Clearance of corticosteroids in yearling coho salmon, Oncorhynchus kisutch, in fresh water and seawater and after stress

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.

Stress response of Salmo salar (Linnaeus 1758) when heavily infested by Caligus rogercresseyi (Boxshall & Bravo 2000) copepodids

The year-round presence of ovigerous females of the parasite Caligus rogercresseyi in the fish farms of southern Chile results in a continuous source of the copepodid (infestive) stage of this louse. The short generation time in spring-summer could lead to high abundances of this copepodid, potentially leading to high infestation levels for fish. Knowing how heavy lice infestations affect Salmo salar can help determine how to time antiparasitic treatments so as to both minimize the treatment impact and reduce lice infestation levels for fish. This study aimed to describe the effects of high infestations of the copepodid stage of C. rogercresseyi on the physiology of S. salar. Two groups of S. salar were used: an infested group (75 copepodids per fish) and a control group (not infested). Sixty-five days after the first infestation, the infested fish group was re-infested at an infestation pressure of 200 copepodids per fish. Sampling was done prior to and following the second infestation, at 56 and 67 days (the latter 2 days following the second infestation). Several physiological variables were measured: cortisol (primary stress response) and glucose, proteins, amino acids, triglycerides, lactate, osmolality levels, and number and diameter of skin mucous cells (secondary stress responses). The plasma cortisol, glucose, and triglyceride levels were altered in the heavily infested fish, as was the diameter of skin mucous cells. These results suggest that heavy infestations of C. rogercresseyi lead to an acute stress response, metabolic reorganization, and increased mucus production in S. salar under heavy infestation conditions.

Cold-induced changes in stress hormone and steroidogenic transcript levels in cunner (Tautogolabrus adspersus), a fish capable of metabolic depression

The cunner (Tautogolabrus adspersus) is a fish with a wide latitudinal distribution that is capable of going into metabolic depression during the winter months, and thus, represents a unique model to investigate the impacts of cold temperatures on the stress response. In this study, we measured resting (pre-stress) plasma cortisol levels in 10 °C and 0 °C acclimated cunner from Newfoundland, and both catecholamine and cortisol levels after they were given a standardized handling stress (i.e. 1 min air exposure). In addition, we cloned and characterized cDNAs for several key genes of the cortisol-axis [cytochrome P450scc, steroidogenic acute regulatory protein (StAR) and a glucocorticoid receptor (GR) most likely to be an ortholog of the teleost GR2], determined the tissue distribution of their transcripts, and measured their constitutive (i.e. pre-stress) transcript levels in individuals acclimated to both temperatures. In cunner acclimated to 0 °C, post-stress epinephrine and norepinephrine levels were much lower (by approximately 9- and 5-fold, respectively) compared to 10 °C acclimated fish, and these fish had relatively low resting cortisol levels (~15 ngml(-1)) and showed a typical post-stress response. In contrast, those acclimated to 10 °C had quite high resting cortisol levels (~75 ngml(-1)) that actually decreased (to ~20 ngml(-1)) post-stress before returning to pre-stress levels. Finally, fish acclimated to 10 °C had higher P450scc transcript levels in the head kidney and lower levels of GR transcript in both the head kidney and liver. Taken together, these results suggest that: (1) temperature has a profound effect on the stress response of this species; and (2) although the ancestors of this species inhabited warm waters (i.e. they are members of the family Labridae), populations of cunner from colder regions may show signs of stress at temperatures as low as 10 °C.

Biochemical reference intervals and pathophysiological changes in Flavobacterium psychrophilum-resistant and -susceptible rainbow trout lines

Host genetic resistance against disease-causing pathogens can be enhanced through family-based selective breeding. At present, there is an incomplete understanding of how artificial selection of fish alters host physiology and response following pathogen exposure. We previously reported the generation of selectively-bred rainbow trout Oncorhynchus mykiss lines with either increased resistance (ARS-Fp-R) or susceptibility (ARS-Fp-S) to bacterial cold water disease (BCWD). This study (1) determined baseline reference-range intervals for packed cell volume (PCV) and 18 plasma biochemistry analytes, and (2) examined pathophysiological changes following infection between the genetic lines. PCV and biochemistry reference-range intervals did not significantly differ between genetic lines; thus data were pooled into a single reference-range population (n = 85). ARS-Fp-R and ARS-Fp-S line fish were intraperitoneally challenged with Flavobacterium psychrophilum, and plasma was collected on Days 1, 3, 6, and 9 post-challenge. Splenic bacterial load was measured using an F. psychrophilum-specific qPCR assay. In both genetic lines, changes were observed in mean PCV, total protein, albumin, glucose, cholesterol, chloride, and calcium, falling outside the established reference intervals and significantly differing from phosphate-buffered saline challenged fish, on at least 1d post-challenge. Mean PCV, total protein, and calcium significantly differed between ARS-Fp-R and ARS-Fp-S line fish on Day 9 post-infection, with values in the ARS-Fp-S line deviating most from the reference interval. PCV, total protein, cholesterol, and calcium negatively correlated with bacterial load. These findings identify divergent pathophysiological responses between ARS-Fp-R and ARS-Fp-S line fish following laboratory challenge that are likely associated with differential survival.

Cortisol and finfish welfare

Previous reviews of stress, and the stress hormone cortisol, in fish have focussed on physiology, due to interest in impacts on aquaculture production. Here, we discuss cortisol in relation to fish welfare. Cortisol is a readily measured component of the primary (neuroendocrine) stress response and is relevant to fish welfare as it affects physiological and brain functions and modifies behaviour. However, we argue that cortisol has little value if welfare is viewed purely from a functional (or behavioural) perspective-the cortisol response itself is a natural, adaptive response and is not predictive of coping as downstream impacts on function and behaviour are dose-, time- and context-dependent and not predictable. Nevertheless, we argue that welfare should be considered in terms of mental health and feelings, and that stress in relation to welfare should be viewed as psychological, rather than physiological. We contend that cortisol can be used (with caution) as a tractable indicator of how fish perceive (and feel about) their environment, psychological stress and feelings in fish. Cortisol responses are directly triggered by the brain and fish studies do indicate cortisol responses to psychological stressors, i.e., those with no direct physicochemical action. We discuss the practicalities of using cortisol to ask the fish themselves how they feel about husbandry practices and the culture environment. Single time point measurements of cortisol are of little value in assessing the stress level of fish as studies need to account for diurnal and seasonal variations, and environmental and genetic factors. Areas in need of greater clarity for the use of cortisol as an indicator of fish feelings are the separation of (physiological) stress from (psychological) distress, the separation of chronic stress from acclimation, and the interactions between feelings, cortisol, mood and behaviour.

Generation of a reference transcriptome for evaluating rainbow trout responses to various stressors

Background

Fish under intensive culture conditions are exposed to a variety of acute and chronic stressors, including high rearing densities, sub-optimal water quality, and severe thermal fluctuations. Such stressors are inherent in aquaculture production and can induce physiological responses with adverse effects on traits important to producers and consumers, including those associated with growth, nutrition, reproduction, immune response, and fillet quality. Understanding and monitoring the biological mechanisms underlying stress responses will facilitate alleviating their negative effects through selective breeding and changes in management practices, resulting in improved animal welfare and production efficiency.

Results

Physiological responses to five treatments associated with stress were characterized by measuring plasma lysozyme activity, glucose, lactate, chloride, and cortisol concentrations, in addition to stress-associated transcripts by quantitative PCR. Results indicate that the fish had significant stressor-specific changes in their physiological conditions. Sequencing of a pooled normalized transcriptome library created from gill, brain, liver, spleen, kidney and muscle RNA of control and stressed fish produced 3,160,306 expressed sequence tags which were assembled and annotated. SNP discovery resulted in identification of ~58,000 putative single nucleotide polymorphisms including 24,479 which were predicted to fall within exons. Of these, 4907 were predicted to occupy the first position of a codon and 4110 the second, increasing the probability to impact amino acid sequence variation and potentially gene function.

Conclusion

We have generated and characterized a reference transcriptome for rainbow trout that represents multiple tissues responding to multiple stressors common to aquaculture production environments. This resource compliments existing public transcriptome data and will facilitate approaches aiming to evaluate gene expression associated with stress in this species.

Whole-body cortisol response of zebrafish to acute net handling stress

Zebrafish, Danio rerio, are frequently handled during husbandry and experimental procedures in the laboratory, yet little is known about the physiological responses to such stressors. We measured the whole-body cortisol levels of adult zebrafish subjected to net stress and air exposure at intervals over a 24 h period; cortisol recovered to near control levels by about 1 h post-net-stress (PNS). We then measured cortisol at frequent intervals over a 1 h period. Cortisol levels were more than 2-fold higher in net stressed fish at 3 min PNS and continued to increase peaking at 15 min PNS, when cortisol levels were 6-fold greater than the control cortisol. Mean cortisol declined from 15 to 60 min PNS, and at 60 min, net-stressed cortisol was similar to control cortisol. Because the age of fish differed between studies, we examined resting cortisol levels of fish of different ages (3, 7, 13, and 19 months). The resting cortisol values among tanks with the same age fish differed significantly but there was no clear effect of age. Our study is the first to report the response and recovery of cortisol after net handling for laboratory-reared zebrafish.

Cortisol response and immune-related effects of Atlantic salmon (Salmo salar Linnaeus) subjected to short- and long-term stress

It is generally considered that stress causes decreased immune function in fish. In this study we examined in Atlantic salmon (Salmo salar Linnaeus) the effects of both short- (a single 15s out of water) and long-term (4 weeks of daily handling 15s out of water) stress on plasma cortisol (free and total) and glucose levels, expression of interleukin-1beta (IL-1beta) and survival of head kidney (HK) macrophages under culture with Aeromonas salmonicida. In the short-term study, samples were collected prior to the application of the stressor, and at 1, 3, 6, 12 and 24h post stress. Free and total plasma cortisol levels and the percentage of free cortisol increased significantly in the stressed group at 1 and 3h post stress. Plasma glucose levels were significantly higher than those of control fish at 1, 3 and 6h post stress. Constitutive expression of IL-1beta in macrophages isolated from head kidneys in stressed fish was significantly higher at 1 and 3h post stress. However, lipopolysaccharide (LPS) stimulated expression of IL-1beta in HK macrophages, exhibited significantly higher fold increases in unstressed fish compared to stressed fish. In the long-term study, with the exception of an increase in plasma glucose levels at 1 week, there were no significant differences in stress parameters between groups. There was a significantly higher constitutive IL-1beta expression in macrophages isolated from stressed fish over the first 2 weeks. At weeks 1, 2 and 3 the magnitude of IL-1beta response of isolated HK macrophages to LPS stimulation was reduced in >90% of the stressed fish. At 4 weeks there was no significant difference in inducible IL-1beta expression between the groups. Macrophages isolated from stressed fish also showed significantly decreased survival when exposed to A. salmonicida. This study shows a clear pattern from repeated handling stress, whereby effects on immune cells begin with increased constitutive expression of IL-1beta, followed by decreased stimulation of leucocytes by extracellular antigen, and finally decreased leukocyte survival when exposed to A. salmonicida. The implications of these changes in the immune system will be discussed with respect to the use of classical indicators of stress to predict possible effects on the immune system of fish.

Exploration of the hypothalamic–pituitary–adrenal function as a tool to evaluate animal welfare

Measuring HPA axis activity is the standard approach to the study of stress and welfare in farm animals. Although the reference technique is the use of blood plasma to measure glucocorticoid hormones (cortisol or corticosterone), several alternative methods such as the measurement of corticosteroids in saliva, urine or faeces have been developed to overcome the stress induced by blood sampling itself. In chronic stress situations, as is frequently the case in studies about farm animal welfare, hormonal secretions are usually unchanged but dynamic testing allows the demonstration of functional changes at several levels of the system, including the sensitization of the adrenal cortex to ACTH and the resistance of the axis to feedback inhibition by corticosteroids (dexamethasone suppression test). Beyond these procedural aspects, the main pitfall in the use of HPA axis activity is in the interpretation of experimental data. The large variability of the system has to be taken into consideration, since corticosteroid hormone secretion is usually pulsatile, follows diurnal and seasonal rhythms, is influenced by feed intake and environmental factors such as temperature and humidity, age and physiological state, just to cite the main sources of variation. The corresponding changes reflect the important role of glucocorticoid hormones in a number of basic physiological processes such as energy metabolism and central nervous system functioning. Furthermore, large differences have been found across species, breeds and individuals, which reflect the contribution of genetic factors and environmental influences, especially during development, in HPA axis functioning. Usually, these results will be integrated with data from behavioral observation, production and pathology records in a comprehensive approach of farm animal welfare.

Changes in free and total plasma cortisol levels in juvenile haddock (Melanogrammus aeglefinus) exposed to long-term handling stress

We measured changes in free and total plasma cortisol levels, plasma glucose, gill hsp70 levels, and growth in haddock (Melanogrammus aeglefinus) subjected to a long-term handling stress (15 s out of water, each day, for 4 weeks), and the effect of this long-term stress on the ability of haddock to respond to an acute stressor. The acute stressor was a single handling stress, and fish were sampled at 1, 6, and 12 h post-stress. During the long-term stress study, free and total plasma cortisol levels increased significantly (10-fold) in the stressed group after the second week. However, the percentage of free cortisol was already significantly elevated by the first week (control 17%, stressed 55%), and remained high during the second week (control 35% and stressed 65%). After 3 and 4 weeks of handling, both free and total cortisol declined in stressed fish to levels that were not significantly different from pre-stress values. Control fish grew significantly more than stressed fish (by 32% and 18%, respectively) over the 4 week study, and condition factor only increased in control fish. Although fish from the control group showed elevated total plasma cortisol levels (to 47 ng mL(-1)) 1 h after the acute stress, and the levels in stressed fish were comparable to those for the control fish, no significant increase in plasma cortisol was measured in the group subjected to the long-term stress. Free plasma cortisol levels did not increase significantly in either group following the acute stress. However, free plasma cortisol levels were significantly higher in long-term stress group, as compared with the control group, at 6 h post-stress. Plasma glucose and gill hsp70 levels were not altered by either the long-term stress or acute stressor. Our data indicate that cortisol (free and total), but not glucose or hsp70, appears to be adequate to assess short- and long-term stress in haddock.

Effects of acute stress on osmoregulation, feed intake, IGF-1, and cortisol in yearling steelhead trout (Oncorhynchus mykiss) during seawater adaptation

Juvenile steelhead (Oncorhynchus mykiss) acclimated to freshwater (FW) were subjected for 3 h to confinement stress in FW, and subsequently saltwater (SW, 25 ppt) was introduced to all tanks. Fish were sampled immediately after the stress treatment, and 1, 7, and 14 days after introduction of SW. Electrolytes, cortisol, glucose, and lactate showed the typical stress response expected after stress treatment in FW. Fish regained osmotic balance within 24 h. Glucose concentrations were increasing throughout the experiment and lactate levels stayed elevated during the time spent in SW. Insulin-like growth factor-1 (IGF-1) did not show an immediate response to stress but after transfer to SW we detected significantly higher concentrations for control fish at days 1 and 14. The differences in IGF-1 levels between stressed and control fish are not reflected in SW adaptability but positive correlations between IGF-1 and electrolyte levels in control fish may indicate its role for osmoregulation. Confinement stress did not impair feed intake subsequently in SW, but our results suggest that feed intake was suppressed by the change of the media from FW to SW.

Rates of cortisol increase and decrease in channel catfish and sunshine bass exposed to an acute confinement stressor

Channel catfish and sunshine bass were exposed to a low-water stress event and allowed to recover in fresh water or a solution of metomidate (dl-1-(1-phenylethyl)-5-(metoxycarbonyl) imidazole hydrochloride), which inhibits the synthesis of cortisol. Change in time of plasma cortisol was used as an index of cortisol secretion and clearance. Plasma cortisol and glucose increased during the exposure to low-water stress in both fish, but the changes of both plasma components were more dramatic in sunshine bass. Exposure to metomidate during recovery resulted in a short-term increase in plasma glucose but differences between controls and metomidate-exposed fish were relatively minor thereafter. Cortisol began to decrease in catfish immediately after the removal of the stress but continued to increase for 15 min in sunshine bass recovering in fresh water and for 5 min in bass recovering in metomidate. Catfish recovering in fresh water had a cortisol elimination rate of -1.28 ng/mL/min compared with -2.45 ng/mL/min for fish recovering in metomidate (P>0.05) while sunshine bass recovering in fresh water had an elimination rate of -6.96 ng/mL/min compared with -4.50 ng/mL/min for fish recovering in metomidate (P>0.05). These data indicate that the rapid decrease of plasma cortisol after removal of the stressor is due to an almost immediate decrease of secretion, tissue uptake and a rapid renal loss due to the absence of a plasma binding protein.

Estradiol impairs hyposmoregulatory capacity in the euryhaline tilapia, Oreochromis mossambicus

Freshwater (FW)-adapted tilapia (Oreochromis mossambicus) were treated with estradiol (E(2)) for 4 days to stimulate protein synthesis and sampled at 0, 4, and 24 h after exposure to 50% seawater (SW). E(2) increased circulating vitellogenin (VTG) levels in large amounts, indicative of unusually high rates of hepatic protein synthesis. E(2) treatment prevented the recovery of plasma osmolality in 50% SW that was evident in the sham group. Plasma sodium concentration was significantly elevated with E(2) in FW, but the levels did not change in 50% SW. Gill Na(+)-K(+)-ATPase activity was significantly lower in the E(2) group compared with sham-injected tilapia in 50% SW. No significant differences were noted in plasma cortisol, thyroxine, triiodothyronine, or glucose concentration with E(2) in 50% SW. E(2) significantly lowered several key liver enzyme activities and also decreased gill lactate dehydrogenase and malate dehydrogenase activities over a 24-h period. Together, our results suggest that E(2) impairs ion regulation in tilapia, partially mediated by a decreased metabolic capacity in liver and gill. The decreased tissue metabolic capacity is likely due to E(2)-induced energy repartitioning processes that are geared toward VTG synthesis at the expense of other energy-demanding pathways.

Elevation of plasma cortisol during the spawning migration of landlocked kokanee salmon (Oncorhynchus nerka kennerlyi)

Kokanee salmon (Oncorhynchus nerka kennerlyi ), a landlocked subspecies of sockeye salmon, exhibited hypothalamic-pituitary interrenal (HPI, adrenal homologue) axis activation and an increase in plasma cortisol concentration up to 639 +/- 55.9 ng/ml in association with upstream migration in the upper Colorado River even though they were not exposed to a change in salinity and lengthy migration. Kokanee salmon were collected at various stages of migration and concomitant sexual maturation. The pattern of cortisol elevation in kokanee is similar to that in ocean-run sockeye salmon (O. nerka nerka). The presence of plasma cortisol elevation in an upstream migrating, landlocked Pacific salmon suggests that stressors previously considered to cause the cortisol increase, such as long-distance migration and changes in salinity, may not be primary causes of the HPI axis activation.

Effects of food deprivation and handling stress on head kidney 17alpha-hydroxyprogesterone 21-hydroxylase activity, plasma cortisol and the activities of liver detoxification enzymes in rainbow trout

The 21-hydroxylation of 17alpha-hydroxyprogesterone is one step in the biosynthesis of corticosteroids. Both 7 days of handling-induced stress and 7 weeks of food deprivation significantly elevated head kidney microsomal 17alpha-hydroxyprogesterone 21-hydroxylase activity in juvenile rainbow trout. The increased 21-hydroxylase activity was not paralleled by changes in plasma cortisol levels induced by handling stress whereas food deprivation for 3 and 7 weeks increased both 21-hydroxylase activity and plasma cortisol levels significantly. Food deprivation in rainbow trout affected detoxification enzyme activities, namely glutathione-S-transferase (GST), uridine-di-phosphate glucuronosyltransferase (UGT) and glutathione reductase (GR) activities in the liver. Together our observations suggest that experimental conditions can affect experimental results, especially the values of parameters like GST, UGT and GR. Furthermore, alterations in the metabolic state of the liver caused by stress or food deprivation can alter the balance between detoxification enzymes in rainbow trout liver.

Cortisol-induced changes in oxygen consumption and ionic regulation in coastal cutthroat trout (Oncorhynchus clarki clarki) parr

The influence of cortisol on oxygen consumption and osmoregulatory variables was examined in coastal cutthroat trout (Oncorhynchus clarki clarki) parr kept in fresh water (FW) and transferred to seawater (SW). Intraperitoneal implants containing cortisol (50 μg g(-1)) in vegetable oil resulted in elevated plasma cortisol titres similar to those observed in fish following a 24h SW exposure. Cortisol treatment significantly increased the oxygen consumption and plasma glucose levels of trout in FW, consistent with the glucocorticoid role of cortisol. Cortisol treatment did not cause any changes in plasma ion concentrations or gill Na(+),K(+)-ATPase activity in FW after 10 days. Cortisol-implanted fish exposed to SW for 24h showed slightly improved ion regulatory ability compare to non-implanted controls. The results of this study suggest that during SW transfer in juvenile salmonids, increases in cortisol may act as both a mineralocorticoid and a glucocorticoid, depending on the developmental state of the fish (e.g., smolt versus parr). Furthermore, the relative energetic costs of osmoregulation and that of the stress associated SW transfer cannot be discerned using whole-animal oxygen consumption rates.

Regulation of interrenal function in freshwater and sea water adapted tilapia (Oreochromis mossambicus)

In teleosts, cortisol is one of the key factors regulating the adaptation to environmental challenges, such as salinity changes. This paper compares interrenal function between fully adapted freshwater (FW) and sea water (SW) specimens of the euryhaline teleost Oreochromis mossambicus (tilapia), combining morphometric and biosynthetic approaches. Interrenal tissue and two tissues producing interrenal secretagogues (ACTH and ANP; atrial natriuretic peptide) were studied. The results demonstrate that sea water adaptation concurs with a sustained stimulation of the interrenal cells, as evidenced by a marked hyperplasia of the cells and the higher initial ex vivo cortisol release in seawater adapted tilapia. This difference was not reflected in ultrastructural differences in the pituitary corticotropes. Plasma ACTH levels were also similar in FW and SW adapted tilapia. Moreover, in vitro data indicate that the ACTH sensitivity of the interrenal cells of both groups was also similar. A second potential interrenal secretagogue (ANP) has recently been implicated in teleost ionic regulation during salinity changes. However, plasma immunoreactive ANP levels and in vitro production of the hormone were also indistinguishable between FW and SW tilapia. ANP pretreatment of tilapia head kidneys in vitro strongly inhibited the response to ACTH, an effect previously undocumented for teleosts. Whereas the sustained stimulation of the interrenal cells under sea water conditions corroborates results obtained with other teleost species, thereby supporting a hypoosmoregulatory role for cortisol, it also is evident that notable species differences exist regarding the regulation of the interrenal gland under these conditions.

Stanniocalcin kinetics in freshwater and seawater european eel (Anguilla anguilla)

The kinetics and hypocalcemic potency of stanniocalcin (STC) were examined in freshwater and seawater eels. The secretion rate and the metabolic clearance rate of STC were calculated from the STC disappearance curve after intra-arterial injection of trout STC. Basal plasma STC concentrations in freshwater and seawater eels did not differ but the STC secretion rate and metabolic clearance rate in seawater eel were 70-75% higher than in FW eel. The increased STC distribution space in seawater eels suggests that the STC receptor density was increased. STC had a higher hypocalcemic potency in seawater than in freshwater eels. These observations support the hypothesis that seawater fish require more hormonal control over transcellular influx of calcium than freshwater fish.

Liver and brain glucocorticoid receptor in rainbow trout, Oncorhynchus mykiss: down-regulation by dexamethasone

Total glucocorticoid binding sites were identified and quantitated in liver and brain of rainbow trout using an exchange method and [3H]dexamethasone as the ligand. Both tissues contained a predominantly cytosolic moiety that bound dexamethasone with high specificity. Binding was saturable, time dependent, and completely reversible. Scatchard analysis showed a linear relationship suggesting that receptors belong to a single class. Dexamethasone down-regulated both liver and brain receptors. Down-regulation was rapid (within hours) and dose dependent (ED50 = 1.5 mg/kg body weight). Dexamethasone-induced down-regulation was not a result of cytoplasm to nuclei translocation or due to increases in tissue concentrations of steroid. Dexamethasone administration resulted in a lowering of Bmax (82.3 +/- 2.5 to 20.6 +/- 10.5 fmol/mg protein) and an increase in Kd (15.6 +/- 0.2 to 44.3 +/- 5.0 nM) suggesting a conformational change in the receptor molecule as part of the mechanism. The brain and liver of the rainbow trout thus have glucocorticoid receptors similar to those described in the mammalian system. Further, these receptors are subjected to autologous regulation similar to their counterparts in other systems.

The biliary accumulation of corticosteroids in rainbow trout,Oncorhynchus mykiss, during acute and chronic stress

The accumulation of immunoreactive corticosteroids in the bile of rainbow trout during stress was monitored by radioimmunoassay and GUMS. Although plasma cortisol levels were elevated by confinement for 1 hour, biliary levels of free and conjugated steroids in the bile were unaffected. However, after 24 hours confinement, in addition to elevated plasma cortisol levels, free and conjugated steroids in the bile were also significantly higher than in control, unstressed fish. The time-course of change in plasma and biliary corticosteroid levels was determined in rainbow trout subject to 96 hours confinement stress. Free steroid levels in the bile of stressed fish were elevated within 2 hours of the onset of stress, while levels of conjugated steroids were significantly elevated within 4 hours of the onset of confinement. Analysis of bile from stressed fish, by GC/MS, established the major conjugated steroids present to be tetrahydrocortisone (230 μg ml(-1) bile), tetrahydrocortisol (75 μg ml(-1)), cortisone (33.5 μg ml(-1)), cortisol (25 μg ml(-1)) and β-cortolone (5 μg ml(-1)). The data are discussed with reference to the role of cortisone and conjugating enzymes in the clearance of cortisol, and further data are presented to suggest that the analysis of biliary steroid content may provide a suitable means of identifying stressed fish under conditions in which an additional sampling stress is unavoidable.

Cortisol effects on plasma electrolytes and thyroid hormones during smoltification in coho salmon Oncorhynchus kisutch

Plasma concentrations of Na+, K+, triiodothyronine (T3), and thyroxine (T4) and muscle water content were measured in yearling coho salmon. Oncorhynchus kisutch, after injection of cortisol in April, May, and June in fresh water (FW) and during acclimation to seawater (SW). Cortisol (17-21 micrograms/g), injected intraperitoneally in a melted cocoa butter suspension, suppressed the rise of plasma Na+ during SW acclimation in April but not in May or June. Muscle water content increased during SW acclimation in cortisol-treated fish in April and June. These observations suggest a hypoosmoregulatory function for cortisol during SW acclimation. Cortisol treatment also induced elevations of plasma K+ in FW (April only) and SW (April and May only). Cortisol treatment increased plasma T3 during SW acclimation in June and T4 in FW in May. The results suggest that cortisol may modify osmoregulatory processes and thyroid hormone activity during smoltification and acclimation to SW in yearling coho salmon.

Cortisol secretion in vitro by the interrenal of coho salmon (Oncorhynchus kisutch) during smoltification: relationship with plasma thyroxine and plasma cortisol

An in vitro system for the incubation of interrenal tissue (head kidney fragments) from coho salmon, Oncorhynchus kisutch, was developed in order to examine changes in interrenal sensitivity to ACTH1-24 during smoltification, using cortisol secretion as the endpoint. Time-course studies indicated that maximal cortisol accumulation in incubation media was achieved after 3 hr exposure to ACTH. There was no correlation between head kidney weight, body weight, or sex and the response of the interrenal to ACTH1-24 in vitro. Approximately monthly or bi-weekly experiments were performed during the smoltification period (February-July): tissue was preincubated in hormone-free media for 3 hr, washed twice, and then challenged with 5 X 10(-10)-5 X 10(-7) M (1.5-1500 ng/ml) ACTH1-24 for 3 hr. The pattern of cortisol secretion was similar in February, early March, and late March in the dose range of 5 X 10(-10)-5 X 10(-8) M ACTH1-24. A marked, significant increase in sensitivity to ACTH and in the steroidogenic capacity of the tissue occurred in April, but by May the response was similar to that in the pre-April period. Enhanced sensitivity and steroidogenic capacity were found in interrenal tissue taken from coho salmon in June and July. Maximal in vitro responsiveness of interrenal tissue to ACTH in April was correlated with peak plasma thyroxine titers and enhanced hypoosmoregulatory ability, but not with peak plasma cortisol titers, which occurred in May.

A protocol for estimation of cortisol plasma clearance in acid-exposed rainbow trout (Salmo gairdneri)

The cortisol metabolic clearance rate (MCR) and degradation rate (DR) were determined in acid (H2SO4)-stressed and control rainbow trout (260 g) cannulated via the dorsal aorta. Recovery from catheterization, as judged by plasma cortisol, glucose, protein, and packed cell volume (PCV), was complete by 6 days. However, serial blood sampling increased plasma cortisol. Furthermore, although no major or consistent diel change in plasma cortisol occurred in terminally sampled free-swimming control trout, fluctuations were observed in serially bled catheterized trout. These findings preclude cortisol MCR estimation by any serial sampling method. Although plasma cortisol was temporarily elevated by constant infusion (70 microliter hr-1) of saline:ethanol vehicle, a satisfactory protocol was established for determining cortisol MCR by infusion of labeled cortisol to constant plasma specific activity. The MCR for control trout in water, pH 7.7, for 7 days was 30.3 +/- 4.2 ml hr-1 100 g-1, which did not differ from that of trout in water, pH 5.0. However, the cortisol DR was greater at pH 5.0 (2.13 +/- 0.46 micrograms hr-1 100 g-1) than at pH 7.7 (0.56 +/- 0.12) due primarily to increased plasma cortisol at pH 5.0.

The interrelationship of cortisol, gill (Na + K) ATPase, and homeostasis during the Parr-Smolt transformation of Atlantic salmon (Salmo salar L.)

Serum cortisol concentrations were measured in juvenile Atlantic salmon (Salmo salar L.) undergoing the parr-smolt transformation in fresh water, at either 1 year (S1 population) or 2 years (S2 population) after hatching. Serum cortisol levels were generally low (less than 10 ng ml-1), but during smoltification became significantly elevated in both populations. In addition, the S2 population showed a small cortisol peak in the autumn prior to smoltification. Simultaneous measurement of gill (Na + K) ATPase activity and serum cortisol concentrations in S2 salmon juveniles revealed that both features rose during smoltification in fresh water. The rise in gill (Na + K) ATPase activity was independent of cortisol levels, and preceded the rise in cortisol titer by approximately 1 month. After seawater transfer, gill enzyme levels remained high while cortisol titers fell sharply. Serum cortisol levels, but not gill (Na + K) ATPase activities, were progressively reduced by acclimation of smolts to increasing salinities. Linear regression studies indicated that, at any one level of gill (Na + K) ATPase, cortisol titer increased with increasing surface area: volume ratio. Extracellular fluid volume (sodium space) was found to decline with increasing gill (Na + K) ATPase activity, and to increase with serum cortisol titers. These results indicate that high serum cortisol levels represent a secondary response caused by the development of hypoosmoregulatory ability while still resident in fresh water. Cortisol does not appear to directly stimulate gill (Na + K) ATPase activity in Atlantic salmon smolts.

Clearance of plasma corticosteroids during smoltification of coho salmon, Oncorhynchus kisutch

The plasma clearance rate (PCR) of radioactivity after a single intracardial injection of 3H-cortisol was elevated during the spring in yearling coho salmon, Oncorhynchus kisutch. Graphical analysis suggested a seasonal correlation between PCR and gill Na/K-ATPase activity. An explanation for this correlation is suggested. The major metabolite of 3H-cortisol in plasma was 3H-cortisone. It appeared rapidly following injection of the original radiotracer.