The impact of social status on the erythrocyte beta-adrenergic response in rainbow trout, Oncorhynchus mykiss

The objective of the present investigation was to determine whether chronic increases in circulating cortisol concentrations, resulting from the occupation of subordinate status in rainbow trout social hierarchies, resulted in an enhancement of the erythrocyte adrenergic response. Rainbow trout (Oncorhynchus mykiss) were confined in fork length matched pairs for 6 h, 18 h, 48 h or 5-7 days, and social status was assigned through observations of behaviour. Erythrocyte adrenergic responsiveness, determined in vitro as changes in water content following incubation with the beta-adrenoreceptor agonist isoproterenol, was significantly greater in subordinate than dominant fish at 48 h of social interactions but not after 5-7 days, nor when assessed as changes in extracellular pH (pHe). However, the activity of the Na+/H+ exchanger (beta-NHE), assessed in vitro as the pHe change following incubation with the permeable cyclic AMP analogue 8-bromo-cyclic AMP, was significantly lower in subordinate fish. The number of erythrocyte membrane-bound adrenergic receptors (Bmax) was significantly higher in subordinate than dominant fish at 48 h, but had decreased by 5-7 days to a value that was not significantly different from that for dominant fish. The apparent dissociation constant (KD) of these receptors was not significantly impacted by either social status or interaction time. Finally, the relative expressions of beta-3b adrenergic receptor (AR) and beta-NHE mRNA were determined using real-time PCR and were found to be minimally affected by social rank. Relative to a control group, beta-3b AR mRNA was significantly up-regulated in both dominant and subordinate trout at all time periods, whereas the expression of beta-NHE was in general significantly down-regulated. Unlike the situation in rainbow trout treated with exogenous cortisol, elevations in circulating cortisol resulting from low social status did not "pre-adapt" the erythrocyte adrenergic response, but rather may have served to offset the potentially adverse effects elicited by plasma catecholamines, which were elevated during social hierarchy formation.     

Stress responsiveness affects dominant-subordinate relationships in rainbow trout.

The magnitude by which plasma cortisol levels increase following exposure to a stressor is a heritable trait in rainbow trout. The relative growth in coculture of F1 lines selected for high responsiveness (HR) and low responsiveness (LR) to a confinement stressor suggested that behavioral characteristics related to food acquisition, aggression, or competitive ability might differ between the two lines. This hypothesis was tested using the F2 generation of the selected lines. The F2 lines clearly exhibited the characteristics of the F1 parents, displaying significantly divergent plasma cortisol responses to a 1-h confinement stressor and a high heritability for the trait. Behavioral differences between the lines were assessed by observing the outcome of staged fights for dominance in size-matched pairs of HR and LR fish. The identification of dominant and subordinate fish within each pair on the basis of their behavior was supported by the levels of blood cortisol in the fish attributed to each group (dominant < subordinate). Fish from the LR line were identified as dominant in significantly more trials than were HR individuals. The results suggest that behavioral attributes that affect the outcome of rank-order fights are closely linked to the magnitude of the plasma cortisol response to stress in rainbow trout. Whether the link is causal or circumstantial is not yet evident.     

Low social status impairs hypoxia tolerance in rainbow trout (Oncorhynchus mykiss)

In the present study, chronic behavioural stress resulting from low social status affected the physiological responses of rainbow trout (Oncorhynchus mykiss) to a subsequent acute stressor, exposure to hypoxia. Rainbow trout were confined in fork-length matched pairs for 48-72?h, and social rank was assigned based on behaviour. Dominant and subordinate fish were then exposed individually to graded hypoxia (final water PO(2), PwO(2)?=?40?Torr). Catecholamine mobilization profiles differed between dominant and subordinate fish. Whereas dominant fish exhibited generally low circulating catecholamine levels until a distinct threshold for release was reached (PwO(2)?=?51.5?Torr corresponding to arterial PO(2), PaO(2)?=?24.1?Torr), plasma catecholamine concentrations in subordinate fish were more variable and identification of a distinct threshold for release was problematic. Among fish that mobilized catecholamines (i.e. circulating catecholamines rose above the 95% confidence interval around the baseline value), however, the circulating levels achieved in subordinate fish were significantly higher (459.9?±?142.2?nmol?L(-1), mean?±?SEM, N?=?12) than those in dominant fish (130.9?±?37.9?nmol?L(-1), N?=?12). The differences in catecholamine mobilization occurred despite similar P(50) values in dominant (22.0?±?1.5?Torr, N?=?6) and subordinate (22.1?±?2.2?Torr, N?=?8) fish, and higher PaO(2) values in subordinate fish under severely hypoxic conditions (i.e. PwO(2)?相似文献   

Does socially induced stress in rainbow trout cause chloride cell proliferation?

At the end of a 2-week confinement period, subordinate rainbow trout Oncorhynchus mykiss had significantly higher circulating concentrations of plasma cortisol than did the dominant fish with which they were paired. Physiological effects linked to elevated plasma cortisol concentrations in subordinate fish included loss of weight and a lowering of condition factor. However, there were no significant differences in gill epithelium chloride cell numbers or blood plasma ion concentrations between dominant and subordinate fish. It is concluded that elevated plasma cortisol concentrations elicited by the social stressors of the present study did not cause proliferation of chloride cells.     

Serotonin, but not melatonin, plays a role in shaping dominant-subordinate relationships and aggression in rainbow trout

The aim of this study was to clarify to what extent the effects of elevated dietary L-tryptophan (Trp) on aggressive behavior and stress responsiveness in rainbow trout are mediated by circulating melatonin and central serotonin (5-HT), respectively. Isolated rainbow trout were paired for 1h a day for 7 days in order to create fish with experience of being dominant and subordinate. Following this week, the fish were tested for aggressive behavior using a resident-intruder test after which they were subjected to one of four treatments: (1) tryptophan, (2) the selective serotonin reuptake inhibitor (SSRI) citalopram, (3) melatonin, and (4) no treatment (controls). After 7 days of treatment, the fish were subjected to a second resident-intruder test. Trp-supplemented feed resulted in a suppression of aggressive behavior in fish with experience of being dominant. Moreover, fish fed Trp-supplemented feed, regardless of social experience, also displayed lower plasma cortisol levels than controls. These effects of elevated dietary Trp were closely mimicked by citalopram treatment, whereas exogenous melatonin had no effect on either aggressive behavior or plasma cortisol. Thus, the effect of elevated dietary Trp on aggressive behavior and stress responses does not appear to be mediated by melatonin even though elevated dietary intake of Trp resulted in an increase in plasma melatonin concentrations.     

Effects of cortisol on aggression and locomotor activity in rainbow trout

Noninvasive administration of cortisol through the diet resulted in relatively rapid (<1.5 h) and highly reproducible increases in plasma cortisol in rainbow trout, comparable to changes seen in fish subjected to substantial stress. Juvenile rainbow trout were reared in isolation for 1 week, before their daily food ration was replaced by a meal of cortisol-treated food corresponding to 6 mg cortisol kg(-1). All fish were observed for 30 min, beginning at 1 or 48 h following the introduction of cortisol-treated food. Additional cortisol (75% of the original dose on Day 2, and 50% on Day 3) was administered to the long-term cortisol-treated group. The resulting blood plasma concentrations of cortisol were similar in short- and long-term treated fish, and corresponded to those previously seen in stressed rainbow trout. Controls were fed similar food without cortisol. Half of the fish from each treatment group (controls and short- and long-term cortisol) were subjected to an intruder test (a smaller conspecific introduced into the aquarium), while half of the fish were observed in isolation. In fish challenged by a conspecific intruder, short-term cortisol treatment stimulated locomotor activity, while long-term treatment inhibited locomotion. Aggressive behavior was also inhibited by long-term cortisol treatment, but not by short-term exposure to cortisol. Cortisol treatment had no effect on locomotor activity in undisturbed fish, indicating that the behavioral effects of cortisol were mediated through interaction with other signal systems activated during the simulated territorial intrusion test. This study demonstrates for the first time that cortisol has time- and context-dependent effects on behavior in teleost fish.     

Physiological causes and consequences of social status in salmonid fish

Social interactions in small groups of juvenile rainbow trout(Oncorhynchus mykiss) lead to the formation of dominance hierarchies.Dominant fish hold better positions in the environment, gaina larger share of the available food and exhibit aggressiontowards fish lower in the hierarchy. By contrast, subordinatefish exhibit behavioural inhibition, including reduced activityand feeding. The behavioural characteristics associated withsocial status are likely the result of changes in brain monoaminesresulting from social interactions. Whereas substantial physiologicalbenefits, including higher growth rates and condition factor,are experienced by dominant trout, low social status appearsto be a chronic stress, as indicated by sustained elevationof circulating cortisol concentrations in subordinate fish.High cortisol levels, in turn, may be responsible for many ofthe deleterious physiological consequences of low social status,including lower growth rates and condition factor, immunosuppressionand increased mortality. Circulating cortisol levels may alsobe a factor in determining the outcome of social interactionsin pairs of rainbow trout, and hence in determining social status.Rainbow trout treated with cortisol were significantly morelikely to become subordinate in paired encounters with smalleruntreated conspecifics.     

Acid-base balance during social interactions in rainbow trout (Oncorhynchus mykiss)

Socially subordinate rainbow trout (Oncorhynchus mykiss) experience chronic stress that impacts upon a variety of physiological functions, including Na(+) regulation. Owing to the tight coupling between Na(+) and Cl(-) uptake and, respectively, H(+) and HCO(3)(-) loss at the gill, ionoregulatory changes associated with social status may affect acid-base regulation. The present study assessed the responses of dominant, subordinate and control trout to hypercapnia (1% CO(2)) to test this hypothesis. Social status appeared to impact net acid excretion (J(net)H(+)) as subordinate individuals failed to increase net acid flux in response to hypercapnia. However, blood acid-base status was found to be unaffected by social status before or during hypercapnic exposure, indicating that subordinate fish were as effective as dominant or control trout in achieving compensation for the acid-base disturbance induced by hypercapnic exposure. Compensation in all groups involved decreasing Cl(-) uptake in response to hypercapnia. The branchial activities of both Na(+),K(+)-ATPase (NKA) and V-type H(+)-ATPase were affected by social interactions and/or exposure to hypercapnia. Branchial NKA activity was higher but V-ATPase activity was lower in control fish than in dominant or subordinate trout. In addition, control and subordinate but not dominant trout exposed to 24h of hypercapnia exhibited significantly higher branchial V-ATPase activity than fish maintained in normocapnia. Collectively, the data suggest that subordinate trout are able to regulate blood pH during a respiratory acidosis.     

Relationship between stress, feeding and plasma ghrelin levels in rainbow trout, Oncorhynchus mykiss

Sexually immature rainbow trout were acclimated to small-volume (1 m³) holding tanks and then exposed to short-term stress to examine the relationship between feeding, stress, plasma ghrelin levels and other plasma stress parameters. Plasma ghrelin levels showed an increase 24 h after a single feed, plasma lactate and glucose levels decreased over the same period and plasma cortisol levels were low and constant. One hour of confinement stress resulted in elevations of plasma cortisol, glucose and lactate and depression of plasma ghrelin levels. In a separate experiment, 2 h of confinement stress also depressed feeding immediately after stress, concomitant with increases in plasma cortisol, lactate and glucose; however, in this case there was no change in plasma ghrelin concentrations. A repeat of the 2-h confinement experiment using fish that had not been acclimated to small-volume holding tanks produced a more marked elevation in plasma cortisol and a stronger suppression of feeding post-stress but in this case also, there was no change in plasma ghrelin levels. The results of this study confirm that feeding in rainbow trout is suppressed by confinement stress although the effect is transitory in this domesticated stock. Similar to that in other fishes, plasma ghrelin levels appear to be modulated by feeding status and may be influenced by stress, suggesting an orexigenic role for ghrelin in rainbow trout.     

Differences in the sensitivity of brown trout, Salmo trutta L., and rainbow trout, Salmo gairdneri Richardson, to physiological doses of cortisol

Interspecific differences in the stress response of fish may be due, in part, to differences in the sensitivity of target tissues to cortisol. The relative response of brown and rainbow trout to a standardized dose of cortisol was assessed by monitoring condition (K factor), the number of circulating lymphocytes and mortality due to disease, following cortisol treatment. Cortisol implantation resulted in a significant decline in K factor and number of circulating lymphocytes in immature brown trout, but not in immature rainbow trout, despite plasma cortisol levels being similar in both cases. Cortisol implantation in mature brown and rainbow trout significantly increased the mortality rate due to bacterial and fungal infection compared with control fish. Furthermore, the mortality rate due to disease was significantly greater in brown trout than rainbow trout, despite both groups receiving similar doses of steroid.     

Stress effects on AVT and CRF systems in two strains of rainbow trout (Oncorhynchus mykiss) divergent in stress responsiveness

The aim for this study was to examine whether the F4 generation of two strains of rainbow trout divergent in their plasma cortisol response to confinement stress (HR: high responder or LR: low responder) would also differ in stress-induced effects on forebrain concentrations of mRNA for corticotropin-releasing factor (CRF), arginine vasotocin (AVT), CRF receptor type 1 (CRF-R1), CRF receptor type 2 (CRF-R2) and AVT receptor (AVT-R). In addition, plasma cortisol concentrations, brainstem levels of monoamines and monoamine metabolites, and behaviour during confinement were monitored. The results confirm that HR and LR trout differ in their cortisol response to confinement and show that fish of these strains also differ in their behavioural response to confinement. The HR trout displayed significantly higher locomotor activity while in confinement than LR trout. Moreover, following 180 min of confinement HR fish showed significantly higher forebrain concentrations of CRF mRNA than LR fish. Also, when subjected to 30 min of confinement HR fish showed significantly lower CRF-R2 mRNA concentrations than LR fish, whereas there were no differences in CRF-R1, AVT or AVT-R mRNA expression between LR and HR fish either at 30 or 180 min of confinement. Differences in the expression of CRF and CRF-R2 mRNA may be related to the divergence in stress coping displayed by these rainbow trout strains.     

Growth depression in socially subordinate rainbow trout Oncorhynchus mykiss: more than a fasting effect

To assess the effects of subordinate social status on digestive function, metabolism, and enzyme activity in salmonid fish, juvenile rainbow trout Oncorhynchus mykiss were paired with size-matched conspecifics (<1.5% difference in fork length) for 5 d. Fish that were fasted for 5 d and fish sampled directly from the holding tank were used as control groups. Both subordinate and fasted fish experienced significant decreases in intestine mass (P = 0.043), and the gall bladder showed marked and significant changes in both size (P = 0.004) and appearance. These findings suggest that the negative effect of social subordination on digestive function reflects in large part a lack of feeding. Hepatic phosphoenolpyruvate carboxykinase activity was significantly higher in subordinate fish relative to dominants, whereas subordinate hepatic pyruvate kinase activity was significantly lower; activities of both enzymes were significantly correlated with plasma cortisol concentrations and behavior scores. Dominant-subordinate differences in the activities of these enzymes were eliminated by administration of the glucocorticoid receptor antagonist RU486, underlining a role for circulating cortisol in eliciting the differences. Significant increases relative to control fish were also detected in red and white muscles from subordinate fish in the activities of protein catabolic enzymes (aspartate aminotransferase, alanine aminotransferase, glutamate dehydrogenase). These differences occurred in the absence of any change in plasma free amino acid or ammonia concentrations, supporting an enhanced turnover of amino acids in muscle in subordinate fish. The results support the hypothesis that changes in metabolism, beyond those elicited by low food consumption, may be responsible at least in part for the low growth rates typical of subordinate fish and that these changes may be related specifically to circulating cortisol levels in subordinate fish.     

Poststress levels of lysozyme and cortisol in adult rainbow trout: heritabilities and genetic correlations

Lysozyme response in stressed rainbow trout was compared with measurements of poststress cortisol activity. Estimates of heritability of lysozyme and cortisol were both moderate-to-high and both traits displayed positive genetic correlations in pair-wise comparisons of stress exposures. Genetic correlations between lysozyme and cortisol in stressed rainbow trout tended to be negative, although insignificant. Neither lysozyme, nor cortisol exhibited significant correlations with serum haemolytic activity. It is concluded that the data do not confirm earlier suggestions that lysozyme should be superior to cortisol in consistency of stress response in rainbow trout.     

The effect of short term confinement stress on binding characteristics of sex steroid binding protein (SBP) in female black bream (Acanthopagrus butcheri) and rainbow trout (Oncorhynchus mykiss)

The effect of short term confinement stress on sex steroid binding protein (SBP) binding characteristics was examined in female black bream (Acanthopagrus butcheri), and rainbow trout (Oncorhynchus mykiss). Black bream were sampled immediately after capture from the wild and again after 1, 6 or 24 h confinement. Rainbow trout were sampled before and after 5 h confinement. Confinement of black bream for 6 h after capture significantly reduced the binding capacity of SBP. Binding affinity also tended to be lower after confinement. There were no differences in binding affinity or capacity of black bream SBP after 1 or 24 h confinement, or rainbow trout SBP after 5 h confinement. Plasma from rainbow trout at 3 and 6 h after treatment with cortisol was compared to plasma from saline-injected controls. No significant differences in binding characteristics were detected, but there was a trend of decreased binding capacity in cortisol-injected fish compared to controls at 6 h post-injection. Relative binding studies indicated that plasma cortisol at concentrations 100x or more greater than plasma estradiol (E(2)) may displace E(2) from SBP in black bream, and act to reduce circulating levels of E(2) through increased clearance of free steroid. Physiological levels of cortisol did not displace E(2) from SBP in trout. The observed changes in SBP and the competition of physiological concentrations of cortisol for SBP binding sites may generate a component of the stress-induced falls in plasma levels of E(2) reported across a range of species.     

Contrasting seasonal modulation of the stress response in male and female rainbow trout

Between June and September the magnitude of the plasma cortisol response of maturing male and female rainbow trout to confinement was indistinguishable. A progressive increase in confinement-induced cortisol levels in both sexes occurred during May to September, associated with the seasonal rise in water temperature. Between September and January a reduction of >50% in the magnitude of the cortisol response to confinement in male fish (but no decline in females) coincided with declining water temperature and significant increases in plasma 11-ketotestosterone and elevated plasma testosterone levels. Plasma oestradiol-17β levels were significantly greater in females than males throughout the study period and this difference was maximal between September and January. However, plasma testosterone was also elevated in females during this period and levels overall were higher than those in male fish. Previous studies have shown oestradiol-17β and testosterone to have diametrically opposed effects on stress responsiveness in trout, with the former enhancing, and the latter suppressing, the cortisol response to a stressor. The relative roles of androgens, estrogen and water temperature in modulating the stress responsiveness of rainbow trout are discussed.     

Stress coping style predicts aggression and social dominance in rainbow trout

Social stress is frequently used as a model for studying the neuroendocrine mechanisms underlying stress-induced behavioral inhibition, depression, and fear conditioning. It has previously been shown that social subordination may result in increased glucocorticoid release and changes in brain signaling systems. However, it is still an open question which neuroendocrine and behavioral differences are causes, and which are consequences of social status. Using juvenile rainbow trout of similar size and with no apparent differences in social history, we demonstrate that the ability to win fights for social dominance can be predicted from the duration of a behavioral response to stress, in this case appetite inhibition after transfer to a new environment. Moreover, stress responsiveness in terms of confinement-induced changes in plasma cortisol was negatively correlated to aggressive behavior. Fish that exhibited lower cortisol responses to a standardized confinement test were markedly more aggressive when being placed in a dominant social position later in the study. These findings support the view that distinct behavioral-physiological stress coping styles are present in teleost fish, and these coping characteristics influence both social rank and levels of aggression.     

Behavioral plasticity in rainbow trout (Oncorhynchus mykiss) with divergent coping styles: when doves become hawks

Consistent and heritable individual differences in reaction to challenges, often referred to as stress coping styles, have been extensively documented in vertebrates. In fish, selection for divergent post-stress plasma cortisol levels in rainbow trout (Oncorhynchus mykiss) has yielded a low (LR) and a high responsive (HR) strain. A suite of behavioural traits is associated with this physiological difference, with LR (proactive) fish feeding more rapidly after transfer to a new environment and being socially dominant over HR (reactive) fish. Following transport from the UK to Norway, a switch in behavioural profile occurred in trout from the 3rd generation; HR fish regained feeding sooner than LR fish in a novel environment and became dominant in size-matched HR–LR pairs. One year after transport, HR fish still fed sooner, but no difference in social dominance was found. Among offspring of transported fish, no differences in feeding were observed, but as in pre-transported 3rd generation fish, HR fish lost fights for social dominance against size-matched LR opponents. Transported fish and their offspring retained their distinctive physiological profile throughout the study; HR fish showed consistently higher post-stress cortisol levels at all sampling points. Altered risk-taking and social dominance immediately after transport may be explained by the fact that HR fish lost more body mass during transport than did LR fish. These data demonstrate that some behavioural components of stress coping styles can be modified by experience, whereas behavioural plasticity is limited by genetic effects determining social position early in life story.     

Changes in selected blood component values of rainbow trout, Salmo gairdneri Richardson, following the blocking of the cortisol stress response with betamethasone and subsequent exposure to phenol or hypoxia

Cortisol was undetectable in rainbow trout plasma 24 h after the fish had been injected with betamethasone. This drug is known to suppress ACTH release and, consequently, cortisol production in teleosts. The blood glucose, chloride, sodium, potassium and total protein concentrations were not significantly affected by betamethasone. However, significantly increased blood packed cell volume values were found.
Betamethasone-injected fish when exposed to hypoxia or phenol had a pattern of blood component changes similar to those found in sham-injected fish also exposed to the pollutants. These changes in turn were similar to those found in exposed but uninjected and unhandled fish. Suppression of rainbow trout plasma cortisol concentrations with betamethasone apparently has no effect on the fish's short-term blood component responses to pollutants under the conditions used in these experiments, but does prevent a stressormediated increase in plasma cortisol concentrations.     

Agonistic encounters and cellular angst: social interactions induce heat shock proteins in juvenile salmonid fish

Juvenile salmonid fish readily form dominance hierarchies when faced with limited resources. While these social interactions may result in profound behavioural and physiological stress, it is unknown if this social stress is evident at the level of the cellular stress response—specifically, the induction of stress or heat shock proteins (Hsps). Thus, the goal of our study was to determine if Hsps are induced during hierarchy formation in juvenile rainbow trout (Oncorhynchus mykiss). To this end, we measured levels of three Hsps, Hsp70, Hsc (heat shock cognate)70 and Hsp90 in the white muscle, liver and brain of trout that had been interacting for 36 h, 72 h or 6 days. Our data indicate that Hsps are induced in both dominant and subordinate fish in a time- and tissue-specific manner. In further mechanistic experiments on fasted and cortisol-treated fish, we demonstrated that high plasma cortisol does not affect Hsp induction in trout white muscle or liver, but both conditions may be part of the mechanism for Hsp induction with social stress in the brain. We conclude that the behavioural and physiological stress experienced by juvenile rainbow trout in dominance hierarchies can be extended to the induction of Hsps.