysmograph for recording of cardiorespiratory function. To determine the contribution of CC to cardiorespiratory variables, selective ablation of chemoreceptor neurons within the retrotrapezoid nucleus (RTN) was performed via injection of saporin toxin conjugated to substance P (SSP-SAP). Vehicle-treated rats (HF+Veh and Sham+Veh) were used as controls for SSP-SAP experiments. Sixty minutes post-EHS, minute ventilation was depressed in sham animals relative to HF animals (ΔV̇e: -5.55 ± 2.10 vs. 1.24 ± 1.35 mL/min 100 g, P < 0.05; Sham+Veh vs. HF+Veh). Furthermore, EHS resulted in autonomic imbalance, cardiorespiratory entrainment, and ventilatory disturbances in HF+Veh but not Sham+Veh rats, and these effects were significantly attenuated by SSP-SAP treatment. Also, the apnea-hypopnea index (AHI) was significantly lower in HF+SSP-SAP rats compared with HF+Veh rats (AHI: 5.5 ± 0.8 vs. 14.4 ± 1.3 events/h, HF+SSP-SAP vs. HF+Veh, respectively, P < 0.05). Finally, EHS-induced respiratory-cardiovascular coupling in HF rats depends on RTN chemoreceptor neurons because it was reduced by SSP-SAP treatment. Overall, EHS triggers ventilatory plasticity and elicits cardiorespiratory abnormalities in HF that are largely dependent on RTN chemoreceptor neurons.

Hypertensive Response to Ischemic Stroke in the Normotensive Wistar Rat

Background and Purpose—

Over 80% of ischemic stroke patients show an abrupt increase in arterial blood pressure in the hours and days following ischemic stroke. Whether this poststroke hypertension is beneficial or harmful remains controversial and the underlying physiological basis is unclear.

Methods—

To investigate the dynamic cardiovascular response to stroke, adult Wistar rats (n=5–8 per group, 393±34 g) were instrumented with telemeters to blood pressure, intracranial pressure, renal sympathetic nerve activity, and brain tissue oxygen in the predicted penumbra (P o 2 ). After 2 weeks of recovery, cardiovascular signals were recorded for a 3-day baseline period, then ischemic stroke was induced via transient middle cerebral artery occlusion, or sham surgery. Cardiovascular signals were then recorded for a further 10 days, and the functional sensorimotor recovery assessed using the cylinder and sticky dot tests.

Results—

Baseline values of all variables were similar between groups. Compared to sham, in the 2 days following stroke middle cerebral artery occlusion produced an immediate, transient rise above baseline in mean blood pressure (21±3 versus 2±4 mm Hg; P <0.001), renal sympathetic nerve activity (54±11% versus 7±4%; P =0.006), and cerebral perfusion pressure (12±5 versus 1±4; P ≤0.001). Intracranial pressure increased more slowly, peaking 3 days after middle cerebral artery occlusion (14±6 versus −1±1 mm Hg; P <0.001). Treating with the antihypertensive agent nifedipine after stroke (1.5–0.75 mg/kg per hour SC) ameliorated poststroke hypertension (12±3 mm Hg on day 1; P =0.041), abolished the intracranial pressure increase (3±1; P <0.001) and reduced cerebral perfusion pressure (10±3 mm Hg; P =0.017). Preventing poststroke hypertension affected neither the recovery of sensorimotor function nor infarct size.

Conclusions—

These findings suggest that poststroke hypertension is immediate, temporally matched to an increase in sympathetic outflow, and elevates cerebral perfusion pressure for several days after stroke, which may enhance cerebral perfusion. Preventing poststroke hypertension does not appear to worsen prognosis after stroke in young, normotensive, and otherwise healthy rats.

Visual Overview—

An online visual overview is available for this article.

Distinct pathways to the parafacial respiratory group to trigger active expiration in adult rats

Active expiration (AE) is part of the breathing phase; it is conditional and occurs when we increase our metabolic demand, such as during hypercapnia, hypoxia, or exercise. The parafacial respiratory group (pFRG) is involved in AE. Data from the literature suggest that excitatory and the absence of inhibitory inputs to the pFRG are necessary to determine AE. However, the source of the inputs to the pFRG that trigger AE remains unclear. We show in adult urethane-anesthetized Wistar rats that the pharmacological inhibition of the medial aspect of the nucleus of the solitary tract (mNTS) or the rostral aspect of the pedunculopontine tegmental nucleus (rPPTg) is able to generate AE. In addition, direct inhibitory projection from the mNTS or indirect cholinergic projection from the rPPTg is able to contact pFRG to trigger AE. The inhibition of the mNTS or the rPPTg under conditions of high metabolic demand, such as hypercapnia (9–10% CO2), did not affect the AE. The present results suggest for the first time that inhibitory sources from the mNTS and a cholinergic pathway from the rPPTg, involving M2/M4 muscarinic receptors, could be important sources to modulate and sustain AE.

The logic behind neural control of breathing pattern

The respiratory rhythm generator is spectacular in its ability to support a wide range of activities and adapt to changing environmental conditions, yet its operating mechanisms remain elusive. We show how selective control of inspiration and expiration times can be achieved in a new representation of the neural system (called a Boolean network). The new framework enables us to predict the behavior of neural networks based on properties of neurons, not their values. Hence, it reveals the logic behind the neural mechanisms that control the breathing pattern. Our network mimics many features seen in the respiratory network such as the transition from a 3-phase to 2-phase to 1-phase rhythm, providing novel insights and new testable predictions.

A spatially dynamic network underlies the generation of inspiratory behaviors

Breathing is a vital rhythmic behavior that originates from neural networks within the brainstem. It is hypothesized that the breathing rhythm is generated by spatially distinct networks localized to discrete kernels or compartments. Here, we provide evidence that the functional boundaries of these compartments expand and contract dynamically based on behavioral or physiological demands. The ability of these rhythmic networks to change in size may allow the breathing rhythm to be very reliable, yet flexible enough to accommodate the large repertoire of mammalian behaviors that must be integrated with breathing.

The ability of neuronal networks to reconfigure is a key property underlying behavioral flexibility. Networks with recurrent topology are particularly prone to reconfiguration through changes in synaptic and intrinsic properties. Here, we explore spatial reconfiguration in the reticular networks of the medulla that generate breathing. Combined results from in vitro and in vivo approaches demonstrate that the network architecture underlying generation of the inspiratory phase of breathing is not static but can be spatially redistributed by shifts in the balance of excitatory and inhibitory network influences. These shifts in excitation/inhibition allow the size of the active network to expand and contract along a rostrocaudal medullary column during behavioral or metabolic challenges to breathing, such as changes in sensory feedback, sighing, and gasping. We postulate that the ability of this rhythm-generating network to spatially reconfigure contributes to the remarkable robustness and flexibility of breathing.

The contribution of endogenous glutamatergic input in the ventral respiratory column to respiratory rhythm

Glutamate is the predominant excitatory neurotransmitter in the ventral respiratory column; however, the contribution of glutamatergic excitation in the individual subregions to respiratory rhythm generation has not been fully delineated. In an adult, in vivo, decerebrate rabbit model during conditions of mild hyperoxic hypercapnia we blocked glutamatergic excitation using the receptor antagonists 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) and d(-)-2-amino-5-phosphonopentanoic acid (AP5). Disfacilitation of the preBötzinger Complex caused a decrease in inspiratory and expiratory duration as well as peak phrenic amplitude and ultimately apnea. Disfacilitation of the Bötzinger Complex caused a decrease in inspiratory and expiratory duration; subsequent disfacilitation of the preBötzinger Complex resulted in complete loss of the respiratory pattern but maintained tonic inspiratory activity. We conclude that glutamatergic drive to the preBötzinger Complex is essential for respiratory rhythm generation. Glutamatergic drive to the Bötzinger Complex significantly affects inspiratory and expiratory phase duration. Bötzinger Complex neurons are responsible for maintaining the silent expiratory phase of the phrenic neurogram.

Inductively coupled, mm-sized, single channel optical neuro-stimulator with intensity enhancer

We introduce a single channel neuro-stimulator consisting of a reflector-coupled microscale light emitting diode (µLED) with an integrated mm-sized wireless receiver (Rx) coil for free-floating, battery-free, untethered optogenetics neuromodulation. The system utilizes a two-coil inductive link to deliver instantaneous power at a low operating frequency (<100 MHz) for continuous optical stimulation with minimized invasiveness and tissue exposure to electromagnetic radiation. Coupling a microscale reflector to the µLED provides significant light intensity enhancement compared to a bare µLED. Our activated stimulators have an operational temperature increase of <1 °C, well below the safety limit of biomedical implants. In vivo experiment and histological analysis verify the efficacy of wireless optical stimulation in the primary visual cortex of rats, using c-Fos biomarker as a reporter of light-evoked neuronal activity.

Role of GABAA Receptors of Parafacial Respiratory Group in Control of Respiration in Rats

The role of GABAergic inhibition in the regulation of activity of the parafacial respiratory group was studied on narcotized mature rats. Microinjections of GABA_A agonist muscimol into the parafacial respiratory group inhibited external respiration and bioelectrical activity of the diaphragmatic muscle resulting in diminished tidal volume and decreased respiration rate accompanied by prolonged inspiration, expiration, and intervals between the inspiratory bursts in the phrenic electromyogram. In contrast, microinjections of GABA_A antagonist bicuculline into this group stimulated respiration and markedly increased its volumetric parameters. In addition, blockade of GABA_A receptors affected the expiratory phase and interburst intervals in the phrenic electromyogram, but produced no effect on respiration rate. These findings attested to contribution of GABA_A-mediated inhibition into the performance of parafacial respiratory group. The study revealed different roles of GABA_A receptors in modulation of the mechanisms of respiration rate control in mature rats and formation of inspiratory motor outputs originating from the parafacial respiratory group.

Distinct parafacial regions in control of breathing in adult rats

Recently, based on functional differences, we subdivided neurons juxtaposed to the facial nucleus into two distinct populations, the parafacial ventral and lateral regions, i.e., pF_V and pF_L. Little is known about the composition of these regions, i.e., are they homogenous or heterogeneous populations? Here, we manipulated their excitability in spontaneously breathing vagotomized urethane anesthetized adult rats to further characterize their role in breathing. In the pF_L, disinhibition or excitation decreased breathing frequency (f) with a concomitant increase of tidal volume (V_T), and induced active expiration; in contrast, reducing excitation had no effect. This result is congruent with pF_L neurons constituting a conditional expiratory oscillator comprised of a functionally homogeneous set of excitatory neurons that are tonically suppressed at rest. In the pF_V, disinhibition increased f with a presumptive reflexive decrease in V_T; excitation increased f, V_T and sigh rate; reducing excitation decreased V_T with a presumptive reflexive increase in f. Therefore, the pF_V, has multiple functional roles that require further parcellation. Interestingly, while hyperpolarization of the pF_V reduces ongoing expiratory activity, no perturbation of pF_V excitability induced active expiration. Thus, while the pF_V can affect ongoing expiratory activity, presumably generated by the pF_L, it does not appear capable of directly inducing active expiration. We conclude that the pF_L contains neurons that can initiate, modulate, and sustain active expiration, whereas the pF_V contains subpopulations of neurons that differentially affect various aspects of breathing pattern, including but not limited to modulation of ongoing expiratory activity.

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.

The Expression of Galanin in the Parafacial Respiratory Group and its Effects on Respiration in Neonatal Rats

The inhibitory peptide galanin is expressed within the retrotrapezoidal nucleus (RTN) - a key central chemoreceptor site that also contains the active expiratory oscillator. It was previously reported that microinjection of galanin into pre-Bötzinger complex - containing the inspiratory oscillator - exerts inhibitory effects on inspiratory motor output and respiratory rhythm. In neonatal rats, the present study aimed to investigate: (1) expression of galanin within the parafacial respiratory group (pFRG), which overlaps anatomically and functionally with the adult RTN, and; (2) effects of galanin on respiratory rhythm using the in vitro brainstem-spinal cord preparation. We showed that 14 ± 2% of Phox2b-immunoreactive (ir) neurons in the parafacial region were also galanin-ir. Galanin peptide expression was confirmed within 3/9 CO₂-sensitive, Phox2b-ir Pre-Inspiratory neurons (Pre-I) recorded in parafacial region. Bath application of galanin (0.1-0.2 µM): (1) decreased the duration of membrane depolarization in both Pre-I and inspiratory pFRG neurons, and; (2) decreased the number of C4 bursts that were associated with each burst in Pre-I neurons within the pFRG. In preparations showing episodic breathing at baseline, the respiratory patterning reverted to the 'normal' pattern of single, uniformly rhythmic C4 bursts (n = 10). In preparations with normal respiratory patterning at baseline, slowing of C4 rhythm (n = 7) resulted although rhythmic bursting in recorded Pre-I neurons remained unperturbed (n = 6). This study therefore demonstrates that galanin is expressed within the pFRG of neonatal rats, including neurons that are intrinsically chemosensitive. Overall the peptide has an inhibitory effect on inspiratory motor output, as previously shown in adults.

The parafacial respiratory group and the control of active expiration

Breathing at rest is typically characterized by three phases: active inspiration, post-inspiration (or stage 1 expiration), and passive expiration (or stage 2 expiration). Breathing during periods of increased respiratory demand, on the other hand, engages active expiration through recruitment of abdominal muscles in order to increase ventilation. It is currently hypothesized that different phases of the respiratory rhythm are driven by three coupled oscillators: the preBötzinger Complex, driving inspiration, the parafacial respiratory group (pFRG), driving active expiration and the post-inspiratory Complex, driving post-inspiration. In this paper we review advances in the understanding of the pFRG and its role in the generation of active expiration across different developmental stages and vigilance states. Recent experiments suggest that the abdominal recruitment varies across development depending on the vigilance state, possibly following the maturation of the network responsible for the generation of active expiration and neuromodulatory systems that influence its activity. The activity of the pFRG is tonically inhibited by GABAergic inputs and strongly recruited by cholinergic systems. However, the sources of these modulatory inputs and the physiological conditions under which these mechanisms are used to recruit active expiration and increase ventilation need further investigation. Some evidence suggests that active expiration during hypercapnia is evoked through disinhibition, while during hypoxia it is elicited through activation of catecholaminergic C1 neurons. Finally, a discussion of experiments indicating that the pFRG is anatomically and functionally distinct from the adjacent and partially overlapping chemosensitive neurons of the retrotrapezoid nucleus is also presented.

Breathing matters

Breathing is a well-described, vital and surprisingly complex behaviour, with behavioural and physiological outputs that are easy to directly measure. Key neural elements for generating breathing pattern are distinct, compact and form a network amenable to detailed interrogation, promising the imminent discovery of molecular, cellular, synaptic and network mechanisms that give rise to the behaviour. Coupled oscillatory microcircuits make up the rhythmic core of the breathing network. Primary among these is the preBötzinger Complex (preBötC), which is composed of excitatory rhythmogenic interneurons and excitatory and inhibitory pattern-forming interneurons that together produce the essential periodic drive for inspiration. The preBötC coordinates all phases of the breathing cycle, coordinates breathing with orofacial behaviours and strongly influences, and is influenced by, emotion and cognition. Here, we review progress towards cracking the inner workings of this vital core.

Confocal calcium imaging analysis of respiratory-related burst activity in the parafacial region

The parafacial respiratory group (pFRG) surrounding the ventrolateral part of the facial motor nucleus is one of respiratory rhythm generators that consists of pre-inspiratory (Pre-I) neurons. Previous studies showed that most of the Pre-I neurons locating in the Phox2b cluster of the rostral ventral medulla were also Phox2b positive and intrinsically CO₂ sensitive. However, it is not clear what percentage of Phox2b-expressing cells in the pFRG of the ventral medulla are Pre-I neurons. To address this issue, we analyzed the activity of Phox2b-positive cells by calcium imaging using a confocal laser microscope in transgenic rats in which Phox2b-positive cells expressed EYFP. We found that more than 60% of the EYFP/Phox2b-positive cells showed Pre-I neuron-like rhythmic burst activity in the parafacial region of newborn rat.

Interdependent feedback regulation of breathing by the carotid bodies and the retrotrapezoid nucleus

The retrotrapezoid nucleus (RTN) regulates breathing in a CO₂ - and state-dependent manner. RTN neurons are glutamatergic and innervate principally the respiratory pattern generator; they regulate multiple aspects of breathing, including active expiration, and maintain breathing automaticity during non-REM sleep. RTN neurons encode arterial PCO2 /pH via cell-autonomous and paracrine mechanisms, and via input from other CO₂ -responsive neurons. In short, RTN neurons are a pivotal structure for breathing automaticity and arterial PCO2 homeostasis. The carotid bodies stimulate the respiratory pattern generator directly and indirectly by activating RTN via a neuronal projection originating within the solitary tract nucleus. The indirect pathway operates under normo- or hypercapnic conditions; under respiratory alkalosis (e.g. hypoxia) RTN neurons are silent and the excitatory input from the carotid bodies is suppressed. Also, silencing RTN neurons optogenetically quickly triggers a compensatory increase in carotid body activity. Thus, in conscious mammals, breathing is subject to a dual and interdependent feedback regulation by chemoreceptors. Depending on the circumstance, the activity of the carotid bodies and that of RTN vary in the same or the opposite directions, producing additive or countervailing effects on breathing. These interactions are mediated either via changes in blood gases or by brainstem neuronal connections, but their ultimate effect is invariably to minimize arterial PCO2 fluctuations. We discuss the potential relevance of this dual chemoreceptor feedback to cardiorespiratory abnormalities present in diseases in which the carotid bodies are hyperactive at rest, e.g. essential hypertension, obstructive sleep apnoea and heart failure.

The Kölliker-Fuse nucleus orchestrates the timing of expiratory abdominal nerve bursting

Coordination of respiratory pump and valve muscle activity is essential for normal breathing. A hallmark respiratory response to hypercapnia and hypoxia is the emergence of active exhalation, characterized by abdominal muscle pumping during the late one-third of expiration (late-E phase). Late-E abdominal activity during hypercapnia has been attributed to the activation of expiratory neurons located within the parafacial respiratory group (pFRG). However, the mechanisms that control emergence of active exhalation, and its silencing in restful breathing, are not completely understood. We hypothesized that inputs from the Kölliker-Fuse nucleus (KF) control the emergence of late-E activity during hypercapnia. Previously, we reported that reversible inhibition of the KF reduced postinspiratory (post-I) motor output to laryngeal adductor muscles and brought forward the onset of hypercapnia-induced late-E abdominal activity. Here we explored the contribution of the KF for late-E abdominal recruitment during hypercapnia by pharmacologically disinhibiting the KF in in situ decerebrate arterially perfused rat preparations. These data were combined with previous results and incorporated into a computational model of the respiratory central pattern generator. Disinhibition of the KF through local parenchymal microinjections of gabazine (GABA_A receptor antagonist) prolonged vagal post-I activity and inhibited late-E abdominal output during hypercapnia. In silico, we reproduced this behavior and predicted a mechanism in which the KF provides excitatory drive to post-I inhibitory neurons, which in turn inhibit late-E neurons of the pFRG. Although the exact mechanism proposed by the model requires testing, our data confirm that the KF modulates the formation of late-E abdominal activity during hypercapnia. NEW & NOTEWORTHY The pons is essential for the formation of the three-phase respiratory pattern, controlling the inspiratory-expiratory phase transition. We provide functional evidence of a novel role for the Kölliker-Fuse nucleus (KF) controlling the emergence of abdominal expiratory bursts during active expiration. A computational model of the respiratory central pattern generator predicts a possible mechanism by which the KF interacts indirectly with the parafacial respiratory group and exerts an inhibitory effect on the expiratory conditional oscillator.