Nervous control of the gills

Unveiling osmoregulation and immunological adaptations in Eleutheronema tetradactylum gills through high-throughput single-cell transcriptome sequencing

The fourfinger threadfin fish (Eleutheronema tetradactylum) is an economically significant species renowned for its ability to adapt to varying salinity environments, with gills serving as their primary organs for osmoregulation and immune defense. Previous studies focused on tissue and morphological levels, whereas ignored the cellular heterogeneity and the crucial gene information related to core cell subsets within E. tetradactylum gills. In this study, we utilized high-throughput single-cell RNA sequencing (scRNA-seq) to analyze the gills of E. tetradactylum, characterizing 16 distinct cell types and identifying unique gene markers and enriched functions associated within each cell type. Additionally, we subdivided ionocyte cells into four distinct subpopulations for the first time in E. tetradactylum gills. By employing weighted gene co-expression network analysis (WGCNA), we further investigated the cellular heterogeneity and specific response mechanisms to salinity fluctuant. Our findings revealed the intricate osmoregulation and immune functions of gill cells, highlighting their crucial roles in maintaining homeostasis and adapting to fluctuating salinity levels. This comprehensive cell-type atlas provides valuable insights into the species adaptive strategies, contributing to the conservation and management of this commercially significant fish as well as other euryhaline species.

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.

Immunohistochemical and ultrastructural study of the immune cell system and epithelial surfaces of the respiratory organs in the bimodally-breathing African sharptooth catfish (Clarias gariepinus Burchell, 1822)

Ach, represents the old neurotransmitter in central and peripheral nervous system. Its muscarinic and nicotinic receptors (mAChRs and nAChRs) constitute an independent cholinergic system that is found in immune cells and playsa key role in regulation of the immune function and cytokine production. Gas exchanging surfaces of the gills and air-breathing organs (ABOs) of the sharptooth catfish Clarias gariepinus were investigated using ultrastructural and confocal immunofluorescence techniques. This study was predominantly focused on the structure of the immune cell types, the expression of their neurotransmitters, including the antimicrobial peptide piscidin 1, and the functional significance of respiratory gas exchange epithelia. A network of immune cells (monocytes, eosinophils, and mast cells) was observed in the gill and theABO epithelia. Eosinophils containing 5HT immunoreactivity were seen in close association with mast cells expressing acetylcholine (Ach), 5HT, nNOS and piscidin 1. A rich and dense cholinergic innervation dispersing across the islet capillaries of the gas exchange barrier, and the localization of Ach in the squamous pavement cells covering the capillaries, were evidenced byVAChT antibodies.We report for the first time that piscidin 1(Pis 1) positive mast cells interact with Pis 1 positive nerves found in the epithelia of the respiratory organs.Pis 1 immunoreactivity was also observed in the covering respiratory epithelium of the ABOs and associated with a role in local mucosal immune defense . The above results anticipate future studies on the neuro-immune interactions at mucosal barrier surfaces, like the gill and the skin of fish, areas densely populated by different immune cells and sensory nerves that constantly sense and adapt to tissue-specific environmental challenges. This article is protected by copyright. All rights reserved.

Expression of acetylcholine, its contribution to regulation of immune function and O2 sensing and phylogenetic interpretations of the African butterfly fish Pantodon buchholzi (Osteoglossiformes, Pantodontidae)

Acetylcholine (Ach) is the main neurotransmitter in the neuronal cholinergic system and also works as a signaling molecule in non-neuronal cells and tissues. The diversity of signaling pathways mediated by Ach provides a basis for understanding the biology of the cholinergic epithelial cells and immune cells in the gill of the species studied. NECs in the gill were not found surprisingly, but specialized cells showing the morphological, histochemical and ultrastructural characteristics of eosinophils were located in the gill filaments and respiratory lamellae. Much remains unknown about the interaction between the nerves and eosinophils that modulate both the release of acetylcholine and its nicotinic and muscarinic receptors including the role of acetylcholine in the mechanisms of O₂ chemosensing. In this study we report for the first time the expression of Ach in the pavement cells of the gill lamellae in fish, the mast cells associated with eosinophils and nerve interaction for both immune cell types, in the gill of the extant butterfly fish Pantodon buchholzi. Multiple roles have been hypothesized for Ach and alpha nAChR in the gills. Among these there are the possible involvement of the pavement cells of the gill lamellae as O₂ chemosensitive cells, the interaction of Ach positive mast cells with eosinophils and interaction of eosinophils with nerve terminals. This could be related to the use of the vesicular acetylcholine transporter (VAChT) and the alpha 2 subunit of the acetylcholine nicotinic receptor (alpha 2 nAChR). These data demonstrate the presence of Ach multiple sites of neuronal and non-neuronal release and reception within the gill and its ancestral signaling that arose during the evolutionary history of this conservative fish species.

Gills versus kidney for ionoregulation in the obligate air-breathing Arapaima gigas, a fish with a kidney in its air-breathing organ

In Arapaima gigas, an obligate air-breather endemic to ion-poor Amazonian waters, a large complex kidney runs through the air-breathing organ (ABO). Previous indirect evidence suggested that the kidney, relative to the small gills, may be exceptionally important in ionoregulation and nitrogen (N) waste excretion, with support of kidney function by direct O2 supply from the airspace. We tested these ideas by continuous urine collection and gill flux measurements in ∼700 g fish. ATPase activities were many-fold greater in kidney than gills. In normoxia, gill Na+ influx and efflux were in balance, with net losses of Cl- and K+ Urine flow rate (UFR, ∼11 ml kg-1 h-1) and urinary ions (< 0.2 mmol l-1) were exceptional, with [urine]:[plasma] ratios of 0.02-0.002 for K+, Na+, and Cl-, indicating strong reabsorption with negligible urinary ion losses. Urinary [ammonia] was very high (10 mmol l-1, [urine]:[plasma] ∼17) indicating strong secretion. The kidney accounted for 21-24% of N excretion, with ammonia dominating (95%) over urea-N through both routes. High urinary [ammonia] was coupled to high urinary [HCO3-]. Aerial hypoxia (15.3 kPa) and aerial hyperoxia (>40.9 kPa) had no effects on UFR, but both inhibited branchial Na+ influx, revealing novel aspects of the osmorespiratory compromise. Aquatic hypoxia (4.1 kPa), but not aquatic hyperoxia (>40.9 kPa), inhibited gill Na+ influx, UFR and branchial and urinary ammonia excretion. We conclude that the kidney is more important than gills in ionoregulation, and is significant in N excretion. Although not definitive, our results do not indicate direct O2 supply from the ABO for kidney function.

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. In the course of the past decade, it has been recognized that lungs contain a population of stem or progenitor cells with an extensive ability to restore tissue; however, despite recent advances in regenerative biology of the lung, the signaling pathways that underlie regeneration are poorly understood. In this Review, we discuss the common evolutionary and embryological origins shared by gills and mammalian lungs. These are evident in homologies in tissue structure, cell populations, cellular function and genetic pathways. An integration of the literature on gill and lung regeneration in vertebrates is presented using a comparative approach in order to outline the challenges that remain in these areas, and to highlight the importance of using aquatic vertebrates as model organisms. The study of gill regeneration in fish and amphibians, which have a high regenerative potential and for which genetic tools are widely available, represents a unique opportunity to uncover common signaling mechanisms that may be important for regeneration of respiratory organs in all vertebrates. This may lead to new advances in tissue repair following lung disease.

Functional and evolutionary perspectives on gill structures of an obligate air-breathing, aquatic snail

Ampullariids are freshwater gastropods bearing a gill and a lung, thus showing different degrees of amphibiousness. In particular, Pomacea canaliculata (Caenogastropoda, Ampullariidae) is an obligate air-breather that relies mainly or solely on the lung for dwelling in poorly oxygenated water, for avoiding predators, while burying in the mud during aestivation, and for oviposition above water level. In this paper, we studied the morphological peculiarities of the gill in this species. We found (1) the gill and lung vasculature and innervation are intimately related, allowing alternation between water and air respiration; (2) the gill epithelium has features typical of a transporting rather than a respiratory epithelium; and (3) the gill has resident granulocytes within intraepithelial spaces that may serve a role for immune defence. Thus, the role in oxygen uptake may be less significant than the roles in ionic/osmotic regulation and immunity. Also, our results provide a morphological background to understand the dependence on aerial respiration of Pomacea canaliculata. Finally, we consider these findings from a functional perspective in the light of the evolution of amphibiousness in the Ampullariidae, and discuss that master regulators may explain the phenotypic convergence of gill structures amongst this molluscan species and those in other phyla.

Air- breathing in fish: Air- breathing organs and control of respiration: Nerves and neurotransmitters in the air-breathing organs and the skin

In fishes, exploitation of aerial gas exchange has evolved independently many times, involving a variety of air-breathing organs. Indeed, air-breathing occurs in at least 49 known families of fish (Graham, 1997). Many amphibious vertebrates, at some stage of their development are actually trimodal breathers that use various combinations of respiratory surfaces to breath both water (skin and/or gill) and air (skin and/or lung). The present review examines the evolutionary implications of air-breathing organs in fishes and the morphology of the peripheral receptors and the neurotransmitter content of the cells involved in the control of air-breathing. Control of breathing, whether gill ventilation or air-breathing, is influenced by feedback from peripheral and/or central nervous system receptors that respond to changes in PO2, PCO2 and/or pH. Although the specific chemoreceptors mediating the respiratory reflexes have not been conclusively identified, studies in water-breathing teleosts have implicated the neuroepithelial cells (NECs) existing in gill tissues as the O2 sensitive chemoreceptors that initiate the cardiorespiratory reflexes in aquatic vertebrates. Some of the air-breathing fishes, such as Protopterus, Polypterus and Amia have been shown to have NECs in the gills and/or lungs, although the role of these receptors and their innervation in the control of breathing is not known. NECs have been also reported in the specialized respiratory epithelia of accessory respiratory organs (ARO's) of some catfish species and in the gill and skin of the mudskipper Periophthalmodon schlosseri. Unlike teleosts matching an O2-oriented ventilation to ambient O2 levels, lungfishes have central and peripheral H+/CO2 receptors that control the acid-base status of the blood.

The innervation of the zebrafish pharyngeal jaws and teeth

Zebrafish (Danio rerio) teeth are increasingly used as a model to study odontogenesis in non-mammalians. Using serial semi-thin section histology and immunohistochemistry, the nerves innervating the pharyngeal jaws and teeth have been identified. The last pair of branchial arches, which are non-gill bearing but which carry the teeth, are innervated by an internal branch of a post-trematic ramus of the vagal nerve. Another, external, branch is probably responsible for the motor innervation of the branchiomeric musculature. Nerve fibres appear in the pulp cavity of the teeth only late during cytodifferentiation, and are therefore likely not involved in early steps of tooth formation. The precise role of the nervous system during continuous tooth replacement remains to be determined. Nonetheless, this study provides the necessary morphological background information to address this question.

A time differential staining technique coupled with full bilateral gill denervation to study ionocytes in fish

Branchial ionocytes (ICs) are the functional units for ionic regulation in fish. In adults, they are found on the filamental and lamellar epithelia of the gill where they transport ions such as Na+, Cl- and Ca2+ via a variety of ion channels, pumps and exchangers. The teleost gill is extrinsically innervated by the facial (VI), glossopharyngeal (IX) and vagus (X) nerves. The IX and X nerves are also the extrinsic source of branchial IC innervation. Here, two techniques used to study the innervation, proliferation and distribution of ICs are described: a time differential staining technique and a full bilateral gill denervation technique. Briefly, goldfish are exposed to a vital mitochondrion-specific dye (e.g., MitoTracker Red) which labels (red fluorescence) pre-existing ICs. Fish were either allowed to recover for 3-5 days or immediately underwent a full bilateral gill denervation. After 3-5 days of recovery, the gills are harvested and fixed for immunohistochemistry. The tissue is then stained with an α-5 primary antibody (targets Na+/K+ ATPase containing cells) in conjunction with a secondary antibody that labels all (both new and pre-existing) ICs green. Using confocal imaging, it was demonstrated that pre-existing ICs appear yellow (labelled with both a viable mitochondrion-specific dye and α-5) and new ICs appear green (labelled with α-5 only). Both techniques used in tandem can be applied to study the innervation, proliferation and distribution of ICs on the gill filament when fish are exposed to environmental challenges.

Biological responses of juvenile European sea bass (Dicentrarchus labrax) exposed to contaminated sediments

Multiple anthropogenic activities present along coastal environments may affect the health status of aquatic ecosystems. In this study, specimens of European sea bass (Dicentrarchus labrax) were exposed for 30 days to highly contaminated sediment collected from the industrial area between Augusta and Priolo (Syracuse, Italy), defined as the most mercury polluted site in the Mediterranean. The aim was to evaluate the responses of juvenile D. labrax to highly contaminated sediments, particularly enriched in Hg, in order to enhance the scarce knowledge on the potential compensatory mechanisms developed by organisms under severe stress conditions. Apoptotic and proliferative activities [cell turnover: Proliferating Cell Nuclear Antigen (PCNA) and FAS Ligand (FasL)], onset of hypoxic condition [hypoxia: Hypoxia Inducibile Factor-1α (HIF-1α)], and changes in the neuroendocrine control mechanisms [neurotransmission: Tyrosine Hydroxylase (TH), Choline Acetyltransferase (ChAT), Acetylcholinesterase (AChE), 5-Hydroxytryptamine (5-HT) and 5-Hydroxytryptamine receptor 3 (5-HT3)] were investigated in sea bass gill tissues. In the specimens exposed to the polluted sediment, the occurrence of altered cell turnover may result in impaired gas exchange that leads to a condition of "functional hypoxia". Changes in neurotransmission pathways were also observed, suggesting a remodeling process as an adaptive response to increase the O2-carrying capacity and restore the normal physiological conditions of the gills. Overall, these findings demonstrated that although chronic exposure to heavy metal polluted sediments alters the functioning of both the nervous and endocrine systems, as well as plasticity of the gill epithelium, fish are able to trigger a series of physiological adjustments or adaptations interfering with specific neuroendocrine control mechanisms that enable their long-term survival.

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.

Confocal imaging of Merkel-like basal cells in the taste buds of zebrafish

The oropharyngeal cavity in fish supports a range of sensory modalities, including detection of chemical and mechanical stimuli. Taste buds are found throughout this tissue and may participate in both processes. We used confocal microscopy and immunohistochemistry to characterize the morphology of Merkel-like cells and their association with other cell types and nerve fibers of the taste bud in the vertebrate model, the zebrafish. In addition, we document procedures for the observation of these structures in whole-tissue preparations from larvae and adults using zebrafish-specific and monoclonal antibodies. A single microvillus Merkel-like cell was found in each taste bud regardless of age or location. Merkel-like cells were neurosecretory, as indicated by labelling with the styryl dye, FM1-43, and the synaptic vesicle marker, SV2. Merkel-like cells were associated with SV2- and calretinin-positive taste receptor cells, received innervation from discoid aggregations of nerve fibers, and retained serotonin-filled synaptic vesicles oriented within the cytoplasm toward adjacent innervation. Moreover, a ring-like formation of nerve endings was identified with the neuronal marker, zn-12 that circumscribed the taste receptor area, surrounding calretinin-immunoreactive taste cell microvilli, and appeared to associate with the nerve plexus adjacent to Merkel-like cells. We suggest that these nerve fibers are somatosensory, perhaps associated with mechanoreception or the common chemical sense.

Localization of neurotransmitters, peptides and nNOS in the pseudobranchial neurosecretory cell system and associated carotid labyrinth of the catfish, Clarias batrachus

The carotid labyrinth is an enigmatic endocrine structure of unknown chemosensory function lying in the gill region of the catfishes. The carotid body is found at the carotid bifurcation of amphibians and all mammalian vertebrates on the evolutionary tree. It is a vascular expansion comprised of a cluster of glomus cells with associated (afferent and efferent) innervations. In the catfish species studied (Clarias batrachus) a neurosecretory cell system consisting of pseudobranchial neurosecretory cells connect the carotid labyrinth or large vessels (both the efferent branchial artery and dorsal aorta), and is likely akin to the glomus cells, but comparing these structures in widely divergent vertebrate species, the conclusion is that the structural components are more elaborate than those of terrestrial vertebrates. However, these cells reveal both an endocrine phenotype (such as the association with capillaries and large vessels) and the presence of regulatory substances such as neurotransmitters and neuropeptides producing good evidence for high levels of conservation of these substances that are present in the glomus cells of mammalian vertebrates. VIP-immunopositive neuronal cell bodies are detected in the periphery of the carotid labyrinth. They are presumptive local neurons that differ from pseudobranchial neurosecretory cells, the latter failing to express VIP in their soma.

Proteomic modification in gills and brains of medaka fish (Oryzias melastigma) after exposure to a sodium channel activator neurotoxin, brevetoxin-1

Although brevetoxins (PbTxs) produced by the marine dinoflagellate Karenia brevis are known to be absorbed across gill membranes and exert their acute toxic effects through an ion-channel mediated pathway in neural tissue, the exact biochemical mechanism concerning PbTxs neurotoxicity in neural tissue and gas-exchange organs has not been well elucidated. In this study, we calculated the LC(50) value of PbTx-1 using the medaka fish model, and presented the molecular responses of sub-acute exposure to PbTx-1 with proteomic method. By adopting two-dimensional electrophoresis, the abundances of 14 and 24 proteins were found to be remarkably altered in the gills and brains, respectively, in response to toxin exposure. Thirteen gill and twenty brain proteins were identified using matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry analysis. These proteins could be categorized into diverse functional classes such as cell structure, macromolecule metabolism, signal transduction and neurotransmitter release. These findings can help to elucidate the possible pathways by which aquatic toxins affect marine organisms within target organs.

Nervous control of photophores in luminescent fishes

Functional studies of the autonomic innervation in the photophores of luminescent fishes are scarce. The majority of studies have involved either the stimulation of isolated photophores or the modulatory effects of adrenaline-induced light emission. The fish skin is a highly complex organ that performs a wide variety of physiological processes and receives extensive nervous innervations. The latter includes autonomic nerve fibers of spinal sympathetic origin having a secretomotor function. More recent evidence indicates that neuropeptide-containing nerve fibers, such as those that express tachykinin and its NK1 receptor, neuropeptide Y, or nitric oxide, may also play an important role in the nervous control of photophores. There is no anatomical evidence that shows that nNOS positive (nitrergic) neurons form a population distinct from the secretomotor neurons with perikarya in the sympathetic ganglia. The distribution and function of the nitrergic nerves in the luminous cells, however, is less clear. It is likely that the chemical properties of the sympathetic postganglionic neurons in the ganglia of luminescent fishes are target-specific, such as observed in mammals.

The area postrema in hindbrain is a central player for regulation of drinking behavior in Japanese eels

It is recognized that fish will drink the surrounding water by reflex swallowing without a thirst sensation. We evaluated the role of the area postrema (AP), a sensory circumventricular organ (CVO) in the medulla oblongata, in the regulation of drinking behavior of seawater (SW) eels. The antidipsogenic effects of ghrelin and atrial natriuretic peptide and hypervolemia and hyperosmolemia (1 M sucrose or 10% NaCl) as well as the dipsogenic effects of angiotensin II and hypovolemia (hemorrhage) were profoundly diminished after AP lesion (APx) in eels compared with sham controls. However, the antidipsogenic effect of urotensin II was not influenced by APx, possibly due to the direct baroreflex inhibition on the swallowing center in eels. When ingested water was drained via an esophageal fistula, water intake increased 30-fold in sham controls but only fivefold in APx eels, suggesting a role for the AP in continuous regulation of drinking by SW eels. After transfer from freshwater to SW, APx eels responded normally with an immediate burst of drinking, but after 4 wk these animals showed a much greater increase in plasma osmolality than controls, suggesting that the AP is involved in acclimation to SW by fine tuning of the drinking rate. Taken together, the AP in the hindbrain of eels plays an integral role in SW acclimation, acting as a conduit of information from plasma for the regulation of drinking, probably without a thirst sensation. This differs from mammals in which sensory CVOs in the forebrain play pivotal roles in thirst regulation.

Presence of paraneuronal pseudobranchial neurosecretory system in the gill region of two air-breathing clupeids, Notopterus chitala and Notopterus notopterus

The pseudobranchial neurosecretory system (PNS) is a system of neurosecretion observed in certain groups of teleosts, which are air-breathing or known to tolerate low oxygen tension in the surrounding water. Like other neuroendocrine cells of gill, cells belonging to this system have also been observed to have a role in condition of hypoxia. Uniformly found in all catfish species, the system was reported to be present in few non-catfish groups also, viz.-Atheriniformes, Channiformes (Devi, 1987), Perciformes, and Clupeiformes (Srivastava et al., 1981; Gopesh, 1983). In an attempt to study the structure and organization of the pseudobranchial neurosecretory system in non-catfish species of teleost, present investigation was undertaken in two species of Notopterus, viz. Notopterus chitala and Notopterus notopterus. The histological observations, using neurosecretion specific stains, undertaken on two clupeids are reported and the findings are discussed in the light of association of PNS with Carotid gland-a structure of intermediate stage in the process of transformation of pseudobranch into the carotid labyrinth, in course of evolution and also the air-breathing habit of the fish.