Microembolism Induces Anhedonia but No Detectable Changes in White Matter Integrity in Aged Rats Christina L. Nemeth 1,2 , David A. Gutman 3 , Waqas Majeed 4,5 , Shella D. Keilholz 3 , Gretchen N. Neigh 1,2 * 1 Department of Psychiatry and Behavioral Science, Emory University, Atlanta, Georgia, United States of America, 2 Department of Physiology, Emory University, Atlanta, Georgia, United States of America, 3 Department of Biomedical Informatics, Emory University, Atlanta, Georgia, United States of America, 4 Coulter Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, Atlanta, Georgia, United States of America, 5 LUMS, School of Science and Engineering, Department of Electrical Engineering, Lahore, Pakistan Abstract Microvascular disease leads to alterations of cerebral vasculature including the formation of microembolic (ME) strokes. Though ME are associated with changes in mood and the severity and progression of cognitive decline, the effect of ME strokes on cerebral microstructure and its relationship to behavioral endpoints is unknown. Here, we used adult and aged male rats to test the hypotheses that ME lesions result in subtle changes to white and gray matter integrity as detected by high-throughput diffusion tensor imaging (DTI) and that these structural disruptions correspond to behavioral deficits. Two weeks post-surgery, aged animals showed depressive-like behaviors in the sucrose consumption test in the absence of altered cerebral diffusivity as assessed by ex-vivo DTI. Furthermore, DTI indices did not correlate with the degree of behavioral disruption in aged animals or in a subset of animals with observed tissue cavitation and subtle DTI alterations. Together, data suggest that behavioral deficits are not the result of damage to brain regions or white matter tracts, rather the activity of other systems may underlie functional disruption and recovery. Citation: Nemeth CL, Gutman DA, Majeed W, Keilholz SD, Neigh GN (2014) Microembolism Induces Anhedonia but No Detectable Changes in White Matter Integrity in Aged Rats. PLoS ONE 9(5): e96624. doi:10.1371/journal.pone.0096624 Editor: Jinglu Ai, St Michael’s Hospital, University of Toronto, Canada Received January 10, 2014; Accepted April 9, 2014; Published May 8, 2014 Copyright: ß 2014 Nemeth et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Funding sources include The American Heart Association (AHA) and The National Alliance for Research in Schizophrenia and Depression (NARSAD). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: gretchen.neigh@emory.edu Introduction Microvascular pathology is common in the aged population and the incidence of microvascular disease is growing [1]. Small vessel pathology, including within the context of microvascular pathol- ogy, and microvascular events (e.g. thickening of arterial walls, microvascular lesions, and microembolic strokes [2,3]) are known contributors to depressive behaviors late in life [4–10], and have been implicated in the progression of cognitive impairment and Alzheimer’s disease [11,12]. Despite these links and the high correlation between microvascular disruption and behavioral deficits, the mechanisms behind this relationship remain a topic of debate [13]. One study found that an estimated 94% of patients with late onset depression (first episode after age 65) exhibited diffuse cerebral lesions [14]. Similar findings linking the presence of infarcts to behavior [7,13,15] led to the development of the Vascular Depression Hypothesis which posits that changes to cerebral vasculature precipitate behavioral changes, such as depression and dementia [6,14]. Due to the slow progression of these symptoms, the phenotypically silent nature of these lesions, and the difficulty of directly addressing this relationship in the clinic, establishing a cause and effect relationship has been difficult. Finally, because ME lesions are typically detected following a larger ischemic event or after death, preventative measures and treatment strategies are greatly hindered. Rodent microembolism (ME) models have been used to successfully recapitulate the cognitive deficits of microvascular pathology [16,17] as well as the clinical features of stroke [18,19]. Importantly, recent work has shown that these lesions are sufficient to induce anhedonic- and anxiety-like behaviors, as well as impairments in spatial memory in adult male rats [20]. In these studies, however, the relationship between microsphere lesions and behavioral disruption were not paralleled by cellular death, macrophage activation, or astrocyte activity measured by conven- tional histology. Therefore, the current study employed diffusion tensor imaging (DTI) as a method of assessing microstructural differences in tissue to complement previous histological findings in this model. In our study, we adopted high-throughput ex vivo imaging, a novel method to more efficiently image multiple perfused brains simultaneously [21,22]. Assessment of DTI indices in microsphere lesioned brains may provide a more translatable correlate compared to slice histology. DTI is a powerful tool that allows for the assessment of microstructural organization, orientation of white matter tracts, and provides detailed information on the integrity of biological tissues. DTI depends on the anisotropy and diffusivity of water molecules through tissue fibers, and thus metrics of DTI, fractional anisotropy (FA) and mean diffusivity (MD), can be used as indices of healthy tracts and tissue microstructure. The sensitivity of DTI allows for the detection of subtle changes in cell, axon, and myelin morphology and has become a useful tool in predicting functional outcome following stroke and other brain injuries [23,24]. Despite growing popularity of DTI for neuroimaging, little is known about the relationship between DTI-derived measures and affective behavior in the context of microvascular pathology. Further, to the PLOS ONE | www.plosone.org 1 May 2014 | Volume 9 | Issue 5 | e96624