Biology Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2749

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End date: 2009Subject: search.filters.subject.Biology, Neuroscience

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    Molecular Mechanisms of Neuronal Development
    (2009) Wang, Philip Yung-cheng; Quinlan, Elizabeth M; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Neuronal development relies on the coordination of various biological mechanisms, including the trafficking and function of neurotransmitter receptors and synaptic cell adhesion molecules (CAMs). In this dissertation, I investigated various distinct, yet related, mechanisms of neuronal development: the roles of synaptic adhesion-like molecules (SALMs) in neurite outgrowth and cell adhesion, and the transient expression of N-methyl D-aspartate receptors (NMDARs) at growth cones of young hippocampal neurons. First, I showed that the SALMs, a newly discovered family of CAMs, regulate changes in neurite outgrowth with distinct morphological characteristics. Through transfections of primary hippocampal neurons, I investigated the roles of each SALM in neurite outgrowth. In addition to neurite outgrowth, SALMs are involved in synapse formation. In a parallel study, I further investigated SALM function in development by examining the formation of SALM-mediated cell-cell contacts, and their implications on synaptogenesis. In my final study, I investigated the transient expression of NMDARs at axonal growth cones of young hippocampal neurons. While NMDAR function at synapses is well known, their roles earlier in development are less characterized. The data indicate that NMDARs are present and functional at axonal growth cones of young hippocampal neurons. Somatic whole-cell recordings of young neurons reveal NMDAR-mediated currents in response to local application of NMDA at axonal growth cones, while calcium imaging experiments show that these NMDARs elicit localized calcium influx. Together, the studies in this dissertation give insights into the recurring phenomena of proteins and mechanisms that have dual/multiple roles throughout neuronal development. While a considerable amount of information is known about various biological events that occur at opposite ends of the developmental spectra, the mechanisms connecting them are often enigmatic, but can be elucidated through examining the proteins that they share in common.
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    COMPARATIVE STUDIES ON THE STRUCTURE OF THE EARS OF DEEP-SEA FISHES
    (2009) Deng, Xiaohong; Popper, Arthur N; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Many deep-sea fishes have sensory adaptations for living at great depths with very limited light. While such adaptations are best known in the visual system, it is likely that there are also adaptations in the auditory system that enable deep-sea fishes to use the "auditory scene." However, there are few data on the inner ear of deep-sea fishes. The purpose of this study was to add to those data. Since deep-sea fishes are rarely taken alive, this study was done through comparative anatomical investigations. Three families were chosen from two major deep-sea fish fauna: benthopelagic and mesopelagic. In Antimora rostrata (family Moridae, deep-sea cods), the inner ear structure and its coupling to the swim bladder were analyzed and compared with similar systems found in shallow-water fishes. Part of the membrane labyrinth is thick and rigid. The elaborate structure of the saccular epithelium and the close contact between the ear and swim bladder suggests enhanced hearing sensitivity. In the family Melamphaidae (bigscales and ridgeheads), five species from three genera show broad interspecific variation in the saccular otolith shapes, including having a long otolithic "stalk" in two genera. The presence of this "stalk" corresponds with a gradual change in the saccular maculae. A special type of ciliary bundle on the saccule may have enhanced sensitivity to bundle displacements. Ears were compared between six species of Macrouridae (grenadiers and rattails) that live at different depths. The saccule/lagena size ratio seems to increase with depth, especially between a mesopelagic and a benthopelagic species in the genus Nezumia, in which the benthopelagic species has an enlarged saccule associated with sound production. These findings support the hypothesis that some deep-sea fishes have evolved specializations for inner ear function. While it is not possible to test hearing in deep-sea fishes, the various adaptations found suggest that at least some such species have evolved specialized structures to enable them to use sound in the deep-sea. Some features in the ears of deep-sea fishes that have never been seen in the ears of other vertebrates, which further reveals the structural diversity of fish inner ears in general.
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    AMPA receptor and synaptic plasticity
    (2009) He, Kaiwen; Lee, Hey-Kyoung; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Long-term changes in synaptic strength, such as long-term potentiation (LTP) and long-term depression (LTD), have been proposed to be the cellular correlates of learning and memory formation. In the hippocampus, an area of the brain associated with memory formation, LTP and LTD require functional modification of AMPA receptors (AMPARs). Since AMPARs are the major ionotropic glutamate receptors in the brain, changing the single channel properties and/or the number at synapses can greatly affect excitatory synaptic function. Recent studies highlight that functional recruitment of Ca2+-permeable AMPARs (CP-AMPARs) at synapses is another key regulatory mechanism that alter excitatory synaptic transmission. By combining electrophysiology, biochemistry, and imaging methods, I found that phosphorylation of the GluR1 subunit of AMPAR on the serine-845 site (GluR1-S845) is critical for the functional recruitment of CP-AMPARs. This has functional consequences as CP-AMPARs can be expressed at synapses by various neuronal activities both in vitro and in vivo, such as by LTP, sensory experiences, brain diseases and drug addiction. On the other hand, dephosphorylation of the GluR1-S845 is necessary for producing long-term synaptic depression, which is accompanied by a loss in functional CP-AMPARs. Interestingly, the GluR1-S845 site is not required for the plasticity of dendritic spine structures, which is considered an important mechanism for long-term synaptic plasticity as well as learning and memory formation. These results suggest that the functional change in synaptic transmission and the structural synaptic plasticity may utilize separate signaling cascades. In a parallel study, I demonstrated that the beta-site cleaving enzyme 1 (BACE1), which cleaves the amyloid precursor protein (APP) to release the amyloid beta peptide (Abeta), is also involved in regulating synaptic plasticity. Using mice lacking the BACE1 gene, I found that BACE1 is involved in specific forms of synaptic plasticity as well as presynaptic function. Abnormal accumulation of Abeta by excessive BACE1 activity is thought responsible for triggering the pathology of Alzheimer's disease (AD). However, my results caution the development of AD therapeutics targeting the BACE1 activity. In summary, my studies demonstrate that the function of AMPA receptors can be regulated in multiple ways, including phosphorylation of a single amino acid, and is critically involved in synaptic plasticity that underlies learning and memory formation.
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    Differential requirements of the hindbrain and mesenchyme on inner ear patterning
    (2009) Liang, Jennifer Kimiko; Lee, Hey-Kyoung; Wu, Doris K; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Microsurgical manipulations were performed in ovo to identify the tissues that are required for conferring inner ear patterning. Our results show that the hindbrain, namely rhombomeres 5 and 6, are required for the formation and patterning of the cochlear duct (basilar papilla). Rhombomere 5 and its underlying notochord appear to be important for the growth of the cochlear duct, whereas rhomobomere 6 and its respective notochord are required for cochlear patterning. Rotating the segment of hindbrain from rhombomere 5 to rhombomere 6 along the anteroposterior axis affects cochlear duct formation but has no effect on the development of vestibular structures. The signaling molecules intrinsic to these tissues are distinct from Sonic Hedgehog, which has been shown to be required for cochlear duct outgrowth. In contrast, otic mesenchyme adjacent to the developing inner ear provides anteroposterior axial information to pattern the anterior and posterior canals and ampullae.
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    A COMPARATIVE STUDY OF FGFR3 SIGNALING DURING THE DEVELOPMENT OF THE ORGAN OF CORTI AND BASILAR PAPILLA
    (2008-12-11) Jacques, Bonnie E; Jeffery, William R; Kelley, Matthew W; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Most age-related hearing loss is the result of the accumulated death of inner ear hair cells over a life span. Human hair cells lack the ability to be regenerated once they die and thus there is a need to understand the processes which regulate hair cell formation. Unlike the mammalian ear, the avian cochlea has the ability to regenerate lost hair cells and thus there exists an ongoing race to find the key to regeneration in the mammalian ear. Human hearing is dependent on the interactions between numerous cell types yet very little is known about the pathways which regulate the development of the functionally essential support cells of the mammalian cochlear sensory epithelium. This study aims to elucidate some of the genetic pathways involved in hair cell and support cell differentiation in the developing cochlea. Specifically, the role of Fgfr3 signaling in pillar cell and hair cell differentiation will be revealed through the use of an in vivo mutant mouse model containing a null Fgf8 gene and in vitro whole organ culturing of the embryonic cochlear sensory epithelia of mice and chickens. The classic localize, activate, inhibit scheme will be employed. This study will demonstrate that Fgf8 and Fgfr3 are expressed by inner hair cells and pillar cells, respectively, and are required throughout development for normal differentiation and pattern formation of the organ of Corti. Inhibition of the receptor or ligand results in the loss of pillar cells and ectopic formation of hair cells, while activation of this pathway inhibits hair cell formation and induces pillar cells or activation of these genes and their proteins have on the formation of hair cell and support cell types. This study also takes a comparative approach by addressing the similarities and differences of the Fgfr3 signaling pathway in the mammalian organ of Corti and the avian basilar papilla. Fgfr inhibition in the developing basilar papilla causes an increase in hair cell density via the direct transdifferentiation of support cells into hair cells suggesting a role for this signaling pathway in the ability to regenerate hair cells.
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    The Gonadotropin Releasing Hormone-3 System in Zebrafish: Early Development and Regulation
    (2008-12-15) Abraham, Eytan; Zohar, Yonathan; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The objective of this study was to expand our understanding of the early development of forebrain Gonadotropin Releasing Hormone (GnRH) neurons in vertebrates in general and in fish in particular. The correct migration during early development of the hypophysiotropic GnRH neurons from the olfactory region to the hypothalamus is crucial for normal gonadal development and reproduction. We developed a Tg(GnRH3:EGFP) zebrafish line in which EGFP is specifically expressed in GnRH3 neurons. Using this line, we have studied in detail the early spatiotemporal development of the GnRH3 system in vivo. In addition, we have studied various factors, including GnRH3, Netrins and Hedgehog to better understand some of the mechanisms that mediate this complex axophilic neuron migration event. Lastly, we have conducted targeted GnRH3 neuron ablation experiments in view of determining the embryonic origin of POA-hypothalamic GnRH3 neurons and the effect of lack of GnRH3 neurons in the CNS. Our findings show that: 1) GnRH neurons first differentiate and express GnRH3 at 24-26 hours post fertilization (hpf) and immediately thereafter begin to extend fibers. 2) GnRH3 neurons project a complex network of fibers, prior the GnRH3 soma migration, to various CNS regions, and to the pituitary. 3) GnRH3 soma begin migrating towards the hypothalamus at 3 days post fertilization (dpf), passing through the terminal nerve (TN), lateral telencephalon, and reaching the hypothalamus by 12 dpf. 4) expression of GnRH3 itself is necessary for the normal early differentiation and fiber extensions of GnRH3 neurons. 5) Netrin1a is directly involved as a chemoattractant in GnRH3 fiber organization and subsequently, in GnRH3 soma migration to the hypothalamus. 6). Netrin2 is required for normal early ZF embryogenesis. 7). Sonic hedgehog a does not serve as a specific factor in the development of the GnRH3 system. 8). GnRH3 neuron regeneration capacity is temporally limited. 9). Successful ablation of olfactory GnRH3 neurons during development results in lack of GnRH3 neurons in the entire sexually mature brain as well as abnormal gonadal development and inability to reproduce. This study expands our understanding vis-à-vis the early events that occur during GnRH3 system development and that regulate this complex process. In a broader sense these findings augment current knowledge regarding the regulation of long range tangential neuron migration during development.
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    Testing a dynamic account of neural processing: Behavioral and electrophsyiological studies of semantic satiation
    (2008-08-13) Tian, Xing; Dougherty, Michael; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In everyday perception, we easily and automatically identify objects. However, there is evidence that this ability results from complicated interactions between levels of perception. An example of hierarchical perception is accessing the meaning of visually presented words through the identification of line segments, letters, lexical entries, and meaning. Studies of word reading demonstrate a dynamic course to identification, producing benefits following brief presentations (excitation) but deficits following longer presentations (habituation). This dissertation investigates hierarchical perception and the role of transient excitatory and habituation dynamics through behavioral and neural studies of word reading. More specifically, the effect of interest is 'semantic satiation', which refers to the gradual loss of meaning when repeating a word. The reported studies test the hypothesis that habituation occurs in the associations between levels. As applied to semantic satiation, this theory supposes that there is not a loss of meaning, but, rather, an inability to access meaning from a repeated word. This application was tested in three behavioral experiments using a speeded matching task, demonstrating that meaning is lost when accessing the meaning of a repeated category label, but is not lost when accessing the category through new exemplars, or when the matching task is changed to simple word matching. To model these results, it is assumed that speeded matching results from detection of novel meaning to the target word after presentation of the cue word. This model was tested by examining neural dynamics with MEG recordings. As predicted by semantic satiation through loss of association, repeated cue words produced smaller M170 responses. M400 responses to the cue also diminished, as expected by a hierarchy in which lower levels drive higher levels. If the M400 corresponds to the post-lexical detection of new meaning, this model predicted that the M400 to targets following repeated cues would increase. This unique prediction was confirmed. These results were tested using a new method of analyzing MEG data that can differentiate between response magnitude versus differences in activity patterns. By considering hierarchical perception and processing dynamics, this work presents a new understanding of transient habituation and a new interpretation of electrophysiological data.
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    Temporal Structure in Zebra Finch Song: Implications for the Motor Code and Learning Process
    (2008-07-07) Glaze, Christopher Mulholland; Carr, Catherine; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    One of the touchstone questions in neuroscience is how the nervous system encodes complex behavioral sequences such as speech. With experience-dependent learning, well-defined anatomy and complex temporal organization, zebra finch song has served as an excellent model system for these questions. Male songs are learned from older males during a sensitive period that includes song memorization and vocal learning guided by auditory feedback. Once learned, song acoustics are hierarchically organized into syllables, continuous stretches of vocalization separated by silent gaps, which are arranged into stereotyped sequences termed motifs; on a finer scale, syllables are composed of one or more notes, vocalizations with a homogenous spectral profile. Although much is known about the song system, progress has been limited by conflicting data on the neural basis of the acoustic hierarchy and the role this organization plays during learning: While behavioral and electrophysiological studies have suggested separate circuits and learning stages for individual syllables and syllable sequence, these models have been challenged by physiological evidence that songs are actually driven by a clock-like mechanism that does not segment songs into different units. We have analyzed and modeled trial-to-trial timing variability in zebra finch song acoustics to investigate whether the hierarchy is in fact represented in the song system and learning process. Using automated template matching and dynamic time warping, we made millisecond-precise timing measurements in tens of thousands of recordings of both adult and juvenile song. In each adult song, we find rendition-to-rendition tempo variability that is spread across syllables and gaps; however syllable lengths stretch and compress with tempo changes proportionally less than gaps, \ie\ they are less ``elastic." Such non-uniformity is at odds with the simplest clock-based model in which songs are driven by a timing mechanism that paces song evenly across syllable-gap sequences. On the other hand, in a subsequent analysis we factored out tempo changes and used the remaining variability to investigate subsyllabic timescales that contradict the hierarchical model as well. Here, we find length variability that is specific to 10-msec song slices and independent of neighboring vocalization, yet correlated across motifs, providing the first behavioral evidence for a 5-10 msec timescale of song representation and an interaction with a neuromodulatory source operating on a much slower timescale. We have developed a model of song production constrained by the timing data; modeling suggests that adult song may be produced by an underlying chain of activity on a single 5-10 msec timescale, but with properties such as synaptic strength that do correspond to the acoustic hierarchy. Finally, we analyzed juvenile song within the same framework and investigated how the timing properties we modeled may develop during sensorimotor learning. The behavioral data indicate a period towards the end of learning in which syllable sequences become more stereotyped, tempo increases selectively among gaps, and independent timing variability falls two- to threefold across syllables and gaps. In remarkable contrast, over this same period we find no changes in patterns of global tempo variability or the fine timescale patterns indicative of chaining mechanisms. Overall, the developmental data suggest a final phase of song learning in which syllable-based representations are consolidated into the longer sequence-based chaining mechanisms proposed for the adult system. A similar process of linking simpler chains to form more functional activity patterns has been proposed for neocortex and other models of sequence learning in mammalian systems. In this respect, adult zebra finch song representations may be most analogous with procedural memory and overlearned sequences such as repetitive speech patterns.
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    Calcium binding proteins and GAD immunoreactivity in the auditory system of Gekko Gecko
    (2008-05-01) Yan, Kai; Carr, Catherine E; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Geckos use vocalizations for intraspecific communication, but little is known about the organization of their central auditory system. We therefore used immunohistochemical techniques to delineate the auditory nuclei of the hindbrain, midbrain and thalamus. We used antibodies against the calcium binding proteins calretinin (CR), parvalbumin (PV), calbindin-D28k (CB) as well as antibodies against Glutamic Acid Decarboxylase (GAD) and synaptic vesicle protein 2 (SV2). Western blots showed that all five antibodies were specific within gecko brain. The fluorescence double labeling of calcium binding proteins yielded partly overlapping but mostly complementary distributions in the gecko auditory pathway. The differential expression of all calcium binding proteins and GAD, as well as SV2, characterized the structures of the gecko auditory system. The results indicate that the auditory structures and organization in Gekko gecko bear many similarities to the other land vertebrates.
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    Performance Under Pressure: Examination of Relevant Neurobiological and Genetic Influence
    (2008-04-28) Goodman, Ronald N; Hatfield, Bradley D.; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Satisfactory human performance demands the complex interaction of multiple factors such as arousal/motivation, emotion expression and regulation, intricate synchronization of central and peripheral motor processes, all recruited in the service of adaptive, moment to moment decision making. The segregation of these various factors aids in the understanding of their complex interactions. Recently, scientific investigation has focused on understanding the integration of these various factors. The complementary role of emotion and cognition in successful human performance is emphasized. As a viable metric of emotion regulation differences in asymmetry of human brain frontal activity have traditionally been utilized to index certain trait predispositions within the approach/withdrawal dimension of emotion/motivation. Researchers have begun to make a case for an acute or state difference in frontal asymmetry. This "Capability Model" posits the neural underpinnings of the relative difference in electrical activity between the left and right frontal lobes as a phasic/situational mechanism possibly sub-serving the integration of emotion and cognition during challenge. The current study demonstrates support for this situational/state model of frontal asymmetry. Thirty channels of EEG were collected along with, skin conductance, heart rate and acoustic startle amplitudes while subjects were engaged in two levels of a working memory task under three increasing levels of stress (final level=electric stimuli/shock). Hierarchical regression results implicate state frontal asymmetry differences as having a mediating role in the adaptive regulation of emotion during enhanced performance on an N-back working memory task but only in the high stress condition. During shock /threat of shock participants with higher state asymmetry scores showed significant attenuation of eye-blink startle magnitudes, faster reaction times and increased accuracy. This suggests an integration of emotion and cognition.