Hypoxia increases intracellular calcium in glutamate-activated horizontal cells of goldfish retina via mitochondrial K channels and intracellular stores.

Central neurons of the common goldfish (Carassius auratus) are exceptional in their capacity to survive Ca-induced excitotoxicity and cell death during hypoxia. Horizontal cells (HCs) are inhibitory interneurons of the retina that are tonically depolarized by the neurotransmitter, glutamate, yet preserve intracellular Ca homeostasis. In HCs isolated from goldfish, and in the absence of glutamatergic input, intracellular Ca concentration ([Ca]) is protected from prolonged exposure to hypoxia by mitochondrial ATP-dependent K (mK) channel activity.

Cell proliferation and regeneration in the gill.

Seminal studies from the early 20th century defined the structural changes associated with development and regeneration of the gills in goldfish at the gross morphological and cellular levels using standard techniques of light and electron microscopy. More recently, investigations using cell lineage tracing, molecular biology, immunohistochemistry and single-cell RNA-sequencing have pushed the field forward and have begun to reveal the cellular and molecular processes that orchestrate cell proliferation and regeneration in the gills. The gill is a multifunctional organ that mediates an array of important physiological functions, including respiration, ion regulation and excretion of waste products.

Identification of signalling pathways involved in gill regeneration in zebrafish.

The occurrence of regeneration of the organs involved in respiratory gas exchange amongst vertebrates is heterogeneous. In some species of amphibians and fishes, the gills regenerate completely following resection or amputation, whereas in mammals, only partial, facultative regeneration of lung tissue occurs following injury. Given the homology between gills and lungs, the capacity of gill regeneration in aquatic species is of major interest in determining the underlying molecular or signalling pathways involved in respiratory organ regeneration.

Neurochemical Signalling Associated With Gill Oxygen Sensing and Ventilation: A Receptor Focused Mini-Review.

Despite the large body of work describing vertebrate ventilatory responses to hypoxia, remarkably little is known about the receptors and afferent pathways mediating these responses in fishes. In this review, we aim to summarize all receptor types to date implicated in the neurotransmission or neuromodulation associated with O sensing in the gills of fish. This includes serotonergic, cholinergic, purinergic, and dopaminergic receptor subtypes.

Single-cell transcriptomic analysis of neuroepithelial cells and other cell types of the gills of zebrafish (Danio rerio) exposed to hypoxia.

The fish gill is a multifunctional organ involved in numerous physiological processes, such as gas exchange and sensing of hypoxia by respiratory chemoreceptors, called neuroepithelial cells (NECs). Many studies have focused on zebrafish (Danio rerio) to investigate the structure, function and development of the gills, yet the transcriptomic profile of most gill cells remains obscure. We present the results of a comprehensive transcriptomic analysis of the gills of zebrafish using single-cell RNA sequencing (scRNA-seq).

Goldfish and crucian carp are natural models of anoxia tolerance in the retina.

Vertebrates need oxygen to survive. The central nervous system has an especially high energy demand, so brain and retinal neurons quickly die in anoxia. But fish of the genus Carassius are exceptionally anoxia-tolerant: the crucian carp (C.

Seasonal changes in membrane structure and excitability in retinal neurons of goldfish (Carassius auratus) under constant environmental conditions.

Seasonal modifications in the structure of cellular membranes occur as an adaptive measure to withstand exposure to prolonged environmental change. Little is known about whether such changes occur independently of external cues, such as photoperiod or temperature, or how they may impact the central nervous system. We compared membrane properties of neurons isolated from the retina of goldfish (Carassius auratus), an organism well adapted to extreme environmental change, during the summer and winter months.

Mitochondrial KATP channels stabilize intracellular Ca2+ during hypoxia in retinal horizontal cells of goldfish (Carassius auratus).

Neurons of the retina require oxygen to survive. In hypoxia, neuronal ATP production is impaired, ATP-dependent ion pumping is reduced, transmembrane ion gradients are dysregulated, and intracellular Ca2+ concentration ([Ca2+]i) increases enough to trigger excitotoxic cell death. Central neurons of the common goldfish (Carassius auratus) are hypoxia tolerant, but little is known about how goldfish retinas withstand hypoxia.

Replacement of mitochondrion-rich cells during regeneration of the gills and opercular epithelium in zebrafish (Danio rerio).

Transport epithelia maintain the volume, ion concentration and acid-base balance of blood and extracellular fluids. In teleost fish, mitochondrion-rich cells (MRCs) are specialized ionocytes that perform this role. These cells are found in epithelia of the gills and buccal surface of the operculum (the bony structure covering the gills).

The development of the O-sensing system in an amphibious fish: consequences of variation in environmental O levels.

Proper development of the O-sensing system is essential for survival. Here, we characterized the development of the O-sensing system in the mangrove rivulus (Kryptolebias marmoratus), an amphibious fish that transitions between hypoxic aquatic environments and O-rich terrestrial environments. We found that NECs formed in the gills and skin of K.

Identification of oxygen-sensitive neuroepithelial cells through an endogenous reporter gene in larval and adult transgenic zebrafish.

In teleost fish, specialized oxygen (O) chemoreceptors, called neuroepithelial cells (NECs), are found in the gill epithelium in adults. During development, NECs are present in the skin before the formation of functional gills. NECs are known for retaining the monoamine neurotransmitter, serotonin (5-HT) and are conventionally identified through immunoreactivity with antibodies against 5-HT or synaptic vesicle protein (SV2).

Retinal horizontal cells of goldfish (Carassius auratus) display subtype-specific differences in spontaneous action potentials in situ.

Horizontal cells (HCs) are neurons of the outer retina, which provide inhibitory feedback onto photoreceptors and contribute to image processing. HCs in teleosts are classified into four subtypes (H1-H4), each having different roles: H1-H3 feed back onto different sets of cones, H4 feed back onto rods, and only H1 store and release the inhibitory neurotransmitter, γ-aminobutyric acid (GABA). Dissociated HCs exhibit spontaneous Ca -based action potentials (APs), yet it is unclear if APs occur in situ, or if all subtypes exhibit APs.

A comparative perspective on lung and gill regeneration.

The ability to continuously grow and regenerate the gills throughout life is a remarkable property of fish and amphibians. Considering that gill regeneration was first described over one century ago, it is surprising that the underlying mechanisms of cell and tissue replacement in the gills remain poorly understood. By contrast, the mammalian lung is a largely quiescent organ in adults but is capable of facultative regeneration following injury.

Regeneration of the gill filaments and replacement of serotonergic neuroepithelial cells in adult zebrafish (Danio rerio).

Respiratory epithelia and chemoreceptors of the gills and mammalian lung derive from the same embryonic structures. While the lung is limited to facultative regeneration, the regenerative capacity of the gill has not been adequately explored. We report regeneration of gill filaments and respiratory lamellae in adult zebrafish (Danio rerio).

Spontaneous action potentials in retinal horizontal cells of goldfish () are dependent upon L-type Ca channels and ryanodine receptors.

Horizontal cells (HCs) are interneurons of the outer retina that undergo graded changes in membrane potential during the light response and provide feedback to photoreceptors. We characterized spontaneous Ca-based action potentials (APs) in isolated goldfish () HCs with electrophysiological and intracellular imaging techniques. Transient changes in intracellular Ca concentration ([Ca]) were observed with fura-2 and were abolished by removal of extracellular Ca or by inhibition of Ca channels by 50 µM Cd or 100 µM nifedipine.

Hydrogen sulphide toxicity and the importance of amphibious behaviour in a mangrove fish inhabiting sulphide-rich habitats.

We investigated amphibious behaviour, hydrogen sulphide (HS) tolerance, and the mechanism of HS toxicity in the amphibious mangrove rivulus (Kryptolebias marmoratus). We found that fish emersed (left water) in response to acutely elevated [HS] (~ 130-200 µmol l). The emersion response to HS may be influenced by prior acclimation history due to acclimation-induced alterations in gill morphology and/or the density and size of neuroepithelial cells (NECs) on the gills and skin.

Insights into the evolution of polymodal chemoreceptors.

Respiratory chemoreceptors in vertebrates are specialized cells that detect chemical changes in the environment or arterial blood supply and initiate autonomic responses, such as hyperventilation or changes in heart rate, to improve O uptake and delivery to tissues. These chemoreceptors are sensitive to changes in O, CO and/or H. In fish and mammals, respiratory chemoreceptors may be additionally sensitive to ammonia, hypoglycemia, and numerous other stimuli.

Purinergic and adenosine receptors contribute to hypoxic hyperventilation in zebrafish (Danio rerio).

The chemoreceptors involved in oxygen sensing in teleost fish are neuroepithelial cells (NECs) in the gills, and are analogous to glomus cells in the mammalian carotid body. Purinergic signalling mechanisms involving the neurotransmitters, ATP and adenosine, have been identified in mediating hypoxic signalling in the carotid body, but these pathways are not well understood in the fish gill. The present study used a behavioural assay to screen for the effects of drugs, that target purinergic and adenosine receptors, on the hyperventilatory response to hypoxia in larval zebrafish (Danio rerio) in order to determine if the receptors on which these drugs act may be involved in hypoxic signalling.

Characterization of ion channels and O sensitivity in gill neuroepithelial cells of the anoxia-tolerant goldfish ().

The neuroepithelial cell (NEC) of the fish gill is an important model for O sensing in vertebrates; however, a complete picture of the chemosensory mechanisms in NECs is lacking, and O chemoreception in vertebrates that are tolerant to anoxia has not yet been explored. Using whole cell patch-clamp recording, we characterized four types of ion channels in NECs isolated from the anoxia-tolerant goldfish. A Ca-dependent K current () peaked at ~20 mV, was potentiated by increased intracellular Ca, and was reduced by 100 μM Cd A voltage-dependent inward current in Ba solution, with peak at 0 mV, confirmed the presence of Ca channels.

Distribution and chronotropic effects of serotonin in the zebrafish heart.

Several lines of evidence suggest that serotonin (5-HT) has a regulatory role in cardiovascular function from embryogenesis through adulthood. However, the reported actions of 5-HT are often contradictory and include bradycardia or tachycardia, hypotension or hypertension, and vasodilation or vasoconstriction. Clarifying such cardiac effects requires further research and may benefit from utilizing a model simpler than the mammalian hearts traditionally used in these studies.

Distribution and morphology of cholinergic cells in the branchial epithelium of zebrafish (Danio rerio).

Acetylcholine is an excitatory neurotransmitter important for oxygen sensing in mammals. A cholinergic mechanism in the fish gill has been implicated in the hyperventilatory response to acute hypoxia; however, the identity and distribution of acetylcholine-containing cells in the gills is poorly defined. We test the hypothesis that cholinergic cells are present in the gill filament epithelium in zebrafish (Danio rerio), a model vertebrate for which oxygen chemoreceptors are well characterized, and that these cells would receive nervous innervation.

Mitogenic action of hypoxia upon cutaneous neuroepithelial cells in developing zebrafish.

In zebrafish, cutaneous neuroepithelial cells (NECs) contain serotonin (5-HT) and are believed to initiate physiological and behavioral responses to hypoxia during embryonic and early larval development, when mature gills and O chemoreceptors are not yet present. The number of skin NECs rapidly declines as embryos develop into larvae, but acclimation to hypoxia leads to retention of a greater number of these cells. We hypothesized that reduction of the partial pressure of oxygen (P ) in water would stimulate mitosis in cutaneous NECs in zebrafish.

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts.

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca Therefore, mechanisms of Ca homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology.

Purinergic and Cholinergic Drugs Mediate Hyperventilation in Zebrafish: Evidence from a Novel Chemical Screen.

A rapid test to identify drugs that affect autonomic responses to hypoxia holds therapeutic and ecologic value. The zebrafish (Danio rerio) is a convenient animal model for investigating peripheral O2 chemoreceptors and respiratory reflexes in vertebrates; however, the neurotransmitters and receptors involved in this process are not adequately defined. The goals of the present study were to demonstrate purinergic and cholinergic control of the hyperventilatory response to hypoxia in zebrafish, and to develop a procedure for screening of neurochemicals that affect respiration.