Browsing by Subject "Neuronal Plasticity"
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Item Activity-Dependent Regulation of Inhibition from Different Inhibitory Subtypes(2007-08-08) Bartley, Aundrea Frances; Gibson, Jay R.Neuronal activity, in the form of action potential firing, is critical in the maturation and maintenance of neocortical circuitry. A negative feedback mechanism by which neuronal circuits adapt to changing levels of average activity on a time scale of hours to days is known as homeostatic plasticity. At the simplest level, homeostatic adaptations occur to maintain firing rate of neurons at a particular set-point. To better understand homeostatic plasticity at the network level, one must understand the activity-dependent adaptations that occur in the different neocortical cells types. To this end, I examined the regulation of inhibitory neurons and their synapses. I used chronic pharmacological block of activity in a neocortical slice cultures to examine the role activity plays in regulating feedback inhibition defined by two biochemical inhibitory neuron subtypes - parvalbumin-positive (Parv+) and somatostatin-positive (Som+). The cellular and synaptic components of local feedback inhibition were examined. I found that chronic activity blockade caused the following: 1) an increase in the intrinsic excitability of Som+ neurons through the downregulation of 2 substhreshold currents. While not thoroughly examined in Parv+ neurons, a similar, but weaker, increase in excitability may occur in these neurons as well. These< changes are consistent with a homeostatic maintenance of firing rate in these neurons. 2) a differential regulation of monosynaptic inhibition based on subtype that was frequency dependent. At low frequency action potential firing, Parv+ mediated inhibitory drive was downregulated while Som+ was unchanged. Both subtypes were likely downregulated at high frequency firing. 3) an upregulation of excitatory drive onto both Parv+ and Som+ neurons. This was most dramatic at low frequency firing where both subtypes displayed an almost 3-fold increase. This is also consistent with homeostatic maintenance of firing rate in inhibitory neurons. 4) based on the above, a clear change in recurrent inhibition occurred at low frequency firing. First, net recurrent inhibition was increased for both subtypes, but the relative influence of the two changed, such that Som+ recurrent inhibition contributed more relative to that of Parv+ circuitry. At high frequency firing, a slight, but less resolvable, increase in net recurrent inhibition may have occurred in both subtypes without any change in relative contribution. 5) all of the synaptic changes were likely due to increases in presynaptic release probability and/or decreases in synapse number.Item Biochemical Characterization Of Delta FosB(2006-12-19) Carle, Tiffany Lynn; Phillips, MegDeltaFosB, the truncated splice variant of FosB, is an important mediator of the long-term plasticity induced in brain by chronic exposure to many types of stimuli, such as repeated administration of drugs of abuse, stress, or compulsive running. Once induced, DeltaFosB persists in the brain for weeks or months following cessation of the chronic stimulus. In addition, DeltaFosB both activates and represses transcription. The biochemical basis of DeltaFosB's persistent expression and dual transcriptional regulation has remained unknown. Both the enhanced protein stability and transcriptional properties are unique to DeltaFosB, compared to FosB, and are critical for its role in neural plasticity. DeltaFosB lacks the C-terminal 101 amino acids of FosB as a result of alternative splicing. The purpose of this work is to biochemically characterize DeltaFosB relative to FosB, to determine how truncation of the FosB C-terminus directs its function. Here, I show that the FosB C-terminus contains two destabilizing elements that promote the degradation of FosB by both proteasome dependent and independent mechanisms. Pulse chase experiments of FosB C-terminal truncation mutants indicate that removal of these C-terminal degrons increases the FosB half-life ~5 fold and prevents its proteasome-mediated degradation and ubiquitylation, properties similar to FosB. These data indicate that alterative splicing specifically removes two destabilizing elements from FosB in order to generate a longer-lived transcription factor, DeltaFosB, in response to chronic perturbations to the brain. Truncation of the C-terminus from FosB also results in differing interaction partners for FosB and DeltaFosB that may contribute to the varying functions of each protein. Specifically, using co-immunoprecipitation assays both in vitro and in vivo, I determined that HDAC1 (histone deacetylase 1) is the preferential binding partner of DeltaFosB compared to FosB. These data suggest an intriguing hypothesis that DeltaFosB interactions with specific HATs and HDACs may be one mechanism by which DeltaFosB mediates both activating and repressive transcriptional activities. DeltaFosB is a unique transcription factor compared to its Fos family members. Truncation of the FosB C-terminal domain liberates DeltaFosB, enabling long-term protein stability and promoting specific interactions with protein partners that are critical for gene regulation important for neural plasticity.Item Calcium-mediated change in neuronal intrinsic excitability in weakly electric fish: biasing mechanisms of homeostatis for those of plasticity(2009-12) George, Andrew Anthony; Zakon, H. H.; Aldrich, Richard W.; Atkinson, Nigel S.; Mihic, S. John; Golding, Nace L.; Dalby, Kevin N.Although the processes used for temporarily storing and manipulating neural information have been extensively studied at the synaptic level far less attention has been given to the underlying cellular and molecular mechanisms that contribute to change in the intrinsic excitability of neurons. More importantly, how do these mechanisms of plasticity integrate with ongoing mechanisms of regulation of neural intrinsic excitability and, in turn, homeostasis of entire neural circuits? In this dissertation I describe the underlying mechanisms that contribute to persistent neural activity and, more globally, sensorimotor adaptation using weakly electric fish as my model system. Weakly electric fish have evolved a behavior adaptation known as the jamming avoidance response (JAR), and it is this adaptation that allows the organism to elevate its own electrical discharge in response to intraspecific interactions and subsequent distortions of the animal’s electric field. The elevation operates over a wide range and in vivo can last tens of hours upon cessation of a jamming stimulus. I demonstrate that the underlying mechanisms of the adaptation are mediated by calcium-dependent signaling in the pacemaker nucleus and that calcium-mediated phosphorylation plays an important role in the regulation of the long-term frequency elevation (LTFE). I demonstrate using an in vitro brain slice preparation from the weakly electric fish, Apteronotus leptorhynchus that the engram of memory formation depends on the cooperativity of calcium-dependent protein kinases and protein phosphatases. In addition, I show that the memory formation (in the form of LTFE) does not depend on the continued flux of calcium, but rather the phosphorylation events downstream of NMDA receptor activation. Moreover, I describe the differences in the expression of protein phosphatases and protein kinases as they relate to species-specific differences in sensorimotor adaptation. It is important to note that this is the first time that the cooperativity between different isoforms of protein kinase C (PKC) have been shown to play a role in graded long-term change in neuronal activity and, in turn, providing the neural basis of species-specific behavior. The neural adaptation of the electromotor system in weakly electric fish provides an excellent model system to study the underlying cellular and molecular events of vertebrate memory formation.Item Rapid Protein Translation Governs Persistent Changes in AMPAR Trafficking in a Form of Long-term Synaptic Plasticity(2010-05-14) Waung, Maggie Wai-Ming; Huber, KimActivation of group 1 metabotropic glutamate receptors (mGluRs) induces long-term depression of glutamatergic synapses (mGluR-LTD). Postsynaptic endocytosis of ionotropic α-amino-5-hydroxy-3-methyl-4-isoxazole propionic acid receptors (AMPARs) accompanies mGluR-LTD, and long-term decreases in AMPAR surface expression most likely mediate this form of synaptic plasticity. In support of this idea, both mGluR-LTD and decreases in AMPA receptors require rapid protein synthesis in dendrites. To understand how newly synthesized proteins maintain decreases in AMPAR surface expression, we examined how mGluRs persistently alter AMPAR trafficking. Using biochemical and immunocytochemical methods in dissociated rat hippocampal cultures, we find that brief activation of mGluRs by the group 1 mGluR selective agonist, DHPG, results in a rapid (10 min) increase in AMPAR endocytosis rate that persists for at least one hour after the removal of agonist. This persistent increase in endocytosis rate is blocked by the protein synthesis inhibitor anisomycin, suggesting that components of the endocytosis machinery are synthesized and necessary for mGluR-LTD. In contrast, treatment of cultures with NMDA, which induces NMDA receptor-dependent LTD causes a long-term (60 min) decrease in AMPAR surface expression, but does not persistently increase endocytosis rate. Recent work has implicated activity-regulated cytoskeletal associated protein (Arc) in the regulation of AMPAR endocytosis through its interactions with endophilin and dynamin, and Arc mRNA is induced in hippocampal CA1 dendrites following behavioral activity. However, little is known about how Arc is locally synthesized at synapses or whether its local synthesis contributes to synaptic plasticity. We find that DHPG induces rapid increases in local and synaptic dendritic Arc protein expression within 10 minutes in hippocampal neurons. Knockdown of Arc by lentiviral delivery of short-hairpin RNA increases basal surface AMPAR expression and synaptic transmission as measured by mEPSC amplitude. Arc knockdown blocks mGluR-induced decreases in surface AMPARs, AMPAR endocytosis as well as mGluR-LTD. Acute inhibition of new Arc translation with antisense nucleotides also blocks mGluR-induced persistent changes in AMPAR trafficking and mGluR-LTD. The involvement of rapid Arc synthesis in mGluR regulation of synaptic function provides a link between behavior-driven neuronal activity and plasticity at the synapse.