Catecholamine neurones in rats modulate sleep, breathing, central chemoreception and breathing variability

The Constrained Disorder Principle Accounts for the Variability That Characterizes Breathing: A Method for Treating Chronic Respiratory Diseases and Improving Mechanical Ventilation

Variability characterizes breathing, cellular respiration, and the underlying quantum effects. Variability serves as a mechanism for coping with changing environments; however, this hypothesis does not explain why many of the variable phenomena of respiration manifest randomness. According to the constrained disorder principle (CDP), living organisms are defined by their inherent disorder bounded by variable boundaries. The present paper describes the mechanisms of breathing and cellular respiration, focusing on their inherent variability. It defines how the CDP accounts for the variability and randomness in breathing and respiration. It also provides a scheme for the potential role of respiration variability in the energy balance in biological systems. The paper describes the option of using CDP-based artificial intelligence platforms to augment the respiratory process's efficiency, correct malfunctions, and treat disorders associated with the respiratory system.

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.

Maternal opioids age-dependently impair neonatal respiratory control networks

Infants exposed to opioids in utero are an increasing clinical population and these infants are often diagnosed with Neonatal Abstinence Syndrome (NAS). Infants with NAS have diverse negative health consequences, including respiratory distress. However, many factors contribute to NAS, confounding the ability to understand how maternal opioids directly impact the neonatal respiratory system. Breathing is controlled centrally by respiratory networks in the brainstem and spinal cord, but the impact of maternal opioids on developing perinatal respiratory networks has not been studied. Using progressively more isolated respiratory network circuitry, we tested the hypothesis that maternal opioids directly impair neonatal central respiratory control networks. Fictive respiratory-related motor activity from isolated central respiratory networks was age-dependently impaired in neonates after maternal opioids within more complete respiratory networks (brainstem and spinal cords), but unaffected in more isolated networks (medullary slices containing the preBötzinger Complex). These deficits were due, in part, to lingering opioids within neonatal respiratory control networks immediately after birth and involved lasting impairments to respiratory pattern. Since opioids are routinely given to infants with NAS to curb withdrawal symptoms and our previous work demonstrated acute blunting of opioid-induced respiratory depression in neonatal breathing, we further tested the responses of isolated networks to exogenous opioids. Isolated respiratory control networks also demonstrated age-dependent blunted responses to exogenous opioids that correlated with changes in opioid receptor expression within a primary respiratory rhythm generating region, the preBötzinger Complex. Thus, maternal opioids age-dependently impair neonatal central respiratory control and responses to exogenous opioids, suggesting central respiratory impairments contribute to neonatal breathing destabilization after maternal opioids and likely contribute to respiratory distress in infants with NAS. These studies represent a significant advancement of our understanding of the complex effects of maternal opioids, even late in gestation, contributing to neonatal breathing deficits, necessary first steps in developing novel therapeutics to support breathing in infants with NAS.

Respiration and rapid eye movement ( REM) sleep substructure: short versus long episodes

Rapid eye movement (REM) sleep in rodents is defined by the presence of theta rhythm in the absence of movement. The amplitude and frequency of theta oscillations have been used to distinguish between tonic and phasic REM sleep. However, tonic REM sleep has not been further subdivided, although characteristics of network oscillations such as cross-frequency coupling between theta and gamma vary within this sub-state. Recently, it has been shown that theta-gamma coupling depends on an optimal breathing rate of ~5 Hz. The frequency of breathing varies strongly throughout REM sleep, and the duration of single REM sleep episodes ranges from several seconds to minutes, whereby short episodes predominate. Here we studied the relation between breathing frequency, accelerometer activity, and the length of REM sleep periods. We found that small movements detected with three-dimensional accelerometry positively correlate with breathing rate. Interestingly, breathing is slow in short REM sleep episodes, while faster respiration regimes exclusively occur after a certain delay in longer REM sleep episodes. Thus, merging REM sleep episodes of different lengths will result in a predominance of slow respiration due to the higher occurrence of short REM sleep periods. Moreover, our results reveal that not only do phasic REM sleep epochs predominantly occur during long REM sleep episodes, but that the long episodes also have faster theta and higher gamma activity. These observations suggest that REM sleep can be further divided from a physiological point of view depending on its duration. Higher levels of arousal during REM sleep, indicated by higher breathing rates, can only be captured in long REM sleep episodes.

The Rostral Ventromedial and Lateral Medulla Are the Major Areas Responsive to Lung Cancer Progression among Brainstem Lung-Innervating Nuclei

In recent years, the information crosstalk between the central nervous system and the periphery has been a hot topic, such as the brain-gut axis, brain-lung axis, etc. Among them, some studies have shown that brainstem nuclei activity can significantly affect the progression of peripheral tumor; however, regarding lung cancer, our understanding of the basic characteristics of the lung-innervating brain nuclei responsive to lung cancer progression remains deficient. Therefore, we used the pseudorabies virus for retrograde labeling of nerves to study the neural circuits between the lung and brain. We then established a mouse orthotopic lung cancer model and used the expression of the c-Fos gene in brain regions to characterize activated brain circuits and compared these results with those of the control group. We focused on c-Fos activity in nuclei associated with retrograde tracing regions of the brainstem. We found over 16 nuclei in the whole brain with direct or indirect lung innervation through neural retrograde labeling with the pseudorabies virus. We further revealed that the neuronal activity of the rostral ventrolateral reticular nucleus (RVL), caudal nucleus of Raphe (raphe obscurus nucleus, ROb), Raphe pallidus nucleus (RPa), and ventral gigantocellular reticular nucleus (GiV) in the rostral ventromedial and lateral medulla were significantly changed in an orthotopic lung cancer mouse model by the immunostaining of c-Fos early responsive protein. Thus, the distinctive rostroventral medulla area, functionally closely related to the vagus nerve, likely plays a role in central neural interaction with peripheral lung tumors and deserves future investigation.

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.

The noradrenergic agent reboxetine plus the antimuscarinic hyoscine butylbromide reduces sleep apnoea severity: a double-blind, placebo-controlled, randomised crossover trial

KEY POINTS

Recent animal and human physiology studies indicate that noradrenergic and muscarinic processes are key mechanisms that mediate pharyngeal muscle control during sleep. The noradrenergic agent reboxetine combined with the anti-muscarinic hyoscine butylbromide has recently been shown to improve upper airway function during sleep in healthy individuals. However, whether these findings translate to the clinically relevant patient population of people with obstructive sleep apnoea (OSA), and the effects of the agents on OSA severity, are unknown. We found that reboxetine plus hyoscine butylbromide reduced OSA severity, including overnight hypoxaemia, via increases in pharyngeal muscle responsiveness, improvements in respiratory control and airway collapsibility without changing the respiratory arousal threshold. These findings provide mechanistic insight into the role of noradrenergic and anti-muscarinic agents on upper airway stability and breathing during sleep and are important for pharmacotherapy development for OSA.

ABSTRACT

The noradrenergic agent reboxetine combined with the anti-muscarinic hyoscine butylbromide has recently been shown to improve upper airway function during sleep in healthy individuals. However, the effects of this drug combination on obstructive sleep apnoea (OSA) severity are unknown. Accordingly, this study aimed to determine if reboxetine plus hyoscine butylbromide reduces OSA severity. Secondary aims were to investigate the effects on key upper airway physiology and endotypic traits. Twelve people with OSA aged 52 ± 13 years, BMI = 30 ± 5 kg/m² , completed a double-blind, randomised, placebo-controlled, crossover trial (ACTRN12617001326381). Two in-laboratory sleep studies with nasal mask, pneumotachograph, epiglottic pressure sensor and bipolar fine-wire electrodes into genioglossus and tensor palatini muscles were performed separated by approximately 1 week. Each participant received either reboxetine (4 mg) plus hyoscine butylbromide (20 mg), or placebo immediately prior to sleep. Polysomnography, upper airway physiology and endotypic estimates of OSA were compared between conditions. Reboxetine plus hyoscine butylbromide reduced the apnoea/hypopnoea index by (mean ± SD) 17 ± 17 events/h from 51 ± 30 to 33 ± 22 events/h (P = 0.005) and nadir oxygen saturation increased by 6 ± 5% from 82 ± 5 to 88 ± 2% (P = 0.002). The drug combination increased tonic genioglossus muscle responsiveness during non-REM sleep (median [25th, 75th centiles]: -0.007 [-0.0004, -0.07] vs. -0.12 [-0.02, -0.40] %maxEMG/cmH₂ O, P = 0.02), lowered loop gain (0.43 ± 0.06 vs. 0.39 ± 0.07, P = 0.01), and improved airway collapsibility (90 [69, 95] vs. 93 [88, 96] %eupnoea, P = 0.02), without changing the arousal threshold (P = 0.39). These findings highlight the important role that noradrenergic and muscarinic processes have on upper airway function during sleep and the potential for pharmacotherapy to target these mechanisms to treat OSA.

A5 noradrenergic neurons and breathing control in neonate rats

The pontine A5 noradrenergic group contributes to the maturation of the respiratory system before birth in rats. These neurons are connected to the neural network responsible for respiratory rhythmogenesis. In the present study, we investigated the participation of A5 noradrenergic neurons in neonates (P7-8 and P14-15) in the control of ventilation during hypoxia and hypercapnia in in vivo experiments using conjugated saporin anti-dopamine beta-hydroxylase (DβH-SAP) to specifically ablate noradrenergic neurons. Thus, DβH-SAP (420 ng/μL) or saporin (SAP, control) was injected into the A5 region of neonatal male Wistar rats. Hypoxia reduced respiratory variability in control animals; however, A5 lesion prevented this effect in P7-8 rats. Our data suggest that noradrenergic neurons of the A5 region in neonate rats do not participate in the control of ventilation under baseline and hypercapnic conditions, but exert an inhibitory modulation on breathing variability under hypoxic challenge in early life (P7-8).

Deficiency of Biogenic Amines Modulates the Activity of Hypoglossal Nerve in the Reserpine Model of Parkinson's Disease

The underlying cause of respiratory impairments appearing in Parkinson's disease (PD) is still far from being elucidated. To better understand the pathogenesis of respiratory disorders appearing in PD, we studied hypoglossal (HG) and phrenic (PHR) motoneuron dysfunction in a rat model evoked with reserpine administration. After reserpine, a decrease in the baseline amplitude and minute HG activity was noted, and no depressive phase of the hypoxic ventilatory response was observed. The pre-inspiratory time of HG activity along with the ratio of pre-inspiratory time to total respiratory cycle time and the ratio of pre-inspiratory to inspiratory amplitude were significantly reduced during normoxia, hypoxia, and recovery compared to sham rats. We suggest that the massive depletion of not only dopamine, but above all noradrenaline and serotonin in the brainstem observed in our study, has an impact on the pre-inspiratory activity of the HG. The shortening of the pre-inspiratory activity of the HG in the reserpine model may indicate a serious problem with maintaining the correct diameter of the upper airways in the preparation phase for inspiratory effort and explain the development of obstructive sleep apnea in some PD patients. Therapies involving the supplementation of amine depletion other than dopamine should be considered.

Locus Coeruleus Acid-Sensing Ion Channels Modulate Sleep-Wakefulness and State Transition from NREM to REM Sleep in the Rat

The locus coeruleus (LC) is one of the essential chemoregulatory and sleep-wake (S-W) modulating centers in the brain. LC neurons remain highly active during wakefulness, and some implicitly become silent during rapid eye movement (REM) sleep. LC neurons are also involved in CO₂-dependent modulation of the respiratory drive. Acid-sensing ion channels (ASICs) are highly expressed in some brainstem chemosensory breathing regulatory areas, but their localization and functions in the LC remain unknown. Mild hypercapnia increases the amount of non-REM (NREM) sleep and the number of REM sleep episodes, but whether ASICs in the LC modulate S-W is unclear. Here, we investigated the presence of ASICs in the LC and their role in S-W modulation and the state transition from NREM to REM sleep. Male Wistar rats were surgically prepared for chronic polysomnographic recordings and drug microinjections into the LC. The presence of ASIC-2 and ASIC-3 in the LC was immunohistochemically characterized. Microinjections of amiloride (an ASIC blocker) and APETx2 (a blocker of ASIC-2 and -3) into the LC significantly decreased wakefulness and REM sleep, but significantly increased NREM sleep. Mild hypercapnia increased the amount of NREM and the number of REM episodes. However, APETx2 microinjection inhibited this increase in REM frequency. These results suggest that the ASICs of LC neurons modulate S-W, indicating that ASICs could play an important role in vigilance-state transition. A mild increase in CO₂ level during NREM sleep sensed by ASICs could be one of the determinants of state transition from NREM to REM sleep.

Respiratory Control by Phox2b-expressing Neurons in a Locus Coeruleus-preBötzinger Complex Circuit

The locus coeruleus (LC) has been implicated in the control of breathing. Congenital central hypoventilation syndrome results from mutation of the paired-like homeobox 2b (Phox2b) gene that is expressed in LC neurons. The present study was designed to address whether stimulation of Phox2b-expressing LC (Phox2b^LC) neurons affects breathing and to reveal the putative circuit mechanism. A Cre-dependent viral vector encoding a Gq-coupled human M3 muscarinic receptor (hM3Dq) was delivered into the LC of Phox2b-Cre mice. The hM3Dq-transduced neurons were pharmacologically activated while respiratory function was measured by plethysmography. We demonstrated that selective stimulation of Phox2b^LC neurons significantly increased basal ventilation in conscious mice. Genetic ablation of these neurons markedly impaired hypercapnic ventilatory responses. Moreover, stimulation of Phox2b^LC neurons enhanced the activity of preBötzinger complex neurons. Finally, axons of Phox2b^LC neurons projected to the preBötzinger complex. Collectively, Phox2b^LC neurons contribute to the control of breathing most likely via an LC-preBötzinger complex circuit.

Respiratory pattern and phrenic and hypoglossal nerve activity during normoxia and hypoxia in 6-OHDA-induced bilateral model of Parkinson's disease

Respiratory disturbances present in Parkinson's disease (PD) are not well understood. Thus, studies in animal models aimed to link brain dopamine (DA) deficits with respiratory impairment are needed. Adult Wistar rats were lesioned with injection of 6-hydroxydopamine (6-OHDA) into the third cerebral ventricle. Two weeks after hypoxic test was performed in whole-body plethysmography chamber, phrenic (PHR) and hypoglossal (HG) nerve activities were recorded in normoxic and hypoxic conditions in anesthetized, vagotomized, paralyzed and mechanically ventilated rats. The effects of activation and blockade of dopaminergic carotid body receptors were investigated during normoxia in anesthetized spontaneously breathing rats. 6-OHDA injection affected resting respiratory pattern in awake animals: an increase in tidal volume and a decrease in respiratory rate had no effect on minute ventilation. Hypoxia magnified the amplitude and minute activity of the PHR and HG nerve of 6-OHDA rats. The ratio of pre-inspiratory to inspiratory HG burst amplitude was reduced in normoxic breathing. Yet, the ratio of pre-inspiratory time to total time of the respiratory cycle was increased during normoxia. 6-OHDA lesion had no impact on DA and domperidone effects on the respiratory pattern, which indicate that peripheral DA receptors are not affected in this model. Analysis of monoamines confirmed substantial striatal depletion of dopamine, serotonin and noradrenaline (NA) and reduction of NA content in the brainstem. In bilateral 6-OHDA model changes in activity of both nerves: HG (linked with increased apnea episodes) and PHR are present. Demonstrated respiratory effects could be related to specific depletion of DA and NA.

A6 neurons simultaneously modulate active expiration and upper airway resistance in rats

NEW FINDINGS

What is the central question of this study? Do A6 neurons modulate active expiratory and airway responses evoked by hypercapnia/acidosis? What is the main finding and its importance? Acute inhibition of A6 neurons reduced active expiratory, inspiratory and the associated oropharyngeal and laryngeal motor responses to hypercapnia/acidosis. A6 neurons provide excitatory synaptic drive contributing to the central generation of inspiratory and expiratory motor activity as well as the control of upper airway resistance.

ABSTRACT

During rest, inspiration is an active phenomenon, whereas expiration is passive. Under conditions of high chemical drive, such as hypercapnia/acidosis, there is an increase in inspiratory activity, expiration becomes active and upper airway resistance is reduced. The locus coeruleus noradrenergic neurons (A6 neurons) are activated when exposed to elevated CO₂ /[H⁺ ] levels and modulate respiratory brainstem neurons regulating ventilation. However, the role of A6 neurons in the control of upper airway resistance is not fully understood. We tested the hypothesis that A6 neurons contribute to the central generation of active inspiratory and expiratory responses and the associated changes in the motor nerves controlling upper airway resistance during hypercapnia/acidosis in rats. Using a perfused brainstem-spinal cord preparation, we inhibited A6 neurons using pharmacogenetics and evaluated the active expiratory (abdominal nerve), laryngeal (cervical vagus nerve), oropharyngeal (hypoglossal nerve) and inspiratory (phrenic nerve) motor nerve responses to hypercapnia/acidosis. Acute inhibition of A6 neurons did not produce significant changes in the respiratory pattern in normocapnia. However, the hypercapnia/acidosis-induced active expiratory response and the associated changes in the motor nerves responsible for control of oropharyngeal and laryngeal resistance, as well as the inspiratory response were all reduced after inhibition of A6 neurons. Our data demonstrate that A6 neurons exert an important excitatory synaptic drive to the central generation of both active inspiratory and expiratory activities and modulate the control of upper airway resistance during hypercapnia/acidosis.

An age- and sex-dependent role of catecholaminergic neurons in the control of breathing and hypoxic chemoreflex during postnatal development

The respiratory system undergoes significant development during the postnatal phase. Maturation of brainstem catecholaminergic (CA) neurons is important for the control and modulation of respiratory rhythmogenesis, as well as for chemoreception in early life. We demonstrated an inhibitory role for CA neurons in CO₂ chemosensitivity in neonatal and juvenile male and female rats, but information regarding their role in the hypoxic ventilatory response (HVR) is lacking. We evaluated the contribution of brainstem CA neurons in the HVR during postnatal (P) development (P7-8, P14-15 and P20-21) in male and female rats through chemical injury with conjugated saporin anti-dopamine beta-hydroxylase (DβH-SAP, 420 ng·μL^-1) injected in the fourth ventricle. Ventilation (V̇_E) and oxygen consumption were recorded one week after the lesion in unanesthetized rats during exposure to normoxia and hypoxia. Hypoxia reduced breathing variability in P7-8 control rats of both sexes. At P7-8, the HVR for lesioned males and females increased 27% and 24%, respectively. Additionally, the lesion reduced the normoxic breathing variability in both sexes at P7-8, but hypoxia partially reverted this effect. For P14-15, the increase in V̇_E during hypoxia was 30% higher for male and 24% higher for female lesioned animals. A sex-specific difference was detected at P20-21, as lesioned males exhibited a 24% decrease in the HVR, while lesioned females experienced a 22% increase. Furthermore, the hypoxia-induced body temperature reduction was attenuated in P20-21 lesioned females. We conclude that brainstem CA neurons modulate the HRV during the postnatal phase, and possibly thermoregulation during hypoxia.

Orexin Neurons Contribute to Central Modulation of Respiratory Drive by Progestins on ex vivo Newborn Rodent Preparations

Dysfunction of central respiratory CO₂/H⁺ chemosensitivity is a pivotal factor that elicits deep hypoventilation in patients suffering from central hypoventilation syndromes. No pharmacological treatment is currently available. The progestin desogestrel has been suggested to allow recovery of respiratory response to CO₂/H⁺ in patients suffering from central hypoventilation, but except the fact that supramedullary regions may be involved, mechanisms are still unknown. Here, we tested in neonates whether orexin systems contribute to desogestrel’s central effects on respiratory function. Using isolated ex vivo central nervous system preparations from newborn rats, we show orexin and almorexant, an antagonist of orexin receptors, supressed strengthening of the increase in respiratory frequency induced by prolonged metabolic acidosis under exposure to etonogestrel, the active metabolite of desogestrel. In parallel, almorexant suppressed the increase and enhanced increase in c-fos expression in respiratory-related brainstem structures induced by etonogestrel. These results suggest orexin signalisation is a key component of acidosis reinforcement of respiratory drive by etonogestrel in neonates. Although stage of development used is different as that for progestin clinical observations, presents results provide clues about conditions under which desogestrel or etonogestrel may enhance ventilation in patients suffering from central hypoventilation syndromes.

Activation of Phox2b-Expressing Neurons in the Nucleus Tractus Solitarii Drives Breathing in Mice

The nucleus tractus solitarii (NTS) is implicated in the control of breathing, but the neuronal phenotype and circuit mechanism involved in such a physiological function remain incompletely understood. This study focused on the respiratory role of paired-like homeobox 2b gene (Phox2b)-expressing NTS neurons and sought to determine whether selective stimulation of this set of neurons activates breathing in male mice. A Cre-dependent vector encoding a Gq-coupled human M3 muscarinic receptor (hM3Dq) was microinjected into the NTS of Phox2b-Cre transgenic mice. The hM3Dq-transduced neurons were pharmacologically activated in conscious mice while respiratory effects were measured by plethysmography. We demonstrate that chemogenetic stimulation of Phox2b-expressing NTS neurons significantly increased baseline minute volume via an increase in respiratory frequency rather than tidal volume. Chemogenetic stimulation also synergized with moderate CO₂ stimulation to enhance pulmonary ventilatory response. Selective ablation of Phox2b-expressing NTS neurons notably attenuated a hypercapnic ventilatory response. Moreover, histological evidence revealed that stimulation of Phox2b-expressing NTS neurons increased neuronal activity of the preBötzinger complex. Finally, we presented the neuroanatomical evidence of direct projection of Phox2b-expressing NTS neurons to putative respiratory central pattern generator. Overall, these findings suggest that selective activation of Phox2b-expressing NTS neurons potentiates baseline pulmonary ventilation via an excitatory drive to respiratory central pattern generator and this group of neurons is also required for the hypercapnic ventilatory response.SIGNIFICANCE STATEMENT The nucleus tractus solitarii (NTS) has been implicated in the control of breathing. The paired-like homeobox 2b gene (Phox2b) is the disease-defining gene for congenital central hypoventilation syndrome and is restrictively present in brainstem nucleus, including the NTS. Using a chemogenetic approach, we demonstrate herein that selective stimulation of Phox2b-expressing NTS neurons vigorously potentiates baseline pulmonary ventilation via an excitatory drive to respiratory central pattern generator in rodents. Genetic ablation of these neurons attenuates the hypercapnic ventilatory response. We also suggest that a fraction of Phox2b-expressing neurons exhibit CO₂ sensitivity and presumably function as central respiratory chemoreceptors. The methodology is expected to provide a future applicability to the patients with sleep-related hypoventilation or apnea.

Locus Coeruleus as a vigilance centre for active inspiration and expiration in rats

At rest, inspiration is an active process while expiration is passive. However, high chemical drive (hypercapnia or hypoxia) activates central and peripheral chemoreceptors triggering reflex increases in inspiration and active expiration. The Locus Coeruleus contains noradrenergic neurons (A6 neurons) that increase their firing frequency when exposed to hypercapnia and hypoxia. Using recently developed neuronal hyperpolarising technology in conscious rats, we tested the hypothesis that A6 neurons are a part of a vigilance centre for controlling breathing under high chemical drive and that this includes recruitment of active inspiration and expiration in readiness for flight or fight. Pharmacogenetic inhibition of A6 neurons was without effect on resting and on peripheral chemoreceptors-evoked inspiratory, expiratory and ventilatory responses. On the other hand, the number of sighs evoked by systemic hypoxia was reduced. In the absence of peripheral chemoreceptors, inhibition of A6 neurons during hypercapnia did not affect sighing, but reduced both the magnitude and incidence of active expiration, and the frequency and amplitude of inspiration. These changes reduced pulmonary ventilation. Our data indicated that A6 neurons exert a CO₂-dependent modulation of expiratory drive. The data also demonstrate that A6 neurons contribute to the CO₂-evoked increases in the inspiratory motor output and hypoxia-evoked sighing.

Correlation between neuroanatomical and functional respiratory changes observed in an experimental model of Parkinson's disease

NEW FINDINGS

What is the central question of this study? What is the relationship between neuroanatomical and functional respiratory changes in an experimental model of Parkinson's disease? What is the main finding and its importance? Sixty days after induction of Parkinson's disease in a rat model, there are decreases in baseline breathing and in the number of neurons, density of the neurokinin-1 receptor and density of astrocytes in the ventrolateral respiratory region. These results provide the first evidence that neuroanatomical changes occur before functional respiratory deficits in a Parkinson's disease model and that there is a positive correlation between those sets of changes. The neuroanatomical changes impair respiratory activity and are presumably a major cause of the respiratory problems observed in Parkinson's disease.

ABSTRACT

We showed previously that 60 days after the induction of Parkinson's disease (PD) in a rat model, there are decreases in baseline breathing and in the number of phox2b-expressing neurons of the retrotrapezoid nucleus (RTN) and nucleus of the solitary tract (NTS), as well as a reduction in the density of the neurokinin-1 receptor (NK1r) in the pre-Bötzinger complex (preBötC) and rostral ventrolateral respiratory group (rVRG). Here, our aim was to evaluate the correlation between neuroanatomical and functional respiratory changes in an experimental model of PD. Male Wistar rats with bilateral injections of 6-hydroxydopamine (6-OHDA, 24 μg μl^-1 ) or vehicle into the striatum had respiratory parameters assessed by whole-body plethysmography 1 day before and 30, 40 or 60 days after the ablation. From the 30th day after the ablation, we observed a reduction in the number of phox2b neurons in the RTN and NTS and a reduction in the density of astrocytes in the rVRG. At 40 days after the ablation, we observed decreases in the density of NK1r in the preBötC and rVRG and of astrocytes in the RTN region. At 60 days, we observed a reduction in the density of astrocytes in the NTS and preBötC regions. The functional data showed changes in the resting and hypercapnia-induced respiratory rates and tidal volume from days 40-60 after injury. Our data suggest that the neuroanatomical changes impair respiratory activity and are presumably a major cause of the respiratory problems observed in PD.

Brainstem Nuclei Associated with Mediating Apnea-Induced Respiratory Motor Plasticity

The respiratory control system is plastic. It has a working memory and is capable of retaining how respiratory stimuli affect breathing by regulating synaptic strength between respiratory neurons. For example, repeated airway obstructions trigger a form of respiratory plasticity that strengthens inspiratory activity of hypoglossal (XII) motoneurons. This form of respiratory plasticity is known as long-term facilitation (LTF) and requires noradrenaline released onto XII motoneurons. However, the brainstem regions responsible for this form of LTF remain unidentified. Here, we used electrophysiology, neuropharmacology and immunohistochemistry in adult rats to identify the brainstem regions involved in mediating LTF. First, we show that repeated airway obstructions induce LTF of XII motoneuron activity and that inactivation of the noradrenergic system prevents LTF. Second, we show that noradrenergic cells in the locus coeruleus (LC), which project to XII motoneurons, are recruited during LTF induction. Third, we show that targeted inactivation of noradrenergic LC cells during LTF induction prevents LTF. And lastly, we show that the nucleus tractus solitarius (NTS), which has known projections to the LC, is critical for LTF because its inactivation prevents LTF. Our results suggest that both the LC and NTS are involved in mediating apnea-induced LTF, and we hypothesize that a NTS → LC → XII circuit mechanism mediates this form of respiratory motor plasticity.

Desogestrel down-regulates PHOX2B and its target genes in progesterone responsive neuroblastoma cells

The paired-like homeobox 2B gene (PHOX2B) encodes a key transcription factor that plays a role in the development of the autonomic nervous system and the neural structures involved in controlling breathing. In humans, PHOX2B over-expression plays a role in the pathogenesis of tumours arising from the sympathetic nervous system such as neuroblastomas, and heterozygous PHOX2B mutations cause Congenital Central Hypoventilation Syndrome (CCHS), a life-threatening neurocristopathy characterised by the defective autonomic control of breathing and involving altered CO₂/H⁺ chemosensitivity. The recovery of CO₂/H⁺ chemosensitivity and increased ventilation have been observed in two CCHS patients using the potent contraceptive progestin desogestrel. Given the central role of PHOX2B in the pathogenesis of CCHS, and the progesterone-mediated effects observed in the disease, we generated progesterone-responsive neuroblastoma cells, and evaluated the effects of 3-Ketodesogestrel (3-KDG), the biologically active metabolite of desogestrel, on the expression of PHOX2B and its target genes. Our findings demonstrate that, through progesterone nuclear receptor PR-B, 3-KDG down-regulates PHOX2B gene expression, by a post-transcriptional mechanism, and its target genes and open up the possibility that this mechanism may contribute to the positive effects observed in some CCHS patients.

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

KEY POINTS

The brainstem catecholaminergic (CA) modulation on ventilation changes with development. We determined the role of the brainstem CA system in ventilatory control under normocapnic and hypercapnic conditions during different phases of development [postnatal day (P)7-8, P14-15 and P20-21] in male and female Wistar rats. Brainstem CA neurones produce a tonic inhibitory drive that affects breathing frequency in P7-8 rats and provide an inhibitory drive during hypercapnic conditions in both males and females at P7-8 and P14-15. In pre-pubertal rats, brainstem CA neurones become excitatory for the CO₂ ventilatory response in males but remain inhibitory in females. Diseases such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome have been associated with abnormalities in the functioning of CA neurones; therefore, the results of the present study contribute to a better understanding of this system.

ABSTRACT

The respiratory network undergoes significant development during the postnatal phase, including the maturation of the catecholaminergic (CA) system. However, postnatal development of this network and its effect on the control of pulmonary ventilation ( V̇E ) is not fully understood. We investigated the involvement of brainstem CA neurones in respiratory control during postnatal development [postnatal day (P)7-8, P14-15 and P20-21], in male and female rats, through chemical injury with conjugated saporin anti-dopamine β-hydroxylase (DβH-SAP). Thus, DβH-SAP (420 ng μL^-1 ), saporin (SAP) or phosphate buffered solution (PBS) was injected into the fourth ventricle of neonatal Wistar rats of both sexes. V̇E and oxygen consumption were recorded 1 week after the injections in unanaesthetized neonatal and juvenile rats during room air and hypercapnia. The resting ventilation was higher in both male and female P7-8 lesioned rats by 33%, with a decrease in respiratory variability being observed in males. The hypercapnic ventilatory response (HCVR) was altered in male and female lesioned rats at all postnatal ages. At P7-8, the HCVR for males and females was increased by 37% and 30%, respectively. For both sexes at P14-15 rats, the increase in V̇E during hypercapnia was 37% higher for lesioned rats. A sex-specific difference in HCRV was observed at P20-21, with lesioned males showing a 33% decrease, and lesioned females showing an increase of 33%. We conclude that brainstem CA neurones exert a tonic inhibitory effect on V̇E in the early postnatal days of the life of a rat, increase variability in P7-8 males and modulate HCRV during the postnatal phase.

Active sleep unmasks apnea and delayed arousal in infant rat pups lacking central serotonin

Sudden infant death syndrome (SIDS), occurring during sleep periods, is highly associated with abnormalities within serotonin (5-HT) neurons, including reduced 5-HT. There is evidence that future SIDS cases experience more apnea and have abnormal arousal from sleep. In rodents, a loss of 5-HT neurons is associated with apnea in early life and, in adulthood, delayed arousal. As the activity of 5-HT neurons changes with vigilance state, we hypothesized that the degree of apnea and delayed arousal displayed by rat pups specifically lacking central 5-HT varies with state. Two-week-old tryptophan hydroxylase 2-deficient (TPH2-/-) and wild-type (WT) rat pups were placed in plethysmographic chambers supplied with room air. At the onset of active (AS) or quiet (QS) sleep, separate groups of rats were exposed to hypercapnia (5% CO₂) or mild hypoxia (~17% O₂) or maintained in room air. Upon arousal, rats received room air. Apnea indexes and latencies to spontaneous arousal from AS and QS were determined for pups exposed only to room air. Arousal latencies were also calculated for TPH2-/- and WT pups exposed to hypoxia or hypercapnia. Compared with WT, TPH2-/- pups hypoventilated in all states but were profoundly more apneic solely in AS. TPH2-/- pups had delayed arousal in response to increasing CO₂, and AS selectively delayed the arousal of TPH2-/- pups, irrespective of the gas they breathed. Thus infants who are deficient in CNS 5-HT may be at increased risk for SIDS in AS because of increased apnea and delayed arousal compared with QS.NEW & NOTEWORTHY Sudden infant death syndrome (SIDS) occurs during sleep and is associated with central serotonin (5-HT) deficiency. We report that rat pups deficient in central 5-HT (TPH2-/-) are profoundly more apneic in active sleep (AS) but not quiet sleep (QS). Unlike control pups, the arousal of TPH2-/- pups in air, CO₂, and hypoxia was delayed in AS compared with QS. Thus for infants deficient in central 5-HT, the risk of SIDS may be higher in AS than in QS.

Role of A5 noradrenergic neurons in the chemoreflex control of respiratory and sympathetic activities in unanesthetized conditions

The A5 area at the ventrolateral pons contains noradrenergic neurons connected with other medullary areas involved in the cardiorespiratory control. Its contribution to the cardiorespiratory regulation was previously evidenced in anesthetized conditions. In the present study, we investigated the involvement of the A5 noradrenergic neurons to the basal and chemoreflex control of the sympathetic and respiratory activities in unanesthetized conditions. A5 noradrenergic neurons were lesioned using microinjections of anti-dopamine β-hydroxylase saporin (anti-DβH-SAP). After 7-8days, we evaluated the arterial pressure levels, heart rate and minute ventilation in freely moving adult rats (280-350g) as well as recorded from thoracic sympathetic (tSN) and phrenic nerves (PN) using the arterially perfused in situ preparation of juvenile rats (80-90g). Baseline cardiovascular, sympathetic and respiratory parameters were similar between control (n=7-8) and A5-lesioned rats (n=5-6) in both experimental preparations. In adult rats, lesions of A5 noradrenergic neurons did not modify the reflex cardiorespiratory adjustments to hypoxia (7% O₂) and hypercapnia (7% CO₂). In the in situ preparations, the sympatho-excitation, but not the PN reflex response, elicited by either the stimulation of peripheral chemoreceptors (ΔtSN: 110±12% vs 58±8%, P<0.01) or hypercapnia (ΔtSN: 9.5±1.4% vs 3.9±1.7%, P<0.05) was attenuated in A5-lesioned rats compared to controls. Our data demonstrated that A5 noradrenergic neurons are part of the circuitry recruited for the processing of sympathetic response to hypoxia and hypercapnia in unanesthetized conditions.

Blood Pressure Regulation by the Rostral Ventrolateral Medulla in Conscious Rats: Effects of Hypoxia, Hypercapnia, Baroreceptor Denervation, and Anesthesia

Current understanding of the contribution of C1 neurons to blood pressure (BP) regulation derives predominantly from experiments performed in anesthetized animals or reduced ex vivo preparations. Here, we use ArchaerhodopsinT3.0 (ArchT) loss-of-function optogenetics to explore BP regulation by C1 neurons in intact, unanesthetized rats. Using a lentivirus that expresses ArchT under the Phox2b-activated promoter PRSx8 (PRSx8-ArchT), ∼65% of transduced neurons were C1 (balance retrotrapezoid nucleus, RTN). Other rats received CaMKII-ArchT3.0 AAV2 (CaMKII-ArchT), which transduced C1 neurons and larger numbers of unidentified glutamatergic and GABAergic cells. Under anesthesia, ArchT photoactivation reduced sympathetic nerve activity and BP and silenced/strongly inhibited most (7/12) putative C1 neurons. In unanesthetized PRSx8-ArchT-treated rats breathing room air, bilateral ArchT photoactivation caused a very small BP reduction that was only slightly larger under hypercapnia (6% FiCO₂), but was greatly enhanced during hypoxia (10 and 12% FiO₂), after sino-aortic denervation, or during isoflurane anesthesia. The degree of hypotension correlated with percentage of ArchT-transduced C1 neurons. ArchT photoactivation produced similar BP changes in CaMKII-ArchT-treated rats. Photoactivation in PRSX8-ArchT rats reduced breathing frequency (F_R), whereas F_R increased in CaMKII-ArchT rats. We conclude that the BP drop elicited by ArchT activation resulted from C1 neuron inhibition and was unrelated to breathing changes. C1 neurons have low activity under normoxia, but their activation is important to BP stability during hypoxia or anesthesia and contributes greatly to the hypertension caused by baroreceptor deafferentation. Finally, C1 neurons are marginally activated by hypercapnia and the large breathing stimulation caused by this stimulus has very little impact on resting BP.SIGNIFICANCE STATEMENT C1 neurons are glutamatergic/peptidergic/catecholaminergic neurons located in the medulla oblongata, which may operate as a switchboard for differential, behavior-appropriate activation of selected sympathetic efferents. Based largely on experimentation in anesthetized or reduced preparations, a rostrally located subset of C1 neurons may contribute to both BP stabilization and dysregulation (hypertension). Here, we used Archaerhodopsin-based loss-of-function optogenetics to explore the contribution of these neurons to BP in conscious rats. The results suggest that C1 neurons contribute little to resting BP under normoxia or hypercapnia, C1 neuron discharge is restrained continuously by arterial baroreceptors, and C1 neuron activation is critical to stabilize BP under hypoxia or anesthesia. This optogenetic approach could also be useful to explore the role of C1 neurons during specific behaviors or in hypertensive models.

The Onset of the Fetal Respiratory Rhythm: An Emergent Property Triggered by Chemosensory Drive?

The mechanisms responsible for the onset of respiratory activity during fetal life are unknown. The onset of respiratory rhythm may be a consequence of the genetic program of each of the constituents of the respiratory network, so they start to interact and generate respiratory cycles when reaching a certain degree of maturation. Alternatively, generation of cycles might require the contribution of recently formed sensory inputs that will trigger oscillatory activity in the nascent respiratory neural network. If this hypothesis is true, then sensory input to the respiratory generator must be already formed and become functional before the onset of fetal respiration. In this review, we evaluate the timing of the onset of the respiratory rhythm in comparison to the appearance of receptors, neurotransmitter machinery, and afferent projections provided by two central chemoreceptive nuclei, the raphe and locus coeruleus nuclei.

Kölliker-Fuse nuclei regulate respiratory rhythm variability via a gain-control mechanism

Respiration varies from breath to breath. On the millisecond timescale of spiking, neuronal circuits exhibit variability due to the stochastic properties of ion channels and synapses. Does this fast, microscopic source of variability contribute to the slower, macroscopic variability of the respiratory period? To address this question, we modeled a stochastic oscillator with forcing; then, we tested its predictions experimentally for the respiratory rhythm generated by the in situ perfused preparation during vagal nerve stimulation (VNS). Our simulations identified a relationship among the gain of the input, entrainment strength, and rhythm variability. Specifically, at high gain, the periodic input entrained the oscillator and reduced variability, whereas at low gain, the noise interacted with the input, causing events known as "phase slips", which increased variability on a slow timescale. Experimentally, the in situ preparation behaved like the low-gain model: VNS entrained respiration but exhibited phase slips that increased rhythm variability. Next, we used bilateral muscimol microinjections in discrete respiratory compartments to identify areas involved in VNS gain control. Suppression of activity in the nucleus tractus solitarii occluded both entrainment and amplification of rhythm variability by VNS, confirming that these effects were due to the activation of the Hering-Breuer reflex. Suppressing activity of the Kölliker-Fuse nuclei (KFn) enhanced entrainment and reduced rhythm variability during VNS, consistent with the predictions of the high-gain model. Together, the model and experiments suggest that the KFn regulates respiratory rhythm variability via a gain control mechanism.

In vitro characterization of noradrenergic modulation of chemosensitive neurons in the retrotrapezoid nucleus

Chemosensitive neurons in the retrotrapezoid nucleus (RTN) regulate breathing in response to CO2/H(+) changes and serve as an integration center for other autonomic centers, including brain stem noradrenergic neurons. Norepinephrine (NE) contributes to respiratory control and chemoreception, and, since disruption of NE signaling may contribute to several breathing disorders, we sought to characterize effects of NE on RTN chemoreception. All neurons included in this study responded similarly to CO2/H(+) but showed differential sensitivity to NE; we found that NE activated (79%), inhibited (7%), or had no effect on activity (14%) of RTN chemoreceptors. The excitatory effect of NE on RTN chemoreceptors was dose dependent, retained in the presence of neurotransmitter receptor blockers, and could be mimicked and blocked by pharmacological manipulation of α1-adrenergic receptors (ARs). In addition, NE-activation was blunted by XE991 (KCNQ channel blocker), and partially occluded the firing response to serotonin, suggesting involvement of KCNQ channels. However, in whole cell voltage clamp, activation of α1-ARs decreased outward current and conductance by what appears to be a mixed effect on multiple channels. The inhibitory effect of NE on RTN chemoreceptors was blunted by an α2-AR antagonist. A third group of RTN chemoreceptors was insensitive to NE. We also found that chemosensitive RTN astrocytes do not respond to NE with a change in voltage or by releasing ATP to enhance activity of chemosensitive neurons. These results indicate NE modulates subsets of RTN chemoreceptors by mechanisms involving α1- and α2-ARs.

Anatomical and functional connections between the locus coeruleus and the nucleus tractus solitarius in neonatal rats

This study was designed to investigate brain connections among chemosensitive areas in newborn rats. Rhodamine beads were injected unilaterally into the locus coeruleus (LC) or into the caudal part of the nucleus tractus solitarius (cNTS) in Sprague-Dawley rat pups (P7-P10). Rhodamine-labeled neurons were patched in brainstem slices to study their electrophysiological responses to hypercapnia and to determine if chemosensitive neurons are communicating between LC and cNTS regions. After 7-10 days, retrograde labeling was observed in numerous areas of the brainstem, including many chemosensitive regions, such as the contralateral LC, cNTS and medullary raphe. Whole-cell patch clamp was done in cNTS. In 4 of 5 retrogradely labeled cNTS neurons that projected to the LC, firing rate increased in response to hypercapnic acidosis (15% CO2), even in synaptic blockade medium (SNB) (high Mg(2+)/low Ca(2+)). In contrast, 2 of 3 retrogradely labeled LC neurons that projected to cNTS had reduced firing rate in response to hypercapnic acidosis, both in the presence and absence of SNB. Extensive anatomical connections among chemosensitive brainstem regions in newborn rats were found and at least for the LC and cNTS, the connections involve some CO2-sensitive neurons. Such anatomical and functional coupling suggests a complex central respiratory control network, such as seen in adult rats, is already largely present in neonatal rats by at least day P7-P10. Since the NTS and the LC play a major role in memory consolidation, our results may also contribute to the understanding of the development of memory consolidation.

An augmented CO2 chemoreflex and overactive orexin system are linked with hypertension in young and adult spontaneously hypertensive rats

KEY POINTS

Activation of central chemoreceptors by CO2 increases sympathetic nerve activity (SNA), arterial blood pressure (ABP) and breathing. These effects are exaggerated in spontaneously hypertensive rats (SHRs), resulting in an augmented CO2 chemoreflex that affects both breathing and ABP. The augmented CO2 chemoreflex and the high ABP are measureable in young SHRs (postnatal day 30-58) and become greater in adult SHRs. Blockade of orexin receptors can normalize the augmented CO2 chemoreflex and the high ABP in young SHRs and normalize the augmented CO2 chemoreflex and significantly lower the high ABP in adult SHRs. In the hypothalamus, SHRs have more orexin neurons, and a greater proportion of them increase their activity with CO2 . The orexin system is overactive in SHRs and contributes to the augmented CO2 chemoreflex and hypertension. Modulation of the orexin system may be beneficial in the treatment of neurogenic hypertension.

ABSTRACT

Activation of central chemoreceptors by CO2 increases arterial blood pressure (ABP), sympathetic nerve activity and breathing. In spontaneously hypertensive rats (SHRs), high ABP is associated with enhanced sympathetic nerve activity and peripheral chemoreflexes. We hypothesized that an augmented CO2 chemoreflex and overactive orexin system are linked with high ABP in both young (postnatal day 30-58) and adult SHRs (4-6 months). Our main findings are as follows. (i) An augmented CO2 chemoreflex and higher ABP in SHRs are measureable at a young age and increase in adulthood. In wakefulness, the ventilatory response to normoxic hypercapnia is higher in young SHRs (mean ± SEM: 179 ± 11% increase) than in age-matched normotensive Wistar-Kyoto rats (114 ± 9% increase), but lower than in adult SHRs (226 ± 10% increase; P < 0.05). The resting ABP is higher in young SHRs (122 ± 5 mmHg) than in age-matched Wistar-Kyoto rats (99 ± 5 mmHg), but lower than in adult SHRs (152 ± 4 mmHg; P < 0.05). (ii) Spontaneously hypertensive rats have more orexin neurons and more CO2 -activated orexin neurons in the hypothalamus. (iii) Antagonism of orexin receptors with a dual orexin receptor antagonist, almorexant, normalizes the augmented CO2 chemoreflex in young and adult SHRs and the high ABP in young SHRs and significantly lowers ABP in adult SHRs. (iv) Attenuation of peripheral chemoreflexes by hyperoxia does not abolish the augmented CO2 chemoreflex (breathing and ABP) in SHRs, which indicates an important role for the central chemoreflex. We suggest that an overactive orexin system may play an important role in the augmented central CO2 chemoreflex and in the development of hypertension in SHRs.

The Structural Connectome of the Human Central Homeostatic Network

Homeostatic adaptations to stress are regulated by interactions between the brainstem and regions of the forebrain, including limbic sites related to respiratory, autonomic, affective, and cognitive processing. Neuroanatomic connections between these homeostatic regions, however, have not been thoroughly identified in the human brain. In this study, we perform diffusion spectrum imaging tractography using the MGH-USC Connectome MRI scanner to visualize structural connections in the human brain linking autonomic and cardiorespiratory nuclei in the midbrain, pons, and medulla oblongata with forebrain sites critical to homeostatic control. Probabilistic tractography analyses in six healthy adults revealed connections between six brainstem nuclei and seven forebrain regions, several over long distances between the caudal medulla and cerebral cortex. The strongest evidence for brainstem-homeostatic forebrain connectivity in this study was between the brainstem midline raphe and the medial temporal lobe. The subiculum and amygdala were the sampled forebrain nodes with the most extensive brainstem connections. Within the human brainstem-homeostatic forebrain connectome, we observed that a lateral forebrain bundle, whose connectivity is distinct from that of rodents and nonhuman primates, is the primary conduit for connections between the brainstem and medial temporal lobe. This study supports the concept that interconnected brainstem and forebrain nodes form an integrated central homeostatic network (CHN) in the human brain. Our findings provide an initial foundation for elucidating the neuroanatomic basis of homeostasis in the normal human brain, as well as for mapping CHN disconnections in patients with disorders of homeostasis, including sudden and unexpected death, and epilepsy.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Impact of maternal physical activity on fetal breathing and body movement--A review

Fetal movements, which include body and breathing movement, are important indicators of fetal well-being and nervous system development. These have been shown to be affected by intrauterine conditions. While maternal physical activity does induce a change in intrauterine conditions and physiology, its impact on fetal movements is still unclear. This paper will provide a brief review of the literature and outline the current knowledge with regards to the effects of maternal exercise on fetal body and breathing movements.

Catecholaminergic neurons projecting to the paraventricular nucleus of the hypothalamus are essential for cardiorespiratory adjustments to hypoxia

Brainstem catecholamine neurons modulate sensory information and participate in control of cardiorespiratory function. These neurons have multiple projections, including to the paraventricular nucleus (PVN), which contributes to cardiorespiratory and neuroendocrine responses to hypoxia. We have shown that PVN-projecting catecholaminergic neurons are activated by hypoxia, but the function of these neurons is not known. To test the hypothesis that PVN-projecting catecholamine neurons participate in responses to respiratory challenges, we injected IgG saporin (control; n = 6) or anti-dopamine β-hydroxylase saporin (DSAP; n = 6) into the PVN to retrogradely lesion catecholamine neurons projecting to the PVN. After 2 wk, respiratory measurements (plethysmography) were made in awake rats during normoxia, increasing intensities of hypoxia (12, 10, and 8% O2) and hypercapnia (5% CO2-95% O2). DSAP decreased the number of tyrosine hydroxylase-immunoreactive terminals in PVN and cells counted in ventrolateral medulla (VLM; -37%) and nucleus tractus solitarii (nTS; -36%). DSAP produced a small but significant decrease in respiratory rate at baseline (during normoxia) and at all intensities of hypoxia. Tidal volume and minute ventilation (VE) index also were impaired at higher hypoxic intensities (10-8% O2; e.g., VE at 8% O2: IgG = 181 ± 22, DSAP = 91 ± 4 arbitrary units). Depressed ventilation in DSAP rats was associated with significantly lower arterial O2 saturation at all hypoxic intensities. PVN DSAP also reduced ventilatory responses to 5% CO2 (VE: IgG = 176 ± 21 and DSAP = 84 ± 5 arbitrary units). Data indicate that catecholamine neurons projecting to the PVN are important for peripheral and central chemoreflex respiratory responses and for maintenance of arterial oxygen levels during hypoxic stimuli.

Dysregulation of locus coeruleus development in congenital central hypoventilation syndrome

Human congenital central hypoventilation syndrome (CCHS), resulting from mutations in transcription factor PHOX2B, manifests with impaired responses to hypoxemia and hypercapnia especially during sleep. To identify brainstem structures developmentally affected in CCHS, we analyzed two postmortem neonatal-lethal cases with confirmed polyalanine repeat expansion (PARM) or Non-PARM (PHOX2B∆8) mutation of PHOX2B. Both human cases showed neuronal losses within the locus coeruleus (LC), which is important for central noradrenergic signaling. Using a conditionally active transgenic mouse model of the PHOX2B∆8 mutation, we found that early embryonic expression (<E10.5) caused failure of LC neuronal specification and perinatal respiratory lethality. In contrast, later onset (E11.5) of PHOX2B∆8 expression was not deleterious to LC development and perinatal respiratory lethality was rescued, despite failure of chemosensor retrotrapezoid nucleus formation. Our findings indicate that early-onset mutant PHOX2B expression inhibits LC neuronal development in CCHS. They further suggest that such mutations result in dysregulation of central noradrenergic signaling, and therefore, potential for early pharmacologic intervention in humans with CCHS.

Regulation of breathing and autonomic outflows by chemoreceptors

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

The role of melanin concentrating hormone (MCH) in the central chemoreflex: a knockdown study by siRNA in the lateral hypothalamus in rats

Melanin concentrating hormone (MCH), a neuropeptide produced mainly in neurons localized to the lateral hypothalamic area (LHA), has been implicated in the regulation of food intake, energy balance, sleep state, and the cardiovascular system. Hypothalamic MCH neurons also have multisynaptic connections with diaphragmatic motoneurons and project to many central chemoreceptor sites. However, there are few studies of MCH involvement in central respiratory control. To test the hypothesis that MCH plays a role in the central chemoreflex, we induced a down regulation of MCH in the central nervous system by knocking down the MCH precursor (pMCH) mRNA in the LHA using a pool of small interfering RNA (siRNA), and measured the resultant changes in breathing, metabolic rate, body weight, and blood glucose levels in conscious rats. The injections of pMCH-siRNA into the LHA successfully produced a ∼62% reduction of pMCH mRNA expression in the LHA and a ∼43% decrease of MCH levels in the cerebrospinal fluid relative to scrambled-siRNA treatment (P = 0.006 and P = 0.02 respectively). Compared to the pretreatment baseline and the scrambled-siRNA treated control rats, knockdown of MCH resulted in: 1) an enhanced hypercapnic chemoreflex (∼42 & 47% respectively; P < 0.05) only in wakefulness; 2) a decrease in body weight and basal glucose levels; and 3) an unchanged metabolic rate. Our results indicate that MCH participates not only in the regulation of glucose and sleep-wake homeostasis but also the vigilance-state dependent regulation of the central hypercapnic chemoreflex and respiratory control.

A HCO(3)(-)-dependent mechanism involving soluble adenylyl cyclase for the activation of Ca²⁺ currents in locus coeruleus neurons

Hypercapnic acidosis activates Ca²⁺ channels and increases intracellular Ca²⁺ levels in neurons of the locus coeruleus, a known chemosensitive region involved in respiratory control. We have also shown that large conductance Ca²⁺-activated K⁺ channels, in conjunction with this pathway, limits the hypercapnic-induced increase in firing rate in locus coeruleus neurons. Here, we present evidence that the Ca²⁺ current is activated by a HCO(3)(-)-sensitive pathway. The increase in HCO(3)(-) associated with hypercapnia activates HCO(3)(-)-sensitive adenylyl cyclase (soluble adenylyl cyclase). This results in an increase in cyclic adenosine monophosphate levels and activation of Ca²⁺ channels via cyclic adenosine monophosphate-activated protein kinase A. We also show the presence of soluble adenylyl cyclase in the cytoplasm of locus coeruleus neurons, and that the cyclic adenosine monophosphate analogue db-cyclic adenosine monophosphate increases Ca²⁺i. Disrupting this pathway by decreasing HCO(3)(-) levels during acidification or inhibiting either soluble adenylyl cyclase or protein kinase A, but not transmembrane adenylyl cyclase, can increase the magnitude of the firing rate response to hypercapnia in locus coeruleus neurons from older neonates to the same extent as inhibition of K⁺ channels. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Ionotropic but not metabotropic glutamatergic receptors in the locus coeruleus modulate the hypercapnic ventilatory response in unanaesthetized rats

AIM

Central chemoreceptors are important to detect changes of CO2/H(+), and the Locus coeruleus (LC) is one of the many putative central chemoreceptor sites. Here, we studied the contribution of LC glutamatergic receptors on ventilatory, cardiovascular and thermal responses to hypercapnia.

METHODS

To this end, we determined pulmonary ventilation (V(E)), body temperatures (T(b)), mean arterial pressure (MAP) and heart rate (HR) of male Wistar rats before and after unilateral microinjection of kynurenic acid (KY, an ionotropic glutamate receptor antagonist, 10 nmol/0.1 μL) or α-methyl-4-carboxyphenylglycine (MCPG, a metabotropic glutamate receptor antagonist, 10 nmol/0.1 μL) into the LC, followed by 60 min of air breathing or hypercapnia exposure (7% CO2).

RESULTS

Ventilatory response to hypercapnia was higher in animals treated with KY intra-LC (1918.7 ± 275.4) compared with the control group (1057.8 ± 213.9, P < 0.01). However, the MCPG treatment within the LC had no effect on the hypercapnia-induced hyperpnea. The cardiovascular and thermal controls were not affected by hypercapnia or by the injection of KY and MCPG in the LC.

CONCLUSION

These data suggest that glutamate acting on ionotropic, but not metabotropic, receptors in the LC exerts an inhibitory modulation of hypercapnia-induced hyperpnea.

Brainstem respiratory networks: building blocks and microcircuits

Breathing movements in mammals are driven by rhythmic neural activity generated within spatially and functionally organized brainstem neural circuits comprising the respiratory central pattern generator (CPG). This rhythmic activity provides homeostatic regulation of gases in blood and tissues and integrates breathing with other motor acts. We review new insights into the spatial-functional organization of key neural microcircuits of this CPG from recent multidisciplinary experimental and computational studies. The emerging view is that the microcircuit organization within the CPG allows the generation of multiple rhythmic breathing patterns and adaptive switching between them, depending on physiological or pathophysiological conditions. These insights open the possibility for site- and mechanism-specific interventions to treat various disorders of the neural control of breathing.

State-dependent modulation of breathing in urethane-anesthetized rats

Respiratory activity is most fragile during sleep, in particular during paradoxical [or rapid eye movement (REM)] sleep and sleep state transitions. Rats are commonly used to study respiratory neuromodulation, but rodent sleep is characterized by a highly fragmented sleep pattern, thus making it very challenging to examine different sleep states and potential pharmacological manipulations within them. Sleep-like brain-state alternations occur in rats under urethane anesthesia and may be an effective and efficient model for sleep itself. The present study assessed state-dependent changes in breathing and respiratory muscle modulation under urethane anesthesia to determine their similarity to those occurring during natural sleep. Rats were anesthetized with urethane and respiratory airflow, as well as electromyographic activity in respiratory muscles were recorded in combination with local field potentials in neocortex and hippocampus to determine how breathing pattern and muscle activity are modulated with brain state. Measurements were made in normoxic, hypoxic, and hypercapnic conditions. Results were compared with recordings made from rats during natural sleep. Brain-state alternations under urethane anesthesia were closely correlated with changes in breathing rate and variability and with modulation of respiratory muscle tone. These changes closely mimicked those observed in natural sleep. Of great interest was that, during both REM and REM-like states, genioglossus muscle activity was strongly depressed and abdominal muscle activity showed potent expiratory modulation. We demonstrate that, in urethane-anesthetized rats, respiratory airflow and muscle activity are closely correlated with brain-state transitions and parallel those shown in natural sleep, providing a useful model to systematically study sleep-related changes in respiratory control.

A Moderate Increase of Physiological CO(2) in a Critical Range during Stable NREM Sleep Episode: A Potential Gateway to REM Sleep

Sleep is characterized as rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. Studies suggest that wake-related neurons in the basal forebrain, posterior hypothalamus and brainstem, and NREM sleep-related neurons in the anterior-hypothalamic area inhibit each other, thus alternating sleep-wakefulness. Similarly, pontine REM-ON and REM-OFF neurons reciprocally inhibit each other for REM sleep modulation. It has been proposed that inhibition of locus coeruleus (LC) REM-OFF neurons is pre-requisite for REM sleep genesis, but it remains ambiguous how REM-OFF neurons are hyperpolarized at REM sleep onset. The frequency of breathing pattern remains high during wake, slows down during NREM sleep but further escalates during REM sleep. As a result, brain CO(2) level increases during NREM sleep, which may alter REM sleep manifestation. It has been reported that hypocapnia decreases REM sleep while hypercapnia increases REM sleep periods. The groups of brainstem chemosensory neurons, including those present in LC, sense the alteration in CO(2) level and respond accordingly. For example, one group of LC neurons depolarize while other hyperpolarize during hypercapnia. In another group, hypercapnia initially depolarizes but later hyperpolarizes LC neurons. Besides chemosensory functions, LC REM-OFF neurons are an integral part of REM sleep executive machinery. We reason that increased CO(2) level during a stable NREM sleep period may hyperpolarize LC neurons including REM-OFF, which may help initiate REM sleep. We propose that REM sleep might act as a sentinel to help maintain normal CO(2) level for unperturbed sleep.

Central chemoreceptors: locations and functions

Central chemoreception traditionally refers to a change in ventilation attributable to changes in CO2/H(+) detected within the brain. Interest in central chemoreception has grown substantially since the previous Handbook of Physiology published in 1986. Initially, central chemoreception was localized to areas on the ventral medullary surface, a hypothesis complemented by the recent identification of neurons with specific phenotypes near one of these areas as putative chemoreceptor cells. However, there is substantial evidence that many sites participate in central chemoreception some located at a distance from the ventral medulla. Functionally, central chemoreception, via the sensing of brain interstitial fluid H(+), serves to detect and integrate information on (i) alveolar ventilation (arterial PCO2), (ii) brain blood flow and metabolism, and (iii) acid-base balance, and, in response, can affect breathing, airway resistance, blood pressure (sympathetic tone), and arousal. In addition, central chemoreception provides a tonic "drive" (source of excitation) at the normal, baseline PCO2 level that maintains a degree of functional connectivity among brainstem respiratory neurons necessary to produce eupneic breathing. Central chemoreception responds to small variations in PCO2 to regulate normal gas exchange and to large changes in PCO2 to minimize acid-base changes. Central chemoreceptor sites vary in function with sex and with development. From an evolutionary perspective, central chemoreception grew out of the demands posed by air versus water breathing, homeothermy, sleep, optimization of the work of breathing with the "ideal" arterial PCO2, and the maintenance of the appropriate pH at 37°C for optimal protein structure and function.

Central chemoreceptors: locations and functions

Central chemoreception traditionally refers to a change in ventilation attributable to changes in CO2/H(+) detected within the brain. Interest in central chemoreception has grown substantially since the previous Handbook of Physiology published in 1986. Initially, central chemoreception was localized to areas on the ventral medullary surface, a hypothesis complemented by the recent identification of neurons with specific phenotypes near one of these areas as putative chemoreceptor cells. However, there is substantial evidence that many sites participate in central chemoreception some located at a distance from the ventral medulla. Functionally, central chemoreception, via the sensing of brain interstitial fluid H(+), serves to detect and integrate information on (i) alveolar ventilation (arterial PCO2), (ii) brain blood flow and metabolism, and (iii) acid-base balance, and, in response, can affect breathing, airway resistance, blood pressure (sympathetic tone), and arousal. In addition, central chemoreception provides a tonic "drive" (source of excitation) at the normal, baseline PCO2 level that maintains a degree of functional connectivity among brainstem respiratory neurons necessary to produce eupneic breathing. Central chemoreception responds to small variations in PCO2 to regulate normal gas exchange and to large changes in PCO2 to minimize acid-base changes. Central chemoreceptor sites vary in function with sex and with development. From an evolutionary perspective, central chemoreception grew out of the demands posed by air versus water breathing, homeothermy, sleep, optimization of the work of breathing with the "ideal" arterial PCO2, and the maintenance of the appropriate pH at 37°C for optimal protein structure and function.