51
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Fournier S, Kinkead R. Noradrenergic modulation of respiratory motor output during tadpole development: Role of alpha-adrenoceptors. ACTA ACUST UNITED AC 2006; 209:3685-94. [PMID: 16943508 DOI: 10.1242/jeb.02418] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Noradrenaline (NA) is an important modulator of respiratory activity. Results from in vitro studies using immature rodents suggest that the effects exerted by NA change during development, but these investigations have been limited to neonatal stages. To address this issue, we used in vitro brainstem preparations of an ectotherm, Rana catesbeiana, at three developmental stages: pre-metamorphic tadpoles, metamorphic tadpoles and fully mature adult bullfrogs. We first compared the effects of NA bath application (0.02-10 micromol l(-1)) on brainstem preparations from both pre-metamorphic (Taylor-Köllros stages VII-XI) and metamorphic tadpoles (TK stages XVIII-XXIII) and adult frogs. The fictive lung ventilation frequency response to NA application was both dose- and stage-dependent. Although no net change was observed in the pre-metamorphic group, NA application decreased fictive lung burst frequency in preparations from more mature animals. These effects were attenuated by application of alpha-adrenoceptor antagonists. Conversely, NA application elicited dose- and stage-dependent increases in fictive buccal ventilation frequency. We then assessed the contribution of alpha-adrenoceptors towards these responses by applying specific agonists (alpha1: phenylephrine; alpha2: clonidine; concentration range from 10 to 200 micromol l(-1) for both). Of the two agonists used, only phenylephrine application consistently mimicked the lung burst frequency response observed during NA application in each stage group. However, both agonists decreased buccal burst frequency, thus suggesting that other (beta) adrenoceptor types mediate this response. We conclude that modulation of both buccal and lung-related motor outputs change during development. NA modulation affects both types of respiratory activities in a distinct fashion, owing to the different adrenoceptor type involved.
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Affiliation(s)
- Stéphanie Fournier
- Department of Pediatrics, Université Laval, Centre de Recherche du Centre Hospitalier Universitaire de Québec, 10 rue de l'Espinay, Québec City, QC G1L 3L5, Canada
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52
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Borday C, Coutinho A, Germon I, Champagnat J, Fortin G. Pre-/post-otic rhombomeric interactions control the emergence of a fetal-like respiratory rhythm in the mouse embryo. ACTA ACUST UNITED AC 2006; 66:1285-301. [PMID: 16967510 DOI: 10.1002/neu.20271] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
How regional patterning of the neural tube in vertebrate embryos may influence the emergence and the function of neural networks remains elusive. We have begun to address this issue in the embryonic mouse hindbrain by studying rhythmogenic properties of different neural tube segments. We have isolated pre- and post-otic hindbrain segments and spinal segments of the mouse neural tube, when they form at embryonic day (E) 9, and grafted them into the same positions in stage-matched chick hosts. Three days after grafting, in vitro recordings of the activity in the cranial nerves exiting the grafts indicate that a high frequency (HF) rhythm (order: 10 bursts/min) is generated in post-otic segments while more anterior pre-otic and more posterior spinal territories generate a low frequency (LF) rhythm (order: 1 burst/min). Comparison with homo-specific grafting of corresponding chick segments points to conservation in mouse and chick of the link between the patterning of activities and the axial origin of the hindbrain segment. This HF rhythm is reminiscent of the respiratory rhythm known to appear at E15 in mice. We also report on pre-/post-otic interactions. The pre-otic rhombomere 5 prevents the emergence of the HF rhythm at E12. Although the nature of the interaction with r5 remains obscure, we propose that ontogeny of fetal-like respiratory circuits relies on: (i) a selective developmental program enforcing HF rhythm generation, already set at E9 in post-otic segments, and (ii) trans-segmental interactions with pre-otic territories that may control the time when this rhythm appears.
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Affiliation(s)
- C Borday
- Neurobiologie Génétique et Intégrative, Institut de Neurobiologie Alfred Fessard, C.N.R.S., 1 av. de la Terrasse, 91198 Gif-sur-Yvette, France
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53
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Vasilakos K, Kimura N, Wilson RJA, Remmers JE. Lung and Buccal Ventilation in the Frog: Uncoupling Coupled Oscillators. Physiol Biochem Zool 2006; 79:1010-8. [PMID: 17041867 DOI: 10.1086/507655] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/19/2006] [Indexed: 11/03/2022]
Abstract
The frog, with two distinct ventilatory acts, provides a useful model to investigate the prospective interaction of two oscillators in generating the respiratory rhythm. Building on evidence supporting the existence of separate oscillators generating buccal and lung ventilation, we have attempted to uncouple the two rhythms in the isolated brain stem preparation. Opioid preferentially inhibits the lung rhythm, suggesting an uncoupling of the lung from the buccal oscillator. Reduction of the superfusate chloride concentration alters both the buccal and the lung rhythms. Joint application of opioid and reduced-chloride superfusate leads to an increase in the variability of the buccal burst-to-lung burst intervals. This increase in variability suggests that chloride-mediated mechanisms are involved in coupling the buccal oscillator to the lung oscillator. Given the results from these interventions, we propose a simple schematic model of the frog respiratory rhythm generator, outlining the coupling of the lung and buccal oscillators.
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54
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Chatonnet F, Borday C, Wrobel L, Thoby-Brisson M, Fortin G, McLean H, Champagnat J. Ontogeny of central rhythm generation in chicks and rodents. Respir Physiol Neurobiol 2006; 154:37-46. [PMID: 16533622 DOI: 10.1016/j.resp.2006.02.004] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2005] [Revised: 01/31/2006] [Accepted: 02/01/2006] [Indexed: 10/24/2022]
Abstract
Recent studies help in understanding how the basic organization of brainstem neuronal circuits along the anterior-posterior (AP) axis is set by the Hox-dependent segmentation of the neural tube in vertebrate embryos. Neonatal respiratory abnormalities in Krox20(-/-), Hoxa1(-/-) and kreisler mutant mice indicate the vital role of a para-facial (Krox20-dependent, rhombomere 4-derived) respiratory group, that is distinct from the more caudal rhythm generator called Pre-Bötzinger complex. Embryological studies in the chick suggest homology and conservation of this Krox20-dependent induction of parafacial rhythms in birds and mammals. Calcium imaging in embryo indicate that rhythm generators may derive from different cell lineages within rhombomeres. In mice, the Pre-Bötzinger complex is found to be distinct from oscillators producing the earliest neuronal activity, a primordial low-frequency rhythm. In contrast, in chicks, maturation of the parafacial generator is tightly linked to the evolution of this primordial rhythm. It seems therefore that ontogeny of brainstem rhythm generation involves conserved processes specifying distinct AP domains in the neural tube, followed by diverse, lineage-specific regulations allowing the emergence of organized rhythm generators at a given AP level.
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Affiliation(s)
- F Chatonnet
- UPR 2216, Neurobiologie Génétique et Integrative, Institut fédératif de Neurobiologie Alfred Fessard, C.N.R.S. 1, Avenue de la terrasse, Gif sur Yvette, 91198 Cedex, France
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55
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Noronha-de-Souza CR, Bícego KC, Michel G, Glass ML, Branco LGS, Gargaglioni LH. Locus coeruleus is a central chemoreceptive site in toads. Am J Physiol Regul Integr Comp Physiol 2006; 291:R997-1006. [PMID: 16644910 DOI: 10.1152/ajpregu.00090.2006] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The locus coeruleus (LC) has been suggested as a CO2chemoreceptor site in mammals. This nucleus is a mesencephalic structure of the amphibian brain and is probably homologous to the LC in mammals. There are no data available for the role of LC in the central chemoreception of amphibians. Thus the present study was designed to investigate whether LC of toads ( Bufo schneideri) is a CO2/H+chemoreceptor site. Fos immunoreactivity was used to verify whether the nucleus is activated by hypercarbia (5% CO2in air). In addition, we assessed the role of noradrenergic LC neurons on respiratory and cardiovascular responses to hypercarbia by using 6-hydroxydopamine lesion. To further explore the role of LC in central chemosensitivity, we examined the effects of microinjection of solutions with different pH values (7.2, 7.4, 7.6, 7.8, and 8.0) into the nucleus. Our main findings were that 1) a marked increase in c-fos-positive cells in the LC was induced after 3 h of breathing a hypercarbic gas mixture; 2) chemical lesions in the LC attenuated the increase of the ventilatory response to hypercarbia but did not affect ventilation under resting conditions; and 3) microinjection with acid solutions (pH = 7.2, 7.4, and 7.6) into the LC elicited an increased ventilation, indicating that the LC of toads participates in the central chemoreception.
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56
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Gargaglioni LH, Milsom WK. Control of breathing in anuran amphibians. Comp Biochem Physiol A Mol Integr Physiol 2006; 147:665-684. [PMID: 16949847 DOI: 10.1016/j.cbpa.2006.06.040] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2006] [Revised: 06/21/2006] [Accepted: 06/24/2006] [Indexed: 11/27/2022]
Abstract
The primary role of the respiratory system is to ensure adequate tissue oxygenation, eliminate carbon dioxide and help to regulate acid-base status. To maintain this homeostasis, amphibians possess an array of receptors located at peripheral and central chemoreceptive sites that sense respiration-related variables in both internal and external environments. As in mammals, input from these receptors is integrated at central rhythmogenic and pattern-forming elements in the medulla in a manner that meets the demands determined by the environment within the constraints of the behavior and breathing pattern of the animal. Also as in mammals, while outputs from areas in the midbrain may modulate respiration directly, they do not play a significant role in the production of the normal respiratory rhythm. However, despite these similarities, the breathing patterns of the two classes are different: mammals maintain homeostasis of arterial blood gases through rhythmic and continuous breathing, whereas amphibians display an intermittent pattern of aerial respiration. While the latter is also often rhythmic, it allows a degree of fluctuation in key respiratory variables that has led some to suggest that control is not as tight in these animals. In this review we will focus specifically on recent advances in studies of the control of ventilation in anuran amphibians. This is the group of amphibians that has attracted the most recent attention from respiratory physiologists.
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Affiliation(s)
- Luciane H Gargaglioni
- Department of Animal Morphology and Physiology, Sao Paulo State University-FCAV at Jaboticabal, SP, Brazil.
| | - William K Milsom
- Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada
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57
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Wilson RJA, Vasilakos K, Remmers JE. Phylogeny of vertebrate respiratory rhythm generators: the Oscillator Homology Hypothesis. Respir Physiol Neurobiol 2006; 154:47-60. [PMID: 16750658 DOI: 10.1016/j.resp.2006.04.007] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2005] [Revised: 04/11/2006] [Accepted: 04/11/2006] [Indexed: 11/30/2022]
Abstract
A revolution is underway in our understanding of respiratory rhythm generation in mammals. Until recently, a major focus of research within the field has centered around the question of locating and elucidating the mechanism of rhythmogenesis of a single respiratory neuronal oscillator which is reiterated bilaterally within the brainstem. Now it appears that each hemisection may contain at least two oscillators that interact to generate the respiratory rhythm in mammals. Comparative studies have hinted at the existence of multiple respiratory oscillators in non-mammalian vertebrates for some time, raising the possibility of homologous oscillators. Here, we consider available tools to identify neuronal oscillators and critically review the evidence for the importance and existence of multiple respiratory oscillators in vertebrates. First focusing on a comparison between frogs and mammals, we then evaluate the hypothesis that ventilatory oscillators in extant tetrapods evolved from ancestral oscillators present in fish (the Oscillator Homology Hypothesis). While supporting data are incomplete, the Oscillator Homology Hypothesis will likely serve as a useful framework to motivate further studies of respiratory rhythm generation in lower vertebrates.
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Affiliation(s)
- Richard J A Wilson
- Department of Physiology and Biophysics, University of Calgary, Calgary, Alta., Canada.
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58
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Forster HV. The parafacial respiratory group (pFRG)/pre-Botzinger complex (preBotC) is the primary site of respiratory rhythm generation in the mammal. J Appl Physiol (1985) 2006. [DOI: 10.1152/japplphysiol.00351.2006] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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59
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Gdovin MJ, Jackson VV, Zamora DA, Leiter JC. Effect of prevention of lung inflation on metamorphosis and respiration in the developing bullfrog tadpole, Rana catesbeiana. ACTA ACUST UNITED AC 2006; 305:335-47. [PMID: 16493648 DOI: 10.1002/jez.a.266] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
We tested the hypothesis that respiratory development would be retarded in tadpoles reared in aquaria in which a barrier prevented access to the air-water interface. To test this hypothesis, we examined swimming behavior and respiration in intact tadpoles and gill and lung respiratory activity and central chemosensory responses in an in vitro brainstem preparation. The "barrier" tadpoles had significantly lower resting gill frequencies and higher lung breath attempts than control tadpoles at the same metamorphic stage. Control tadpoles swam greater distances and spent more time in the upper one third of the aquaria, while barrier tadpoles spent significantly more time at the bottom of the aquaria. There was significantly greater mortality for barrier tadpoles compared to control animals in the earliest and latest metamorphic stages. Mean body weight was significantly greater, and metamorphic rate was reduced in barrier tadpoles. Neither control nor barrier tadpole brainstem preparations demonstrated a gill ventilatory response to CO(2); however, both control and barrier preparations possessed significant lung frequency responses to central CO(2) chemoreceptor stimulation. Bath application of the GABA(A) and glycine receptor antagonists, bicuculline and strychnine, had greater effects on control tadpole gill burst activity and produced a similar large-amplitude bursting pattern in both control and barrier tadpoles, that was insensitive to CO(2) chemoreceptor stimulation. We conclude that development of the respiratory pattern was perturbed by the barrier, but the major effect was on gill ventilation rather than lung ventilation as we had expected.
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Affiliation(s)
- Matthew J Gdovin
- Department of Biology, University of Texas at San Antonio, 6900 North Loop 1604 West, San Antonio, Texas 78249, USA.
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60
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Feldman JL, Del Negro CA. Looking for inspiration: new perspectives on respiratory rhythm. Nat Rev Neurosci 2006; 7:232-42. [PMID: 16495944 PMCID: PMC2819067 DOI: 10.1038/nrn1871] [Citation(s) in RCA: 602] [Impact Index Per Article: 33.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Recent experiments in vivo and in vitro have advanced our understanding of the sites and mechanisms involved in mammalian respiratory rhythm generation. Here we evaluate and interpret the new evidence for two separate brainstem respiratory oscillators and for the essential role of emergent network properties in rhythm generation. Lesion studies suggest that respiratory cell death might explain morbidity and mortality associated with neurodegenerative disorders and ageing.
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Affiliation(s)
- Jack L Feldman
- Department of Neurobiology, David Geffen School of Medicine at the University of California, Los Angeles, BOX 951763, Los Angeles, California 90095-1763, USA.
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61
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Reid SG. Chemoreceptor and pulmonary stretch receptor interactions within amphibian respiratory control systems. Respir Physiol Neurobiol 2006; 154:153-64. [PMID: 16504604 DOI: 10.1016/j.resp.2006.01.016] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2005] [Revised: 01/24/2006] [Accepted: 01/27/2006] [Indexed: 10/25/2022]
Abstract
The hypercapnic drive to breathe in amphibians is generally greater than hypoxic ventilatory drive and a variety of interdependent control systems function to regulate both the hypoxic and hypercapnic ventilatory responses. During exposure to hypercapnic conditions, breathing increases in response to input from central chemoreceptors (sensitive to CSF pH/CO(2) levels) and peripheral chemoreceptors (sensitive to arterial blood O(2) and CO(2)). On the other hand, olfactory CO(2) receptors in the nasal epithelium inhibit breathing during exposure to acute hypercapnia. Further complexity arises from the CO(2)-sensitive nature of the pulmonary stretch receptors (PSR) which provide both tonic (stimulates lung inflation at low lung volumes; deflation at higher volumes) and phasic (generally excitatory) feedback. This review focuses on interactions between the various populations of chemoreceptors and interactions between chemoreceptors and PSR. Differences between various levels of experimental reduction (i.e., in vitro; in situ; in vivo) are highlighted as are the effects of chronic respiratory challenges on acute hypoxic and hypercapnic chemoreflexes.
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Affiliation(s)
- Stephen G Reid
- Centre for the Neurobiology of Stress, Department of Life Sciences, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ont., Canada.
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62
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Feldman JL, Janczewski WA. Point:Counterpoint: The parafacial respiratory group (pFRG)/pre-Botzinger complex (preBotC) is the primary site of respiratory rhythm generation in the mammal. Point: the PFRG is the primary site of respiratory rhythm generation in the mammal. J Appl Physiol (1985) 2006; 100:2096-7; discussion 2097-8, 2103-8. [PMID: 16767809 DOI: 10.1152/japplphysiol.00119.2006] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Affiliation(s)
- Jack L Feldman
- Department of Neurobiology, David Geffen School of Medicine at UCLA, Los Angeles, CA, USA.
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63
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Borday C, Chatonnet F, Thoby-Brisson M, Champagnat J, Fortin G. Neural tube patterning by Krox20 and emergence of a respiratory control. Respir Physiol Neurobiol 2005; 149:63-72. [PMID: 16203212 DOI: 10.1016/j.resp.2005.02.014] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2004] [Revised: 02/16/2005] [Accepted: 02/17/2005] [Indexed: 11/15/2022]
Abstract
Recent data begin to bridge the gap between developmental events controlling hindbrain neural tube regional patterning and the emergence of breathing behaviour in the fetus and its vital adaptive function after birth. In vertebrates, Hox paralogs and Hox-regulating genes orchestrate, in a conserved manner, the transient formation of developmental compartments in the hindbrain, the rhombomeres, in which rhythmic neuronal networks of the brainstem develop. Genetic inactivation of some of these genes in mice leads to pathological breathing at birth pointing to the vital importance of rhombomere 3 and 4 derived territories for maintenance of the breathing frequency. In chick embryo at E7, we investigated neuronal activities generated in neural tube islands deriving from combinations of rhombomeres isolated at embryonic day E1.5. Using a gain of function approach, we reveal a role of the transcription factor Krox20, specifying rhombomeres 3 and 5, in inducing a rhythm generator at the parafacial level of the hindbrain. The developmental genes selecting and regionally coordinating the fate of CNS progenitors may hold further clues to conserved aspects of neuronal network formation and function. However, the most immediate concern is to take advantage of early generated rhythmic activities in the hindbrain to pursue their downstream cellular and molecular targets, for it seems likely that it will be here that rhythmogenic properties will eventually take on a vital role at birth.
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Affiliation(s)
- C Borday
- UPR 2216 Neurobiologie Génétique et Integrative, Institut fédératif de Neurobiologie Alfred Fessard, C.N.R.S., 1, Avenue de la Terrasse, 91198 Gif sur Yvette Cedex, France
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64
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Hedrick MS. Development of respiratory rhythm generation in ectothermic vertebrates. Respir Physiol Neurobiol 2005; 149:29-41. [PMID: 15914099 DOI: 10.1016/j.resp.2005.03.019] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2004] [Revised: 03/17/2005] [Accepted: 03/18/2005] [Indexed: 11/30/2022]
Abstract
Compared with birds and mammals, very little is known about the development and regulation of respiratory rhythm generation in ectothermic vertebrates. The development and regulation of respiratory rhythm generation in ectothermic vertebrates (fish, amphibians and reptiles) should provide insight into the evolution of these mechanisms. One useful model for examining the development of respiratory rhythm generation in ectothermic vertebrates has emerged from studies with the North American bullfrog (Rana catesbeiana). A major advantage of bullfrogs as a comparative model for respiratory rhythm generation is that respiratory output may be measured at all stages of development, both in vivo and in vitro. An emerging view of recent studies in developing bullfrogs is that many of the mechanisms of respiratory rhythm generation are very similar to those seen in birds and mammals. The overall conclusion from these studies is that respiratory rhythm generation during development may be highly conserved during evolution. The development of respiratory rhythm generation in mammals may, therefore, reflect the antecedent mechanisms seen in ectothermic vertebrates. The main focus of this brief review is to discuss recent data on the development of respiratory rhythm generation in ectothermic vertebrates, with particular emphasis on the North American bullfrog (R. catesbeiana) as a model.
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Affiliation(s)
- Michael S Hedrick
- Department of Biological Sciences, California State University, East Bay, Hayward, CA 94542, USA.
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65
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Hedrick MS, Chen AK, Jessop KL. Nitric oxide changes its role as a modulator of respiratory motor activity during development in the bullfrog (Rana catesbeiana). Comp Biochem Physiol A Mol Integr Physiol 2005; 142:231-40. [PMID: 16023875 DOI: 10.1016/j.cbpb.2005.06.004] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2005] [Revised: 06/10/2005] [Accepted: 06/12/2005] [Indexed: 10/25/2022]
Abstract
Nitric oxide (NO) is a unique chemical messenger that has been shown to play a role in the modulation of breathing in amphibians and other vertebrates. In the post-metamorphic tadpole and adult amphibian brainstem, NO, acting via the neuronal isoform of nitric oxide synthase (nNOS), is excitatory to the generation of lung burst activity. In this study, we examine the modulation of breathing by NO during development of the amphibian brainstem. Isolated brainstem preparations from pre-metamorphic and late-stage post-metamorphic tadpoles (Rana catesbeiana) were used to determine the role of NO in modulating central respiratory neural activity. Respiratory neural activity was monitored with suction electrodes recording extracellular activity of cranial nerve rootlets that innervate respiratory musculature. Brainstems were superfused with an artificial cerebrospinal fluid (aCSF) at 20-22 degrees C containing l-nitroarginine (l-NA; 1-10 mM), a non-selective NOS inhibitor. In pre-metamorphic tadpoles, l-NA increased fictive gill ventilation frequency and amplitude, and increased lung burst frequency. By contrast, l-NA applied to the post-metamorphic tadpole brainstem had little effect on fictive buccal activity, but significantly decreased lung burst frequency and the frequency of lung burst episodes. These data indicate that early in development, NO provides a tonic inhibitory input to gill and lung burst activity, but as development progresses, NO provides an excitatory input to lung ventilation. This changing role for NO coincides with the shift in importance in the different respiratory modes during development in amphibians; that is, pre-metamorphic tadpoles rely predominantly on gill ventilation whereas post-metamorphic tadpoles have lost the gills and are obligate air-breathers primarily using lungs for gas exchange. We hypothesize that NO provides a tonic input to the respiratory CPG during development and this changing role reflects the modulatory influence of NO on inhibitory or excitatory modulators or neurotransmitters involved in the generation of respiratory rhythm.
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Affiliation(s)
- Michael S Hedrick
- Department of Biological Sciences, California State University, East Bay, Hayward, CA 94542 USA.
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66
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67
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Vasilakos K, Wilson RJA, Kimura N, Remmers JE. Ancient gill and lung oscillators may generate the respiratory rhythm of frogs and rats. ACTA ACUST UNITED AC 2005; 62:369-85. [PMID: 15551345 DOI: 10.1002/neu.20102] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Though the mechanics of breathing differ fundamentally between amniotes and "lower" vertebrates, homologous rhythm generators may drive air breathing in all lunged vertebrates. In both frogs and rats, two coupled oscillators, one active during the inspiratory (I) phase and the other active during the preinspiratory (PreI) phase, have been hypothesized to generate the respiratory rhythm. We used opioids to uncouple these oscillators. In the intact rat, complete arrest of the external rhythm by opioid-induced suppression of the putative I oscillator, that is, pre-Bötzinger complex (PBC) oscillator, did not arrest the putative PreI oscillator. In the unanesthetized frog, the comparable PreI oscillator, that is, the putative buccal/gill oscillator, was refractory to opioids even though the comparable I oscillator, the putative lung oscillator, was arrested. Studies in en bloc brainstem preparations derived from both juvenile frogs and metamorphic tadpoles confirmed these results and suggested that opioids may play a role in the clustering of lung bursts into episodes. As the frog and rat respiratory circuitry produce functionally equivalent motor outputs during lung inflation, these data argue for a close homology between the frog and rat oscillators. We suggest that the respiratory rhythm of all lunged vertebrates is generated by paired coupled oscillators. These may have originated from the gill and lung oscillators of the earliest air breathers.
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Affiliation(s)
- Konstantinon Vasilakos
- Department of Medicine, University of Calgary, 3330 Hospital Drive, Calgary, Alberta, Canada T2N 4N1
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68
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69
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The Cardiorespiratory System in Tropical Fishes: Structure, Function, and Control. ACTA ACUST UNITED AC 2005. [DOI: 10.1016/s1546-5098(05)21006-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
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70
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Gargaglioni LH, Branco LGS. Nucleus isthmi and control of breathing in amphibians. Respir Physiol Neurobiol 2004; 143:177-86. [PMID: 15519554 DOI: 10.1016/j.resp.2004.03.018] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/04/2004] [Indexed: 10/26/2022]
Abstract
Despite recent advances, the mechanisms of neurorespiratory control in amphibians are far from understood. One of the brainstem structures believed to play a key role in the ventilatory control of anuran amphibians is the nucleus isthmi (NI). This nucleus is a mesencephalic structure located between the roof of the midbrain and the cerebellum, which differentiates during metamorphosis; the period when pulmonary ventilation develops in bullfrogs. It has been recently suggested that the NI acts to inhibit hypoxic and hypercarbic drives in breathing by restricting increases in tidal volume. This data is similar to the influence of two pontine structures of mammals, the locus coeruleus and the nucleus raphe magnus. The putative mediators for this response are glutamate and nitric oxide. Microinjection of kynurenic acid (an ionotropic receptor antagonist of excitatory amino acids) and l-NAME (a non-selective NO synthase inhibitor) elicited increases in the ventilatory response to hypoxia and hypercarbia. This article reviews the available data on the role of the NI in the control of ventilation in amphibians.
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Affiliation(s)
- Luciane H Gargaglioni
- Department of Animal Morphology and Physiology, FCAV-UNESP at Jaboticabal, SP, Brazil.
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Milsom WK, Chatburn J, Zimmer MB. Pontine influences on respiratory control in ectothermic and heterothermic vertebrates. Respir Physiol Neurobiol 2004; 143:263-80. [PMID: 15519560 DOI: 10.1016/j.resp.2004.05.008] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/04/2004] [Indexed: 11/24/2022]
Abstract
Respiratory rhythm generators appear both evolutionarily and developmentally as paired segmental rhythm generators in the reticular formation, associated with the motor nuclei of cranial nerves V, VII, IX, X, and XII. Those associated with the Vth and VIIth motor nuclei are "pontine" in origin and in fishes that employ a buccal suction/force pump for breathing the primary pair of respiratory rhythm generators are associated with the trigeminal nuclei. In amphibians, while the basic respiratory pump remains the same, the dominant site of respiratory rhythm generation has been assumed by the facial, glossopharyngeal and vagal motor nuclei. In reptiles, birds and mammals, in general there is a switch to an aspiration pump driven by thoraco-lumbar muscles innervated by spinal nerves. In these groups, the critical sites necessary for respiratory rhythmogenesis now sit near the ponto-medullary border, in the parafacial region (which may underlie expiratory-dominated, intercostal-abdominal breathing in non-mammalian tetrapods) and in a more caudal region, the preBotzinger complex (which may underlie inspiratory-dominated diaphragmatic breathing in mammals).
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Affiliation(s)
- William K Milsom
- Department of Zoology, University of British Columbia, Vancouver, BC, Canada V6T 1Z4.
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72
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Reid SG, West NH. Modulation of breathing by phasic pulmonary stretch receptor feedback in an amphibian, Bufo marinus. Respir Physiol Neurobiol 2004; 142:165-83. [PMID: 15450478 DOI: 10.1016/j.resp.2004.06.003] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/07/2004] [Indexed: 10/26/2022]
Abstract
This study examined the role of phasic pulmonary stretch receptor (PSR) feedback in ventilatory control, breath clustering and breath timing in decerebrate, paralysed and artificially-ventilated cane toads (Bufo marinus) under conditions designed to minimise tonic PSR feedback. Fictive breathing was recorded as trigeminal motor output to the buccal musculature. Artificial tidal ventilation, with hypercarbic gas mixtures, was either continuous or activated by the fictive breaths and was manipulated to provide differing amounts/patterns of phasic PSR feedback. The results demonstrate that increased amounts of phasic PSR feedback increase overall breathing frequency. Within multi-breath episodes there was an increase in the instantaneous breathing frequency during the later stages of the episode. The temporal relationship between a fictive breath and lung inflation influenced the duration of the pause between fictive breaths. The data indicate that phasic PSR feedback stimulates breathing by enhancing the occurrence of breathing episodes in this species but does not appear to modify the instantaneous breathing frequency during an episode.
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Affiliation(s)
- Stephen G Reid
- Department of Life Sciences, and the Centre for the Neurobiology of Stress, University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ont., Canada M1C 1A4.
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73
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Duffin J. Functional organization of respiratory neurones: a brief review of current questions and speculations. Exp Physiol 2004; 89:517-29. [PMID: 15258123 DOI: 10.1113/expphysiol.2004.028027] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
This article presents a short overview of current knowledge about the medullary respiratory neurones and the generation of breathing rhythm. The background respiratory neurophysiology of the medulla and pons is briefly reviewed, with some current ideas about the organization of the pontine-medullary respiratory control system and its development. Questions and speculations about the organization and generation of respiratory rhythm are included, with a view to stimulating experiments to provide answers.
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Affiliation(s)
- James Duffin
- Department of Physiology, University of Toronto, Medical Sciences Building, Room 3326, 1 King's College Circle, Toronto, Ontario, M5S 1A8, Canada.
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74
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Taylor BE, Harris MB, Leiter JC, Gdovin MJ. Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs. Am J Physiol Regul Integr Comp Physiol 2004; 285:R1461-72. [PMID: 14615406 DOI: 10.1152/ajpregu.00256.2003] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.
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Affiliation(s)
- Barbara E Taylor
- Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire 03756-0001, USA
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75
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Abstract
Respiratory neuronal networks in vertebrates appear to be able to generate a variety of rhythmic patterns in vivo, leading to the biological diversity of eupneic patterns as well as to life-threatening dyspneic patterns. Eupnea is best viewed as the collection of respiratory strategies preventing potential dyspneas, the major (and perhaps the only) criterion for a definition being that eupnea allows survival. Specific criteria can then be derived from the physiological identification of neurobiological mechanisms underlying identified dyspneic patterns, by exaggerating (pro-dyspneic mechanisms) or suppressing them (anti-dyspneic mechanisms). Because eupnea is vital, and one of the major targets of evolutionary pressure, identification of dyspnea-related neuronal systems seems to be important to understand the normal biological organization of the respiratory neuronal system.
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Affiliation(s)
- Jean Champagnat
- U.P.R. 2216, Neurobiologie Génétique et Intégrative, IFR 2118 Institut de Neurobiologie Alfred Fessard, C.N.R.S., 1, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France.
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76
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Winmill RE, Hedrick MS. Developmental changes in the modulation of respiratory rhythm generation by extracellular K+ in the isolated bullfrog brainstem. JOURNAL OF NEUROBIOLOGY 2003; 55:278-87. [PMID: 12717698 DOI: 10.1002/neu.10212] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
This study tested the hypothesis that voltage-dependent, respiratory-related activity in vitro, inferred from changes in [K(+)](o), changes during development in the amphibian brainstem. Respiratory-related neural activity was recorded from cranial nerve roots in isolated brainstem-spinal cord preparations from 7 premetamorphic tadpoles and 10 adults. Changes in fictive gill/lung activity in tadpoles and buccal/lung activity in adults were examined during superfusion with artificial CSF (aCSF) with [K(+)](o) ranging from 1 to 12 mM (4 mM control). In tadpoles, both fictive gill burst frequency (f(gill)) and lung burst frequency (f(lung)) were significantly dependent upon [K(+)](o) (r(2) > 0.75; p < 0.001) from 1 to 10 mM K(+), and there was a strong correlation between f(gill) and f(lung) (r(2) = 0.65; p < 0.001). When [K(+)](o) was raised to 12 mM, there was a reversible abolition of fictive breathing. In adults, fictive buccal frequency (f(buccal)), was significantly dependent on [K(+)](o) (r(2) = 0.47; p < 0.001), but [K(+)](o) had no effect on f(lung) (p > 0.2), and there was no significant correlation between f(buccal) and f(lung). These data suggest that the neural networks driving gill and lung burst activity in tadpoles may be strongly voltage modulated. In adults, buccal activity, the proposed remnant of gill ventilation in adults, also appears to be voltage dependent, but is not correlated with lung burst activity. These results suggest that lung burst activity in amphibians may shift from a "voltage-dependent" state to a "voltage-independent" state during development. This is consistent with the hypothesis that the fundamental mechanisms generating respiratory rhythm in the amphibian brainstem change during development. We hypothesize that lung respiratory rhythm generation in amphibians undergoes a developmental change from a pacemaker to network-driven process.
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Affiliation(s)
- Rachel E Winmill
- Department of Biological Sciences, California State University, Hayward, Hayward, California 94542, USA
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77
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Gargaglioni LH, Branco LGS. Role of glutamate in the nucleus isthmi on the hypoxia- and hypercarbia-induced hyperventilation of toads. Respir Physiol Neurobiol 2003; 135:47-58. [PMID: 12706065 DOI: 10.1016/s1569-9048(03)00037-5] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain that has been reported to participate in CO(2) chemoreception and in the ventilatory response to hypoxia. In the present study, we assessed the role of glutamatergic transmission inside the NI on the hypoxic and hypercarbic drive to breathing. We compared the respiratory responses to 7 and 5% inspired O(2) and 3% inspired CO(2) after microinjecting 10 nmol/100 nl of kynurenic acid (an antagonist of L-glutamate receptors) into the NI of toads (Bufo paracnemis). Kynurenic acid had no effect under resting conditions. Both hypoxia and hypercarbia elicited an increase in ventilation in all groups, with hypoxia acting on tidal volume (V(T)) and hypercarbia on frequency (f). The microinjection of kynurenic acid into the NI caused an increased ventilatory response to hypoxia and hypercarbia due to a higher V(T). We conclude that glutamatergic transmission in the NI has an inhibitory effect when the respiratory drive is high, acting on V(T).
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Affiliation(s)
- Luciane H Gargaglioni
- Department of Morphology, Estomatology and Physiology, Faculdade de Odontologia de Ribeirão Preto, Dental School of Ribeirão Preto, University of São Paulo, 14040-904, SP, Ribeirão Preto, Brazil
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78
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Mellen NM, Janczewski WA, Bocchiaro CM, Feldman JL. Opioid-induced quantal slowing reveals dual networks for respiratory rhythm generation. Neuron 2003; 37:821-6. [PMID: 12628172 PMCID: PMC3210017 DOI: 10.1016/s0896-6273(03)00092-8] [Citation(s) in RCA: 270] [Impact Index Per Article: 12.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Current consensus holds that a single medullary network generates respiratory rhythm in mammals. Pre-Bötzinger Complex inspiratory (I) neurons, isolated in transverse slices, and preinspiratory (pre-I) neurons, found only in more intact en bloc preparations and in vivo, are each proposed as necessary for rhythm generation. Opioids slow I, but not pre-I, neuronal burst periods. In slices, opioids gradually lengthened respiratory periods, whereas in more intact preparations, periods jumped nondeterministically to integer multiples of the control period (quantal slowing). These findings suggest that opioid-induced quantal slowing results from transmission failure of rhythmic drive from pre-I neurons to preBötC I networks, depressed below threshold for spontaneous rhythmic activity. Thus, both I (in the slice), and pre-I neurons are sufficient for respiratory rhythmogenesis.
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Affiliation(s)
- Nicholas M Mellen
- Department of Neurobiology, University of California, Los Angeles, P.O. Box 951763, Los Angeles, CA 90095, USA.
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79
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Hedrick MS, Winmill RE. Excitatory and inhibitory effects of tricaine (MS-222) on fictive breathing in isolated bullfrog brain stem. Am J Physiol Regul Integr Comp Physiol 2003; 284:R405-12. [PMID: 12414435 DOI: 10.1152/ajpregu.00418.2002] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
This study examined the direct effects of tricaine methanesulfonate (MS-222), a sodium-channel blocking local anesthetic, on respiratory motor output using an in vitro brain stem preparation of adult North American bullfrogs (Rana catesbeiana). Bullfrogs were anesthetized with halothane, and the brain stem was removed and superfused with artificial cerebrospinal fluid containing MS-222 at concentrations ranging from 0.1 to 1,000 micro M. At the lowest concentration of MS-222, respiratory frequency (fR) increased significantly (P < 0.05), but at higher concentrations, fR progressively decreased and was abolished in all preparations at 1,000 micro M (P < 0.01). Respiratory burst amplitude and burst duration were not affected by MS-222. The frequency of nonrespiratory neural activity did not significantly change with the addition of MS-222 below 1,000 micro M. These data indicate that MS-222 has a significant, direct effect on respiratory motor output from the central nervous system, producing both excitation and inhibition of fictive breathing. The results are consistent with other studies demonstrating that low concentrations of anesthetics generally cause excitation followed by depression at higher concentrations. Although the mechanisms underlying the excitatory effects of MS-222 in this study are unclear, they may include increased excitatory neurotransmission and/or disinhibition of inputs to the respiratory central pattern generator.
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Affiliation(s)
- Michael S Hedrick
- Department of Biological Sciences, California State University, Hayward, California 94542, USA.
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80
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Abstract
Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.
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Affiliation(s)
- Jack L. Feldman
- Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California 90095-1763
| | - Gordon S. Mitchell
- Department of Comparative Biosciences, University of Wisconsin, Madison, Wisconsin 53706
| | - Eugene E. Nattie
- Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire 03756-0001
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81
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Johnson SM, Wilkerson JER, Wenninger MR, Henderson DR, Mitchell GS. Role of synaptic inhibition in turtle respiratory rhythm generation. J Physiol 2002; 544:253-65. [PMID: 12356896 PMCID: PMC2290555 DOI: 10.1113/jphysiol.2002.019687] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
In vitro brainstem and brainstem-spinal cord preparations were used to determine the role of synaptic inhibition in respiratory rhythm generation in adult turtles. Bath application of bicuculline (a GABA(A) receptor antagonist) to brainstems increased hypoglossal burst frequency and amplitude, with peak discharge shifted towards the burst onset. Strychnine (a glycine receptor antagonist) increased amplitude and frequency, and decreased burst duration, but only at relatively high concentrations (10-100 microM). Rhythmic activity persisted during combined bicuculline and strychnine application (50 microM each) with increased amplitude and frequency, decreased burst duration, and a rapid onset-decrementing burst pattern. The bicuculline-strychnine rhythm frequency decreased during mu-opioid receptor activation or decreased bath P(C)(O(2)). Synaptic inhibition blockade in the brainstem of brainstem-spinal cord preparations increased burst amplitude in spinal expiratory (pectoralis) nerves and nearly abolished spinal inspiratory activity (serratus nerves), suggesting that medullary expiratory motoneurons were mainly active. Under conditions of synaptic inhibition blockade in vitro, the turtle respiratory network was able to produce a rhythm that was sensitive to characteristic respiratory stimuli, perhaps via an expiratory (rather than inspiratory) pacemaker-driven mechanism. Thus, these data indicate that the adult turtle respiratory rhythm generator has the potential to operate in a pacemaker-driven manner.
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Affiliation(s)
- Stephen M Johnson
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison 53706, USA.
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82
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Perry SF, Wilson RJ, Straus C, Harris MB, Remmers JE. Which came first, the lung or the breath? Comp Biochem Physiol A Mol Integr Physiol 2001; 129:37-47. [PMID: 11369532 DOI: 10.1016/s1095-6433(01)00304-x] [Citation(s) in RCA: 87] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Lungs are the characteristic air-filled organs (AO) of the Polypteriformes, lungfish and tetrapods, whereas the swimbladder is ancestral in all other bony fish. Lungs are paired ventral derivatives of the pharynx posterior to the gills. Their respiratory blood supply is the sixth branchial artery and the venous outflow enters the heart separately from systemic and portal blood at the sinus venosus (Polypteriformes) or the atrium (lungfish), or is delivered to a separate left atrium (tetrapods). The swimbladder, on the other hand, is unpaired, and arises dorsally from the posterior pharynx. It is employed in breathing in Ginglymodi (gars), Halecomorphi (bowfin) and in basal teleosts. In most cases, its respiratory blood supply is homologous to that of the lung, but the vein drains to the cardinal veins. Separate intercardiac channels for oxygenated and deoxygenated blood are lacking. The question of the homology of lungs and swimbladders and of breathing mechanisms remains open. On the whole, air ventilatory mechanisms in the actinopterygian lineage are similar among different groups, including Polypteriformes, but are distinct from those of lungfish and tetrapods. However, there is extreme variation within this apparent dichotomy. Furthermore, the possible separate origin of air breathing in actinopterygian and 'sarcopterygian' lines is in conflict with the postulated much more ancient origin of vertebrate air-breathing organs. New studies on the isolated brainstem preparation of the gar (Lepisosteus osseus) show a pattern of efferent activity associated with a glottal opening that is remarkably similar to that seen in the in-vitro brainstem preparation of frogs and tadpoles. Given the complete lack of evidence for AO in chondrichthyans, and the isolated position of placoderms for which buoyancy organs of uncertain homology have been demonstrated, it is likely that homologous pharyngeal AO arose in the ancestors of early bony fish, and was pre-dated by behavioral mechanisms for surface (water) breathing. The primitive AO may have been the posterior gill pouches or even the modified gills themselves, served by the sixth branchial artery. Further development of the dorsal part may have led to the respiratory swimbladder, whereas the paired ventral parts evolved into lungs.
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Affiliation(s)
- S F Perry
- Institut für Zoologie, Universität Bonn, Poppelsdorfer Schloss, 53115, Bonn, Germany.
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