The dipsogenic effect of the renin-angiotensin system in elasmobranch fish

The Amphibious Mudskipper: A Unique Model Bridging the Gap of Central Actions of Osmoregulatory Hormones Between Terrestrial and Aquatic Vertebrates

Body fluid regulation, or osmoregulation, continues to be a major topic in comparative physiology, and teleost fishes have been the subject of intensive research. Great progress has been made in understanding the osmoregulatory mechanisms including drinking behavior in teleosts and mammals. Mudskipper gobies can bridge the gap from aquatic to terrestrial habitats by their amphibious behavior, but the studies are yet emerging. In this review, we introduce this unique teleost as a model to study osmoregulatory behaviors, particularly amphibious behaviors regulated by the central action of hormones. Regarding drinking behavior of mammals, a thirst sensation is aroused by angiotensin II (Ang II) through direct actions on the forebrain circumventricular structures, which predominantly motivates them to search for water and take it into the mouth for drinking. By contrast, aquatic teleosts can drink water that is constantly present in their mouth only by reflex swallowing, and Ang II induces swallowing by acting on the hindbrain circumventricular organ without inducing thirst. In mudskippers, however, through the loss of buccal water by swallowing, which appears to induce buccal drying on land, Ang II motivates these fishes to move to water for drinking. Thus, mudskippers revealed a unique thirst regulation by sensory detection in the buccal cavity. In addition, the neurohypophysial hormones, isotocin (IT) and vasotocin (VT), promote migration to water via IT receptors in mudskippers. VT is also dipsogenic and the neurons in the forebrain may mediate their thirst. VT regulates social behaviors as well as osmoregulation. The VT-induced migration appears to be a submissive response of subordinate mudskippers to escape from competitive and dehydrating land. Together with implications of VT in aggression, mudskippers may bridge the multiple functions of neurohypophysial hormones. Interestingly, cortisol, an important hormone for seawater adaptation and stress response in teleosts, also stimulates the migration toward water, mediated possibly via the mineralocorticoid receptor. The corticosteroid system that is responsive to external stressors can accelerate emergence of migration to alternative habitats. In this review, we suggest this unique teleost as an important model to deepen insights into the behavioral roles of these hormones in relation to osmoregulation.

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

Hormonal control of drinking behavior in teleost fishes; insights from studies using eels

Marine teleost fishes drink environmental seawater to compensate for osmotic water loss, and the amount of water intake is precisely regulated to prevent dehydration or hypernatremia. Unlike terrestrial animals in which thirst motivates a series of drinking behaviors, aquatic fishes can drink environmental water by reflex swallowing without searching for water. Hormones are key effectors for the regulation of drinking. In particular, angiotensin II and atrial natriuretic peptide are likely candidates for physiological regulators because of their potent dipsogenic and antidipsogenic activities, respectively. In the eel, these hormones act on the area postrema in the medulla oblongata, a circumventricular structure without blood-brain barrier, which then regulates the activity of the glossopharyngeal-vagal motor complex. These motor neurons in the hindbrain innervate the upper esophageal sphincter muscle and other swallowing-related muscles in the pharynx and esophagus for regulation of drinking. Thus, the neural circuitry for drinking in fishes appears to be confined within the hindbrain. This simple mechanism is much different from that of terrestrial animals in which thirst sensation is induced through hormonal actions on the subfornical organ and organum vasculosum of the lamina terminalis that are located in the forebrain. It seems that the neural and hormonal mechanism that regulates drinking behavior has evolved from fishes depending on the availability of water in their natural habitats.

Evolution of melanocortin receptors in cartilaginous fish: melanocortin receptors and the stress axis in elasmobranches

There is general agreement that the presence of five melanocortin receptor genes in tetrapods is the result of two genome duplications that occurred prior to the emergence of the gnathostomes, and at least one local gene duplication that occurred early in the radiation of the ancestral gnathostomes. Hence, it is assumed that representatives from the extant classes of gnathostomes (i.e., Chondrichthyes, Actinopterygii, Sarcopterygii) should also have five paralogous melanocortin genes. Current studies on cartilaginous fishes indicate that while there is evidence for five paralogous melanocortin receptor genes in this class, to date all five paralogs have not been detected in the genome of a single species. This mini-review will discuss the ligand selectivity properties of the melanocortin-3 receptor of the elephant shark (subclass Holocephali) and the ligand selectivity properties of the melanocortin-3 receptor, melanocortin-4 receptor, and the melanocortin-5 receptor of the dogfish (subclass Elasmobranchii). The potential relationship of these melanocortin receptors to the hypothalamus/pituitary/interrenal axis will be discussed.

Purification, structural characterization, and myotropic activity of a peptide related to des-Arg(9)-bradykinin from an elasmobranch fish, the little skate, Leucoraja erinacea

A bradykinin (BK)-related peptide was isolated from heat-denaturated plasma from an elasmobranch fish, the little skate, Leucoraja erinacea after incubation with porcine pancreatic kallikrein. The primary structure of the peptide (H-Gly-Ile-Thr-Ser-Trp-Leu-Pro-Phe-OH; skate BK) shows limited structural similarity to the mammalian B1 receptor agonist, des-Arg(9)-BK. The myotropic activities of synthetic skate BK, and the analog skate [Arg(9)]BK, were examined in isolated skate vascular and intestinal smooth muscle preparations. Skate BK produced a concentration-dependent constriction of the mesenteric artery (EC(50)=4.37x10(-8)M; maximum response=103.4+/-10.23% of the response to 60mM KCl) but the response to skate [Arg(9)]BK was appreciably weaker (response to 10(-6)M=73.0+/-23.4% of the response to 60mM KCl). Neither the first branchial gill arch nor the ventral aorta responded to either purified peptide. Skate BK also produced a concentration-dependent constriction of intestinal smooth muscle preparations (EC(50)=2.74x10(-7)M; maximum response 31.0+/-12.2% of the response to 10(-5)M acetylcholine). Skate [Arg(9)]BK was without effect on the intestinal preparation. The data provide evidence for the existence of the kallikrein-kinin system in a phylogenetically ancient vertebrate group and the greater potency of skate BK compared with the analog skate [Arg(9)]BK suggests that the receptor mediating vascular responses resembles the mammalian B1 receptor more closely than the B2 receptor.

Body fluid volume regulation in elasmobranch fish

This review addresses an often overlooked aspect of elasmobranch osmoregulation, i.e., control of body fluid volume. More specifically the review addresses the impact of changes in blood volume in elasmobranchs exposed to different environmental salinities. Measurement of blood volume in the European lesser-spotted dogfish, Scyliorhinus canicula, following acute and chronic exposure to 80% and 120% seawater (SW) is reported. In 80%, 100% and 120% SW-adapted S. canicula, blood volume was 6.3+/-0.2, 5.6+/-0.2 and 4.6+/-0.2 mL 100 g(-1) body mass, respectively. Blood volume was significantly higher and lower in 80% and 120% SW-acclimated animals compared to 100% SW controls. Comparisons are made between these results and previously published data. The role of drinking and volume regulation in elasmobranchs is discussed. For the first time measured water reabsorption rates and solute flux rates across the elasmobranch intestinal epithelia are presented. Water reabsorption rates did not differ between 100% SW-adapted bamboo shark, Chiloscyllium plagiosum, and fish acutely transferred to 140% SW. For the most part net solute flux rates and direction for both the 100% and 140% SW groups were the same with the exception of a net efflux of chloride and potassium in the 140% group and influx of these ions in the 100% adapted group. The significance of the intestine as part of the overall elasmobranch osmoregulatory strategy is discussed as is the role of the kidneys, rectal gland and gills in the regulation of body fluid volume in this class of vertebrates.

Sequence, circulating levels, and expression of C-type natriuretic peptide in a euryhaline elasmobranch, Carcharhinus leucas

The present study has examined expression and circulating levels of C-type natriuretic peptide (CNP) in the euryhaline bull shark, Carcharhinus leucas. Complementary DNA and deduced amino acid sequence for CNP in C. leucas were determined by RACE methods. Homology of CNP amino acid sequence in C. leucas was high both for proCNP and for mature CNP when compared with previously identified elasmobranch CNPs. Mature CNP sequence in C. leucas was identical to that in Triakis scyllia and Scyliorhinus canicula. Levels of expression of CNP mRNA were significantly decreased in the atrium but did not change in either the brain or ventricle following acclimation to a SW environment. However, circulating levels of CNP significantly increased from 86.0+/-7.9 fmol ml(-1) in FW to 144.9+/-19.5 fmol ml(-1) in SW. The results presented demonstrate that changes in environmental salinity influences both synthesis of CNP from the heart and also circulating levels in C. leucas. Potential stimulus for release and modes of action are discussed.

Hormonal regulation of the fish gastrointestinal tract

The gastrointestinal tracts (GIT) of fish and other vertebrates are challenged with a diversity of functional demands caused by changes and differences in dietary inputs and environmental conditions. This contribution reviews how hormonal regulation plays an essential role in modulating the GIT functions of fish to match changes in functional demands. Exemplary is how hormones produced by the GIT, the associated organs (e.g., pancreas), and other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads) modulate the digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs. Hormones regulate the other GIT functions of osmoregulation (secretion and absorption of electrolytes and water), immunity, endocrine secretions, metabolism, and the elimination of toxic metabolites and environmental contaminants to match changes in environmental conditions and physiological states. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.

Genomic characterization and expression analysis of the first nonmammalian renin genes from zebrafish and pufferfish

Renin is a key enzyme in the renin-angiotensin system (RAS), a pathway which plays an important physiological role in blood pressure and electrolyte homeostasis. The origin of the RAS is believed to have accompanied early evolution of vertebrates. However, renin genes have so far only been unequivocally identified in mammals. Whether or not a bona fide renin gene exists in nonmammalian vertebrates has been an intriguing question of physiological and evolutionary interest. Using a genomic analytical approach, we identified renin genes in two nonmammalian vertebrates, zebrafish (Danio rerio) and pufferfish (Takifugu rubripes). Phylogenetic analysis demonstrates that the predicted fish renins cluster together with mammalian renins to form a distinct subclass of vertebrate aspartyl proteases. RT-PCR results confirm generation of the predicted zebrafish mRNA and its expression in association with the opisthonephric kidney of adult zebrafish. Comparative in situ hybridization analysis of wild-type and developmental mutants indicates that renin expression is first detected bilaterally in cells of the interrenal primordia at 24 h postfertilization, which subsequently migrate to lie adjacent to, but distinct from, the glomerulus of the developing pronephric kidney. Our report provides the first molecular evidence for the existence of renin genes in lower vertebrates. The observation that the earliest renin-expressing cells, arising during ontogeny of this teleost vertebrate, are of adrenocortical lineage raises an interesting hypothesis as regards the origin of renin-expressing cells in the metanephric kidney of higher vertebrates.

Urea based osmoregulation and endocrine control in elasmobranch fish with special reference to euryhalinity

Since the landmark contributions of Homer Smith and co-workers in the 1930s there has been a considerable advance in our knowledge regarding the osmoregulatory strategy of elasmobranch fish. Smith recognised that urea was retained in the body fluids as part of the 'osmoregulatory ballast' of elasmobranch fish so that body fluid osmolality is raised to a level that is iso- or slightly hyper-osmotic to that of the surrounding medium. From studies at that time he also postulated that many marine dwelling elasmobranchs were not capable of adaptation to dilute environments. However, more recent investigations have demonstrated that, at least in some species, this may not be the case. Gradual acclimation of marine dwelling elasmobranchs to varying environmental salinities under laboratory conditions has demonstrated that these fish do have the capacity to acclimate to changes in salinity through independent regulation of Na(+), Cl(-) and urea levels. This suggests that many of the presumed stenohaline marine elasmobranchs could in fact be described as partially euryhaline. The contributions of Thomas Thorson in the 1970s demonstrated the osmoregulatory strategy of a fully euryhaline elasmobranch, the bull shark, Carcharhinus leucas, and more recent investigations have examined the mechanisms behind this strategy in the euryhaline elasmobranch, Dasyatis sabina. Both partially euryhaline and fully euryhaline species utilise the same physiological processes to control urea, Na(+) and Cl(-) levels within the body fluids. The role of the gills, kidney, liver, rectal gland and drinking process is discussed in relation to the endocrine control of urea, Na(+) and Cl(-) levels as elasmobranchs acclimate to different environmental salinities.

Osmotic and volaemic effects on drinking rate in elasmobranch fish

An increase in drinking rate of two species of marine elasmobranch fish, Scyliorhinus canicula and Triakis scyllia, acclimated to 80% sea water was observed following the introduction of 100 % sea water to experimental tanks. The drinking response in both species was found to be maximal within 6 h, and a significant increase was sustained for up to 24 h in T. scyllia. Plasma osmolality was significantly increased within 6 h following introduction of 100 % sea water, and this increase was principally due to elevated plasma Na+ and Cl- concentrations. Administration of 2 mol l-1 mannitol, 75 % sucrose and vehicle(elasmobranch Ringer) did not induce a significant increase or decrease in the drinking rate of S. canicula. However, injection of 20 % NaCl was found to decrease drinking rate significantly in S. canicula 60 min after administration. Controlled haemorrhage of approximately 5.7 % of total blood volume in S. canicula induced a rapid 36-fold increase in drinking over basal levels. The present study demonstrates a physiological dipsogenesis in response to hypovolaemia in marine elasmobranch fish as part of their overall iso/hyperosmoregulatory strategy.