The control of breathing in fishes - historical perspectives and the path ahead

The study of breathing in fishes has featured prominently in Journal of Experimental Biology (JEB), particularly during the latter half of the past century. Indeed, many of the seminal discoveries in this important sub-field of comparative respiratory physiology were reported first in JEB. The period spanning 1960-1990 (the 'golden age of comparative respiratory physiology') witnessed intense innovation in the development of methods to study the control of breathing. Many of the guiding principles of piscine ventilatory control originated during this period, including our understanding of the dominance of O2 as the driver of ventilation in fish. However, a critical issue - the identity of the peripheral O2 chemoreceptors - remained unanswered until methods for cell isolation, culture and patch-clamp recording established that gill neuroepithelial cells (NECs) respond to hypoxia in vitro. Yet, the role of the NECs and other putative peripheral or central chemoreceptors in the control of ventilation in vivo remains poorly understood. Further progress will be driven by the implementation of genetic tools, most of which can be used in zebrafish (Danio rerio). These tools include CRISPR/Cas9 for selective gene knockout, and Tol2 systems for transgenesis, the latter of which enables optogenetic stimulation of cellular pathways, cellular ablation and in vivo cell-specific biosensing. Using these methods, the next period of discovery will see the identification of the peripheral sensory pathways that initiate ventilatory responses, and will elucidate the nature of their integration within the central nervous system and their link to the efferent motor neurons that control breathing.

Evolution of vertebrate respiratory central rhythm generators

Tracing the evolution of the central rhythm generators associated with ventilation in vertebrates is hindered by a lack of information surrounding key transitions. To begin with, central rhythm generation has been studied in detail in only a few species from four vertebrate groups, lamprey, anuran amphibians, turtles, and mammals (primarily rodents). Secondly, there is a lack of information regarding the transition from water breathing fish to air breathing amniotes (reptiles, birds, and mammals). Specifically, the respiratory rhythm generators of fish appear to be single oscillators capable of generating both phases of the respiratory cycle (expansion and compression) and projecting to motoneurons in cranial nerves innervating bucco-pharyngeal muscles. In the amniotes we find oscillators capable of independently generating separate phases of the respiratory cycle (expiration and inspiration) and projecting to pre-motoneurons in the ventrolateral medulla that in turn project to spinal motoneurons innervating thoracic and abdominal muscles (reptiles, birds, and mammals). Studies of the one group of amphibians that lie at this transition (the anurans), raise intriguing possibilities but, for a variety of reasons that we explore, also raise unanswered questions. In this review we summarize what is known about the rhythm generating circuits associated with breathing that arise from the different rhombomeric segments in each of the different vertebrate classes. Assuming oscillating circuits form in every pair of rhombomeres in every vertebrate during development, we trace what appears to be the evolutionary fate of each and highlight the questions that remain to be answered to properly understand the evolutionary transitions in vertebrate central respiratory rhythm generation.

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.

Central control of air breathing in fishes

The diversity of sites and surfaces that are utilized for gas transfer from air to blood in fish is remarkable. While a few species do utilize their gills for gas exchange in air, this is a rare occurrence and most air-breathing fish utilize other surfaces including air-breathing organs and lungs. At present almost nothing is known about the central sites that initiate and regulate air breathing although hypotheses can be put forward based on our rudimentary understanding of the sites involved in water breathing in lampreys and teleost fishes, and those involved in air breathing in pre-metamorphic anuran ampibians. The pumps involved in producing both water and air breathing in fishes are highly conserved, a buccal pump, assisted by pharyngeal and/or parabranchial/opercular pumps, produce both forms of ventilation. What varies between species are the manner in which air breaths are produced (in two versus four phases), and the 'valving' involved in producing water flow over the gills versus air flow in and out of air-breathing organs. The latter suggests that a major step in the evolution of air breathing was the evolution of the mechanisms that control the flow of the respiratory medium. The neural matrix that underlies the co-ordination of the pump and the valving events remains enigmatic and in much need of further research.

Isolated adult turtle brainstems exhibit central hypoxic chemosensitivity

During hypoxia, red-eared slider turtles increase ventilation and decrease episodic breathing, but whether these responses are due to central mechanisms is not known. To test this question, isolated adult turtle brainstems were exposed to 240 min of hypoxic solution (bath PO₂ = 32.6 ± 1.2 mmHg) and spontaneous respiratory-related motor bursts (respiratory event) were recorded on hypoglossal nerve roots. During hypoxia, burst frequency increased during the first 15 min, and then decreased during the remaining 35-240 min of hypoxia. Burst amplitude was maintained for 120 min, but then decreased during the last 120 min. The number of bursts/respiratory event decreased within 30 min and remained decreased. Pretreatment with either prazosin (α₁-adrenergic antagonist) or MDL7222 (5-HT₃ antagonist) blocked the hypoxia-induced short-term increase and the longer duration decrease in burst frequency. MDL7222, but not prazosin, blocked the hypoxia-induced decrease in bursts/respiratory event. Thus, during bath hypoxia, isolated turtle brainstems continued to produce respiratory motor output, but the frequency and pattern were altered in a manner that required endogenous α₁-adrenergic and serotonin 5-HT₃ receptor activation. This is the first example of isolated reptile brainstems exhibiting central hypoxic chemosensitivity similar to other vertebrate species.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

Environmentally induced return to juvenile-like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

KEY POINTS

The degree to which developmental programmes or environmental signals determine physiological phenotypes remains a major question in physiology. Vertebrates change environments during development, confounding interpretation of the degree to which development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) produces phenotypes. Tadpoles mainly breathe water for gas exchange and frogs may breathe water or air depending on their environment and are, therefore, exemplary models to differentiate the degree to which life-stage vs. environmental context drives developmental phenotypes associated with neural control of lung breathing. Using isolated brainstem preparations and patch clamp electrophysiology, we demonstrate that adult bullfrogs acclimatized to water-breathing conditions do not exhibit CO₂ and O₂ chemosensitivity of lung breathing, similar to water-breathing tadpoles. Our results establish that phenotypes associated with developmental stage may arise from plasticity per se and suggest that a developmental trajectory coinciding with environmental change obscures origins of stage-dependent physiological phenotypes by masking plasticity.

ABSTRACT

An unanswered question in developmental physiology is to what extent does the environment vs. a genetic programme produce phenotypes? Developing animals inhabit different environments and switch from one to another. Thus a developmental time course overlapping with environmental change confounds interpretations as to whether development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) generates phenotypes. Tadpoles of the American bullfrog, Lithobates catesbeianus, breathe water at early life-stages and minimally use lungs for gas exchange. As adults, bullfrogs rely on lungs for gas exchange, but spend months per year in ice-covered ponds without lung breathing. Aquatic submergence, therefore, removes environmental pressures requiring lung breathing and enables separation of adulthood from environmental factors associated with adulthood that necessitate control of lung ventilation. To test the hypothesis that postmetamorphic respiratory control phenotypes arise through permanent developmental changes vs. reversible environmental signals, we measured respiratory-related nerve discharge in isolated brainstem preparations and action potential firing from CO₂ -sensitive neurons in bullfrogs acclimatized to semi-terrestrial (air-breathing) and aquatic-overwintering (no air-breathing) habitats. We found that aquatic overwintering significantly reduced neuroventilatory responses to CO₂ and O₂ involved in lung breathing. Strikingly, this gas sensitivity profile reflects that of water-breathing tadpoles. We further demonstrated that aquatic overwintering reduced CO₂ -induced firing responses of chemosensitive neurons. In contrast, respiratory rhythm generating processes remained adult-like after submergence. Our results establish that phenotypes associated with life-stage can arise from phenotypic plasticity per se. This provides evidence that developmental time courses coinciding with environmental changes obscure interpretations regarding origins of stage-dependent physiological phenotypes by masking plasticity.