Browsing by Subject "Plasticity"

Now showing 1 - 9 of 9
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    AMPA Glutamate Receptor Trafficking in Models of Disease
    (2013-08) Miller, Eric
    The signaling between neurons in the brain underlies many crucial processes - from the beating of the heart to remembering that you have a meeting at noon. Indeed, it is proposed that changes in the way neurons communicate with each other form the basis of learning and memory. In this dissertation I will explore the ways in which neurological diseases can affect these integral neuronal functions and explain the changes on a cellular and molecular level. Deficits in AMPAR signaling are found in numerous diseases. I will explore the mechanisms of these deficits using in vitro models of three diseases: opioid-related cognitive deficits and addiction, Alzheimer's disease, and Parkinson's disease-related dementia. Background information is presented in the first chapter. In the second chapter the signaling pathways underlying morphine-induced synaptic deficits are delineated. I found that calcineurin is necessary for both functional and structural deficits in AMPAR signaling, while CaMKII is necessary for only the structural deficits. The role of tau in synaptic deficits cause by soluble Abeta; oligomers, which are found at elevated levels in Alzheimer's disease patients, are probed in the third chapter. I found that treatment with soluble Abeta; oligomers leads to phosphorylation- dependent mislocalization of tau to dendritic spines. Furthermore, treatment with soluble Abeta; oligomers leads to decreases in AMPAR signaling that require calcineurin activity and GluR1 residue S845, much like the mechanisms of AMPAR internalization in neurons treated with morphine. The fourth chapter unveils a novel role of tau and GSK3 in synaptic deficits found in neurons expressing A53T alpha-synuclein. In fact, I discovered that tau is involved in AMPAR signaling deficits found in neurons expressing A53T alpha- synuclein. Furthermore, both mislocalization of tau and synaptic deficits require phosphorylation of tau by GSK3. This dissertation shows that divergent pathways mediate structural and functional plasticity found in neurons exposed to morphine. Also, I show that deficits in AMPAR signaling in both Alzheimer's disease and Parkinson's disease involve tau mislocalization. These findings shed new light on the signaling pathways involved in AMPAR signaling deficits found in neurological diseases and provide new therapeutic targets for pharmacological interventions.
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    Analytical and Experimental Nanomechanical Approaches to Understanding the Ductile-to-Brittle Transition
    (2015-10) Hintsala, Eric
    This dissertation presents progress towards understanding the ductile-to-brittle transition (DBT) using a mixture of nanomechanical experiments and an analytical model. The key concept is dislocation shielding of crack tips, which is occurs due to a dislocation back stress. In order to properly evaluate the role of these interactions, in-situ experiments are ideal by reducing the number of interacting dislocations and allowing direct observation of cracking behavior and the dislocations themselves. First, in-situ transmission electron microscope (TEM) compression experiments of plasma-synthesized silicon nanocubes (NCs) are presented which shows plastic strains greater than 50% in a semi-brittle material. The mechanical properties are discussed and plasticity mechanisms are identified using post-mortem imaging with a combination of dark field and high-resolution imaging. This observations help to develop a back stress model which is used to fit the hardening regime. This represents the first study of its kind where back stresses are used in a discrete manner to match hardening rates. However, the important measurable quantities for evaluating the DBT include fracture toughness values and energetic activation parameters for cracking and plasticity. In order to do this, a new method for doing in-situ fracture experiments is explored. This method is pre-notched three point bending experiments, which were fabricated by focused ion beam (FIB) milling. Two different materials are evaluated: a model ductile material, Nitronic 50, an austenitic steel alloy, and a model brittle material, silicon. These experiments are performed in-situ scanning electron microscope (SEM) and TEM and explore different aspects including electron backscatter diffraction (EBSD) to track deformation in SEM scale experiments, pre-notching using a converged TEM beam to produce sharper notches better replicating natural cracks, etching procedures to reduce residual FIB damage and elevated temperature experiments. Lastly, an analytical method to predict DBTs is presented which can account for effects of strain rate, temperature and impurity presence. The model is tested by pre-existing data on macroscopic compact tension specimens of single crystal Fe-3%Si. Next, application of the model to nano/micro scale fracture toughness experiments is explored and the large number of confounding variables is discussed in detail. A first attempt at fitting is also presented.
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    Behavioral and neurobiological consequences of intermittent exposure to addictive drugs.
    (2010-02) Rothwell, Patrick Eldredge
    These studies were undertaken to better understand how repeated exposure to addictive drugs leads to adaptations in brain function and behavior related to the development of addiction. They are predicated on evidence that the mere presence of a drug in the body is not the sole determinant of adaptation - rather, the pattern of drug exposure is a key variable, with intermittent exposure making the brain reward system increasingly sensitive to drugs and leaving individuals susceptible to relapse. These experiments were designed to examine whether events occurring during the offset of drug action may contribute to the unique effects of intermittent drug exposure. The first series of experiments develops a set of behavioral measures that can be used to resolve and quantify a state of acute withdrawal caused by the offset of drug action. The second series of experiments utilizes these measures to investigate whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization - a specific consequence of intermittent drug exposure related to adaptations in the brain reward system. The final series of experiments describes a specific synaptic adaptation in a key component of the brain reward system (the nucleus accumbens) that is caused by intermittent drug exposure, related to the development of psychomotor sensitization, and reversed by experiences linked to relapse. The results of these studies suggests new and provocative interactions between neural circuits mediating reward and aversion, which may help identify and explain forms of neural plasticity that underlie the development of drug addiction.
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    Developmental environment contributes to rapid trait shifts among newly colonized subterranean habitats
    (2022-08) Swanson, Nathan
    Recent colonization events to extreme environments provide unique opportunities to study the early steps of adaptation and the potential for rapid convergent evolution. However, phenotypic shifts in recent colonization may also be due to plasticity in response to changes in the rearing environment. Here we analyzed a suite of morphological and behavioral traits of paired surface, subterranean, and facultatively subterranean Mexican Tetra, Astyanax mexicanus from recent introductions in two separate watersheds outside of their native range. We find a variety of phenotypic and behavioral shifts between subterranean and surface populations that mimic more established subterranean populations in Mexico. Despite this rapid morphological divergence, we find that most of these traits are due to plasticity in response to rearing environments, as common-garden, lab-raised fish do not maintain the phenotypes from the parental populations, and lab-born fish resemble each other for most traits more than any wild population. Interestingly and similar to wild-caught fish, subterranean-derived, lab-born subterranean fish exhibit more wall-following behavior than their lab-born surface counterparts, suggesting that this trait is genetically determined and rapidly diverging between subterranean and surface populations. Thus, our study sheds light on the early steps to subterranean evolution, is indicative of potential rapid behavioral evolution of navigational tactics and suggests that plasticity in traits involving exploratory behavior may facilitate or be in response to subterranean invasions.
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    Modifying Pharmaceutical Properties of Levofloxacin by Crystal Engineering
    (2024-05) Huang, Pin-Syuan
    The commercial form of a fluoroquinolone antibiotic, levofloxacin (Lev), is a hydrochloride salt (Lev-HCl). Lev-HCl possesses an intense bitter taste, which presents a challenge for developing an oral tablet with high patient compliance. We approached this challenge by preparing a sweet salt of Lev with an artificial sweetener, Acesulfame (Acs), through an anion exchange reaction. Solid-state properties of an anhydrous Lev-Acs salt were characterized using various analytical techniques. With a degradation temperature at about 260 °C, Lev-Acs is thermally more stable than Lev-HCl. Lev-Acs also exhibits approximately 3 orders of magnitude lower aqueous solubility than Lev-HCl. Both the lower aqueous solubility and the presence of a sweetener make Lev-Acs an excellent candidate for taste-masking. Lev-Acs exhibits superior tabletability at pressures below 150 MPa, attributed to its high plasticity. The results suggest that Lev-Acs holds promise for formulating a palatable tablet, addressing challenges associated with Lev-HCl. Based on analysis of five pairs of stoichiometric hydrates and corresponding anhydrates, it was hypothesized that higher plasticity of a hydrate is caused by a lower crystal packing efficiency and density. In these systems, all hydrates exhibit higher plasticity and lower packing efficiency. Thus, an example of a hydrate with a higher packing efficiency exhibiting lower plasticity would strengthen this hypothesis. Ideally, this can be observed for channel hydrates, where filling the channel space by water molecules increases crystal packing efficiency. In the absence of such an ideal model system, we have tested this hypothesis using a channeled hemi-methanol solvate of a levofloxacin acesulfame salt. Our results confirm this hypothesis since, compared to the isostructural anhydrate, the hemi-methanol solvate exhibits higher packing efficiency and lower plasticity. The higher plasticity of the solvate is confirmed by both crystal structure analysis and energy framework calculations. If this correlation between crystal packing efficiency and plasticity is robust, we can objectively predict material plasticity of structurally related crystals based on crystal packing efficiency.
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    Modulating Human Cortical Plasticity via Transcranial Direct Current Stimulation: Basic & Clinical Applications
    (2019-12) Boroda, Elias
    As humans we have a unique ability to study, and even to modify the makeup of our own existence. The concept of changing oneself has always intrigued me, and it was what initially piqued my interest in the study of the human brain. In my estimation, the brain was where most of our “existence” derived from (I’ve changed my mind about that somewhat since then), and therefore learning about it, and how to modify it, would be quite an interesting undertaking. My passion for this topic led me to work with Dr. Kelvin Lim, who at the time was building momentum for studying the clinical potential of non-invasive neuromodulation. Over the course of 5 years working with Kelvin, I was able to learn a significant amount regarding neuromodulation, research and science as a whole. This dissertation describes two of my main projects. These studies focus on researching the basic and clinical applications of transcranial direct current stimulation (tDCS) as a means to modulate human brain plasticity. The first project, described in chapter II, was a basic science study which aimed to investigate how tDCS interacts with functional brain state. Previous literature has reported on the ability of tDCS to modulate plasticity, both in humans and in animal models. However, given the non-focal nature of tDCS, there is an open debate as to how specific outcomes (physiological or behavioral) are achieved. Recently, a hypothesis has been proposed that active brain networks or populations of neurons are preferentially susceptible to the influence of electric fields over inactive networks or groups of cells. This ‘activity-selectivity’ hypothesis has not been thoroughly tested in studies using physiological measures. In this study I use a novel electrophysiological paradigm to investigate the impact of tDCS on plasticity in the auditory cortex. The unique features of the paradigm allowed me to analyze stimulus specific effects of tDCS, making it possible to test the ‘activity-selectivity’ hypothesis using a novel physiological measure. The third chapter of the thesis describes a clinical trial where we used tDCS in combination with cognitive training to treat impaired executive functions in children with fetal alcohol spectrum disorders (FASD). Exposure to alcohol in the womb impairs neuroplasticity in the developing brain and often leads to severe cognitive deficits later in life. Cognitive training is one of a few treatment options for these deficits, however treatment times are long and difficult to complete. Research has shown that pairing cognitive training with tDCS enhances efficacy and can allow for a shorter intervention. However, tDCS has not been tried in children with FASD and it is not clear if it would be tolerated or efficacious in this population. With this in mind, we conducted a first of its kind clinical trial in children with FASD to test the tolerability and feasibility of tDCS augmented cognitive training and its effects on executive functioning. In sum, this dissertation describes two of my major studies which describe the characteristics and the use of tDCS in both a basic and clinical setting. I believe that the findings generated by these studies will make a significant and positive effect on the field of tDCS and its use in the clinic.
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    Modulation of the Junctional Conductance of Retinal AII Amacrine Cell Electrical Synapses
    (2023-10) Cable, Chloe
    Retinal AII amacrine cells are extensively coupled together by electrical synapses. Changes to the strength of these synapses affect how signals are routed through rod and cone retinal pathways during scotopic and photopic vision. Plasticity at these electrical synapses have not, to date, been characterized using electrophysiological approaches. We investigated the effects of adenosine (AR) and N-methyl-D-aspartate receptor (NMDAR) activation on the electrical coupling between AII cells using dual whole-cell patch-clamp electrophysiology in mouse retinal slices. While neither AR activation nor inhibition affected junctional conductance, NMDAR activation substantially decreased junctional conductance between AII cells. Relieving the Mg2+ block of NMDARs through bath application of Mg2+-free solution or by depolarizing AII cells to 0 mV reduced junctional conductance. Exogenous application of NMDA decreased conductance between cells, a decrease which was blocked by the non-selective NMDAR antagonist APV but not by Ro 25-6981, a selective GluN2B-NMDAR antagonist. Addition of either D-serine or glycine, both NMDAR coagonists, without NMDA, reduced the junctional conductance and addition of either coagonist to NMDA-treated retinas further decreased conductance. Experiments were conducted in inositol 1,4,5-trisphosphate receptor type 2 KO and serine racemase KO mice and in WT mice with D-amino acid oxidase to reduce retinal D-serine levels. Under these conditions, the NMDA-mediated conductance decrease was maintained, indicating that D-serine is not necessary for NMDAR-mediated plasticity. These results demonstrate that NMDAR activation results in a decrease in electrical coupling between AII amacrine cells and suggests that both D-serine and glycine can serve as NMDAR coagonists for this plasticity.
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    Relevance of inhibitory G protein-dependent signaling in prelimbic pyramidal neurons to cocaine-related behavior
    (2022-01) Rose, Timothy
    Drugs of abuse share the ability to enhance DA levels within the mesocorticolimbic system. This increased DA neurotransmission triggers persistent adaptations throughout the brain that are believed to underlie the detrimental behaviors that define addiction. For example, chronic cocaine exposure causes a suppression of inhibitory G protein-dependent signaling mediated by the GABAB receptor (GABABR) and G protein-gated inwardly rectifying K+ (GIRK/Kir3) channel in pyramidal neurons of the prelimbic cortex (PL), a cell population important for executive function. As GIRK-dependent signaling is crucial for tempering excitatory input in neurons, the loss of this “inhibitory brake" may drive neuronal hyperexcitability and foster the development of addiction-related behavior. The goal of this thesis is to examine the contribution of GIRK channels in PL pyramidal neurons to behaviors that may be relevant to addiction, and to further understand the regulatory mechanisms that control inhibitory signaling mediated by GABABRs and GIRK channels.To test the prediction that a loss of GIRK channel activity in pyramidal neurons promotes neuronal hyperexcitability, we employed a viral genetic approach to selectively ablate a critical GIRK channel subunit (GIRK1) in PL pyramidal neurons. GIRK channel ablation blunted GABABR-GIRK currents in, and elevated the excitability of, PL pyramidal neurons – electrophysiological outcomes that closely resemble the effects of repeated cocaine exposure. To examine the behavioral consequences of elevated PL pyramidal neuron excitability, we used complementary viral approaches to model the impact of acute (chemogenetic) and persistent (GIRK channel ablation) excitation of PL pyramidal neurons on PL-dependent behaviors, including acute cocaine-induced locomotion and trace fear conditioning. We found that GIRK channel ablation enhanced the motor-stimulatory effect of cocaine, but did not impact baseline activity or trace fear learning. In contrast, selective chemogenetic excitation of PL pyramidal neurons increased baseline and cocaine-induced activity and disrupted trace fear learning. These effects were mirrored in male mice by selective excitation of PL pyramidal neurons projecting to the ventral tegmental area, a brain region important for reward behavior. Collectively, these data show that manipulations enhancing the excitability of PL pyramidal neurons, and specifically those projecting to the VTA, recapitulate behavioral hallmarks of repeated cocaine exposure in mice. Withdrawal from prolonged cocaine exposure has been correlated with negative affective behaviors, as well as formation of persistent drug-related memories that drive drug-seeking behavior. Therefore, we next modeled the impact of viral-mediated GIRK, or GABABR, ablation in PL pyramidal neurons on mood-related behaviors and cocaine conditioned place preference (CPP). While GIRK ablation did not impact anxiety- or depression-related behavior, the manipulation impaired the extinction of cocaine CPP in male mice. In contrast, GABABR ablation was without effect. Since an impairment in extinction may result in prolonged drug-seeking behavior, we next assessed whether strengthening GIRK channel activity could enhance the extinction of cocaine CPP. As predicted, overexpression of GIRK2 in PL pyramidal neurons facilitated extinction of cocaine CPP in male mice. Together, these findings highlight a unique, sex-specific role for GIRK channels in PL pyramidal neurons in tempering cocaine conditioned responding. Despite established links between GIRK channel plasticity and disease, the basic mechanisms that regulate GIRK-dependent signaling in PL pyramidal neurons are not fully understood. One important regulator of GIRK channel activity is the regulator of G protein signaling (RGS) protein, and specifically RGS6 and RGS7 (RGS6/7). RGS6/7 facilitate the termination of inhibitory G protein-dependent signaling, and are thus critical for maintaining the high temporal resolution of GABABR-GIRK signaling. While both RGS6/7 are expressed in the PFC, little is known about their functional roles in the PL. After establishing that RGS6/7 are coexpressed in most PL pyramidal neurons, we next examined their contribution to synaptically-evoked and baclofen-activated GABABR-GIRK currents using constitutive RGS6–/– and RGS7–/– mice. We found that RGS6/7 differentially regulate GIRK channel activity; RGS6 regulates the amplitude, while RGS7 regulates the kinetics and sensitivity, of GIRK-dependent signaling. These shed light on the functional compartmentalization mechanisms that are critical for ensuring high temporal resolution of neuronal inhibitory G protein-dependent signaling. Overall, the work in this thesis suggests that GIRK-dependent signaling in PL pyramidal neurons represents an “inhibitory brake” on cellular excitability that is critical for excitation/inhibition balance and optimal behavioral function. Although the weakening of this inhibition following repeated cocaine exposure may promote neuronal hyperexcitability and addiction-related behavior, therapeutic interventions that restore inhibitory tone may confer resilience to these effects.
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    Repeated morphine exposure activates synaptogenesis and other neuroplasticity-related gene networks in the dorsomedial prefrontal cortex of male and female rats
    (2023-04) Liu, Shirelle
    Opioid abuse is a chronic disorder likely involving stable neuroplastic modifications. While a number of molecules contributing to these changes have been identified, the broader spectrum of genes and gene networks that are affected by repeated opioid administration remain understudied.In this study, Next-Generation RNA-sequencing (RNA-seq) was employed followed by quantitative chromatin immunoprecipitation to investigate changes in gene expression and their regulation in adult male and female rats’ dorsomedial prefrontal cortex (dmPFC) after a regimen of daily injection of morphine (5.0 mg/kg; 10 days). Ingenuity Pathway Analysis (IPA) was used to analyze affected molecular pathways, gene networks, and associated regulatory factors. A complementary behavioral study evaluated the effects of the same morphine injection regimen on locomotor activity, pain sensitivity, and somatic withdrawal signs. Behaviorally, repeated morphine injection induced locomotor hyperactivity and hyperalgesia in both sexes. 90% of differentially expressed genes (DEGs) in morphine-treated rats were upregulated in both males and females, with a 35% overlap between sexes. A substantial number of DEGs play roles in synaptic signaling and neuroplasticity. Chromatin immunoprecipitation revealed enrichment of H3 acetylation, a transcriptionally activating chromatin mark. Although broadly similar, some differences were revealed in the gene ontology networks enriched in females and males. The results cohere with findings from previous studies based on a priori gene selection. This study also reveals novel genes and molecular pathways that are upregulated by repeated morphine exposure, with some common to males and females and others that are sex-specific.