Ontogenesis of oxygen chemoreception in aquatic vertebrates

Establishment and identification of the gill cell line from the blunt snout bream (Megalobrama amblycephala) and its application in studying gill remodeling under hypoxia

To probe the mechanisms of gill remodeling in blunt snout bream under hypoxic conditions, we selected gill tissue for primary cell culture to establish and characterize the first blunt snout bream gill cell line, named MAG. The gill cells were efficiently passaged in M199 medium supplemented with 8% antibiotics and 15% fetal bovine serum at 28 °C, exhibiting primarily an epithelial-fibroblast mixed type. Additionally, the MAG cells (17th generation) were subjected to four experimental conditions-normoxia, hypoxia 12 h, hypoxia 24 h, and reoxygenation 24 h (R24h)-to evaluate the effects of hypoxia and reoxygenation on MAG cells during gill remodeling. We found that the MAG cell morphology underwent shrinkage and mitochondrial potential gradually lost, even leading to gradual apoptosis with increasing hypoxia duration and increased reactive oxygen species (ROS) activity. Upon reoxygenation, MAG cells gradually regain cellular homeostasis, accompanied by a decrease in ROS activity. Analysis of superoxide dismutase (SOD), glutathione (GSH), lactate dehydrogenase (LDH), catalase (CAT), anti-superoxide anion, and other enzyme activities revealed enhanced antioxidant enzyme activity in MAG cells during hypoxia, aiding in adapting to hypoxic stress and preserving cell morphology. After reoxygenation, the cells gradually returned to normoxic levels. Our findings underscore the MAG cells can be used to study hypoxic cell apoptosis during gill remodeling. Therefore, the MAG cell line will serve as a vital in vitro model for exploring gill remodeling in blunt snout bream under hypoxia.

Fish gill chemosensing: knowledge gaps and inconsistencies

In this review, we explore the inconsistencies in the data and gaps in our knowledge that exist in what is currently known regarding gill chemosensors which drive the cardiorespiratory reflexes in fish. Although putative serotonergic neuroepithelial cells (NEC) dominate the literature, it is clear that other neurotransmitters are involved (adrenaline, noradrenaline, acetylcholine, purines, and dopamine). And although we assume that these agents act on neurons synapsing with the NECs or in the afferent or efferent limbs of the paths between chemosensors and central integration sites, this process remains elusive and may explain current discrepancies or species differences in the literature. To date it has been impossible to link the distribution of NECs to species sensitivity to different stimuli or fish lifestyles and while the gills have been shown to be the primary sensing site for respiratory gases, the location (gills, oro-branchial cavity or elsewhere) and orientation (external/water or internal/blood sensing) of the NECs are highly variable between species of water and air breathing fish. Much of what has been described so far comes from studies of hypoxic responses in fish, however, changes in CO2, ammonia and lactate have all been shown to elicit cardio-respiratory responses and all have been suggested to arise from stimulation of gill NECs. Our view of the role of NECs is broadening as we begin to understand the polymodal nature of these cells. We begin by presenting the fundamental picture of gill chemosensing that has developed, followed by some key unanswered questions about gill chemosensing in general.

Developing zebrafish utilize taste-signaling pathways for oxygen chemoreception

A fundamental requirement for all animals is to sense and respond to changes in environmental O2 availability. Low O2 (hypoxia) typically stimulates breathing, a universal and critical response termed the hypoxic ventilatory response (HVR). In this study, we test the hypothesis that taste-signaling pathways are used for O2 sensing and activation of the HVR. We show that Merkel-like cells (MLCs), which are part of the taste-bud complex, function as O2 chemoreceptor cells in larval zebrafish and that transduction of the O2 signal uses taste-signaling pathways. Specifically, MLCs responded to hypoxia in vivo with an increase in Ca2+ activity that can drive the HVR. In addition, MLCs transmit O2 signals to afferent cranial nerves IX and X (nIX/X), which project into the area postrema within the hindbrain and synapse with interneurons that are in contact with vagal motor neurons. Hypoxia or chemo-activation of nIX/X caused Ca2+ activity to increase within the area postrema and elicited hyperventilation. The results provide the first demonstration of an O2 signaling pathway that commences with the activation of taste receptors (MLCs) to yield a critical physiological reflex, the HVR.

Whole-Genome Methylation Sequencing of Large Yellow Croaker (Larimichthys crocea) Liver Under Hypoxia and Acidification Stress

Large yellow croaker (Larimichthys crocea) is an important aquaculture species in China. This study analysed whole-genome methylation differences in liver tissues of young fish under different hypoxic and acidification conditions. Differentially methylated regions (DMRs) and differentially methylated genes (DMGs) were identified. Gene ontology (GO) and Kyoto encyclopaedia of genes and genomes (KEGG) enrichment analyses of DMGs were conducted to explore the mechanism of coping with hypoxic acidification. The main methylation type was CG, accounting for > 70% of total methylation, significantly higher than CHG and CHH methylation types. GO enrichment analysis of DMGs revealed strong enrichment of nervous system development, cell periphery, plasma membrane, cell junction organisation, cell junction, signalling receptor activity, molecular sensor activity, cell-linked tissue junction organisation, cell-cell adhesion and nervous system development. KEGG enrichment analysis of DMR-related genes identified cell adhesion molecules, cortisol synthesis and secretion and aldosterone synthesis and secretion as the three key pathways regulating the physiological responses to hypoxia and acidification. Long-term hypoxic and acidification stress affected the immune system, nervous system and stress responses of large yellow croaker. Whole-genome sequencing analysis of exposed tissues was used to investigate changes that occur in L. crocea in response to hypoxic and acidic conditions at the DNA methylation level. The findings contribute to our comprehensive understanding of functional methylation in large yellow croaker and will support future research on the response mechanisms of this species under different environmental pressures.

Insights into the control and consequences of breathing adjustments in fishes-from larvae to adults

Adjustments of ventilation in fishes to regulate the volume of water flowing over the gills are critically important responses to match branchial gas transfer with metabolic needs and to defend homeostasis during environmental fluctuations in O₂ and/or CO₂ levels. In this focused review, we discuss the control and consequences of ventilatory adjustments in fish, briefly summarizing ventilatory responses to hypoxia and hypercapnia before describing the current state of knowledge of the chemoreceptor cells and molecular mechanisms involved in sensing O₂ and CO₂. We emphasize, where possible, insights gained from studies on early developmental stages. In particular, zebrafish (Danio rerio) larvae have emerged as an important model for investigating the molecular mechanisms of O₂ and CO₂ chemosensing as well as the central integration of chemosensory information. Their value stems, in part, from their amenability to genetic manipulation, which enables the creation of loss-of-function mutants, optogenetic manipulation, and the production of transgenic fish with specific genes linked to fluorescent reporters or biosensors.

Does blood flow limit acute hypoxia performance in larval zebrafish (Danio rerio)?

Oxygen uptake (ṀO₂) in larval zebrafish prior to maturation of the gill relies on cutaneous O₂ transfer. Under normoxic conditions, rates of cutaneous O₂ transfer are unaffected by haemoglobin availability but are diminished in fish lacking a functional circulatory system, suggesting that internal convection is critically involved in setting the resting ṀO₂ in zebrafish larvae, even when relying on cutaneous O₂ transfer. The reliance of ṀO₂ on blood circulation led to the first objective of the current study, to determine whether loss of internal convection would reduce acute hypoxia performance (as determined by measuring critical PO₂; P_crit) in larval zebrafish under conditions of moderate hypoxia (PO₂ = 55 mmHg) at 28.5 and 34 °C. Internal convection was eliminated by preventing development of blood vessels using morpholino knockdown of vascular endothelial growth factor (VEGF); these fish are termed VEGF morphants. Breathing frequency (f_V) and heart rate (f_H) also were measured (at 28.5 °C) to determine whether any detriment in performance might be linked to cardiorespiratory dysfunction. Although ṀO₂ was reduced in the VEGF morphants, there was no significant effect on P_crit at 28.5 °C. Raising temperature to 34 °C resulted in the VEGF morphants exhibiting a higher P_crit than the shams, suggesting an impairment of hypoxia tolerance in the morphants at the higher temperature. The usual robust increase in f_V during hypoxia was absent or attenuated in VEGF morphants at 4 and 5 days post fertilization (dpf), respectively. Resting f_H was reduced in the VEGF morphants and unlike the sham fish, the morphants did not exhibit hypoxic tachycardia at 4 or 5 dpf. The number of cutaneous neuroepithelial cells (presumptive O₂ chemoreceptors) was significantly higher in the VEGF morphants and thus the cardiorespiratory impairment in the morphants during hypoxia was unlikely related to inadequate peripheral O₂ sensing.

The role of TASK-2 channels in CO2 sensing in zebrafish (Danio rerio)

Peripheral chemosensitivity in fishes is thought to be mediated by serotonin-enriched neuroepithelial cells (NECs) that are localized to the gills of adults and the integument of larvae. In adult zebrafish (Danio rerio), branchial NECs are presumed to mediate the cardiorespiratory reflexes associated with hypoxia or hypercapnia, whereas in larvae, there is indirect evidence linking cutaneous NECs to hypoxic hyperventilation and hypercapnic tachycardia. No study yet has examined the ventilatory response of larval zebrafish to hypercapnia, and regardless of developmental stage, the signaling pathways involved in CO₂ sensing remain unclear. In the mouse, a background potassium channel (TASK-2) contributes to the sensitivity of chemoreceptor cells to CO₂. Zebrafish possess two TASK-2 channel paralogs, TASK-2 and TASK-2b, encoded by kcnk5a and kcnk5b, respectively. The present study aimed to determine whether TASK-2 channels are expressed in NECs of larval zebrafish and whether they are involved in CO₂ sensing. Using immunohistochemical approaches, TASK-2 protein was observed on the surface of NECs in larvae. Exposure of larvae to hypercapnia caused cardiac and breathing frequencies to increase, and these responses were blunted in fish experiencing TASK-2 and/or TASK-2b knockdown. The results of these experiments suggest that TASK-2 channels are involved in CO₂ sensing by NECs and contribute to the initiation of reflex cardiorespiratory responses during exposure of larvae to hypercapnia.

The paraneuronal gill neuroendocrine system in ocellated puffer fish Leiodon cutcutia

Pseudobranchial neurosecretory system (PSNS) is the third Neuroendocrine (NE) system found in the gill region of fishes in close association with pseudobranch/carotid labyrinth/carotid gland and can suitably be placed under the category of "Diffused NE system (DNES)." The cells belonging to this system fall under the category of "Paraneurons," a concept proposed by Fujita and coworkers. It is found uniformly in all the catfish species and some other noncatfish group of teleosts as Atheriniformes, Channiformes, Perciformes, and Clupeiformes. The fishes, in which the PSNS is present, belong to different breathing habits. Most of these have the capacity to tolerate low O₂ conditions. Leiodon cutcutia although not an air-breathing fish, is known to retain air in its stomach for varied periods when threatened. In an attempt to verify the veracity of this system in a fish of another peculiar breathing habit, ocellated puffer fish L. cutcutia (order Tetradontiformes) was investigated. The histological observations undertaken on L. cutcutia revealed the presence of a well-developed extrabranchial NE system. The findings are discussed in the light of the association of PSNS with chemosensory system and its evolution in fishes, especially in the view of the transition from aquatic to terrestrial life.

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Role of internal convection in respiratory gas transfer and aerobic metabolism in larval zebrafish ( Danio rerio)

Purely diffusive O₂ transport typically is insufficient to sustain aerobic metabolism in most multicellular organisms. In animals that are small enough, however, a high surface-to-volume ratio may allow passive diffusion alone to supply sufficient O₂ transfer. The purpose of this study was to explore the impacts of internal convection on respiratory gas transfer in a small complex organism, the larval zebrafish ( Danio rerio). Specifically, we tested the hypothesis that internal convection is required for the normal transfer of the respiratory gases O₂ and CO₂ and maintenance of resting aerobic metabolic rate in larvae at 4 days postfertilization (dpf). Morpholino knockdown of the vascular endothelial growth factor (VEGF) or cardiac troponin T (TNNT2) proteins allowed an examination of gas transfer in two independent models lacking internal convection. With the use of a scanning micro-optrode technique to measure regional epithelial O₂ fluxes ( Jo₂), it was demonstrated that larvae lacking convection exhibited reduced Jo₂ in regions spanning the head to the trunk. Moreover, the acute loss of internal convection caused by heart stoppage resulted in reduced rates of cutaneous Jo₂, an effect that was reversed upon the restoration of internal convection. With the use of whole body respirometry, it was shown that loss of internal convection was associated with reduced resting rates of O₂ consumption and CO₂ excretion in larvae at 4 dpf. The results of these experiments clearly demonstrate that internal convection is required to maintain resting rates of respiratory gas transfer in larval zebrafish.

Hypoxic acclimation leads to metabolic compensation after reoxygenation in Atlantic salmon yolk-sac alevins

Hypoxia is common in aquatic environments and has substantial effects on development, metabolism and survival of aquatic organisms. To understand the physiological effects of hypoxia and its dependence on temperature, metabolic rate ( [Formula: see text] ) and cardiorespiratory function were studied in response to acute hypoxia (21→5kPa) at different measurement temperatures (T_a; 4, 8 and 12°C) in Salmo salar alevins that were incubated under normoxic conditions (P_O2=21kPa) or following hypoxic acclimation (P_O2=10kPa) as well as two different temperatures (4°C or 8°C). Hypoxic acclimation lead to a developmental delay manifested through slower yolk absorption. The general response to acute hypoxia was metabolic depression (~60%). Hypoxia acclimated alevins had higher [Formula: see text] s when measured in normoxia than alevins acclimated to normoxia. [Formula: see text] s were elevated to the same degree (~30% per 4°C change) irrespective of T_a. Under severe, acute hypoxia (~5kPa) and irrespective of T_a or acclimation, [Formula: see text] s were similar between most groups. This suggests that despite different acclimation regimes, O₂ transport was limited to the same degree. While cardiorespiratory function (heart-, ventilation rate) was unchanged in response to acute hypoxia after normoxic acclimation, hypoxic acclimation led to cardiorespiratory changes predominantly in severe hypoxia, indicating earlier onset and plasticity of cardiorespiratory control mechanisms. Although [Formula: see text] in normoxia was higher after hypoxic acclimation, at the respective acclimation P_O2, [Formula: see text] was similar in normoxia and hypoxia acclimated alevins. This is indicative of metabolic compensation to an intrinsic [Formula: see text] at the acclimation condition in hypoxia-acclimated alevins after re-exposure to normoxia.

Evolution of the hypoxia-sensitive cells involved in amniote respiratory reflexes

The evolutionary origins of the hypoxia-sensitive cells that trigger amniote respiratory reflexes - carotid body glomus cells, and 'pulmonary neuroendocrine cells' (PNECs) - are obscure. Homology has been proposed between glomus cells, which are neural crest-derived, and the hypoxia-sensitive 'neuroepithelial cells' (NECs) of fish gills, whose embryonic origin is unknown. NECs have also been likened to PNECs, which differentiate in situ within lung airway epithelia. Using genetic lineage-tracing and neural crest-deficient mutants in zebrafish, and physical fate-mapping in frog and lamprey, we find that NECs are not neural crest-derived, but endoderm-derived, like PNECs, whose endodermal origin we confirm. We discover neural crest-derived catecholaminergic cells associated with zebrafish pharyngeal arch blood vessels, and propose a new model for amniote hypoxia-sensitive cell evolution: endoderm-derived NECs were retained as PNECs, while the carotid body evolved via the aggregation of neural crest-derived catecholaminergic (chromaffin) cells already associated with blood vessels in anamniote pharyngeal arches.

Role of endogenous carbon monoxide in the control of breathing in zebrafish (Danio rerio)

Carbon monoxide (CO) is a gaseous signaling molecule and is produced in vivo from the intracellular breakdown of heme via the heme oxygenase (HO) family of enzymes. In this study we investigated the role of the HO-1/CO system in the control of ventilation in zebrafish, Danio rerio Immunohistochemistry revealed the presence of HO-1 in the chemoreceptive neuroepithelial cells (NECs) of larvae (4 days postfertilization) and adults, indicating the potential for endogenous CO production in the NECs. Hypoxia (20 min, water Po₂ of 30 mmHg) caused a significant increase in HO-1 activity in whole larvae and in the gills of adult fish. Zebrafish with reduced HO-1 activity (via HO-1 knockdown in larvae or zinc protoporphyrin IX treatment in adults) exhibited increased ventilation frequency (V_f) under normoxic but not hypoxic conditions. The addition of exogenous CO restored resting V_f in fish with diminished CO production, and in some cases (e.g., hypoxic sham larvae) CO modestly reduced V_f below resting levels. Larval fish were treated with phenylhydrazine (PHZ) to eliminate the potential confounding effects of CO-hemoglobin interactions that might influence ventilation. PHZ treatment did not cause changes in V_f of normoxic larvae, and the addition of CO to PHZ-exposed larvae resulted in a significant decrease in sham and HO-1-deficient fish under normoxic conditions. This study demonstrates for the first time that CO plays an inhibitory role in the control of breathing in larval and adult zebrafish.

A TALEN-Exon Skipping Design for a Bethlem Myopathy Model in Zebrafish

Presently, human collagen VI-related diseases such as Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy (BM) remain incurable, emphasizing the need to unravel their etiology and improve their treatments. In UCMD, symptom onset occurs early, and both diseases aggravate with ageing. In zebrafish fry, morpholinos reproduced early UCMD and BM symptoms but did not allow to study the late phenotype. Here, we produced the first zebrafish line with the human mutation frequently found in collagen VI-related disorders such as UCMD and BM. We used a transcription activator-like effector nuclease (TALEN) to design the col6a1^ama605003-line with a mutation within an essential splice donor site, in intron 14 of the col6a1 gene, which provoke an in-frame skipping of exon 14 in the processed mRNA. This mutation at a splice donor site is the first example of a template-independent modification of splicing induced in zebrafish using a targetable nuclease. This technique is readily expandable to other organisms and can be instrumental in other disease studies. Histological and ultrastructural analyzes of homozygous and heterozygous mutant fry and 3 months post-fertilization (mpf) fish revealed co-dominantly inherited abnormal myofibers with disorganized myofibrils, enlarged sarcoplasmic reticulum, altered mitochondria and misaligned sarcomeres. Locomotion analyzes showed hypoxia-response behavior in 9 mpf col6a1 mutant unseen in 3 mpf fish. These symptoms worsened with ageing as described in patients with collagen VI deficiency. Thus, the col6a1^ama605003-line is the first adult zebrafish model of collagen VI-related diseases; it will be instrumental both for basic research and drug discovery assays focusing on this type of disorders.

Aquatic surface respiration and swimming behaviour in adult and developing zebrafish exposed to hypoxia

Severe hypoxia elicits aquatic surface respiration (ASR) behaviour in many species of fish, where ventilation of the gills at the air-water interface improves O2 uptake and survival. ASR is an important adaptation that may have given rise to air breathing in vertebrates. The neural substrate of this behaviour, however, is not defined. We characterized ASR in developing and adult zebrafish (Danio rerio) to ascertain a potential role for peripheral chemoreceptors in initiation or modulation of this response. Adult zebrafish exposed to acute, progressive hypoxia (PO2 from 158 to 15 mmHg) performed ASR with a threshold of 30 mmHg, and spent more time at the surface as PO2 decreased. Acclimation to hypoxia attenuated ASR responses. In larvae, ASR behaviour was observed between 5 and 21 days postfertilization with a threshold of 16 mmHg. Zebrafish decreased swimming behaviour (i.e. distance, velocity and acceleration) as PO2 was decreased, with a secondary increase in behaviour near or below threshold PO2. In adults that underwent a 10-day intraperitoneal injection regime of 10 µg g−1 serotonin (5-HT) or 20 µg g−1 acetylcholine (ACh), an acute bout of hypoxia (15 mmHg) increased the time engaged in ASR by 5.5 and 4.9 times, respectively, compared to controls. Larvae previously immersed in 10 µmol l−1 5-HT or ACh also displayed an increased ASR response. Our results support the notion that ASR is a behavioural response that is reliant upon input from peripheral O2 chemoreceptors. We discuss implications for the role of chemoreceptors in the evolution of air breathing.

A role for nitric oxide in the control of breathing in zebrafish (Danio rerio)

Nitric oxide (NO) is a gaseous neurotransmitter, which in adult mammals, modulates the acute hypoxic ventilatory response; its role in the control of breathing in fish during development is unknown. We addressed the interactive effects of developmental age and NO in the control of piscine breathing by measuring the ventilatory response of zebrafish (Danio rerio) adults and larvae to NO donors and by inhibiting endogenous production of NO. In adults, sodium nitroprusside (SNP), a NO donor, inhibited ventilation; the extent of the ventilatory inhibition was related to the pre-existing ventilatory drive, with the greatest inhibition exhibited during exposure to hypoxia (PO2=5.6 kPa). Inhibition of endogenous NO production using L-NAME supressed the hypoventilatory response to hyperoxia, supporting an inhibitory role of NO in adult zebrafish. Neuroepithelial cells, the putative oxygen chemoreceptors of fish, contain neuronal nitric oxide synthase (nNOS). In zebrafish larvae at 4 days post fertilization, SNP increased ventilation in a concentration-dependent manner. Inhibition of NOS activity with L-NAME or knockdown of nNOS inhibited the hypoxic (PO2=3.5 kPa) ventilatory response. Immunohistochemistry revealed the presence of nNOS in the NECs of larvae. Taken together, these data suggest that NO plays an inhibitory role in the control ventilation in adult zebrafish, but an excitatory role in larvae.

Serotonin is required for pharyngeal arch morphogenesis in zebrafish

Serotonin (5-HT) is not only a neurotransmitter but also a mediator of developmental processes in vertebrates. In this study, we analyzed the importance of 5-HT during zebrafish development. The expression patterns of three zebrafish tryptophan hydroxylase isoforms (Tph1A, Tph1B, Tph2), the rate-limiting enzymes in 5-HT synthesis, were analyzed and compared to the appearance and distribution of 5-HT. 5-HT was found in the raphe nuclei correlating with tph2 expression and in the pineal gland correlating with tph1a and tph2 expression. tph2 deficient fish generated with antisense morpholino oligonucleotides exhibited morphogenesis defects during pharyngeal arch development. The correct specification of neural crest cells was not affected in tph2 morphants as shown by the expression of early markers, but the survival and differentiation of pharyngeal arch progenitor cells were impaired. An organizing role of 5-HT in pharyngeal arch morphogenesis was suggested by a highly regular pattern of 5-HT positive cells in this tissue. Moreover, the 5-HT2B receptor was expressed in the pharyngeal arches and its pharmacological inhibition also induced defects in pharyngeal arch morphogenesis. These results support an important role of Tph2-derived serotonin as a morphogenetic factor in the development of neural crest derived tissues.

Rh protein expression in branchial neuroepithelial cells, and the role of ammonia in ventilatory control in fish

Bill Milsom has made seminal contributions to our understanding of ventilatory control in a wide range of vertebrates. Teleosts are particularly interesting, because they produce a 3rd, potentially toxic respiratory gas (ammonia) in large amounts. Fish are well known to hyperventilate under high environmental ammonia (HEA), but only recently has the potential role of ammonia in normal ventilatory control been investigated. It is now clear that ammonia can act directly as a ventilatory stimulant in trout, independent of its effects on acid-base balance. Even in ureotelic dogfish sharks, acute elevations in ammonia cause increases in ventilation. Peripherally, the detection of elevated ammonia resides in gill arches I and II in trout, and in vitro, neuroepithelial cells (NECs) from these arches are sensitive to ammonia, responding with elevations in intracellular Ca(2+) ([Ca(2+)]i). Centrally, hyperventilatory responses to ammonia correlate more closely with concentrations of ammonia in the brain than in plasma or CSF. After chronic HEA exposure, ventilatory responsiveness to ammonia is lost, associated with both an attenuation of the [Ca(2+)]i response in NECs, and the absence of elevation in brain ammonia concentration. Chronic exposure to HEA also causes increases in the mRNA expression of several Rh proteins (ammonia-conductive channels) in both brain and gills. "Single cell" PCR techniques have been used to isolate the individual responses of NECs versus other gill cell types. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O2 uptake in fish.

Serotonergic and cholinergic elements of the hypoxic ventilatory response in developing zebrafish

The chemosensory roles of gill neuroepithelial cells (NECs) in mediating the hyperventilatory response to hypoxia are not clearly defined in fish. While serotonin (5-HT) is the predominant neurotransmitter in O(2)-sensitive gill NECs, acetylcholine (ACh) plays a more prominent role in O(2) sensing in terrestrial vertebrates. The present study characterized the developmental chronology of potential serotonergic and cholinergic chemosensory pathways of the gill in the model vertebrate, the zebrafish (Danio rerio). In immunolabelled whole gills from larvae, serotonergic NECs were observed in epithelia of the gill filaments and gill arches, while non-serotonergic NECs were found primarily in the gill arches. Acclimation of developing zebrafish to hypoxia (P(O2)=75 mmHg) reduced the number of serotonergic NECs observed at 7 days post-fertilization (d.p.f.), and this effect was absent at 10 d.p.f. In vivo administration of 5-HT mimicked hypoxia by increasing ventilation frequency (f(V)) in early stage (7-10 d.p.f.) and late stage larvae (14-21 d.p.f.), while ACh increased f(V) only in late stage larvae. In time course experiments, application of ketanserin inhibited the hyperventilatory response to acute hypoxia (P(O2)=25 mmHg) at 10 d.p.f., while hexamethonium did not have this effect until 12 d.p.f. Cells immunoreactive for the vesicular acetylcholine transporter (VAChT) began to appear in the gill filaments by 14 d.p.f. Characterization in adult gills revealed that VAChT-positive cells were a separate population of neurosecretory cells of the gill filaments. These studies suggest that serotonergic and cholinergic pathways in the zebrafish gill develop at different times and contribute to the hyperventilatory response to hypoxia.

Comparative embryology of the carotid body

Vertebrate carotid bodies and related structures (branchial arch oxygen chemoreceptors in fishes, carotid labyrinth in amphibians, chemoreceptors in the wall of the common carotid and its branches in birds) develop in embryos when neural crest cells, blood vessels, and nerve fibers from sympathetic and cranial nerve ganglia invade mesenchymal primordia in the wall of the 3rd branchial arch. This review focuses on literature published since the 1970s investigating similarities and differences in the embryological development of 3rd arch oxygen chemoreceptors, especially between mammals and birds, but also considering reptiles, amphibians and fishes.

Serotonergic neuroepithelial cells of the skin in developing zebrafish: morphology, innervation and oxygen-sensitive properties

In teleost fish, O(2) chemoreceptors of the gills (neuroepithelial cells or NECs) initiate cardiorespiratory reflexes during hypoxia. In developing zebrafish, hyperventilatory and behavioural responses to hypoxia are observed before development of gill NECs, indicating that extrabranchial chemoreceptors mediate these responses in embryos. We have characterised a population of cells of the skin in developing zebrafish that resemble O(2)-chemoreceptive gill NECs. Skin NECs were identified by serotonin immunolabelling and were distributed over the entire skin surface. These cells contained synaptic vesicles and were associated with nerve fibres. Skin NECs were first evident in embryos 24-26 h post-fertilisation (h.p.f.), and embryos developed a behavioural response to hypoxia between 24 and 48 h.p.f. The total number of NECs declined with age from approximately 300 cells per larva at 3 days post-fertilisation (d.p.f.) to ~120 cells at 7 d.p.f., and were rarely observed in adults. Acclimation to hypoxia (30 mmHg) or hyperoxia (300 mmHg) resulted in delayed or accelerated development, respectively, of peak resting ventilatory frequency and produced changes in the ventilatory response to hypoxia. In hypoxia-acclimated larvae, the temporal pattern of skin NECs was altered such that the number of cells did not decrease with age. By contrast, hyperoxia produced a more rapid decline in NEC number. The neurotoxin 6-hydroxydopamine degraded catecholaminergic nerve terminals that made contact with skin NECs and eliminated the hyperventilatory response to hypoxia. These results indicate that skin NECs are sensitive to changes in O(2) and suggest that they may play a role in initiating responses to hypoxia in developing zebrafish.

Neuroepithelial cells of the gill and their role in oxygen sensing

A highly sensitive oxygen (O(2)) sensing mechanism is critical for the survival of all vertebrate species. In fish, this requirement is fullfilled by the neuroepithelial cells (NECs) of the gill. NECs are neurotransmitter-containing chemosensory cells that are diffusely distributed within a thin epithelial layer of the filaments and respiratory lamellae of all gill arches, and are innervated by afferent fibers from the central nervous system. In acute cell culture, NECs respond immediately, and in a dose-dependent manner, to acute changes in O(2) tension. Thus, hypoxic stimulation of gill NECs appears to initiate the production of adaptive, cardiorespiratory reflexes that contribute to the maintenance of O(2) uptake in order to meet metabolic demands. This review covers the current evidence for the status of NECs as the primary peripheral O(2) sensors in fish. We have included an overview of the phylogeny of O(2) sensing structures among vertebrate groups, and morphological and physiological evidence for the importance of NECs in O(2) sensing.

Peripheral chemoreceptors in air- versus water- breathers

Among the vertebrates, peripheral chemoreceptors have evolved to play a key role in matching oxygen delivery to the metabolic needs of the body cells and tissues. Specialized neuroepithelial cells (NECs) distributed within the gill filaments and/or lamellae of water-breathers appear to subserve this function by initiating an increase in ventilation in response to lowering of blood or water PO(2) (hypoxia). It is only recently, however, that these cells have become amenable for detailed investigations using electrophysiological tools. By contrast, the well-studied specialized neuroendocrine cells (i.e. glomus or type I cells) located principally in the carotid body of air-breathers initiate a similar reflex ventilatory response to hypoxia so as to maintain blood PO(2) homeostasis. In some species, however, the carotid body is immature and relatively insensitive to hypoxia at birth; it is during this period that their sympathoadrenal counterparts in the adrenal medulla act as key PO(2) receptors, critical for the proper transition to air-breathing life. It is becoming increasingly clear that in general these chemoreceptors act as polymodal receptors, i.e. capable of detecting several sensory modalities including high CO(2)/H(+) or acid hypercapnia. Given the phylogenetic and ontogenetic evidence pointing to homology between the mammalian carotid artery and the first gill arch of teleosts, the question arises whether the mechanisms of chemosensing are conserved among these cell types. This review examines some of the anatomical and functional similarities among these peripheral chemoreceptors, while raising the possibility that the fundamental mechanisms of O(2) and CO(2)/H(+) sensing arose first in water-breathers and are conserved among the vertebrates.

Neuroepithelial cells and the hypoxia emersion response in the amphibious fish Kryptolebias marmoratus

Teleost fish have oxygen-sensitive neuroepithelial cells (NECs) in the gills that appear to mediate physiological responses to hypoxia, but little is known about oxygen sensing in amphibious fish. The mangrove rivulus, Kryptolebias marmoratus, is an amphibious fish that respires via the gills and/or the skin. First, we hypothesized that both the skin and gills are sites of oxygen sensing in K. marmoratus. Serotonin-positive NECs were abundant in both gills and skin, as determined by immunohistochemical labelling and fluorescence microscopy. NECs retained synaptic vesicles and were found near nerve fibres labelled with the neuronal marker zn-12. Skin NECs were 42% larger than those of the gill, as estimated by measurement of projection area, and 45% greater in number. Moreover, for both skin and gill NECs, NEC area increased significantly (30-60%) following 7 days of exposure to hypoxia (1.5 mg l(-1) dissolved oxygen). Another population of cells containing vesicular acetylcholine transporter (VAChT) proteins were also observed in the skin and gills. The second hypothesis we tested was that K. marmoratus emerse in order to breathe air cutaneously when challenged with severe aquatic hypoxia, and this response will be modulated by neurochemicals associated chemoreceptor activity. Acute exposure to hypoxia induced fish to emerse at 0.2 mg l(-1). When K. marmoratus were pre-exposed to serotonin or acetylcholine, they emersed at a significantly higher concentration of oxygen than untreated fish. Pre-exposure to receptor antagonists (ketanserin and hexamethonium) predictably resulted in fish emersing at a lower concentration of oxygen. Taken together, these results suggest that oxygen sensing occurs at the branchial and/or cutaneous surfaces in K. marmoratus and that serotonin and acetylcholine mediate, in part, the emersion response.

Ammonia sensing by neuroepithelial cells and ventilatory responses to ammonia in rainbow trout

Ammonia, the third respiratory gas in teleost fish, acts as an acute stimulant to ventilation in ammoniotelic rainbow trout. We investigated whether this sensitivity is maintained in trout chronically exposed (1+ months) to high environmental ammonia [HEA, 250 μmol l–1 (NH4)2SO4] in the water, and whether gill neuroepithelial cells (NECs) are involved in ammonia sensing. Hyperventilation was induced both by acute external (NH4)2SO4 exposure [250 or 500 μmol l–1 (NH4)2SO4] and by intra-arterial (NH4)2SO4 injection (580 μmol kg–1 of ammonia) in control trout, but these responses were abolished in chronic HEA animals. Hyperventilation in response to acute ammonia exposure persisted after bilateral removal of each of the four gill arch pairs separately or after combined removal of arches III and IV, but was delayed by removal of gill arch I, and eliminated by combined removal of arches I and II. NECs, identified by immunolabeling against 5-HT, were mainly organized in two lines along the filament epithelium in all four gill arches. In control trout, NECs were slightly smaller but more abundant on arches I and II than on arches III and IV. Chronic HEA exposure reduced the density of the NECs on all four arches, and their size on arches I and II only. Fura-2 fluorescence imaging was used to measure intracellular free calcium ion concentration ([Ca2+]i) responses in single NECs in short-term (24–48 h) culture in vitro. [Ca2+]i was elevated to a comparable extent by perfusion of 30 mmol l–1 KCl and 1 mmol l–1 NH4Cl, and these [Ca2+]i responses presented in two different forms, suggesting that ammonia may be sensed by multiple mechanisms. The [Ca2+]i responses to high ammonia were attenuated in NECs isolated from trout chronically exposed to HEA, especially in ones from gill arch I, but responses to high K+ were unchanged. We conclude that the hyperventilatory response to ammonia is lost after chronic waterborne HEA exposure, and that NECs, especially the ones located in gill arches I and II, are probably ammonia chemoreceptors that participate in ventilatory modulation in trout.

Ontogeny of the corticotropin-releasing factor system in rainbow trout and differential effects of hypoxia on the endocrine and cellular stress responses during development

To further our understanding of the development of the stress axis and the responsiveness of embryonic and larval fish to environmental stressors, this study examined the ontogeny of whole-body cortisol levels and of the corticotropin-releasing factor (CRF) system in rainbow trout, as well as the endocrine and cellular stress responses to hypoxia. After depletion of a maternal deposit, de novo synthesis of cortisol increased exponentially between the 'eyed' stage and first feeding. Whole body CRF mRNA levels dominated over those of the related peptide urotensin I (UI) from hatch through complete yolk sac absorption. The mRNA levels of CRF-binding protein (CRF-BP) closely paralleled those of CRF and UI throughout ontogeny except at first feeding when an increase in CRF gene expression was not matched by change in CRF-BP transcript abundance. In the hypoxia challenge, fish were exposed to 15% O(2) saturation for either 90 min or 24h at three key developmental stages: hatch, swim up and first feeding. While the embryos were unaffected, chronic hypoxia elicited a transient 2-fold increase in whole-body cortisol levels in the larval stages. The hypoxia challenge also generally suppressed the mRNA levels of CRF and CRF-BP, had no effect on the expression of UI, but had a marked stimulatory effect on heat shock protein 70 (Hsp70) gene expression. Taken together, these results suggest a role for the CRF system in the ontogenic regulation of corticosteroidogenesis and show that hypoxia has developmental stage-specific effects on the endocrine and cellular stress responses in rainbow trout.

First aid kit for hypoxic survival: sensors and strategies

Survival success under conditions of acute oxygen deprivation depends on efficiency of the central and peripheral chemoreception, optimization of oxygen extraction from the hypoxic environment and its delivery to the periphery, and adjustments of energy production and consumption. This article uses a comparative approach to assess the efficiency of adaptive strategies used by anoxia-tolerant and hypoxia-sensitive species to support survival during the first minutes to 1 h of oxygen deprivation. An aquatic environment is much more demanding in terms of diurnal and seasonal variations of the ambient oxygen availability from anoxia to hyperoxia than is an air environment. Therefore, fishes and aquatic turtles have developed a number of adaptive responses, which are lacking in most of the terrestrial mammals, to cope with these extreme conditions. These include efficient central and peripheral chemoreception, acute changes in respiratory rate and amplitude, and acute increase of the gas-exchange interface. A special set of adaptive mechanisms are engaged in reduction of the energy expenditure of the major oxygen-consuming organs: the brain and the heart. Both reduction of ATP consumption and a switch to alterative energy sources contribute to the maintenance of ATP and ion balance in hypoxia-tolerant animals. Hypoxia and hyperoxia are conditions favoring development of oxidative stress. Efficient protection from oxidation in anoxia-tolerant species includes reduction in the glutamate levels in the brain, stabilization of the mitochondrial function, and maintenance of nitric oxide production under conditions of oxygen deprivation. We give an overview of the current state of knowledge on some selected molecular and cellular acute adaptive mechanisms. These include the mechanisms of chemoreception in adult and neonatal mammals and in fishes, acute metabolic adaptive responses in the brain, and the role of nitrite in the preservation of heart function under hypoxic conditions.

Onset and early development of hypoxic ventilatory responses and branchial neuroepithelial cells in Xenopus laevis

Onset and ontogeny of the O₂ chemoreceptive control of ventilation was investigated in Xenopus laevis. The density and size of branchial serotonin-immunoreactive neuroepithelial cells (5-HT-IR NECs) were also determined using confocal immunofluorescent microscopy. Larvae started gill ventilation at 3 days post-fertilization (dpf), and, at this early stage, acute hypoxic exposure produced an increase in frequency from 28 ± 4 to 60 ± 2 beats x min⁻¹. Concurrent with the onset of ventilatory responses, 5-HT-IR NECs appeared in the gill filament bud. Lung ventilation began at 5 dpf and exhibited a 3-fold increase in frequency during acute hypoxia. At 10 dpf, gill ventilatory sensitivity to hypoxia increased, as did NEC density, from 15 ± 1 (5 dpf) to 29 ± 2 (10 dpf) cells x mm of filament⁻¹. Unlike ventilation frequency, gill ventilation amplitude and lung expired volume were unaltered by acute hypoxia. Chronic exposure to moderate hypoxia, at a P(O₂) of 110 mmHg, attenuated acute responses to moderate hypoxia at 10 and 14 dpf but had no effect at more severe hypoxia or at other stages. Chronic hypoxia also stimulated 5-HT-IR NECs growth at 21 dpf. Collectively, larvae at 5 dpf exhibited strong O₂-driven gill and lung ventilatory responses, and between 10 and 21 dpf, the early hypoxic responses can be shaped by the ambient P(O₂).

Ammonia as a stimulant to ventilation in rainbow trout Oncorhynchus mykiss

Ammonia is the third most important respiratory gas in ammoniotelic fish after oxygen and carbon dioxide. We here investigated the effects of elevated plasma ammonia on ventilation in freshwater rainbow trout. Intact trout fitted with indwelling dorsal aortic catheters were given injections (over 5 min) of Cortland saline, isotonic high ammonia solutions (NH(4)HCO(3), (NH(4))(2)SO(4), NH(4)OH at pH 8.0, and NH(4)OH at pH 9.0), and other solutions as controls for acid-base effects, while ventilatory rate (VR) and buccal pressure amplitude (DeltaP(buccal)) were recorded. All high ammonia solutions resulted in immediate elevations of plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a), and increases in ventilatory DeltaP(buccal) and VR to different degrees. However, while Pa(O2) remained constant, in every case there was a confounding change in one or more components of acid-base status (decreases in pH(a) or increases in [HCO(3)(-)](a) or Pa(CO2) in different treatments), although the ventilatory responses to ammonia injections were generally larger than could be explained by changes in acid-base status. Therefore a series was performed in which normal blood perfusion of the gills was replaced by ventral aortic perfusion with either Cortland saline or Cortland saline plus high ammonia in which pH, [HCO(3)(-)], P(CO2), and P(O2) remained unchanged. Although ventilation was depressed in these anaesthetized, spontaneously ventilating preparations, perfusion with high ammonia saline increased DeltaP(buccal). In a final series, trout were infused for 24h with Cortland saline, isotonic NH(4)HCO(3), or isotonic (NH(4))(2)SO(4) solutions. The two ammonia solutions both caused persistent elevations in VR and DeltaP(buccal), together with similar large increases in plasma Tamm(a), Pa(NH3), and [NH(4)(+)](a). As there was no changes in Pa(O2), pH(a), Pa(CO2), or [HCO(3)(-)](a) in the (NH(4))(2)SO(4) infusion series, this, together with the ventral aortic perfusion experiment, provides the most convincing evidence that ammonia stimulates ventilation. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O(2) uptake, and propose that ammonia-induced hyperventilation may also facilitate ammonia excretion in rainbow trout.

Cardio-respiratory control during early development in the model animal zebrafish

Independent of species, the cardiovascular system is the first functioning component of developing vertebrate embryos. One of the main hypotheses is the assumption that larval and juvenile stages of fish and amphibians are not just smaller versions of an adult phenotype. In this review, the cardiovascular and respiratory responses to environmental, genetic and epigenetic perturbations are discussed in detail to understand the relationships between cardiac and respiratory performance, haematopoiesis for embryonic or larval stages with special focus on the popular model animal, the zebrafish. Zebrafish are tiny animals which have many advantages as a model organism in analysis of the cardio-respiratory system. It obtains sufficient amounts of oxygen via bulk diffusion, in contrast to convection-dependent mammals. It is possible to study genetic mutants even with extreme defective phenotypes of the cardio-respiratory system in order to understand its developmental and physiological mechanisms. It has become apparent that the cardio-respiratory system and its control starts functioning very early during development, long before oxygen uptake becomes diffusion limited in zebrafish. Finally, recent improvements in imaging techniques for the use of fish models relevant for developmental physiology and biomedical research are discussed.

Anoxia-induced suspended animation in budding yeast as an experimental paradigm for studying oxygen-regulated gene expression

A lack of oxygen can force many organisms to enter into recoverable hypometabolic states. To better understand how organisms cope with oxygen deprivation, our laboratory previously had shown that when challenged with anoxia, both the nematode Caenorhabditis elegans and embryos of the zebrafish Danio rerio enter into suspended animation, in which all life processes that can be observed by light microscopy reversibly halt pending the restoration of oxygen (P. A. Padilla and M. B. Roth, Proc. Natl. Acad. Sci. USA 98:7331-7335, 2001, and P. A. Padilla, T. G. Nystul, R. A. Zager, A. C. Johnson, and M. B. Roth, Mol. Biol. Cell 13:1473-1483, 2002). Here, we show that both sporulating and vegetative cells of the budding yeast Saccharomyces cerevisiae also enter into a similar state of suspended animation when made anoxic on a nonfermentable carbon source. Transcriptional profiling using cDNA microarrays and follow-on quantitative real-time PCR analysis revealed a relative derepression of aerobic metabolism genes in carbon monoxide (CO)-induced anoxia when compared to nitrogen (N(2)) gas-induced anoxia, which is consistent with the known oxygen-mimetic effects of CO. We also found that mutants deleted for components of the mitochondrial retrograde signaling pathway can tolerate prolonged exposure to CO but not to N(2). We conclude that the cellular response to anoxia is dependent on whether the anoxic gas is an oxygen mimetic and that the mitochondrial retrograde signaling pathway is functionally important for mediating this response.

Lateral line, otic and epibranchial placodes: developmental and evolutionary links?

Two embryonic cell populations, the neural crest and cranial ectodermal placodes, between them give rise to many of the unique characters of vertebrates. Neurogenic placode derivatives are vital for sensing both external and internal stimuli. In this speculative review, we discuss potential developmental and evolutionary relationships between two placode series that are usually considered to be entirely independent: lateral line placodes, which form the mechanosensory and electroreceptive hair cells of the anamniote lateral line system as well as their afferent neurons, and epibranchial placodes (geniculate, petrosal and nodose), which form Phox2b(+) visceral sensory neurons with input from both the external and internal environment. We illustrate their development using molecular data we recently obtained in shark embryos, and we describe their derivatives, including the possible geniculate placode origin of a mechanosensory sense organ associated with the first pharyngeal pouch/cleft (the anamniote spiracular organ/amniote paratympanic organ). We discuss how both lateral line and epibranchial placodes can be related in different ways to the otic placode (which forms the inner ear and its afferent neurons), and how both are important for protective somatic reflexes. Finally, we put forward a highly speculative proposal about the original function of the cells whose evolutionary descendants today include the derivatives of the lateral line, otic and epibranchial placodes, namely that they produced sensory receptors and neurons for Phox2b-dependent protective reflex circuits. We hope this review will stimulate both debate and a fresh look at possible developmental and evolutionary relationships between these seemingly disparate and independent placodes.

Neurotransmitter localization in the neuroepithelial cells and unipolar neurons of the respiratory tract in the bichir, Polypterus bichir bichir G. ST-HIL

Immunohistochemical localisation of neurotransmitters was used to determine the distribution of unipolar neurons and neuroepithelial cells (NECs) in the respiratory tract of the bichir, Polypterus bichir bichir. NECs were commonly encountered in the mucociliated epithelium of the lung. Unipolar neurons were located in the submucosal and muscle layers of the glottis. The results suggest the presence of tyrosine hydroxylase (TH) and nNOS immunoreactivities in NECs. In addition, ACh-E/nNOS and TH/nNOS nerve fibers were also found associated with these cells. Unipolar neuronal cells showed a chemical code including the presence of 5-HT, ACh-E, peptides and P2x2 receptors. The present findings indicate that nitric oxide (NO) is a primitive transmitter of neuroepithelial oxygen-sensitive chemoreceptor cells together with acetylcholine. The coexistence of ACh-E with other substances in the unipolar neurons, but not with NO, may be a property of vagal postganglionic neurons since the emergence of the cranial autonomic pathways in the earliest vertebrates. It would be interesting to know about the provenance of the nerves in contact with NECs, which appear to have a complex innervation pattern.

Innervation and Neurotransmitter Localization in the Lung of the Nile bichirPolypterus bichir bichir

Anatomical and functional studies of the autonomic innervation in the lung of dipnoan fishes and the bichirs are lacking. The present immunohistochemical studies demonstrated the presence of nerve fibers in the muscle layers of the lung of the bichir, Polypterus bichir bichir, and identified the immunoreactive elements of this innervation. Tyrosine hydroxylase, acetylcholinesterase, and peptide immunoreactivity was detected in the intramural nerve fibers. Extensive innervation was present in the submucosa where adenylatecyclase/activating polypeptide 38, substance P, P(2)X(2), and 5-hydroxytryptamine (5-HT)-immunoreactive nerve fibers mainly supplied blood vessels. A collection of monopolar neurons located in the submucosal and the muscular layers of the glottis expressed a variety of various transmitters. These neurons may be homologous to ganglion cells in the branchial and pharyngeal rami of the vagus in fishes. Nerves containing 5-HT and P(2)X(2) receptor immunoreactivity projected to the lung epithelium. Associated with neuroepithelial cells in mucociliated epithelium, were neuronal nitric oxide synthase-immunopositive axons. The physiological function of this innervation is not known. The present study shows that the pattern of autonomic innervation of the bichir lung may by similar in its elements to that in tetrapods.