THE DEVELOPMENT OF LASTING IMPAIRMENTS: A MILD PEDIATRIC BRAIN INJURY ALTERS GENE EXPRESSION, DENDRITIC MORPHOLOGY, AND SYNAPTIC CONNECTIVITY IN THE PREFRONTAL CORTEX OF RATS R. MYCHASIUK, a * H. HEHAR, a I. MA, a B. KOLB b AND M. J. ESSER a a Alberta Children’s Hospital Research Institute, University of Calgary, Faculty of Medicine, Calgary, Canada b Canadian Centre for Behavioural Neuroscience, University of Lethbridge, Lethbridge, Canada Abstract—Apart from therapeutic discovery, the study of mild traumatic brain injury (mTBI) has been focused on two challenges: why do a majority of individuals recover with little concern, while a considerable proportion suffer with persistent and often debilitating symptomology; and, how do mild injuries significantly increase risk for an early-onset neurodegeneration? Owing to a lack of observa- ble damage following mTBI, this study was designed to determine if there were changes in neuronal morphology, synaptic connectivity, and epigenetic patterning that could contribute to the manifestation of persistent neurological dysfunction. Prefrontal cortex tissue from male and female rats was used for Golgi-Cox analysis along with the profiling of changes in gene expression (BDNF, DNMT1, FGF2, IGF1, Nogo-A, OXYR, and TERT) and telomere length (TL), follow- ing a single mTBI or sham injury in the juvenile period. Golgi-Cox analysis of dendritic branch order, dendritic length, and spine density demonstrate that an early mTBI increases complexity of pyramidal neurons in the mPFC. Furthermore, there are also substantial changes in the expression levels of the seven genes of interest and TL fol- lowing a single mild injury in this brain region. The results from the neuroanatomical measures and changes in gene expression indicate that the mTBI disrupts normal pruning processes that are typically underway at this point in devel- opment. In addition, there are significant interactions between the social environment and epigenetic processes that work in concert to perpetuate neurological dysfunction. Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: Golgi-Cox, qRT-PCR, telomere, neurodegenera- tion, pruning, epigenetics. INTRODUCTION Mild traumatic brain injuries (mTBI) are relatively common occurrences in childhood with at least 10% of children experiencing an injury by 10 years of age (Faul et al., 2010). While many of these children recover without inci- dence, a significant proportion suffers from lingering symptomology and present with persistent cognitive and neurological impairments (Barlow et al., 2010). However, research is currently unable to explain how a mild brain injury in the pediatric period is capable of producing enduring impairments and in some situations a prolonged risk for neurological diseases. As there is generally no detectable damage to the brain, mild brain injuries are generally diagnosed based upon their pathological char- acteristics (e.g. result from forceful biomechanical move- ment of the head; acceleration/deceleration) and resultant symptomology (e.g. confusion, disorientation, loss of consciousness less than 30 min, etc.) (McCrory et al., 2013). Histological studies tend to corroborate beliefs regarding single mild injuries and often describe very few detectable pathologies; in contrast, individuals with histories of repetitive mTBI often exhibit extensive neurofibrillary tangles, amyloid-ß and tau pathologies, along with brain atrophy and white matter degeneration (for review see (Smith et al., 2013)). Rendering it even more difficult to generate a mechanistic relationship between early mTBIs and longer-term outcomes is a lack of pediatric research. The majority of experimental investigations regarding pathophysiological outcomes associated with mTBI have been conducted on adult brain tissue (e.g. Hamberger et al., 2009; Viano et al., 2009, 2012; Shultz et al., 2011; Jeter et al., 2013). Owing to the great deal of heterogeneity that exists between the developing and adult brain, mild insults early in life may impact disease trajectories in a vastly different manner. The purpose of this study was to investigate the neuroanatomical changes and modifications to gene expression that occur in the prefrontal cortex following a single mild brain injury in juvenile rats. The prefrontal cortex was the primary area of exploration because it is significantly correlated with the functional impairments generally reported after mTBI and it is undergoing substantial growth and plasticity during childhood and adolescence (Bock et al., 2005; Kolb et al., 2012). mTBIs are often associated with deficits in executive control and http://dx.doi.org/10.1016/j.neuroscience.2014.12.034 0306-4522/Ó 2014 IBRO. Published by Elsevier Ltd. All rights reserved. * Corresponding author. Address: Alberta Children’s Hospital Research Institute, University of Calgary, Faculty of Medicine, Heritage Medical Research Building Room 274, 3330 Hospital Drive NW, Calgary, AB T2N 1N4, Canada. Tel: +1-403-210-7178, fax: +1-403- 955-7649. E-mail address: rmmychas@ucalgary.ca (R. Mychasiuk). Abbreviations: ANOVA, analysis of variance; mTBI, mild traumatic brain injury; TL, telomere length. Neuroscience 288 (2015) 145–155 145