Glomerular actions of arginine vasotocin in the in situ perfused trout kidney

Hypo-Osmoregulatory Roles of Vasotocinergic and Isotocinergic Systems in the Intestines of Two European Sea Bass Lineages

European sea bass (Dicentrarchus labrax) are a major aquaculture species that live in habitats with fluctuating salinities that are sometimes higher than in seawater (SW). Atlantic and West-Mediterranean genetic lineages were compared regarding intestinal neuropeptide receptor expression in SW (36%) and following a two-week transfer to hypersalinity (HW, 55%). Phylogenetic analysis revealed seven neuropeptide receptors belonging to the arginine vasotocine (AVTR) family and two isotocin receptors (ITR). Among AVTR paralogs, the highest mRNA levels were recorded for v1a2, with a two- to fourfold upregulation in the European sea bass intestinal sections after transfer of fish to HW. Principal component analysis in posterior intestines showed that v1a2 expression grouped together with the expression and activity of main ion transporters and channels involved in solute-coupled water uptake, indicating a possible role of this receptor in triggering water absorption. v1a1 expression, however, was decreased or did not change after transfer to hypersaline water. Among ITR paralogs, itr1 was the most expressed paralog in the intestine and opposite expression patterns were observed following salinity transfer, comparing intestinal sections. Overall, different expression profiles were observed between genetic lineages for several analyzed genes which could contribute to different osmotic stress-related responses in D. labrax lineages.

Revealing the Increased Stress Response Behavior through Transcriptomic Analysis of Adult Zebrafish Brain after Chronic Low to Moderate Dose Rates of Ionizing Radiation

Simple Summary

The increasing use of radiopharmaceuticals for medical diagnostics and radiotherapy raises concerns regarding health risks for both humans and the environment. Additionally, in the context of major nuclear accidents like in Chernobyl and Fukushima, very little is known about the effects of chronic exposure to low and moderate dose rates of ionizing radiation (IR). Many studies demonstrated the sensibility of the developmental brain, but little data exists for IR at low dose rates and their impact on adults. In this study, we characterized the molecular mechanisms that orchestrate stress behavior caused by chronic exposure to low to moderate dose rates of IR using the adult zebrafish model. We observed the establishment of a congruent stress response at both the molecular and individual levels.

Abstract

High levels of ionizing radiation (IR) are known to induce neurogenesis defects with harmful consequences on brain morphogenesis and cognitive functions, but the effects of chronic low to moderate dose rates of IR remain largely unknown. In this study, we aim at defining the main molecular pathways impacted by IR and how these effects can translate to higher organizational levels such as behavior. Adult zebrafish were exposed to gamma radiation for 36 days at 0.05 mGy/h, 0.5 mGy/h and 5 mGy/h. RNA sequencing was performed on the telencephalon and completed by RNA in situ hybridization that confirmed the upregulation of oxytocin and cone rod homeobox in the parvocellular preoptic nucleus. A dose rate-dependent increase in differentially expressed genes (DEG) was observed with 27 DEG at 0.05 mGy/h, 200 DEG at 0.5 mGy/h and 530 DEG at 5 mGy/h. Genes involved in neurotransmission, neurohormones and hypothalamic-pituitary-interrenal axis functions were specifically affected, strongly suggesting their involvement in the stress response behavior observed after exposure to dose rates superior or equal to 0.5 mGy/h. At the individual scale, hypolocomotion, increased freezing and social stress were detected. Together, these data highlight the intricate interaction between neurohormones (and particularly oxytocin), neurotransmission and neurogenesis in response to chronic exposure to IR and the establishment of anxiety-like behavior.

Circulating Isotocin, not Angiotensin II, is the Major Dipsogenic Hormone in Eels

Angiotensin II (AngII) is generally known as the most important dipsogenic hormone throughout vertebrates, while two other neurohypophysial hormones, vasopressin and oxytocin, are not dipsogenic in mammals. In this study, we found that systemic isotocin, but not vasotocin, is the potent dipsogenic hormone in eels. When injected intra-arterially into conscious eels, isotocin, vasotocin and AngII equally increased ventral aortic pressure dose-dependently at 0.03-1.0 nmol/kg, but only isotocin induced copious drinking. The dipsogenic effect was dose-dependent and occurred significantly at as low as 0.1 nmol/kg. By contrast, a sustained inhibition of drinking occurred after AngII, probably due to baroreflexogenic inhibition. No such inhibition was observed after isotocin despite similar concurrent hypertension. The baroreceptor may exist distal to the gill circulation because the vasopressor effect occurred at both ventral and dorsal aorta after AngII but only at ventral aorta after isotocin. By contrast, intra-cerebroventricular (i.c.v.) injection of isotocin had no effect on drinking or blood pressure, but AngII increased drinking and aortic pressure dose-dependently at 0.03-0.3 nmol/eel. Lesioning of the area postrema (AP), a sensory circumventricular organ, abolished drinking induced by peripheral isotocin, but not i.c.v. AngII. Collectively, isotocin seems to be a major circulating hormone that induces swallowing through its action on the AP, while AngII may be an intrinsic brain peptide that induces drinking through its action on a different circumventricular site, possibly a recently identified blood-brain barrier-deficient structure in the antero-ventral third ventricle of eels, as shown in birds and mammals.

Reproductive hormones modulate differentially brain and ovarian vasotocin receptor gene expression in early and late recrudescent catfish, Heteropneustes fossilis

Investigations on the role of the reproductive hormones on VT receptor gene expression are lacking in teleosts. Previously we reported that gonadotropin and steroid hormones modulate the secretion and gene expression of brain and ovarian vasotocin (VT) in the catfish Heteropneustes fossilis. In continuation, in the present study we investigated the role of estradiol-17β (E₂), the maturation-inducing steroid (MIS) 17α, 20β-dihydroxy-4-pregnen-3-one (17, 20β-DP), and human chorionic gonadotropin (hCG) on the expression of VT receptor genes (v1a1, v1a2 and v2a) in the brain and ovary of the catfish in early (previtellogenic, preparatory) and late (post vitellogenic, prespawning) phases of the ovarian cycle. The steroid treatments (in vivo and in vitro) modulated only the v1a1 and v1a2 expression in both tissues, but not the v2a expression. The E₂-induced modulation of the v1a1 and v1a2 gene expression varied with the reproductive phase. In the preparatory phase, E₂ up regulated the expression of brain and ovarian v1a1 and v1a2 gene expression, the response varied with the dose and duration. In the prespawning phase, E₂ inhibited the expression in a dose- and duration-dependent manner. On the other hand, 17, 20β-DP up regulated the expression of brain and ovarian v1a1 and v1a2 in both phases, and the response was higher in the prespawning phase and varied with dose and duration. In contrast to the steroid effects, the hCG treatment modulated the expression of all the VT receptor genes only in the prespawning phase and the response varied with dose and duration. The results indicate differential modulatory roles of steroid hormones and hCG on the VT receptor gene expression, to mediate VT's reproductive or osmoregulatory functions. While the hCG effect on v1a type receptor expression may be steroid- dependent, that of v2a expression seems to be steroid-independent.

Arginine Vasotocin and Cortisol Co-regulate Vasotocinergic, Isotocinergic, Stress, and Thyroid Pathways in the Gilthead Sea Bream (Sparus aurata)

In teleosts, a complex interaction between several endocrine axes modulates physiological functions related to metabolism, stress, and osmoregulation. Although many studies in fish underline the interconnection between the hypothalamic–pituitary–interrenal (HPI) and hypothalamic–pituitary–thyroid (HPT) endocrine axes, their relationship with the vasotocinergic and isotocinergic systems remains unknown. The aim of the present study is therefore to shed light on the potential cross-regulations between HPT, HPI, and the vasotocinergic and isotocinergic axes in gilthead sea bream (Sparus aurata) at hypothalamic, hypophyseal, and plasma levels. Sea breams were administered with intraperitoneal slow-release implants containing different doses of vasotocin (the active peptide in vasotocinergic system) or cortisol (the last component of HPI axis). Plasma osmolality was higher in active neuropeptides vasotocin (Avt)-treated fish, indicating an osmoregulatory function of this hormone. Low concentrations of Avt increased hypothalamic arginine vasotocin precursor (avt) mRNA levels and increased Avt storage in the pituitary. Avt treatment down-regulated hypothalamic arginine vasotocin receptor v1a-type (avtrv1a), suggesting a negative paracrine co-regulation of the HPI axis due to the close location of avtrv1a and adrenocorticotropin hormone (Acth) cells in the anterior pituitary. Furthermore, the up-regulation observed in arginine vasotocin receptor v2-type (avtrv2) suggests their involvement in metabolic and cortisol-related pathways in the hypothalamus. The decrease in isotocin (It) pituitary storage and the up-regulation of it receptor, observed in the Avt-treated group, reinforce the idea of an interconnection between the vasotocinergic and isotocinergic systems. Cortisol and Avt administration each inhibited the HPI axis, down-regulating crh gene expression in the absence of variations in corticotropin releasing hormone binding protein (crhbp). Finally, both hormonal treatments activated the HPT axis via up-regulation of trh and down-regulation of thrb. Our results provide evidence for strong interactions among the Avt/It, HPI, and HPT axes of marine teleosts, particularly at the hypothalamic level.

Evidence for a role of arginine vasotocin receptors in the gill during salinity acclimation by a euryhaline teleost fish

The nonapeptide arginine vasotocin (AVT) regulates osmotic balance in teleost fishes, but its mechanisms of action are not fully understood. Recently, it was discovered that nonapeptide receptors in teleost fishes are differentiated into two V1a-type, several V2-type, and two isotocin (IT) receptors, but it remains unclear which receptors mediate AVT's effects on gill osmoregulation. Here, we examined the role of nonapeptide receptors in the gill of the euryhaline Amargosa pupfish (Cyprinodon nevadensis amargosae) during osmotic acclimation. Transcripts for the teleost V1a-type receptor v1a2 were upregulated over fourfold in gill 24 h after transferring pupfish from 7.5 ppt to seawater (35 ppt) or hypersaline (55 ppt) conditions and downregulated after transfer to freshwater (0.3 ppt). Gill transcripts for the nonapeptide degradation enzyme leucyl-cystinyl aminopeptidase (LNPEP) also increased in fish acclimating to 35 ppt. To test whether the effects of AVT on the gill might be mediated by a V1a-type receptor, we administered AVT or a V1-type receptor antagonist (Manning compound) intraperitoneally to pupfish before transfer to 0.4 ppt or 35 ppt. Pupfish transferred to 35 ppt exhibited elevated gill mRNA abundance for cystic fibrosis transmembrane conductance regulator (cftr), but that upregulation diminished under V1-receptor inhibition. AVT inhibited the increase in gill Na⁺/Cl^- cotransporter 2 (ncc2) transcript abundance that occurs following transfer to hypoosmotic environments, whereas V1-type receptor antagonism increased ncc2 mRNAs even without a change in salinity. These findings indicate that AVT acts via a V1-type receptor to regulate gill Cl^- transport by inhibiting Cl^- uptake and facilitating Cl^- secretion during seawater acclimation.

Identification of an oxytocinase/vasopressinase-like leucyl-cystinyl aminopeptidase (LNPEP) in teleost fish and evidence for hypothalamic mRNA expression linked to behavioral social status

The vasotocin/vasopressin and isotocin/mesotocin/oxytocin family of nonapeptides regulate social behaviors and physiological functions associated with reproductive physiology and osmotic balance. While experimental and correlative studies provide evidence for these nonapeptides as modulators of behavior across all classes of vertebrates, mechanisms for nonapeptide inactivation in regulating these functions have been largely overlooked. Leucyl-cystinyl aminopeptidase (LNPEP) - also known as vasopressinase, oxytocinase, placental leucine aminopeptidase (P-LAP), and insulin-regulated aminopeptidase (IRAP) - is a membrane-bound zinc-dependent metalloexopeptidase enzyme that inactivates vasopressin, oxytocin, and select other cyclic polypeptides. In humans, LNPEP plays a key role in the clearance of oxytocin during pregnancy. However, the evolutionary diversity, expression distribution, and functional roles of LNPEP remain unresolved for other vertebrates. Here, we isolated and sequenced a full-length cDNA encoding a LNPEP-like polypeptide of 1033 amino acids from the ovarian tissue of Amargosa pupfish, Cyprinodon nevadensis. This deduced polypeptide exhibited high amino acid identity to human LNPEP both in the protein's active domain that includes the peptide binding site and zinc cofactor binding motif (53.1% identity), and in an intracellular region that distinguishes LNPEP from other aminopeptidases (70.3% identity). Transcripts encoding this LNPEP enzyme (lnpep) were detected at highest relative abundance in the gonads, hypothalamus, forebrain, optic tectum, gill and skeletal muscle of adult pupfish. Further evaluation of lnpep transcript abundance in the brain of sexually-mature pupfish revealed that lnpep mRNAs were elevated in the hypothalamus of socially subordinate females and males, and at lower abundance in the telencephalon of socially dominant males compared to dominant females. These findings provide evidence of an association between behavioral social status and hypothalamic lnpep transcript abundance and suggest that variation in the rate of VT/IT peptide inactivation by LNPEP may be a contributing component in the mechanism whereby nonapeptides regulate social behavior.

Impact of dehydration on the forebrain preoptic recess walls in the mudskipper, Periophthalmus modestus: a possible locus for the center of thirst

The forebrain lamina terminalis has not yet been examined for the role of osmosensing in teleosts, although the thirst center is well known to be present in this vascular permeable forebrain region in mammals. Here, we examined vascular permeability and neuronal responsiveness to dehydration in the lamina terminalis of the mudskipper, a euryhaline goby. Evans blue and N-hydroxysulfosuccinimide-biotin both bind to blood proteins, and are impermeable to the blood-brain barrier. Intraperitoneal injection of these probes stained the walls of the preoptic recess (PR) of the third ventricle, indicating increased vascular permeability in this region. When mudskippers kept in isotonic brackish water (ca. 11 psu) were challenged to seawater (ca. 34 psu) for 3 h, body water content showed a 1 % decrease, compared with mudskippers without hypertonic challenge. Simultaneously, the number of immunohistochemically identified cFos-expressing neurons in the anterior parvocellular preoptic nucleus (PPa) of the PR walls increased in a site-specific manner by approximately 1.6-fold compared with controls. Thus, these findings indicate that PPa neurons are activated, following dehydration in mudskippers. Taken together, the vascularly permeable PR walls may be involved in osmosensing, as in the mammalian thirst center.

Is there convergence in the molecular pathways underlying the repeated evolution of sociality in African cichlids?

Despite wide variation in the complexity of social interactions across taxa, the basic behavioral components of sociality appear to be modulated by conserved hormone pathways. Specifically, the nonapeptide hormones oxytocin and vasopressin and their receptors have been implicated in regulating diverse social behaviors across vertebrates. Here, we took advantage of the repeated evolution of cooperative breeding in African cichlids to investigate whether there are consistent brain gene expression patterns of isotocin and arginine vasotocin (teleost homologues of oxytocin and vasopressin), as well as their receptors, between four closely related pairs of social (cooperative) and non-social (non-cooperative) species. We first found that the coding sequences for the five genes studied were highly conserved across the eight species. This is the first study to examine the expression of both isotocin receptors, and so we performed a phylogenetic analysis that suggests that these two isotocin receptors are paralogues that arose during the teleost genome duplication. When we then examined brain gene expression patterns relative to social system, we found that there were whole-brain gene expression differences between the social and non-social species in many of the species pairs. However, these relationships varied in both the direction and magnitude among the four species pairs. In conclusion, our results suggest high sequence conservation and species-specific gene expression patterns relative to social behavior for these candidate hormone pathways in the cichlid fishes.

Water transport and functional dynamics of aquaporins in osmoregulatory organs of fishes

Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.

Tubular localization and expressional dynamics of aquaporins in the kidney of seawater-challenged Atlantic salmon

Most vertebrate nephrons possess an inherited ability to secrete fluid in normal or pathophysiological states. We hypothesized that renal aquaporin expression and localization are functionally regulated in response to seawater and during smoltification in Atlantic salmon and thus reflect a shift in renal function from filtration towards secretion. We localized aquaporins (Aqp) in Atlantic salmon renal tubular segments by immunohistochemistry and monitored their expressional dynamics using RT-PCR and immunoblotting. Three aquaporins: Aqpa1aa, Aqp1ab and Aqp8b and two aquaglyceroporins Aqp3a and Aqp10b were localized in the kidney of salmon. The staining for all aquaporins was most abundant in the proximal kidney tubules and there was no clear effect of salinity or developmental stage on localization pattern. Aqp1aa and Aqp3a were abundant apically but extended throughout the epithelial cells. Aqp10b was expressed apically and along the lateral membrane. Aqp8b was mainly basolateral and Aqp1ab was located in sub-apical intracellular compartments. mRNAs of aqp8b and aqp10b were higher in FW smolts compared to FW parr, whereas the opposite was true for aqp1aa. Aqp mRNA levels changed in response to both SW and sham transfer. Protein levels, however, were stable for most paralogs. In conclusion, aquaporins are abundant in salmon proximal renal tubules and may participate in water secretion and thus urine modification as suggested for other vertebrates. Further studies should seek to couple functional measurements of single nephrons to expression and localization of Aqps in the salmonid kidney.

Molecular neuroanatomy and chemoarchitecture of the neurosecretory preoptic-hypothalamic area in zebrafish larvae

The paraventricular nucleus (PVN) in mammals is the main hypothalamic nucleus controlling hormone release in the pituitary and plays pivotal roles in homeostasis. While the location of a PVN-homologous region has been described in adult fish as the neurosecretory preoptic area (NPO), this region has not been clearly defined in larval zebrafish due to the difficulty in defining cytoarchitectonic nuclear boundaries in the larval brain. Here we identify the precise location of the larval zebrafish NPO using conserved transcription factor and neuropeptide gene expressions. Our results identify the dorsal half of the preoptic-hypothalamic orthopedia a (otpa) domain as the larval NPO and the homologous region to the mammalian PVN. Further, by reconstructing the locations of cells producing zebrafish neuropeptides found in the mammalian PVN (CCK, CRH, ENK, NTS, SS, VIP, OXT, AVP), we provide the first 3D arrangement map of NPO neuropeptides in the larval zebrafish brain. Our results show striking conservation of transcription factor expression (otp, arx, dlx5a, isl1) in and around the NPO/PVN together with neuropeptide expression within it. Finally, we describe the exact anatomical location of cells producing Oxt and Avp in the adult zebrafish. Thus, our results identify the definitive borders and extent of the PVN homologous region in larval zebrafish and serve as an important basis for cross-species comparisons of PVN/NPO structure and function.

Diverse mechanisms for body fluid regulation in teleost fishes

Teleost fishes are the major group of ray-finned fishes and represent more than one-half of the total number of vertebrate species. They have experienced in their evolution an additional third-round whole genome duplication just after the divergence of their lineage, which endowed them with an extra adaptability to invade various aquatic habitats. Thus their physiology is also extremely diverse compared with other vertebrate groups as exemplified by the many patterns of body fluid regulation or osmoregulation. The key osmoregulatory organ for teleosts, whose body fluid composition is similar to mammals, is the gill, where ions are absorbed from or excreted into surrounding waters of various salinities against concentration gradients. It has been shown that the underlying molecular physiology of gill ionocytes responsible for ion regulation is highly variable among species. This variability is also seen in the endocrine control of osmoregulation where some hormones have distinct effects on body fluid regulation in different teleost species. A typical example is atrial natriuretic peptide (ANP); ANP is secreted in response to increased blood volume and acts on various osmoregulatory organs to restore volume in rainbow trout as it does in mammals, but it is secreted in response to increased plasma osmolality, and specifically decreases NaCl, and not water, in the body of eels. The distinct actions of other osmoregulatory hormones such as growth hormone, prolactin, angiotensin II, and vasotocin among teleost species are also evident. We hypothesized that such diversity of ionocytes and hormone actions among species stems from their intrinsic differences in body fluid regulation that originated from their native habitats, either fresh water or seawater. In this review, we summarized remarkable differences in body fluid regulation and its endocrine control among teleost species, although the number of species is still limited to substantiate the hypothesis.

Variations in the expression of vasotocin and isotocin receptor genes in the gilthead sea bream Sparus aurata during different osmotic challenges

The dynamic changes in mRNA expression levels for vasotocin (AVT) and isotocin (IT) receptor gene levels were assessed in a time-course response study in immature male specimens of the gilthead sea bream (Sparus aurata) submitted to hyper- (55‰ salinity) and hypo-osmotic (5‰ salinity) challenges. Two different cDNAs for the AVT receptor and one for the IT receptor (V1a2-type and V2-type AVTR, and ITR, respectively) were cloned by screening an S. aurata brain cDNA library. Genes for these receptors were expressed differentially and is nearly ubiquitously in 26 of the examined tissues. In the gills, both environmental salinity challenges up-regulated AVTR V1a2-type gene expression concomitantly with mRNA expression protein activity of Na(+), K(+)-ATPase gene expression and protein, whereas the AVTR V2-type and cystic fibrosis transmembrane conductance regulator (CFTR) mRNA levels were associated with mRNAs environmental salinity, indicating a possible connection between AVTRs and these transporters. In kidney, AVTR V1a2-type gene expression peaked rapidly and lasted only a short time (12-24h) in response to both osmotic challenges. In contrast, AVTR V2-type mRNA levels were enhanced in specimens exposed to hyperosmotic conditions, whereas they decreased under hypoosmotic environments, suggesting an antidiuretic role related to the vasoconstriction function. In the hypothalamus, only the expression of the AVTR V2-type gene was enhanced at 7 and 14 days under both experimental conditions. In the liver, both AVTRs had increased mRNA levels, with the upregulation of their AVTR V2-type gene increasing faster than the V1a2-type. The ITR gene was not sensitive to variations of external salinity in any of the analyzed tissues. Our results demonstrate the involvement of the vasotocinergic, but not the isotocinergic, pathway as well as the hypothalamic function, in the adjustments of both osmoregulatory and metabolic processes after osmotic challenges.

Fish as research tools: alternatives to in vivo experiments

The use of fish in scientific research is increasing worldwide, due to both the rapid expansion of the fish farming industry and growing awareness of questions concerning the humane use of mammalian models in basic research and chemical testing. As fish are lower on the evolutionary scale than mammals, they are considered to be less sentient. Fish models are providing researchers, and those concerned with animal welfare, with opportunities for adhering to the Three Rs principles of refinement, reduction and replacement. However, it should be kept in mind that fish should also be covered by the principles of the Three Rs. Indeed, various studies have shown that fish are capable of nociception, and of experiencing pain in a manner analogous to that in mammals. Thus, emphasis needs to be placed on the development of alternatives that replace, as much as possible, the use of all living vertebrate animals, including fish. This review gives the first comprehensive and critical overview of the existing alternatives for live fish experimental studies. The alternative methods described range from cell and tissue cultures, organ and perfusion models, and embryonic models, to in silico computer and mathematical models. This article aspires to guide scientists in the adoption of the correct alternative methods in their research, and, whenever possible, to reduce the use of live fish.

The hypothalamic neuropeptide oxytocin is required for formation of the neurovascular interface of the pituitary

The hypothalamo-neurohypophyseal system (HNS) is the neurovascular structure through which the hypothalamic neuropeptides oxytocin and arginine-vasopressin exit the brain into the bloodstream, where they go on to affect peripheral physiology. Here, we investigate the molecular cues that regulate the neurovascular contact between hypothalamic axons and neurohypophyseal capillaries of the zebrafish. We developed a transgenic system in which both hypothalamic axons and neurohypophyseal vasculature can be analyzed in vivo. We identified the cellular organization of the zebrafish HNS as well as the dynamic processes that contribute to formation of the HNS neurovascular interface. We show that formation of this interface is regulated during development by local release of oxytocin, which affects endothelial morphogenesis. This cell communication process is essential for the establishment of a tight axovasal interface between the neurons and blood vessels of the HNS. We present a unique example of axons affecting endothelial morphogenesis through secretion of a neuropeptide.

The role of aquaporins in the kidney of euryhaline teleosts

Water balance in teleost fish is maintained with contributions from the major osmoregulatory organs: intestine, gills, and kidney. Overall water fluxes have been studied in all of these organs but not until recently has it become possible to approach the mechanisms of water transport at the molecular level. This mini-review addresses the role of the kidney in osmoregulation with special emphasis on euryhaline teleosts. After a short review of current knowledge of renal functional morphology and regulation, we turn the focus to recent molecular investigations of the role of aquaporins in water and solute transport in the teleost kidney. We conclude that there is much to be achieved in understanding water transport and its regulation in the teleost kidney and that effort should be put into systematic mapping of aquaporins to their tubular as well as cellular localization.

Identification of multiple vasotocin receptor cDNAs in teleost fish: sequences, phylogenetic analysis, sites of expression, and regulation in the hypothalamus and gill in response to hyperosmotic challenge

Vasopressin and its homolog vasotocin regulate hydromineral balance, stress responses, and social behaviors in vertebrates. In mammals, the functions of vasopressin are mediated via three classes of membrane-bound receptors: V1a-type, V1b-type and V2-type. To date, however, only a single class of vasotocin receptor has been identified in teleost fish. Here, cDNAs encoding three putative vasotocin receptors - two distinct V1a-type receptor paralogs (V1a1 and V1a2) and a previously undescribed V2-type receptor (V2) - and a single isotocin receptor were isolated and sequenced from the Amargosa pupfish (Cyprinodon nevadensis amargosae). RT-PCR revealed that mRNAs for these receptors differed in expression patterns with V1a1 mRNAs abundant in the brain, pituitary and testis, V1a2 transcripts at greatest levels in brain, heart and muscle, V2 transcripts most common in the gills, heart and kidney, and isotocin receptor mRNAs abundant in the midbrain, pituitary and gonads. In response to an acute hyperosmotic challenge, pro-vasotocin and V2 mRNA levels in the hypothalamus decreased, while transcripts of V1a1 in the hypothalamus and V1a2 in the gills increased. Partial transcripts for structurally related V2-type, as well as multiple V1a-type, receptors were also identified in other teleosts, suggesting that multiple vasotocin receptors may be present in many Actinopterygii fishes.

Comparative renal physiology of exotic species

In this article the osmoregulatory, acid-base homeostasis, and excretory functions of the renal system of invertebrates and vertebrates are reviewed. The mammalian renal system is the most highly evolved in terms of the range of functions performed by the kidneys. Renal physiology in other animals can be very different, and a sound knowledge of these differences is important for understanding health and disease processes that involve the kidneys, as well as ion and water homeostasis. Many animals rely on multiple organs along with the kidneys to maintain osmotic, ionic, and pH balance. Some animals rely heavily on postrenal modification of urine to conserve water and salt balance; this can influence the interpretation of disease signs and treatment modalities.

Renal regulation of plasma glucose in the freshwater rainbow trout

This study examined the effects of prolonged hyperglycemia on renal handling of glucose and explored the in vivo pharmacological effects of phlorizin on glucose transport in the rainbow trout. The transport of glucose was examined by experimentally elevating the rate of renal glucose reabsorption via infusion of the fish with exogenous glucose at a rate of 70 micromol kg(-1) h(-1) and by inactivating the glucose transporters via the simultaneous administration of phlorizin (1 micromol kg(-1) h(-1)). Glucose was reabsorbed against a concentration gradient, until plasma glucose levels reached approximately 22 micromol l(-1) and the transport maximum of glucose in the kidney (approximately 145 micromol kg(-1) h(-1)) was exceeded. At this point, glucose was lost to the urine, resulting in glucosuria. Glucosuria affected water reabsorption, approximately doubling the water clearance ratio, and resulted in osmotic diuresis. This in turn reduced Na+ reabsorption, increasing the amount lost to the urine from 0.5% to 2% of the filtered load. Glucose reabsorption was found to be correlated with Na+ reabsorption, though the latter was almost 10-fold higher than glucose transport rates. Phlorizin treatment reduced glucose reabsorption, although it did not block it entirely until 48-72 h of infusion. The glucosuria resulting from the blockade of the glucose transporters resulted in a similar osmotic diuresis and a greater Na+ loss to the urine (9% of filtered load). The results are discussed with respect to the net renal ;wasting' of glucose and the detrimental osmoregulatory and ionoregulatory effects associated with glucosuria caused by carbohydrate-rich diets.

Development of an in situ perfused kidney preparation for elasmobranch fish: action of arginine vasotocin

Acclimation of the European lesser-spotted dogfish Scyliorhinus canicula to reduced environmental salinity [85-70% seawater (SW)] induced a significant diuresis in addition to a significant decrease in plasma osmolality in vivo. The threshold for this diuresis was determined to be 85% SW. Therefore, S. canicula acclimated to 85% SW was selected for further study as a diuretic model in the development of an in situ perfused kidney preparation. The renal role of arginine vasotocin (AVT) in the in situ perfused trunk preparation was investigated. In SW, perfusion of 10(-9) and 10(-10) M AVT resulted in a glomerular antidiuresis and decreases in tubular transport maxima for glucose and perfusate flow. In 85% SW, 10(-10) M AVT had no significant effect on these renal parameters with the exception of transport maxima for glucose and perfusate flow. Tubular parameters remained unchanged by either 10(-9) or 10(-10) M AVT. The results demonstrate that the perfused kidney preparation was a viable tool for the investigation of renal parameters in elasmobranch fish and that AVT induced a glomerular antidiuresis.

Evidence for an intrarenal renin-angiotensin system in the rainbow trout, Oncorhynchus mykiss

Physiological and molecular approaches were used to investigate the existence of an intrarenal renin-angiotensin system (RAS) in rainbow trout. Inhibition of angiotensin-converting enzyme by captopril (5 x 10(-4 )M) rapidly decreased vascular resistance of the trunk of the trout, perfused at 19 mmHg, resulting in an increased perfusate flow rate and a decreased intrarenal dorsal aortic pressure. A profound diuresis occurred in the in situ perfused kidney and reflected both increased glomerular filtration rates and decreased water reabsorption (osmolyte reabsorption was unchanged). Renal and vascular parameters recovered once captopril treatment was stopped. Diuretic and vascular effects of captopril on the in situ trout kidney concur with an inhibition of known vasoconstrictor and antidiuretic actions of angiotensin II. However, at a higher perfusion pressure (28 mmHg), captopril had no effect on intrarenal aortic pressure or perfusate and urine flow rates, suggesting that the trout intrarenal RAS is activated by low perfusion pressures/flows. Existence of the renal RAS in trout was further supported by evidence for angiotensinogen gene expression in kidney as well as liver.