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.

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 seconds, every 10 minutes, 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 twofold 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 (hypoxia-inducible factor 1 alpha) mRNA expression in catecholaminergic presympathetic 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 presympathetic 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.

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

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

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 (GABAA receptor agonist, 1 mM) or 8-OH-DPAT (5-HT1A 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.

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.

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.

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.

Postnatal intermittent hypoxia enhances phrenic and reduces vagal upper airway motor activities in rats by epigenetic mechanisms.

NEW FINDINGS: What is the central question of this study? What are the alterations in respiratory motor activity that may underlie ventilatory dysfunctions in juvenile and adult animals exposed to postnatal chronic intermittent hypoxia? What is the main finding and its importance? Postnatal chronic intermittent hypoxia modifies the motor activity to pumping and upper airway respiratory muscles in rats, mediated by epigenetic DNA hypermethylation, enhancing resting pulmonary ventilation and predisposing to collapse of the upper airways in juvenile and adult life. ABSTRACT: Periods of apnoea, commonly observed in prematures and newborns, are an important risk factor for the development of cardiorespiratory diseases in adulthood. In the present study, we evaluated changes in pulmonary ventilation and respiratory motor pattern in juvenile and adult rats exposed to postnatal chronic intermittent hypoxia (pCIH). Newborn male Holtzman rats (P1) were submitted to pCIH (6% O2 for 30 s, every 9 min, 8 h a day (09.30-17.30 h)) during their first 10 days of life, while control animals were maintained under normoxic conditions (20.8% O2 ). Thereafter, animals of both groups were maintained under normoxia until the experiments. Unanaesthetized juvenile pCIH rats (n = 27) exhibited elevated tidal volume and respiratory irregularities (P < 0.05) compared to control rats (n = 7). Decerebrate, arterially perfused in situ preparations of juvenile pCIH rats (n = 11) displayed augmented phrenic nerve (PN) burst amplitude and reduced central vagus nerve activity in comparison to controls (n = 10). At adulthood, pCIH rats (n = 5) showed enhanced tidal volume (P < 0.05) and increased respiratory variability compared to the control group (n = 5). The pCIH-induced changes in ventilation and respiratory motor outputs were prevented by treatment with the DNA methyltransferase inhibitor decitabine (1 mg kg-1 , i.p.) during the exposure to pCIH. Our data demonstrate that pCIH in rats impacts, in a persistent way, control of the respiratory pattern, increasing PN activity to the diaphragm and reducing the vagal-related activity to laryngeal muscles, which, respectively, may contribute to improve resting pulmonary ventilation and predispose to collapse of the upper airways during quiet breathing.

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.

Importance of the commissural nucleus of the solitary tract in renovascular hypertension.

The rodent renovascular hypertension model has been used to investigate the mechanisms promoting hypertension. The importance of the carotid body for renovascular hypertension has been demonstrated. As the commissural NTS (cNTS) is the first synaptic site in the central nervous system that receives information from carotid body chemoreceptors, we evaluated the contribution of cNTS to renovascular hypertension in the present study. Normotensive male Holtzman rats were implanted with a silver clip around the left renal artery to induce two-kidney, one-clip (2K1C) hypertension. Six weeks later, isoguvacine (a GABAA agonist) or losartan (an AT1 antagonist) was injected into the cNTS, and the effects were compared with carotid body removal. Immunohistochemistry for Iba-1 and GFAP to label microglia and astrocytes, respectively, and RT-PCR for components of the renin-angiotensin system and cytokines in the NTS were also performed 6 weeks after renal surgery. The inhibition of cNTS with isoguvacine or the blockade of AT1 receptors with losartan in the cNTS decreased the blood pressure and heart rate of 2K1C rats even more than carotid body removal did. The mRNA expression of NOX2, TNF-α and IL-6, microglia, and astrocytes also increased in the cNTS of 2K1C rats compared to that of normotensive rats. These results indicate that tonically active neurons within the cNTS are essential for the maintenance of hypertension in 2K1C rats. In addition to signals from the carotid body, the present results suggest that angiotensin II directly activates the cNTS and may also induce microgliosis and astrogliosis within the NTS, which, in turn, cause oxidative stress and neuroinflammation.