Evolution of central respiratory chemoreception: a new twist on an old story

Paraneuronal pseudobranchial neurosecretory system in tank goby Glossogobius giuris with special reference to novel neurohaemal contact complex

Various putative oxygen chemosensory cells are reported to be present throughout the vertebrate body performing pivotal roles in respiration by initiating responses during acute hypoxia. Since air-breathing fishes often are exposed to the oxygen-deficient milieu, in such conditions various chemosensory cells operate in an orchestrated fashion. The Pseudobranchial neurosecretory system (PSNS) a newly discovered system, is one of these. It has been placed in the category of "Diffuse NE systems (DNES)". It is found in all the catfish species and in some other non-catfish group of teleosts. In catfishes, it is present in close association with the carotid labyrinth- a chemosensory structure, known in fish and amphibians. The presence of this system in Glossogobius giuris, in association with the pseudobranch, a structure considered to be precursor of carotid labyrinth, is a significant finding. In an attempt to study the structure and organization of the pseudobranchial neurosecretory system in a non-catfish species of teleost, the present investigation was undertaken on a goby G. giuris. The histological observations, using a neurosecretion-specific stain, revealed the presence of this system in G. giuris. The findings are discussed in the light of the association of PSNS with pseudobranch and the type of "neurohaemal contact complex" formed between this neurosecretory system and the elements of the circulatory system.

Buccal rhythmogenesis and CO2 sensitivity in Lithobates catesbeianus tadpole brainstems across metamorphosis

Bullfrog tadpoles ventilate both the buccal cavity and lung. In isolated brainstems, the midbrain/pons influences CO₂ responsiveness and timing of lung ventilatory bursting, depending on larval development. However, little is known about midbrain/pons influences on buccal burst patterns. As such, we investigated how removal of this region affects buccal burst shape and CO₂ responsiveness across development. We measured facial nerve activity in brainstems isolated from tadpoles during early and late developmental stages, under normal and elevated levels of CO₂. Brainstems were either left intact or transected by removing the midbrain/pons. In late stage preparations, buccal burst pattern differed between intact and reduced preparations, and bursts were responsive to elevated CO₂ in these reduced preparations. These results suggest the midbrain/pons affects tadpole buccal burst pattern and CO₂ responsiveness, perhaps similar to its influences on lung ventilation.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Learning to Air-Breathe: The First Steps

Air-breathing in vertebrates has evolved many times among the bony fish while in water. Its appearance has had a fundamental impact on the regulation of ventilation and acid-base status. We review the physico-chemical constraints imposed by water and air, place the extant air-breathing fish into this framework, and show how that the advantages of combining control of ventilation and acid-base status are only available to the most obligate of air-breathing fish, thus highlighting promising avenues for research.

Developmental Maturation of Functional Coupling Between Ventilatory Oscillators in the American Bullfrog

Many vital motor behaviors - including locomotion, swallowing, and breathing - appear to be dependent upon the activity of and coordination between multiple endogenously rhythmogenic nuclei, or neural oscillators. Much as the functional development of sensory circuits is shaped during maturation, we hypothesized that coordination of oscillators involved in motor control may likewise be maturation-dependent, i.e., coupling and coordination between oscillators change over development. We tested this hypothesis using the bullfrog isolated brainstem preparation to study the metamorphic transition of ventilatory motor patterns from early rhythmic buccal (water) ventilation in the tadpole to the mature pattern of rhythmic buccal and lung (air) ventilation in the adult. Spatially distinct oscillators drive buccal and lung bursts in the isolated brainstem; we found these oscillators to be active but functionally uncoupled in the tadpole. Over the course of metamorphosis, the rhythms produced by the buccal and lung oscillators become increasingly tightly coordinated. These changes parallel the progression of structural and behavioral changes in the animal, with adult levels of coupling arising by the metamorphic stage (forelimb eruption). These findings suggest that oscillator coupling undergoes a maturation process similar to the refinement of sensory circuits over development.

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

Evolution of lung breathing from a lungless primitive vertebrate

Air breathing was critical to the terrestrial radiation and evolution of tetrapods and arose in fish. The vertebrate lung originated from a progenitor structure present in primitive boney fish. The origin of the neural substrates, which are sensitive to metabolically produced CO2 and which rhythmically activate respiratory muscles to match lung ventilation to metabolic demand, is enigmatic. We have found that a distinct periodic centrally generated rhythm, described as "cough" and occurring in lamprey in vivo and in vitro, is modulated by central sensitivity to CO2. This suggests that elements critical for the evolution of breathing in tetrapods, were present in the most basal vertebrate ancestors prior to the evolution of the lung. We propose that the evolution of breathing in all vertebrates occurred through exaptations derived from these critical basal elements.

The phylogeny of central chemoreception

Respiratory chemoreceptors responsive to changes in CO(2)/H(+) appear to be present in all vertebrates from fish to birds and mammals. They appear to have arisen first in the periphery sensitive to the external environment. Thus, in most fish CO(2)/H(+) chemoreceptors reside primarily in the gills and respond to changes in aquatic rather than arterial P(CO)₂ . In the air-breathing tetrapods (amphibians, mammals, reptiles and birds), the branchial arches regress developmentally and the derivatives of the branchial arteries are now exclusively internal. The receptors associated with these arteries now sense only arterial (not environmental) P(CO)₂/pH . Central CO(2)/H(+) chemoreception also appears to have arisen with the advent of air breathing, presumably as a second line of defense. These receptors may have arisen multiple times in association with several (but not all) of the independent origins of air breathing in fishes. There is strong evidence for multiple central sites of CO(2)/H(+) sensing, at least in amphibians and mammals, suggesting that it may not only have originated multiple times in different species but also multiple times within a single species.

Sensory innervation of the Gills: O2-sensitive chemoreceptors and mechanoreceptors

Physical characteristics of water (O(2) solubility and capacitance) dictate that cardiovascular and ventilatory performance be controlled primarily by the need for oxygen uptake rather than carbon dioxide excretion, making O(2) receptors more important in fish than in terrestrial vertebrates. An understanding of the anatomy and physiology of mechanoreception and O(2) chemoreception in fishes is important, because water breathing is the primitive template upon which the forces of evolution have modified into the various cardioventilatory modalities we see in extant terrestrial species. Key to these changes are the O(2)-sensitive chemoreceptors and mechanoreceptors, their mechanisms and central pathways.

Amphibians as Animal Models for Laboratory Research in Physiology

The concept of animal models is well honored, and amphibians have played a prominent part in the success of using key species to discover new information about all animals. As animal models, amphibians offer several advantages that include a well-understood basic physiology, a taxonomic diversity well suited to comparative studies, tolerance to temperature and oxygen variation, and a greater similarity to humans than many other currently popular animal models. Amphibians now account for approximately 1/4 to 1/3 of lower vertebrate and invertebrate research, and this proportion is especially true in physiological research, as evident from the high profile of amphibians as animal models in Nobel Prize research. Currently, amphibians play prominent roles in research in the physiology of musculoskeletal, cardiovascular, renal, respiratory, reproductive, and sensory systems. Amphibians are also used extensively in physiological studies aimed at generating new insights in evolutionary biology, especially in the investigation of the evolution of air breathing and terrestriality. Environmental physiology also utilizes amphibians, ranging from studies of cryoprotectants for tissue preservation to physiological reactions to hypergravity and space exploration. Amphibians are also playing a key role in studies of environmental endocrine disruptors that are having disproportionately large effects on amphibian populations and where specific species can serve as sentinel species for environmental pollution. Finally, amphibian genera such as Xenopus, a genus relatively well understood metabolically and physiologically, will continue to contribute increasingly in this new era of systems biology and "X-omics."

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models

Carbon dioxide (CO(2)) excretion and acid-base regulation in fish are linked, as in other animals, though the reversible reactions of CO(2) and the acid-base equivalents H(+) and HCO(3)(-): CO(2)+H(2)O<-->H(+)+HCO(3)(-). These relationships offer two potential routes through which acid-base disturbances may be regulated. Respiratory compensation involves manipulation of ventilation so as to retain CO(2) or enhance CO(2) loss, with the concomitant readjustment of the CO(2) reaction equilibrium and the resultant changes in H(+) levels. In metabolic compensation, rates of direct H(+) and HCO(3)(-) exchange with the environment are manipulated to achieve the required regulation of pH; in this case, hydration of CO(2) yields the necessary H(+) and HCO(3)(-) for exchange. Because ventilation in fish is keyed primarily to the demands of extracting O(2) from a medium of low O(2) content, the capacity to utilize respiratory compensation of acid-base disturbances is limited and metabolic compensation across the gill is the primary mechanism for re-establishing pH balance. The contribution of branchial acid-base exchanges to pH compensation is widely recognized, but the molecular mechanisms underlying these exchanges remain unclear. The relatively recent application of molecular approaches to this question is generating data, sometimes conflicting, from which models of branchial acid-base exchange are gradually emerging. The critical importance of the gill in acid-base compensation in fish, however, has made it easy to overlook other potential contributors. Recently, attention has been focused on the role of the kidney and particularly the molecular mechanisms responsible for HCO(3)(-) reabsorption. It is becoming apparent that, at least in freshwater fish, the responses of the kidney are both flexible and essential to complement the role of the gill in metabolic compensation. Finally, while respiratory compensation in fish is usually discounted, the few studies that have thoroughly characterized ventilatory responses during acid-base disturbances in fish suggest that breathing may, in fact, be adjusted in response to pH imbalances. How this is accomplished and the role it plays in re-establishing acid-base balance are questions that remain to be answered.

Ancient gill and lung oscillators may generate the respiratory rhythm of frogs and rats

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.

Monitoring carbon dioxide in critical care: the newest vital sign?

Carbon dioxide (CO(2)) monitoring can yield substantial information about cardiac and pulmonary function. Because capnography is not complicated, it is relatively easy to apply in a wide variety of clinical settings. A new area of CO(2) measurement is sublingual CO(2). Although outcomes data are as yet unavailable for this technology, its ease of use makes it an attractive monitoring tool to assess severity of illness and predict patients' responses to therapy. This article describes the pathophysiology and clinical applications of the technologies and describes why they may well be "the newest vital signs."

Genesis and control of the respiratory rhythm in adult mammals

The neural mechanisms responsible for respiratory rhythmogenesis in mammals were studied first in vivo in adults and subsequently in vitro in neonates. In vitro data have suggested that the pacemaker neurons are the kernel of the respiratory network. These data are reviewed, and their relevance to adults is discussed.

Central respiratory activity of the tadpole in vitro brain stem is modulated diversely by nitric oxide

Nitric oxide (NO) is a potent central neuromodulator of respiration, yet its scope and site of action are unclear. We used 7-nitroindazole (7-NI), a selective inhibitor of endogenous neuronal NO synthesis, to investigate the neurogenesis of respiration in larval bullfrog (Rana catesbeiana) isolated brain stems. 7-NI treatment (0.0625-0.75 mM) increased the specific frequency of buccal ventilation (BV) events, indicating influence on BV central rhythm generators (CRGs). The drug reduced occurrence, altered burst shape, and disrupted clustering of lung ventilation (LV) events, without altering their specific frequency. LV burst occurrence and clustering also differed between pH conditions. We conclude that NO has diverse effects on respiratory rhythmogenesis, being necessary for the expression of respiratory rhythms, inhibiting the frequency of BV CRG, and affecting both shape and clustering of LV bursts through conditional modulation of LV CRG. We confirm central chemosensitivity in these preparations and demonstrate chemomodulation of LV burst clustering and occurrence but not specific frequency. Results support distinct oscillators underlying LV and BV CRGs.