A Rapid Neurological Assessment Protocol for Repeated Mild Traumatic Brain Injury in Awake Rats.

Preclinical models for mild traumatic brain injury (mTBI) need to recapitulate several essential clinical features associated with mTBI, including a lack of significant neuropathology and the onset of neurocognitive symptoms normally associated with mTBI. Here we show how to establish a protocol for reliably and repeatedly inducing a mild awake closed head injury (ACHI) in rats, with no mortality or clinical indications of persistent pain. Moreover, we implement a new rapid neurological assessment protocol (NAP) that can be completely conducted within 1 min of each impact. This ACHI model will help to rectify the paucity of data on how repeated mTBI (r-mTBI) impacts the juvenile brain, an area of significant concern in clinical populations where there is evidence that behavioral sequelae following injury can be more persistent in juveniles. In addition, the ACHI model can help determine if r-mTBI early in life can predispose the brain to exhibiting greater neuropathology (i.e., chronic traumatic encephalopathy) later in life and can facilitate the identification of critical periods of vulnerability to r-mTBI across the lifespan. This article describes the protocol for administering an awake closed head mTBI (i.e., ACHI) to rats, as well as how to perform a rapid NAP following each ACHI. Methods for administering the ACHI to individual subjects repeatedly are described, as are the methods and scoring system for the NAP. The goal of this article is to provide a standardized set of procedures allowing the ACHI and NAP protocols to be used reliably by different laboratories. © 2019 by John Wiley & Sons, Inc.

Repeated mild traumatic brain injury can cause acute neurologic impairment without overt structural damage in juvenile rats.

Repeated concussion is becoming increasingly recognized as a serious public health concern around the world. Moreover, there is a greater awareness amongst health professionals of the potential for repeated pediatric concussions to detrimentally alter the structure and function of the developing brain. To better study this issue, we developed an awake closed head injury (ACHI) model that enabled repeated concussions to be performed reliably and reproducibly in juvenile rats. A neurological assessment protocol (NAP) score was generated immediately after each ACHI to help quantify the cumulative effects of repeated injury on level of consciousness, and basic motor and reflexive capacity. Here we show that we can produce a repeated ACHI (4 impacts in two days) in both male and female juvenile rats without significant mortality or pain. We show that both single and repeated injuries produce acute neurological deficits resembling clinical concussion symptoms that can be quantified using the NAP score. Behavioural analyses indicate repeated ACHI acutely impaired spatial memory in the Barnes maze, and an interesting sex effect was revealed as memory impairment correlated moderately with poorer NAP score performance in a subset of females. These cognitive impairments occurred in the absence of motor impairments on the Rotarod, or emotional changes in the open field and elevated plus mazes. Cresyl violet histology and structural magnetic resonance imaging (MRI) indicated that repeated ACHI did not produce significant structural damage. MRI also confirmed there was no volumetric loss in the cortex, hippocampus, or corpus callosum of animals at 1 or 7 days post-ACHI. Together these data indicate that the ACHI model can provide a reliable, high throughput means to study the effects of concussions in juvenile rats.

Effects of Voluntary Exercise on Cell Proliferation and Neurogenesis in the Dentate Gyrus of Adult FMR1 Knockout Mice.

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disability that can be traced to a single gene mutation. This disorder is caused by the hypermethylation of the Fmr1 gene, which impairs translation of Fragile X Mental Retardation Protein (FMRP). In Fmr1 knockout (KO) mice, the loss of FMRP has been shown to negatively impact adult hippocampal neurogenesis, and to contribute to learning, memory, and emotional deficits. Conversely, physical exercise has been shown to enhance cognitive performance, emotional state, and increase adult hippocampal neurogenesis. In the current experiments, we used two different voluntary running paradigms to examine how exercise impacts adult neurogenesis in the dorsal and ventral hippocampal dentate gyrus (DG) of Fmr1 KO mice. Immunohistochemical analyses showed that short-term (7 day) voluntary running enhanced cell proliferation in both wild-type (WT) and Fmr1 KO mice. In contrast, long-term (28 day) running only enhanced cell proliferation in the whole DG of WT mice, but not in Fmr1 KO mice. Interestingly, cell survival was enhanced in both WT and Fmr1 KO mice following exercise. Interestingly we found that running promoted cell proliferation and survival in the ventral DG of WTs, but promoted cell survival in the dorsal DG of Fmr1 KOs. Our data indicate that long-term exercise has differential effects on adult neurogenesis in ventral and dorsal hippocampi in Fmr1 KO mice. These results suggest that physical training can enhance hippocampal neurogenesis in the absence of FMRP, may be a potential intervention to enhance learning and memory and emotional regulation in FXS.

Diffusion MRI abnormalities in adolescent rats given repeated mild traumatic brain injury.

OBJECTIVE: Mild traumatic brain injury (mTBI) is a serious health concern in the adolescent population. Repeated mTBI may result in more pronounced deficits, and has been associated with long-term neurological consequences including neurodegeneration. As such, there is a critical need for the development of objective mTBI biomarkers to help guide medical management. Diffusion-weighted imaging (DWI) is an advanced magnetic resonance imaging (MRI) technique that may detect brain abnormalities after mTBI. Diffusion tensor imaging (DTI) is the most commonly applied DWI method, and initial studies have reported DTI changes in mTBI patients. Furthermore, new DWI methods (e.g., track-weighted imaging; TWI) are being developed that may also be sensitive to mTBIs, but remain to be comprehensively studied. METHODS: This study utilized the Awake Closed Head Injury (ACHI) model of mTBI to investigate changes in DTI and TWI following repeated mTBI in adolescent male and female rats. A total of four ACHI impacts, two/day over two consecutive days, were delivered beginning on postnatal day 25. At 1 day and 7 days postinjury, rats were euthanized and brains were collected for DWI analyses. RESULTS: Rats given repeated mTBI displayed changes in fractional anisotropy and radial diffusivity (i.e., DTI measures), as well as track density (i.e., TWI). INTERPRETATION: These findings are consistent with initial DTI findings in mTBI patients, suggest that TWI may complement DTI, support the utility of DWI measures as biomarkers in mTBI, and further validate the ACHI rat model of mTBI.

Mild Traumatic Brain Injury Produces Long-Lasting Deficits in Synaptic Plasticity in the Female Juvenile Hippocampus.

Mild traumatic brain injury (mTBI) is becoming recognized as a significant concern in modern society. In particular, youth is being increasingly seen as a vulnerable time period for mTBI, as this is the final developmental period for the brain and typically involves robust synaptic reorganization and axonal myelination. Another issue that is being hotly debated is whether mTBI differentially impacts the male and female brain. To examine the impact of mTBI in the juvenile brain, we measured hippocampal synaptic plasticity using a closed-head mTBI model in male and female Long-Evans rats (25-28 days of age) at either 1 h, 1 day, 7 days, or 28 days post-injury. In female rats, the dentate gyrus (DG) region ipsilateral to the impact showed a significant reduction in long-term potentiation (LTP) at 1 day, which persisted to 28 days following injury. In male rats, the deficit in LTP was maximal in the CA1 and DG subfields ipsilateral to the impact site 7 days post-injury; however, these deficits did not persist to 28 days post-injury. These data indicate that mTBI can produce more immediate and persistent impairments in synaptic plasticity in the female brain.

The potential for animal models to provide insight into mild traumatic brain injury: Translational challenges and strategies.

Mild traumatic brain injury (mTBI) is a common health problem. There is tremendous variability and heterogeneity in human mTBI, including mechanisms of injury, biomechanical forces, injury severity, spatial and temporal pathophysiology, genetic factors, pre-injury vulnerability and resilience factors, and clinical outcomes. Animal models greatly reduce this variability and heterogeneity, and provide a means to study mTBI in a rigorous, controlled, and efficient manner. Rodent models, in particular, are time- and cost-efficient, and they allow researchers to measure morphological, cellular, molecular, and behavioral variables in a single study. However, inter-species differences in anatomy, morphology, metabolism, neurobiology, and lifespan create translational challenges. Although the term "mild" TBI is used often in the pre-clinical literature, clearly defined criteria for mild, moderate, and severe TBI in animal models have not been agreed upon. In this review, we introduce current issues facing the mTBI field, summarize the available research methodologies and previous studies in mTBI animal models, and discuss how a translational research approach may be useful in advancing our understanding and management of mTBI.

Sustained Arc expression in adult-generated granule cells.

The dentate gyrus (DG) plays a critical role in memory formation and maintenance. Fitting this specialized role, the DG has many unique characteristics. In addition to being one of the few places in which new neurons are continually added in adulthood, the region also shows a unique long-term sustained transcriptional response of the immediate-early gene Arc to sensory input. Although we know that adult-generated granule cells are reliably recruited into behaviorally-driven neuronal network, it remains unknown whether they display robust late-phase sustained transcription in response to activity like their developmentally-generated counterparts. Since this late-phase of transcription is required for enduring plasticity, knowing if sustained transcription appears as soon as these cells are incorporated provides information on their potential for plasticity. To address this question, adult F344 rats were injected with BrdU (50mg/kg/day for 5 days) and 4 weeks later explored a novel environment. Arc expression in both BrdU- and BrdU+ neurons was determined 0.5h, 1h, 2h, 6h, 8h, 12h, or 24h following this behavior. Recently-generated granule cells showed a robust sustained Arc expression following a discrete behavioral experience. These data provide information on a potential mechanism to sculpt the representations of events occurring within hours of each other to create uncorrelated representations of episodes despite a highly excitable population of neurons.

The Benefits of Exercise on Structural and Functional Plasticity in the Rodent Hippocampus of Different Disease Models.

In this review, the benefits of physical exercise on structural and functional plasticity in the hippocampus are discussed. The evidence is clear that voluntary exercise in rats and mice can lead to increases in hippocampal neurogenesis and enhanced synaptic plasticity which ultimately result in improved performance in hippocampal-dependent tasks. Furthermore, in models of neurological disorders, including fetal alcohol spectrum disorders, traumatic brain injury, stroke, and neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's disease exercise can also elicit beneficial effects on hippocampal function. Ultimately this review highlights the multiple benefits of exercise on hippocampal function in both the healthy and the diseased brain.