Chronic mild stress impairs cognition in mice: from brain homeostasis to behavior

Magnetic resonance imaging of hippocampal subfields in posttraumatic stress disorder

CONTEXT

Most neuroimaging studies of posttraumatic stress disorder (PTSD) have focused on potential abnormalities in the whole hippocampus, but the subfields of this structure, which have distinctive histological characteristics and specialized functions, have not been investigated. Studies of individual subfields may clarify the role of the hippocampus in PTSD.

OBJECTIVE

To determine if PTSD is associated with structural alterations in specific subfields of the hippocampus.

DESIGN

Case-control study.

PARTICIPANTS

A total of 17 male veterans with combat trauma and PTSD (mean [SD] age, 41 [12] years) and 19 age-matched male veterans without PTSD who were recruited from the outpatient mental health clinic of the San Francisco Veterans Affairs Medical Center and by advertising in the community.

INTERVENTIONS

High-resolution magnetic resonance imaging at 4 T.

MAIN OUTCOME MEASURE

Volumes of hippocampal subfields.

RESULTS

Posttraumatic stress disorder was associated with 11.4% (1.5%) (P = .02) smaller mean (SD) cornu ammonis 3 (CA3)/dentate gyrus subfield volumes, irrespective of age-related alterations, whereas other subfields were spared. Age was associated with reduced volume of the CA1 subfield (P = .03). Total hippocampal volume was also reduced in PTSD by a mean (SD) of 6.5% (0.6%) but, related to both PTSD (P = .05) and age (P = .01), was consistent with the measurements in the subfields.

CONCLUSIONS

The findings indicate for the first time in humans that PTSD is associated with selective volume loss of the CA3/dentate gyrus subfields, consistent with animal studies, implying that chronic stress suppresses neurogenesis and dendritic branching in these structures.

Susceptibility of a potential animal model for pathological anxiety to chronic mild stress

When anxiety-related behaviour in animals appears to lack adaptive value, it might be defined as pathological. Adaptive behaviour can be assessed for example by changes in behavioural responses over time, i.e. habituation. Thus, non-adaptive anxiety would be reflected by a lack of habituation. Recently, we found that 129P3/J mice are characterised by non-adaptive avoidance behaviour after repeated test exposure. The present study was aimed at investigating the sensitivity of the behavioural profile of these animals to exposure to a chronic mild stress (CMS) paradigm followed by repeated exposure to the modified hole board test. If the behavioural profile of 129P3/J mice mirrors pathological anxiety, their behavioural habituation under repeated test exposure conditions should be affected by CMS treatment. The results confirm the profound lack of habituation with respect to anxiety-related behaviour in both control and CMS treated mice. Additionally, CMS treated animals revealed a lower exploratory behaviour, reduced locomotor activity and increased arousal-related behaviour over time when compared to control individuals, proving an extension of their impaired habituation behaviour. Although no effects of CMS treatment on plasma corticosterone levels were found, higher immediate early gene expression in the bed nucleus of the stria terminalis and the ventrolateral periaqueductal grey in CMS treated mice indicated that 129P3/J mice are susceptible to the negative effects of CMS treatment at both the behavioural and the functional level. These results support the hypothesis that 129P3/J mice might be an interesting model for pathological anxiety.

Evaluation of 5-ethynyl-2'-deoxyuridine staining as a sensitive and reliable method for studying cell proliferation in the adult nervous system

Recently, a novel method for detection of DNA synthesis has been developed based on the incorporation of 5-ethynyl-2'-deoxyuridine (EdU), a thymidine analogue, into cellular DNA and the subsequent reaction of EdU with a fluorescent azide in a copper-catalyzed [3+2] cycloaddition ("Click" reaction). In the present study, we evaluated this method for studying cell proliferation in the adult central nervous system in comparison with the "gold standard" method of 5-bromo-2'-deoxyuridine (BrdU) staining using two behavioral paradigms, voluntary exercise and restraint stress. Our data demonstrate that the number of EdU-positive cells in the dentate gyrus of the hippocampus (DG) slightly increased in an EdU dose-dependent manner in both the control and voluntary exercise (running) mouse groups. The number of EdU-labeled cells was comparable to the number of BrdU-labeled cells in both the control and running mice. Furthermore, EdU and BrdU co-localized to the same cells within the DG. Voluntary exercise significantly increased the number of EdU- and BrdU-positive cells in the DG. In contrast, restraint stress significantly decreased the number of EdU-positive cells. The EdU-positive cells differentiated into mature neurons. EdU staining is compatible with immunohistochemical staining of other antigens. Moreover, our data demonstrated EdU staining can be combined with BrdU staining, providing a valuable tool of double labeling DNA synthesis, e.g., for tracking the two populations of neurons generated at different time points. In conclusion, our results suggest that EdU staining is a fast, sensitive and reproducible method to study cell proliferation in the central nervous system.

Adverse stress, hippocampal networks, and Alzheimer's disease

Recent clinical data have implicated chronic adverse stress as a potential risk factor in the development of Alzheimer's disease (AD) and data also suggest that normal, physiological stress responses may be impaired in AD. It is possible that pathology associated with AD causes aberrant responses to chronic stress, due to potential alterations in the hypothalamic-pituitary-adrenal (HPA) axis. Recent study in rodent models of AD suggests that chronic adverse stress exacerbates the cognitive deficits and hippocampal pathology that are present in the AD brain. This review summarizes recent findings obtained in experimental AD models regarding the influence of chronic adverse stress on the underlying cellular and molecular disease processes including the potential role of glucocorticoids. Emerging findings suggest that both AD and chronic adverse stress affect hippocampal neural networks in a similar fashion. We describe alterations in hippocampal plasticity, which occur in both chronic stress and AD including dendritic remodeling, neurogenesis, and long-term potentiation. Finally, we outline potential roles for oxidative stress and neurotrophic factor signaling as the key determinants of the impact of chronic stress on the plasticity of neural networks and AD pathogenesis.

A neurotransmitter system that regulates macrophage pro-inflammatory functions

Neurotransmitters released through peripheral and autonomic nerves play an important role in the signaling from the cells of the nervous system to lymphocytes, macrophages and other cells of the immune system. Macrophages are related to numerous physiological and pathological inflammatory processes since their cytokines play an important role in the defensive responses against invasive microorganisms, atherosclerosis progress, insulin resistance, behavior deviation, hematopoiesis feedback, degenerative chronic diseases and the stimulation of the hypothalamus-hypophysis-adrenal axis. Production of pro-inflammatory cytokines by macrophages is the main target for the modulatory activity of diverse neurotransmitters. In this brief review, we show how some neurotransmitters released by the central or the autonomic nervous systems down-regulate peripheral macrophages' inflammatory functions to balance immune protective mechanisms, although they can also promote the collateral progress of diverse diseases. The possible therapeutic uses of some neurotransmitters and the agonists or antagonist of their respective receptors are included as well.

A reward increases running-wheel performance without changing cell proliferation, neuronal differentiation or cell death in the dentate gyrus of C57BL/6 mice

Exercise is one of the best-known stimulators of adult hippocampal neurogenesis, but it is not known if voluntary changes in the intensity of exercise are accompanied by changes in neurogenesis. In this study we investigated whether a reward influences the performance in a running wheel and the rate of cell proliferation, neuronal differentiation and cell death in C57BL/6 mice. Mice had free access to a running wheel during the first week of the experiment. In the second week, animals were rewarded for their performance and compared to normal voluntary running and control mice. A reward significantly increased the performance by 78% when compared to the non-rewarded performance of the first week. The performance of the non-rewarded runners remained relatively constant. Fourteen days of exercise significantly increased cell proliferation by 27% and the number of doublecortin immunoreactive cells by 46%. A reward and the associated increase of performance did not modulate proliferation, cell death or the number of cells entering the neuronal lineage. We suggest that, in C57BL/6 mice, either exercise increases adult hippocampal neurogenesis to a ceiling value, which is reached by a performance at or below the level achieved by voluntary wheel running, or that a possible positive effect of increased running-wheel activity is balanced by stress resulting from rewarded running, which is no longer performed on a strictly voluntary basis.

Chronic mild stress (CMS) in mice: of anhedonia, 'anomalous anxiolysis' and activity

Background

In a substantial proportion of depressed patients, stressful life events play a role in triggering the evolution of the illness. Exposure to stress has effects on different levels in laboratory animals as well and for the rat it has been shown that chronic mild stress (CMS) can cause antidepressant-reversible depressive-like effects. The adoption of the model to the mouse seems to be problematic, depending on the strain used and behavioural endpoint defined. Our aim was to evaluate the applicability of CMS to mice in order to induce behavioural alterations suggested to reflect depression-like symptoms.

Methodology/Principal Findings

A weekly CMS protocol was applied to male mice of different mouse strains (D2Ola, BL/6J and BL/6N) and its impact on stress-sensitive behavioural measures (anhedonia-, anxiety- and depression-related parameters) and body weight was assessed. Overnight illumination as commonly used stressor in CMS protocols was particularly investigated in terms of its effect on general activity and subsequently derived saccharin intake. CMS application yielded strain-dependent behavioural and physiological responses including ‘paradox’ anxiolytic-like effects. Overnight illumination was found to be sufficient to mimic anhedonic-like behaviour in BL/6J mice when being applied as sole stressor.

Conclusions/Significance

The CMS procedure induced some behavioural changes that are compatible with the common expectations, i.e. ‘anhedonic’ behaviour, but in parallel behavioural alterations were observed which would be described as ‘anomalous’ (e.g. decreased anxiety). The results suggest that a shift in the pattern of circadian activity has a particular high impact on the anhedonic profile. Changes in activity in response to novelty seem to drive the ‘anomalous’ behavioural alterations as well.

A trans-dimensional approach to the behavioral aspects of depression

Depression, a complex mood disorder, displays high comorbidity with anxiety and cognitive disorders. To establish the extent of inter-dependence between these behavioral domains, we here undertook a systematic analysis to establish interactions between mood [assessed with the forced-swimming (FST) and sucrose consumption tests (SCT)], anxiety [elevated-plus maze (EPM) and novelty suppressed feeding (NSF) tests] and cognition (spatial memory and behavioral flexibility tests) in rats exposed to unpredictable chronic-mild-stress (uCMS). Expectedly, uCMS induced depressive-like behavior, a hyperanxious phenotype and cognitive impairment; with the exception of the measure of anxiety in the EPM, these effects were attenuated by antidepressants (imipramine, fluoxetine). Measures of mood by the FST and SCT were strongly correlated, whereas no significant correlations were found between the different measures of anxiety (EPM and NSF); likewise, measures of cognition by spatial memory and behavioral flexibility tests were poorly correlated. Inter-domain analysis revealed significant correlations between mood (FST and SCT) and anxiety-like behavior (NSF, but not EPM). Furthermore, significant correlations were found between cognitive performance (reverse learning task) and mood (FST and SCT) and anxiety-like behavior (NSF). These results demonstrate interactions between different behavioral domains that crosscut the disciplines of psychiatry and neurology.