Does Individual Variation in Stress Responses and Agonistic Behavior Reflect Divergent Stress Coping Strategies in Juvenile Rainbow Trout?

Catecholamines, cardiac natriuretic peptides and chromogranin A: evolution and physiopathology of a 'whip-brake' system of the endocrine heart

In the past 50 years, extensive evidence has shown the ability of vertebrate cardiac non-neuronal cells to synthesize and release catecholamines (CA). This formed the mindset behind the search for the intrinsic endocrine heart properties, culminating in 1981 with the discovery of the natriuretic peptides (NP). CA and NP, co-existing in the endocrine secretion granules and acting as major cardiovascular regulators in health and disease, have become of great biomedical relevance for their potent diagnostic and therapeutic use. The concept of the endocrine heart was later enriched by the identification of a growing number of cardiac hormonal substances involved in organ modulation under normal and stress-induced conditions. Recently, chromogranin A (CgA), a major constituent of the secretory granules, and its derived cardio-suppressive and antiadrenergic peptides, vasostatin-1 and catestatin, were shown as new players in this framework, functioning as cardiac counter-regulators in 'zero steady-state error' homeostasis, particularly under intense excitatory stimuli, e.g. CA-induced myocardial stress. Here, we present evidence for the hypothesis that is gaining support, particularly among human cardiologists. The actions of CA, NP and CgA, we argue, may be viewed as a hallmark of the cardiac capacity to organize 'whip-brake' connection-integration processes in spatio-temporal networks. The involvement of the nitric oxide synthase (NOS)/nitric oxide (NO) system in this configuration is discussed. The use of fish and amphibian paradigms will illustrate the ways that incipient endocrine-humoral agents have evolved as components of cardiac molecular loops and important intermediates during evolutionary transitions, or in a distinct phylogenetic lineage, or under stress challenges. This may help to grasp the old evolutionary roots of these intracardiac endocrine/paracrine networks and how they have evolved from relatively less complicated designs. The latter can also be used as an intellectual tool to disentangle the experimental complexity of the mammalian and human endocrine hearts, suggesting future investigational avenues.

Subclinical effects of saxitoxin and domoic acid on aggressive behaviour and monoaminergic turnover in rainbow trout (Oncorhynchus mykiss)

The algal produced neurotoxins saxitoxin and domoic acid may have serious effects on marine life and can be responsible for the intoxication of for instance sea mammals, sea birds and fish. Given that farmed fish cannot escape algal blooms, they may be more susceptible to intoxication than wild stocks. In the present study, subclinical effects of saxitoxin and domoic on aggressive behaviour and monoaminergic systems in the brain of the rainbow trout (Oncorhynchus mykiss) were investigated. The resident-intruder test was used to measure aggression where only the resident fish were subjected to the toxins and analysed for monoamines and their metabolites. The resident-intruder test was carried out on two consecutive days. On day one basal aggression was measured in the four groups. On day two three of the groups were injected with subclinical doses of one of the following: saxitoxin (1.752 microg/kg bw), domoic (0.75 mg/kg bw) or 0.9% saline solution. This was performed 30 min prior to the aggression test. Handling stress and injection affected aggressive behaviour, cortisol and the serotonergic system in telencephalic brain regions. Cortisol levels were elevated in all of the injected groups when compared to the control group. An increase in serotonergic turnover was evident when all injected groups were pooled and compared to the control group. All together this suggests that the handling stress in connection with the injection was similar in all of the three injected groups. In contrast to both the undisturbed control group and the toxin-injected groups, the saline-injected group displayed a reduction in aggressive behaviour which was evident in increased attack latency. Furthermore the domoic injected group displayed more aggressive attacks towards their conspecifics than the saline-injected group. Consequently the two toxins appear to mask the stress induced alteration in aggressive behaviour. Monoamine levels and monoaminergic turnover could not be demonstrated to be directly affected by the two toxins at the given doses in the investigated brain regions (dorsal and ventral parts of telencephalon, optic tectum, locus coeruleus, raphe nucleus, molecular and granular layer of cerebellum). This could indicate that the toxins mediate aggressive behaviour either through other systems than the monoaminergic systems, such as neuroactive amino acids, or that the mediation occurs in other brain regions.

Stress coping styles and singing behavior in the short-tailed singing mouse (Scotinomys teguina)

Stress coping styles have been characterized as a proactive/reactive dichotomy in laboratory and domesticated animals. In this study, we examined the prevalence of proactive/reactive stress coping styles in wild-caught short-tailed singing mice (Scotinomys teguina). We compared stress responses to spontaneous singing, a social and reproductive behavior that characterizes this species. To establish proactive/reactive profiles for singing mice, we measured exploratory and anxiety behavior using an open-field behavioral test. We examined correlations between open-field behaviors and fecal corticosterone (CORT) metabolites, baseline plasma CORT, and stress-induced CORT. Mice with proactive behavioral responses in the open-field had higher fecal CORT titers than reactive males, but did not differ in baseline or stress-induced plasma CORT. We suggest that individual differences in CORT metabolism may contribute to this surprising pattern. Males that sang in the open-field were behaviorally proactive and had lower stress-induced CORT, indicating a link between stress responses and singing in this species. Overall, the data demonstrate that singing mice offer an interesting model for exploring how stress reactivity can shape social behaviors.

Coping strategies in a strongly schooling fish, the common carp Cyprinus carpio

Individual common carp Cyprinus carpio were screened repeatedly for risk taking (rate of exploration of a novel, potentially dangerous environment) and for competitive ability (success in gaining access to a spatially restricted food source). Marked differences in behaviour were evident, and significant consistency in individual responses across trials was found for both risk taking and competitive ability. In addition, there was a significant positive relationship between individual performance in these two contexts, with fish that explored more quickly in the novel environment tending to be among the first to gain access to restricted food. In two follow-up studies, resting metabolic rate, blood lactate and glucose and the expression of the cortisol receptor gene in the head kidney and brain were compared in fish from the two extremes of the risk-taking spectrum. Mass-specific metabolic rate was significantly higher in risk-taking than in risk-avoiding fish, while plasma lactate and glucose concentrations and expression of the cortisol receptor gene were lower. It was concluded that a behavioural syndrome based on boldness and aggression exists in C. carpio, as it does in many other animals, and that this is associated with differences in metabolic and stress physiology (down to the genomic level) similar to those described in animals with different coping strategies.

Effect of different stress factors on some physiological parameters of Nile tilapia (Oreochromis niloticus)

This study was conducted to determine the effect of different stress factors on some physiological measurements of Nile tilapia (Oreochromis niloticus). A total number of 160 Nile tilapia, the body weight ranging between 100 and 120 g, were exposed to three stress factors of hypoxia, overcrowding and starvation for different periods 24, 72 and 144 h. The results of cortisol level were 134.15, 144.27, 154.12 ng/ml and 140.18 ng/ml for control, hypoxia, overcrowding and starvation, respectively, while after 144 h did not show significant difference among treatments compared with control group. In contrast, the values of T3 and T4 observed reduction with significant difference that T3 ranged between the highest value 122.12 ng/ml for control group to lowest value of starvation group 94.35, 93.81 and 88.46 ng/ml after 24, 72 and 144 h. Also, similar trend of results observed in T4 and blood glucose among treatments. And the enzymatic activity of lactate dehydrogenise (LDH) increased in hypoxic group, while a significant reduction appeared in overcrowding and starved fish compared to control group. The pyruvate kinase (PK) activity decreased in hypoxic group but increased in other group.

Behavioral Profile Predicts Dominance Status in Mountain Chickadees

Individual variation in stable behavioral traits may explain variation in ecologically-relevant behaviors such as foraging, dispersal, anti-predator behavior, and dominance. We investigated behavioral variation in mountain chickadees (Poecile gambeli), a North American parid that lives in dominance-structured winter flocks, using two common measures of behavioral profile: exploration of a novel room and novel object exploration. We related those behavioral traits to dominance status in male chickadees following brief, pair-wise encounters. Low-exploring birds (birds that visited less than four locations in the novel room) were significantly more likely to become dominant in brief, pairwise encounters with high-exploring birds (i.e., birds that visited all perching locations within a novel room). On the other hand, there was no relationship between novel object exploration and dominance. Interestingly, novel room exploration was also not correlated with novel object exploration. These results suggest that behavioral profile may predict the social status of group-living individuals. Moreover, our results contradict the idea that novel object exploration and novel room exploration are always interchangeable measures of individuals' sensitivity to environmental novelty.

Cohesive social behaviour shortens the stress response: the effects of conspecifics on the stress response in lake sturgeon Acipenser fulvescens

An examination was made of whether social interactions can have a beneficial effect through the attenuation of the stress response in a social species. In the first experiment, one larger (mean +/-s.e. 194.0 +/- 12.5 g) and seven smaller (32.0 +/- 2.6 g) juvenile lake sturgeon Acipenser fulvescens were placed in tanks to determine whether a classic dominance effect would be established based on body size (n = 6). Large fish did not establish a territory or aggressively interact with smaller fish, as there were no significant differences in nearest-neighbour distances and an absence of aggressive behaviour (biting, chasing and pushing). In the second experiment, it was hypothesized that the presence of conspecifics would have a beneficial effect through an attenuation of the stress response. Fish in groups or isolation were stressed by a brief aerial exposure (30 s), and blood plasma was measured at regular time intervals (0, 20, 40, 60, 120 and 240 min) following the stressor via an implanted cannula (n = 9-11). The presence of conspecifics did not affect the peak cortisol response, however, the overall cortisol response was shorter in duration compared to fish in isolation. Furthermore, secondary stress variables (plasma ions and glucose) showed differences between fish in groups and isolation. The results of these experiments suggest that social interaction plays an important and beneficial role in regulating the stress response in cohesive social species such as A. fulvescens.

Individual variation in endocrine systems: moving beyond the 'tyranny of the Golden Mean'

Twenty years ago, Albert Bennett published a paper in the influential book New directions in ecological physiology arguing that individual variation was an 'underutilized resource'. In this paper, I review our state of knowledge of the magnitude, mechanisms and functional significance of phenotypic variation, plasticity and flexibility in endocrine systems, and argue for a renewed focus on inter-individual variability. This will provide challenges to conventional wisdom in endocrinology itself, e.g. re-evaluation of relatively simple, but unresolved questions such as structure-function relationships among hormones, binding globulins and receptors, and the functional significance of absolute versus relative hormone titres. However, there are also abundant opportunities for endocrinologists to contribute solid mechanistic understanding to key questions in evolutionary biology, e.g. how endocrine regulation is involved in evolution of complex suites of traits, or how hormone pleiotropy regulates trade-offs among life-history traits. This will require endocrinologists to embrace the raw material of adaptation (heritable, individual variation and phenotypic plasticity) and to take advantage of conceptual approaches widely used in evolutionary biology (selection studies, reaction norms, concepts of evolutionary design) as well as a more explicit focus on the endocrine basis of life-history traits that are of primary interest to evolutionary biologists (cf. behavioural endocrinology).

Frequency distribution of coping strategies in four populations of brown trout (Salmo trutta)

In a challenging situation some animals respond by active avoidance, aggression and an activation of the sympathetic nervous system whereas others respond by immobility, low levels of aggression and a predominant adrenocortical stress response. When consistent over time and across situations such inter-individual differences in behavioural and physiological stress responses are referred to as stress coping strategies. In a previous study we reported the existence of two distinct stress coping strategies in a sea-ranched brown trout (Salmo trutta) population. Using the same method, we here show that four brown trout populations with different origin, but reared under identical conditions, differ in their endocrine stress response, behaviour during hypoxia and aggression. Further more, if individuals are classified as high- and low responsive based on post-stress blood plasma noradrenalin levels (indicator of sympathetic reactivity) the frequency distribution shows that populations with hatchery origin are biased towards having higher frequencies of high responsive individuals. However, the number of high responsive trout ranges from 14-48% in the different populations which shows that generally the frequency is biased towards lower levels of high responsive individuals. We discuss different frequency-dependent mechanisms that maintain multiple phenotypes in populations and speculate about differences in selection regime among the studied populations.

Adrenocortical responses in zebra finches (Taeniopygia guttata): individual variation, repeatability, and relationship to phenotypic quality

Although individual variation is a key requirement for natural selection, little is known about the magnitude and patterns of individual variation in endocrine systems or the functional significance of that variation. Here we describe (1) the extent and repeatability of inter-individual variation in adrenocortical responses and (2) its relationship to sex-specific phenotypic quality, such as song duration and frequency and timing of egg laying. We measured adrenocortical responses to a standardized stressor in zebra finches (Taeniopygia guttata) at two life history stages: approximately day 16 (nestlings) and 3 months of age (sexually mature adults). Subsequently, we assessed phenotypic (reproductive) quality of all individuals as adults. Marked inter-individual variation in the adrenocortical response was seen in both sexes and ages, e.g., stress-induced corticosterone ranged from 2.2 to 62.5 ng/mL in nestlings and 5.0-64.0 ng/mL in adults. We found sex differences in (a) inter-individual variation in the adrenocortical response, (b) repeatability, and (c) relationships between corticosterone levels and phenotypic quality. In males, variation in nestling corticosterone was weakly but positively correlated with brood size and negatively correlated with nestling mass (though this relationship was dependent on one individual). There was no significant correlation of adrenocortical responses between two stages in males and adult phenotypic quality was significantly correlated only with adult corticosterone levels. In contrast, in females there was no relationship between nestling corticosterone and brood size or mass but adrenocortical response was repeatable between two stages (r2=0.413). Phenotypic quality of adult females was correlated with nestling baseline and adrenocortical response.

The Relationships among Sprint Performance, Voluntary Swimming Activity, and Social Dominance in Juvenile Rainbow Trout

The specific objectives of this study were to determine whether sprint performance in juvenile rainbow trout is correlated with either voluntary swimming activity or aggressive behaviors and to determine the reciprocal: the effect of swimming activity and aggression on sprint performance. Sprint performance was assessed by rapidly accelerating trout (5-7-cm fork length) to a fixed velocity (40, 42, or 45 cm s(-1)) and then holding them at that velocity until fatigue. There was considerable interindividual variation in sprint performance not explained by variations in body size, but intraindividual performance was highly repeatable over at least 2 mo. Voluntary swimming was measured as the frequency of transits (voluntary transit activity, VTA) between two identical tanks via a connecting channel with two different flow regimes: zero or minimum velocity (0 or 2.5 cm s(-1)) and high velocity (84 cm s(-1)). There was a strong correlation between sprint performance and VTA in minimal current but no correlation in high current. Furthermore, sprint performance did not predict the outcome of dominance encounters. Experience with rapid acceleration, especially when voluntary, led to a pronounced improvement in sprint performance in proportion to the number of acceleration events. Social dominance encounters had a more complex effect: a significant reduction in sprint performance in previously high-performance sprinters and the reverse for low performers. We propose that there are four independent axes of interindividual variation in juvenile rainbow trout: spontaneous and rheotaxis-stimulated locomotor activity, aggressive activity, and the trainability of sprint performance. The independence of these axes has the potential to produce a much larger diversity in behavioral and ultimately physiological phenotypes than would be produced if the axes were linked.

Genetically Determined Variation in Stress Responsiveness in Rainbow Trout: Behavior and Neurobiology

It is becoming increasingly recognized that the diversity in stressors, their intensity, predictability and the context in which they are experienced, will result in behavioral and physiological responses just as diverse. In addition, stress responses are characterized by individual variations where the physiological and behavioral reactions are associated in such a manner that distinct stress coping styles encompassing suites of correlated traits can be identified. These are often referred to as proactive and reactive stress coping styles. Proactive coping is characterized by more aggression, higher general activity and higher sympathetic activation, whereas reactive coping is characterized by immobility, lack of initiative and a higher parasympathetic/hypothalamic activation. Stable coping styles appear to coexist within populations, and these strategies appear to be largely innate. Moreover, the physiological and behavioral traits of coping styles appear to be heritable. These stress coping styles have proven to play a major role in competitive ability and subsequent social position in different species of vertebrates. However, there are also studies showing that social position can affect parameters encompassing the stress coping style of individuals. In this regard it is important, but not always easy, to distinguish between causes and effects of behavioral and physiological responses to stressors. The question raised is to what extent and rigidness stress coping styles are guided by genetic factors.

Variable neuroendocrine responses to ecologically-relevant challenges in sticklebacks

Variable neuroendocrine responses to ecologically-relevant challenges in sticklebacks. PHYSIOL BEHAV 00(0) 000-000, 2006. Here, we compare the behavioral, endocrine and neuroendocrine responses of individual sticklebacks exposed to either an unfamiliar conspecific or to a predator. We found that the two stressors elicited a similar hypothalamic-pituitary-interrenal response as assessed by whole-body concentrations of cortisol, but produced quite different patterns of change in brain monoamine and monoamine metabolite content as assessed by concentrations of serotonin (5-HT), dopamine (DA), norepinephrine (NE) and the monoamine metabolites 5-hydroxyindole acetic acid (5-HIAA), homovanillic acid (HVA) and 3-4-dihydroxyphenylacetic acid (DOPAC). For example, relative to baseline levels, NE levels were elevated in individuals exposed to a predator but were lower in individuals confronted by a challenging conspecific. Levels of monoamine neurotransmitters in specific regions of the brain showed extremely close links with behavioral characteristics. Frequency of attacking a conspecific and inspecting a predator were both positively correlated with concentrations of NE. However, whereas serotonin was negatively correlated with frequency of attacking a conspecific, it was positively associated with predator inspection. The data indicate that the qualitative and quantitative nature of the neuroendocrine stress response of sticklebacks varies according to the nature of the stressor, and that interindividual variation in behavioural responses to challenge are reflected by neuroendocrine differences.

HPA activity and neotic and anxiety-like behavior vary among Peromyscus species

Behaviorally plastic species are more likely to invade and endure in new areas, and behaviorally plastic individuals tend to be attracted to novelty (i.e., neophilic). Furthermore, neophilic behaviors are often influenced by glucocorticoids. Thus in addition to environmental conditions and vicariant events, behavioral plasticity and its endocrinological mediators may influence the extent of vertebrate geographic distributions. Some species of mice in the genus, Peromyscus, occupy most of North America whereas others are restricted to small areas. We predicted that one widespread species (Peromyscus maniculatus) would interact more with novel objects, more readily explore novel environments, and possess hypo-responsive HPA axes compared to species with small ranges. Our hypothesis was not supported, but given the small number of species in this study and the high anxiety-like behavior in captive P. maniculatus, it is premature to reject the hypothesis that behavioral flexibility affects geographic distribution in Peromyscus. Indeed, behavioral and HPA axis variation was complementary among species, which is opposite of the pattern typically detected within species, suggesting that future studies of glucocorticoid mediation of neotic and anxiety-like behaviors in Peromyscus would be valuable.

Evolutionary background for stress-coping styles: relationships between physiological, behavioral, and cognitive traits in non-mammalian vertebrates

Reactions to stress vary between individuals, and physiological and behavioral responses tend to be associated in distinct suites of correlated traits, often termed stress-coping styles. In mammals, individuals exhibiting divergent stress-coping styles also appear to exhibit intrinsic differences in cognitive processing. A connection between physiology, behavior, and cognition was also recently demonstrated in strains of rainbow trout (Oncorhynchus mykiss) selected for consistently high or low cortisol responses to stress. The low-responsive (LR) strain display longer retention of a conditioned response, and tend to show proactive behaviors such as enhanced aggression, social dominance, and rapid resumption of feed intake after stress. Differences in brain monoamine neurochemistry have also been reported in these lines. In comparative studies, experiments with the lizard Anolis carolinensis reveal connections between monoaminergic activity in limbic structures, proactive behavior in novel environments, and the establishment of social status via agonistic behavior. Together these observations suggest that within-species diversity of physiological, behavioral and cognitive correlates of stress responsiveness is maintained by natural selection throughout the vertebrate sub-phylum.

Serotonergic characteristics of rainbow trout divergent in stress responsiveness

Juvenile rainbow trout divergent in their cortisol response to confinement stress (HR: high responsive or LR: low responsive fish) were exposed to either 1 or 3 h of confinement stress. Untreated fish served as control. After the exposure blood and brain samples were collected. From the blood samples, the levels of cortisol and catecholamines were determined, while the brain serotonergic and monoamineoxidase (MAO) activity was determined in four different brain areas (brain stem, hypothalamus, telencephalon and optic tectum). Our results show that the LR fish responds to handling stress with a higher increase in plasma epinephrine compared to HR fish. Our results also show that confinement stress leads to a larger increase in the serotonergic activity in the brain stem and telencephalon in LR fish compared to HR fish. These results support the hypothesis that stress coping strategies similar to those described in mammals also exists in fish. Further, our results have shown that the MAO activity increases in optic tectum and hypothalamus of rainbow trout during confinement stress, while it remains unchanged or decreases in brain stem and telencephalon. Moreover, the MAO activity does not differ between the two selection lines. This indicates that MAO participates actively in the stress response without contributing to the differences in stress coping strategies.

Behavioral indicators of stress-coping style in rainbow trout: Do males and females react differently to novelty?

It is becoming increasingly clear that individual differences in the behavioral response to stressful situations are associated with distinct physiological profiles, and stress coping characteristics are of fundamental importance to fitness and life history. Teleost fishes display considerable variation in reproductive strategy, but sex differences in stress-coping style have not been described previously in fish. Prior to sexual maturation, the glucocorticoid response to stress is not affected by sex in salmonid fish. Nevertheless, behavior in novel and stressful situations differed between immature male and female rainbow trout (Oncorhynchus mykiss). When tested 1 week following transport to a new rearing facility, females resumed feeding after transfer to social isolation quicker than males. The locomotor response to acute confinement stress also varied between sexes, with females settling down and ceasing to move in a panic-like manner quicker than males. There was a strong correlation between behavior in the two test situations: individuals that readily resumed feeding behavior in a new environment also moved less in the acute stress test. Thus, the time to resume feeding after a stressful experience is a precise indicator of stress-coping style in salmonid fish, which is likely to reflect the dynamics of neuroendocrine stress responses. Furthermore, these observations could reflect a sex difference in the response to novel and stressful situations, which occur even in the absence of differences in glucocorticoid responsiveness.

The Integration of Behaviour into Comparative Physiology

Comparative physiology has traditionally focused on the physiological responses of animals to their physicochemical environment. In recent years, awareness has increased among physiologists of the potential for behavioural factors, such as the social environment of the animal, to affect physiological condition and responses. This recognition has led to an emerging trend within the field toward using multidisciplinary approaches that incorporate both behavioural and physiological techniques. Research areas in which the integrated study of behaviour and physiology has been particularly fruitful include the physiology of the social environment, sensory physiology and behaviour, and physiological constraints on behavioural ecology. The manner in which incorporating behavioural considerations has informed the physiological data collected is discussed for each of these areas using specific examples.