Elsevier

NeuroImage

Volume 105, 15 January 2015, Pages 312-322
NeuroImage

Stress-induced alterations in large-scale functional networks of the rodent brain

https://doi.org/10.1016/j.neuroimage.2014.10.037Get rights and content

Highlights

  • Chronic stress impacts large-scale functional connectivity networks in the rat brain.

  • Stress increases connectivity in somatosensory, visual, and default mode networks.

  • Chronic stress does not induce major changes in gray matter volume in the rat brain.

  • Stress increases the volume and diffusivity of the lateral ventricles.

Abstract

Stress-related psychopathology is associated with altered functioning of large-scale brain networks. Animal research into chronic stress, one of the most prominent environmental risk factors for development of psychopathology, has revealed molecular and cellular mechanisms potentially contributing to human mental disease. However, so far, these studies have not addressed the system-level changes in extended brain networks, thought to critically contribute to mental disorders. We here tested the effects of chronic stress exposure (10 days immobilization) on the structural integrity and functional connectivity patterns in the brain, using high-resolution structural MRI, diffusion kurtosis imaging, and resting-state functional MRI, while confirming the expected changes in neuronal dendritic morphology using Golgi-staining. Stress effectiveness was confirmed by a significantly lower body weight and increased adrenal weight. In line with previous research, stressed animals displayed neuronal dendritic hypertrophy in the amygdala and hypotrophy in the hippocampal and medial prefrontal cortex. Using independent component analysis of resting-state fMRI data, we identified ten functional connectivity networks in the rodent brain. Chronic stress appeared to increase connectivity within the somatosensory, visual, and default mode networks. Moreover, chronic stress exposure was associated with an increased volume and diffusivity of the lateral ventricles, whereas no other volumetric changes were observed. This study shows that chronic stress exposure in rodents induces alterations in functional network connectivity strength which partly resemble those observed in stress-related psychopathology. Moreover, these functional consequences of stress seem to be more prominent than the effects on gross volumetric change, indicating their significance for future research.

Introduction

Stress has a major impact on brain functioning. Whereas the stress response first and foremost constitutes a highly adaptive mechanism that enables an organism to respond optimally to potential threats, dysregulation or prolonged exposure to stress can cumulate in psychopathology, such as post-traumatic stress disorder or depression (de Kloet et al., 2005).

Implementing a top–down approach, neuroimaging studies in humans have investigated the functional and structural abnormalities observed in the mentally diseased brain. Patient studies revealed functional impairments and volumetric reductions of the hippocampus and prefrontal cortex, and a hyperactive amygdala (Drevets et al., 2008, Pitman et al., 2012). However, recent advances in the field have elicited a shift away from such region-of-interest-based approaches towards network based approaches, in which the brain is regarded as a set of functional networks, each representing a unique brain function. Importantly, these analyses revealed that stress-related psychopathology is also characterized by alterations in structural integrity and functional connectivity patterns throughout the brain, which might in fact distinguish the healthy from the diseased individual (Whitfield-Gabrieli and Ford, 2012, Admon et al., 2013, Patel et al., 2012).

To elucidate the potential neural underpinnings of stress-related illnesses, animal research has investigated the effects of prolonged (i.e., chronic) stress exposure on neuronal function and structure, using a bottom–up approach. Chronic stress was shown to affect both brain function and structure in a region-specific manner. Higher-order cognitive function, involving the hippocampus and medial prefrontal cortex, was shown to be deteriorated following chronic stress (McEwen, 2001, Pavlides et al., 2002, Liston et al., 2006, Cerqueira et al., 2007), which was accompanied by a reduction in hippocampal volume (Lee et al., 2009), and dendritic hypotrophy (reduced dendritic length and number of branch points) in hippocampal and medial prefrontal neurons (Woolley et al., 1990, Watanabe et al., 1992, Cook and Wellman, 2004, Radley et al., 2004). Conversely, amygdala neurons were shown to display dendritic hypertrophy, and an increased anxiety phenotype (Vyas et al., 2002). Remarkably, similar to the amygdala, chronic stress was shown to enhance plasticity of the most ventral part of the hippocampus, contrary to its dorsal part (Suvrathan et al., 2013, Maggio and Segal, 2007).

Despite the detailed knowledge about regional effects of chronic stress, it is currently entirely unknown whether these microscopic effects observed in animals translate to altered large-scale connectivity as observed in the diseased human brain. We here set out to investigate the effects of chronic stress exposure on the large-scale functional connectivity patterns and structural integrity of the rodent brain. Implementing a controlled design, we exposed male rats to 10 days of chronic immobilization stress, and tested its effects on functional connectivity networks as identified by independent component analysis (ICA) of resting-state functional MRI (rs-fMRI). Additionally, we performed post-mortem high-resolution structural MRI and diffusion kurtosis imaging (DKI) to assess structural changes resulting from stress exposure. To confirm the presence of the expected chronic stress-induced changes in dendritic morphology in the hippocampus, amygdala, and prefrontal cortex, a subset of rats was used for Golgi staining.

Section snippets

Stress manipulation

Thirty-six male Wistar rats (RccHan™, Harlan) were housed in groups of three animals per cage with ad libitum access to food and water. Animals were kept in a temperature-controlled room (22–24 °C), with a light/dark cycle of 12 h (lights on at 7:00 A.M.). At the beginning of the experiments, animals were approximately 3 months old and weighed 325–400 g. The rats were randomly assigned to one of two experimental groups, entering either a chronic immobilization stress (CIS) or control protocol for

Physiology and neuronal morphology

At the start of the experiment (day 1), the control and stress groups did not differ on average weight (p > 0.2). However, over the course of the experiment the groups' weight developed differentially as indicated by a time × group interaction in weight gain (F(10,25) = 25.2, p < 0.001). Whereas the control animals significantly gained weight over the course of the experiment (F(10,8) = 51.0, p < 0.001), the stressed animals did not (F(10,8) = 1.6, p > 0.2), which resulted in a significantly lower body weight

Discussion

Here we show that chronic stress alters large-scale functional connectivity in the rodent brain. Functional connectivity was significantly increased in the somatosensory, visual, and default mode network (DMN) following stress. Moreover, chronic stress induced higher mean diffusivity in the lateral ventricles, which was related to an increase in their volume, while no differences were found in hippocampal volume or shape, nor in the volume of any other brain regions. Thus, this study shows that

Acknowledgments

This work was supported by grants of the Netherlands Organization for Scientific Research (NWO) (021.002.053), and the Utrecht University High Potential Program. We thank C. Beckmann and E. Hermans for the useful discussions on the data analyses, and W. Mol, B. Jongbloets, H. Karst, T. Bezemer, K. van den Hoven, M. Terpstra, S. Versteeg and A. van der Sar for their practical assistance.

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    Both authors contributed equally to the manuscript.

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