MRI uncovers disrupted hippocampal microstructure that underlies memory impairments after early-life adversity

Abstract

Memory and related cognitive functions are progressively impaired in a subgroup of individuals experiencing childhood adversity and stress. However, it is not possible to identify vulnerable individuals early, a crucial step for intervention. In this study, high-resolution magnetic resonance imaging (MRI) and intra-hippocampal diffusion tensor imaging (DTI) were employed to examine for structural signatures of cognitive adolescent vulnerabilities in a rodent model of early-life adversity. These methods were complemented by neuroanatomical and functional assessments of hippocampal network integrity during adolescence, adulthood and middle-age. The high-resolution MRI identified selective loss of dorsal hippocampal volume, and intra-hippocampal DTI uncovered disruption of dendritic structure, consistent with disrupted local connectivity, already during late adolescence in adversity-experiencing rats. Memory deteriorated over time, and stunting of hippocampal dendritic trees was apparent on neuroanatomical analyses. Thus, disrupted hippocampal neuronal structure and connectivity, associated with cognitive impairments, are detectable via non-invasive imaging modalities in rats experiencing early-life adversity. These high-resolution imaging approaches may constitute promising tools for prediction and assessment of at-risk individuals in the clinic. © 2016 Wiley Periodicals, Inc.

Keywords: brain networks; cognitive vulnerabilities; diffusion tensor imaging (DTI); early-life stress; magnetic resonance imaging (MRI).

FIGURE 1

Chronic early-life stress (CES) provokes incipient and overt memory impairments. A: Using object recognition tests, both control (CTL) and (CES) rats (n =6/group) spent more time exploring the novel (N) compared to familiar (F) object at ages 4 and 8 months. However, by 12 months, CES rats failed to distinguish the objects. B: Already at 2 months, stress challenge uncovered incipient impairments of object memory in CES rats. 24 h after a short (5 h) stress, CES+ rats, but not CTL+ rats failed to spend more time exploring the novel vs. the familiar object (n =8–11/group). C: The impaired memory did not result from anxiety (see text) or altered stress-responses in CES rats: the time-course of plasma corticosterone was comparable between groups. (n =12/group/time-point). Statistical significance is denoted by *P < 0.05 compared to CTL, ^#P < 0.05 compared to CTL, CTL+, and CES. D: Hippocampus-dependent spatial memory (object location/placement, OL) was impaired in CES rats already at age 2 months. These rats failed to spend more time exploring the novel compared to familiar locations, indicated by the discrimination index (DI) values. These deficits were apparent with (CES+) or without (CES) exposure to a short stress 24 h earlier. Error bars indicate SEM. [Color figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

CES reduces the volume of dorsal hippocampus subregions involved in memory. Volumetric analyses of brain and hippocampus were performed on 3D-RARE coronal MRI sections (78 μm; see Methods). A: Representative regions of interest illustrate boundaries of left hippocampus (red), right hippocampus (yellow) and brain (green). B: Left mid-dorsal hippocampal volumes were reduced (left) with corresponding increase of volumes of lateral cerebral ventricles in the same mid-dorsal sections (right) in CES rats (see Methods for precise definitions and boundaries). C: three-dimensional rendering of hippocampus illustrates decreased mid-dorsal hippocampal volume in CES rats. The color-coded scale indicates the average percent of volume reduction of each hemisphere in the CES vs. CTL rats. D: The sub-regional volume reduction is insufficient to alter total volumes of the whole left or right hippocampi or E: of brain and cerebral ventricles. Statistical significance is denoted by *P < 0.05 vs. CTL. Error bars indicate SEM. [Color figure can be viewed at wileyonlinelibrary.com]

FIGURE 3

Diffusion tensor imaging (DTI) identifies microstructural alterations provoked by CES. In images obtained using high-resolution, 30-direction DTI, two regions of interest (ROI) within CA1 dendritic layers were delineated and analyzed. A: Stratum radiatum (SR) delineated on fractional anisotropy (FA) maps B: via anatomical landmarks (see Methods). Because directional vectors within CA1-SR vary along the mediolateral axis, the region was divided into two ROIs. Group-specific differences were confined to ROI-1. C: FA (water mobility asymmetry) was significantly (P < 0.01) increased in 7- to 8-week old CES rats compared to controls (CTL). D: FA represents the anisotropy from three eigenvalues (λ), illustrated by the diffusion ellipsoid. Therefore, E: the contribution of each eigenvalue (λ1, λ2, λ3) to the increased FA is shown. Increased FA is a result of trends for increased λ1 and λ2 values. Statistical significance is denoted by *P < 0.05 vs. CTL. Error bars indicate SEM. SP, stratum pyramidale; ML, molecular layer; GL, granule cell layer. [Color figure can be viewed at wileyonlinelibrary.com]

FIGURE 4

Dendritic tree abnormalities in CES hippocampus underlie the volume loss and abnormal microstructure identified by MRI and DTI. A: Representative neurons from each group illustrate reduced dendritic arborization in CES CA1 neurons. B: Total apical dendritic lengths in hippocampal areas CA1 and CA3 were significantly reduced in CES rats (n =3–4/group). C: Complexity of dendritic arbors was quantified using Sholl analyses, counting numbers of dendritic intersections in apical dendritic hippocampal CA1 and D: CA3 regions. Dendritic complexity was diminished in CES rats vs. CTL (n =6–10 neurons/rat). Statistical significance is denoted by *P < 0.05 vs. CTL. Error bars indicate SEM. [Color figure can be viewed at wileyonlinelibrary.com]