Hypoxia sensing in the body: An update on the peripheral and central mechanisms

An adequate supply of O2 is essential for the maintenance of cellular activity. Systemic or local hypoxia can be experienced during decreased O2 availability or associated with diseases, or a combination of both. Exposure to hypoxia triggers adjustments in multiple physiological systems in the body to generate appropriate homeostatic responses. However, with significant reductions in the arterial partial pressure of O2, hypoxia can be life-threatening and cause maladaptive changes or cell damage and death. To mitigate the impact of limited O2 availability on cellular activity, O2 chemoreceptors rapidly detect and respond to reductions in the arterial partial pressure of O2, triggering orchestrated responses of increased ventilation and cardiac output, blood flow redistribution and metabolic adjustments. In mammals, the peripheral chemoreceptors of the carotid body are considered to be the main hypoxic sensors and the primary source of excitatory feedback driving respiratory, cardiovascular and autonomic responses. However, current evidence indicates that the CNS contains specialized brainstem and spinal cord regions that can also sense hypoxia and stimulate brain networks independently of the carotid body inputs. In this manuscript, we review the discoveries about the functioning of the O2 chemoreceptors and their contribution to the monitoring of O2 levels in the blood and brain parenchyma and mounting cardiorespiratory responses to maintain O2 homeostasis. We also discuss the implications of the chemoreflex-related mechanisms in paediatric and adult pathologies.

Exploring the role of the Kölliker-Fuse nucleus in breathing variability by mathematical modelling

The Kölliker-Fuse nucleus (KF), which is part of the parabrachial complex, participates in the generation of eupnoea under resting conditions and the control of active abdominal expiration when increased ventilation is required. Moreover, dysfunctions in KF neuronal activity are believed to play a role in the emergence of respiratory abnormalities seen in Rett syndrome (RTT), a progressive neurodevelopmental disorder associated with an irregular breathing pattern and frequent apnoeas. Relatively little is known, however, about the intrinsic dynamics of neurons within the KF and how their synaptic connections affect breathing pattern control and contribute to breathing irregularities. In this study, we use a reduced computational model to consider several dynamical regimes of KF activity paired with different input sources to determine which combinations are compatible with known experimental observations. We further build on these findings to identify possible interactions between the KF and other components of the respiratory neural circuitry. Specifically, we present two models that both simulate eupnoeic as well as RTT-like breathing phenotypes. Using nullcline analysis, we identify the types of inhibitory inputs to the KF leading to RTT-like respiratory patterns and suggest possible KF local circuit organizations. When the identified properties are present, the two models also exhibit quantal acceleration of late-expiratory activity, a hallmark of active expiration featuring forced exhalation, with increasing inhibition to KF, as reported experimentally. Hence, these models instantiate plausible hypotheses about possible KF dynamics and forms of local network interactions, thus providing a general framework as well as specific predictions for future experimental testing. KEY POINTS: The Kölliker-Fuse nucleus (KF), a part of the parabrachial complex, is involved in regulating normal breathing and controlling active abdominal expiration during increased ventilation. Dysfunction in KF neuronal activity is thought to contribute to respiratory abnormalities seen in Rett syndrome (RTT). This study utilizes computational modelling to explore different dynamical regimes of KF activity and their compatibility with experimental observations. By analysing different model configurations, the study identifies inhibitory inputs to the KF that lead to RTT-like respiratory patterns and proposes potential KF local circuit organizations. Two models are presented that simulate both normal breathing and RTT-like breathing patterns. These models provide testable hypotheses and specific predictions for future experimental investigations, offering a general framework for understanding KF dynamics and potential network interactions.

Maternal high-fat diet changes breathing pattern and causes excessive sympathetic discharge in juvenile offspring rat

Early life over-nutrition, as experienced in maternal obesity, is a risk factor for developing cardiorespiratory and metabolic diseases. Here we investigated the effects of high-fat diet (HFD) consumption on the breathing pattern and sympathetic discharge to blood vessels in juvenile offspring from dams fed with HFD (O-HFD). Adult female Holtzman rats were given a standard diet (SD) or HFD from 6 wk before gestation to weaning. At weaning (P21), the male offspring from SD dams (O-SD) and O-HFD received SD until the experimental day (P28-P45). Nerve recordings performed in decerebrated in situ preparations demonstrated that O-HFD animals presented abdominal expiratory hyperactivity under resting conditions and higher vasoconstrictor sympathetic activity levels. The latter was associated with blunted respiratory-related oscillations in sympathetic activity, especially in control animals. When exposed to elevated hypercapnia or hypoxia levels, the O-HFD animals mounted similar ventilatory and respiratory motor responses as the control animals. Hypercapnia and hypoxia exposure also increased sympathetic activity in both groups but did not reinstate the respiratory-sympathetic coupling in the O-HFD rats. In freely behaving conditions, O-HFD animals exhibited higher resting pulmonary ventilation and larger variability of arterial pressure levels than the O-SD animals due to augmented sympathetic modulation of blood vessel diameter. Maternal obesity modified the functioning of cardiorespiratory systems in offspring at a young age, inducing active expiration and sympathetic overactivity under resting conditions. These observations represent new evidence about pregnancy-related complications that lead to the development of respiratory distress and hypertension in children of obese mothers.NEW & NOTEWORTHY Maternal obesity is a risk factor for developing cardiorespiratory and metabolic diseases. This study highlights the changes on the breathing pattern and sympathetic discharge to blood vessels in juvenile offspring from dams fed with HFD. Maternal obesity modified the functioning of cardiorespiratory systems in offspring, inducing active expiration and sympathetic overactivity. These observations represent new evidence about pregnancy-related complications that lead to the development of respiratory distress and hypertension in children of obese mothers.

Acute inhibition of nicotinamide adenine dinucleotide phosphate oxidase in the commissural nucleus of the solitary tract reduces arterial pressure and renal sympathetic nerve activity in renovascular hypertension

BACKGROUND

A growing body of evidence suggests that oxidative stress plays a role in the pathophysiology of hypertension. However, the involvement of the reactive oxygen species (ROS) in the commissural nucleus of the solitary tract (commNTS) in development the of hypertension remains unclear.

METHOD

We evaluated the hemodynamic and sympathetic responses to acute inhibition of NADPH oxidase in the commNTS in renovascular hypertensive rats. Under anesthesia, male Holtzman rats were implanted with a silver clip around the left renal artery to induce 2-kidney 1-clip (2K1C) hypertension. After six weeks, these rats were anesthetized and instrumented for recording mean arterial pressure (MAP), renal blood flow (RBF), renal vascular resistance (RVR), and renal sympathetic nerve activity (RSNA) during baseline and after injection of apocynin (nicotinamide adenine dinucleotide phosphate oxidase inhibitor), NSC 23766 (RAC inhibitor) or saline into the commNTS.

RESULTS

Apocynin into the commNTS decreased MAP, RSNA, and RVR in 2K1C rats. NSC 23766 into the commNTS decreased MAP and RSNA, without changing RVR in 2K1C rats.

CONCLUSION

These results demonstrate that the formation of ROS in the commNTS is important to maintain sympathoexcitation and hypertension in 2K1C rats and suggest that NADPH oxidase in the commNTS could be a potential target for therapeutics in renovascular hypertension.

Sympathetic dysregulation induced by postnatal intermittent hypoxia

STUDY OBJECTIVES

Exposure to postnatal chronic intermittent hypoxia (pCIH), as experienced in sleep-disordered breathing, is a risk factor for developing cardiorespiratory diseases in adulthood. pCIH causes respiratory instability and motor dysfunction that persist until adult life. In this study, we investigated the impact of pCIH on the sympathetic control of arterial pressure in rats.

METHODS AND RESULTS

Neonate male Holtzman rats (P0-1) were exposed to pCIH (6% O2 for 30 s, every 10 min, 8 h/day) during their first 10-15 days of life, while control animals were maintained under normoxia. In early adult life (P25-40), freely behaving pCIH animals (n=13) showed higher baseline arterial pressure levels linked to augmented sympathetic-mediated variability than control animals (n=12, P<0.05). Using decerebrated in situ preparations, we found that juvenile pCIH rats exhibited a two-fold increase in thoracic sympathetic nerve activity (n=14) and elevated firing frequency of ventromedullary presympathetic neurons (n=7) compared to control rats (n=6-7, P<0.05). This pCIH-induced sympathetic dysregulation was associated with increased HIF-1α (hypoxic inducible factor) mRNA expression in catecholaminergic pre-sympathetic neurons (n=5,P<0.05). At older age (P90-99), pCIH rats displayed higher arterial pressure levels and larger depressor responses to ganglionic blockade (n=6-8, P<0.05), confirming the sympathetic overactivity state.

CONCLUSIONS

pCIH facilitates the vasoconstrictor sympathetic drive by mechanisms associated with enhanced firing activity and HIF-1α expression in ventromedullary pre-sympathetic neurons. This excessive sympathetic activity persists until adulthood resulting in high blood pressure levels and variability, which contribute to developing cardiovascular diseases.

The carotid body: A novel key player in neuroimmune interactions

The idea that the nervous system communicates with the immune system to regulate physiological and pathological processes is not new. However, there is still much to learn about how these interactions occur under different conditions. The carotid body (CB) is a sensory organ located in the neck, classically known as the primary sensor of the oxygen (O2) levels in the organism of mammals. When the partial pressure of O2 in the arterial blood falls, the CB alerts the brain which coordinates cardiorespiratory responses to ensure adequate O2 supply to all tissues and organs in the body. A growing body of evidence, however, has demonstrated that the CB is much more than an O2 sensor. Actually, the CB is a multimodal sensor with the extraordinary ability to detect a wide diversity of circulating molecules in the arterial blood, including inflammatory mediators. In this review, we introduce the literature supporting the role of the CB as a critical component of neuroimmune interactions. Based on ours and other studies, we propose a novel neuroimmune pathway in which the CB acts as a sensor of circulating inflammatory mediators and, in conditions of systemic inflammation, recruits a sympathetic-mediated counteracting mechanism that appears to be a protective response.

The carotid body detects circulating tumor necrosis factor-alpha to activate a sympathetic anti-inflammatory reflex

Recent evidence has suggested that the carotid bodies might act as immunological sensors, detecting pro-inflammatory mediators and signalling to the central nervous system, which, in turn, orchestrates autonomic responses. Here, we confirmed that the TNF-α receptor type I is expressed in the carotid bodies of rats. The systemic administration of TNF-α increased carotid body afferent discharge and activated glutamatergic neurons in the nucleus tractus solitarius (NTS) that project to the rostral ventrolateral medulla (RVLM), where many pre-sympathetic neurons reside. The activation of these neurons was accompanied by an increase in splanchnic sympathetic nerve activity. Carotid body ablation blunted the TNF-α-induced activation of RVLM-projecting NTS neurons and the increase in splanchnic sympathetic nerve activity. Finally, plasma and spleen levels of cytokines after TNF-α administration were higher in rats subjected to either carotid body ablation or splanchnic sympathetic denervation. Collectively, our findings indicate that the carotid body detects circulating TNF-α to activate a counteracting sympathetic anti-inflammatory mechanism.

Advancing respiratory-cardiovascular physiology with the working heart-brainstem preparation over 25 years

Twenty‐five years ago, a new physiological preparation called the working heart–brainstem preparation (WHBP) was introduced with the claim it would provide a new platform allowing studies not possible before in cardiovascular, neuroendocrine, autonomic and respiratory research. Herein, we review some of the progress made with the WHBP, some advantages and disadvantages along with potential future applications, and provide photographs and technical drawings of all the customised equipment used for the preparation. Using mice or rats, the WHBP is an in situ experimental model that is perfused via an extracorporeal circuit benefitting from unprecedented surgical access, mechanical stability of the brain for whole cell recording and an uncompromised use of pharmacological agents akin to in vitro approaches. The preparation has revealed novel mechanistic insights into, for example, the generation of distinct respiratory rhythms, the neurogenesis of sympathetic activity, coupling between respiration and the heart and circulation, hypothalamic and spinal control mechanisms, and peripheral and central chemoreceptor mechanisms. Insights have been gleaned into diseases such as hypertension, heart failure and sleep apnoea. Findings from the in situ preparation have been ratified in conscious in vivo animals and when tested have translated to humans. We conclude by discussing potential future applications of the WHBP including two‐photon imaging of peripheral and central nervous systems and adoption of pharmacogenetic tools that will improve our understanding of physiological mechanisms and reveal novel mechanisms that may guide new treatment strategies for cardiorespiratory diseases.

Lesion of Serotonergic Afferents to the Retrotrapezoid Nucleus Impairs the Tachypneic Response to Hypercapnia in Unanesthetized Animals

Hypercapnia promotes an increase in pulmonary ventilation due to the stimulation of brainstem chemosensory cells that are connected to the respiratory network. Among these cells are the raphe serotonergic neurons which widely send projections to distinct central respiratory compartments. Nevertheless, the physiological role of specific raphe serotonergic projections to other chemosensitive sites on the emergence of hypercapnia ventilatory response in vivo still remains to be elucidated. Here we investigated whether the ventilatory response to hypercapnia requires serotonergic inputs to the chemosensitive cells of the retrotrapezoid nucleus (RTN) in the ventrolateral medulla. To test this, pulmonary ventilation was evaluated under baseline conditions and during hypercapnia (7% CO₂) in unanesthetized juvenile Holtzman rats (60-90 g) that received bilateral microinjections of either vehicle (control) or anti-SERT-SAP (0.1 mM, 10 pmol/100 nl) toxin in the RTN to retrogradely destroy serotonergic afferents to this region. Fifteen days after microinjections, baseline ventilation was not different between anti-SERT-SAP (n = 8) and control animals (n = 9). In contrast, the ablation of RTN-projecting serotonergic neurons markedly attenuated the hypercapnia-induced increase in respiratory frequency which was correlated with reduced numbers of serotonergic neurons in the raphe obscurus and magnus, but not in the raphe pallidus. The increase in tidal volume during hypercapnia was not significantly affected by anti-SERT-SAP microinjections in the RTN. Our data indicate that serotoninergic neurons that send projections to the RTN region are required for the processing of ventilatory reflex response during exposure to high CO₂ in unanesthetized conditions.

Differential modulation of active expiration during hypercapnia by the medullary raphe in unanesthetized rats

Active expiration represents an important mechanism to improve ventilation in conditions of augmented ventilatory demand, such as hypercapnia. While a rostral ventromedullary region, the parafacial respiratory group (pFRG), has been identified as a conditional expiratory oscillator, little is known about how central chemosensitive sites contribute to modulate active expiration under hypercapnia. In this study, we investigated the influence of the medullary raphe in the emergence of phasic expiratory abdominal activity during hypercapnia in unanesthetized adult male rats, in a state-dependent manner. To do so, reverse microdialysis of muscimol (GABA_A receptor agonist, 1 mM) or 8-OH-DPAT (5-HT_1A agonist, 1 mM) was applied in the MR during sleep and wakefulness periods, both in normocapnic (room air) and hypercapnic conditions (7% CO₂). Electromyography (EMG) of diaphragm and abdominal muscles was performed to measure inspiratory and expiratory motor outputs. We found that active expiration did not occur in room air exposure during wakefulness or sleep. However, hypercapnia did recruit active expiration, and differential effects were observed with the drug dialyses in the medullary raphe. Muscimol increased the diaphragm inspiratory motor output and also increased the amplitude and frequency of abdominal expiratory rhythmic activity during hypercapnia in wakefulness periods. On the other hand, the microdialysis of 8-OH-DPAT attenuated hypercapnia-induced active expiration in a state-dependent manner. Our data suggest that the medullary raphe can either inhibit or potentiate respiratory motor activity during hypercapnia, and the balance of these inhibitory or excitatory outputs may determine the expression of active expiration.

Inhibitory control of active expiration by the Bötzinger complex in rats

KEY POINTS

Contraction of abdominal muscles at the end of expiration during metabolic challenges (such as hypercapnia and hypoxia) improves pulmonary ventilation. The emergence of this active expiratory pattern requires the recruitment of the expiratory oscillator located on the ventral surface of the medulla oblongata. Here we show that an inhibitory circuitry located in the Bötzinger complex is an important source of inhibitory drive to the expiratory oscillator. This circuitry, mediated by GABAergic and glycinergic synapses, provides expiratory inhibition that restrains the expiratory oscillator under resting condition and regulates the formation of abdominal expiratory activity during active expiration. By combining experimental and modelling approaches, we propose the organization and connections within the respiratory network that control the changes in the breathing pattern associated with elevated metabolic demand.

ABSTRACT

The expiratory neurons of the Bötzinger complex (BötC) provide inhibitory inputs to the respiratory network, which, during eupnoea, are critically important for respiratory phase transition and duration control. Here, we investigated how the BötC neurons interact with the expiratory oscillator located in the parafacial respiratory group (pFRG) and control the abdominal activity during active expiration. Using the decerebrated, arterially perfused in situ preparations of juvenile rats, we recorded the activity of expiratory neurons and performed pharmacological manipulations of the BötC and pFRG during hypercapnia or after the exposure to short-term sustained hypoxia - conditions that generate active expiration. The experimental data were integrated in a mathematical model to gain new insights into the inhibitory connectome within the respiratory central pattern generator. Our results indicate that the BötC neurons may establish mutual connections with the pFRG, providing expiratory inhibition during the first stage of expiration and receiving excitatory inputs during late expiration. Moreover, we found that application of GABAergic and glycinergic antagonists in the BötC caused opposing effects on abdominal expiratory activity, suggesting complex inhibitory circuitry within the BötC. Using mathematical modelling, we propose that the BötC network organization and its interactions with the pFRG restrain abdominal activity under resting conditions and contribute to abdominal expiratory pattern formation during active expiration observed during hypercapnia or after the exposure to short-term sustained hypoxia.

Leptin: Master Regulator of Biological Functions that Affects Breathing

Obesity is a global epidemic in developed countries accounting for many of the metabolic and cardiorespiratory morbidities that occur in adults. These morbidities include type 2 diabetes, sleep-disordered breathing (SDB), obstructive sleep apnea, chronic intermittent hypoxia, and hypertension. Leptin, produced by adipocytes, is a master regulator of metabolism and of many other biological functions including central and peripheral circuits that control breathing. By binding to receptors on cells and neurons in the brainstem, hypothalamus, and carotid body, leptin links energy and metabolism to breathing. In this comprehensive article, we review the central and peripheral locations of leptin's actions that affect cardiorespiratory responses during health and disease, with a particular focus on obesity, SDB, and its effects during early development. Obesity-induced hyperleptinemia is associated with centrally mediated hypoventilation with decrease CO₂ sensitivity. On the other hand, hyperleptinemia augments peripheral chemoreflexes to hypoxia and induces sympathoexcitation. Thus, "leptin resistance" in obesity is relative. We delineate the circuits responsible for these divergent effects, including signaling pathways. We review the unique effects of leptin during development on organogenesis, feeding behavior, and cardiorespiratory responses, and how undernutrition and overnutrition during critical periods of development can lead to cardiorespiratory comorbidities in adulthood. We conclude with suggestions for future directions to improve our understanding of leptin dysregulation and associated clinical diseases and possible therapeutic targets. Lastly, we briefly discuss the yin and the yang, specifically the contribution of relative adiponectin deficiency in adults with hyperleptinemia to the development of metabolic and cardiovascular disease. © 2020 American Physiological Society. Compr Physiol 10:1047-1083, 2020.

Renovascular hypertension elevates pulmonary ventilation in rats by carotid body-dependent mechanisms

The two kidney-one clip (2K1C) renovascular hypertension depends on the renin-angiotensin system and sympathetic overactivity. The maintenance of 2K1C hypertension also depends on inputs from the carotid bodies (CB), which when activated stimulate the respiratory activity. In the present study, we investigated the importance of CB afferent activity for the ventilatory responses in 2K1C hypertensive rats and for phrenic and hypoglossal activities in in situ preparations of normotensive rats treated with angiotensin II. Silver clips were implanted around the left renal artery of male Holtzman rats (150 g) to induce renovascular hypertension. Six weeks after clipping, hypertensive 2K1C rats showed, in conscious state, elevated resting tidal volume and minute ventilation compared with the normotensive group. 2K1C rats also presented arterial alkalosis, urinary acidification, and amplified hypoxic ventilatory response. Carotid body removal (CBR), 2 wk before the experiments (4th week after clipping), significantly reduced arterial pressure and pulmonary ventilation in 2K1C rats but not in normotensive rats. Intra-arterial administration of angiotensin II in the in situ preparation of normotensive rats increased phrenic and hypoglossal activities, responses that were also reduced after CBR. Results show that renovascular hypertensive rats exhibit increased resting ventilation that depends on CB inputs. Similarly, angiotensin II increases phrenic and hypoglossal activities in in situ preparations of normotensive rats, responses that also depend on CB inputs. Results suggest that mechanisms that depend on CB inputs in renovascular hypertensive rats or during angiotensin II administration in normotensive animals increase respiratory drive.

Modulation of hypercapnic respiratory response by cholinergic transmission in the commissural nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is an important area of the brainstem that receives and integrates afferent cardiorespiratory sensorial information, including those from arterial chemoreceptors and baroreceptors. It was described that acetylcholine (ACh) in the commissural subnucleus of the NTS (cNTS) promotes an increase in the phrenic nerve activity (PNA) and antagonism of nicotinic receptors in the same region reduces the magnitude of tachypneic response to peripheral chemoreceptor stimulation, suggesting a functional role of cholinergic transmission within the cNTS in the chemosensory control of respiratory activity. In the present study, we investigated whether cholinergic receptor antagonism in the cNTS modifies the sympathetic and respiratory reflex responses to hypercapnia. Using an arterially perfused in situ preparation of juvenile male Holtzman rats, we found that the nicotinic antagonist (mecamylamine, 5 mM), but not the muscarinic antagonist (atropine, 5 mM), into the cNTS attenuated the hypercapnia-induced increase of hypoglossal activity. Furthermore, mecamylamine in the cNTS potentiated the generation of late-expiratory (late-E) activity in abdominal nerve induced by hypercapnia. None of the cholinergic antagonists microinjected in the cNTS changed either the sympathetic or the phrenic nerve responses to hypercapnia. Our data provide evidence for the role of cholinergic transmission in the cNTS, acting on nicotinic receptors, modulating the hypoglossal and abdominal responses to hypercapnia.

Centrally acting adrenomedullin in the long-term potentiation of sympathetic vasoconstrictor activity induced by intermittent hypoxia in rats

NEW FINDINGS

What is the central question of this study? Adrenomedullin in the rostral ventrolateral medulla (RVLM) increases sympathetic activity; given that adrenomedullin is released during hypoxia, what are the effects of its agonism and antagonism in the RVLM after chronic intermitent hypoxia (CIH) exposure? What is the main finding and its importance? CIH exposure sensitizes adrenomedullin-dependent mechanisms in the RVLM, supporting its role as a sympathoexcitatory neuromodulator. A novel mechanism was identified for the generation of sympathetic overdrive and hypertension associated with hypoxia, providing potential guidance on new therapeutic approaches for controlling sympathetic hyperactivity in diseases such as sleep apnoea and neurogenic hypertension.

ABSTRACT

Adrenomedullin in the rostral ventrolateral medulla (RVLM) has been shown to increase sympathetic activity whereas the antagonism of its receptors inhibited this autonomic activity lowering blood pressure in conditions of hypertension. Given that hypoxia is a stimulant for releasing adrenomedullin, we hypothesized that the presence of this peptide in the RVLM associated with chronic intermittent hypoxia (CIH) would cause sympathetic overdrive. Juvenile male rats (50-55 g) submitted to CIH (6% oxygen every 9 min, 8 h day^-1 for 10 days) were studied in an arterially perfused in situ preparation where sympathetic activity was recorded. In control rats (n = 6), exogenously applied adrenomedullin in the RVLM raised baseline sympathetic activity when combined with episodic activation of peripheral chemoreceptors (KCN 0.05%, 5 times every 5 min). This sympathoexcitatory response was markedly amplified in rats previously exposed to CIH (n = 6). The antagonism of adrenomedullin receptors in the RVLM caused a significant reduction in sympathetic activity in the CIH group (n = 7), but not in controls (n = 8). The transient reflex-evoked sympathoexcitatory response to peripheral chemoreceptor stimulation was not affected by either adrenomedullin or adrenomedullin receptor antagonism in the RVLM of control and CIH rats. Our findings indicate that CIH sensitizes the sympathoexcitatory networks within the RVLM to adrenomedullin, supporting its role as an excitatory neuromodulator when intermittent hypoxia is present. These data reveal novel state-dependent mechanistic insights into the generation of sympathetic overdrive and provide potential guidance on possible unique approaches for controlling sympathetic discharge in diseases such as sleep apnoea and neurogenic hypertension.

High-fat diet increases respiratory frequency and abdominal expiratory motor activity during hypercapnia

Breathing disorders are commonly observed in association with obesity. Here we tested whether high-fat diet (HFD) impairs the chemoreflex ventilatory response. Male Holtzman rats (300-320 g) were fed with standard chow diet (SD) or HFD for 12 weeks. Then, tidal volume (VT), respiratory frequency (fR) and pulmonary ventilation (VE) were determined in conscious rats during basal condition, hypercapnia (7% or 10% CO2) or hypoxia (7% O2). The mean arterial pressure (MAP), heart rate (HR) and baroreflex sensitivity were also evaluated in conscious rats. A group of anesthetized rats was used for the measurements of the activity of inspiratory (diaphragm) and expiratory (abdominal) muscles under the same gas conditions. Baseline fR, VT and VE were similar between SD and HFD rats. During hypercapnia, the increase of fR was exacerbated in conscious HFD rats (60 ± 3, vs. SD: 47 ± 3 Δ breaths.min-1, P < 0.05). In anesthetized rats, hypercapnia strongly increased abdominal muscle activity in HFD group (238 ± 27, vs. basal condition: 100 ± 0.3%; P < 0.05), without significant change in SD group (129 ± 2.1, vs. basal condition: 100 ± 0.8%; P = 0.34). The ventilatory responses to hypoxia were similar between groups. In conscious HFD rats, MAP and HR were elevated and the baroreflex function was impaired (P < 0.05). These data demonstrated that 12 weeks of HFD exaggerate the ventilatory response activated by hypercapnia. The mechanisms involved in these responses need more investigation in future studies.

Interaction between the retrotrapezoid nucleus and the parafacial respiratory group to regulate active expiration and sympathetic activity in rats

The retrotrapezoid nucleus (RTN) contains chemosensitive cells that distribute CO₂-dependent excitatory drive to the respiratory network. This drive facilitates the function of the respiratory central pattern generator (rCPG) and increases sympathetic activity. It is also evidenced that during hypercapnia, the late-expiratory (late-E) oscillator in the parafacial respiratory group (pFRG) is activated and determines the emergence of active expiration. However, it remains unclear the microcircuitry responsible for the distribution of the excitatory signals to the pFRG and the rCPG in conditions of high CO₂. Herein, we hypothesized that excitatory inputs from chemosensitive neurons in the RTN are necessary for the activation of late-E neurons in the pFRG. Using the decerebrated in situ rat preparation, we found that lesions of neurokinin-1 receptor-expressing neurons in the RTN region with substance P-saporin conjugate suppressed the late-E activity in abdominal nerves (AbNs) and sympathetic nerves (SNs) and attenuated the increase in phrenic nerve (PN) activity induced by hypercapnia. On the other hand, kynurenic acid (100 mM) injections in the pFRG eliminated the late-E activity in AbN and thoracic SN but did not modify PN response during hypercapnia. Iontophoretic injections of retrograde tracer into the pFRG of adult rats revealed labeled phox2b-expressing neurons within the RTN. Our findings are supported by mathematical modeling of chemosensitive and late-E populations within the RTN and pFRG regions as two separate but interacting populations in a way that the activation of the pFRG late-E neurons during hypercapnia require glutamatergic inputs from the RTN neurons that intrinsically detect changes in CO₂/pH.

Short-Term Sustained Hypoxia Elevates Basal and Hypoxia-Induced Ventilation but Not the Carotid Body Chemoreceptor Activity in Rats

Exposure to chronic sustained hypoxia (SH), as experienced in high altitudes, elicits an increase in ventilation, named ventilatory acclimatization to hypoxia (VAH). We previously showed that rats exposed to short-term (24 h) SH exhibit enhanced abdominal expiratory motor activity at rest, accompanied by augmented baseline sympathetic vasoconstrictor activity. In the present study, we investigated whether the respiratory and sympathetic changes elicited by short-term SH are accompanied by carotid body chemoreceptor sensitization. Juvenile male Holtzman rats (60-80 g) were exposed to SH (10% O₂ for 24 h) or normoxia (control) to examine basal and hypoxic-induced ventilatory parameters in unanesthetized conditions, as well as the sensory response of carotid body chemoreceptors in artificially perfused in situ preparations. Under resting conditions (normoxia/normocapnia), SH rats (n = 12) exhibited higher baseline respiratory frequency, tidal volume, and minute ventilation compared to controls (n = 11, P < 0.05). SH group also showed greater hypoxia ventilatory response than control group (P < 0.05). The in situ preparations of SH rats (n = 8) exhibited augmented baseline expiratory and sympathetic activities under normocapnia, with additional bursts in abdominal and thoracic sympathetic nerves during late expiratory phase that were not seen in controls (n = 8, P < 0.05). Interestingly, basal and potassium cyanide-induced afferent activity of carotid sinus nerve (CSN) was similar between SH and control rats. Our findings indicate that the maintenance of elevated resting ventilation, baseline sympathetic overactivity, and enhanced ventilatory responses to hypoxia in rats exposed to 24 h of SH are not dependent on increased basal and sensorial activity of carotid body chemoreceptors.

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.

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.

Neurogenic hypertension and the secrets of respiration

Despite recent advances in the knowledge of the neural control of cardiovascular function, the cause of sympathetic overactivity in neurogenic hypertension remains unknown. Studies from our laboratory point out that rats submitted to chronic intermittent hypoxia (CIH), an experimental model of neurogenic hypertension, present changes in the central respiratory network that impact the pattern of sympathetic discharge and the levels of arterial pressure. In addition to the fine coordination of respiratory muscle contraction and relaxation, which is essential for O₂ and CO₂ pulmonary exchanges, neurons of the respiratory network are connected precisely to the neurons controlling the sympathetic activity in the brain stem. This respiratory-sympathetic neuronal interaction provides adjustments in the sympathetic outflow to the heart and vasculature during each respiratory phase according to the metabolic demands. Herein, we report that CIH-induced sympathetic over activity and mild hypertension are associated with increased frequency discharge of ventral medullary presympathetic neurons. We also describe that their increased frequency discharge is dependent on synaptic inputs, mostly from neurons of the brain stem respiratory network, rather than changes in their intrinsic electrophysiological properties. In perspective, we are taking into consideration the possibility that changes in the central respiratory rhythm/pattern generator contribute to increased sympathetic outflow and the development of neurogenic hypertension. Our experimental evidence provides support for the hypothesis that changes in the coupling of respiratory and sympathetic networks might be one of the unrevealed secrets of neurogenic hypertension in rats.

The Kölliker-Fuse nucleus acts as a timekeeper for late-expiratory abdominal activity

While the transition from the inspiratory to the post-inspiratory (post-I) phase is dependent on the pons, little attention has been paid to understanding the role of the pontine respiratory nuclei, specifically the Kölliker-Fuse nucleus (KF), in transitioning from post-I to the late expiratory (late-E) activity seen with elevated respiratory drive. To elucidate this, we used the in situ working heart-brainstem preparation of juvenile male Holtzman rats and recorded from the vagus (cVN), phrenic (PN) and abdominal nerves (AbN) during baseline conditions and during chemoreflex activation [with potassium cyanide (KCN; n=13) or hypercapnia (8% CO2; n=10)] to recruit active expiration. Chemoreflex activation with KCN increased PN frequency and cVN post-I and AbN activities. The inhibition of KF with isoguvacine microinjections (10mM) attenuated the typical increase in PN frequency and cVN post-I activity, and amplified the AbN response. During hypercapnia, AbN late-E activity emerged in association with a significant reduction in expiratory time. KF inhibition during hypercapnia significantly decreased PN frequency and reduced the duration and amplitude of post-I cVN activity, while the onset of the AbN late-E bursts occurred significantly earlier. Our data reveal a negative relationship between KF-induced post-I and AbN late-E activities, suggesting that the KF coordinates the transition between post-I to late-E activity during conditions of elevated respiratory drive.

Generation of active expiration by serotoninergic mechanisms of the ventral medulla of rats

Abdominal expiratory activity is absent at rest and is evoked during metabolic challenges, such as hypercapnia and hypoxia, or after the exposure to intermittent hypoxia (IH). The mechanisms engaged during this process are not completely understood. In this study, we hypothesized that serotonin (5-HT), acting in the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG), is able to generate active expiration. In anesthetized (urethane, ip), tracheostomized, spontaneously-breathing adult male Holtzman rats we microinjected a serotoninergic agonist and antagonist bilaterally in the RTN/pFRG and recorded diaphragm and abdominal muscle activities. We found that episodic (3 times, 5 min apart), but not single microinjections of 5-HT (1 mM) in the RTN/pFRG elicited an enduring (>30 min) increase in abdominal activity. This response was amplified in vagotomized rats and blocked by previous 5-HT receptor antagonism with ketanserin (10 µM). Episodic 5-HT microinjections in the RTN/pFRG also potentiated the inspiratory and expiratory reflex responses to hypercapnia. The antagonism of 5-HT receptors in the RTN/pFRG also prevented the long-term facilitation (>30 min) of abdominal activity in response to acute IH exposure (10 × 6-7% O for 45 s every 5 min). Our findings indicate the activation of serotoninergic mechanisms in the RTN/pFRG is sufficient to increase abdominal expiratory activity at resting conditions and required for the emergence of active expiration after IH in anesthetized animals.

Chemoreception and neuroplasticity in respiratory circuits

The respiratory central pattern generator must respond to chemosensory cues to maintain oxygen (O₂) and carbon dioxide (CO₂) homeostasis in the blood and tissues. To do this, sensorial cells located in the periphery and central nervous system monitor the arterial partial pressure of O₂ and CO₂ and initiate respiratory and autonomic reflex adjustments in conditions of hypoxia and hypercapnia. In conditions of chronic intermittent hypoxia (CIH), repeated peripheral chemoreceptor input mediated by the nucleus of the solitary tract induces plastic changes in respiratory circuits that alter baseline respiratory and sympathetic motor outputs and result in chemoreflex sensitization, active expiration, and arterial hypertension. Herein, we explored the hypothesis that the CIH-induced neuroplasticity primarily consists of increased excitability of pre-inspiratory/inspiratory neurons in the pre-Bötzinger complex. To evaluate this hypothesis and elucidate neural mechanisms for the emergence of active expiration and sympathetic overactivity in CIH-treated animals, we extended a previously developed computational model of the brainstem respiratory-sympathetic network to reproduce experimental data on peripheral and central chemoreflexes post-CIH. The model incorporated neuronal connections between the 2nd-order NTS neurons and peripheral chemoreceptors afferents, the respiratory pattern generator, and sympathetic neurons in the rostral ventrolateral medulla in order to capture key features of sympathetic and respiratory responses to peripheral chemoreflex stimulation. Our model identifies the potential neuronal groups recruited during peripheral chemoreflex stimulation that may be required for the development of inspiratory, expiratory and sympathetic reflex responses. Moreover, our model predicts that pre-inspiratory neurons in the pre-Bötzinger complex experience plasticity of channel expression due to excessive excitation during peripheral chemoreflex. Simulations also show that, due to positive interactions between pre-inspiratory neurons in the pre-Bötzinger complex and expiratory neurons in the retrotrapezoid nucleus, increased excitability of the former may lead to the emergence of the active expiratory pattern at normal CO₂ levels found after CIH exposure. We conclude that neuronal type specific neuroplasticity in the pre-Bötzinger complex induced by repetitive episodes of peripheral chemoreceptor activation by hypoxia may contribute to the development of sympathetic over-activity and hypertension.

Maternal protein restriction increases respiratory and sympathetic activities and sensitizes peripheral chemoreflex in male rat offspring

BACKGROUND

Maternal protein restriction in rats increases the risk of adult offspring arterial hypertension through unknown mechanisms.

OBJECTIVES

The aims of the study were to evaluate the effects of a low-protein (LP) diet during pregnancy and lactation on baseline sympathetic and respiratory activities and peripheral chemoreflex sensitivity in the rat offspring.

METHODS

Wistar rat dams were fed a control [normal-protein (NP); 17% protein] or an LP (8% protein) diet during pregnancy and lactation, and their male offspring were studied at 30 d of age. Direct measurements of baseline arterial blood pressure (ABP), heart rate (HR), and respiratory frequency (Rf) as well as peripheral chemoreflex activation (potassium cyanide: 0.04%) were recorded in pups while they were awake. In addition, recordings of the phrenic nerve (PN) and thoracic sympathetic nerve (tSN) activities were obtained from the in situ preparations. Hypoxia-inducible factor 1α (HIF-1α) expression was also evaluated in carotid bifurcation through a Western blotting assay.

RESULTS

At 30 d of age, unanesthetized LP rats exhibited enhanced resting Rf (P = 0.001) and similar ABP and HR compared with the NP rats. Despite their similar baseline ABP values, LP rats exhibited augmented low-frequency variability (∼91%; P = 0.01). In addition, the unanesthetized LP rats showed enhanced pressor (P = 0.01) and tachypnoeic (P = 0.03) responses to peripheral chemoreflex activation. The LP rats displayed elevated baseline tSN activity (∼86%; P = 0.02) and PN burst frequency (45%; P = 0.01) and amplitude (53%; P = 0.001) as well as augmented sympathetic (P = 0.01) and phrenic (P = 0.04) excitatory responses to peripheral chemoreflex activation compared with the NP group. Furthermore, LP rats showed an increase of ∼100% in HIF-1α protein density in carotid bifurcation compared with NP rats.

CONCLUSION

Sympathetic-respiratory overactivity and amplified peripheral chemoreceptor responses, potentially through HIF-1α-dependent mechanisms, precede the onset of hypertension in juvenile rats exposed to protein undernutrition during gestation and lactation.

Control of respiratory and cardiovascular functions by leptin

Leptin, a peptide hormone produced by adipose tissue, acts in brain centers that control critical physiological functions such as metabolism, breathing and cardiovascular regulation. The importance of leptin for respiratory control is evident by the fact that leptin deficient mice exhibit impaired ventilatory responses to carbon dioxide (CO2), which can be corrected by intracerebroventricular leptin replacement therapy. Leptin is also recognized as an important link between obesity and hypertension. Humans and animal models lacking either leptin or functional leptin receptors exhibit many characteristics of the metabolic syndrome, including hyperinsulinemia, insulin resistance, hyperglycemia, dyslipidemia and visceral adiposity, but do not exhibit increased sympathetic nerve activity (SNA) and have normal to lower blood pressure (BP) compared to lean controls. Even though previous studies have extensively focused on the brain sites and intracellular signaling pathways involved in leptin effects on food intake and energy balance, the mechanisms that mediate the actions of leptin on breathing and cardiovascular function are only beginning to be elucidated. This mini-review summarizes recent advances on the effects of leptin on cardiovascular and respiratory control with emphasis on the neural control of respiratory function and autonomic activity.

Peripheral chemoreceptors and cardiorespiratory coupling: a link to sympatho-excitation

NEW FINDINGS

What is the topic of this review? Chronic intermittent hypoxia (CIH), as observed in patients with obstructive sleep apnoea, is associated with the development of sympathetically mediated arterial hypertension. Nevertheless, the mechanisms underpinning the augmented sympathetic outflow in CIH still remain under investigation. What advances does it highlight? In this report, I present experimental evidence supporting the hypothesis that changes in the function of the respiratory network and coupling with the sympathetic nervous system may be considered as a novel and relevant mechanism for the increase in baseline sympathetic outflow in animals submitted to CIH. Chronic intermittent hypoxia (CIH) has been identified as a relevant risk factor for the development of enhanced sympathetic outflow and arterial hypertension. Several studies have highlighted the importance of peripheral chemoreceptors for the cardiovascular changes elicited by CIH. However, the effects of CIH on the central mechanisms regulating sympathetic outflow are not fully elucidated. Our research group has explored the hypothesis that the enhanced sympathetic drive following CIH exposure is, at least in part, dependent on alterations in the respiratory network and its interaction with the sympathetic nervous system. In this report, I discuss the changes in the discharge profile of baseline sympathetic activity in rats exposed to CIH, their association with the generation of active expiration and the interactions between expiratory and sympathetic neurones after CIH conditioning. Together, these findings are consistent with the theory that mechanisms of central respiratory-sympathetic coupling are a novel factor in the development of neurogenic hypertension.

Vagal afferent control of abdominal expiratory activity in response to hypoxia and hypercapnia in rats

In the present study, we tested the hypothesis that vagal afferent information modulates the pattern of expiratory response to hypercapnia and hypoxia. Simultaneous recordings of airflow, diaphragmatic (DIA) and oblique abdominal muscle (ABD) activities were performed in anesthetized (urethane, 1.2g/kg), tracheostomized, spontaneously breathing male Wistar rats (290-320g, n=12). The animals were exposed to hypercapnia (7 and 10% CO2 for 5min) and hypoxia (7% O2 for 1min) before and after bilateral vagotomy. We verified that the percentage increase in DIA burst amplitude elicited by hypercapnia and hypoxia episodes was similar between intact and vagotomized rats (P>0.05). In contrast, hypercapnia and hypoxia promoted a marked increase in ABD activity in vagotomized, but not in intact rats (P<0.01). These amplified expiratory motor changes after vagotomy were associated with enhanced expiratory airflow (P<0.01) and augmented tidal volume responses (P<0.01). Our data indicates that, in anesthetized conditions, the removal of peripheral afferent inputs facilitates the processing of active expiration in response to hypercapnia and hypoxia in rats.

The nucleus of the solitary tract and the coordination of respiratory and sympathetic activities

It is well known that breathing introduces rhythmical oscillations in the heart rate and arterial pressure levels. Sympathetic oscillations coupled to the respiratory activity have been suggested as an important homeostatic mechanism optimizing tissue perfusion and blood gas uptake/delivery. This respiratory-sympathetic coupling is strengthened in conditions of blood gas challenges (hypoxia and hypercapnia) as a result of the synchronized activation of brainstem respiratory and sympathetic neurons, culminating with the emergence of entrained cardiovascular and respiratory reflex responses. Studies have proposed that the ventrolateral region of the medulla oblongata is a major site of synaptic interaction between respiratory and sympathetic neurons. However, other brainstem regions also play a relevant role in the patterning of respiratory and sympathetic motor outputs. Recent findings suggest that the neurons of the nucleus of the solitary tract (NTS), in the dorsal medulla, are essential for the processing and coordination of respiratory and sympathetic responses to hypoxia. The NTS is the first synaptic station of the cardiorespiratory afferent inputs, including peripheral chemoreceptors, baroreceptors and pulmonary stretch receptors. The synaptic profile of the NTS neurons receiving the excitatory drive from afferent inputs is complex and involves distinct neurotransmitters, including glutamate, ATP and acetylcholine. In the present review we discuss the role of the NTS circuitry in coordinating sympathetic and respiratory reflex responses. We also analyze the neuroplasticity of NTS neurons and their contribution for the development of cardiorespiratory dysfunctions, as observed in neurogenic hypertension, obstructive sleep apnea and metabolic disorders.

Short-term sustained hypoxia induces changes in the coupling of sympathetic and respiratory activities in rats

Individuals experiencing sustained hypoxia (SH) exhibit adjustments in the respiratory and autonomic functions by neural mechanisms not yet elucidated. In the present study we evaluated the central mechanisms underpinning the SH-induced changes in the respiratory pattern and their impact on the sympathetic outflow. Using a decerebrated arterially perfused in situ preparation, we verified that juvenile rats exposed to SH (10% O2) for 24 h presented an active expiratory pattern, with increased abdominal, hypoglossal and vagal activities during late-expiration (late-E). SH also enhanced the activity of augmenting-expiratory neurones and depressed the activity of post-inspiratory neurones of the Bötzinger complex (BötC) by mechanisms not related to changes in their intrinsic electrophysiological properties. SH rats exhibited high thoracic sympathetic activity and arterial pressure levels associated with an augmented firing frequency of pre-sympathetic neurones of the rostral ventrolateral medulla (RVLM) during the late-E phase. The antagonism of ionotropic glutamatergic receptors in the BötC/RVLM abolished the late-E bursts in expiratory and sympathetic outputs of SH rats, indicating that glutamatergic inputs to the BötC/RVLM are essential for the changes in the expiratory and sympathetic coupling observed in SH rats. We also observed that the usually silent late-E neurones of the retrotrapezoid nucleus/parafacial respiratory group became active in SH rats, suggesting that this neuronal population may provide the excitatory drive essential to the emergence of active expiration and sympathetic overactivity. We conclude that short-term SH induces the activation of medullary expiratory neurones, which affects the pattern of expiratory motor activity and its coupling with sympathetic activity.

Coupling of respiratory and sympathetic activities in rats submitted to chronic intermittent hypoxia

The respiratory modulation of sympathetic activity relies on the balance between excitatory and inhibitory inputs from the brainstem respiratory network to presympathetic neurons of the rostral ventrolateral medulla. This central respiratory-sympathetic contributes for the generation of respiratory-related rhythmical oscillations in heart rate and arterial pressure levels, whose functional effects on the blood gas exchange/perfusion and cardiac work remain to be elucidated. Herein, we discuss the experimental evidence describing the potential neural mechanisms underlying the entrainment between respiratory and sympathetic activities at baseline conditions as well as under conditions of metabolic challenges. We also discuss the possible implications of changes in the strength or pattern of the central respiratory-sympathetic coupling in the genesis of sympathetic overactivity and neurogenic hypertension, including that associated with the exposure to chronic intermittent hypoxia.

Physiological and pathophysiological interactions between the respiratory central pattern generator and the sympathetic nervous system

Respiratory modulation seen in the sympathetic nerve activity (SNA) implies that the respiratory and sympathetic networks interact. During hypertension elicited by chronic intermittent hypoxia (CIH), the SNA displays an enhanced respiratory modulation reflecting strengthened interactions between the networks. In this chapter, we review a series of experimental and modeling studies that help elucidate possible mechanisms of sympatho-respiratory coupling. We conclude that this coupling significantly contributes to both the sympathetic baroreflex and the augmented sympathetic activity after exposure to CIH. This conclusion is based on the following findings. (1) Baroreceptor activation results in perturbation of the respiratory pattern via transient activation of postinspiratory neurons in the Bötzinger complex (BötC). The same BötC neurons are involved in the respiratory modulation of SNA, and hence provide an additional pathway for the sympathetic baroreflex. (2) Under hypercapnia, phasic activation of abdominal motor nerves (AbN) is accompanied by synchronous discharges in SNA due to the common source of this rhythmic activity in the retrotrapezoid nucleus (RTN). CIH conditioning increases the CO2 sensitivity of central chemoreceptors in the RTN which results in the emergence of AbN and SNA discharges under normocapnic conditions similar to those observed during hypercapnia in naïve animals. Thus, respiratory-sympathetic interactions play an important role in defining sympathetic output and significantly contribute to the sympathetic activity and hypertension under certain physiological or pathophysiological conditions, and the theoretical framework presented may be instrumental in understanding of malfunctioning control of sympathetic activity in a variety of disease states.

Glial cells modulate the synaptic transmission of NTS neurons sending projections to ventral medulla of Wistar rats

There is evidence that sympathoexcitatory and respiratory responses to chemoreflex activation involve ventrolateral medulla-projecting nucleus tractus solitarius (NTS) neurons (NTS-VLM neurons) and also that ATP modulates this neurotransmission. Here, we evaluated whether or not astrocytes is the source of endogenous ATP modulating the synaptic transmission in NTS-VLM neurons. Synaptic activities of putative astrocytes or NTS-VLM neurons were recorded using whole cell patch clamp. Tractus solitarius (TS) stimulation induced TS-evoked excitatory postsynaptic currents (TS-eEPSCs) in NTS-VLM neurons as well in NTS putative astrocytes, which were also identified by previous labeling. Fluoracetate (FAC), an inhibitor of glial metabolism, reduced TS-eEPSCs amplitude (-85.6 ± 16 vs. -39 ± 7.1 pA, n = 12) and sEPSCs frequency (2.8 ± 0.5 vs. 1.8 ± 0.46 Hz, n = 10) in recorded NTS-VLM neurons, indicating a gliomodulation of glutamatergic currents. To verify the involvement of endogenous ATP a purinergic antagonist was used, which reduced the TS-eEPSCs amplitude (-207 ± 50 vs. -149 ± 50 pA, n = 6), the sEPSCs frequency (1.19 ± 0.2 vs. 0.62 ± 0.11 Hz, n = 6), and increased the paired-pulse ratio (PPR) values (∼20%) in NTS-VLM neurons. Simultaneous perfusion of Pyridoxalphosphate-6-azophenyl-2',5'-disulfonic acid (iso-PPADS) and FAC produced reduction in TS-eEPSCs similar to that observed with iso-PPADS or FAC alone, indicating that glial cells are the source of ATP released after TS stimulation. Extracellular ATP measurement showed that FAC reduced evoked and spontaneous ATP release. All together these data show that putative astrocytes are the source of endogenous ATP, which via activation of presynaptic P2X receptors, facilitates the evoked glutamate release and increases the synaptic transmission efficacy in the NTS-VLM neurons probably involved with the peripheral chemoreflex pathways.

Sympathoexcitation during chemoreflex active expiration is mediated by l-glutamate in the RVLM/Bötzinger complex of rats

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla/Bötzinger/pre-Bötzinger complexes (RVLM/BötC/pre-BötC) on the respiratory modulation of sympathoexcitatory response to peripheral chemoreflex activation (chemoreflex) was evaluated in the working heart-brain stem preparation of juvenile rats. We identified different types of baro- and chemosensitive presympathetic and respiratory neurons intermingled within the RVLM/BötC/pre-BötC. Bilateral microinjections of kynurenic acid (KYN) into the rostral aspect of RVLM (RVLM/BötC) produced an additional increase in frequency of the phrenic nerve (PN: 0.38 ± 0.02 vs. 1 ± 0.08 Hz; P < 0.05; n = 18) and hypoglossal (HN) inspiratory response (41 ± 2 vs. 82 ± 2%; P < 0.05; n = 8), but decreased postinspiratory (35 ± 3 vs. 12 ± 2%; P < 0.05) and late-expiratory (24 ± 4 vs. 2 ±1%; P < 0.05; n = 5) abdominal (AbN) responses to chemoreflex. Likewise, expiratory vagal (cVN; 67 ± 6 vs. 40 ± 2%; P < 0.05; n = 5) and expiratory component of sympathoexcitatory (77 ± 8 vs. 26 ± 5%; P < 0.05; n = 18) responses to chemoreflex were reduced after KYN microinjections into RVLM/BötC. KYN microinjected into the caudal aspect of the RVLM (RVLM/pre-BötC; n = 16) abolished inspiratory responses [PN ( n = 16) and HN ( n = 6)], and no changes in magnitude of sympathoexcitatory ( n = 16) and expiratory (AbN and cVN; n = 10) responses to chemoreflex, producing similar and phase-locked vagal, abdominal, and sympathetic responses. We conclude that in relation to chemoreflex activation 1) ionotropic glutamate receptors in RVLM/BötC and RVLM/pre-BötC are pivotal to expiratory and inspiratory responses, respectively; and 2) activation of ionotropic glutamate receptors in RVLM/BötC is essential to the coupling of active expiration and sympathoexcitatory response.

Contribution of the retrotrapezoid nucleus/parafacial respiratory region to the expiratory-sympathetic coupling in response to peripheral chemoreflex in rats

Central mechanisms of coupling between respiratory and sympathetic systems are essential for the entrainment between the enhanced respiratory drive and sympathoexcitation in response to hypoxia. However, the brainstem nuclei and neuronal network involved in these respiratory-sympathetic interactions remain unclear. Here, we evaluated whether the increase in expiratory activity and expiratory-modulated sympathoexcitation produced by the peripheral chemoreflex activation involves the retrotrapezoid nucleus/parafacial respiratory region (RTN/pFRG). Using decerebrated arterially perfused in situ rat preparations (60-80 g), we recorded the activities of thoracic sympathetic (tSN), phrenic (PN), and abdominal nerves (AbN) as well as the extracellular activity of RTN/pFRG expiratory neurons, and reflex responses to chemoreflex activation were evaluated before and after inactivation of the RTN/pFRG region with muscimol (1 mM). In the RTN/pFRG, we identified late-expiratory (late-E) neurons (n = 5) that were silent at resting but fired coincidently with the emergence of late-E bursts in AbN after peripheral chemoreceptor activation. Bilateral muscimol microinjections into the RTN/pFRG region (n = 6) significantly reduced basal PN frequency, mean AbN activity, and the amplitude of respiratory modulation of tSN (P < 0.05). With respect to peripheral chemoreflex responses, muscimol microinjections in the RTN/pFRG enhanced the PN inspiratory response, abolished the evoked late-E activity of AbN, but did not alter either the magnitude or pattern of the tSN reflex response. These findings indicate that the RTN/pFRG region is critically involved in the processing of the active expiratory response but not of the expiratory-modulated sympathetic response to peripheral chemoreflex activation of rat in situ preparations.