Cortisol inhibits the ACTH-releasing activity of urotensin I, CRF and sauvagine observed with superfused goldfish pituitary cells

Long-term stress induced cortisol downregulation, growth reduction and cardiac remodeling in Atlantic salmon

Stress and elevated plasma cortisol in salmonids have been linked with pathological remodeling of the heart and deterioration of fitness and welfare. However, these associations were based on biomarkers that fail to provide a retrospective view of stress. This study is the first whereby the association of long-term stress, using scale cortisol as a chronic stress biomarker, with cardiac morphology and growth performance of wild Atlantic salmon (Salmo salar) is made. Growth, heart morphology, plasma and scale cortisol levels, and expression of genes involved in cortisol regulation of the hypothalamic-pituitary-interrenal axis of undisturbed fish (control) were compared with those of fish exposed daily to stress for 8 weeks. Though scale cortisol levels showed a time-dependent accumulation in both groups, plasma and scale cortisol levels of stress group fish were 29.1% and 25.0% lower than those of control fish, respectively. These results correlated with the overall upregulation of stress-axis genes involved in the systemic negative feedback of cortisol, and local feedback via 11β-hydroxysteroid dehydrogenases, glucocorticoid and mineralocorticoid receptors in the stress treatment at the hypothalamus and pituitary level. These lower cortisol levels were, however, counterintuitive in terms of the growth performance as stress group fish grew 33.7% slower than control fish, which probably influenced the 8.4% increase in relative ventricle mass in the stress group. Though compact myocardium area between the treatments was comparable, these parameters showed significant linear correlations with scale cortisol levels, indicating the involvement of chronic stress in cardiac remodeling. These findings underscore the importance of scale cortisol as biomarker when associating chronic stress with long-term processes including cardiac remodeling.

Negative feedback regulation in the hypothalamic-pituitary-interrenal axis of rainbow trout (Oncorhynchus mykiss) subjected to chronic social stress

The formation of dominance hierarchies in pairs of juvenile rainbow trout (Oncorhynchus mykiss) results in subordinate individuals exhibiting chronically elevated plasma cortisol concentrations. Cortisol levels reflect a balance between cortisol production, which is coordinated by the hypothalamic-pituitary-interrenal (HPI) axis in teleost fish, and negative feedback regulation and hormone clearance, which act to lower cortisol levels. However, the mechanisms contributing to the longer-term elevation of cortisol levels during chronic stress are not well established in fishes. The current study aimed to determine how subordinate fish maintain elevated cortisol levels, by testing the prediction that negative feedback and clearance mechanisms are impaired by chronic social stress. Plasma cortisol clearance was unchanged by social stress based on a cortisol challenge trial, hepatic abundance of the cortisol-inactivating enzyme 11-beta hydroxysteroid dehydrogenase type 2 (11βHSD2), and tissue fate of labelled cortisol. The capacity for negative feedback regulation in terms of transcript and protein abundances of corticosteroid receptors in the preoptic area (POA) and pituitary appeared stable. However, changes in 11βHSD2 and mineralocorticoid receptor (MR) expression suggest subtle regulatory changes in the pituitary that may alter negative feedback. The chronic cortisol elevation observed during social subordination likely is driven by HPI axis activation and compounded by dysregulated negative feedback.

Domestication of farmed fish via the attenuation of stress responses mediated by the hypothalamus-pituitary-inter-renal endocrine axis

Human-directed domestication of terrestrial animals traditionally requires thousands of years for breeding. The most prominent behavioral features of domesticated animals include reduced aggression and enhanced tameness relative to their wild forebears, and such behaviors improve the social tolerance of domestic animals toward both humans and crowds of their own species. These behavioral responses are primarily mediated by the hypothalamic-pituitary-adrenal (inter-renal in fish) (HPA/I) endocrine axis, which is involved in the rapid conversion of neuronal-derived perceptual information into hormonal signals. Over recent decades, growing evidence implicating the attenuation of the HPA/I axis during the domestication of animals have been identified through comprehensive genomic analyses of the paleogenomic datasets of wild progenitors and their domestic congeners. Compared with that of terrestrial animals, domestication of most farmed fish species remains at early stages. The present review focuses on the application of HPI signaling attenuation to accelerate the domestication and genetic breeding of farmed fish. We anticipate that deeper understanding of HPI signaling and its implementation in the domestication of farmed fish will benefit genetic breeding to meet the global demands of the aquaculture industry.

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.

Experimental manipulation of perceived predation risk and cortisol generates contrasting trait trajectories in plastic crucian carp

Most animals constitute potential prey and must respond appropriately to predator-mediated stress in order to survive. Numerous prey also adaptively tailor their response to the prevailing level of risk and stress imposed by their natural enemies, i.e. they adopt an inducible defence strategy. Predator exposure may activate the stress axis, and drive the expression of anti-predator traits that facilitate survival in a high-risk environment (the predation-stress hypothesis). Here, we quantified two key morphological anti-predator traits, body morphology and coloration, in crucian carp reared in the presence or absence of a predator (pike) in addition to experimental manipulation of physiological stress via implants containing either cortisol or a cortisol inhibitor. We found that predator-exposed fish expressed a deeper-bodied phenotype and darker body coloration as compared with non-exposed individuals. Skin analyses revealed that an increase in the amount of melanophores caused the dramatic colour change in predator-exposed fish. Increased melanization is costly, and the darker body coloration may act as an inducible defence against predation, via a conspicuous signal of the morphological defence or by crypsis towards dark environments and a nocturnal lifestyle. By contrast, the phenotype of individuals carrying cortisol implants did not mirror the phenotype of predator-exposed fish but instead exhibited opposite trajectories of trait change: a shallow-bodied morphology with a lighter body coloration as compared with sham-treated fish. The cortisol inhibitor did not influence the phenotype of fish i.e. neither body depth nor body coloration differed between this group and predator-exposed fish with a sham implant. However, our results illuminate a potential link between stress physiology and morphological defence expression.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

Stress Effects on the Mechanisms Regulating Appetite in Teleost Fish

The homeostatic regulation of food intake relies on a complex network involving peripheral and central signals that are integrated in the hypothalamus which in turn responds with the release of orexigenic or anorexigenic neuropeptides that eventually promote or inhibit appetite. Under stress conditions, the mechanisms that control food intake in fish are deregulated and the appetite signals in the brain do not operate as in control conditions resulting in changes in the expression of the appetite-related neuropeptides and usually a decreased food intake. The effect of stress on the mechanisms that regulate food intake in fish seems to be mediated in part by the corticotropin-releasing factor (CRF), an anorexigenic neuropeptide involved in the activation of the HPI axis during the physiological stress response. Furthermore, the melanocortin system is also involved in the connection between the HPI axis and the central control of appetite. The dopaminergic and serotonergic systems are activated during the stress response and they have also been related to the control of food intake. In addition, the central and peripheral mechanisms that mediate nutrient sensing capacity and hence implicated in the metabolic control of appetite are inhibited in fish under stress conditions. Finally, stress also affects peripheral endocrine signals such as leptin. In the present minireview, we summarize the knowledge achieved in recent years regarding the interaction of stress with the different mechanisms that regulate food intake in fish.

Stress in Atlantic salmon: response to unpredictable chronic stress

Combinations of stressors occur regularly throughout an animal's life, especially in agriculture and aquaculture settings. If an animal fails to acclimate to these stressors, stress becomes chronic, and a condition of allostatic overload arises with negative results for animal welfare. In the current study, we describe effects of exposing Atlantic salmon parr to an unpredictable chronic stressor (UCS) paradigm for 3 weeks. The paradigm involves exposure of fish to seven unpredictable stressors three times a day. At the end of the trial, experimental and control fish were challenged with yet another novel stressor and sampled before and 1 h after that challenge. Plasma cortisol decreased steadily over time in stressed fish, indicative of exhaustion of the endocrine stress axis. This was confirmed by a lower cortisol response to the novel stressor at the end of the stress period in chronically stressed fish compared with the control group. In the preoptic area (POA) and pituitary gland, chronic stress resulted in decreased gene expression of 11βhsd2, gr1 and gr2 in the POA and increased expression of those genes in the pituitary gland. POA crf expression and pituitary expression of pomcs and mr increased, whereas interrenal gene expression was unaffected. Exposure to the novel stressor had no effect on POA and interrenal gene expression. In the pituitary, crfr1, pomcs, 11βhsd2, grs and mr were down-regulated. In summary, our results provide a novel overview of the dynamic changes that occur at every level of the hypothalamic-pituitary gland-interrenal gland (HPI) axis as a result of chronic stress in Atlantic salmon.

In vitro effect of cortisol and urotensin I on arginine vasotocin and isotocin secretion from pituitary cells of gilthead sea bream Sparus aurata

This study aimed at determining whether in vitro secretion of two neuropeptides, arginine vasotocin (AVT) and isotocin (IT), from pituitary cells of gilthead sea bream Sparus aurata was affected by cortisol and urotensin (UI). Pituitary cells were exposed to 1·4 × 10(-8) , 1·4 × 10(-7) and 0·4 × 10(-6)  M cortisol and 10(-12) , 10(-10) and 10(-8)  M UI for 6, 24 and 48 h, respectively. AVT and IT contents were determined in the culture media by high-performance liquid chromatography (HPLC). An increase in AVT secretion and a decrease in IT secretion were observed at all cortisol doses. UI increased AVT secretion after 6 h of incubation at all doses. After 24 h, however, only the highest dose of UI still displayed an effect. IT secretion was not influenced by UI. It was thus demonstrated that cortisol does influence AVT and IT secretion from S. aurata pituitary cells, while UI regulates AVT secretion, as a component of hypothalamic-pituitary-interrenal (HPI) axis in this species.

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.

Recombinant human leptin attenuates stress axis activity in common carp (Cyprinus carpio L.)

Proper functioning of the endocrine stress axis requires communication between the stress axis and other regulatory mechanisms. We here describe an intimate interplay between the stress axis and recombinant human leptin (rhLeptin) in a teleostean fish, the common carp Cyprinus carpio. Restraint stress (by netting up to 96h) increased plasma cortisol but did not affect hepatic leptin expression. Perifusion of pituitary glands or head kidneys with rhLeptin revealed direct effects of rhLeptin on both tissues. RhLeptin suppresses basal and CRF-induced ACTH-secretion in a rapid and concentration-dependent manner. The rhLeptin effect persisted for over an hour after administration had been terminated. RhLeptin decreases basal interrenal cortisol secretion in vitro, and by doing so attenuates ACTH-stimulated cortisol production; rhLeptin does not affect interrenal ACTH-sensitivity. Our findings show that the endocrine stress axis activity and leptin are inseparably linked in a teleostean fish, a notion relevant to further our insights in the evolution of leptin physiology in vertebrates.

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.

Effects of chronic manipulation of adrenocorticotropic hormone levels in Chinook salmon on expression of interrenal steroidogenic acute regulatory protein and steroidogenic enzymes

The effects of chronic exposure to adrenocorticotropic hormone (ACTH) or the synthetic glucocorticoid dexamethasone (DEX) on the expression of genes involved in cortisol synthesis were examined using quantitative RT-PCR and immunohistochemistry. Juvenile Chinook salmon were treated with either ACTH via micro-osmotic pumps or with DEX via a lipid-based sustained release vehicle. Plasma cortisol levels were significantly elevated in ACTH-treated fish after 1 day, with a significant reduction in this effect with increasing treatment duration. ACTH also appeared to cause progressive hyperplasia of interrenal cells. Steroidogenic acute regulatory protein (StAR) and cytochrome P450 side chain cleavage enzyme (P450scc) transcripts but not 3β-hydroxysteroid dehydrogenase-isomerase (3β-HSD) or cytochrome P450 11β-hydroxylase (P45011β) transcripts in head kidneys significantly increased after 5 days of ACTH treatment. Significant linear relationships between plasma cortisol levels and transcript levels were identified at day 1 and day 5 for StAR, and day 5 for P450scc. Increased immunoreactivity for P450scc was observed in interrenal cells of ACTH-treated fish after 5 and 10 days. No effect of ACTH on 3β-HSD immunoreactivity was apparent at any time point. P45011β immunoreactivity was more intense after 5 days treatment with ACTH. DEX significantly reduced resting plasma cortisol levels and induced interrenal cell atrophy. Although no significant effect of treatment with DEX was found for any transcript, immunoreactivity for P450scc and P45011β appeared to be reduced. These results indicate that StAR and P450scc are subject to transcriptional regulation by chronic changes in ACTH levels.

Modulation of cortisol levels, endocannabinoid receptor 1A, proopiomelanocortin and thyroid hormone receptor alpha mRNA expressions by probiotics during sole (Solea solea) larval development

In the present study, we investigated whether the use of Enterococcus faecium IMC 511 as a probiotic can modulate neuroendocrine system responses during the larval rearing of Solea solea; to this end, the gene expression patterns of proopiomelanocortin (POMC), endocannabinoid receptor 1A (CB1A), and thyroid receptor alpha (TRα) were quantified, and whole-body cortisol levels were measured. Probiotic treatment up-regulated transcription of all selected genes and cortisol concentrations on day 10 post hatch (ph), while on day 30 ph experimental groups showed significantly lower levels of both POMC and CB1A compared to those of the control group. These changes were no longer evident on day 60 ph, when POMC, CB1A, TRα gene expression and cortisol titers were found to be similar in all experimental groups. Our results suggest that metabolic responses to probiotic treatment can be modulated through the activation of genes selected for functional interaction between the hypothalamic-pituitary-thyroid (HPT) axis and the melanocortin and the endocannabinoid systems. Furthermore, the observed (30 ph) down-regulation of both POMC and CB1A gene expression coupled with up-regulation of TRα mRΝΑ levels suggest the activation of a compensatory mechanism that promotes growth and development and perhaps modulates food intake.

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.

Cloning of sole proopiomelanocortin (POMC) cDNA and the effects of stocking density on POMC mRNA and growth rate in sole, Solea solea

Proopiomelanocortin (POMC) is an important gene implicated in different functions, such as the stress response of the hypothalamus-pituitary-adrenal axis. The aim of the present study was to determine whether farming conditions, such as stocking density, can be considered a powerful stressor influencing in turn the growth rate in juvenile fish. Thus, POMC cDNA expression was investigated during adaptation to farming conditions in sole (Solea solea), as a model for studying the effects of rearing densities on stress response; different stocking densities (50, 100, and 250 animals/m(2)) were applied and, after 7 and 21 days, the fishes were examined for body weight and plasma cortisol levels as indicators of stress. In addition, proopiomelanocortin was cloned and sequenced from the brain of sole, allowing semi-quantitative RT-PCR to be performed to evaluate POMC mRNA expression in brain tissue. There was a significant increase in cortisol levels in fish reared at high stocking densities of 250/m(2) compared to fish reared at control densities of 100 and 50/m(2), in both experimental times, i.e., 7 and 21 days. The high stocking densities were also found to decrease the specific growth rate of fish. Moreover, it was demonstrated that the highest stocking density induced a significant decrease in sole POMC mRNA expression. It is concluded that POMC and cortisol are both involved in the stress response due to high rearing densities, during which cortisol may serve as a negative regulator of POMC.

Distribution of corticotropin-releasing hormone in the developing zebrafish brain

Corticotropin-releasing hormone (CRH) plays a central role in the physiological regulation of the hypothalamus-pituitary-adrenal/interrenal axis mediating endocrine, behavioral, autonomic, and immune responses to stress. Despite the wealth of knowledge about the physiological roles of CRH, the genetic mechanisms by which CRH neurons arise during development are poorly understood. As a first step toward analyzing the molecular and genetic pathways involved in CRH lineage specification, we describe the developmental distribution of CRH neurons in the embryonic zebrafish, a model organism for functional genomics and developmental biology. We searched available zebrafish expressed sequence tag (EST) databases for CRH-like sequences and identified one EST that contained the complete zebrafish CRH open reading frame (ORF). The CRH precursor sequence contained a signal peptide, the CRH peptide, and a cryptic peptide with a conserved sequence motif. RT-PCR analysis showed crh expression in a wide range of adult tissues as well as during embryonic and larval stages. By whole-mount in situ hybridization histochemistry, discrete crh-expressing cell clusters were found in different parts of the embryonic zebrafish brain, including telencephalon, preoptic region, hypothalamus, posterior tuberculum, thalamus, epiphysis, midbrain tegmentum, and rostral hindbrain and in the neural retina. The localization of crh mRNA within the preoptic region is consistent with the central role of CRH in the teleost stress response through activation of the hypothalamic-pituitary-interrenal axis. The widespread distribution of CRH-synthesizing cells outside the preoptic region suggests additional functions of CRH in the embryonic zebrafish brain.

Corticotropin-releasing factor and neuropeptide Y mRNA levels are modified by glucocorticoids in rainbow trout, Oncorhynchus mykiss

The primary stress response involves neuronal activation that ultimately leads to the release of glucocorticoids. Circulating glucocorticoids are thought to influence their own synthesis and release through a negative feedback mechanism that inhibits the activity of the hypothalamic and pituitary components of the stress axis. This study was designed to address the hypothesis that glucocorticoids modify corticotropin-releasing factor (CRF) and neuropeptide Y (NPY) mRNA levels in the rainbow trout (Oncorhynchus mykiss) brain. Cortisol implantation significantly reduced CRF1 and NPY mRNA levels in fish exposed to an isolation stress. In contrast, cortisol implantation did not prevent the stress-induced elevation of CRF1 and NPY mRNA levels during confinement. Treatment with the glucocorticoid receptor antagonist RU-486 reduced CRF1 mRNA levels in both isolated and confined fish, but had no effect on NPY mRNA. Although the cytochrome P450 inhibitor metyrapone reduced ACTH-induced cortisol secretion in vitro, plasma cortisol levels were elevated in isolated trout treated with metyrapone. Nevertheless, metyrapone implantation increased CRF1 and NPY mRNA levels in confined fish. Together, these results implicate cortisol as a modulator of CRF and NPY mRNA levels in the preoptic area of the trout brain, but that cortisol is only one such regulating mechanism.

Regulation of behavioral responses by corticotropin-releasing factor

In the wild, animals survive by responding to perceived threats with adaptive and appropriate changes in their behaviors and physiological states. The exact nature of these responses depends on species-specific factors plus the external context and internal physiological states associated with the stressful condition. The neuroendocrine mechanisms that control context-dependent stress responses are poorly understood for most animals, but some progress has been made recently. Corticotropin-releasing factor (CRF) plays an important role in mediating neuroendocrine, autonomic, and behavioral responses to stress. Across many vertebrate taxa, CRF not only stimulates the HPA axis by increasing the secretion of ACTH and glucocorticoid hormones, but also acts centrally by modifying neurotransmitter systems and behaviors. CRF or one of several CRF-related neuropeptides acts to stimulate locomotor activity during periods of acute stress. This behavioral activation consists of anxiety-related non-ambulatory motor activity, ambulatory locomotion, or swimming depending on the species and context. CRF-related neuropeptides increase swimming behaviors in amphibians and fish, apparently by activating brainstem serotonergic systems because the administration of fluoxetine (a selective serotonin re-uptake inhibitor) greatly enhances CRF-induced locomotor activity. Thus, our working model is that CRF, in part via interactions with brainstem serotonergic systems, modulates context-dependent behavioral responses to perceived threats, including both anxiety-related risk assessment behaviors and fight-or-flight locomotor responses.

Modulation of catecholamine release and cortisol secretion by social interactions in the rainbow trout, Oncorhynchus mykiss

The objective of the present study was to investigate the effect of social status on the ability of rainbow trout to secrete the stress hormones, cortisol, and catecholamines. Rainbow trout were confined in pairs for six days to permit the formation of dominance hierarchies. An in situ saline-perfused posterior cardinal vein (PCV) preparation was then used to assess cortisol secretion or release of the catecholamine hormones, adrenaline and noradrenaline, in response to the inclusion of appropriate secretagogues in the perfusate. Fish identified as subordinate on the basis of their behaviour showed a characteristic elevation of circulating plasma cortisol concentrations when compared with dominant fish. When the interrenal cells were stimulated in situ with adrenocorticotropic hormone, subordinate fish displayed a significantly lower rate of cortisol secretion than dominant fish. However, social status had no significant effect on either adrenaline or noradrenaline secretion rates upon stimulation of the chromaffin cells in situ with acetylcholine. These results suggest that the chronic elevation of plasma cortisol associated with subordinate social status in rainbow trout reduces the sensitivity of the cortisol-secreting interrenal cells, presumably through negative feedback mechanisms.

The hypothalamic-pituitary-interrenal axis and the control of food intake in teleost fish

Although environmental, social and physical stressors have been shown to inhibit food intake and feeding behavior in fish, little is known about the mechanisms that mediate the appetite-suppressing effects of stress. Since the hypothalamic-pituitary-interrenal (HPI) axis is activated in response to most forms of stress in fish, components of this axis may be involved in mediating the food intake reductions elicited by stress. Recent investigations into the brain regulation of food intake in fish have identified several signals with orexigenic and anorexigenic properties. Among these appetite-regulating signals are related neuropeptides that can activate the HPI axis, namely corticotropin-releasing factor (CRF) and urotensin I (UI). Central injections of CRF or UI, or treatments that result in an increase in hypothalamic CRF and UI gene expression, can elicit dose-dependent decreases in food intake that can be reversed by pre-treatment with a CRF-receptor antagonist. Evidence also suggests that cortisol, the end product of HPI activation in most fishes (i.e. Osteichthyes), may be involved in the regulation of food intake. Overall, while elements of the HPI axis may mediate some of the appetite-suppressing effects of stress, it is undetermined how either CRF-related peptides, cortisol, or other elements of the stress response interact with the complex circuitry of the hypothalamic feeding center.

Differential expression of corticotropin-releasing factor (CRF) and urotensin I precursor genes, and evidence of CRF gene expression regulated by cortisol in goldfish brain

Corticotropin-releasing factor (CRF) and urotensin I (UI) precursor cDNAs were cloned and sequenced from a goldfish brain cDNA library in order to investigate the distribution of CRF and UI mRNAs in goldfish brain and the regulation of CRF and UI gene expression. The CRF (966-bp) and UI (769-bp) cDNAs encode 163- and 146-amino acid precursors, respectively, and consist of a signal peptide sequence, a cryptic region, and a 41-amino acid mature peptide at the carboxy terminal. The deduced amino acid sequences of the CRF and UI peptides exhibit a sequence identity of 54%. Northern blot analysis revealed a single size of CRF (1.3 kb) and UI (2.0 kb) mRNAs, which are expressed in the telencephalon-preoptic, hypothalamic, optic tectum-thalamus, and posterior brain regions, but not in the pituitary. In addition, while the CRF gene is strongly expressed in the olfactory bulbs, the UI gene is not. In brain regions in which both genes are expressed, the mRNA levels of CRF were three- to sevenfold higher that those of UI. While the low expression levels of the UI gene prevented further analysis of its regulation, the regulation of CRF gene expression by cortisol was examined. In response to intraperitoneal implants of cortisol (300 microg/g BW) the level of CRF mRNA in the telencephalon-preoptic region decreased to 69% of control values at 6 and 24 h posttreatment. In sham-treated fish, in parallel with a transient injection stress-elicited increase in plasma cortisol, CRF mRNA levels declined to 72% of control value at 6 h postinjection and recovered after 24 h. Injection of the glucocorticoid antagonist, RU-486 (100 microg/g BW), prevented the reduction in CRF gene expression associated with the injection stress at 6 h and increased CRF mRNA levels to 145% of control value after 24 h. In contrast, the various implants had no effect on CRF mRNA levels in either the hypothalamus or the optic tectum-thalamus region. These results provide evidence of differential expression of the CRF and UI genes in hypothalamic and extrahypothalamic regions of goldfish brain. Furthermore, they demonstrate that stress levels of plasma cortisol can lead to a decrease in CRF gene expression that is mediated by glucocorticoid receptors in the telencephalon-preoptic region and give an indication of the regional specificity of the regulation of CRF gene expression by cortisol.

Rainbow trout (Oncorhynchus mykiss) urotensin-I: structural differences between urotensins-I and urocortins

In bony fishes, both corticotropin-releasing factor (CRF) and urotensin-I play a role in the regulation of interrenal glucocorticoid release. The rainbow trout, Oncorhynchus mykiss, is a useful model for understanding the mechanisms of stress and the hypothalamo-pituitary-interrenal axis because of its phylogenetic position at the base of the euteleostei and its popularity as a food fish. Urotensin-I may act as a glucocorticoid releaser in a mechanism phylogenetically older than that of CRF. The structural and functional relationships of trout urotensin-I have been investigated. The transcript was cloned from a trout brain hypothalamic cDNA library. A single positive clone was isolated and sequenced. It possesses 3218 bases and has the longest 3' untranslated region of all urotensins-I and CRF transcripts found to date. In comparison to the other fish orthologues, it has the closest sequence identity to the mammalian urocortins. The transcript appears to be differentially processed in brain and urophysis as determined by Northern blot analysis and the presence of polyadenylation signals in the 3' untranslated region. Synthetic trout urotensin-I activated both human CRF-R1 and -R2 receptor-transfected CHO cells with a potency similar to that of white sucker (Catostomus commersoni) urotensin-I. Both fish neuropeptides possessed an order of magnitude less potency than human urocortin in CRF-R2 transfected cells.

Corticotropin releasing factor in the brain of the gymnotiform fish, Apteronotus leptorhynchus: immunohistochemical studies combined with neuronal tract tracing

The expression of corticotropin-releasing factor (CRF) has been studied by immunohistochemistry in the brain of the gymnotiform fish, Apteronotus leptorhynchus. Labeled somata were found exclusively in the posterior subdivision of the nucleus preopticus periventricularis and in the hypothalamus anterioris, where these cells form a continuous cluster of neurons. Combination of anti-peptide immunohistochemistry with an in vitro tract-tracing technique confirmed that at least some of these neurons project to the pituitary. Additional terminal fields were present in the following areas of the telencephalon and the diencephalon: ventral subdivision of the ventral telencephalon, supracommissural subdivision of the ventral telencephalon, anterior subdivision of the nucleus preopticus periventricularis, inferior subdivision of the nucleus recessus lateralis, central posterior/prepacemaker nucleus, hypothalamus dorsalis and lateralis, medial subdivision 2 of the nucleus recessus lateralis, and in the region between the dorsal edge of the nucleus tuberis anterior on the one side and both the glomerular nucleus and the central nucleus of the inferior lobe on the other side. It is likely that the projection of CRF-expressing neurons of the posterior subdivision of the nucleus preopticus periventricularis/hypothalamus anterioris to the pituitary provides, similarly as in other fishes, the neural substrate for the activation of the hypothalamo-pituitary adrenal axis through CRF. In addition to this function, CRF may be involved in the regulation of several other processes, including neural control of communicatory behavior exerted by neurons of the central posterior/prepacemaker nucleus.

Cloning and expression of two proopiomelanocortin mRNAs in the common carp (Cyprinus carpio L.)

Proopiomelanocortin (POMC) is the precursor for a number of biologically active peptides such as adrenocorticotropic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. It is well known that these peptides are involved in the stress response in fish as well as in mammals. We have cloned two different carp POMC cDNAs called, POMC-I and POMC-II. The nucleotide sequences of 955 bp for POMC-I and 959 bp for POMC-II share 93.5% identity in their cDNAs, and the deduced amino acid sequences (both 222 amino acids) are 91.4% identical. In the ACTH and beta-MSH domain, two amino acid substitutions are found, whereas alpha-MSH and beta-endorphin are identical. For beta-MSH, the serine replacement (in POMC-I) by a glycine (in POMC-II) results in a putative amidation site Pro-X-Gly for POMC-II. We used RT-PCR to show that both POMC mRNAs are expressed in the hypophysis, hypothalamus and other parts of the brain of a single fish. Furthermore, in a phylogenetic tree based on POMC sequences the divergence of carp POMC-I and -II from tetraploid animals (salmon, trout and xenopus) is demonstrated.

In vitro thyrotropin-releasing activity of corticotropin-releasing hormone-family peptides in coho salmon, Oncorhynchus kisutch

Investigations of hypothalamic regulation of fish thyrotropin (TSH) secretion and subsequent thyroid activity have been impeded by the lack of a reliable assay for TSH. Using a recently developed radioimmunoassay (RIA) for coho salmon TSH we employed an in vitro pituitary cell culture technique to examine regulation of TSH secretion by corticotropin-releasing hormone (CRH) family peptides [ovine CRH (oCRH), carp urotensin I (UI), and frog sauvagine (SV)] as well as thyrotropin-releasing hormone (TRH), salmon growth hormone-releasing hormone (sGHRH), and salmon gonadotropin-releasing hormone (sGnRH). At concentrations of 0.01 to 100 nM, TRH, sGHRH, and sGnRH did not stimulate TSH secretion from coho salmon pituitary cells. However, at these same concentrations, both oCRH and SV caused a significant and concentration-dependent increase in TSH secretion; whereas, UI was highly stimulatory at all concentrations tested. In a related experiment we examined the effect of alpha-helical CRF(9-41) on oCRH-stimulated TSH release by pituitary cells. alpha-Helical CRF(9-41) is an analogue of CRH that has been shown by others to antagonize the adrenocorticotropic hormone (ACTH)-releasing activity of CRH in goldfish. Preincubation of cells with 1 microM alpha-helical CRF(9-41) for 4 h caused a significant suppression of the TSH-releasing activity of oCRH at 1.0 and 10 nM concentrations. The results of these experiments demonstrate the potency of a CRH-like peptide in the hypothalamic regulation of TSH in fish and reveal similarities in the inhibition of the response of both the thyroid and interrenal axis of fish to alpha-helical CRF(9-41).

Evidence for ultra-short-loop feedback in ACTH-induced interrenal steroidogenesis in coho salmon: acute self-suppression of cortisol secretion in vitro

Interrenal tissues from coho salmon (Oncorhynchus kisutch) were incubated in a defined medium under blood-gas atmosphere at 17 degrees. Rates of cortisol secretion by tissues incubated in media containing 50 mU/ml porcine-ACTH were initially much greater than those of resting tissues in hormone-free media, but after 3 to 6 hr returned to resting rates. The time course of cortisol accumulation in ACTH-containing media was the same when tissues were incubated in different volumes; the final concentrations of cortisol in these incubations were similar to each other and resembled peak in vivo concentrations in juvenile coho subjected to acute stress. Cortisol secretion rates of tissues sequentially transferred to fresh ACTH-containing media every 6 hr did not return to resting levels but remained elevated for at least 24 hr. Cortisol secretion in response to ACTH was attenuated or completely abolished in tissues incubated in media containing exogenous cortisol; this effect was reversible and dose-dependent. Our results suggest that in coho salmon, cortisol may exert ultra-short-loop negative feedback directly at the level of the interrenal gland to effect self-suppression.

Effects of ovine CRF on amphibian pituitary ACTH and TSH cells in vivo: a quantitative ultrastructural study

The subcellular responses of amphibian adrenocorticotropic (ACTH) and thyrotropic (TSH) pituitary cells to the in vivo administration of ovine corticotropin-releasing factor was investigated. For this purpose, groups of six Rana perezi adult frog (three males and three females) were given daily injections of ovine CRF and sacrificed at 6 hr, 24 hr, and 4 days after the first injection. Immunogold staining, applied to ultrathin sections using antisera to human ACTH (1-39) and human beta-TSH identified ACTH and TSH cells, respectively. Morphometry was used to evaluate the volume density (Vv) changes of the rough endoplasmic reticulum, Golgi complex, and secretory granules and the numerical density of the latter. CRF significantly reduced the Vv of the secretory granules in both cell types, taken as indicative of short-term enhanced hormonal release. The peptide also stimulated the development of the cellular biosynthetic machinery, although this effect was detected at an earlier stage in ACTH cells than in TSH cells. These results show for the first time the occurrence of cellular response of amphibian adrenocorticotropes and thyrotropes to CRF and suggesting that this peptide regulates ACTH and TSH production. Moreover, each type of cell differed in its sensitivity to the peptide. After long-term treatment the cytological response of ACTH cells to CRF seemed to decrease, while TSH cells remained active.

Isolation, Amino-Acid Sequence, Synthesis and Biological Properties of Urotensin I from Hippoglossoides elassodon

Abstract A 41-residue urotensin I neuropeptide (H-UI) was isolated from urophyses of the marine teleost Hippoglossoides elassodon (the flathead sole). The peptide was recognized by its partial cross-reactivity in a radioimmunoassay developed for Catostomus (sucker) Ul (S-UI), and was purified by reversed-phase high-performance liquid chromatography. The amino-acid sequence was shown to be H-Ser-Glu-Glu-Pro-Pro-Met-Ser-lle-Asp-Leu-Thr-Phe-His-Met-Leu-Arg-Asn-Met-lle-His-Arg-Ala-Lys-Met-Glu-Gly-Glu-Arg-Glu-Gln-Ala-Leu-lle-Asn-Arg-Asn-Leu-Leu-Asp-Glu-Val-NH(2). H-UI is 66% homologous with S-UI and 63% homologous with Cyprinus (carp) Ul (C-UI). Like S- and C-UI, H-UI is about 50% homologous with the frog skin peptide sauvagine and with Catostomus and mammalian corticotropin-releasing factors. H-UI had similar vasodilatory effects in mammals, and similar adrenocorticotropin-releasing effects (in rat and goldfish) to S-UI, C-UI, sauvagine and the corticotropin-releasing factors, but had relatively low potency (e.g. 10% to 30% of the vasodilatory potency of S- and C-UI) in all the bioassay systems studied.

CRF, urotensin I, and sauvagine stimulate the release of POMC-derived peptides from goldfish neurointermediate lobe cells

In teleost fishes, the melanotropes of the neurointermediate lobe of the pituitary gland release numerous peptides--adrenocorticotropin (ACTH), melanotropin (MSH), lipotropin (LPH), corticotropin-like intermediate lobe peptide (CLIP), and endorphin--which are derived from the precursor molecule proopiomelanocortin. Superfused, isolated, dispersed goldfish neurointermediate lobe cell columns were used to investigate the release of immunoreactive (ir) alpha-MSH and ir ACTH from goldfish melanotropes. Stimulation of neurointermediate lobe cell columns with pulses of the structurally homologous peptides, Catostomus urotensin I (UI), ovine corticotropin-releasing factor (oCRF), or sauvagine, produced a significant increase in the concomitant release of ir alpha-MSH and ir ACTH. UI was two to three times as potent as ovine CRF or sauvagine. These studies suggest that CRF- and UI-like peptides stimulate the secretory activity of teleost melanotropes.

Use of a pituitary cell dispersion method and primary culture system for the studies of gonadotropin-releasing hormone action in the goldfish, Carassius auratus. I. Initial morphological, static, and cell column perifusion studies

Two cell dispersion methods for excised goldfish pituitary glands were tested, and a cultured dispersed cell system based on trypsin enzymatic tissue digestion was developed and characterized. Controlled trypsin/DNase treatment of goldfish pituitary gland yielded dispersed cells of high viability (trypsin blue exclusion test) that responded to gonadotropin (GTH)-releasing hormone (GnRH) challenges with GTH secretion in a time- and dose-dependent manner following overnight culture. Electron microscopy revealed that cell preparations produced by the trypsin dispersion were free of cell debris and nerve terminals. The dispersed pituitary cells also retained distinct morphological and immunological identities. Under static incubation conditions, 2-hr treatments with 0.1 nM to 1 microM [Trp7,Leu8]-GnRH (sGnRH) and [D-Arg6,Pro9-N-ethylamide]-sGnRH (sGnRHa) stimulated GTH release with similar efficacy, but with ED50S of 1.92 +/- 0.48 and 0.19 +/- 0.08 nM, respectively. [His5,Trp7,Tyr8]-GnRH (cGnRH-II) stimulated GTH release in a nonsigmoidal, but dose-dependent manner, and with a higher efficacy than sGnRH. In contrast, sGnRH, sGnRHa, and cGnRH-II were equipotent in inducing growth hormone (GH) secretion in static culture studies and with ED50S of 0.29 +/- 0.13, 0.18 +/- 0.11, and 0.19 +/- 0.17 nM, respectively. When trypsin/DNase-dispersed cells cultured overnight with cytodex beads were tested in a cell column perifusion system, dose-related increase in GTH secretion, as well as GH release, were also observed with 0.5 to 50 nM sGnRH. These results suggest that trypsin-dispersed goldfish pituitary cells can be used effectively to study the actions of GnRH on teleost pituitary either in short-term static incubation or column perifusion studies. Differences in the GTH and GH responses to the two native GnRH forms, sGnRH and cGnRH-II, are also indicated.

Steroidogenic role of the caudal neurosecretory system in the flounder, Platichthys flesus

Transfer of flounders from seawater (SW) to fresh water (FW) resulted in a small reduction in circulating cortisol levels and urophysial protein storage. Transfer of flounders from FW to SW resulted in a larger increase in plasma cortisol and specific urophysial protein storage. Over the first 4 days after transfer from FW to SW there was a positive correlation between the observed changes in urophysial urotensin I (UI) content and plasma cortisol. This apparent steroidogenic effect of UI was supported by the increases in plasma cortisol observed following iv injection of crude flounder urophysial gland extract and synthetic Catostomus commersoni UI. The study supports a contribution of the caudal neurosecretory system to the control of interrenal steroidogenesis as part of the integrated osmoregulatory physiology of euryhaline species like the flounder.

Neuropeptides regulating the activity of goldfish corticotropes and melanotropes

The goldfish (Carassius auratus) has proven an advantageous model for investigations of the neuroendocrine regulation of pituitary hormone secretion in teleost fishes. Investigations examining the secretion of adrenocorticotropin (ACTH) and melanocyte-stimulating hormone (MSH) from pituitary cellsin vitro have been used to identify neuropeptides influencing goldfish corticotrope and melanotrope activity. Ovine CRF, urotensin I (UI), arginine vasotocin (AVT), isotocin and angiotensins I and II stimulate the release of ACTH from corticotropesin vitro. Thyrotropin-releasing hormone (TRH), oCRF, UI and neuropeptide Y stimulate the release of MSH from melanotropesin vitro. Immunocytochemical studies have revealed the presence of separate CRF- and UI-immunoreactive perikarya in the hypothalamus suggesting the existence of two structurally similar, yet distinct, hypothalamic CRF-UI-like peptides. Interactions of AVT and CRF in the regulation of ACTH secretion is suggested from studies demonstrating the co-localization of AVT- and CRF-immunoreactivities in perikarya of the preoptic-hypophyseal system. These investigations demonstrate that the secretory activity of goldfish corticotropes and melanotropes is influenced by a diversity of neuropeptides of hypothalamic origin.

Quantitative changes of CRF-like immunoreactivity in eels treated with reserpine and cortisol

Immunocytochemical techniques were applied to brain and pituitary sections of European eels after experimental manipulation of the pituitary-interrenal activity. A corticotropin-releasing factor (CRF) antiserum allowed the identification of a CRF-like peptide in the preoptic nucleus (PON) and rostral and caudal neurohypophysis (NH). CRF-immunoreactivity (ir) was not affected in solvent-injected eels compared to noninjected eels. Reserpine induced a stimulation of the pituitary interrenal axis, decreased ir-CRF in the rostral NH, but did not affect hypothalamic ir-CRF. Cortisol reduced the immunostaining of hypothalamic CRF-ir perikarya and perikarya cross-sectional area. In the rostral NH, CRF-ir fibers decreased in number and almost disappeared in long-term treated eels. The immunostaining of ACTH cells with ACTH antiserum was greatly reduced. These data suggest that cortisol induces a marked reduction in the activity of the CRF-corticotrop axis. The intensity of the ir-CRF staining observed in the caudal NH, close to the intermediate lobe (IL) was not significantly affected in reserpine-treated eels, and only slightly reduced in long-term cortisol-treated eels. The intensity of ir-CRF staining in the caudal NH did not correlate with melanocorticotropic activity or plasma cortisol level. These data suggest that immunoreactive CRF fibers in the rostral and caudal NH are differently regulated.

Urotensin I- and CRF-like peptides in Catostomus commersoni brain and pituitary--HPLC and RIA characterization

Two distinct neuronal systems, containing urotensin I-like immunoreactivity (UI-LI) and CRF-LI respectively, have previously been demonstrated immunocytochemically in the brain and pituitary of the teleost Catostomus commersoni. In the present studies, we used HPLC followed by UI- and CRF-RIA to further characterize these UI- and CRF-LI substances. HPLC of Catostomus brain extracts (including extracts modified by dilute H2O2 oxidation or dilute acid cleavage) suggests that the major form of brain UI-LI is very likely identical to the previously characterized urophysial UI; the small amount of pituitary UI-LI is not yet fully characterized. The behavior of brain CRF-LI on reversed phase (RP) extraction suggests that it may exist largely in precursor form. Pituitary CRF-LI, however, behaved as expected for a peptide on RP extraction, and eluted from HPLC as essentially a single (though somewhat broad) peak. The HPLC behavior of pituitary CRF-LI, and its crossreactivity with various CRF antisera, suggest that it represents authentic fish CRF, apparently similar to (but not identical with) human/rat CRF.

Inhibition of CRF- and urotensin I-stimulated ACTH release from goldfish pituitary cell columns by the CRF analogue alpha-helical CRF-(9-41)

alpha-Helical CRF-(9-41) is an analogue of corticotropin-releasing factor (CRF) that antagonizes CRF-stimulated ACTH release in rats. In the present study, alpha-helical CRF-(9-41) was tested to determine whether it would antagonize the ACTH-releasing activity of CRF or urotensin I (UI) observed with superfused, dispersed goldfish anterior pituitary cells. At a concentration of 4 microM, alpha-helical CRF-(9-41) completely blocked the ACTH-releasing activity of 100 nM CRF or 100 nM UI. The inhibitor by itself showed little intrinsic ACTH-releasing activity. This investigation reveals similarities in the CRF-antagonism of alpha-helical CRF-(9-41) in the teleost and mammalian pituitary in vitro. It also provides are similar and suggests that alpha-helical CRF-(9-41) may be useful as a tool to investigate the effects of CRF-like and UI-like peptides in teleost fishes.

Regulation of MSH release from the neurointermediate lobe of Xenopus laevis by CRF-like peptides

Immunocytochemical studies showed the presence of a fiber system containing a CRF-like peptide in the median eminence and in the neural lobe of the pituitary gland of Xenopus laevis. During in vitro superfusion of neurointermediate lobe tissue, CRF, sauvagine and urotensin I induced a rapid and dose-dependent stimulation of secretion of MSH and endorphin. Tissue of white-background adapted animals displayed a remarkably higher sensitivity to CRF and sauvagine than tissue from animals that were adapted to a black background. During superfusion of isolated melanotrope cells in suspension, it was shown that CRF and sauvagine exerted their effect directly on the melanotrope cell. We therefore conclude that there is morphological and biochemical evidence to consider a CRF-like peptide as a physiological MSH-releasing factor.

Stimulation by angiotensins I and II of ACTH release from goldfish pituitary cell columns

Dispersed, superfused goldfish anterior pituitary cell columns were stimulated with pulses of salmon angiotensin I (sAI), human angiotensin I (hAI), and human angiotensin II (hAII). Human AII stimulated the greatest release of ACTH. The dose-response curves for hAI and sAI were similar and revealed that hAI and sAI were about one-tenth as potent as hAII in stimulating ACTH release. In mammals, AI must be converted to AII in order to stimulate ACTH release. In goldfish, the angiotensin-converting enzyme inhibitor captopril, which inhibits the conversion of AI to AII, was not able to block sAI-stimulated ACTH release. This finding suggests that the angiotensin receptor of the goldfish corticotrope is less discriminating than that of the mammalian corticotrope and recognizes both AI and AII. This hypothesis was supported by the observation that sarcosine analogs of AII, which block AII-stimulated ACTH release in mammals, failed to block hAII-stimulated ACTH release in goldfish. Saralasin showed negligible, [Sar1,Thr8]-AII slight, and [Sar1, Ile8]-AII moderate, intrinsic ACTH-releasing activity. These findings suggest that the ACTH-releasing activity of angiotensin appeared early in the evolution of the vertebrate pituitary.

The effects of stress on plasma ACTH, alpha-MSH, and cortisol levels in salmonid fishes

Handling and confinement caused a steady increase in the plasma ACTH level in both coho salmon and rainbow trout. Within 2 min plasma ACTH levels had increased significantly, and by 30 min they were 5- to 8-fold higher than the basal ACTH level in unstressed fish. This type of stress also caused a pronounced elevation in plasma cortisol, which lagged behind the ACTH increase, although the degree of change was greater, the level rising between 20- and 50-fold. The plasma alpha-MSH level was unaffected by handling and confinement stress. A second series of experiments assessed the effects of a more severe stress, which consisted of 5 min out of water, during which the fish were restrained, followed by 25-min confinement in a small volume of water. This caused a very rapid, pronounced increase in the plasma ACTH level of sterile rainbow trout, the level reaching a peak at 5 min, and remaining elevated for the next 25 min. Plasma cortisol levels, which were low at the beginning of the experiment, remained so for the first 5 min, and rose thereafter. This type of stress also caused a rapid and pronounced elevation of the plasma alpha-MSH level. It rose in a very similar way, and at the same time, as the plasma ACTH level, but instead of remaining elevated it fell during the 25 min of confinement which followed the 5 min of restraint, to finish one-third of the peak value reached after 5 min.

Comparative effects of corticotropin-releasing factor, arginine vasopressin, and related neuropeptides on the secretion of ACTH and alpha-MSH by frog anterior pituitary cells and neurointermediate lobes in vitro

The ability of corticoliberin (CRF), urotensin I, sauvagine, arginine-vasopressin (AVP), and mesotocin to stimulate ACTH release by frog anterior pituitary cells and alpha-melanotropin (MSH) by frog neurointermediate lobe was studied in vitro using a perifusion technique. CRF and AVP were found to be potent stimulators of ACTH secretion, whereas urotensin I and sauvagine were totally inactive. In opposition to recent findings in the rat. CRF did not modify alpha-MSH secretion by the frog neurointermediate lobe. Mesotocin, which is present in the parenchymal cells of the frog pars intermedia, had no effect on alpha-MSH release in vitro. No potentiation of CRF-induced ACTH release was observed when anterior pituitary cells were incubated with a combination of AVP and CRF. Together with the recent elucidation of a CRF-like molecule in the frog diencephalon, these results suggest that, in Amphibia, CRF and AVP exert their stimulatory action specifically on distal lobe corticotrophs.

The fish neuropeptide urotensin I: its physiology and pharmacology

Significant structural and biological homologies between urotensin I (UI), ovine hypothalamic corticotropin releasing factor (oCRF) and the frog skin peptide sauvagine (SVG) have been investigated and compared in fishes and mammals. In mammals, urotensin and the related peptides exert uniquely selective mesenteric vasodilatation, oCRF having approximately equal to 4% the activity of the other two. All three peptides are equipotent in stimulation of ACTH secretion in the rat in vivo and in vitro. UI is significantly more potent than the other two related peptides in stimulation of ACTH secretion in the goldfish pituitary. Immunocytochemical demonstration of UI not only in the caudal spinal cord but also in the brain, mainly in the lateral tuberal region and of an oCRF-like substance in the preoptic nucleus and pituitary, suggests that ACTH secretion in fishes may be controlled by two similar but distinct UI- or oCRF-like peptides.