Adolescents experiencing early-life adversity are vulnerable to cognitive impairments that commonly herald neuropsychiatric illness¹. Previously, anatomical studies have identified dendritic alterations within the CA1 region of the rodent hippocampus following chronic early life stress (CES)². We undertook high resolution MRI studies that could have the potential detect the anatomical abnormalities in rodents.CES experimental paradigm²: postnatal day 2 (P2), dams and pups of the CES group were placed in cages with plastic-coated aluminum mesh bottoms and no bedding material. Nesting material was one paper towel that was shredded by the dam to construct a rudimentary nest. Control (CTL) dams/litters resided in bedded cages containing sanitary chips. CTL/CES cages were undisturbed during P2–P9. CTL and CES brains (n=9/group) were perfusion fixed using 4% PFA at 60d after CES, postfixed and stored at 4°C in 0.1 M phosphate buffer. Ex vivo brains underwent MRI using an 11.7 T Bruker Avance with a 256³ matrix, 2cm field of view and 78µm slice thickness using a 3D Rapid Acquisition with Relaxation Enhancement (3D RARE) image acquisition with a TR/TE=2388/15ms and a single average (time=5hr). Volumetric brain, dorsal and ventral hippocampal analysis was performed on coronal slices using Cheshire image software (Hayden Image/Processing Group) by blinded investigators using anatomically defined landmarks. The hippocampus and other regions of interest were manually delineated on every fourth slice. To calculate volumes, interpolated areas were computed using actual areas with a cubic spline function (MATLAB MathWorks). High-resolution Diffusion Tensor Imaging (DTI) was acquired on a 9.4 T Bruker using an 4-shot EPI sequence with a matrix 128² and then zero-filled to 256² .The scan parameters were: 50 slices at 0.5mm thick, a 1.92cm FOV, four averages, b values of 0 (5 images) and 3000 s/mm² (30 images in non-collinear directions) with a TE/TE=12500/36ms for an acquisition time of 2hr. DTI images were analyzed using DSI Studio. Fractional anisotropy (FA) and primary, secondary and tertiary diffusion eigenvector maps were calculated. Regions of interest (ROI) were drawn bilaterally in the dendritc CA1 regions of the dorsal hippocampus on two adjacent slices. Eigenvalue and FA ROI statistics were then extracted from each brain using FSL tools by superimposing ROIs over the corresponding parametric maps generated by dtifit. Correlative histology was also performed.We observed a significant reduction in dorsal hippocampal volumes between groups (n=9/group) with CES (4.84±0.12 mm³) vs. CTL rats (5.26±0.15 mm³; t16=2.19, p=0.04; Fig. 1). Volume loss in CES hippocampi was confined to the dorsal hippocampus while total and ventral hippocampal volumes were similar across groups. No change in brain volume was found. However, the selective reduction of dorsal hippocampal volumes were associated with a commensurate increase of the volume of the dorsal portions of the lateral ventricles in the CES group (CES: 5.17±0.40 mm³, CTL:4.19±0.23 mm³, t16=2.12 , p=0.05). A significant increase in FA in the dendritic layer of hippocampal CA1 was found in CES rats in the left hippocampus (p<0.01) but not in the right (p=0.2) compared to CTL rats (Fig. 2). No differences in the left or right hippocampus for anterior-posterior direction (λ3) or radial diffusivity (RD) we found. However, we observed a significant increase in FA between the left and right hippocampal dendritic regions in λ2 (p<0.01).

We observed reductions in dorsal hippocampal volumes without overt ventral hippocampal or brain volume changes. We also found commensurate increases in ventricular volumes. The volume of cortical brain regions, including the hippocampus, is a sum of the volumes of cell bodies, axons, dendrites, glia and extracellular matrix. It has been estimated that dendrites contribute to ~35% of cortical volume³. Hence, aberrant development, or loss, of dendritic branching and length might be expected to translate into significant loss of hippocampal volume in the affected region.

We found increased FA values in the apical dendritic region of the CA1, in line with the concept that water diffusion was less constrained in the dendritic layers of CES rats, a factor governed by dendritic microstructure and morphology. The increased FA is a result of combined trends in changes along the plane of the main apical dendritic stem (λ1) as well as water mobility within the plane of the commissuralassociational and Schaeffer collateral branching (λ2).

We report that CES results in MR-observable signatures, such dorsal hippocampal volume loss and increased water mobility (FA) within the apical dendritic regions. Using MRI, The structural consequences of early-life adversity observed in rodents might be instructive about the human condition.