Christina Sørensen

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.

Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter function

A study of two species of coral reef fish demonstrates that the anticipated increase in atmospheric carbon dioxide directly interferes with neurotransmitter function in their larvae, a hitherto unrecognized problem for marine fishes. Predicted future CO2 levels have been found to alter sensory responses and behaviour of marine fishes. Changes include increased boldness and activity, loss of behavioural lateralization, altered auditory preferences and impaired olfactory function1,2,3,4,5. Impaired olfactory function makes larval fish attracted to odours they normally avoid, including ones from predators and unfavourable habitats1,3. These behavioural alterations have significant effects on mortality that may have far-reaching implications for population replenishment, community structure and ecosystem function2,6. However, the underlying mechanism linking high CO2 to these diverse responses has been unknown. Here we show that abnormal olfactory preferences and loss of behavioural lateralization exhibited by two species of larval coral reef fish exposed to high CO2 can be rapidly and effectively reversed by treatment with an antagonist of the GABA-A receptor. GABA-A is a major neurotransmitter receptor in the vertebrate brain. Thus, our results indicate that high CO2 interferes with neurotransmitter function, a hitherto unrecognized threat to marine populations and ecosystems. Given the ubiquity and conserved function of GABA-A receptors, we predict that rising CO2 levels could cause sensory and behavioural impairment in a wide range of marine species, especially those that tightly control their acid–base balance through regulatory changes in HCO3− and Cl− levels.

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.

Melanin-based skin spots reflect stress responsiveness in salmonid fish

Within animal populations, genetic, epigenetic and environmental factors interact to shape individual neuroendocrine and behavioural profiles, conferring variable vulnerability to stress and disease. It remains debated how alternative behavioural syndromes and stress coping styles evolve and are maintained by natural selection. Here we show that individual variation in stress responsiveness is reflected in the visual appearance of two species of teleost fish; rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Salmon and trout skin vary from nearly immaculate to densely spotted, with black spots formed by eumelanin-producing chromatophores. In rainbow trout, selection for divergent hypothalamus-pituitary-interrenal responsiveness has led to a change in dermal pigmentation patterns, with low cortisol-responsive fish being consistently more spotted. In an aquaculture population of Atlantic salmon individuals with more spots showed a reduced physiological and behavioural response to stress. Taken together, these data demonstrate a heritable behavioural-physiological and morphological trait correlation that may be specific to alternative coping styles. This observation may illuminate the evolution of contrasting coping styles and behavioural syndromes, as occurrence of phenotypes in different environments and their response to selective pressures can be precisely and easily recorded.

Forebrain cell proliferation, behavior, and physiology of zebrafish, Danio rerio, kept in enriched or barren environments.

Comparative studies on neural plasticity in non-mammalian vertebrates are increasingly promoted as an important complement to mammalian models. In teleost fishes the number of brain cells increases with age, body weight, and body length throughout life. Neurogenesis persists to a large degree, and both neuron replacement and net brain growth occur during adulthood. Whether environmental factors affect brain cell proliferation has however been scarcely investigated in this animal group. In the current study adult male zebrafish were kept in social isolation in different environments (enriched vs. barren) for one week. Telencephalic cell proliferation was investigated by proliferating cell nuclear antigen (PCNA) immunohistochemistry. Higher numbers of PCNA positive nuclei and significantly increased inter-individual variability was observed in fish kept in aquaria enriched with artificial plants and gravel. Zebrafish rapidly regained feed intake after transfer to social isolation. Whole-body cortisol levels were also generally low in isolated fish, although slightly elevated in fish from enriched environments. In summary, this study demonstrates that environmental alterations can rapidly alter cell cycle dynamics in the zebrafish brain. Furthermore, the results support the idea that mild short-term stressors and concomitant small increases in corticosteroid exposure stimulate brain cell proliferation.

Neural plasticity and stress coping in teleost fishes.

Physiological and behavioural responses to environmental change are individually variable traits, which manifest phenotypically and are subject to natural selection as correlated trait-clusters (coping styles, behavioural syndromes, or personality traits). Comparative research has revealed a range of neuroendocrine-behavioural associations which are conserved throughout the vertebrate subphylum. Regulatory mechanisms universally mediate a switch between proactive (e.g. active/aggressive) and reactive (e.g. conservation/withdrawal) behaviour in response to unpredictable and uncontrollable events. Thresholds for switching from active coping to behavioural inhibition are individually variable, and depend on experience and genetic factors. Such factors affect physiological stress responses as well as perception, learning, and memory. Here we review the role of an important contributor to neural processing, the set of biochemical, molecular, and structural processes collectively referred to as neural plasticity. We will concentrate on work in teleost fishes, while also elucidating conserved aspects. In fishes, environmental and physiological control of brain cell proliferation and neurogenesis has received recent attention. This work has revealed that the expression of genes involved in CNS plasticity is affected by heritable variation in stress coping style, and is also differentially affected by short- and long-term stress. Chronic stress experienced by subordinate fish in social hierarchies leads to a marked suppression of brain cell proliferation. Interestingly, typically routine dependent and inflexible behaviour in proactive individuals is also associated with low transcription of neurogenesis related genes. The potential for these findings to illuminate stress-related neurobiological disorders in other vertebrates is also discussed.

Attenuation of stress-induced anorexia in brown trout (Salmo trutta) by pre-treatment with dietary l-tryptophan.

The general consensus is that brain serotonin (5-HT) inhibits feed intake in teleost fishes and other vertebrates. Dietary manipulations with the 5-HT precursor tryptophan (TRP) have, however, yielded contradictory effects on feed intake, while studies of the endocrine response to stress indicate that the effects of TRP-enriched feed are context dependent. A characteristic behavioural response to stress is a reduction in feed intake, and in the present study we investigated whether pre-treatment with TRP-enriched feed affected stress-induced changes in feeding behaviour in brown trout (Salmo trutta). After acclimatisation in observation aquaria, isolated fish were fed control or TRP-supplemented feed for 7 d, whereupon they were transferred to a novel environment, in which all fish were fed control feed. Transfer to a new environment resulted in decreased feeding in both the TRP pre-treated and the control-treated group. However, this decrease was more pronounced in the control-treated group. Previous experiments have concluded that stimulation of brain 5-HT systems by TRP enhancement does not affect feed intake in salmonid fishes, but in these studies food intake was observed in unstressed animals only. The present study suggests that pre-treatment with dietary TRP attenuates stress-induced anorexia. Hence, it appears that the effect of dietary manipulations of TRP on feeding behaviour is dependent on the stress levels experienced by experimental animals. These behavioural data are discussed in the context of the involvement of 5-HT in appetite regulation.

Social Regulation of Neurogenesis in Teleosts

Salmonid fishes such as the rainbow trout (Oncorhynchus mykiss) are frequently used to study behavioral and neuroendocrine effects of socially induced stress. A predictable aggressive response to territorial intrusion, a well described neuroanatomy, and many essential similarities in the stress response in fishes and other vertebrates are among the advantages of this comparative model. One conspicuous difference when compared to mammals, however, is that in telost fish and other non-mammalian vertebrates, neurogenesis persists into adulthood to a much higher degree. Very little is known about the functional significance of individual differences in the rate of brain cell proliferation in fish, or whether structural changes in the fish brain are influenced by the social environment. In this paper we discuss the observation that brain cell proliferation is reduced in subordinate fish, focusing in particular on whether such individual variation reflects a difference in coping style or is indeed a response to social interactions.

Cortisol reduces cell proliferation in the telencephalon of rainbow trout (Oncorhynchus mykiss)

The fish brain grows throughout life, and new cells are added continuously in all major brain areas. As in mammals, the rate of adult brain cell proliferation in fish can be regulated by external factors including environmental complexity and interaction with conspecifics. We have recently demonstrated that the stress experienced by subordinate rainbow trout in social hierarchies leads to a marked suppression of brain cell proliferation in the telencephalon, and that this is accompanied by an increase in plasma levels of cortisol. Corticosteroid hormones are known to suppress adult neurogenesis in mammals, and to investigate whether this is also the case in fish, rainbow trout were fed feed containing either a low or a high dose of cortisol for 6 days. Compared to control animals receiving regular feed, both cortisol treated groups had significantly elevated cortisol levels 24h after the last feeding, with the high group having levels comparable to those previously reported in socially stressed fish. To quantify cell proliferation, immunohistochemistry for proliferating cell nuclear antigen (PCNA) was performed to identify actively cycling cells. The density of PCNA-positive nuclei in the telencephalon was reduced by about 50% in both cortisol treated groups. The effect of cortisol on brain cell proliferation did not reflect a general down regulation of growth, as only the high cortisol group had reduced growth rate, and there was no correlation between brain cell proliferation and growth rate in any group. These results indicate that the reduced proliferative activity seen in brains of socially stressed fish is mediated by cortisol, and that there is a similar suppressive effect of cortisol on brain cell proliferation in the teleost forebrain as in the mammalian hippocampus.

Social stress reduces forebrain cell proliferation in rainbow trout (Oncorhynchus mykiss).

Compared to mammals, teleost fish have a very high rate of adult brain cell proliferation. Still little is known about how this process is regulated in comparative models, and what its functional implications are. We investigated the effect of stressful social interaction on brain cell proliferation in size matched rainbow trout pairs after the formation of stable social hierarchies. After 4 days of interaction, socially subordinate fish displayed common signs of chronic stress including reduced feeding behaviour, elevated plasma cortisol levels, and up-regulated brain stem 5-HT activity. The number of newborn cells in the telencephalon was quantified using immunohistochemistry for the exogenously administered S-phase marker BrdU. Subordinate fish had 40% fewer BrdU-positive telencephalic cells compared to isolated controls, while dominant individuals showed a non-significant tendency towards reduced cell proliferation. Cell proliferation in subordinate animals correlated negatively with aggression received immediately after hierarchy formation, indicating that the level of cell division suppression is related to the severity of the social stressor. These findings are comparable to findings in mammalian models of psychosocial stress, indicating that the suppressive effect of social stress on brain cell proliferation is conserved, and thus likely confers adaptive benefits throughout the vertebrate subphylum.