ASSESSMENT OF ALTERATIONS IN NEUROMETABOLITES AND BRAIN MICROSTRUCTURE AFTER REPETITIVE MILD TRAUMATIC BRAIN INJURY AND ITS ASSOCIATION WITH THE BEHAVIORAL OUTCOME

Abstract

Traumatic brain injury (TBI) is a leading cause of mortality and morbidity worldwide, affecting both civilian populations and military personnel. Survivors of TBI often experience long-term, disabling changes in cognition, motor function, and personality. The Glasgow Coma Scale (GCS) is a commonly used tool for assessing the level of consciousness and severity of TBI, evaluating three key components: eye-opening response, motor response, and verbal response. Based on the score, TBI is classified into three categories: mild, moderate, or severe. In addition to the GCS, imaging techniques such as CT and MRI scans are often used to assess the extent of injury, particularly in cases of mild TBI. TBI is inherently heterogeneous, meaning it can manifest in various ways depending on the individual. To model this complexity in humans, preclinical studies have used experimental models that replicate the key pathophysiological features of different types of human TBI. Despite the identification of numerous promising neuroprotective agents in experimental studies, none have led to significant improvements in long-term clinical outcomes. Several factors contribute to these translational failures, including differences in the types of data collected (such as histopathological, behavioral, and imaging data) and the timing of data collection (ranging from hours to weeks post-TBI) between clinical and experimental studies. Additionally, preclinical testing should involve multiple experimental TBI models, preferably across different species, to better simulate human conditions. Repetitive mild traumatic brain injury (rmTBI) refers to a condition where a person sustains multiple mild TBIs over time. Athletes in contact sports like football, hockey, and soccer, military personnel exposed to blasts, and individuals in high-risk activities for falls or head impacts are all vulnerable to rmTBI. The cumulative effects of repeated mild TBIs mean that each successive injury may have a more significant impact on brain function and recovery. Symptoms of mild TBI, such as headaches, dizziness, memory issues, concentration problems, mood changes, or fatigue, can be subtle and temporary, making detection difficult. While each mild TBI may heal relatively quickly, when injuries occur consecutively, recovery can become delayed or incomplete. Some individuals may experience persistent post- concussive symptoms or develop long-term issues. Repeated mild TBIs also increase the likelihood of subsequent injuries and lower the threshold for more severe symptoms. This cumulative effect raises v concerns about long-term consequences, such as chronic traumatic encephalopathy (CTE), a degenerative brain disease linked to repeated brain trauma. Metabolomics, the study of metabolites, offers a way to identify biomarkers associated with TBI. These biomarkers can be used to predict the severity, progression, and recovery of the injury. Nuclear magnetic resonance (NMR) metabolomics plays a vital role in TBI research, helping to understand the metabolic changes that occur following injury. In this study, NMR metabolomics was used to standardize the classification of diffuse TBI in rats and assess its impact on both the brain and serum. Two types of injury models were used: (1) blunt trauma using the modified Marmarou’s weight drop model, and (2) blast trauma using a compression- driven shock wave tube. Chapter 1 provides an overview of TBI, the history of NMR spectroscopy, its basic principles, and its application in metabolomics and TBI research. It also discusses various injury models, including those for repetitive TBI. Chapter 2 explores graded traumatic brain injury (TBI) in animal models, which simulate varying levels of injury severity, from mild to moderate to severe. This chapter offers valuable insights into the pathophysiological mechanisms of diffuse graded TBI and its effect on the metabolic homogeneity of the hippocampus at acute time points, using 1H-NMR metabolomics. Chapter 3 focuses on the differential effects of diffuse brain injury on regions distant from the impact site, such as the hippocampus, thalamus, and striatum, at acute, early sub-acute, and sub-acute time points. Using 1H-NMR metabolomics, this chapter aims to deepen our understanding of the metabolic changes and pathophysiology of TBI in these brain regions. Chapter 4 examines the metabolic changes associated with repetitive blast concussions, specifically in female rats. This study investigates how repetitive blast injury reprograms metabolism and explores the interplay between metabolic changes and epigenetic alterations following such injuries. Chapter 5 investigates TBI-induced metabolic alterations over time, from acute to chronic stages, following both mild and repetitive mild TBI. Using 1H-NMR metabolomics to analyze serum metabolic vi changes, the study also employs fecal 16S rRNA sequencing to explore changes in the gut microbiome following injury. Chapter 6 summarizes the behavioral changes observed in animals subjected to two types of TBI: diffuse blunt injury and blast-induced injury. This chapter highlights significant behavioral differences between the two injury groups, particularly at chronic time points following the initial injury. Chapter 7 provides a comprehensive summary of the thesis findings, offering key insights into the pathophysiological and behavioral changes induced by blast and blunt TBI over time.