Differential increase in forebrain and caudal neurosecretory system corticotropin-releasing factor and urotensin I gene expression associated with seawater transfer in rainbow trout

[The caudal neurosecretory system, the other "neurohypophysial" system in fish]

The caudal neurosecretory system (CNSS) is a neuroendocrine complex whose existence is specific to fishes. Structurally, it has many similarities with the hypothalamic-neurohypophyseal complex of other vertebrates. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most important being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is totally unknown and its role is still poorly understood. Paradoxically, it is almost no longer studied today. Recent developments in imaging and genome editing could make it possible to resume investigations on CNSS in order to solve the mysteries that still surround it.

Toxicity in Takifugu rubripes exposed to acute ammonia: Effects on immune responses, brain neurotransmitter levels, and thyroid endocrine hormones

Exposure to ammonia can cause convulsions, coma, and death. In this study, we investigate the effects of ammonia exposure on immunoregulatory and neuroendocrine changes in Takifugu rubripes. Fish were sampled at 0, 12, 24, 48, and 96 h following exposure to different ammonia concentrations (0, 5, 50, 100, and 150 mg/L). Our results showed that exposure to ammonia significantly reduced the concentrations of C3, C4, IgM, and LZM whereas the heat shock protein 70 and 90 levels significantly increased. In addition, the transcription levels of Mn-SOD, CAT, GRx, and GR in the liver were significantly upregulated following exposure to low ammonia concertation, however, downregulated with increased exposure time. These findings suggest that ammonia poisoning causes oxidative damage and suppresses plasma immunity. Ammonia exposure also resulted in the elevation and depletion of the T3 and T4 levels, respectively. Furthermore, ammonia stress induced an increase in the corticotrophin-releasing hormone, adrenocorticotropic hormone, and cortisol levels, and a decrease in dopamine, noradrenaline, and 5-hydroxytryptamine levels in the brain, illustrating that ammonia poisoning can disrupt the endocrine and neurotransmitter systems. Our results provide insights into the mechanisms underlying the neurotoxic effects of ammonia exposure, which helps to assess the ecological and environmental health risks of this contaminant in marine fish.

Neuroendocrine Regulation of Plasma Cortisol Levels During Smoltification and Seawater Acclimation of Atlantic Salmon

Diadromous fishes undergo dramatic changes in osmoregulatory capacity in preparation for migration between freshwater and seawater. One of the primary hormones involved in coordinating these changes is the glucocorticoid hormone, cortisol. In Atlantic salmon (Salmo salar), cortisol levels increase during the spring smoltification period prior to seawater migration; however, the neuroendocrine factors responsible for regulating the hypothalamic-pituitary-interrenal (HPI) axis and plasma cortisol levels during smoltification remain unclear. Therefore, we evaluated seasonal changes in circulating levels of cortisol and its primary secretagogue-adrenocorticotropic hormone (ACTH)-as well as transcript abundance of the major regulators of HPI axis activity in the preoptic area, hypothalamus, and pituitary between migratory smolts and pre-migratory parr. Smolts exhibited higher plasma cortisol levels compared to parr across all timepoints but circulating ACTH levels were only elevated in May. Transcript abundance of preoptic area corticotropin-releasing factor b1 and arginine vasotocin were ~2-fold higher in smolts compared to parr in February through May. Smolts also had ~7-fold greater hypothalamic transcript abundance of urotensin 1 (uts-1a) compared to parr in May through July. When transferred to seawater during peak smolting in May smolts rapidly upregulated hypothalamic uts-1a transcript levels within 24 h, while parr only transiently upregulated uts-1a 96 h post-transfer. In situ hybridization revealed that uts-1a is highly abundant in the lateral tuberal nucleus (NLT) of the hypothalamus, consistent with a role in regulating the HPI axis. Overall, our results highlight the complex, multifactorial regulation of cortisol and provide novel insight into the neuroendocrine mechanisms controlling osmoregulation in teleosts.

Social effects on AVT and CRF systems

Stress and aggression have negative effects on fish welfare and productivity in aquaculture. Thus, research to understand aggression and stress in farmed fish is required. The neuropeptides arginine-vasotocin (AVT) and corticotropin-releasing factor (CRF) are involved in the control of stress and aggression. Therefore, we investigated the effect of agonistic interactions on the gene expression of AVT, CRF and their receptors in juvenile rainbow trout (Oncorhynchus mykiss). The social interactions lead to a clear dominant-subordinate relationship with dominant fish feeding more and being more aggressive. Subordinate fish had an upregulation of the AVT receptor (AVT-R), an upregulation of CRF mRNA levels, and higher plasma cortisol levels. The attenuating effect of AVT on aggression in rainbow trout is proposed to be mediated by AVT-R, and the attenuating effect of the CRF system is proposed to be mediated by CRF.

GABAA receptor-mediated inhibition of Dahlgren cells electrical activity in the olive flounder, Paralichthys olivaceus

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system. We investigated its potential role as a neurotransmitter in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the flounder Paralichthys olivaceus. The application of GABA in vitro resulted in a decrease in electrical activity of Dahlgren cells, followed by an increase of the number of silent cells, together with a decreased firing frequency of all three activity patterns (tonic, phasic, bursting). GABA_A receptor agonist etomidate decreased Dahlgren cell firing activity, in a similar way to GABA. The response to GABA was blocked by the GABA_A receptor antagonist bicuculline. GABA_A receptor gamma2 subunit (Gabrg2) and chloride channel (Clcn2) mRNA expression were significantly upregulated in the CNSS after GABA superfusion. These data suggest that GABA may modulate CNSS activity in vivo mediated by GABA_A receptors.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Inhibitory role of taurine in the caudal neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

Taurine plays role in neural development and physiological functions such as endocrine regulation in the central nervous system (CNS), and it is one of the most abundant free amino acid there. We investigated its potential effect as a neurotransmitter in the group of neuroendocrine Dahlgren cells at flounder Paralichthys olivaceus caudal neurosecretory system (CNSS). The application of taurine in vitro led to a reduction in electrical activity of Dahlgren cells, followed by a rise in the number of silent cells, at the same time the frequency of all three activity patterns (tonic, phasic, bursting) in Dahlgren cells was reduced. Both strychnine (a glycine receptor antagonist) and bicuculline (a GABA_A receptor antagonist) can block the response to taurine separately. Transcriptome sequencing analysis showed the existence of glycine receptor (GlyR) and GABA_A receptor (GABA_AR) in the flounder CNSS, and the GlyR, GABA_AR, and Cl^- channel mRNA expression were significantly raised after taurine superfusion according to quantitative RT-PCR results. These data indicate that taurine may mediate Dahlgren cell population of CNSS activity in vivo through GlyR and GABA_AR, thereby, regulating stress-response.

Cortisol is an osmoregulatory and glucose-regulating hormone in Atlantic sturgeon, a basal ray-finned fish

Our current understanding of the hormonal control of ion regulation in aquatic vertebrates comes primarily from studies on teleost fishes, with relatively little information on more basal fishes. We investigated the role of cortisol in regulating seawater tolerance and its underlying mechanisms in an anadromous chondrostean, the Atlantic sturgeon (Acipenser oxyrinchus). Exposure of freshwater-reared Atlantic sturgeon to seawater (25 ppt) resulted in transient (1-3 day) increases in plasma chloride, cortisol and glucose levels and long-term (6-14 day) increases in the abundance of gill Na⁺/K⁺/2Cl^- cotransporter (NKCC), which plays a critical role in salt secretion in teleosts. The abundance of gill V-type H⁺-ATPase, which is thought to play a role in ion uptake in fishes, decreased after exposure to seawater. Gill Na⁺/K⁺-ATPase activity did not increase in 25 ppt seawater, but did increase in fish gradually acclimated to 30 ppt. Treatment of Atlantic sturgeon in freshwater with exogenous cortisol resulted in dose-dependent increases in cortisol, glucose and gill NKCC and H⁺-ATPase abundance. Our results indicate that cortisol has an important role in regulating mechanisms for ion secretion and uptake in sturgeon and provide support for the hypothesis that control of osmoregulation and glucose by corticosteroids is a basal trait of jawed vertebrates.

Dynamic Responses of the Caudal Neurosecretory System (CNSS) Under Thermal Stress in Olive Flounder (Paralichthys olivaceus)

Temperature is a critical environmental factor that affect most biological and physiological processes in fish. The caudal neurosecretory system (CNSS) is unique to fish and is proved to maintain homeostasis during seasonal alterations. However, the dynamic expression and secretion pattern of its major hormones, corticotrophin-releasing hormone (CRH), urotensin I (UI), and urotensin II (UII), and their response to thermal stress has not been studied. CRH, UII and cortisol in plasma, gene expression levels of CRH, UI, and UII in the CNSS of olive flounder (Paralichthys olivaceus) were therefore characterized. UI- and UII-positive Dahlgren cells, as well as cell proliferation in the CNSS, were also quantified. The results showed that plasma cortisol and CRH were increased in both low temperature (LT) and high temperature (HT) groups. However, there was no difference in plasma UI and UII during thermal stress. In CNSS, CRH, UI, and UII mRNA levels were all significantly elevated in response to acute hypothermal stress and recovered back to the control (normal) level after 8 days of adaptation. During hyperthermal challenge, gene expression of CRH and UI only significantly increased after 8-days of transfer but no change in UII was observed. We also demonstrated an increasing percent of UI-positive Dahlgren cells in the CNSS of 8-days hyperthermal stressed fish. However, no BrdU-labeled Dahlgren cells were found among the three treatment groups. Collectively, our results demonstrate that the CNSS is subjected to dynamic responses under thermal stress and expands upon the role of the CNSS in thermoregulation. The dynamic responses of hormone levels and the gene expression of CRH, UI and UII in CNSS are all involved in the process of hyper- or hypo-thermal stress and adaptation.

Dynamic Expression and Regulation of Urotensin I and Corticotropin-Releasing Hormone Receptors in Ovary of Olive Flounder Paralichthys olivaceus

Urotensin I (UI), a fish corticotropin-releasing hormone (CRH) like peptide, has been found throughout vertebrate species that has great effects on adaptive physiology comprising stress-related responses, and osmotic regulation by binding with CRH type I receptor (CRHR1) and CRH type II receptor (CRHR2) in fish. Dynamic expression and regulation of UI and CRH receptors in the olive flounder ovarian follicle were studied so as to make further efforts to understand the role of UI in the development of teleost ovary. The results showed that stage-specific change in UI mRNA levels in ovarian follicles of olive flounder. UI and CRHR1 mRNA levels were higher in stage III follicles (300∼500 μm diameter) compared to stage II (90∼300 μm diameter) and IV (500∼800 μm diameter) follicles, however, the levels of CRHR2 mRNA were decreased in line with the ovarian development from stage II to stage IV. A strong signal of UI protein was observed in the follicular cells and oocyte in stage III and IV follicles by immunohistochemistry. In vitro treatment of olive flounder ovarian follicles with human chorionic gonadotropin (hCG) showed that the mRNA expression of UI increased significantly at low concentration and decreased at high concentration at 6 h, but the CRHR1 and CRHR2 mRNA did not change obviously. In addition, the results of incubation with 17α, 20β-dihydroxy-4-oregnen-3-one (DHP) show that UI and CRHR1 mRNA expression were elevated with increasing concentrations at 9 h. All above results indicated that UI and CRH receptors may have a vital effect on olive flounder ovarian development.

Gene expression and hormone secretion profile of urotensin I associated with osmotic challenge in caudal neurosecretory system of the euryhaline flounder, Platichthys flesus

The caudal neurosecretory system (CNSS) is a part of stress response system, a neuroendocrine structure unique to fish. To gain a better understanding of the physiological roles of CNSS in fluid homeostasis, we characterized the tissue distribution of urotensin I (UI) expression in European flounder (Platichthys flesus), analyzed the effect chronic exposure to seawater (SW) or freshwater (FW), transfer from SW to FW, and reverse transfer on mRNA levels of UI, L-type Ca²⁺ channels and Ca-activated K⁺ channels transcripts in CNSS. The tissue distribution demonstrated that the CNSS is dominant sites of UI expression, and UI mRNA level in fore brain appeared greater than other non-CNSS tissues. There were no consistent differences in CNSS UI expression or urophysis UI content between SW- and FW-adapted fish in July and September. After transfer from SW to FW, at 8 h CNSS UI expression was significantly increased, but urophysis UI content was no significantly changes. At 24 h transfer from SW to FW, expression of CNSS UI was no apparent change and urophysis UI content was reduced. At 8 h and 24 h after transfer from FW to SW UI expression and urophysis UI content was no significantly effect. The expression of bursting dependent L-type Ca²⁺ channels and Ca-activated K⁺ channels in SW-adapted fish significantly decreased compared to those in FW-adapted. However, there were no differences in transfer from SW to FW or from FW to SW at 8 h and 24 h. Thus, these results suggest CNSS UI acts as a modulator in response to osmotic stress and plays important roles in the body fluid homeostasis.

The evolutionary consequences for seawater performance and its hormonal control when anadromous Atlantic salmon become landlocked

Populations of anadromous fish have become landlocked in relatively recent geological history (<14,000 years), but the evolutionary impacts of this altered lifecycle on traits underlying seawater performance have not been established. In order to examine the effects of relaxed selection on seawater traits, anadromous and landlocked Atlantic salmon were reared under identical conditions and examined for differences in seawater performance and its underlying physiological and endocrine control during the time of spring downstream migration. Salinity tolerance, survival and initial growth in seawater were greater in anadromous than in landlocked salmon. Abundance of the seawater isoform of gill Na+/K+-ATPase increased in spring in both strains but was greater in anadromous salmon. Hormones associated with seawater acclimation (adrenocorticotropic hormone, cortisol and growth hormone) increased in spring in both strains but were higher in anadromous salmon, whereas plasma thyroid hormones did not differ. Hypothalamic urotensin I mRNA levels also increased in spring and were higher in the anadromous strain. The results provide evidence that salinity tolerance and associated physiological traits are regulated by seasonal stimulation of the hypothalamic-pituitary-interrenal axis, and that relaxed selection on seawater entry traits has decreased this stimulation in landlocked salmon.

Nitric Oxide and the Neuroendocrine Control of the Osmotic Stress Response in Teleosts

The involvement of nitric oxide (NO) in the modulation of teleost osmoresponsive circuits is suggested by the facts that NO synthase enzymes are expressed in the neurosecretory systems and may be regulated by osmotic stimuli. The present paper is an overview on the research suggesting a role for NO in the central modulation of hormone release in the hypothalamo-neurohypophysial and the caudal neurosecretory systems of teleosts during the osmotic stress response. Active NOS enzymes are constitutively expressed by the magnocellular and parvocellular hypophysiotropic neurons and the caudal neurosecretory neurons of teleosts. Moreover, their expression may be regulated in response to the osmotic challenge. Available data suggests that the regulatory role of NO appeared early during vertebrate phylogeny and the neuroendocrine modulation by NO is conservative. Nonetheless, NO seems to have opposite effects in fish compared to mammals. Indeed, NO exerts excitatory effects on the electrical activity of the caudal neurosecretory neurons, influencing the amount of peptides released from the urophysis, while it inhibits hormone release from the magnocellular neurons in mammals.

Promoter methylation and Hoxd4 regulate UII mRNA tissue-specific expression in olive flounder (paralichthys olivaceus)

The peptide urotensin II (UII) mediates multiple physiology effects in mammals and fishes, and UII expression shows a tissue-specific pattern. However the mechanism is still unknown. In the present study high level of UII mRNA was detected in the caudal neurosecretory system (CNSS) of the olive flounder when compared to other tissues. We examined whether epigenetic mechanisms of DNA methylation are involved in UII gene expression. Methylation DNA immune precipitation (MeDIP) assay showed low methylation of UII promoter in CNSS tissue compared with muscle and spinal cord. Methylation of UII promoter was further assessed through bisulphate sequencing analysis. Low level methylation (31%) in CpG island of UII promoter was detected in CNSS tissue, while methylation status in muscle and spinal cord was 89% and 91%, respectively. In addition, high conserved sites of Hoxd4 in UII promoter were found. Activation of Hoxd4 mRNA using transretinoic acid (RA) resulted in 18-fold increase of UII mRNA expression in CNSS and high locomotor activity in medaka, confirming that Hoxd4 is also involved in UII gene transcriptional regulation. Taken together, our data provide the first evidence of the epigenetic mechanism of promoter methylation in transcriptional regulation of UII expression in a tissue-specific manner, and Hoxd4 may also participate in UII gene transcription in flounder.

Modulatory effect of glutamate GluR2 receptor on the caudal neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

A neuromodulatory role for glutamate has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined the potential role of glutamate as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) in the euryhaline flounder Paralichthys olivaceus. In pharmacological experiments in vitro, glutamate (Glu) caused an increase in electrical activity of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The glutamate substrate, glutamine (Gln), led to increased firing frequency, cell recruitment and enhanced bursting activity. The glutamate effect was not blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, or the GluR1/GluR3 (AMPA) receptor antagonist IEm1795-2HBr, but was blocked by the broad-spectrum α-amino-3-hydroxy- 5- methyl-4-isoxazo-lepropionic acid (AMPA) receptor antagonist ZK200775. Our transcriptome sequencing study revealed three AMPA receptor (GluR1, GluR2 and GluR3) in the olive flounder CNSS. Quantitative RT-PCR revealed that GluR2 receptor mRNA expression was significant increased following dose-dependent superfusion with glutamate in the CNSS. GluR1 and GluR3 receptor mRNA expression were decreased following superfusion with glutamate. L-type Ca²⁺ channel mRNA expression had a significant dose-dependent decrease following superfusion with glutamate, compared to the control. In the salinity challenge experiment, acute transfer from SW to FW, GluR2 receptor mRNA expression was significantly higher than the control at 2 h. These findings suggest that GluR2 is one of the mechanisms which can medicate glutamate action within the CNSS, enhancing electrical activity and hence secretory output.

CRF and urocortin 3 protect the heart from hypoxia/reoxygenation-induced apoptosis in zebrafish

Fish routinely experience environmental hypoxia and have evolved various strategies to tolerate this challenge. Given the key role of the CRF system in coordinating the response to stressors and its cardioprotective actions against ischemia in mammals, we sought to characterize the cardiac CRF system in zebrafish and its role in hypoxia tolerance. We established that all genes of the CRF system, the ligands CRFa, CRFb, urotensin 1 (UTS1), and urocortin 3 (UCN3); the two receptor subtypes (CRFR1 and CRFR2); and the binding protein (CRFBP) are expressed in the heart of zebrafish: crfr1 > crfr2 = crfbp > crfa > ucn3 > crfb > uts1 In vivo, exposure to 5% O2 saturation for 15 min and 90 min of recovery resulted in four- to five-fold increases in whole heart crfb and ucn3 mRNA levels but did not affect the gene expression of other CRF system components. In vitro, as assessed by monitoring caspase 3 activity and the number of terminal deoxynucleotidyl transferase dUTP nick-end labeling-positive cells, pretreatment of excised whole hearts with CRF or UCN3 for 30 min prevented the increase in apoptosis associated with exposure to 1% O2 saturation for 30 min with a 24-h recovery. Lastly, the addition of the nonselective CRF receptor antagonist αh-CRF(9-41) prevented the cytoprotective effects of CRF. We show that the CRF system is expressed in fish heart, is upregulated by hypoxia, and is cytoprotective. These findings identify a novel role for the CRF system in fish and a new strategy to tolerate hypoxia.

Three urocortins in medaka: identification and spatial expression in the central nervous system

The urocortin (UCN) group of neuropeptides includes urocortin 1/sauvagine/urotensin 1 (UTS1), urocortin 2 (UCN2) and urocortin 3 (UCN3). In recent years, evidence has accumulated showing that UCNs play pivotal roles in mediating stress response and anxiety in mammals. Evidence has also emerged regarding the evolutionary conservation of UCNs in vertebrates, but very little information is available about UCNs in non-mammalian vertebrates. Indeed, at present, there are no reports of the empirical identification of ucn2 in non-mammalian vertebrates or of the distribution of ucn2 and ucn3 expression in the adult central nervous system (CNS) of these animals. To gain insight into the evolutionary nature of UCNs in vertebrates, we cloned uts1, ucn2 and ucn3 in a teleost fish, medaka and examined the spatial expression of these genes in the adult brain and spinal cord. Although all known UCN2 genes except those in rodents have been reported to likely lack the necessary structural features to produce a functional pre-pro-protein, all three UCN genes in medaka, including ucn2, displayed all of these features, suggesting their functionality. The three UCN genes exhibited distinct spatial expression patterns in the medaka brain: uts1 was primarily expressed in broad regions of the dorsal telencephalon, ucn2 was expressed in restricted regions of the thalamus and brainstem and ucn3 was expressed in discrete nuclei throughout many regions of the brain. We also found that these genes were all expressed throughout the medaka spinal cord, each with a distinct spatial pattern. Given that many of these regions have been implicated in stress responses and anxiety, the three UCNs may serve distinct physiological roles in the medaka CNS, including those involved in stress and anxiety, as shown in the mammalian CNS.

Chronic cortisol and the regulation of food intake and the endocrine growth axis in rainbow trout

To gain a better understanding of the mechanisms by which cortisol suppresses growth during chronic stress in fish, we characterized the effects of chronic cortisol on food intake, mass gain, the expression of appetite-regulating factors, and the activity of the GH/IGF axis. Fish given osmotic pumps that maintained plasma cortisol levels at ∼70 or 116 ng/ml for 34 days were sampled 14, 28 and 42 days post-implantation. Relative to shams, the cortisol treatments reduced food intake by 40-60% and elicited marked increases in liver leptin (lep-a1) and brain preoptic area (POA) corticotropin-releasing factor (crf) mRNA levels. The cortisol treatments also elicited 40-80% reductions in mass gain associated with increases in pituitary gh, liver gh receptor (ghr), liver igfI and igf binding protein (igfbp)-1 and -2 mRNA levels, reduced plasma GH and no change in plasma IGF1. During recovery, while plasma GH and pituitary gh, liver ghr and igfI gene expression did not differ between treatments, the high cortisol-treated fish had lower plasma IGF1 and elevated liver igfbp1 mRNA levels. Finally, the cortisol-treated fish had higher plasma glucose levels, reduced liver glycogen and lipid reserves, and muscle lipid content. Thus, our findings suggest that the growth-suppressing effects of chronic cortisol in rainbow trout result from reduced food intake mediated at least in part by increases in liver lep-a1 and POA crf mRNA, from sustained increases in hepatic igfbp1 expression that reduce the growth-promoting actions of the GH/IGF axis, and from a mobilization of energy reserves.

Dynamic expression pattern of corticotropin-releasing hormone, urotensin I and II genes under acute salinity and temperature challenge during early development of zebrafish

Corticotropin-releasing hormone (CRH), urotensin I (UI) and urotensin II (UII) are found throughout vertebrate species from fish to human. To further understand the role of crh, uI and uII in teleosts during development, we investigated the expression pattern of crh, uI, uIIα and uIIβ genes, and their response to acute salinity and temperature challenge during early development of zebrafish, Danio rerio. The results reveal that crh, uI, uIIα and uIIβ mRNA are detected from 0hpf, and the expression levels increase to a maximum at 6 days post fertilization (dpf), with the exception of uIIα that peak at 5dpf. Exposure of zebrafish embryos and larvae to acute osmotic (30ppt) stress for 15 min failed to modify expression levels of crh, uI, uIIα and uIIβ mRNA from levels in control fish except at 6dpf when uIIα and uIIβ were significantly (P < 0.05) modified. Exposure of embryos and larvae to a cold (18 °C) or hot stress (38 °C) generally down-regulated mRNA levels of crh, uI, uIIα and uIIβ apart from at 3dpf. The results indicate that the contribution of crh, uI, uIIα and uIIβ genes to the stress response in zebrafish may be stressor-specific during early development. Overall, the results from this study provide a basis for further research into the developmental and stressor-specific function of crh, uI, uIIα and uIIβ in zebrafish.

Localization and characterization of val-opsin isoform-expressing cells in the brain of adult zebrafish

In addition to vision, light information is used to regulate a range of animal physiology. Such nonimage-forming functions of light are mediated by nonvisual photoreceptors expressed in distinct neurons in the retina and the brain in most vertebrates. A nonvisual photoreceptor vertebrate ancient long opsin (VAL-opsin) possesses two functional isoforms in the zebrafish, encoded by valopa and valopb, which has received little attention. To delineate the neurochemical identities of valop cells and to test for colocalization of the valop isoforms, we used in situ hybridization to characterize the expression of the valop genes along with that of neurotransmitters and a neuropeptide known to be present at the sites of valop expression. Double labeling showed that the thalamic valop population coexpresses valopa and valopb. All the thalamic valop cells overlapped with a GABAergic cell mass that continues from the anterior nucleus to the intercalated thalamic nucleus. A novel valopa cell population found in the superior raphe was serotonergic in nature. A valopb cell population in the Edinger-Westphal nucleus was identified as containing thyrotropin-releasing hormone. Valopb cells localized in the hindbrain intermediate reticular formation were noncholinergic in nature (nonmotorneurons). Thus, the presence of valop cell populations in different brain regions with coexpression of neurotransmitters and neuropeptides and the colocalization of valop isoforms in the thalamic cell population indicate regulatory and functional complexity of VAL-opsin in the brain of the zebrafish.

Crowding stress inhibits serotonin 1A receptor-mediated increases in corticotropin-releasing factor mRNA expression and adrenocorticotropin hormone secretion in the Gulf toadfish

Stimulation of the serotonin 1A (5-HT1A) receptor subtype by 5-HT has been shown to result in an elevation in plasma corticosteroid levels in both mammals and several species of teleost fish, including the Gulf toadfish (Opsanus beta); however, in the case of teleost fish, it is not clearly known at which level of the hypothalamic-pituitary-interrenal axis the 5-HT1A receptor is stimulated. Additionally, previous investigations have revealed that chronic elevations of plasma cortisol mediate changes in brain 5-HT1A receptor mRNA and protein levels via the glucocorticoid receptor (GR); thus, we hypothesized that the function of centrally activated 5-HT1A receptors is reduced or abolished as a result of chronically elevated plasma cortisol levels and that this response is GR mediated. Our results are the first to demonstrate that intravenous injection of the 5-HT1A receptor agonist, 8-OH-DPAT, stimulates a significant increase in corticotropin-releasing factor (CRF) precursor mRNA expression in the hypothalamic region and the release of adrenocorticotropic hormone (ACTH) from the pituitary of teleost fish compared to saline-injected controls. We also provide evidence that cortisol, acting via GRs, attenuates the 5-HT1A receptor-mediated secretion of both CRF and ACTH.

Duress without stress: Cryptobia infection results in HPI axis dysfunction in rainbow trout

Despite clear physiological duress, rainbow trout (Oncorhynchus mykiss) infected with the pathogenic haemoflagellate Cryptobia salmositica do not appear to mount a cortisol stress response. Therefore, we hypothesized that the infection suppresses the stress response by inhibiting the key effectors of the hypothalamic-pituitary-interrenal (HPI) axis. To test this, we characterized the basal activity of the HPI axis and the cortisol response to air exposure in saline- and parasite-injected fish. All fish were sampled at 4 and 6 weeks post-injection (wpi). While both the treatment groups had resting plasma cortisol levels, the parasite-infected fish had lower levels of plasma ACTH than the control fish. Relative to the control fish, the infected fish had higher mRNA levels of brain pre-optic area corticotrophin-releasing factor (CRF) and pituitary CRF receptor type 1, no change in pituitary POMC-A1, -A2 and -B gene expression, higher and lower head kidney melanocortin 2 receptor mRNA levels at 4 and 6 wpi respectively and reduced gene expression of key proteins regulating interrenal steroidogenesis: StAR, cytochrome P450scc and 11β-hydroxylase. The parasite-infected fish also had a reduced plasma cortisol response to a 60-s air exposure stressor. Superfusion of the head kidney tissues of the parasite-infected fish led to significantly lower ACTH-stimulated cortisol release rates than that observed in the control fish. These novel findings show that infection of rainbow trout with C. salmositica results in complex changes in the transcriptional activity of both central and peripheral regulators of the HPI axis and in a reduction in the interrenal capacity to synthesize cortisol.

Daily rhythms of urotensin I and II gene expression and hormone secretion in the caudal neurosecretory system of the euryhaline flounder (Platichthys flesus)

The caudal neurosecretory system (CNSS) is a unique neuroendocrine structure for environmental adaptation in fish, and is the major site of expression and secretion of urotensin I (UI) and II (UII). This study examined daily changes in mRNA expression and the secretion profile of UI and UII in the CNSS. Daily rhythms were observed in mRNA level of CNSS UI, urophysis UI, plasma UII, glucose, potassium and sodium. No statistically significant (Cosinor, P>0.05) diel rhythmicity in mRNA level of CNSS UII, urophysis UII, cortisol, lactate, osmolality and chloride were detected. The calculated acrophase of sodium, cortisol, plasma UII, urophysis UII, urophysis UI and mRNA level of CNSS UI rhythms were recorded at 13:04 h, 13:39 h, 14:45 h, 15:27 h, 14:41 h and 14:39 h, respectively and a positive relationship was evident among them. The acrophase of glucose and potassium rhythms were recorded at 18:57 h and 22:35 h, respectively. The glucose levels increased progressively at the onset of the UII surge at 15:00 h and reached peak values at dusk. The results support the hypothesis that the CNSS may play a role in the control of co-ordinated daily changes in energy mobilization, nutritional behavior and osmoregulatory systems in euryhaline flounder. Our findings described for the first time the existence of daily rhythms of CNSS hormone expression and secretion in Platichthys flesus. These results reveal the importance of taking into account the time of day when assessing stress responses and evaluating UI and UII as physiological indicators of stress in this species.

Central and peripheral glucocorticoid receptors are involved in the plasma cortisol response to an acute stressor in rainbow trout

Cortisol, the primary circulating corticosteroid in teleosts, is elevated during stress following activation of the hypothalamus-pituitary-interrenal (HPI) axis. Cortisol exerts genomic effects on target tissues in part by activating glucocorticoid receptors (GR). Despite a well-established negative feedback loop involved in plasma cortisol regulation, the role of GR in the functioning of the HPI axis during stress in fish is still unclear. We used mifepristone (a GR antagonist) to suppress GR signaling in rainbow trout (Oncorhynchus mykiss) and assessed the resultant changes to HPI axis activity. We show for the first time that mifepristone caused a functional knockdown of GR by depleting protein expression 40-75%. The lower GR protein expression corresponded with a compensatory up-regulation of GR mRNA levels across tissues. Mifepristone treatment completely abolished the stressor-induced elevation in plasma cortisol and glucose levels seen in the control fish. A reduction in corticotropin-releasing factor (CRF) mRNA abundance in the hypothalamic preoptic area was also observed, suggesting that GR signaling is involved in maintaining basal CRF levels. We further characterized the effect of mifepristone treatment on the steroidogenic capacity of interrenal tissue in vitro. A marked reduction in cortisol production following adrenocorticotropic hormone stimulation of head kidney pieces was observed from mifepristone treated fish. This coincided with the suppression of steroidogenic acute regulatory protein, but not P450 side chain cleavage mRNA abundances. Overall, our results underscore a critical role for central and peripheral GR signaling in the regulation of plasma cortisol levels during stress in fish.

Effect of cortisol on permeability and tight junction protein transcript abundance in primary cultured gill epithelia from stenohaline goldfish and euryhaline trout

Primary cultured gill epithelia from goldfish and rainbow trout were used to investigate a role for cortisol in the regulation of paracellular permeability and tight junction (TJ) protein transcript abundance in representative stenohaline versus euryhaline freshwater (FW) fish gills. Glucocorticoid and mineralocorticoid receptors are expressed in cultured goldfish gill preparations and cortisol treatment (100, 500 and 1000 ng/mL) dose-dependently elevated transepithelial resistance (TER) and reduced paracellular [(3)H]PEG-4000 flux across cultured goldfish gill epithelia. Despite these dose-dependent 'tightening' effects of cortisol, the response of goldfish TJ protein transcripts (i.e. occludin, claudin b, c, d, e, h, 7, 8d and 12, and ZO-1) were surprisingly small, with only claudin c and h, and ZO-1 transcript levels significantly decreasing at a dose of 1000 ng/mL. Extending the duration of cortisol exposure from 24 to 48 or 96 h (at 500 ng/mL) did little to alter this phenomenon. By comparison, exposing primary cultured trout gill epithelia (i.e. a euryhaline fish gill model) to 500 ng/mL cortisol resulted in a qualitatively similar, but quantitatively stronger epithelial 'tightening' response. Furthermore, transcript abundance of orthologous trout TJ proteins (i.e. occludin, and claudin 30, 28b, 3a, 7, 8d and 12) significantly elevated as would be expected in a 'tighter' epithelium. Taken together, data suggest a conservative role for cortisol in the endocrine regulation of paracellular permeability across the goldfish gill that may relate to stenohalinity.

Immunomodulatory role of urotensins in teleost Channa punctatus

The present study, for the first time in ectothermic vertebrates, reports the immunoregulatory role of urotensins I and II (UI and UII). Urotensins decreased the phagocytosis and nitrite production by splenic phagocytes. On superoxide production, UI had stimulatory while UII showed inhibitory effect. UI exerted its effect on phagocytes through corticotrophin-releasing factor (CRF) receptor as its non-specific antagonist astressin completely blocked the effect of UI on phagocytosis, nitrite release and superoxide production. Among the antagonists for specific CRF receptor 1 and 2, only CRF receptor 1 antagonist NBI 27914 abolished the effect of urotensin I. On the other hand, UII mediated its effect through urotensin receptor (UT receptor) since its antagonist urantide antagonized the effect of UII on phagocytosis, superoxide and nitrite release. These findings provide the direct evidence on physiological role of UI and UII through CRF receptor 1 and UT receptor, respectively in control of fish immune responses.

The endocrinology of stress in fish: an environmental perspective

Much of the understanding of the endocrine basis of stress in fish comes from studies of cultured stocks of teleosts; there is comparatively little information on stress responses in wild stock, and less still on chondrosteans and elasmobranchs. This understanding is being refined through increasing understanding of molecular processes underlying endocrine events, with molecular tools offering ready examination of parts of the endocrine pathway that have been resistant to easy measurement of hormone products. An assessment of the timecourse of activation of the hypothalamic-pituitary-interrenal axis shows generally strong independence of temperature, with most teleosts showing measurable increase in plasma cortisol within 10 min of stress. Chondrostean and elasmobranch responses are less well described, but in chondrosteans at least, the response pattern appears to be similar to teleosts. The short latency for increases in corticosteroids following exposure to a stressor means that sampling of wild fish needs to occur rapidly after encounter. Several techniques including underwater sampling and rapid line capture are suitable for this, as is measurement of steroid release to the water by undisturbed fish, albeit possibly with a reduced range of applications. Basal cortisol values in wild teleosts are typically <10 ng mL(-1), but a number of species show values orders of magnitude higher in unstressed fish. Variability in corticosteroid levels arises from a range of factors in addition to stress including, sex and maturity, time of day or since feeding, and season. These factors need to be understood for the sensible assessment of stress responses in wild fish. Studies on free-living birds suggest that environmental stress resides mainly around unpredictable change, and the limited data available for fish support this view. The effect of unpredictable event such as floods or storms are difficult to assess in wild fish due to the difficulty in sampling at these times, and would be predicted to impose environmental stress as in terrestrial systems; however, this has yet to be demonstrated. There is scope for use of stress responses to be used as a measure of environmental quality but only if the basic response to environmental stress is well understood first. Development of this understanding remains a priority for this field of research.

Localized expression of urocortin genes in the developing zebrafish brain

The corticotropin-releasing hormone (CRH) family consists of four paralogous genes, CRH and urocortins (UCNs) 1, 2, and 3. In a previous study, we analyzed CRH in the teleost model organism zebrafish and its transcript distribution in the embryonic brain. Here, we describe full-length cDNAs encoding urotensin 1 (UTS1), the teleost UCN1 ortholog, and UCN3 of zebrafish. Major expression sites of uts1 in adult zebrafish are the caudal neurosecretory system and brain. By using RT-PCR analysis, we show that uts1 mRNA is also present in ovary, maternally contributed to the embryo, and expressed throughout embryonic development. Expression of ucn3 mRNA was detected in a range of adult tissues and during developmental stages from 24 hours post fertilization onward. Analysis of spatial transcript distributions by whole-mount in situ hybridization revealed limited forebrain expression of uts1 and ucn3 during early development. Small numbers of uts1-synthesizing neurons were found in subpallium, hypothalamus, and posterior diencephalon, whereas ucn3-positive cells were restricted to telencephalon and retina. The brainstem was the main site of uts1 and ucn3 synthesis in the embryonic brain. uts1 Expression was confined to the midbrain tegmentum; distinct hindbrain cell groups, including locus coeruleus and Mauthner neurons; and the spinal cord. ucn3 Expression was localized to the optic tectum, serotonergic raphe, and distinct rhombomeric cell clusters. The prominent expression of uts1 and ucn3 in brainstem is consistent with proposed roles of CRH-related peptides in stress-induced modulation of locomotor activity through monoaminergic brainstem neuromodulatory systems.

Ontogeny of the corticotropin-releasing factor system in zebrafish

The corticotropin-releasing factor (CRF) system in fish functions to maintain homeostasis during stress in part by regulating cortisol production via the hypothalamus-pituitary-interrenal (HPI) axis. Towards understanding the role of the CRF system in vertebrate development, we describe the ontogeny of the CRF system, cortisol, and the stress response in the zebrafish, Danio rerio. Early embryonic expression of mRNA encoding CRF, urotensin I (UI), CRF-binding protein (CRF-BP), and two CRF receptors (CRF-R1 and CRF-R2) suggest a function in the early organization of the developing embryo. The expression patterns of CRF, UI, and CRF-BP in the larval brain are consistent with the adult distribution patterns for these genes and support HPI-axis independent functions. The relative amounts of CRF and UI mRNA in the heads and tails of developing and adult zebrafish suggest that CRF functions primarily in the brain while UI also plays an important role in the caudal neurosecretory system. The amount of cortisol in developing zebrafish is low and relatively constant through the first 6 days of development. The commencement of feeding after 4 dpf, however, significantly increases basal cortisol production. Finally, we show that zebrafish larvae are able to respond to an osmotic stressor as early as 3 dpf. Overall, results from this study establish the zebrafish as a model species for research on stress during ontogeny and offer new insights into an HPI-axis independent function for the CRF system during embryogenesis.

Gene expression endpoints following chronic waterborne copper exposure in a genomic model organism, the zebrafish, Danio rerio

Although copper (Cu) is an essential micronutrient for all organisms, in excess, waterborne Cu poses a significant threat to fish from the cellular to population level. We examined the physiological and gene expression endpoints that chronic waterborne Cu exposure (21 d) imposes on soft-water acclimated zebrafish at two environmentally relevant concentrations: 8 microg/l (moderate) and 15 microg/l (high). Using a 16,730 65-mer oligonucleotide customized zebrafish microarray chip related to metal metabolism and toxicity to assess the transcriptomic response, we found that 573 genes in the liver responded significantly to Cu exposure. These clustered into three distinct patterns of expression. There was distinct upregulation of a majority of these genes under moderate Cu exposure and a significant downregulation under high Cu exposure. Microarray results were validated by qPCR of eight genes; two genes, metallothionein 2 (mt2) and Na(+)-K(+)-ATPase 1a1 (atp1a1), displayed increased expression under both Cu exposures, indicative of potential genetic endpoints of Cu toxicity, whereas the remaining six genes demonstrated opposing effects at each Cu exposure. Na(+)-K(+)-ATPase enzyme activity decreased during Cu exposure, which may be linked to Cu's competitive effects with Na(+). Whole body cortisol levels were significantly increased in Cu-exposed fish, which prompted an analysis of the promoter region of all significantly regulated genes for glucocorticoid (GRE) and metal (MRE) response elements to dissociate metal- and stress-specific gene responses. Of the genes significantly regulated, 30% contained only a GRE sequence, whereas 2.5% contained only a consensus MRE. We conclude that the indirect effects of Cu exposure regulate gene expression to a much greater degree than the direct effects.

Sequences, expression patterns and regulation of the corticotropin-releasing factor system in a teleost

Corticotropin-releasing factor (CRF) is well known for its role in regulating the stress response in vertebrate species by controlling release of glucocorticoids. CRF also influences other physiological processes via two distinct CRF receptors (CRF-Rs) and is co-regulated by a CRF binding protein (CRFBP). Although CRF was first discovered in mammals, it is important for the regulation of the stress response, motor behavior, and appetite in all vertebrates. However, it is unclear how the actions of CRF, CRF-Rs, and CRFBP are coordinated. To approach this problem, we cloned and identified CRF, CRF-Rs, and CRFBP in a teleost fish model system, Astatotilapia burtoni and mapped their expression patterns in the body and brain. We found that CRF, CRFBP, and CRF-Rs gene sequences are highly conserved across vertebrates, suggesting that the CRF system plays an essential role in survival. Members of the CRF system are expressed widely in the brain, retina, gill, spleen, muscle, and kidney, thereby implicating them in a variety of bodily functions, including vision, respiration, digestion, and movement. We also found that following long-term social stress, mRNA expression of CRF in the brain and CRF type 1 receptor in the pituitary decrease whereas CRFBP in the pituitary increases via a homeostatic mechanism.

Feeding behavior of fish and its control

Feeding behavior is a complex behavior that is closely associated with food intake. Fish have a wide variety of feeding habits and feeding patterns making them good experimental models for the study of the regulation of feeding behavior. The aquatic nature of fish often creates challenges in the study of feeding behavior and different approaches have been used by researchers, including field studies, observations of free-living animals, and laboratory experiments. Feeding behavior is regulated by a number of environmental factors and also by complex homeostatic mechanisms that involve central and peripheral hormonal factors as well as metabolites. This review summarizes our current knowledge on the control of feeding behavior of fish, with emphasis on the methodology used and on the endocrine and metabolic regulation of feeding.

Effects of freshwater and saltwater adaptation and dietary salt on fluid compartments, blood pressure, and venous capacitance in trout

Trout are of interest in defining the relationship between fluid and salt balance on cardiovascular function because they thrive in freshwater (FW; volume loading, salt depleting), saltwater (SW; volume depleting, salt loading), and FW while fed a high-salt diet (FW-HS; volume and salt loading). The effects of chronic (>2 wk) adaptation to these three protocols on blood volume (51Cr red cell space), extracellular fluid volume (99mTc-diethylene triaminepenta-acetic acid space), arterial (dorsal aortic; PDA) and venous (ductus Cuvier; Pven) blood pressure, mean circulatory filling pressure (zero-flow Pven), and vascular capacitance were examined in the present study on unanesthetized rainbow trout. Blood volume, extracellular fluid volume, PDA, Pven, and mean circulatory filling pressure progressively increased in the order SW < FW < FW-HS. Vascular capacitance in SW fish appeared to be continuous with the capacitance curve of FW fish and reflect a passive volume-dependent unloading of the venous system of FW fish. Vascular capacitance curves for FW-HS fish were displaced upward and parallel to those of FW fish, indicative of an active increase in unstressed blood volume without any change in vascular compliance. These studies are the first in any vertebrate to measure the relationship between fluid compartments and cardiovascular function during independent manipulation of volume and salt balance, and they show that volume, but not salt, balance is the primary determinant of blood pressure in trout. They also present a new paradigm with which to investigate the relative contributions of water and salt balance in cardiovascular homeostasis.

Distribution and regional stressor-induced regulation of corticotrophin-releasing factor binding protein in rainbow trout (Oncorhynchus mykiss)

The corticotrophin-releasing factor (CRF) system plays a key role in the co-ordination of the physiological response to stress in vertebrates. Although the binding protein (BP) for CRF-related peptides, CRF-BP, is an important player in the many functions of the CRF system, the distribution of CRF-BP and the impact of stressors on its expression in fish are poorly understood. In the present study, we describe the distribution of CRF-BP in the brain and peripheral tissues of rainbow trout (Oncorhynchus mykiss) using a combination of real-time reverse transcriptase-polymerase chain reaction, in situ hybridisation and immunohistochemistry. Our results indicate a widespread and highly localised distribution of CRF-BP in the central nervous system, but do not support a significant peripheral production of the protein. Major expression sites in the brain include the area ventralis telencephali, nucleus preopticus, anterior and lateral tuberal nuclei, and the posterior region of the pituitary pars distalis. We further characterise changes in CRF-BP gene expression in three discrete brain regions after exposure to 8 h and 24 h of social stress or hypoxia. The plasma cortisol concentration in subordinate fish was much higher than in dominant fish and controls, and was indicative of a relatively severe stressor. By contrast, the increase in plasma cortisol concentration in fish exposed to hypoxia was characteristic of the response to a mild stressor. Changes in CRF-BP gene expression were only observed after 24 h of either stressor, and were region-specific. CRF-BP mRNA in the telencephalon increased in both subordinate fish and fish exposed to hypoxia, but CRF-BP in the preoptic area only increased after 24 h of hypoxia exposure. In the hypothalamus, CRF-BP mRNA levels decreased in dominant fish relative to controls after 24 h. Taken together, our results support a diverse role for CRF-BP in the central actions of the fish CRF system, but a negligible role in the peripheral functions of circulating CRF-related peptides. Furthermore, the differential changes in forebrain CRF-BP mRNA appear to occur independently of the hypothalamic-pituitary-inter-renal axis.

Seasonal changes in CRF-I and urotensin I transcript levels in masu salmon: correlation with cortisol secretion during spawning

Pacific salmon employ a semelparous reproductive strategy where sexual maturation is followed by rapid senescence and death. Cortisol overproduction has been implicated as the central physiologic event responsible for the post-spawning demise of these fish. Cortisol homeostasis is regulated through the action of hormones of the hypothalamus-pituitary-interrenal (HPI) axis. These include corticotropin-releasing factor (CRF) and urotensin-I (UI). In the present study, masu salmon (Oncorhynchus masou) were assayed for changes in the levels CRF-I and UI mRNA transcripts by quantitative real-time PCR (qRT-PCR). These results were compared to plasma cortisol levels in juvenile, adult, and spawning masu salmon to identify specific regulatory factors that appear to be functionally associated with changes in cortisol levels. Intramuscular implantation of GnRH analog (GnRHa) capsules was also used to determine whether GnRH influences stress hormone levels. In both male and female masu salmon, spawning fish experienced a 5- to 7-fold increase in plasma cortisol levels relative to juvenile non-spawning salmon. Changes in CRF-I mRNA levels were characterized by 1-2 distinctive short-term surges in adult masu salmon. Conversely, seasonal changes in UI mRNA levels displayed broad and sustained increases during the pre-spawning and spawning periods. The increases in UI mRNA levels were positively correlated (R(2)=0.21 male and 0.26 female, p<0.0001) with levels of plasma cortisol in the pre-spawning and spawning periods. Despite the importance of GnRH in sexual maturation and reproduction, the administration of GnRHa to test animals failed to produce broad changes in CRF-I, UI or plasma cortisol levels. These findings suggest a more direct role for UI than for CRF-I in the regulation of cortisol levels in spawning Pacific salmon.

Corticotropin releasing factor induces anxiogenic locomotion in trout and alters serotonergic and dopaminergic activity

Corticotropin releasing factor (CRF) and serotonin (5-HT) are strongly linked to stress and anxiety in vertebrates. As a neuromodulator in the brain, CRF has anxiogenic properties often characterized by increased locomotion and stereotyped behavior in familiar environments. We hypothesized that expression of anxiogenic behavior in response to CRF will also be exhibited in a teleost fish. Rainbow trout were treated with intracerebroventricular (icv) injections of artificial cerebrospinal fluid (aCSF), 500 or 2000 ng ovine CRF, or not injected. Treatment with either dose of CRF elicited greater locomotion and pronounced head shaking behavior but did not influence water column position. Locomotor and head shaking behaviors may be analogous to the increased stereotypy evoked by icv CRF in rats and may reflect the expression of stress/anxiety behavior. Injection with either aCSF or CRF produced significant increases in plasma cortisol. The absence of behavioral changes in aCSF-injected fish suggests that the behavioral responses following CRF were not due to cortisol. Treatment with 2000 ng CRF significantly increased serotonin, 5-HIAA and dopamine concentrations in the subpallium and raphé and increased 5-HIAA in the preoptic hypothalamus (POA). Concurrent effects of CRF on central monoamines, locomotion and head shaking in trout suggest that anxiogenic properties of CRF are evolutionarily conserved. In addition, positive linear correlations between locomotion and serotonergic and dopaminergic function in the subpallium, POA and raphé nuclei suggest a locomotory function for these monoamines.

Seasonal changes in peptide, receptor and ion channel mRNA expression in the caudal neurosecretory system of the European flounder (Platichthys flesus)

The caudal neurosecretory system (CNSS) of the euryhaline flounder Platichthys flesus has suggested roles in osmoregulatory, reproductive and nutritional adaptation, as fish migrate between seawater (winter) and brackish/freshwater (summer) environments. This study examined seasonal changes in mRNA expression profile of functionally important genes in the CNSS. cDNAs encoding neuropeptides, receptors and ion channels were cloned by reverse transcriptase polymerase chain reaction (RT-PCR) and screening of a flounder CNSS cDNA library. The expression profile of cloned genes was determined by real-time RT-PCR at 2-month intervals throughout the year in CNSS from seawater-adapted fish. Plasma cortisol (measured by radioimmunoassay) showed a peak in April, the time of spawning. Expression levels of mRNA for peptides urotensins I and II (UI, UII) and corticotropin releasing factor (CRF) all showed a seasonal cycle, with lowest expression in April and highest in August-October. The expression of CRF2(UI), UT(UII) and CRF1 receptors was not correlated with corresponding peptide expression. Receptors for potential neuromodulators of CNSS activity also displayed a seasonal mRNA expression profile. Glucocorticoid, 5-hydroxytryptamine, kappa-opioid and glutamate receptor expression peaked around April, suggesting that modulation of electrical activity of the neurosecretory Dahlgren cells is of particular importance at this time. Expression of mRNA for L-type Ca(2+) and Ca-activated K(+) channels was lower during the summer months. These channels underlie electrical bursting activity in Dahlgren cells. Ion channel mRNA expression was also lower in CNSS from flounder fully adapted to freshwater as opposed to seawater, consistent with previously reported observations of reduced bursting activity in Dahlgren cells from freshwater-adapted CNSS. These findings support the hypothesis that the CNSS is functionally reprogrammed to cope with changes in physiological challenge as fish migrate between sea and estuaries in winter and spring.

Evidence for nitric oxide role in the caudal neurosecretory system of the European flounder, Platichthys flesus

A neuromodulatory role for nitric oxide has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined its potential as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the euryhaline flounder. Immunocytochemistry using an antibody raised against human neuronal nitric oxide synthase (NOS) indicated the presence of NOS in the Dahlgren cells. Quantitative RT-PCR, using a flounder-specific probe, revealed NOS mRNA expression in the CNSS. In July, though not in September, NOS mRNA expression was significantly higher in fish fully adapted to seawater, compared to freshwater-adapted fish. Following acute transfer of fish from freshwater to seawater, NOS mRNA expression was elevated at 8h and then recovered by 24h. In pharmacological experiments in vitro, application of NO donors (SNAP, SNP) caused an increase in electrical activity (firing frequency) of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The NOS substrate, l-arginine, led to increased firing frequency, cell recruitment and enhanced bursting activity. However, this effect was not blocked by the NOS inhibitor L-NAME. These findings suggest that NO acts as a modulator within the CNSS, potentially enhancing electrical activity and hence secretory output. A role in supporting adaptation to hyperosmotic conditions is also indicated.

Fish caudal neurosecretory system: a model for the study of neuroendocrine secretion

The caudal neurosecretory system (CNSS) is unique to fish and has suggested homeostatic roles in osmoregulation and reproduction. Magnocellular neuroendocrine Dahlgren cells, located in the terminal segments of the spinal cord, project to a neurohaemal organ, the urophysis, from which neuropeptides are released. In the euryhaline flounder Platichthys flesus Dahlgren cells synthesise at least four peptides, including urotensins I and II and CRF. These peptides are differentially expressed with co-localisation of up to three in a single cell. Dahlgren cells display a range of electrical firing patterns, including characteristic bursting activity, which is dependent on L-type Ca(2+) and Ca-activated K(+)channels. Activity is modulated by a range of extrinsic and intrinsic neuromodulators. This includes autoregulation by the secreted peptides themselves, leading to enhanced bursting. Electrophysiological and mRNA expression studies have examined changes in response to altered physiological demands. Bursting activity is more robust and more Dahlgren cells are recruited in seawater compared to freshwater adapted fish and this is mirrored by a reduction in mRNA expression for L-type Ca(2+) and Ca-activated K(+) channels. Acute seawater/freshwater transfer experiments support a role for UII in adaptation to hyperosmotic conditions. Responses to stress suggest a shared role for CRF and UI, released from the CNSS. We hypothesise that the Dahlgren cell population is reprogrammed, both in anticipation of and in response to changed physiological demands, and this is seen as changes in gene expression profile and electrical activity. The CNSS shows striking parallels with the hypothalamic-neurohypophysial system, providing a highly accessible system for studies of neuroendocrine mechanisms. Furthermore, the presence of homologues of urotensins throughout the vertebrates has sparked new interest in these peptides and their functional evolution.

Localization of corticotropin-releasing factor, urotensin I, and CRF-binding protein gene expression in the brain of the zebrafish,Danio rerio

Our current understanding of the corticotropin-releasing factor (CRF) system distribution in the teleost brain is restricted by limited immunohistochemical studies and a lack of complete transcriptional distribution maps. The present study used in situ hybridization to localize and compare CRF, urotensin I (UI), and CRF-binding protein (CRF-BP) expression in the brain of adult zebrafish (Danio rerio). All three peptides were localized in the preoptic area, periventricular hypothalamic and tectal regions, and dorsal part of the trigeminal motor nucleus. CRF and UI were both expressed in several nuclei of the dorsal telencephalon, whereas CRF and CRF-BP were both expressed in the ventral nucleus of the ventral telencephalon. Sole expression of CRF and CRF-BP was apparent in the olfactory bulbs and superior raphe nucleus, respectively, whereas only UI was observed in the corpus mamillare, nucleus of the medial longitudinal fascicle, dorsal tegmental nucleus, nucleus lateralis valvulae, and nucleus interpeduncularis. A major finding of this study was the general regional overlapping of CRF-BP with its ligands and a tendency to be expressed in tandem with CRF rather than UI. Overall, the mRNA expression patterns outlined in this study support the stress-related neuroendocrine, autonomic, and behavioral functions generally ascribed to the vertebrate CRF system and suggest some unique functional roles for CRF and UI in the teleost brain.

Corticotropin releasing factor (CRF) receptor signaling in the central nervous system: new molecular targets

Corticotropin-releasing factor (CRF) and the related urocortin peptides mediate behavioral, cognitive, autonomic, neuroendocrine and immunologic responses to aversive stimuli by activating CRF(1) or CRF(2) receptors in the central nervous system and anterior pituitary. Markers of hyperactive central CRF systems, including CRF hypersecretion and abnormal hypothalamic-pituitary-adrenal axis functioning, have been identified in subpopulations of patients with anxiety, stress and depressive disorders. Because CRF receptors are rapidly desensitized in the presence of high agonist concentrations, CRF hypersecretion alone may be insufficient to account for the enhanced CRF neurotransmission observed in these patients. Concomitant dysregulation of mechanisms stringently controlling magnitude and duration of CRF receptor signaling also may contribute to this phenomenon. While it is well established that the CRF(1) receptor mediates many anxiety- and depression-like behaviors as well as HPA axis stress responses, CRF(2) receptor functions are not well understood at present. One hypothesis holds that CRF(1) receptor activation initiates fear and anxiety-like responses, while CRF(2) receptor activation re-establishes homeostasis by counteracting the aversive effects of CRF(1) receptor signaling. An alternative hypothesis posits that CRF(1) and CRF(2) receptors contribute to opposite defensive modes, with CRF(1) receptors mediating active defensive responses triggered by escapable stressors, and CRF(2) receptors mediating anxiety- and depression-like responses induced by inescapable, uncontrollable stressors. CRF(1) receptor antagonists are being developed as novel treatments for affective and stress disorders. If it is confirmed that the CRF(2) receptor contributes importantly to anxiety and depression, the development of small molecule CRF(2) receptor antagonists would be therapeutically useful.

The corticotropin-releasing factor system as a mediator of the appetite-suppressing effects of stress in fish

A characteristic feature of the behavioural response to intensely acute or chronic stressors is a reduction in appetite. In fish, as in other vertebrates, the corticotropin-releasing factor (CRF) system plays a key role in coordinating the neuroendocrine, autonomic, and behavioural responses to stress. The following review documents the evidence implicating the CRF system as a mediator of the appetite-suppressing effects of stress in fish. Central injections of CRF or the related peptide, urotensin I (UI), or pharmacological treatments or stressors that result in an increase in forebrain CRF and UI gene expression, can elicit dose-dependent reductions in food intake that are at least partially reversed by pre-treatment with a CRF receptor antagonist. In addition, the appetite suppressing effects of various environmental, pathological, physical, and social stressors are associated with elevated levels of forebrain CRF and UI gene expression and with an activation of the hypothalamic-pituitary-interrenal (HPI) stress axis. In contrast, although stressors can also be associated with an increase in caudal neurosecretory system CRF and UI gene expression and an endocrine role for CRF-related peptides has been suggested, the physiological effects of peripheral CRF-related peptides on the gastrointestinal system and in the regulation of appetite have not been investigated. Overall, while CRF and UI appear to participate in the stress-induced changes in feeding behaviour in fish, the role of other know components of the CRF system is not known. Moreover, the extent to which the anorexigenic effects of CRF-related peptides are mediated through the hypothalamic feeding center, the HPI axis and cortisol, or via actions on descending autonomic pathways remains to be investigated.