Pramod Dash - US grants

When organisms learn, their memory can last either for a short or long- period of time. Studies show that short-term memory (lasting minutes to hours) involves covalent modifications of pre-existing proteins. In contrast, long-term memory (lasting hours to days) appears to involve new protein synthesis and gene expression. Inhibition of translation and transcription during the acquisition phase blocks the formation of long- term memory without any effect on short-term memory. Moreover, long-term memory is often accompanied and perhaps mediated by morphological changes in neurons. The biochemical events leading to long-term memory and the associated morphological changes are virtually unknown. While a few genes and proteins have been shown to be altered in their expression following long-term training, it has been difficult to investigate causal relationships between long-term memory formation and the functions of these identified genes and proteins. The purpose of this project is to identify and characterize transcription factors and immediate-early genes activated by long-term training and to investigate their functional role in memory formation and the associated morphological changes. We will carry out these experiments in the marine mollusc Aplysia californica. Because of its simple and accessible nervous system, Aplysia has been one of the major systems used in memory research. A critical locus for the storage of memory for long-term sensitization of the gill- and siphon- withdrawal reflex is the connection between the sensory neurons and the motor neurons which can be reconstituted in dissociated cell cultures. Previous studies in intact animals and in sensory-motor neuron cultures indicate that cAMP mediated gene induction may be necessary for long-term memory formation as well as the associated morphological changes in Aplysia. The project has four specific aims. Aim 1 is to test the hypothesis that the transcription factor CREB (cAMP responsive element binding protein) is part of the biochemical pathway by which long-term memory is induced in Aplysia. Aim 2 is to test the hypothesis that immediate-early genes are involved in the induction of long-term memory. Aim 3 is to test the hypothesis that morphological changes seen following long-term training are caused by activation of CREB protein. Aim 4 is to isolate the cDNA clone encoding Aplysia CREB protein and to test the hypothesis that microinjection of the Aplysia protein into sensory neurons can induce long-term facilitation and the associated morphological changes. A combination of biochemical, electrophysiological and behavioral experiments in intact animals, in semi-intact ganglia and in sensory-motor neuron cultures will be to test the above hypotheses. Understanding the biochemical pathway(s) involved in the formation of memory will provide insights into mechanisms responsible for establishment of long-lasting plasticities in the brain. This information also would be of great value in understanding the consequences of some morphological diseases associated with memory deficits as well as injury to the brain.

DESCRIPTION The long term goal of the research is to identify some of the molecular mechanisms underlying post-traumatic memory deficits. Memory deficits are one of the most persistent consequences of traumatic brain injury (TBI) in humans. Mild to moderate TBI can cause memory dysfunction that lasts for months to years in the absence of any physical injury to the brain. Unfortunately, no demonstrable effective therapies for human TBI are available. In order to investigate the mechanism(s) of spatial memory dysfunction following moderate TBI, the applicant proposes to use an experimental brain injury model in rats. Memory is initially stored in a transient state and later consolidated into more long-lasting forms. Recent studies show that in both vertebrates and invertebrates, expression of Ca++/cAMP-mediated genes via CREB (Ca++/cAMP response element binding protein) is involved in memory consolidation. The applicant's hypothesis is that: the observed spatial memory deficits following moderate TBI are produced by alterations in Ca++/cAMP-mediated gene expression via CREB. The applicant proposes six related Specific Aims to test this hypothesis. He will measure the activities of Ca++/calmodulin-dependent protein kinase and cAMP-dependent protein kinase following moderate brain injury and use specific inhibitors to evaluate their in vivo roles. He will investigate the activation of CREB and the CREB antagonists (cAMP early repressor protein) and use antisense techniques to assess their contributions to spatial memory dysfunction. Finally, the applicant will examine the expression of dynorphin, a gene whose expression can be regulated by Ca++/cAMP and utilize a specific antagonist for dynorphin action to measure its role on memory deficits. The results obtained from these studies may lead to development of new molecular therapies for memory deficits associated with TBI and other neurological conditions.

DESCRIPTION (Adapted from applicant's abstract): The long-term goal of our research is to elucidate some of the biochemical and molecular events underlying the formation and storage of memory. Memory is initially stored in a transient state and later converted into more long-lasting forms. Long-term memory, unlike short-term memory, requires gene expression and protein synthesis. Inhibition of RNA or protein synthesis during, or shortly after, training blocks the formation of long-term memory without any effect on short-term memory. The identification of proteins and genes which are activated during training and participate in long-term memory storage is an intense area of current research. The goals of this proposal are: 1) to identify the second messenger kinases activated during behavioral training and examine their causal role, and 2) to investigate if the transcription factor CREB (Ca2+/cAMP response element binding protein) participates in the formation of long-term memory. Understanding the molecular basis of memory storage will be of value in the future development of pharmacological agents to treat amnesia associated with neurological disorders and other mental health related problems.

Striking randomly, traumatic brain injury (TBI) is a "silent epidemic" which affects two to four million persons each year. The consequences of TBI in these patients, having a mean age of 29.5 years, pose a tremendous loss to family and society in terms of potential productive years of life. Unfortunately, no effective therapies for human head injury are available. The injury to the CNS can be categorized as either primary or secondary. Primary injury results from immediately physical damage as a consequence of trauma and is difficult, it not impossible, to prevent. Secondary injuries are delayed pathological events occurring within minutes, hours, or days after the primary trauma and lead to further damage of the nervous system. Inflammatory responses are major components of secondary injury and are thought to be key contributors to TBI pathophysiology. Prostaglandins, potent mediators of inflammation, are produced via the action of cyclooxygenase-1 and 2 [Cox-1 and COX-2, also known as PGH synthase 1 and 2]. Cox-1 is constitutively expressed in most tissue and is responsible for the physiological production of prostaglandins. In contrast, COX-2 is inducible and is responsible for the elevated production of prostaglandins. A considerable amount of evidence from several experimental systems indicates that COX-2 plays a critical role in inflammation. However, the mechanism(s) of inflammatory responses following TBI has not been elucidated. Our preliminary studies indicate that enhanced Cox-2 expression is associated with experimental TBI. Based on these and other findings, the proposal has three specific aims: (1) to test the hypothesis that induction of Cox-2 contributes to TBI pathophysiology, (2) to test the hypothesis that TBI-induced activation of NFkappaB [nuclear factor kappa B], ATF [activating transcription factor], and/or C/EBP [CCAAT/enhancer binding protein] lead to Cox-2 induction; and (3) to test the hypothesis that the anti- inflammatory agents methylprednisolone and IL-10 attenuate Cox-2 expression. The experiments outlined in this proposal will provide a unique opportunity to both unravel the cellular and molecular mechanisms of TBI- induced inflammation and provide the foundation for therapeutic strategies which will be invaluable in the treatment of TBI.

DESCRIPTION (provided by applicant): Almost two million people sustain traumatic brain injury each year in the United States alone, with most reporting deficits in working memory. Working memory involves the "online" storage of information necessary for performing cognitive operations. The prefrontal cortex, which is required for working memory, is highly developed in humans and its function is often impaired by brain trauma. Studies performed in humans, monkeys and rodents have shown that dopamine signaling plays a critical role in working memory. Either too little or too much dopamine receptor D1 stimulation impairs working memory performance. Recently, a few studies have examined the expression of the rate-limiting enzyme for dopamine synthesis, tyrosine hydroxylase (TH), and the plasma membrane dopamine transporter (DAT) in rat frontal cortex following traumatic brain injury (TBI). However, very little is known about how TBI affects the normalcy of dopamine signaling in the prelimbic/infralimbic (PL/IL) cortices, structures required for working memory performance in rodents. This proposal will test the overall hypothesis that enhanced D1 receptor-mediated signaling in the PL/IL cortices is causally related to TBI-associated working memory deficits. Consequently, attenuation of dopamine synthesis or D1 receptor-activated events in these areas will alleviate these deficits.. The Specific Aims of the proposal are: 1) To determine if dopamine biosynthesis in the PL/IL cortices is increased following TBI and to assess its role in working memory deficits. 2) To determine if TBI alters tissue dopamine content, or receptor levels in the PL/IL cortices and to assess the contribution of dopamine to working memory deficits, and 3) To determine if dopamine-activated intracellular events in the PL/IL cortices ifollowing TBI contributes to working memory deficits. An understanding of the mechanisms by which TBI alters dopamine signaling is critical for the development of mechanism-based intelligent pharmacological treatments for working memory deficits for brain trauma patients.

[unreadable] DESCRIPTION (provided by applicant): The prefrontal cortex (PFC) plays a critical role for behaviors that require a high level of mental integration. Damage to this structure results in an inability to select, maintain, and associate temporally disconnected stimuli. Although these dysfunctions have been attributed to disruptions in prefrontal activity and an inability to learn, they also could, however, result from a deficit in prefrontal memory storage. Our preliminary data show that in addition to the hippocampus, the medial RFC is involved in long-term memory storage for trace fear conditioning, a hippocampal-dependent associative learning paradigm. Training-related activation of extracellular signal-regulated kinase (ERK) occurred in the mPFC prior to that observed in the hippocampus, suggesting that mPFC plasticity may not depend on hippocampal information storage (or plasticity). As dopamine has been repeatedly shown to be necessary for proper prefrontal function, dopamine D1 receptor activity may modulate mPFC memory storage. Based on these observations and others, the present proposal will test the general hypothesis that information is stored long-term within the prefrontal cortex as a direct result of training in higher cognitive tasks. The three Specific Aims are: (1) To examine if molecular correlates of long-term memory occur in the mPFC as a result of trace fear conditioning (2) To examine the contribution of mPFC dopamine receptors in long-term memory for trace fear conditioning. (3) To examine the interaction between hippocampal and mPFC plasticity during memory storage for trace fear conditioning. Presently, there is an incomplete understanding of the process of memory storage. Investigation of PFC memory storage could be crucial to our understanding of this process and to the contribution of memory storage within the mPFC in guiding complex behaviors. The study will explore the role of the prefrontal cortex in long-term memory storage. As the function of the prefrontal cortex is often compromised as a result of neurological and psychiatric disorders, in aging, and following brain injury, an understanding of the role of this structure in long-term memory will provide information that will help guide treatment strategies for these conditions. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): This is the second revision of a proposal entitled "Tuberous Sclerosis Complex in Memory Formation". Elucidation of the molecular events underlying the formation and storage of memory is not only relevant for understanding information processing in the brain but also for the development of therapies to treat memory disorders. Memory formation is initiated by neurotransmitters and neurotrophic factors released as a result of neuronal activity during the learning experience. Engaged neurotrophic receptors activate three key intracellular signaling cascades: Ras-Erk, PLCgamma and PI3-kinase. Although the PLCgamma and Ras- Erk pathways have been extensively examined in long-term spatial memory formation, the role of the equally important PI3K cascade in memory formation has received less attention. Recent genetic and biochemical experiments show that tuberous sclerosis protein -2 (TSC2) is phosphorylated by AKT, a key protein kinase activated by PI3K. The TSC1-TSC2 complex regulates the activity of the mammalian target for rapamycin (mTOR), which increases translation of specific mRNA through its action on ribosomal S6 kinase (S6K, also called p70s6k) and/or 4EBP1 (4E binding protein 1). The dominant role of mTOR in PI3K signaling is evident from experimental findings demonstrating that tumors caused by constitutive activation of the PI3K cascade can be targeted by rapamycin, a highly selective inhibitor for mTOR. In addition to its regulation by AKT phosphorylation, the TSC-mTOR pathway is regulated by adenosine monophosphate-activated kinase (AMPK) which acts as an energy sensor for the cell. In this proposal, we will examine in Specific Aim 1: if the TSC-mTOR pathway is activated following training and is required for spatial memory storage in the hippocampus, in Specific Aim 2: if the memory enhancing effect of glucose is in part acting through the TSC- mTOR pathway in the hippocampus, and in Specific Aim 3: if conditional tsc2 (-/-) mice have impaired spatial memory, and if these impairments can be rescued by manipulation of the mTOR pathway. The results from this study will reveal key molecular events that participate in spatial memory by investigating the activity and role of the TSC-mTOR pathway in the hippocampus. The results from these studies will provide a crucial step towards development of pharmacological and/or molecular strategies to treat memory disorders.

Abstract Studies performed in rats, mice, non-human primates, and human patients have demonstrated that the hippocampus, a structure within the temporal lobe, plays a critical role in learning and memory, and damage to this structure can result in profound impairments. As this basic cognitive function is critical for day-to-day activities, learning and memory dysfunction makes it difficult to hold a job, manage one's finances, and plan daily activities. These problems severely compromise the quality of life for persons with traumatic brain injury, can hamper the effectiveness of rehabilitation, and hinder a return to an independent lifestyle. Using experimental models of brain injury, a number of investigators including us have shown that traumatic brain injury causes hippocampal cell death and dysfunction that underlies learning and memory deficits. Through a series of experimentats, we have identified two compounds that are capable of increasing the expression of cytoprotective genes, which are endogenous to a number of cell types including neurons and are activated by the transcription factor Nrf2. Our working hypothesis is that post-TBI administration of these newly identified compounds will reduce secondary pathologies and improve learning and memory by increasing the expression of Nrf2-driven genes. We will use a combination of biochemical, molecular, genetic and behavioral tests to examine if post-injury administration of these compounds can decrease blood-brain barrier permeability, offer neuroprotection, and improve learning and memory. If successful, the results from this mechanism-based study may pave the way for clinical testing in patients who have sustained a traumatic brain injury.

? DESCRIPTION (provided by applicant): Both clinical and experimental studies have suggested that inflammation is a key player in the progression of traumatic brain injury (TBI)-associated pathologies and neural repair. Uncontrolled inflammation can lead to exacerbated tissue damage and can hinder the repair process. While the role of local inflammation originating in the injured brain has been examined in some detail, the contribution of systemic inflammation to TBI outcome is less established. It has been demonstrated that systemic inflammation is mediated, in large part, by the spleen, which is regulated by the efferent component of the vagus nerve. Previous studies have shown that stimulation of the vagus nerve can reduce BBB permeability, cerebral edema and improve learning after TBI, suggesting a role for systemic inflammation in TBI outcome. However, the mechanism(s) by which vagus nerve activity exerts these effects is not understood. Recent studies have shown that this effect requires splenic nicotinic alpha 7 nicotinic acetylcholine receptor (alpha7nAChR). We propose to test the hypothesis that loss of alpha 7 nicotinic cholinergic signaling worsens, while augmentation of alpha7nAChR signaling improves, inflammation, blood-brain barrier (BBB) integrity and cognitive outcome. Three Specific Aims are outlined to test our hypothesis. Aim1: To examine if alpha7nAChRs regulate TBI-associated inflammation. Aim 2: To test if alpha7nAChR signaling regulates BBB permeability and cerebral edema following TBI. Aim 3: To determine if post-TBI administration of alpha7nAChR agonists improves learning and memory and offers neuroprotection. The results from these studies will not only test a novel mechanism underlying TBI pathology, but will test the therapeutic potential of mechanism-based agents as a treatment for TBI.

? DESCRIPTION (provided by applicant): Moderate-severe traumatic brain injury (TBI) often causes neuronal death and neurocognitive impairments, with both immature and mature neurons being vulnerable to the injury. The endoplasmic reticulum (ER) plays a major role in the folding of membrane and secreted proteins and in calcium storage and intracellular calcium homeostasis. Its function can disrupted in response to decreased in response to a number of stimuli including reduced glucose levels, hypoxia, and altered calcium levels, all of which have been observed after TBI. Disrupted ER function (often referred to as ER stress) can result in the accumulation of misfolded proteins. One of the signaling pathways activated in response to ER stress is double-stranded RNA-activated protein kinase-like endoplasmic reticulum kinase (Perk). Once activated, Perk phosphorylates the translation initiation factor eIF2a, which acts to reduce global protein synthesis while permitting the synthesis of chaperones involved in protein folding. If ER function cannot be restored, Perk leads to the increased expression of CCAAT/enhancer-binding protein homologous protein (CHOP), a mediator of cell death. We have observed that TBI increases eIF2a phosphorylation and enhances CHOP expression. In order to examine the translational potential of targeting Perk-eIF2a-CHOP pathway, we have obtained preliminary experimental results to indicate that post injury administration of guanabenz (a FDA-approved drug that acts to inhibit eIF2a phosphatase) reduces neuronal loss and improves neurocognitive outcome. Furthermore, post-injury administration a chemical chaperone (4-phenylbuteric acid (4-PBA), an FDA-approved drug) also improved outcome. Based on these and other observations, we propose to test the hypothesis that post-TBI administration of guanabenz, 4-PBA, and their combination will effectively reduce loss of both mature and immature neurons and improve neurocognitive function. Aim 1: To determine efficacy of guanabenz and its therapeutic time window. Aim 2: To define the optimal dose and therapeutic time window for 4-PBA. Aim 3: To evaluate if the combination of guanabenz and 4-PBA is more efficacious. The results from these studies provide the basis for future clinical studies to determine if individual drugs alone or in combination can be used to improve outcome after TBI.

? DESCRIPTION (provided by applicant): Although the brain has a high metabolic demand, it has a low capacity for storing energy and is dependent on a continuous supply of glucose from the circulation. Disturbed brain metabolism is a well- characterized secondary pathology of Traumatic Brain Injury (TBI). A biphasic change in brain glucose metabolism has been documented in which an acute, transient increase in brain glucose metabolism is followed by a prolonged suppression in metabolism lasting for days. This decreased glucose metabolism occurs in the absence of ischemia and in brain regions critical to cognition such as the hippocampus and the neocortex. There are four primary classes of mammalian glucose transporters, Glut 1-4, with Glut3 being the principal glucose transporter for neurons. However, a recent study has demonstrated that Glut3 expression patterns did not correlate with brain glucose uptake in a linear manner, a disparity that was especially evident in the hippocampus. Glut4 is expressed at high levels in muscle and adipose tissue, and is unique in that under basal conditions, a relatively large proportion of the transporter is retained in intracellular Glut4- containing storage vesicles (GSVs). Recent studies have shown that the energy sensor AMP-activated protein kinase (AMPK) can mobilize Glut4 by phosphorylating the regulatory molecule AS160. As TBI alters brain glucose metabolism, it is plausible that this will result in a change in AMPK activity and Glut4 mobilization within the injured brain. While the role of Glut4 in energy metabolism in the periphery is well appreciated, its role in TBI pathophysiology has not been examined. We propose to test the hypotheses that Glut4 acts to protect neurons and lessen TBI-associated memory impairments. Its mobilization by activators of AMPK will offer neuroprotection and improve memory in brain injured animals. We outline three Specific Aims to test the above hypotheses. Specific Aim 1: Determine if manipulating Glu4 activity/levels can influence neuroprotection. Specific Aim 2: Determine if increasing Glut4 mobilization offers neuroprotection. Specific Aim 3: Examine if activators of AMPK can improve memory in TBI animals. These experiments will examine if modulating Glut4 expression can be used a potential therapeutic and will reveal the influence of this transporter on the neuronal death and dysfunction associated with TBI. The proposed studies have translational relevance and, if successful, may have implications for other neurological disorders and diseases in which altered brain metabolism are features.

Abstract Traumatic brain injury (TBI) remains a serious health concern in the United States, with nearly one out of every 225 people suffering a brain injury each year. The frontal and temporal lobes are highly vulnerable to TBI and damage to these areas presents a myriad of cognitive and behavioral impairments including learning and memory dysfunction. Problems with memory can interfere with keeping a job, planning one's day-to-day activities, and living an independent life. Memory impairments from TBI can result from death and dysfunction of cells resident to the hippocampus (a structure that resides in the core of the temporal lobe) and other brain structures. Both clinical and experimental studies have shown that metabolic dysfunction and lack of energy production in the injured brain contribute to secondary injury, hinders repair and gives rise to poor outcome. Mitochondria are the ?energy powerhouses? of cells and have been recently shown to be highly dynamic. They constantly combine (i.e. fusion) and divide (i.e. fission) based on the energy needs of the cell. Mitochondrial fusion is regulated by the mitochondrial GTPases optic atrophy1 (Opa1) and mitofusin (Mfn)1/2, while fission is primarily regulated by the cytosolic GTPase dynamin-related protein1 (Drp1). In healthy cells, these two processes exist in a dynamic equilibrium. Excessive mitochondrial fission caused by aberrant Drp1 activity diminishes the ability of mitochondria to produce sufficient energy and has been implicated in cell death, dysfunction and neurodegeneration. The proposed research aims to investigate if altered mitochondrial dynamics plays a causal role in the neuronal pathology and poor outcome after TBI. We hypothesize that TBI increases mitochondrial fission for a discrete time widow following injury and that attenuating fission during this period will enhance mitochondrial function, decrease neuronal damage and improve cognitive function. Three Specific Aims have been proposed: Aim 1. To determine the time course for changes in mitochondrial dynamics and function following TBI in male and female mice. Aim 2. To determine cell-specific changes in mitochondrial morphology after TBI. Aim 3. Investigate if decreasing mitochondrial fission following TBI reduces neuronal loss and improves memory function. By investigating pathological changes in mitochondrial dynamics and function, these studies will provide an innovative perspective on mechanisms of metabolic dysfunction that occurs both in experimental TBI and human patients, and may lead to novel mitochondrial- targeted therapeutic approaches to improve patient outcome.

ABSTRACT Currently, cellular alterations associated with pathological conditions are studied using low complexity immunohistochemical (IHC) assays, typically utilizing 2-5 antibodies, that only reveal a tiny subset of the alterations that are occurring, lack comprehensive cellular context, and do not provide quantitative readouts of cellular changes throughout the tissue. For example, a injury or disease can initiate a complex web of pathological alterations across cell types, and at multiple scales ranging from individual cells to multi-cellular units and the layered brain cytoarchitecture. However, technological limitations are hindering a more comprehensive global understanding of these pathological changes. This lack of understanding is hampering our ability to intelligently design effective treatment regimens, and may have contributed to the failures of clinical trials that targeted a single cell type or specific protein. To bridge this gap in our understanding, we propose to develop a Comprehensive Brain Cellular Alteration Profiling Toolkit (CBAT), a carefully validated and broadly applicable image analysis toolkit with unprecedented potential to accelerate investigation & development of next-generation treatments for brain diseases. CBAT, in association with a flexible and modular protocol for highly multiplexed IHC, will enable simultaneous profiling of all major brain cell types and their functional/pathological status (e.g., resting, reactive, apoptotic) across whole brain sections. It will provide quantitative readouts of cellular alterations at multiple scales ranging from individual cells of all types to multi- cellular units (e.g. niches), brain cell layers, and brain regions. Comprehensive cellular profiling and measurements generated using CBAT will enable a deeper understanding of pathological cellular changes that will enable accelerated design, testing, and optimization of therapeutic interventions. Further, it will reduce overall experimental costs by replacing a large number of less-informative assays with a single comprehensive assay. In the longer term, it will enhance our ability to conduct the systems-level investigations that will be required for fully understanding, and successfully treating, multiple brain pathologies. To achieve these goals, we propose the following aims: Aim 1: Develop and validate a flexible, scalable, extensible, and reproducible method for comprehensive whole slide imaging of all the major brain cell types in stereotactically aligned rat whole brain sections; Aim 2: Develop and validate a turnkey software system profiling cell identify and status at multiple scales ranging from individual cells to multi-cellular units, brain cell layers, and brain anatomic regions; and Aim 3: Test the utility of the CBAT system to comprehensively profile concussion biology, and assess the effectiveness of a drug combination to reduce newly identified pathologies. After its development and validation, CBAT will be disseminated to the research community at no cost for use in their specific research projects.

Project Summary/Abstract Traumatic brain injury (TBI) is a leading cause of death and disability with approximately 1.7 million incidents occurring each year. There is an urgent need to develop new treatments that would limit brain pathology and improve overall outcome. Recently, a signaling pathway has been described that by which the brain regulates systemic inflammation. Specifically, this pathway acts on ?7 nicotinic acetylcholine receptors (?7nAChR) present on peripheral immune cells to decrease inflammation. Activation of these receptors using chemical agonists decreases TBI-triggered inflammation, reduces blood-brain barrier permeability and improves cognitive outcome. The ?7nAChR agonist AZD0328, developed by AstraZeneca, has been shown to improve cognitive function in normal rodents and non-human primates. However, it has not been tested if AZD0328 can reduce TBI-triggered inflammation, reduce brain pathology, or improve outcome. We propose to test the overall hypothesis that AZD0328 will reduce inflammation in rats following TBI (primary outcome). Further, it is anticipated that AZD0328 will reduce BBB permeability, brain pathology and improve cognitive outcome. If beneficial effects are observed, we will proceed to the planning of a Phase II efficacy study. We propose two aims to test our hypothesis: Aim 1 (UG3): To examine if AZD0328 reduces peripheral and central inflammation and improves outcome in traumatically brain injured rats. Aim 2 (UH3): Clinical Trial Planning.

Abstract. The cognitive impairments that occur after a concussion (or mild TBI) can be long-lasting, and can interfere with every day activities. These deficits, especially memory dysfunction, are often due to perturbations of hippocampal function. In vivo recordings of neural activity in behaving animals have demonstrated that the firing of a subset of pyramidal neurons in the hippocampus increases when an animal moves through its environment. These cells, referred to as ?place cells?, display localized firing patterns (i.e. place fields) that the animal uses to recognize an environment. Thus, a failure to form stable place fields has been linked to learning and memory dysfunction. Evidence has shown that theta oscillations (a rhythmic firing pattern seen in the hippocampus) play an important role in modulating place field stability, and in learning and memory. This rhythm is established by connections between inhibitory neurons present in the medial septum and the hippocampus. We present supportive results to indicate that the number of parvalbumin-expressing inhibitory neurons in the CA1 subfield of the hippocampus is decreased after a fluid percussion injury (FPI), an effect that occurs in the absence of overt loss of pyramidal neurons. Associated with this loss, electrophysiological recordings revealed a decrease in theta power and place cell instability that are evident for weeks after brain injury. The transcription factor cAMP response element binding protein (CREB) is phosphorylated and increases neuroplasticity-related gene expression following phosphorylation by specific protein kinases, and has been shown to be critical for place cell stability. Based on these results, we propose to test the hypothesis that stimulation of hippocampal CA1 pyramidal neurons at theta frequency or pharmacological potentiation of CREB will increase place cell function and improve memory formation in the chronic stage of FPI. The results from these proposed studies will reveal the neural basis for memory dysfunction and potential pharmacological strategy to restore neural function and improve learning and memory during subacute/chronic stage of traumatic brain injury.