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Ventrolateral pons mediates short-term depression of respiratory frequency after brief hypoxia
Abstract
The respiratory response to hypoxia is dynamic in the adult anesthetized Sprague-Dawley rat. Hypoxia elicits acute increases in both tidal volume (VT) and respiratory frequency (fR) followed by short-term increases in VT and short-term decreases in fR. After brief hypoxia (<1 min), recovery of the breathing pattern is again dynamic, where both VT and fR decrease immediately, but where VT remains above, and fR drops below, baseline. These acute changes are followed by a short-term progressive decrease in VT and increase in fR to baseline. We have identified a potential neural mechanism that depends on the integrity of the ventrolateral (vl) pons. Our studies show that: (a) blockade of activity in the vl pons prevents the short-term decrease in fR after hypoxia (b) stimulation of the vl pons decreases fR, and (c) vl pontine expiratory neurons are activated after hypoxia. These neurons may not be acting through alpha(2) -adrenergic receptors, but their effect does depend on NMDA-type receptor function. We conclude that the vl pons is a critical element in the pontomedullary network that generates and modulates the fR response to acute hypoxia.
... Following exposure of mice to a HX challenge, the return to room-air results in respiratory patterns classified as short-term potentiation (STP), in which ventilation remains elevated (Powell et al., 1998) or post-hypoxic frequency decline (PHFD) in which breathing frequency falls below baseline (Dick and Coles, 2000). One mechanism for initiation and propagation of unusual breathing patterns is the disturbed balance between STP and PHFD with the dominance of PHFD (i.e., absence of STP and expression of PHFD) promoting disordered breathing (Powell et al., 1998;Yamauchi et al., 2008a,b,c;Strohl, 2003;Younes, 2008;Yamauchi et al., 2010). ...
... The mechanisms responsible for post-HX disordered breathing have received considerable attention. At present, evidence is in favor of disturbances in central signaling (Wilkinson, 1997;Strohl, 2003) including the pons (Coles and Dick, 1996;Dick and Coles, 2000) rather than processes within the carotid bodies (Vizek et al., 1987;Brown et al., 1993) although it is evident that carotid body chemoafferents play an essential role in the expression of sleep apnea . Despite considerable normal physiology, the C57BL6 mouse is of major interest to sleep-apnea researchers because it displays disordered breathing (irregular breaths and irregular breathing patterns including apneas, sighs, sniffs) during sleep and wakefulness and markedly disordered breathing upon return to room-air after exposure to HX gas challenges (Han et al., 2000(Han et al., , 2001(Han et al., , 2002Tagaito et al., 2001;Yamauchi et al., 2008aYamauchi et al., ,b,c, 2010. ...
... The parameters are reproducible within a strain and distinguish breathing differences among strains; and may be valuable for detection of non-eupneic breathing during the various stages of the awake-sleep cycle. The use of Rinx and Rinx/f R may be considered as a first-level phenotyping for breathing at rest, during a gas challenge and upon return to room-air, and would complement the detailed analyses of waveforms as elegantly performed by several groups (Dick and Coles, 2000;Tankersley et al., 2000, Strohl, 2003Balbir et al., 2006). Moreover, use of Rinx and Rinx/f R may be valuable in pharmacological studies aimed at uncovering the mechanisms of disordered breathing, in assessment of gene-by-drug interactions, and in studies designed to establish the efficacy of drugs on respiratory disorders such as sleep apnea. ...
C57BL6 mice display non-eupneic breathing and spontaneous apneas during wakefulness and sleep as well as markedly disordered breathing following cessation of a hypoxic challenge. We examined whether (1) C57BL6 mice display marked non-eupneic breathing following hypercapnic or hypoxic-hypercapnic challenges, and (2) compared the post-hypoxia changes in non-eupneic breathing of C57BL6 mice to those of B6AF1 (57BL6 dam × A/J sire) and Swiss-Webster mice, which display different ventilatory responses than C57BL6 mice. C57BL6 mice displayed marked increases in respiratory frequency and non-eupneic breathing upon return to room-air after hypoxic (10% O2, 90% N2), hypercapnic (5% CO2, 21% O2, 74% N2) and hypoxic-hypercapnic (10% O2, 5% CO2, 85% N2) challenges. B6AF1 mice displayed less tachypnea and reduced non-eupneic breathing post-hypoxia, whereas Swiss-Webster mice displayed robust tachypnea with minimal increases in non-eupneic breathing post-hypoxia. These studies demonstrate that non-eupneic breathing increases after physiologically-relevant hypoxic-hypercapnic challenge in C57BL6 mice and suggest that further studies with these and B6AF1 and Swiss-Webster mice will help define the genetics of non-eupneic breathing.
... On return to air, after the 5 min hypoxic exposure, ventilation remained elevated in Mecp2 T158A/+ and Mecp2 R168X/+ mice while it rapidly returned to baseline in wild type mice (Fig. 4). Additionally, respiratory frequency in Mecp2 +/+ mice decreased below that of the pre-hypoxia baseline (Fig. 5), a characteristic of rodents termed post-hypoxic frequency decline (Dick and Coles, 2000). In the 1st minute of recovery, frequency fell 22 ± 8 bpm in wild-type while it remained elevated 17 ± 7 bpm above baseline in Mecp2 T158A/+ animals (F = 4.757; p = 0.047). ...
... Furthermore, the absence of a post-hypoxic decline in respiratory frequency (PHFD) (Fig. 4) robustly distinguished both Mecp2 T158A/+ and Mecp2 R168X/+ mice from their respective wild type controls. In wild-type mice, PHFD is mediated mainly through the prolongation of T E compared to baseline as shown previously (Dick and Coles, 2000;Song and Poon, 2009). PHFD requires intact signaling from neurons in the ventrolateral pons and depends on N-methyl-D-aspartic acid (NMDA) receptor function (Dick and Coles, 2000). ...
... In wild-type mice, PHFD is mediated mainly through the prolongation of T E compared to baseline as shown previously (Dick and Coles, 2000;Song and Poon, 2009). PHFD requires intact signaling from neurons in the ventrolateral pons and depends on N-methyl-D-aspartic acid (NMDA) receptor function (Dick and Coles, 2000). While impaired NMDA receptor signaling has been demonstrated in symptomatic, but not asymptomatic, Mecp2 Jae/+ female mice (Song et al., 2011), the extent to which alterations in NMDA function in the ventrolateral pons and EPSCs in the NTS contribute to hypoxic ventilatory response in Mecp2 T158A/+ and Mecp2 R168X/+ mice has yet to be studied. ...
Respiratory disturbances are a primary phenotype of the neurological disorder, Rett syndrome (RTT), caused by mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). Mouse models generated with null mutations in Mecp2 mimic respiratory abnormalities in RTT girls. Large deletions, however, are seen in only ∼10% of affected human individuals. Here we characterized respiration in heterozygous females from two mouse models that genetically mimic common RTT point mutations, a missense mutation T158A (Mecp2(T158A/+)) or a nonsense mutation R168X (Mecp2(R168X/+)). MeCP2 T158A shows decreased binding to methylated DNA, while MeCP2 R168X retains the capacity to bind methylated DNA but lacks the ability to recruit complexes required for transcriptional repression. We found that both Mecp2(T158A/+) and Mecp2(R168X/+) heterozygotes display augmented hypoxic ventilatory responses and depressed hypercapnic responses, compared to wild type controls. Interestingly, the incidence of apnea was much greater in Mecp2(R168X/+) heterozygotes, 189 per hour, than Mecp2(T158A/+) heterozygotes, 41 per hour. These results demonstrate that different RTT mutations lead to distinct respiratory phenotypes, suggesting that characterization of the respiratory phenotype may reveal functional differences between MeCP2 mutations and provide insights into the pathophysiology of RTT.
... It is to be noted that the prolongation of inspiratory duration (Ti) does not result in the profound apneusis that followed similar dl pontine intervention (191). However, as observed for the KF-area, stimulating the vagi reverses the apneusis (191) and in spontaneously breathing rats baseline-breathing patterns are also altered only slightly following vl pontine interventions in the vagi intact rat (69,82). Activation of neurons in the vl pons with glutamate prolongs expiratory duration (Te) (192). ...
... Recording revealed respiratory modulated activity particularly post-I and E activity. These activities are responsive to hypoxia (82,154). The expiratory-facilitatory effect evoked by stimulation of the vl pons is consistent with the recording of expiratory-modulated activity within. ...
... In contrast, when rats with bilateral vl pontine interventions were exposed to hypoxia, Ti and Te decreased with no changes in peak respiratory frequency. However, Te does not decrease after hypoxia (69,82). Thus, even though normal breathing pattern was altered following vl pontine lesions, the acute response remains during hypoxia and only the posthypoxic pattern is altered (69,82). ...
Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival. © 2012 American Physiological Society. Compr Physiol 2:2443-2469, 2012.
... Following exposure to HXC, the return to room-air resulted in respiratory patterns that can be classified as short-term potentiation, in which ventilation remains elevated (Powell et al., 1998;Getsy et al., 2014) or post-hypoxic frequency decline, in which breathing frequency falls below baseline (Dick and Coles, 2000). Our C57BL6 mice displayed robust short-term potentiation upon return to room-air, accompanied by a substantial prolonged phase of disordered breathing (e.g., elevated rejection index). ...
... Our C57BL6 mice displayed robust short-term potentiation upon return to room-air, accompanied by a substantial prolonged phase of disordered breathing (e.g., elevated rejection index). The mechanisms responsible for post-HXC disordered breathing have received considerable investigation, and at present, evidence is in favor of disturbances in central signaling (Wilkinson et al., 1997;Strohl, 2003) including, the pons area of the brainstem (Coles and Dick, 1996;Dick and Coles, 2000) rather than processes within the carotid bodies (Vizek et al., 1987;Brown et al., 1993), even though it is evident that carotid body chemoafferents play a vital role in the expression of disordered breathing such as, sleep apnea (Smith et al., 2003). The post-HXC responses were similar in our SHAM and CSCX mice, suggesting that diminished input to the SCG and (presumably decreased activity of SCG cells) do not have an obvious impact on the post-HXC responses, including the disordered breathing. ...
The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O2 and 90% N2) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF50) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.
... Une hypoxie continue est caractérisée par une réponse aiguë (augmentation de la fréquence respiratoire ainsi que de l'amplitude des décharges des nerfs phréniques), suivie immédiatement par une potentialisation à court-terme, puis dans les minutes ou les jours qui suivent l'exposition à l'hypoxie, par une dépression à moyen-terme au niveau de la sortie motrice phrénique (Eldridge et Millhorn, 1986, Powell et al., 1998Coles et Dick, 1996 ;Hayashi et al., 1993 ;Dick et Coles, 2000), connue sous le nom de déclin de la fréquence posthypoxique (Coles et Dick, 1996 ;Powell et al., 1998) (Figure 24). Cette réponse à l'hypoxie semble être dominée initialement par une sensibilisation des chémorécepteurs périphériques (Bisgard et Neubauer, 1995), suivie par une augmentation progressive de l'intégration centrale des neurones chémoafférents carotidiens (Dwinell et Powell, 1999 ;Soulier et al., 1997). ...
... Représentation du déclin de la fréquence respiratoire post-hypoxique.A. Activité phrénique chez un rat témoin. B. Activité phrénique en conditions hypoxiques, puis retour à la normale, mais le fréquence de décharge a dimunué par rapport à A. Adapté d'aprèsDick et Coles, 2000. ...
Résumé : Ce travail doctoral a permis d’étudier les phénomènes de plasticité anatomo-fonctionnelle et
moléculaire survenant après lésion unilatérale cervicale de la moelle épinière.
Une section latérale cervicale est suffisante pour abolir l’activité de l’hémidiaphragme ipsilatéral.
Néanmoins, après un délai post-lésionnel court (7 jours), une activité phrénique ipsilatérale peut être
détectée. Celle-ci dépend de voies descendantes contralatérales croisées, situées latéralement, et de
voies afférentes ipsilatérales. A 3 mois post-lésionnels, cette activité est renforcée par des « nouvelles »
voies descendantes spinales médianes.
Au niveau moléculaire, les neurones respiratoires axotomisés (et d’autres neurones bulbaires non
axotomisés) expriment c-Jun en réponse à une hémisection spinale cervicale, ce qui traduit un potentiel
de plasticité intrinsèque. Dans la zone lésionnelle, les taux de certains effecteurs de plasticité (GAP-43,
BDNF) sont cependant diminués.
Ces résultats dans leur ensemble montrent que le réseau respiratoire développe, après lésion spinale, des
processus de neuroplasticité qui pourront être exploités dans le cadre de stratégies à « vocation
réparatrice ».
Mots clés : C-Jun, Groupe respiratoire ventral, Lésion spinale, Diaphragme, Nerf phrénique,
Cross Phrenic Phenomenon, Reconnexions fonctionnelles, Plasticité respiratoire, GAP-43, BDNF.
... This post-hypoxia depression of fr (post-hypoxic fr decline. PHFD) [54], is an active neural process that depends on the ventrolateral pons [52,53,55,56]. In VEH rats, Ti, EIP, and PEF re-turned to pre-values upon return to room-air whereas fr, Vt, Vm and PIF declined below pre-levels. ...
... Although fr during each episode of hypoxia was depressed in MOR rats, baseline fr gradually rose during N1 -N3 such that fr was higher than in VEH rats during N3. Since the PHFD observed in VEH rats was absent in MOR rats, morphine may have directly interfered with pontomedullary systems responsible for this phenomenon [52,53,55,56]. Although resting Vt returned to baseline levels during N1-N3 in MOR rats, the responses during H1 -H3 changed in that the increases in Vt following H1 were slightly greater than those of VEH rats whereas they were greater than VEH rats during H2 and H3. ...
The aim of this study was to determine whether morphine depresses the ventilatory responses elicited by a hypoxic challenge (10% O2, 90% N2) in conscious rats at a time when the effects of morphine on arterial blood gas (ABG) chemistry, Alveolar-arterial (A-a) gradient and minute ventilation (VM) had completely subsided. In vehicle-treated rats, each episode of hypoxia stimulated ventilatory function and the responses generally subsided during each normoxic period. Morphine (5 mg/kg, i.v.) induced an array of depressant effects on ABG chemistry, A-a gradient and VM (via decreases in tidal volume). Despite resolution of these morphine-induced effects, the first episode of hypoxia elicited substantially smaller increases in VM than in vehicle-treated rats, due mainly to smaller increases in frequency of breathing. The pattern of ventilatory responses during subsequent episodes of hypoxia and normoxia changed substantially in morphine-treated rats. It is evident that morphine has latent deleterious effects on ventilatory responses elicited by hypoxic challenge.
... To gain insight into this issue, much attention has been focused on events during hypoxia, yet the acute period of reoxygenation rather than hypoxia itself may also be a critical period of vulnerability for respiratory function. It is welldocumented that during reoxygenation and the immediate period following, breathing is centrally depressed [20,29,30,31,32,33]. Our study demonstrates that a gender difference exists in the acute recovery of respiration following severe hypoxia. ...
... Unlike the hypoxic augmentation, the post-hypoxic ventilatory depression appears to be primarily of central origin. Although early reports suggested that the ventrolateral pons is required for post-hypoxic ventilatory depression in vivo [29,30,31], in vitro studies indicate that the preBötC is a major contributor to the depression as a dramatic post-hypoxic depression is present even in the absence of the pons [19,20]. Thus, while the post-hypoxic ventilatory depression likely involves several interacting CNS areas, our in vivo and in vitro experiments are consistent in their key findings. ...
The preBötzinger complex (preBötC) is a critical neuronal network for the generation of breathing. Lesioning the preBötC abolishes respiration, while when isolated in vitro, the preBötC continues to generate respiratory rhythmic activity. Although several factors influence rhythmogenesis from this network, little is known about how gender may affect preBötC function. This study examines the influence of gender on respiratory activity and in vitro rhythmogenesis from the preBötC. Recordings of respiratory activity from neonatal mice (P10-13) show that sustained post-hypoxic depression occurs with greater frequency in males compared to females. Moreover, extracellular population recordings from the preBötC in neonatal brainstem slices (P10-13) reveal that the time to the first inspiratory burst following reoxygenation (TTFB) is significantly delayed in male rhythmogenesis when compared to the female rhythms. Altering activity of ATP sensitive potassium channels (KATP) with either the agonist, diazoxide, or the antagonist, tolbutamide, eliminates differences in TTFB. By contrast, glucose supplementation improves post-hypoxic recovery of female but not male rhythmogenesis. We conclude that post-hypoxic recovery of respiration is gender dependent, which is, in part, centrally manifested at the level of the preBötC. Moreover, these findings provide potential insight into the basis of increased male vulnerability in a variety of conditions such as Sudden Infant Death Syndrome (SIDS).
... The return to room-air in mice having undergone hypoxic gas challenges often results in an abrupt dramatic increase in Freq, TV and therefore MV in mice [40][41][42][43][44][45][46]86 that can result in unstable breathing. [86][87][88][89] The mechanisms responsible for post-hypoxia alterations in breathing have received considerable investigation and at present, evidence favors disturbances in central signaling 87,90 including the pons 91,92 rather than processes within the carotid bodies. 93,94 The present study demonstrated that C57BL/6 WT mice displayed the expected abrupt increase frequency of breathing, tidal volume and minute ventilation upon return to room-air which returned to baseline within 5 min. ...
Histone deacetylase 6 (HDAC6) is a class II histone deacetylase that is predominantly localized in the cytoplasm of cells. HDAC6 associates with microtubules, regulating acetylation of tubulin and other proteins. The possibility that HDAC6 participates in hypoxic signaling is supported by evidence that (1) hypoxic gas challenges cause microtubule depolymerization, (2) expression of hypoxia inducible factor alpha (HIF)-1α is regulated by microtubule alterations in response to hypoxia, and (3) inhibition of HDAC6 prevents HIF-1α expression and protects tissue from hypoxic/ischemic insults. The aim of this study was to address whether the absence of HDAC6 alters ventilatory responses during and/or after hypoxic gas challenges (10% O 2 , 90% N 2 for 15 min) in adult male wild-type (WT) C57BL/6 mice and HDAC6 knock-out (KO) mice. Key findings were that (1) baseline values for frequency of breathing, tidal volume, inspiratory and expiratory times and end expiratory pause were different between KO mice and WT mice, (2) ventilatory responses during hypoxic challenge were more robust in KO mice than WT mice for parameters including frequency of breathing, minute ventilation, inspiratory and expiratory durations, peak inspiratory and expiratory flows, inspiratory and expiratory drives, and (3) responses upon return to room-air were markedly different in KO mice than WT mice for frequency of breathing, minute ventilation, inspiratory and expiratory durations, end expiratory (but not end inspiratory) pauses, peak inspiratory and expiratory flows, and inspiratory or expiratory drives. These data suggest that HDAC6 may have a fundamentally important role in regulating the neural responses to hypoxia.
... Dans les minutes suivant cet épisode d'hypoxie continue, l'amplitude de décharge des nerfs phréniques revient à des valeurs pré-hypoxie, alors que la fréquence respiratoire est significativement réduite (Bach et al., 1999). Ce phénomène de dépression de l'activité phrénique post-hypoxie est connu sous le nom de déclin de la fréquence post-hypoxie (Dick and Coles, 2000) (Figure 25). Il est aboli après lésion du pont au niveau ventrolatéral et à proximité de l'aire adrénergique A5, d'où sa modulation possible via les récepteurs α2-adrénergiques (Powell et al., 1998;Bach et al., 1999). ...
Les traumas spinaux cervicaux induisent non seulement une insuffisance respiratoire en raison de la déafférentation des motoneurones innervant le diaphragme, mais aussi des déficits cardiovasculaires dus à la réduction des efférences sympathiques innervant le cœur et le système vasculaire. La mise au point de thérapeutiques visant à améliorer ces fonctions suite à une lésion spinale est donc nécessaire. Une partie de ce travail doctoral a donc consisté à améliorer les connaissances sur la physiopathologie des traumas spinaux. Au niveau fonctionnel, nous avons montré qu’en plus d’une paralysie hémidiaphragmatique permanente, une hémisection spinale unilatérale en C2 induit aussi un déficit systolique évalué par échocardiographie et caractérisée par une réduction de la fraction d’éjection du ventricule gauche. Cette lésion partielle conduit de plus à une réponse inflammatoire au niveau des motoneurones phréniques (localisés dans la moelle épinière C3-C6), mais aussi au niveau lésionnel (C1-C3), où la production de pièges extracellulaires a pu être observée par les neutrophiles infiltrants et la microglie activée par cytométrie en flux. La seconde partie de ce travail doctoral s’est concentrée sur l’évaluation de l’efficacité de la stimulation magnétique transcrânienne répétée sur la neuromodulation du réseau respiratoire. Nous avons ainsi montré que délivré de façon aigu, notre protocole permet d’augmenter l’excitabilité du réseau phrénique. Délivré de façon chronique après trauma spinal, il permet de renforcer l’activité de l’hémidiaphragme intact afin de mieux compenser pour la perte d’activité de l’hémidiaphragme lésé. Malgré une absence d’effet sur l’activité basale de l’hémidiaphragme paralysé, un renforcement des voies croisées phréniques a pu être observé après traitement, laissant entrevoir de nouvelles thérapeutiques potentielles visant à induire une récupération fonctionnelle respiratoire après trauma spinal cervical.
... Since these mechanisms rely on time-dependent operations such as gating and filtering, they can be viewed and referred to as a "swallow gate." Also, the respiratory motor output expresses many forms of plasticity (Dick and Coles, 2000;Mitchell and Johnson, 2003;Morris et al., 2003) whereas (fictive) swallows are often considered "stereotypic events" with characteristic and consistent form (Doty, 1951(Doty, , 1968Kawasaki et al., 1964;Sumi, 1970;Miller and Loizzi, 1974;Miller, 1982). Since execution of protective reflexes such as cough and swallow also engage elements of the respiratory CPG (Gestreau et al., 1996(Gestreau et al., , 2005Shannon et al., 1998;Bolser et al., 2006), we hypothesized that the swallow CPG could also express plasticity, such as an enhancement of swallowing motor output upon sustained stimulation of afferent fibers. ...
Swallow-breathing coordination safeguards the lower airways from tracheal aspiration of bolus material as it moves through the pharynx into the esophagus. Impaired movements of the shared muscles or structures of the aerodigestive tract, or disruptions in the interaction of brainstem swallow and respiratory central pattern generators (CPGs) result in dysphagia. To maximize lower airway protection these CPGs integrate respiratory rhythm generation signals and vagal afferent feedback to synchronize swallow with breathing. Despite extensive study, the roles of central respiratory activity and vagal feedback from the lungs as key elements for effective swallow-breathing coordination remain unclear. The effect of altered timing of bronchopulmonary vagal afferent input on swallows triggered during electrical stimulation of the superior laryngeal nerves or by injection of water into the pharyngeal cavity was studied in decerebrate, paralyzed, and artificially ventilated cats. We observed two types of single swallows that produced distinct effects on central respiratory-rhythm across all conditions: post-inspiratory type swallows disrupted central-inspiratory activity without affecting expiration, whereas expiratory type swallows prolonged expiration without affecting central-inspiratory activity. Repetitive swallows observed during apnea reset the E2 phase of central respiration and produced facilitation of swallow motor output nerve burst durations. Moreover, swallow initiation was negatively modulated by vagal feedback and was reset by lung inflation. Collectively, these findings support a novel model of reciprocal inhibition between the swallow CPG and inspiratory or expiratory cells of the respiratory CPG where lung distension and phases of central respiratory activity represent a dual peripheral and central gating mechanism of swallow-breathing coordination.
... Yet, it is important to note that the present testing procedure was defined by three sequential phases related with the air conditions (normoxia-hypoxia-recovery normoxia). Following the hyperventilation caused by a hypoxic condition, all pups exhibited a marked posthypoxic breathing frequency decline that has also been observed in adult animals and that is related with a short-term depression of the phrenic nerve motor output (Powell et al. 1998;Dick and Coles 2000). This phenomenon was associated with an 11-fold increase in the number of apneas relative to those recorded in the initial normoxic phase of the test (Fig. 3e). ...
Rationale:
The effects of early ethanol exposure upon neonatal respiratory plasticity have received progressive attention given a multifactorial perspective related with sudden infant death syndrome or hypoxia-associated syndromes. The present preclinical study was performed in 3-9-day-old pups, a stage in development characterized by a brain growth spurt that partially overlaps with the 3rd human gestational trimester.
Methods:
Breathing frequencies and apneas were examined in pups receiving vehicle or a relatively moderate ethanol dose (2.0 g/kg) utilizing a whole body plethysmograph. The experimental design also considered possible associations between drug administration stress and exteroceptive cues (plethysmographic context or an artificial odor). Ethanol exposure progressively exerted a detrimental effect upon breathing frequencies. A test conducted at PD9 when pups were under the state of sobriety confirmed ethanol's detrimental effects upon respiratory plasticity (breathing depression).
Results:
Pre-exposure to the drug also resulted in a highly disorganized respiratory response following a hypoxic event, i.e., heightened apneic episodes. Associative processes involving drug administration procedures and placement in the plethysmographic context also affected respiratory plasticity. Pups that experienced intragastric administrations in close temporal contiguity with such a context showed diminished hyperventilation during hypoxia. In a 2nd test conducted at PD9 while pups were intoxicated and undergoing hypoxia, an attenuated hyperventilatory response was observed. In this test, there were also indications that prior ethanol exposure depressed breathing frequencies during hypoxia and a recovery normoxia phase.
Conclusion:
As a whole, the results demonstrated that brief ethanol experience and stress-related factors significantly disorganize respiratory patterns as well as arousal responses linked to hypoxia in neonatal rats.
... This observation is in line with our previous studies (performed in Epo transgenic mice lines) showing that Epo, by modulation of the catecholaminergic content, contribute to alter the HVR, while no differences were observed in basal ventilation Soliz et al., 2005Soliz et al., , 2007. Moreover, it was shown that higher NE content in A5, but lower NE content in A2C2 and A1C1 are associated with augmentation of the ventilation (Champagnat et al., 1979;Dick and Coles, 2000;Hilaire and Duron, 1999). In fact, in the opposite way, local application of NE within the A5 (simulating the impact of A6 over A5), silence the A5 neurons (by reducing the own secretion of NE) and increase the frequency of the respiratory rhythm generator (Hilaire et al., 2004). ...
The Locus coeruleus (LC) is a pontine area that contributes to the CO2/pH chemosensitivity. LC cells express erythropoietin (Epo) receptors (EpoR), and Epo in the brainstem is a potent normoxic and hypoxic respiratory stimulant. However, a recent study showed that the intra-cisternal injection (ICI) of Epo antagonist does not alter the hypercapnic ventilatory response in mice. As ICI leads to a widespread dispersal of the product throughout the brainstem, in this work we evaluated the specific impact of Epo in the LC-mediated ventilatory response to CO2 (by whole body plethysmography) in juvenile male Wistar rats. Normocapnic and hypercapnic ventilation were evaluated before and after unilateral microinjection of Epo (1 ng/100 nL) into the LC. To evaluate the long-term effect of Epo, the HcVR was re-evaluated 24 h later. Our results show that Epo attenuates the hypercapnic ventilation. We conclude that Epo in the LC tunes the hypercapnia-induced hyperpnea.
... A5 neurons and breathing. A5 neurons, or the "A5 region," may exert a tonic inhibitory effect on breathing in neonatal rats (40) and may facilitate phasic expiratory inhibition in adult rats exposed to hypoxia (posthypoxic frequency decline) (30,32,70). The present study did not reveal any effect of brief A5 neuronal stimulation on breathing. ...
Combined optogenetic activation of the retrotrapezoid nucleus (RTN, a CO2/proton-activated brainstem nucleus) with nearby catecholaminergic neurons (C1 and A5), or selective C1 neuron stimulation, increases blood pressure (BP) and breathing, causes arousal from non-REM sleep and triggers sighs. Here we wished to determine which of these physiological responses are elicited when RTN neurons are selectively activated. The left rostral RTN and nearby A5 neurons were transduced with channelrhodopsin-2 (ChR2(+)) using a lentiviral vector. Very few C1 cells were transduced. BP, breathing, EEG and neck EMG were monitored. During non-REM sleep, photostimulation of ChR2(+) neurons (20s, 2-20Hz) instantly increased VE without changing BP (13 rats). VE and BP were unaffected by light in nine control (ChR2(-)) rats. Photostimulation produced no sighs and caused arousal (EEG desynchronization) more frequently in ChR2(+) than ChR2(-) rats (62 ± 5% of trials vs. 25 ± 2%; p < 0.0001). Six ChR2(+) rats then received spinal injections of a saporin-based toxin that spared RTN neurons but destroyed surrounding catecholaminergic neurons. Photostimulation of the ChR2(+) neurons produced the same ventilatory stimulation before and after lesion but arousal was no longer elicited. Overall (all ChR2(+) rats combined), ΔVE correlated with the number of ChR2(+) RTN neurons whereas arousal probability correlated with the number of ChR2(+) catecholaminergic neurons. In conclusion, RTN neurons activate breathing powerfully and, unlike the C1 cells, have minimal effects on BP and have a weak arousal capability at best. A5 neuron stimulation produces little effect on breathing and BP, but does appear to facilitate arousal.
Copyright © 2015, Journal of Applied Physiology.
... Taken together, these results suggest that the lack of VGLUT3 disrupts a common mechanism accounting for both in vitro and in vivo findings. However, the decrease in PB frequency in vitro (frequently referred to as 'short-term depression' under these experimental conditions; Dick & Coles, 2000;Powell et al. 1998) and the decrease in ventilation in vivo had markedly different time courses, which may at least partially reflect different mechanisms. Therefore, caution must be used in attempting to integrate both results within a unifying framework, as previously noted (Bissonnette, 2000). ...
Key points
Hypoxic stress is an important cause of morbidity and mortality in neonates.
We examined the role of VGLUT3, an atypical transporter of glutamate present in serotonergic neurons involved in breathing and heat production, in the response to hypoxia.
The respiratory responses to chemical stimuli and the turnover of serotonin in the brainstem were impaired in newborn mice lacking VGLUT3.
Under cold conditions, metabolic rate, body temperature, baseline breathing and the ventilatory response to hypoxia were disrupted.
Thus, VGLUT3 expression is required for optimal response to hypoxic stress in neonates.
Abstract Neonates respond to hypoxia initially by increasing ventilation, and then by markedly decreasing both ventilation (hypoxic ventilatory decline) and oxygen consumption (hypoxic hypometabolism). This latter process, which vanishes with age, reflects a tight coupling between ventilatory and thermogenic responses to hypoxia. The neurological substrate of hypoxic hypometabolism is unclear, but it is known to be centrally mediated, with a strong involvement of the 5‐hydroxytryptamine (5‐HT, serotonin) system. To clarify this issue, we investigated the possible role of VGLUT3, the third subtype of vesicular glutamate transporter. VGLUT3 contributes to glutamate signalling by 5‐HT neurons, facilitates 5‐HT transmission and is expressed in strategic regions for respiratory and thermogenic control. We therefore assumed that VGLUT3 might significantly contribute to the response to hypoxia. To test this possibility, we analysed this response in newborn mice lacking VGLUT3 using anatomical, biochemical, electrophysiological and integrative physiology approaches. We found that the lack of VGLUT3 did not affect the histological organization of brainstem respiratory networks or respiratory activity under basal conditions. However, it impaired respiratory responses to 5‐HT and anoxia, showing a marked alteration of central respiratory control. These impairments were associated with altered 5‐HT turnover at the brainstem level. Furthermore, under cold conditions, the lack of VGLUT3 disrupted the metabolic rate, body temperature, baseline breathing and the ventilatory response to hypoxia. We conclude that VGLUT3 expression is dispensable under basal conditions but is required for optimal response to hypoxic stress in neonates.
... This response is thought to be of central origin (Lawson and Long, 1983;Vizek et al., 1987;Feldman et al., 2003), persists in brainstem-spinal cord preparations from rodents (e.g., Völker et al., 1995;Nsegbe et al., 2004) as well as in neonatal mice transverse medullary slice preparations (Telgkamp and Ramirez, 1999), and may reflect central oxygen sensing in specific brainstem sites (Voituron et al., 2006;Gestreau et al., 2010). In addition, upon removal of the hypoxic stimulus, an immediate decline in respiratory frequency and tidal volume occurs (Coles and Dick, 1996), where the decreased respiratory frequency undershoots baseline frequency (post-hypoxic frequency decline) (Dick and Coles, 2000). Although previous studies in adult mice had shown that brain Epo increases the ventilatory response to acute and chronic hypoxia (Soliz et al., 2005(Soliz et al., , 2007b, this is the first demonstration that Epo is also involved in the neural network controlling the hypoxic brainstem circuitry in newborn mice. ...
... In contrast, in Smith et al. (1984) as with Wilson (2007, 2009) and the present study, a hypoadditive central-peripheral chemoreceptor interaction was achieved by reversing the order of central and peripheral chemoreceptor stimulation/disfacilitation.This subtle difference in the stimulation sequence could result in very different response dynamics and plasticity in the central circuits that modulate the breathing pattern. For example, acute hypoxia is known to elicit multiple time-dependent responses in breathing pattern during and after the hypoxia period, such as short-term potentiation of inspiratory drive due to corresponding time-dependent augmentation of inspiratory amplitude and shortening of T I , as well as short-term depression of respiratory frequency (post-hypoxic frequency decline) due to corresponding time-dependent prolongation of T E (Day and Wilson, 2005;Dick and Coles, 2000;Poon and Siniaia, 2000;Song and Poon, 2009b). Such peripheral chemoreceptor-induced neuroplasticity may interact with central chemoreceptor input in a complex manner leading to distinct steady-state responses in T I , T E and inspiratory amplitude depending on the timing and magnitude of the central and peripheral chemoreceptor inputs. ...
A negative influence of central chemosensitivity on peripheral chemoreflex response has been demonstrated recently in a decerebrate-vagotomized rat preparation in situ with separate carotid body and brainstem perfusions. Here, we report similar negative influences of hypercapnia on the hypoxic respiratory response in anesthetized, spontaneously breathing rats before and after vagotomy and anesthetized, artificially ventilated rats after vagotomy. Baseline breathing patterns and responsiveness to hypercapnia and hypoxia varied widely between the three respiratory modes. Despite this, the responses in inspiratory amplitude and expiratory duration (and hence respiratory frequency and neural ventilation) to hypoxia varied inversely with the background CO2 level in all three groups. Results demonstrate a hypoadditive hypercapnic-hypoxic interaction in vivo that resembles the hypoadditive central-peripheral chemoreceptor interaction in situ for these respiratory variables in the rat, regardless of differences in vagal feedback, body temperature and ventilation method. These observations stand in contrast to previous reports of hyperadditive peripheral-central chemoreceptor interaction.
... Hypoxia triggers various forms of short-and long-term respiratory plasticity. The acute hypoxic response is an initial increase in respiratory frequency (Cross and Oppe 1952;Dick and Coles 2000;Lawson and Long 1983;Powell et al. 1998;Richter DW et al. 1991), which is largely preserved in rhythmically active slices (Blitz and Ramirez 2002;Ramirez et al. 1998;Telgkamp and Ramirez 1999). Depending on the duration and severity of hypoxia, this initial frequency increase is followed by a secondary respiratory depression and, if hypoxia persists, finally leads to central apnea (Ramirez et al. 1998;Richter DW et al. 1991). ...
Rett syndrome (RTT) patients suffer from respiratory arrhythmias with frequent apneas causing intermittent hypoxia. In a RTT mouse model (methyl-CpG-binding protein 2-deficient mice; Mecp2(-/y)) we recently discovered an enhanced hippocampal susceptibility to hypoxia and hypoxia-induced spreading depression (HSD). In the present study we investigated whether this also applies to infant Mecp2(-/y) brain stem, which could become life-threatening due to failure of cardiorespiratory control. HSD most reliably occurred in the nucleus of the solitary tract (NTS) and the spinal trigeminal nucleus (Sp5). HSD susceptibility of the Mecp2(-/y) NTS and Sp5 was increased on 8 mM K(+)-mediated conditioning. 5-HT(1A) receptor stimulation with 8-hydroxy-2-(di-propylamino)tetralin (8-OH-DPAT) postponed HSD by up to 40%, mediating genotype-independent protection. The deleterious impact of HSD on in vitro respiration became obvious in rhythmically active slices, where HSD propagation into the pre-Bötzinger complex (pre-BötC) immediately arrested the respiratory rhythm. Compared with wild-type, the Mecp2(-/y) pre-BötC was invaded less frequently by HSD, but if so, HSD occurred earlier. On reoxygenation, in vitro rhythms reappeared with increased frequency, which was less pronounced in Mecp2(-/y) slices. 8-OH-DPAT increased respiratory frequency but failed to postpone HSD in the pre-BötC. Repetitive hypoxia facilitated posthypoxic recovery only if HSD occurred. In 57% of Mecp2(-/y) slices, however, HSD spared the pre-BötC. Although this occasionally promoted residual hypoxic respiratory activity ("gasping"), it also prolonged the posthypoxic recovery, and thus the absence of central inspiratory drive, which in vivo would lengthen respiratory arrest. In view of the breathing disorders in RTTs, the increased hypoxia susceptibility of MeCP2-deficient brain stem potentially contributes to life-threatening disturbances of cardiorespiratory control.
... The modulatory influences of pontine structures on respiratory pattern and rhythm generation, heart rate, blood pressure and cardiorespiratory coupling are well recognized (Cohen, 1971;von Euler, 1977;Oku and Dick, 1992;Paton et al., 1998;Dick and Coles, 2000;Alheid et al., 2004;Dutschmann and Herbert, 2006;Smith et al., 2007;Baekey et al., 2008;Padley et al., 2008;Segers et al., 1985;. It was shown that pontine transection eliminates respiratory sinus arrhythmia, respiratory modulation of BP (Traube-Hering waves) and postinspiratory discharges from vagal efferents . ...
Cardiorespiratory coupling can be significantly influenced by both pontine and vagal modulation of medullary motor and premotor areas. We investigated influences of the pontine intertrigeminal region (ITR) and peripheral vagal pathways on the coupling between systolic blood pressure (SBP) and respiration in 9 anesthetized rats. Glutamate injection into the ITR perturbed both respiration and SBP and decreased SBP-respiratory coherence (0.95±0.01 vs 0.89±0.02; (p=0.01). Intravenous infusion of serotonin (5-HT) produced apnea and hypertension and also decreased SBP-respiratory coherence (0.95±0.01 vs 0.72±0.06; p=0.04). Bilateral vagotomy eliminated the cardiorespiratory coherence perturbations induced by central (glutamate injection into the ITR: 0.89±0.03 vs 0.86±0.03; p=0.63) and peripheral (5-HT infusion: 0.89±0.03 vs 0.88±0.02; p=0.98) pharmacologic manipulations. Glutamate stimulation of the ITR postvagotomy increased the relative spectral power density of SBP in the respiratory frequency range (0.25±0.08 vs 0.55±0.06; p=0.01). The data suggest that SBP-respiratory coupling is largely mediated within the central nervous system, with vagal systems acting in a way that disrupts coherence during transient cardiorespiratory disturbances. Although decreased cardiorespiratory coherence may increase cardiac work during perturbations, this may be physiologically advantageous in restoring homeostatic equilibrium of respiration and blood pressure.
... We found that most double-labeled neurons were localized in commissural and medial NTS. Therefore, we conclude that peripheral chemoafferents were forwarded to the pneumotaxic center mainly through the direct pathway from second-order relay neurons in commissural and medial NTS, although indirect pathways via higher-order relay neurons in ventrolateral NTS where dorsal respiratory neuron group resides, VLM where ventral respiratory neuron group resides, and A5 that mediates the post-hypoxic respiratory frequency decline (Dick et al., 2000) were certainly also involved. The existence of this direct NTS-pontine chemoafferent pathway was further supported by our electrophysiological experiments, in which electrical stimulation at the dl-pons antidromically activated some hypoxia-excited neurons in commissural and medial NTS. ...
Hypoxic respiratory and cardiovascular responses in mammals are mediated by peripheral chemoreceptor afferents which are relayed centrally via the solitary tract nucleus (NTS) in dorsomedial medulla to other cardiorespiratory-related brainstem regions such as ventrolateral medulla (VLM). Here, we test the hypothesis that peripheral chemoafferents could also be relayed directly to the Kölliker-Fuse/parabrachial complex in dorsolateral pons, an area traditionally thought to subserve pneumotaxic and cardiovascular regulation. Experiments were performed on adult Sprague-Dawley rats. Brainstem neurons with axons projecting to the dorsolateral pons were retrogradely labeled by microinjection with choleras toxin subunit B (CTB). Neurons involved in peripheral chemoreflex were identified by hypoxia-induced c-Fos expression. We found that double-labeled neurons (i.e. immunopositive to both CTB and c-Fos) were localized mostly in the commissural and medial subnuclei of NTS and to a lesser extent in the ventrolateral NTS subnucleus, VLM and ventrolateral pontine A5 region. Extracellular recordings from the commissural and medial NTS subnuclei revealed that some hypoxia-excited NTS neurons could be antidromically activated by electrical stimulations at the dorsolateral pons. These findings demonstrate that hypoxia-activated afferent inputs are relayed to the Kölliker-Fuse/parabrachial complex directly via the commissural and medial NTS and indirectly via the ventrolateral NTS subnucleus, VLM and A5 region. These pontine-projecting peripheral chemoafferent inputs may play an important role in the modulation of cardiorespiratory regulation by dorsolateral pons.
... For example, the PPT reciprocally innervates the parabrachial complex (Gilbert and Lydic, 1991;Quattrochi et al. 1998;Semba and Fibiger, 1992) and neurons of the parabrachical complex together with the Kolliker-Fuse nucleus regulate respiratory phase switching. In addition, stimulation of the medial parabrachial or Kölliker-Fuse areas produces expiratory facilitation and apnea, whereas activation of the lateral parabrachial region can produce tachypnea (Chamberlin and Saper, 1994;Dick and Coles, 2000;Takayama and Miura, 1993). Thus, it is reasonable to speculate that activation of medial parabrachial neurons following PPT stimulation may account for apnea, whereas activation of the lateral parabrachial neurons may account for tachypnea. ...
Functionally distinct areas were mapped within the pedunculopontine tegmentum (PPT) of 42 ketamine/xylazine anesthetized rats using local stimulation by glutamate microinjection (10 mM, 5-12 nl). Functional responses were classified as: (1) apnea; (2) tachypnea; (3) hypertension (HTN); (4) sinus tachycardia; (5) genioglossus electromyogram activation or (6) pontine-waves (p-waves) activation.We found that short latency apneas were predominantly elicited by stimulation in the lateral portion of the PPT, in close proximity to cholinergic neurons. Tachypneic responses were elicited from ventral regions of the PPT and HTN predominated in the ventral portion of the antero-medial PPT. We observed sinus tachycardia after stimulation of the most ventral part of the medial PPT at the boundary with nucleus reticularis pontis oralis, whereas p-waves were registered predominantly following stimulation in the dorso-caudal portion of the PPT. Genioglossus EMG activation was evoked from the medial PPT. Our results support the existence of the functionally distinct areas within the PPT affecting respiration, cardiovascular function, EEG and genioglossus EMG.
... The mechanism of PHFD is still unclear. It is reported that the actions of inhibitory α 2 receptors on both brainstem respiratory neurones (Bach et al. 1999) and pontine respiratory group participate in the response (Dick & Coles, 2000). It still remains to be determined if AIH has any functional or histological effects on brainstem or pontine respiratory neurones. ...
Acute intermittent hypoxia (AIH) elicits long-term increases in respiratory and sympathetic outflow (long-term facilitation, LTF). It is still unclear whether sympathetic LTF is totally dependent on changes in respiration, even though respiratory drive modulates sympathetic nerve activity (SNA). In urethane-anaesthetized, vagotomized mechanically ventilated Sprague-Dawley rats, we investigated the effect of ten 45 s episodes of 10% O2-90% N(2) on splanchnic sympathetic nerve activity (sSNA) and phrenic nerve activity (PNA). We then tested whether or not hypoxic sympathetic chemoreceptor and baroreceptor reflexes were changed 60 min after AIH. We found that 17 animals manifested a sustained increase of sSNA (+51.2+/-4.7%) 60 min after AIH, but only 10 of these rats also expressed phrenic LTF compared with the time controls (rats not exposed to hypoxia, n=5). Inspiratory triggered averages of integrated sSNA showed respiratory modulation of SNA regardless of whether or not phrenic LTF had developed. The hypoxic chemoreceptor reflex was enhanced by 60 min after the development of AIH (peak change from 76.9+/-13.9 to 159.5+/-24.9%). Finally, sympathetic baroreceptor reflex sensitivity increased after sympathetic LTF was established (Gainmax from 1.79+/-0.18 to 2.60+/-0.28% mmHg1). Our findings indicate that respiratory-sympathetic coupling does contribute to sympathetic LTF, but that an additional tonic increase of sympathetic tone is also present that is independent of the level of PNA. Sympathetic LTF is not linked to the change in baroreflex function, since the baroreflex appears to be enhanced rather than impaired, but does play an important role in the enhancement of the hypoxic chemoreflex.
... The increased expiratory modulation of cells in postulated reciprocal inhibitory networks and expiratory-phase MWROs seen with vagotomy is consistent with participation in posthypoxic frequency decline. Posthypoxic frequency decline is also abolished with pontine lesions (17,18,23). ...
Previous models have attributed changes in respiratory modulation of pontine neurons after vagotomy to a loss of pulmonary stretch receptor "gating" of an efference copy of inspiratory drive. Recently, our group confirmed that pontine neurons change firing patterns and become more respiratory modulated after vagotomy, although average peak and mean firing rates of the sample did not increase (Dick et al., J Physiol 586: 4265-4282, 2008). Because raphé neurons are also elements of the brain stem respiratory network, we tested the hypotheses that after vagotomy raphé neurons have increased respiratory modulation and that alterations in their firing patterns are similar to those seen for pontine neurons during withheld lung inflation. Raphé and pontine neurons were recorded simultaneously before and after vagotomy in decerebrated cats. Before vagotomy, 14% of 95 raphé neurons had increased activity during single respiratory cycles prolonged by withholding lung inflation; 13% exhibited decreased activity. After vagotomy, the average index of respiratory modulation (eta(2)) increased (0.05 +/- 0.10 to 0.12 +/- 0.18 SD; Student's paired t-test, P < 0.01). Time series and frequency domain analyses identified pontine and raphé neuron firing rate modulations with a 0.1-Hz rhythm coherent with blood pressure Mayer waves. These "Mayer wave-related oscillations" (MWROs) were coupled with central respiratory drive and became synchronized with the central respiratory rhythm after vagotomy (7 of 10 animals). Cross-correlation analysis identified functional connectivity in 52 of 360 pairs of neurons with MWROs. Collectively, the results suggest that a distributed network participates in the generation of MWROs and in the coordination of respiratory and vasomotor rhythms.
... Compared to WT controls, tg21 mice show altered catecholaminergic content in brainstem, higher levels in pons, but lower levels in medulla. These data are in agreement with the report showing that increased hypoxic ventilation-by mean the augmentation of the respiratory frequency-is associated with higher catecholamine level in pontial A5 cell group [41,42] and with lower catecholamine level in the medullary A1C1 and A2C2 cell groups [43]. Thus, our results suggest that higher Epo level modulates the catecholamine synthesis in brainstem. ...
Numerous factors involved in general homeostasis are able to modulate ventilation. Classically, this comprises several kind of molecules, including neurotransmitters and steroids that are necessary for fine tuning ventilation under different conditions such as sleep, exercise, and acclimatization to high altitude. Recently, however, we have found that erythropoietin (Epo), the main regulator of red blood cell production, influences both central (brainstem) and peripheral (carotid bodies) respiratory centers when the organism is exposed to hypoxic conditions. Here, we summarize the effect of Epo on the respiratory control in mammals and highlight the potential implication of Epo in the ventilatory acclimatization to high altitude, as well as in the several respiratory sickness and syndromes occurring at low and high altitude.
... The dependence of the hypoxic response on an intact pons has been reported by other investigators. Bilateral lesions in the dl pons decrease the response to hypoxia (St John, 1979;Mizusawa et al., 1995;Song and Poon, 2009a, b) as well as the loss of post-hypoxic changes in patterning (Coles and Dick, 1996;Dick and Coles, 2000). Our results emphasize the role of the pons in coupling the relationship between breathing and sympathetic nerve activity. ...
Histone deacetylase 6 (HDAC6) is a class II histone deacetylase that is predominantly localized in the cytoplasm of cells. HDAC6 associates with microtubules and regulates acetylation of tubulin and other proteins. The possibility that HDAC6 participates in hypoxic signaling is supported by evidence that 1) hypoxic gas challenges cause microtubule depolymerization, 2) expression of hypoxia inducible factor alpha (HIF-1α) is regulated by microtubule alterations in response to hypoxia, and 3) inhibition of HDAC6 prevents HIF-1α expression and protects tissue from hypoxic/ischemic insults. The aim of this study was to address whether the absence of HDAC6 alters ventilatory responses during and/or after hypoxic gas challenge (10% O 2 , 90% N 2 for 15 min) in adult male wildtype (WT) C57BL/6 mice and HDAC6 knock-out (KO) mice. Key findings were that 1) baseline values for frequency of breathing, tidal volume, inspiratory and expiratory times, and end expiratory pause were different between knock-out mice and wildtype mice, 2) ventilatory responses during hypoxic challenge were more robust in KO mice than WT mice for recorded parameters including, frequency of breathing, minute ventilation, inspiratory and expiratory durations, peak inspiratory and expiratory flows, and inspiratory and expiratory drives, and 3) responses upon return to room-air were markedly different in KO compared to WT mice for frequency of breathing, minute ventilation, inspiratory and expiratory durations, end expiratory pause (but not end inspiratory pause), peak inspiratory and expiratory flows, and inspiratory and expiratory drives. These data suggest that HDAC6 may have a fundamentally important role in regulating the hypoxic ventilatory response in mice.
Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.
Superior cervical ganglia (SCG) post-ganglionic neurons receive pre-ganglionic drive via the cervical sympathetic chain (CSC). The SCG projects to structures like the carotid bodies (e.g., vasculature, chemosensitive glomus cells), upper airway (e.g., tongue, nasopharynx) and to parenchyma and cerebral arteries throughout the brain. We previously reported that a hypoxic gas challenge elicited an array of ventilatory responses in sham-operated (SHAM) freely-moving adult male C57BL6 mice and that responses were altered in mice with bilateral transection of the cervical sympathetic chain (CSCX). Since the CSC provides pre-ganglionic innervation to the SCG, we presumed that mice with superior cervical ganglionectomy (SCGX) would respond similarly to hypoxic gas challenge as CSCX mice. However, while SCGX mice had altered responses during hypoxic gas challenge that occurred in CSCX mice (e.g., more rapid occurrence of changes in frequency of breathing and minute ventilation), SCGX mice displayed numerous responses to hypoxic gas challenge that CSCX mice did not, including reduced total increases in frequency of breathing, minute ventilation, inspiratory and expiratory drives, peak inspiratory and expiratory flows, and appearance of non-eupneic breaths. In conclusion, hypoxic gas challenge may directly activate sub-populations of SCG cells, including sub-populations of post-ganglionic neurons and small intensely fluorescent (SIF) cells, independently of CSC drive, and that SCG drive to these structures dampens the initial occurrence of the hypoxic ventilatory response, while promoting the overall magnitude of the response. The multiple effects of SCGX may be due to loss of innervation to peripheral and central structures with differential roles in breathing control.
Serotonergic neuroepithelial cells (NECs) in larval zebrafish are believed to be O2 chemoreceptors. Serotonin (5-HT) within these NECs has been implicated as a neurotransmitter mediating the hypoxic ventilatory response (HVR). Here, we use knockout approaches to discern the role of 5-HT in regulating the HVR by targeting the rate limiting enzyme for 5-HT synthesis, tryptophan hydroxylase (Tph). Using transgenic lines, we determined that Tph1a is expressed in skin and pharyngeal arch NECs, as well as in pharyngeal arch Merkel-like cells (MLCs), whereas Tph1b is expressed predominately in MLCs. Knocking out the two tph1 paralogs resulted in similar changes in detectable serotonergic cell density between the two mutants, yet their responses to hypoxia (35 mmHg) were different. Larvae lacking Tph1a (tph1a−/− mutants) displayed a higher ventilation rate when exposed to hypoxia compared to wild-types, whereas tph1b−/− mutants exhibited a lower ventilation rate suggesting that 5-HT located in locations other than NECs, may play a dominant role in regulating the HVR.
Advances in our understanding of brain mechanisms for the hypoxic ventilatory response, coordinated changes in blood pressure, and the long-term consequences of chronic intermittent hypoxia as in sleep apnea, such as hypertension and heart failure, are giving impetus to the search for therapies to "erase" dysfunctional memories distributed in the carotid bodies and central nervous system. We review current network models, open questions, sex differences, and implications for translational research.
That hypoxia modulates breathing has long been recognized. Prior to the discovery of the function of the arterial chemoreceptors in 1930 (1), the locus of this phenomenon had been assigned to the central nervous system (CNS). With that discovery, however, full attention was paid to the transducer role of the carotid bodies in the ventilatory response to hypoxia, and only in the last two decades or so has attention returned to the consideration that the CNS may serve as the initial transducer as well as an important modulator in the increasingly large array of recognized ventilatory responses to acute and more prolonged hypoxia.
DefinitionChange in sound energy into some other form, usually heat, in passing through a medium or striking a surface.Acoustics
The respiratory regulatory system is one of the most extensively studied homeostatic systems in the body. Despite its deceptively mundane physiological function, the mechanism underlying the robust control of the motor act of breathing in the face of constantly changing internal and external challenges throughout one's life is still poorly understood. Traditionally, control of breathing has been studied with a highly reductionist approach, with specific stimulus-response relationships being taken to reflect distinct feedback/feedforward control laws. It is assumed that the overall respiratory response could be described as the linear sum of all unitary stimulus-response relationships under a Sherringtonian framework. Such a divide-and-conquer approach has proven useful in predicting the independent effects of specific chemical and mechanical inputs. However, it has limited predictive power for the respiratory response in realistic disease states when multiple factors come into play. Instead, vast amounts of evidence have revealed the existence of complex interactions of various afferent-efferent signals in defining the overall respiratory response. This thesis aims to explore the nonlinear interaction of afferents in respiratory control. In a series of computational simulations, it was shown that the respiratory response in humans during muscular exercise under a variety of pulmonary gas exchange defects is consistent with an optimal interaction of mechanical and chemical afferents. This provides a new understanding on the impacts of pulmonary gas exchange on the adaptive control of the exercise respiratory response. Furthermore, from a series of in-vivo neurophysiology experiments in rats, it was discovered that certain respiratory neurons in the dorsolateral pons in the rat brainstem integrate central and peripheral chemoreceptor afferent signals in a hypoadditive manner. Such nonlinear interaction evidences classical (Pavlovian) conditioning of chemoreceptor inputs that modulate the respiratory rhythm and motor output. These findings demonstrate a powerful gain modulation function for control of breathing by the lower brain. The computational and experimental studies in this thesis reveal a form of associative learning important for adaptive control of respiratory regulation, at both behavioral and neuronal levels. Our results shed new light for future experimental and theoretical elucidation of the mechanism of respiratory control from an integrative modeling perspective.
This Workshop addressed emerging issues of central integration of chemoreceptor inputs during challenges such as hypoxia,
hypercapnia and exercise. Classical chemoreflex models assume that respiratory output increases in direct proportion to the
sum of chemoreceptor inputs via a hard-wired neural controller, with additive interactions between afferent and efferent pathways
(Figure lA). While this structure has proved satisfactory in explaining the chemical control of breathing, it seems to fail
in other respects such as exercise. Increasing evidence now points to the respiratory controller probably being plastic-wired rather than hard-wired, with inputs and outputs demonstrating redundancy. These recent findings have ushered in a gradual
paradigm shift toward an adaptive respiratory controller model structure (see Figure 1B).
The coordination of CuII ions in Anderson and Keggin type heteropolyanion compounds in their polycrystalline and corresponding magnetically diluted forms {(NH4)4[Zn(OH)6Mo6O18]·nH2O/Cu2+; ((CH3)4N)6[ZnO4W12O36]·nH2O/Cu2+} have been investigated by means of chemical, spectroscopic (EPR, IR), diffraction (X-ray) and quantum-mechanical methods. Simple model compounds with known Cu—O coordination have been used for comparison. In both the Anderson ion [Cu(OH)6Mo6O18]4– and the Keggin ion [SiO4W11O30CuO5(OH2)]6– the copper coordination polyhedra consist of six oxygen atoms, forming a distorted octahedron. As expected, the magnetically diluted Anderson compound exhibits a pronounced dynamic Jahn–Teller effect.
We have been studying the role of changes of pHi as part of the signaling pathway of hypercapnia in neonatal rat brainstem neurons from both chemosensitive (NTS-nucleus tractus solitarius, VLM-ventrolateral medulla, LC-locus coeruleus) and non-chemosensitive (IO-inferior olive, Hyp-hypoglossal) regions. We are testing the model of chemotransduction that proposes that hypercapnia acidifies neurons, which leads to inhibition of K+ channels, resulting in neuronal depolarization and increased firing rate. We measure pHi in individual neurons within brain slices using fluorescence imaging microscopy [1,2]. We developed a new technique to measure pHi and Vmsimultaneously by loading pH-sensitive fluorescent dyes using perforated patch (amphotericin B) pipettes in LC neurons [3]. All studies were done at 37°C.
Neurons from all brainstem regions have high intrinsic buffering power, around 45 mM/pH unit for NTS, VLM, IO and Hyp neurons [1] and about 90 mM/pH unit for LC neurons [3]. The only pH-regulating transporter available for recovery from an acid load in these neurons is Na+/H+ exchange (NHE) [1,2]. The only HCO3-dependent transporter present is the acidifying Cl-/HCO3- exchanger, which has been found in VLM, IO, Hyp and some NTS neurons [1]. Interestingly, the NHE in neurons from chemosensitive regions (eg, NTS and VLM) is more sensitive to inhibition by decreased pHo than neurons from non-chemosensitive regions (eg, IO and Hyp) [1].
We observed different responses to hypercapnia in neurons from chemosensitive vs. non-chemosensitive regions. Hypercapnic acidosis (HA–10% CO2, pHo 7.15) resulted in a maintained fall of pHi in neurons from chemosensitive areas whereas pHi recovery from acidosis was evident in neurons from non-chemosensitive areas [2]. Under conditions of isohydric hypercapnia (IH–10% CO2, 52 mM HCO3-, pHo 7.45), pHi recovery was seen in neurons from all regions [2], indicating that pH-regulating mechanisms are present in all these neurons but that they are inhibited more fully by decreased pHo in neurons from chemosensitive regions. The same pHi responses to HA (15% CO2, pHo 6.8) and IH (15% CO2, 77 mM HCO3-, pHo 7.45) were seen in LC neurons where pHi and Vm were measured simultaneously. The changes of pHi ccurred before the changes in neuronal firing rate [3]. In LC neurons, HA resulted in a larger fall of pHi (0.27 vs. 0.15 pH unit) and a larger increase in firing rate (1.31 vs. 1.06 Hz) than seen with IH. Notably, increased firing rate was accompanied by membrane depolarization (2.5 mV) in response to HA but by membrane hyperpolarization (2.3 mV) in response to IH. Other acid challenges involving decreased pHo (acidified HEPES; isocapnic acidosis–5% CO2, 7 mM HCO3-, pHo 6.9) resulted in increased firing rate with depolarization while acid challenges involving constant pHo (50 mM propionate, pHo 7.45) resulted in slightly increased firing rate with membrane hyperpolarization.
In summary, the response of Vm to a fall of pHi is dependent on whether changes or not and is not well correlated with pHo increased firing rate in LC neurons. The parameter that best correlates with increased firing rate in response to an acid challenge is the change of pHi, indicating that pHi is likely involved as part of the chemosensitive signaling pathway in LC neurons.
Erythropoietin (Epo) was originally discovered as a cytokine able to increase the production of red blood cells upon conditions of reduced oxygen availability. Now we know that Epo does far more than "only" augmenting the number of erythrocytes. Since the demonstration that Epo (and its receptor) is expressed in the mammalian brain, several elegant experiments were performed to reveal the function of this molecule in the neuronal tissue. Accordingly to its anti-apoptotic, neurotrophic and proliferative effects in the bone marrow, it was suitably suggested that upon pathological conditions Epo exerts neuroprotective functions (i.e. reducing the infarct volume of stroke, thus allowing better and faster recovery). We considered however, that Epo in brain might also exert a physiological function. Indeed, we found that Epo is an important modulator of the respiratory control system. By using adult mice we showed that Epo increases the hypoxic ventilatory response by interacting with both the central respiratory network (brainstem) as well as the main peripheral sensory organs detecting systemic hypoxia, the carotid bodies. More recently, our research turned to examine the exciting hypothesis that Epo is also implicated in the regulation of the neuronal control of ventilation during the postnatal development. The objective of this review is to summarize the role and mode of action of Epo on respiratory control in adult mammals and highlight the potential pathways by which this cytokine achieve this function. Additionally, we review recent evidences showing that Epo play a crucial role in setting the respiratory motor output (measured on the isolated brainstem spinal cord preparation, en bloc technique) during the early postnatal life.
The respiratory control consists of the controller (brain), controlled system (lungs and chest wall), and sensors (e.g. carotid body). This system will maintain pH and to a lesser extent oxygenation within a fairly narrow range despite changes in metabolism and environmental conditions with growth, development, and activities of daily living. Ventilatory behavior (the expression of frequency, tidal volume, and ventilation) is one measurable output in this respiratory system. Many combinations of tidal volume and frequency are found among species [1] and depend only in part on the mechanical properties of the controlled system. In the rat or the mouse there is a broad range of frequency values that are energetically similar [5; 10]. Thus, both genes and environment can modify ventilatory behavior in a global manner without extracting significiant energy cost in regard to gas exchange. © 2008 Springer Science+Business Media, LLC. All rights reserved.
We recently reported that neurons in the dorsocaudal chemosensitive area, the solitary complex (SC), are stimulated by reactive oxygen species (ROS) during acute hyperbaric oxygen (HBO2), suggesting that oxidative stress modulates cardiorespiratory networks [1,2]. Hyperoxia is known to have various effects on car-diorespiratory function [3,4], and it has been proposed that central hypoventilation syndromes, for example, SIDS, are due in part to increased oxidative stress of neurons after birth [5]. Thus, we have tested the hypothesis that excitability of CO2-chemosensitive neurons in the caudal SC is altered during oxidative stress at normobaric and hyperbaric pressures.
Intracellular recordings were made from SC neurons in 300 μm thick medullary slices prepared from weaned and adult rats. Control medium was equilibrated with 95% O2 -5% CO2 at barometric pressure (PB) ~1 atmosphere absolute (ATA) (medium PO2 ~720 Torr, PCO2 ~38 Torr). A slice was maintained in a hyperbaric chamber and exposed to one or more of the following conditions (~37°C): 1)normobaric hypercapnia (8–20 mins 15% CO2 in O2) to test for CO2 chemosensitivity; 2)normocapnic HBO2 (10–20 mins 98.3% O2 -1.65% CO2 at PB 3.3 ATA) and 3)pro-oxidants at PB ~1ATA (8–10 mins 500 μM Chloramine-T or 1 mM N-chlorosuccinimide) to test for the effects of ROS; and 4)hypercapnic HBO2 (10–20 mins 95% O2 -5% CO2 at PB 3.2ATA) to test for the effects of hypercapnic acidosis on sensitivity to HBO2[6,7,9]. During HBO2, tissue PO2 (mean ± S.E.) at the core of the slice (150 μm depth) increased from 291 ± 20 to 1517 ± 15 Torr [6].
Focusing on the caudal SC, 62% (18/29) of the neurons tested were depolarized by normobaric hypercapnia. Of these, 78% (14/18) were stimulated by normocapnic HBO2 and pro-oxidants. When hypercapnia and HBO2 were combined (hypercapnic HBO2), the firing rate response was greater to both stimuli than to the sum of their individual responses. Most neurons (9/11) that were CO2 insensitive were also unresponsive to HBO2 and/or pro-oxidants.
We conclude that acute exposure to oxidative stress, either by increased tissue PO2 at hyperbaric pressure or pro-oxidants at normobaric pressure, stimulates CO2-chemosensitive neurons in the SC, suggesting that central chemosensitivity may likewise be affected by oxidative stress. It is unclear, however, if the strong neuronal excitation observed when hypercapnic acidosis and HBO2 are combined is due to increased lipid peroxidation during intracellular acidosis [7,8,9] or to the effect of increased ROS on CO2 chemosensitivity.
In the rat, a mostly nocturnal animal, activity, body temperature and metabolic rate increase during the dark hours of the day. Since all these variables are known to influence breathing, it is expected that also pulmonary ventilation (V̇e) will present a circadian pattern. In rats chronically instrumented for measurements of body temperature and activity by telemetry, carbon dioxide production and oxygen consumption (V̇O2) were measured continuously for several days by an open-circuit method, while V̇e was monitored by a modification of the barometric technique. Tidal volume, frequency, and V̇e increased in the dark (D) compared to the light (L) hours, with minor L–D differences in V̇e/V̇O2. L–D differences in activity were not responsible for the circadian pattern of V̇e. Both in hypoxia and in hypercapnia the degree of hyperventilation (percent increase in V̇e/V̇O2) was essentially independent of the time of the day, despite the fact that in hypoxia, differently from hypercapnia, the amplitude of the circadian pattern of all variables decreased, activity being the least affected, and body temperature the most. These effects of hypoxia, which occurred before and after sino-aortic denervation and did not compromise the period of the biological clock, may be mediated by the hypothalamic thermoregulatory centers. The data of these experiments and of others reviewed in this article indicate that (1) breathing and its control mechanisms accompany the daily oscillations of numerous physiological variables, and (2) the advantages of a biological clock do not compromise the adequacy of the hyperventilatory responses to chemical challenges.
Current models propose that a neuronal network in the ventrolateral medulla generates the basic respiratory rhythm and that this ventrolateral respiratory column (VRC) is profoundly influenced by the neurons of the pontine respiratory group (PRG). However, functional connectivity among PRG and VRC neurons is poorly understood. This study addressed four model-based hypotheses: 1) the respiratory modulation of PRG neuron populations reflects paucisynaptic actions of multiple VRC populations; 2) functional connections among PRG neurons shape and coordinate their respiratory-modulated activities; 3) the PRG acts on multiple VRC populations, contributing to phase-switching; and 4) neurons with no respiratory modulation located in close proximity to the VRC and PRG have widely distributed actions on respiratory-modulated cells. Two arrays of microelectrodes with individual depth adjustment were used to record sets of spike trains from a total of 145 PRG and 282 VRC neurons in 10 decerebrate, vagotomized, neuromuscularly blocked, ventilated cats. Data were evaluated for respiratory modulation with respect to efferent phrenic motoneuron activity and short-timescale correlations indicative of paucisynaptic functional connectivity using cross-correlation analysis and the "gravity" method. Correlogram features were found for 109 (3%) of the 3,218 pairs composed of a PRG and a VRC neuron, 126 (12%) of the 1,043 PRG-PRG pairs, and 319 (7%) of the 4,340 VRC-VRC neuron pairs evaluated. Correlation linkage maps generated for the data support our four motivating hypotheses and suggest network mechanisms for proposed modulatory functions of the PRG.
Using mice, we demonstrated that when oxygen supply is lowered, erythropoietin (Epo), the main regulator of red blood cell production, modulates the ventilatory response by interacting with central (brainstem) and peripheral (carotid bodies) respiratory centers. We showed that enhanced Epo levels in the brainstem increased the hypoxic ventilatory response, and that intracerebroventricular injection of an Epo antagonist (soluble Epo receptor) abolished the ventilatory acclimatization to hypoxia. More recently, we have found that the impact of Epo on ventilation occurs in a sex-dependent manner. Keeping in mind that women are less susceptible to several respiratory sicknesses and syndromes than men, we suggest that Epo plays a key role in sexually-dimorphic hypoxic ventilation. Accordingly, we foresee that Epo has a potential therapeutic use as treatment for hypoxia-associated ventilatory diseases.
The parabrachial complex, also known as the pneumotaxic center or pontine respiratory group, has long been recognized as an important participant in respiratory control. One line of evidence supporting this idea is the demonstration of changes in breathing pattern following injection of neuroactive substances into or near the parabrachial complex. However, it is not yet known exactly which cell groups and projections mediate those responses. In order to address this issue, we explored the topographic organization of respiratory responses to chemical stimulation of the parabrachial complex of the rat and examined the descending projections of the most sensitive sites. Injection of glutamate (5-100 pmol) at specific sites in or near the parabrachial nucleus produced three distinct site-specific response patterns. First, hyperpnea followed glutamate injection into far rostral and midcaudal areas of the Kölliker-Fuse nucleus and most of the lateral parabrachial nucleus, including the external lateral, central lateral, dorsal lateral, and superior lateral subnuclei. Threshold hyperpneic effects were manifested as single, deepened breaths of premature onset. Suprathreshold doses of glutamate at these locations produced tachypnea. Neurons in these sites projected to the ventral respiratory group in the ventrolateral medulla. Second, the most intense inspiratory facilitatory responses were seen at mid to rostral levels of the Kölliker-Fuse nucleus, near the ventrolateral tip of the superior cerebellar peduncle. Even at threshold doses of glutamate, exhalation was incomplete, resulting in a breathing pattern that resembled apneusis (an inspiratory cramp). This site contained an especially dense cluster of neurons that projected either to the ventrolateral medulla or to the dorsal respiratory group in the nucleus of the solitary tract, but not to both areas. The third type of response, decreases in respiratory rate, occurred following glutamate injection at the most lateral and ventral boundaries of the Kölliker-Fuse nucleus. The most sensitive apneic sites were not found in the parabrachial nucleus but along the dorsal and medial edge of the principal sensory trigeminal nucleus and extending ventrally between the sensory and motor trigeminal nuclei. Scattered neurons in these sites were retrogradely labeled from the ventral but not the dorsal respiratory group. These results indicate that there are anatomically and functionally distinct cell populations in and near the parabrachial complex that, when chemically stimulated, can produce specific and sometimes opposing effects on respiration. The predominant effect of lateral parabrachial stimulation is respiratory facilitation, while inhibitory effects are elicited by trigeminal injections of glutamate.
The purpose of this study was to examine the effects of alpha 2-adrenoceptor agonists in the control of breathing with goats that were either awake (n = 7) or anesthetized and artificially ventilated (n = 11). Awake goats infused intravenously with either of the alpha 2-agonists clonidine (1.0-6.0 micrograms/kg) or guanabenz (15.0-63.0 micrograms/kg) exhibited two distinct ventilatory patterns. One pattern was characterized by tachypnea in which respiratory frequency and minute ventilation increased to approximately 50% above control values. A second ventilatory pattern consisted of slow breathing with reductions of respiratory frequency and minute ventilation and highly variable expiratory duration intervals. These two patterns were unaffected by bilateral carotid body denervation. In anesthetized goats, alpha 2-agonists also caused an arrhythmia in phrenic nerve activity that was similar to the slow breathing pattern seen in awake goats. Respiratory disturbances were abolished by the selective alpha 2-receptor antagonist SKF-86466 (100-500 micrograms/kg), indicating that the effects are mediated by alpha 2-receptors. The results suggest that stimulation of alpha 2-adrenoceptors generally has an inhibitory effect on breathing in goats. The disruption of ventilation with clonidine or guanabenz suggests that alpha 2-adrenoceptors may play an important role in the control of central respiratory rhythm.
Posthypoxic frequency decline (PHFD) refers to the undershoot in respiratory frequency that follows brief hypoxic exposures. Lateral pontine neurons are required for PHFD. The neurotransmitters involved in the circuit that activate and/or are released by these pontine neurons regulating PHFD are unknown. We hypothesized that N-methyl-D-aspartate (NMDA) receptors are required for PHFD, because of the similarity in respiratory pattern after blocking lateral pontine activity or NMDA receptors. Furthermore, we hypothesized that the location of these NMDA receptors could be visualized by optimizing binding affinity with spermidine. In vagotomized, anesthetized rats (n = 16), cardiorespiratory responses to hypoxia (8% O2, 30-90 s) were recorded before and after dizocilpine (10 microg-1 mg/kg iv), and NMDA receptors were mapped with [3H]dizocilpine (n = 6). Dizocilpine elicited a dose-related effect on PHFD, blocking PHFD at high doses. Resting arterial blood pressure and breathing frequency decreased with high doses of dizocilpine, but the respiratory response to hypoxia remained intact. Our novel anatomical data indicate that NMDA receptors were widespread but distributed differentially in the brain stem. We conclude that NMDA receptors are located in pontine and medullary respiratory-related regions and that PHFD requires NMDA-receptor activation.
The aim of this study was to determine whether post-hypoxic frequency decline (PHFD) requires central activation of alpha2-adrenergic receptors. PHFD is defined as the undershoot in respiratory frequency that occurs immediately following brief hypoxic periods. Adult anesthetized, vagotomized rats were exposed to hypoxia (8% O2, mean=45 s) before and after intracerebroventricular (i.c.v.) infusion of vehicle or alpha2-antagonist. The efficacy of the i.c.v. antagonist was assessed by recording the response to intravenous injection of alpha2-agonist before and after the infusion. We compared breathing frequencies before, during, and after hypoxia, both before and after treatments. The decline in breathing frequency after hypoxia was not prevented by the alpha2-antagonists, RX 821002 or SK&F-86466. Guanabenz, an alpha2-agonist, prolonged baseline expiration and potentiated PHFD. Prior treatment with SK&F-86466 blocked the agonist-evoked response which was also reversed by subsequent administration of SK&F-86466. We conclude that PHFD does not require the activation of alpha2-adrenergic receptors, but that alpha2-adrenergic receptors can modulate resting and post-hypoxic respiratory frequency.
Apnea is an important protective response to upper airway irritation, but the central mechanisms responsible for eliciting sensory-induced apnea are not well understood. Recent studies have emphasized the Kölliker-Fuse nucleus in producing apnea and proposed a trigeminoparabrachial pathway for mediating these reflexes. However, in our earlier study of apneic responses produced by glutamate stimulation in the dorsolateral pons, we found that apnea was elicited from the area just ventral to the Kölliker-Fuse nucleus, rather than within it. Because this region was not known to be involved in respiratory control, we combined chemical microstimulation with both anterograde and retrograde axonal tracing to characterize the sites in the pons that produce apneic responses. We found that apneic sites were consistently associated with the intertrigeminal region, between the principal sensory and motor trigeminal nuclei. Injections of anterograde tracer at these sites labeled terminals in the ventral respiratory group, in the ventrolateral medulla. Injection of retrograde tracer into this target region in the ventrolateral medulla disclosed a previously unrecognized population of neurons among the trigeminal motor rootlets. Injection of retrograde tracer into this intertrigeminal region demonstrated inputs from portions of the spinal trigeminal nucleus and the nucleus of the solitary tract that have been associated with producing sensory apnea. Our observations suggest that the intertrigeminal region receives a convergence of sensory inputs capable of driving apneic responses and that it may represent a common link between input from different portions of the airway and the respiratory neurons that mediate apneic reflexes.
We tested the hypothesis that the post-hypoxia frequency decline of phrenic nerve activity following brief, isocapnic hypoxic episodes in rats is diminished by prior hypoxic episodes and alpha2-adrenoreceptor antagonism. Anesthetized (urethane), artificially ventilated (FIO2=0.50) and vagotomized rats were presented with two or three, 5 min episodes of isocapnic hypoxia (FIO2 approximately 0.11), separated by 30 min of control, hyperoxic conditions. Phrenic nerve discharge, end-tidal CO2, and arterial blood gases were measured before during and after hypoxia. The average maximum frequency decline, measured 5 min after the first hypoxic episode, was 26+/-7 bursts/min below pre-hypoxic baseline values (a 70+/-16% decrease). By 30 min post-hypoxia, frequency had returned to baseline. Two groups of rats were then administered either: (1) saline (sham) or (2) the alpha2-receptor antagonist, RX821002 HCl (2-[2-(2-Methoxy-1,4-benzodioxanyl)] imidazoline hydrochloride; 0.25 mg/kg, i.v.). Isocapnic hypoxia was repeated 10 min later. In sham rats, the post-hypoxia frequency decline (PHFD) was significantly attenuated relative to the initial (control) response. However, PHFD was attenuated significantly more in RX821002-treated vs. sham rats (-3+/-3 bursts/min vs. -12+/-4 bursts/min @ 5 min post hypoxia for RX821002 and sham-treated, respectively; p<0.05). We conclude that the magnitude of PHFD is dependent on the prior history of hypoxia and that alpha2 adrenoreceptor activation plays a role in its underlying mechanism.
1. Superfused brain stem-spinal cord preparations of newborn rats, which continue to show a rhythmic respiratory activity in vitro, were used to analyse the mechanisms whereby the A5 noradrenergic area modulates the activity of the medullary respiratory rhythm generator in the newborn. 2. In preparations including the pons (ponto-medullary preparations), noradrenaline (NA, 25-100 microM) added to the bathing medium either increased (n = 29/50) or decreased (n = 21/50) the respiratory frequency and elicited a tonic discharge in the cervical ventral roots in 50% of the experiments. Double-bath experiments showed that the increases in respiratory frequency were due to NA acting on the pons, whereas the decreases in respiratory frequency were due to NA acting on the medulla. The NA-induced increases in respiratory frequency were attributed to inhibition of A5 neurons by NA and therefore to withdrawal of A5 inhibition on the medullary rhythm respiratory generator. The NA-induced decreases in respiratory frequency seemed to mimic the effects of endogenous NA on the A5 medullary targets. 3. Noradrenaline-induced tonic activity (i) could be induced after elimination of the pons but not on isolated spinal cord, (ii) could be elicited by alpha 1- but not alpha 2-agonists, (iii) could be blocked by alpha 1- but not alpha 2-antagonists. The tonic activity therefore originated from activation of alpha 1 receptors located in the medulla but its importance in respiratory function is doubtful. 4. In medullary preparations (elimination of the pons by transection), the effects of NA agonists and antagonists on respiratory frequency were analysed. Significant decreases in respiratory frequency were induced by NA, adrenaline, phenylephrine and alpha-methyl-NA, but not by the agonists classified as alpha 2 (clonidine and guanfacine), alpha 1 (6-fluoro-NA) and beta (isoprenaline). Since yohimbine, idazoxan and piperoxane (alpha 2 antagonists) blocked the NA-induced decreases in respiratory frequency whereas prazosin (alpha 1-antagonist) did not, it is postulated that alpha 2-receptors may be involved in modulating respiratory frequency. 5. Stimulation, lesion and NA microejection experiments showed the complexity of the mechanisms mediating NA-induced changes in respiratory activity but suggested that the main site of NA action is located in the rostral ventrolateral medulla, where electrical stimulations triggered inspiration prematurely, lesions suppressed the NA-induced decrease in respiratory frequency, and localized application of NA led to an immediate decrease in the respiratory frequency.(ABSTRACT TRUNCATED AT 400 WORDS)
Development of short-term potentiation (STP) of respiration, which leads to the respiratory 'afterdischarge', was studied in anesthetized, paralyzed, vagotomized and glomectomized cats. Phrenic nerve activity was used as an index of respiratory output. Respiratory output was increased and the potentiating mechanism activated by electrical stimulation of a carotid sinus nerve (CSN). Development of STP was determined from the magnitude of potentiation after various durations (0 to 60 sec) of stimulation. The average time constant (TC) for the development of the potentiation was 9 sec, whereas the TC for its decay (afterdischarge) was 46.1 +/- 3.9 sec. The magnitude of potentiation is dependent upon the number of pulses in the stimulus train. We conclude that the development of short-term potentiation of respiration is relatively slow but much faster than the decay, or afterdischarge. We suggest that the slow increase of respiration during a stimulation and the decay afterwards are due to a common mechanism, short-term potentiation of neural activity in respiratory control pathways.
In anaesthetized, bivagotomized and artificially ventilated rats, the respiratory effects of systemic injection of MK-801, a non-competitive N-methyl-D-aspartate antagonist, were studied. In all the experiments (n = 11), the injection increased the inspiratory duration and decreased the expiratory duration. In 4 experiments, the inspiratory duration was drastically lengthened, resulting in an apneustic-like breathing pattern. These results demonstrate that apneusis is difficult but possible to induce in rats and suggest that termination of inspiration is controlled via central mechanisms in which NMDA-like receptors are involved.
The involvement of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in the control of inspiratory termination was studied in paralyzed decerebrated cats. Cats were either vagotomized, or had intact vagus nerves and were ventilated with a ventilator driven by the discharge of the phrenic nerve. The systemic administration of NMDA antagonists acting non-competitively (MK-801, ketamine, phencyclidine) or competitively (2-amino-7-phosphonoheptanoic acid: AP7), produced an apneusis in vagotomized animals or in animals transiently deprived of vagal pulmonary feedback by the 'no inflation test'. After NMDA receptor blockade, the inspiratory phase could be terminated by lung inflation or sensory stimulation. Thus pharmacologically distinct mechanisms control the termination of inspiration: vagal afferents which are NMDA-independent, and a central mechanism acting through the activation of NMDA receptors. The apneustic pattern induced by NMDA receptor blockade was characterized by a decrease of the amplitude of integrated phrenic nerve activity, the persistence of CO2 sensitivity and an enhancement of apneusis by anaesthesia. After injection of NMDA antagonists there was a decrease of the duration of expiration which thereafter remained constant and dissociated from inspiratory duration. The possible mechanisms by which NMDA receptors may contribute to respiratory rhythmogenesis are discussed.
Respiratory activity was recorded on hypoglossal nerve or ventral cervical roots during in vitro experiments performed in the superfused brainstem-cervical cord preparation of newborn rats. Section and coagulation experiments revealed that the medullary respiratory generator was tonically inhibited by a structure located in the caudal ventrolateral pons. Electrical and pharmacological stimulations located this structure more precisely between the superior olivary nuclei and the sensory nucleus of the Vth nerve, i.e. in an area containing the A5 noradrenergic nucleus. Norepinephrine and alpha 2-antagonists (yohimbine, idazoxan) added to the bathing medium modified the respiratory frequency. Norepinephrine decreased respiratory frequency whereas norepinephrine antagonists increased respiratory rate. The electrical stimulation of the caudal ventrolateral pons which inhibited the respiratory rhythm under normal bathing medium became ineffective after alpha 2-antagonist. The results herein suggest that a noradrenergic inhibitory drive, originating from the A5 area or surrounding structures modulates the activity of the medullary respiratory generator. This hypothesis is discussed in relation to A5 involvement in cardiovascular regulation.
1. The relationships between the depth of a breath and the durations of the inspiratory and expiratory phases have been studied in cat and in man during rebreathing, and in cat using artificial inflations of different magnitudes and timings.
2. In the cat, the apparent volume threshold for termination of inspiration (Hering—Breuer threshold) decreased with time from the onset of the inspiratory phase.
3. Both in rebreathing experiments and with artificially imposed inflations in the cat, the inspiratory duration T I was dependent upon tidal volume V T , and this dependence could be expressed by a hyperbolic relationship of the form ( V T ‐ V 0 ) T I = C where V 0 and C are constants.
4. The time course of this ‘Hering—Breuer’ threshold was dependent on intact vagus nerves. After vagotomy the inspiratory duration remained essentially constant with changes in tidal volume produced either by artificial inflation or by the increased respiratory drive due to accumulation of CO 2 during rebreathing.
5. In man during rebreathing, the relation between volume and inspiratory duration typically showed two different characteristics. 1, at tidal volumes up to 1·5–2 times eupnoeic values, inspiratory duration did not change as tidal volume increased in response to increased P CO 2 . This range of operation has been designated range 1. 2, as tidal volume increased above this range 1 a second range designated range 2 was observed where inspiratory duration was volume dependent in the same manner as in the cat.
6. In cat under pentobarbitone anaesthesia, a range 1 operation was not seen except after vagotomy. However, under urethane anaesthesia a range 1 plus a range 2 operation could be seen.
7. The differences between cat and man appeared to be largely quantitative rather than qualitative.
8. In both cat and man, expiratory duration was dependent on inspiratory duration, usually with a linear relationship.
9. The experimental results were assembled in the form of an inspiratory characteristic and a timing relationship that serve as a model of the respiratory mechanisms controlling the depth and rate of breathing. The model predicts that the depth and duration of a breath are related in a definite manner fixed by the system characteristics and that ventilation is adjusted by setting the appropriate operating point on these characteristics. The operating point is determined primarily by how quickly lung volume increases, i.e. the rate of increase of inspiratory motor activity.
Although superficially similar, HVD appears to arise from different mechanisms in the awake animal as compared with the anesthetized animal. Consequently, the good evidence supporting a central site of action for hypoxia in the genesis of HVD in the anesthetized animal cannot be used as evidence for a central site of action for hypoxia in the awake animal. In their paper on HVD in the awake cat, Long et al. (10) conclude: "The striking similarity between feline and human ventilatory responses to moderate hypoxia illustrated by this and previous experiments leads us to believe that it is likely similar mechanisms apply to both species. Thus it seems probable that working out the mechanisms of the ventilatory response to hypoxia in the awake cat will go a long way toward solving the same problems in humans." This activity should not neglect a consideration of whether adaptation at the carotid body during sustained hypoxia may be involved in the genesis of HVD in the awake state.
Although lesion experiments in the vagotomized cat have indicated that neurons in the parabrachial region of the dorsolateral pons contribute to the mechanisms involved in terminating inspiration, a similar role for pontine structures in the rat has been questioned since pontomedullary transections in the anesthetized rat failed to prolong inspiration. In the present study, lesions of the parabrachial pons of the decerebrate, unanesthetized rat produced an increase in the duration of inspiration to 400% of control and a doubling of the duration of expiration, suggesting a role for this pontine area in the regulation of the timing of the phases of respiration.
1. An attempt has been made to test the hypothesis that, in the caudal part of nucleus tractus solitarii (NTS) where carotid sinus nerve (CSN) afferents project, L-glutamate (Glut) modulates the hypoxic ventilatory response. 2. Unanaesthetized, peripherally chemodenervated (carotid body denervated; CBD) and sham-operated, freely moving rats were used. During peripheral chemoreceptor stimulation by hypoxia (10% O2 for 30 min) or doxapram (Dox) infusion (2 mg kg-1 (30 min)-1), ventilation was recorded and successively, under the same conditions, the extracellular Glut concentration ([Glut]o) in the caudal NTS was measured by in vivo microdialysis. [Glut]o was also measured during hyperoxic hypercapnia (10% CO2-30% O2 for 30 min). 3. Furthermore, the effects on ventilation of exogenous Glut, the NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 or the ionotropic receptor antagonist kynurenate microinjected into the caudal NTS were investigated in sham-operated rats. 4. In sham-operated rats, both ventilation and [Glut]o in NTS were increased during peripheral chemoreceptor stimulation. On the other hand, no increases in either ventilation or Glut release were observed in CBD rats. In spite of ventilatory augmentation during hypercapnia, no response of [Glut]o to hypercapnia was observed in either group. 5. Local Glut application into NTS increased ventilation. Pretreatment with MK-801 or kynurenate reduced the hypoxic ventilatory response. This reduction in ventilation was mainly due to the decrease in tidal volume. 6. These results suggest that hypoxia induced the release of Glut in NTS and that this effect was mediated by arterial chemosensory input.
Inhibition of neural activity in the caudal ventrolateral pons (A5 area) by microinjection of muscimol (Mus) attenuates (-65%) the carotid sympathetic chemoreflex (SChR) without altering the concomitant activation of the phrenic nerve (PND). The present study, performed in urethan-anesthetized rats, explores the possibility that activation of the noradrenergic (NE) neurons of the A5 area is involved in the SChR. The NE neuron-selective toxin 6-hydroxydopamine (6-OHDA) was microinjected bilaterally into the spinal cord at T2 level (4 micrograms). This dose reduced the SChR by 55% (n = 5) 90 min after injection, while 0.4 microgram of 6-OHDA produced no effect (n = 5). In seven rats that had received 250 micrograms 6-OHDA intracisternally 2 wk before, Mus injections into the A5 area failed to attenuate the SChR. These rats also had a lower resting mean arterial pressure than controls (97 vs. 112 mmHg). Spinal intrathecal injection of alpha-adrenergic receptor antagonists (prazosin, 10 and 20 micrograms) or phentolamine (20 and 40 micrograms) attenuated resting sympathetic nerve discharge (SND) and SChR in a roughly proportional manner (25-40%); the beta-adrenergic antagonist nadolol (10 and 20 microgram(s) intrathecally) attenuated the SChR selectively but modestly (-10%). The results are generally compatible with the hypothesis that A5 NE neurons and particularly their spinal cord projection could play a facilitating role in the SChR. However, clear evidence that A5 cells contribute selectively to sympathoactivation during chemoreceptor stimulation by releasing NE in the spinal cord could not be obtained.
The mass discharges of the splanchnic sympathetic (SND) and phrenic nerves (PND) were recorded in urethananesthetized rats with resected vagal and aortic nerves. Carotid chemoreceptor (CC) stimulation with N2 inhalation (4-12 s) or cyanide (50-100 micrograms/kg iv) activated SND in bursts synchronized with the postinspiratory phase (mean SND increase: 105 +/- 8%), raised AP, and increased PND rate and amplitude (n = 40). Brain transection at superior collicular level produced no effect. The sympathetic (SChR) and respiratory chemoreflexes (RChR) were reduced after transections through the pons. Lesions of the dorsolateral pons (dl-pons) produced CO2-dependent apneusis and/or tachypnea at rest. After such lesions, CC stimulation produced expiratory apnea and a 30% increase in SChR due to tonic activation of SND. In contrast, bilateral lesions of the ventrolateral pons (vl-pons) reduced the SChR by 54-76%. Muscimol (Mus) injections (bilateral, 175 pmol/side) into vl-pons did not change resting SND, MAP, baroreflex, and RChR but reduced the SChR (54-82%). In conclusion, under anesthesia: 1) the pathway of the carotid chemoreflex is confined to the pons and medulla, 2) the dl-pons exerts indirect control over the SChR via its role in respiratory rhythmogenesis, and 3) neurons in the vl-pons contribute selectively to the SChR but not to PND activation during CC activation.
Electrical and chemical lesions in the ventrolateral pons produced apneustic breathing in anesthetized, vagotomized, paralyzed, ventilated adult rats (n = 13). Apneustic breathing did not develop if the vagi remained intact and was reversed partially with vagal (proximal end) stimulation. Physiologically, these data are similar to those obtained following dorsolateral pontine lesion in rat and other mammalian species and support the hypothesis that pontine neurons influence breathing similarly across mammalian species.
The objectives were to determine 1) respiratory responses to carotid chemoreceptor inputs in anesthetized rats and 2) whether the cerebellar vermis plays a role in these responses. A carotid sinus nerve was stimulated (20 Hz) with five 2-min trains, each separated by approximately 3 min. During stimulation, respiratory frequency (f), peak amplitude of integrated phrenic nerve activity (integral of Phr), and their product (f x integral of Phr) immediately increased. As stimulation continued, integral of Phr progressively increased to a plateau [short-term potentiation (STP)], but f and f x integral of Phr decreased [short-term depression (STD)] to a value still above control. Upon stimulus termination, integral of Phr progressively decreased but remained above control; f and f x integral of Phr transiently decreased below baseline. After the final stimulation, integral of Phr remained above control for at least 30 min [long-term facilitation (LTF)]. Repeated 5-min episodes of isocapnic hypoxia also elicited STP, STD, and LTF. Vermalectomy lowered the CO2-apneic threshold and eliminated LTF. In conclusion, carotid chemoreceptor activation in rats elicits STP and LTF similar to that in cats; the vermis may play a role in LTF. A new response, STD, was observed.
Sympathetic nerve discharge (SND), phrenic nerve discharge (PND), and unit activity of locus ceruleus (LC) and of putative A5 noradrenergic cells were recorded in vagotomized rats anesthetized with urethan. SND was activated by stimulation of carotid chemoreceptors with hypoxia (N2 inhalation, 5-15 s or 12% O2 inhalation, 2-5 min) and displayed a prominent central respiratory modulation during the hypoxic challenge (postinspiratory pattern). LC cells were also activated by peripheral chemoreceptor stimulation. The discharge of most LC units (28 of 31) exhibited central respiratory modulation. 15 LC units had a postinspiratory pattern and 11 had an inspiratory one. Putative A5 cells were also excited by hypoxia and also displayed a clear central respiratory modulation (mostly postinspiratory pattern). These experiments indicate that 1) the firing rate of most pontine noradrenergic cells is increased by peripheral chemoreceptor stimulation, and 2) pontine noradrenergic neurons receive afferent information of a respiratory nature, possibly from their ventrolateral medullary inputs.
Our purpose was to characterize the pontile components of the brain stem ventilatory control system in rats. This study was precipitated by reports that this pontile component might differ fundamentally from that of other species. Efferent activity of the phrenic nerve was recorded in anesthetized, vagotomized, paralyzed, and ventilated adult rats. As in other species, electrical stimulations of the rostral pons caused premature terminations and/or onsets of phrenic activity in eupnea. Electrolytic lesions of rostrolateral pons resulted in apneusis, characterized by significant prolongations of the phrenic burst. Some effective lesions were in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus, the site of the pneumotaxic center. Other lesions resulting in apneusis were ventral to the pneumotaxic center. As in cats, lesions in the caudal pontile reticular formation caused the duration of the apneustic neural inspiration to return toward that of eupnea. Again, as in other species, gradual alterations from eupnea to gasping in the rat were recorded during hypoxia, which was induced by ventilation with carbon monoxide. We conclude that the brain stem respiratory control system is similarly organized in rats and other mammalian species. These results have implications for contemporary hypotheses concerning the neurogenesis of ventilatory activity.
Stimulation of carotid body chemoreceptors by saline saturated with 100% CO2 elicited an increase in mean arterial pressure, respiratory rate, tidal volume, and minute ventilation (VE). Microinjections of L-glutamate into a midline area 0.5-0.75 mm caudal and 0.3-0.5 mm deep with respect to the calamus scriptorius increased VE. Histological examination showed that the site was located in the commissural nucleus of the nucleus tractus solitarii (NTS). The presence of excitatory amino acid receptors [N-methyl-D-aspartic acid (NMDA); kainate, quisqualate/alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and trans 1-amino-cyclopentane-trans-1,3-dicarboxylic acid (ACPD)] in this area was demonstrated by microinjections of appropriate agonists. Simultaneous blockade of NMDA and non-NMDA receptors by combined injections of DL-2-aminophosphonoheptanoate (AP-7; 1 nmol) and 6,7-dinitro-quinoxaline-2,3-dione (DNQX; 1 nmol) abolished the responses to stimulation of carotid body on either side. Combined injections of AP-7 and DNQX did not produce a nonspecific depression of neurons because the responses to another agonist, carbachol, remained unaltered. Inhibition of the neurons in the aforementioned area with microinjections of muscimol (which hyperpolarizes neuronal cell bodies but not fibers of passage) also abolished the responses to subsequent carotid body stimulation on either side.(ABSTRACT TRUNCATED AT 250 WORDS)
Immediate early genes, like c-fos, are believed to be involved in triggering the expression of other genes such as those involved in the synthesis of neurochemicals. Exposure of unanesthetized rats to oxygen deprivation induces activation of the c-fos gene within the nucleus tractus solitarius, resulting in expression of fos-like immunoreactive protein (Fos). Prior administration of MK-801, a nonselective antagonist of the N-methyl-D-aspartate (NMDA)-sensitive glutamate receptor (1 or 2 mg/kg), significantly attenuated but did not completely block hypoxia-induced Fos expression. However, blockade of muscarinic receptors by atropine sulfate (2, 10 or 25 mg/kg) had no measurable effects on Fos expression induced by oxygen deprivation. These results suggest that an NMDA receptor signalling pathway is partly involved in programming the expression of early response genes that regulate various aspects of the response to oxygen deprivation.
1. The parabrachial nucleus (PBN) is thought to play an important role in cardiorespiratory control. However, the circumstances under which it affects ventilation are still not known. The purpose of the present study was to investigate how the PBN modulates the ventilatory responses to hypercapnia, hypoxia or a resistive load in awake rats with chemical lesions of the PBN. 2. In three groups of rats (with lateral PBN lesion, with Kölliker-Fuse nucleus lesion and control), ventilation was measured under various conditions. 3. There was no difference in the breathing of normal room air in any of the groups. However, the lesioned groups showed a reduced ventilatory response to hyperoxic hypercapnia (inspired CO2 fractions (FI,CO2) of 3, 5, 8 and 10%) and to graded hypoxia (inspired O2 fractions (FI,O2) of 16, 12, 10 and 8%) compared with the control group. The control group showed a biphasic response to sustained hypoxia (FI,O2 at 10% for 30 min), known as 'hypoxic depression', while the lesioned groups showed moderate ventilatory exaggeration throughout hypoxia. In response to a resistive load, the lateral PBN lesion group showed no change in ventilatory compensation. 4. The PBN appeared to have a considerable influence on ventilation stimulated in various ways during wakefulness.
1. The breathing pattern following acute hypoxia (arterial O2 pressure (Pa,O2), 27.4 +/- 7.7 mmHg) was measured in intact, anaesthetized and spontaneously breathing adult rats (n = 4) and in anaesthetized, vagotomized, paralysed and ventilated animals (n = 14). Measurements were made both before and after bilateral lesions or chemical inactivation of neurones in the lateral pons. Respiratory motor activity was recorded as an index of the respiratory cycle. We tested the hypothesis that the ventrolateral pons is required for expression of post-hypoxic frequency decline, defined as a decrease in respiratory frequency below steady-state baseline levels following brief exposures to hypoxia. 2. We identified an area in the ventrolateral pons where brief (1 ms) low current (< or = 20 microA) pulses evoked a short-latency inhibitor of phrenic nerve activity. At this site, bilateral electrical or chemical lesions (n = 3) were performed, or neural activity was inhibited by focal injections of 10 mM muscimol (n = 9). In six control animals, neural activity was inhibited by muscimol injections into the lateral pons, dorsal to the target site. 3. Prior to pontine intervention, respiratory frequency decreased below baseline levels following 20-110 s of 8% O2. The decrease in frequency resulted from a prolongation of expiration (up to 276%), which gradually returned to baseline levels (tau = 45 s). 4. Following lesions or inhibition of neural activity in the ventrolateral pons, baseline inspiratory (TI) and expiratory (TE) durations were altered, albeit minimally, in the animals with intact vagus nerves. Expiratory duration following hypoxia was not different from baseline levels either in vagotomized (P = 0.18) or intact (P > 0.05) animals. In contrast, injections of muscimol at more dorsal sites did not alter the decrease in frequency normally seen following hypoxia. 5. Histological examination revealed that effective lesion or injection sites were within the lateral pontine tegmental field and included portions of the noradrenergic A5 cell group. 6. We conclude that the mechanism responsible for post-hypoxic frequency decline involves an active neural process that depends on the integrity of the ventrolateral pons.
Activation of neurons in the ventrolateral (vl) pons was hypothesized to alter the breathing pattern because previous studies demonstrated apneusis after inhibiting neuronal activity with bilateral muscimol (10 mM) microinjections into the vl pons (17). The excitatory amino acid L-glutamate (10 mM) was microinjected (10-100 nl) into the vl pons in anesthetized, vagotomized, paralyzed, and ventilated adult rats (n = 8). In four of these animals, the target site was approached from the ventral surface of the pons to avoid penetrating the dorsolateral (dl) pons. The expiratory phase was prolonged transiently and concurrently with the microinjection. The location of the injection sites included the A5 area, was independent of the approach, and was distinct from the dl pons. These results complement our previous data and indicate that neurons located in the vl pons influence respiration specifically by prolonging expiration when activated and by delaying the inspiratory-to-expiratory phase transition when inhibited.
The ventilatory response to hypoxia depends on the pattern and intensity of hypoxic exposure and involves several physiological mechanisms. These mechanisms differ in their effect (facilitation or depression) on different components of ventilation (tidal volume and frequency) and in their time course (seconds to years). Some mechanisms last long enough to affect future ventilatory responses to hypoxia, indicating 'memory' or functional plasticity in the ventilatory control system. A standard terminology is proposed to describe the different time domains of the hypoxic ventilatory response (HVR) and to promote integration of results from different experimental preparations and laboratories. In general, the neurophysiological and neurochemical basis for short time domains of the HVR (seconds and minutes) are understood better than longer time domains (days to years), primarily because short time domains are studied in the laboratory more easily. Understanding the mechanisms for different time domains of the HVR has important implications for both basic and clinical science.
Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.
1. In some anaesthetized preparations, eupnoea is eliminated following a blockade or destruction of neurons in a rostral medullary pre-Botzinger complex. 2. Neurons in this region might underlie the neurogenesis of eupnoea, or be the source of an input which is necessary for eupnoea to be expressed. If the latter, any apnoea following ablation of the pre-Botzinger complex might be reversed by an augmentation in 'tonic input.' Contrariwise, this apnoea should be permanent if the neuronal activities of the pre- Botzinger complex are an exclusive generator of the eupnoeic rhythm. 3. Decerebrate, vagotomized, paralysed and ventilated adult rats were studied. Efferent activity of the phrenic nerve was recorded as an index of ventilatory activity. 4. Blockade or destruction of neuronal activities of the pre-Botzinger complex by unilateral and/or bilateral injections of muscimol or kainic acid eliminated eupnoea only transiently. Eupnoea returned following activation of the peripheral chemoreceptors and spontaneously over time. 5. Results do not support the concept that neuronal activities of the pre-Botzinger complex play an exclusive role in the neurogenesis of eupnoea in vivo. Rather, these neuronal activities appear to provide a tonic input to the ponto-medullary circuit which generates eupnoea and/or appear to be one component of this circuit.
Carotid body afferents terminate on nTS neurons project to the ventrolateral pons
- B K Freeman
- D L Kunze
- S K Coles
- T E Dick
Freeman, B.K., Kunze, D.L., Coles, S.K., Dick, T.E., 2000. Carotid body afferents terminate on nTS neurons project to the ventrolateral pons. FASEB J. 14, A392.
Non-NMDA receptors mediate synaptic transmission in mNTS
- Andresen
Andresen, M.C., Yang, M.Y., 1990. Non-NMDA receptors mediate synaptic transmission in mNTS. Am. J. Physiol. 259, H1307 – H1311.
Noradrenergic modulation of the medullary respiratory rhythm generator by the pontine A5 area Respiratory Control: Central and Periph-eral Mechanisms
- G Hilaire
- R Monteau
- S Errchidi
- D Morin
- J M Emard
Hilaire, G., Monteau, R., Errchidi, S., Morin, D., Cottel-Emard, J.M., 1993. Noradrenergic modulation of the medullary respiratory rhythm generator by the pontine A5 area. In: Speck, D.F., Dekin, M.S., Revelette, W.R., Fra-zier, D.T. (Eds.), Respiratory Control: Central and Periph-eral Mechanisms. The University Press of Kentucky, Lexington, KY, pp. 43 – 46.
A 'pneumo-taxic centre' in the ventrolateral pons of adult rats
- T E Dick
- S K Coles
- J S Jodkowski
Dick, T.E., Coles, S.K., Jodkowski, J.S., 1995. A 'pneumo-taxic centre' in the ventrolateral pons of adult rats. In: Trouth, O., Millis, R.M., Kiwull-Schö ne, H., Schlä, M.E. (Eds.), Ventral Brainstem Mechanisms and Control Functions. Marcel Dekker, New York, pp. 723 – 740.
Dorsolateral pons modulates the respiratory response to hypoxia in rats: comparison with the ventrolateral pons
- S K Coles
- T E Dick
Carotid body afferents terminate on nTS neurons project to the ventrolateral pons
- Freeman
A ‘pneumotaxic centre’ in the ventrolateral pons of adult rats
- Dick