Basal and stress-induced differences in HPA axis, 5-HT responsiveness, and hippocampal cell proliferation in two mouse lines

Experiential and genetic contributions to depressive- and anxiety-like disorders: clinical and experimental studies

Stressful events have been implicated in the precipitation of depression and anxiety. These disorders may evolve owing to one or more of an array of neuronal changes that occur in several brain regions. It seems likely that these stressor-provoked neurochemical alterations are moderated by genetic determinants, as well as by a constellation of experiential and environmental factors. Indeed, animal studies have shown that vulnerability to depressive-like behaviors involve mechanisms similar to those associated with human depression (e.g., altered serotonin, corticotropin releasing hormone and their receptors, growth factors), and that the effects of stressors are influenced by previous stressor experiences, particularly those encountered early in life. These stressor effects might reflect sensitization of neuronal functioning, phenotypic changes of processes that lead to neurochemical release or receptor sensitivity, or epigenetic processes that modify expression of specific genes associated with stressor reactivity. It is suggested that depression is a life-long disorder, which even after effective treatment, has a high rate of re-occurrence owing to sensitized processes or epigenetic factors that promote persistent alterations of gene expression.

Aggressive and non-aggressive personalities differ in oxidative status in selected lines of mice (Mus musculus)

Mice selected for aggression and coping (long attack latency (LAL), reactive coping strategy; short attack latency (SAL), pro-active coping strategy) are a useful model for studying the physiological background of animal personalities. These mice also show a differential stress responsiveness, especially in terms of hypothalamic-pituitary-adrenal axis reactivity, to various challenges. Since the stress response can increase the production of reactive oxygen species, we predicted that the basic oxidative status of the lines could differ. We found that LAL showed higher serum antioxidant capacity (OXY) than SAL, while no differences emerged for reactive oxygen metabolites (ROMs) or the balance between ROMs and OXY, reflecting oxidative stress. Moreover, the lines showed inverse relationships between ROMs or OXY and body mass corrected for age. The results indicate that variation in oxidative status is heritable and linked to personality. This suggests that different animal personalities may be accompanied by differences in oxidative status, which may predict differences in longevity.

Deactivation of the limbic system during acute psychosocial stress: evidence from positron emission tomography and functional magnetic resonance imaging studies

BACKGROUND

Stress-induced metabolic changes can have detrimental health effects. Newly developed paradigms to investigate stress in neuroimaging environments allow the assessment of brain activation changes in association with the perception of and the metabolic response to stress.

METHODS

We exposed human subjects to a psychosocial stressor in one positron emission tomography (n = 10) and one functional magnetic resonance imaging (fMRI; n = 40) experiment.

RESULTS

We observed a profound deactivation of limbic system components including hippocampus, hypothalamus, medio-orbitofrontal cortex and anterior cingulate cortex in subjects who reacted to the stressor with a significant increase of the endocrine stress marker cortisol. Further, in the fMRI study, the degree of deactivation in the hippocampus was correlated with the release of cortisol in response to the stress task.

CONCLUSIONS

The observed deactivation of limbic system structures suggests elevated activation at rest and during nonstressful situations. A model is proposed where the observed reduction in limbic system activity is essential for the initiation of the stress response.

Individual Variation in Coping with Stress: A Multidimensional Approach of Ultimate and Proximate Mechanisms

Ecological studies on feral populations of mice, fish and birds elucidate the functional significance of phenotypes that differ individually in their behavioral and neuroendocrine response to environmental challenge. Within a species, the capacity to cope with environmental challenges largely determines individual survival in the natural habitat. Recent studies indicate that individual variation within a species may buffer the species for strong fluctuations in the natural habitat. A conceptual framework will be presented that is based on the view that individual variation in aggressive behavior can be considered more generally as a variation in actively coping with environmental challenges. Highly aggressive individuals adopt a proactive coping style whereas low levels of aggression indicate a more passive or reactive style of coping. Coping styles have now been identified in a range of species and can be considered as trait characteristics that are stable over time and across situations. The dimension of coping style seems to be independent of an emotionality dimension. Hence, in the analysis of the proximate mechanisms of stress and adaptation, one has to consider the possibility that the mechanisms which determine the type of stress response might be independent from those underlying the magnitude of the response. The two coping styles differ in a number of important neurobiological and neuroendocrine systems. For example, proactive males differ significantly from reactive males in the homeostatic control of serotonergic activity resulting in completely opposite dose response relationships of various serotonergic drugs. The results so far show that proactive coping is characterized by a strong inhibitory control of the 5-HT neuron via its somatodendritic 5-HT(1A) autoreceptor. It is hypothesized that the regulation of serotonin release is causally related to coping style rather than emotionality. Understanding the functional individual variation as it occurs in nature and the underlying neurobiology and neuroendocrinology is fundamental in understanding individual vulnerability to stress related disease.

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 teleost 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.

Neurobiological Mechanisms of Aggression and Stress Coping: A Comparative Study in Mouse and Rat Selection Lines

Aggression causes major health and social problems and constitutes a central problem in several psychiatric disorders. There is a close relationship between the display of aggression and stress coping strategies. In order to gain more insight into biochemical pathways associated with aggression and stress coping, we assessed behavioral and neurobiological responses in two genetically selected rodent models, namely wild house mice selectively bred for a short (SAL) and long (LAL) attack latency and Wistar rats bred for high (HAB) or low (LAB) anxiety-related behavior. Compared to their line counterparts, the SAL mice and the LAB rats display a high level of intermale aggression associated with a proactive coping style. Both the SAL mice and the LAB rats show a reduced hypothalamic-pituitary-adrenal (HPA) axis response to non-social stressors. However, when exposed to social stressors (resident-intruder, sensory contact), SAL mice show an attenuated HPA response, whereas LAB rats show an elevated HPA response. In both rodent lines, the display of aggression is associated with high neuronal activation in the central amygdala, but reduced neuronal activation in the lateral septum. Furthermore, in the lateral septum, SAL mice have a reduced vasopressinergic fiber network, and LAB rats show a decreased vasopressin release during the display of aggression. Moreover, the two lines show several indications of an increased serotonergic neurotransmission. The relevance of these findings in relation to high aggression and stress coping is discussed. In conclusion, exploring neurobiological systems in animals sharing relevant behavioral characteristics might be a useful approach to identify general mechanisms of action, which in turn can improve our understanding of specific behavioral symptoms in human psychiatric disorders.

Differential effects of stress on adult hippocampal cell proliferation in low and high aggressive mice

Male wild house mice selected for a long (LAL) or a short (SAL) latency to attack a male intruder generally show opposing behavioural coping responses to environmental challenges. LAL mice, unlike SAL mice, adapt to novel challenges with a highly reactive hypothalamic-pituitary-adrenal axis and show an enhanced expression of markers for hippocampal plasticity. The present study aimed to test the hypothesis that these features of the more reactive LAL mice are reflected in parameters of hippocampal cell proliferation. The data show that basal cell proliferation in the subgranular zone (SGZ) of the dentate gyrus, assessed by the endogenous proliferation marker Ki-67, is lower in LAL than in SAL mice. Furthermore, application of bromodeoxyuridine (BrdU) over 3 days showed an almost two-fold lower cell proliferation rate in the SGZ in LAL versus SAL mice. Exposure to forced swimming resulted, 24 h later, in a significant reduction in BrdU + cell numbers in LAL mice, whereas cell proliferation was unaffected by this stressor in SAL mice. Plasma corticosterone and dentate gyrus glucocorticoid receptor levels were higher in LAL than in SAL mice. However, no differences between the SAL and LAL lines were found for hippocampal NMDA receptor binding. In conclusion, the data suggest a relationship between coping responses and hippocampal cell proliferation, in which corticosterone may be one of the determinants of line differences in cell proliferation responses to environmental challenges.

Social stress, immune functions and disease in rodents

The link between social factors, stress and health has been the focus of many interdisciplinary studies mostly because: (i) animals, including humans, often live in societies; (ii) positive and negative social relationships affect disease and well being; (iii) physiological alterations, which parallel social interactions also modulate immune and neuroendocrine functions. This review will focus on studies conducted on laboratory and wild rodents where social factors such as dyadic interactions, individual housing and differential group housing were investigated. The results obtained allow one to conclude that social factors in rodents are causally linked with immune disorders/disease susceptibility. In particular, lower lymphocyte proliferation and antigen-specific-IgG, granulocytosis and lymphopenia, as well as higher tumor induction and progression, are reliably associated with negative social events. Finally, due to the increasing utilization of social stress-based animal models the reliability of the concept of "social stress" and its evolutionary context are re-evaluated.

Low inborn anxiety correlates with high intermale aggression: link to ACTH response and neuronal activation of the hypothalamic paraventricular nucleus

Aggression constitutes a central problem in several psychopathologies, including anxiety and depression disorders and antisocial behaviors. In particular, the activity of the hypothalamic-pituitary-adrenocortical (HPA) axis has been associated with aggression-related disorders. The present study assessed whether genetically determined levels of anxiety-related behavior influence the level of intermale aggression and whether this is associated with differences in neuroendocrine responsiveness and neuronal activation in the brain. Adult male Wistar rats bred for high (HAB) or low (LAB) anxiety-related behavior were used, as well as non-selected rats (NAB) with an intermediate anxiety level. LAB residents displayed more aggressive behavior than HAB and NAB residents during the resident-intruder (RI) test. Moreover, an inverse correlation was found between the level of anxiety and the level of aggression. The plasma corticotropin (ACTH) response to RI-test exposure was significantly higher in LABs than in HABs and NABs, indicating that a higher level of aggression was linked to an elevated hormonal stress response. Furthermore, LAB residents showed more neuronal activation in the parvocellular part of the hypothalamic paraventricular nucleus (PVN) than HAB residents 1 h after the RI-test. In addition, a tendency toward a higher number of c-Fos-positive cells in LABs compared with HABs was observed in the medial amygdala, hypothalamic attack area and central amygdala, areas relevant for the regulation of aggression. These data demonstrate that low trait anxiety is correlated with high intermale aggression. Furthermore, the increased neuronal activation of the PVN along with the higher ACTH responsiveness might underlie the display of high aggression.

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.

Activational levels of androgens influence risk assessment behaviour but do not influence stress-induced suppression in hippocampal cell proliferation in adult male rats

Adult male, but not female, rats demonstrate a suppression in hippocampal cell proliferation in response to an acute predator odour stress. The present study examined the effect of activational levels of androgens on stress-induced changes in hippocampal cell proliferation and defensive and non-defensive behaviours in adult male rats. Adult male Sprague-Dawley rats were castrated and exposed to trimethylthiazoline (TMT, the main component of fox feces). Androgen status did not significantly affect TMT-induced suppression in hippocampal cell proliferation or expression of defensive burying. However, castrated males did not show an increase in duration of stretch attends (a risk assessment behaviour) in response to TMT. The results of this study suggest that activational levels of androgens in the male rat do not directly regulate the sex difference in stress-induced suppression of hippocampal cell proliferation but do regulate risk assessment behaviour.

Alterations in stress reactivity after long-term treatment with paroxetine in women with posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is typically accompanied by both acute and chronic alterations in the stress response. These alterations have mostly been described in individuals under baseline conditions, but studies have also used a challenge model to assess the role of the hypothalamic-pituitary-adrenal (HPA) axis in the stress response. The purpose of this article was to assess the effect of long-term treatment with the selective reuptake inhibitor (SSRI), paroxetine, on stress reactivity in patients with PTSD. We assessed diurnal salivary cortisol and urinary cortisol as well as cortisol, heart rate, and behavioral responses to a standardized cognitive stress challenge, in 13 female patients with chronic PTSD before and after 12 months of paroxetine treatment. Treatment resulted in a significant decrease in PTSD symptoms. Twenty-four-hour urinary cortisol was lower compared to base line after successful treatment. Treatment resulted in a decrease of salivary cortisol levels on all time points on a diurnal curve. Despite similar stress perception, cortisol response to the cognitive stress challenge resulted in a 26.5% relative decrease in stress-induced salivary cortisol with treatment. These results suggest that successful treatment with SSRI in chronic PTSD is associated with a trend for a decrease in baseline diurnal cortisol and with reduced cortisol reactivity to stress.

Agomelatine, a new antidepressant, induces regional changes in hippocampal neurogenesis

BACKGROUND

Antidepressant treatments increase neural plasticity and adult neurogenesis, especially in the hippocampus. Here, we determined the effects of agomelatine (S-20098), a new antidepressant, on various phases of neurogenesis in the dentate gyrus of adult rat.

METHODS

Animals were injected with agomelatine for different time periods. Immunostaining for bromodeoxyuridine, neuron specific nuclear protein, and glial fibrillary acid protein, as well as for the highly polysialylated form of neuronal cell adhesion molecule and doublecortin, was used to detect changes in cell proliferation, neurogenesis, and survival. Cell death was estimated by terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling and cresyl violet staining.

RESULTS

Chronic (3 weeks) but not acute (4 hours) or subchronic (1 week) administration of agomelatine increased cell proliferation and neurogenesis in the ventral dentate gyrus, a region notably implicated in response to emotion, which is consistent with the antidepressant-anxiolytic properties of the drug. Extending agomelatine treatment over several weeks, however, increases survival of newly formed neurons in the entire dentate gyrus. Finally, agomelatine treatment does not affect mature granule cells.

CONCLUSIONS

This study shows that an antidepressant can affect differentially various stages of neurogenesis in the dorsal and ventral hippocampus. Altogether, these changes lead to a pronounced augmentation in the total number of new granule cells.

The effect of chronic exposure to highly aggressive mice on hippocampal gene expression of non-aggressive subordinates

Exposure to a chronic psychosocial stressor changes the behavioral and neuroendocrine response pattern and causes structural changes in the rodent hippocampus. However, the underlying molecular mechanism of these changes induced by chronic stress is largely unknown. Recently, it was shown that exposure to a dominant highly aggressive mouse in the sensory contact model induced long-lasting stress symptoms in subordinate mice genetically selected for long attack latency (LAL mice). The aim of the present study was to study the effect of chronic stress on hippocampal gene expression in these subordinate LAL mice. GeneChips (Affymetrix) were used to compare gene expression profiles of LAL mice exposed to a sensory contact stressor for 25 days and their controls (one array per mouse, n=5 per line). After this stress paradigm, 131 genes were found differentially expressed (P<0.01). Strikingly, all of these genes showed a subtle downregulation in response to a chronic stressor. Interestingly, a significant overrepresentation of genes encoding structural components of ribosomes were found, suggesting diminished protein biosynthesis in the hippocampus of chronically stressed LAL mice. In addition, several genes of the NFkappaB signaling cascade, a pathway crucially involved in neuronal viability and neurite growth, were found to be downregulated. Together, we hypothesize that reduced NFkappaB signaling and diminished protein biosynthesis form part of the molecular mechanisms by which a chronic psychosocial stressor induces structural alterations in hippocampus of LAL mice.

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.

Effect of neonatal handling on serotonin 1A sub-type receptors in the rat hippocampus

Serotonin 1A sub-type receptors play an important role in the etiopathogenesis of depression, which is known to occur more often in females than males. Early experiences can be a predisposing factor for depression; however, the underlying cellular processes remain unknown. In an effort to address such issues, we employed neonatal handling, an experimental model of early experience, which has been previously shown to render females more vulnerable to display enhanced depression-like behavior in response to chronic stress, while it increases the ability of males to cope. In rat pre-pubertal (30 days of age) and adult (90 days) hippocampus, of both males and females, the effect of neonatal handling on serotonin 1A sub-type receptor mRNA and protein levels was determined by in situ hybridization and immunohistochemistry, respectively, while the number of binding sites was determined by in vitro autoradiography using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin as the ligand. Our results revealed a significant sex difference in serotonin 1A sub-type receptor mRNA, protein and binding sites, with females having higher levels than males. Handling resulted in statistically significant decreased numbers of cells positive for serotonin 1A sub-type receptor mRNA or protein, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites in the area 4 of Ammon's horn and dentate gyrus of both pre-pubertal males and females. In adult animals the number of serotonin 1A sub-type receptor mRNA positive cells was increased as a result of handling in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of males, while it was decreased only in the area 4 of Ammon's horn of females. Furthermore, the number of serotonin sub-type 1A receptor immunopositive cells, as well as [(3)H]8-hydroxy-2(di-n-propylamino)tetralin binding sites was increased in the area 1 of Ammon's horn, area 4 of Ammon's horn and dentate gyrus of handled males, whereas it was decreased in these same brain areas in the handled females. We can thus infer that neonatal handling results in alterations in postsynaptic serotonergic neurotransmission, which may contribute to the sex dimorphic effects of handling as to the vulnerability toward depression-like behavior in response to chronic stressful stimuli.

Behavioral specificity of non-genomic glucocorticoid effects in rats: effects on risk assessment in the elevated plus-maze and the open-field

The rapid effects of glucocorticoids on various behaviors suggest that these hormones play a role in rapidly coping with challenging situations. The variety of behaviors affected in different situations raise, however, questions regarding the specificity and roles of glucocorticoids in controlling behavior. To clarify this issue, we assessed the rapid behavioral effects of glucocorticoids in the elevated plus-maze (EPM) and the open-field (OF) tests in male rats. Both tests measure three different kinds of behavioral responses: locomotion, anxiety-like behaviors (central area and open arm exploration in the OF and EPM tests, respectively), and risk assessment (investigating aversive areas in a stretched attend posture). The acute inhibition of glucocorticoid synthesis by metyrapone decreased risk assessment but did not affect locomotion and anxiety-like behaviors. Corticosterone administration increased risk assessment, without affecting locomotion and anxiety-like behaviors. Moreover, plasma corticosterone levels measured immediately after testing strongly correlated with the intensity of risk assessment. The effects of corticosterone were rapid, as occurred even when the hormone was injected 2 min before behavioral testing. In addition, the effect was resistant to protein synthesis inhibition. These data demonstrate that glucocorticoids are able to increase specifically risk assessment behaviors by non-genomic mechanisms in two different, novelty-related, non-social challenging situations. Thus, glucocorticoids appear to rapidly induce specific behavioral adjustments to meet immediate requirements set by the challenge. These data support earlier assumptions on the role of glucocorticoids in coping, and it can be hypothesized that the rapid activation of the HPA-axis may play a role in forming coping responses.

Hypothalamus-pituitary-adrenal system regulation and suicidal behavior in depression

BACKGROUND

One of the most demanding tasks in psychiatry is to protect patients from suicidal attempts. Preventive strategies could be improved by increasing our knowledge on the pathophysiologic disturbances underlying this behavior. More than 70-80% of suicides occur in the context of depressive disorders, in which dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis is one of the most prominent neurobiological findings. So far data on the involvement of the HPA axis in the pathophysiology of suicidal behavior in depressed patients are controversial.

METHODS

In this retrospective study, we administered the combined dexamethasone-suppression/CRH stimulation (Dex/CRH) test to 310 patients with a depressive syndrome characterized at admission for acute and past suicidal behavior within the first 10 days after hospitalization.

RESULTS

Suicidal behavior in depressed patients, including past and recent suicide attempts as well as suicidal ideation, was associated with a lower adrenocorticotropin and cortisol response in the combined Dex/CRH test, with lowest hormone levels observed in patients with a recent suicide attempt.

DISCUSSION

The findings suggest that suicidal behavior may alter HPA axis regulation in depressed patients. Large-scale prospective studies assessing neuroendocrine changes may help to develop predictors for an early identification of patients at risk for committing suicide.