One feature of the brain is that different parts of it respond to different stimuli. This means not all brain regions or neurons within those regions are active at a given moment. This feature of the brain gives it the ability to encode and store a wide range of stimuli that are then used to make predictions about a changing external environment. Activation of non-overlapping neural populations is fundamental to the ability to encode a wide range of stimuli to represent a changing environment. To examine the limits of this idea we used genetic tools to label active cell populations following a neutral stimulus presentation or a learned negative association with the same stimulus. The study examined the degree of similarity between these active populations by comparing key features of the active neurons including gene expression and monosynaptic inputs. Another feature of the brain is its ability to store information. In a neural population recently activated by a salient stimulus, molecular processes occur that result in the formation and maintenance of a memory. Collectively these processes are referred to as plasticity, and act on short and long time scales to strengthen the connections between active neurons and weaken the connections between inactive ones. Plasticity processes are not only necessary for the formation and storage of memories but also for wiring up the nervous system during development. A molecule called ZIP has been shown to erase memories months after formation and specifically affects plasticity on longer time scales. However, the effects of ZIP on the developing brain are not well understood and difficult to study using ZIP’s typical delivery method of injection into the brain. To facilitate a developmental study of ZIP’s effects, we made a genetic tool that can specify where and when ZIP is delivered to the brain. Results of the study indicated that males were particularly vulnerable to ZIP during early development while females were unaffected. Together these results provide insight into the limits of information coding potential at the anatomical level and reveal a fundamental difference in plasticity processes in males and females. / 10000-01-01

No description available.

The genetic causes of intellectual disability (ID) and autism spectrum disorder (ASD) are frequently associated with mutations in genes that encode synaptic proteins. A recent screen of ID patients has revealed that approximately 4% of individuals carry spontaneous autosomal-dominant de novo mutations in the SYNGAP1 gene. This gene encodes the synaptic GTPase activating protein (SYNGAP) a known regulator of Ras signalling. Investigations into the pathological consequences of Syngap1 haploinsufficiency (Syngap+/−) in mice have reported abnormalities in behaviour, synaptic plasticity and dendritic spine development. These are analogous to findings from the mouse model of fragile X syndrome (FXS; Fmr1-/y), the most common inherited form of ID. One of the prominent phenotypes reported in the mouse model of FXS is that a form of hippocampal long-term depression (LTD) mediated by the activation of Group 1 (Gp1) metabotropic glutamate (mGlu) receptors is enhanced and independent of new protein synthesis (Huber et al. 2002; Nosyreva et al. 2006). The cause of these synaptic plasticity deficits together with other cognitive abnormalities observed in FXS are thought to arise, in part, from excessive protein synthesis, the consequence of altered mGlu5 receptor signalling via the Ras-ERK1/2 signalling pathway. Enhanced protein synthesis rates in Fmr1-/y mice can be corrected by either inhibiting mGlu5 receptors or reducing Ras and subsequent ERK1/2 activity (Osterweil et al. 2013). In this thesis mGluR-dependent LTD was examined at Schaffer collateral/commissural inputs to CA1 pyramidal neurones in hippocampal slices obtained from Fmr1-/y, Syngap+/− and Fmr1-/ySyngap+/− double mutant mice. Extracellular field recordings reveal that acute application of the Gp1 mGluR agonist dihydroxyphenylglycine (DHPG) induces a form of mGluR-dependent LTD that is enhanced and independent of new protein synthesis in CA1 of Fmr1-/y mice. In Syngap+/− mice, the magnitude of mGluR-dependent LTD is also significantly increased relative to WT littermates and insensitive to protein synthesis inhibitors. Furthermore, in the Fmr1-/ySyngap+/− double mutant, Syngap haploinsufficiency occludes the increase in mGluR-dependent LTD caused by the loss of FMRP. In addition, metabolic labelling studies reveal basal protein synthesis rates to be modestly enhanced in the hippocampus of Fmr1-/y mice compared to WT mice. Importantly this phenotype translates to the rat model of FXS. In Syngap+/- hippocampal slices, basal protein synthesis rates are also significantly elevated compared to WT counterparts. Interestingly, elevated basal protein synthesis rates in Syngap+/- mice could be corrected in the hippocampus by similarly pharmacological strategies employed in Fmr1-/y mice. The comparable neuropathophysiology we observe between Syngap+/− and Fmr1-/y mice suggests that SYNGAP and fragile X mental retardation protein (FMRP) may converge on similar biochemical pathways raising the intriguing possibility that therapeutic strategies used in the treatment of FXS may also be of benefit in treating individuals with ID caused by mutations in SYNGAP1.

Maintaining the intrinsic excitability of neurons is crucial for stable brain activity. This can be achieved by the homeostatic regulation of membrane ion channel conductances, although it is not well understood how these processes influence broader aspects of neuron and network function. One of the many mechanisms which contribute towards this task is the modulation of potassium channel conductances by activity-dependent nitric oxide signalling. Here, we first investigate this mechanism in a conductance-based neuron model. By fitting the model to experimental data we find that nitric oxide signalling improves synaptic transmission fidelity at high firing rates, but that there is an increase in the metabolic cost of action potentials associated with this improvement. Although the improvement in function had been observed previously in experiment, the metabolic constraint was unknown. This additional constraint provides a plausible explanation for the selective activation of nitric oxide signalling only at high firing rates. In addition to mediating homeostatic control of intrinsic excitability, nitric oxide can diffuse freely across cell membranes, providing a unique mechanism for neurons to communicate within a network, independent of synaptic connectivity. We next conduct a theoretical investigation of the distinguishing roles of diffusive homeostasis mediated by nitric oxide in comparison with canonical non-diffusive homeostasis in cortical networks. We find that both forms of homeostasis robustly maintain stable activity. However, the resulting networks differ, with diffusive homeostasis maintaining substantial heterogeneity in activity levels of individual neurons, a feature disrupted in networks with non-diffusive homeostasis. This results in networks capable of representing input heterogeneity, and linearly responding over a broader range of inputs than those undergoing non-diffusive homeostasis. We further show that diffusive homeostasis interferes less than non-diffusive homeostasis in the synaptic weight dynamics of networks undergoing Hebbian plasticity. Overall, these results suggest a novel homeostatic mechanism for maintaining stable network activity while simultaneously minimising metabolic cost and conserving network functionality.

Dopaminergic systems regulate many brain functions and dysfunction of dopaminergic neurotransmission is thought to underlie numerous disorders, including schizophrenia, attention deficit hyperactivity disorder (ADHD), depression and Alzheimer’s disease. In the hippocampus, a dopaminergic projection from the ventral tegmental area (VTA) is proposed to be essential for controlling entry of sensory information into long-term memory through novelty and salience detection. However, the effects of the VTA-dopamine system on hippocampal synaptic transmission are largely under-explored and the underlying mechanisms are unclear. The goal of this project was to investigate mechanisms involved in dopaminergic modulation of hippocampal neurophysiology. Specifically, I (1) examined if dopamine modulates hippocampal synaptic transmission in a region- and input-specific manner, and (2) studied the signaling mechanisms underlying such modulation. In the first aim for the study, I tested whether SKF38393, a dopamine D1-like receptor agonist, differentially affects excitatory synaptic transmission in perforant path synapses onto dentate gyrus granule cells and whether such effects differ from those at area CA1 synapses. I found that SKF38393 produced a concentration-dependent increase in field excitatory postsynaptic potential (fEPSP) in both subregions, but that higher concentrations were needed in the dentate gyrus to produce comparable effects. This synaptic enhancement was long-lasting and largely irreversible which suggests it may be a form of long term enhancement (LTP). Also, the increase in synaptic transmission at medial perforant path synapses was larger than in the lateral perforant path. Importantly, effects in the dentate gyrus, unlike those in CA1, differed substantially along the dorsoventral axis, with effects being significantly larger at the dorsal compared to the ventral pole. In the second aim, various combinations of D1 and D2-like receptor agonists and antagonists as well as inhibitors of second messenger systems, demonstrated that differential mechanisms were required for initiation and maintenance of SKF38393-mediated early and late-phase enhancement and that a novel non-canonical phospholipase-C (PLC) dependent signaling pathway may be involved. Based on recent discoveries in other brain regions, we hypothesized that multiple subcellular signaling pathways may contribute to PLC activation which may include but are not limited to D1(5)-D2 heteromers and Gβγ complex. In conclusion, this work uncovers novel dopaminergic signaling pathways regulating hippocampal physiology, which will lead to development of better (functionally selective) therapeutic agents.

Neurons in the central nervous system are typically not replaced and must therefore maintain their choice of fate and their synaptic connections throughout the life of an organism. I have used Drosophila genetics to analyze genes that prevent neurons from switching fates and allow them to form synapses onto target neurons. The Drosophila fly eye is composed of approximately 750 ommatidia, each comprising eight photoreceptor neurons (R1-R8) surrounded by non-neuronal accessory cells. These photoreceptor neurons undergo a well-defined developmental specification process and form synapses at defined locations in the brain. I have taken advantage of this system to investigate two questions: 1) how do neurons maintain their fate after specification? and 2) how do neurons form stable synapses? For the first half of my dissertation, I have focused my research on a gene, Sce, that I have shown is essential to prevent R7 neurons from undergoing a late switch in cell fate. Sce is an integral component of the Polycomb Group (PcG) complex that is essential for maintaining repression of multiple genes throughout the genome. I found that PcGs are required to prevent R7s from derepression of the R8-specific transcription factor Senseless. For the second half of my dissertation, I focused on the gene syd-1 that was identified to be required for proper presynaptic formation of R7 neurons. Previous studies in Caenorhabditis elegans suggested that Syd-1 acts upstream of Liprin-α and that Liprin-α promotes presynaptic development by binding the kinesin Kif1a to promote axon transport. I used live image analysis to show that, unlike Liprin-α, Syd-1 is not necessary to promote axon transport. Instead, we show that in R7s, Syd-1 acts upstream of Trio, and our results suggest that Syd-1's function is to promote Trio activity. This dissertation includes both my previously published and co-authored materials. / 10000-01-01

No description available.

Synapses are an early pathological target in a wide range of neurodegenerative conditions including adult-onset Alzheimer’s and Parkinson’s, and diseases of childhood such as spinal muscular atrophy and neuronal ceroid lipofuscinoses (NCLs). However, our understanding of the mechanisms regulating the stability of synapses and their exceptional vulnerability to neurodegenerative stimuli remains in its infancy. To address this, we have used the NCLs to model the molecular alterations underpinning synaptic vulnerability. Our primary objective is to identify novel regulators of synaptic stability as well as highlight novel therapeutic targets which may prove effective across multiple neurodegenerative conditions where synapses are an early pathological target. The NCLs, are the most frequent autosomal-recessive disease of childhood. There are currently 14 individual genes whose mutations result in similar phenotypes including blindness, cognitive/motor deficits, seizures and premature death. This suggests that despite the difference in the initiating mutation and the degenerative processes across this collective group are likely to impact on overlapping pathways. Focusing on two murine models of NCL; one with an infantile onset - CLN1 disease (Ppt1-/-) and one with a juvenile onset - CLN3 disease (Cln3-/-) we made use of the temporo-spatial synaptic vulnerability pattern in these mice to plan proteomic and in silico analyses. This pipeline was utilised to identify perturbed protein candidates and pathways correlating with differential regional synaptic vulnerability. This ultimately allowed the generation of a list of candidate proteins, some of which were relevant to human NCL as they were altered in post mortem brain samples. Interestingly, many of the correlative candidates also appear to show conserved alterations in both NCL forms examined and other neurodegenerative diseases. Next, candidates were genetically and/or pharmacologically targeted to study their modulatory effects on neuronal stability in vivo. This was done using CLN3 Drosophila as a rapid screening assay and led to the successful characterisation of a subset of candidates as either enhancers or suppressors of the CLN3-induced phenotype in vivo. As well as identifying regulators of neuronal stability, following a similar pipeline, we identified a set of putative biomarkers of disease progression in muscle and blood in the Ppt1- /- mice, a subset of which appeared conserved in Cln3-/- mice. One of these conserved candidates presented the same directionality of change in human post mortem brain samples, indicating its relevance to the human NCL. Following this workflow from spatio-temporal profiling of murine synaptic populations, to in silico analyses and in vivo phenotypic assessment, we demonstrate that we can identify multiple protein candidates capable of modulating neuronal stability in vivo and identified putative biomarkers that tracked disease progression.

The brain is a complex system par excellence. Its intricate structure has become clearer recently, and it has been reported that it shares some properties common to complex networks, such as the small-world property, the presence of hubs, and assortative mixing, among others. These properties provide the brain with a robust architecture appropriate for efficient information transmission across different brain regions. Nevertheless, how these topological properties emerge in neural networks is still an open question. Moreover, in the last decade the observation of neuronal avalanches in neocortical circuits suggested the presence of self-organised criticality in neural systems. The occurrence of this kind of dynamics implies several benefits to neural computation. However, the mechanisms that give rise to critical behaviour in these systems, and how they interact with other neuronal processes such as synaptic plasticity are not fully understood. In this thesis, we study self-organised criticality and neural systems in the context of complex networks. Our work differs from other similar approaches by stressing the importance of analysing the influence of hubs, high clustering coefficients, and synaptic plasticity into the collective dynamics of the system. Additionally, we introduce a metric that we call node success to assess the effectiveness of a spike in terms of its capacity to trigger cascading behaviour. We present a synaptic plasticity rule based on this metric, which enables the system to reach the critical state of its collective dynamics without the need to fine-tune any control parameter. Our results suggest that retro-synaptic signals could be responsible for the emergence of self-organised criticality in brain networks. Furthermore, based on the measure of node success, we find what kind of topology allows nodes to be more successful at triggering cascades of activity. Our study comprises four different scenarios: i) static synapses, ii) dynamic synapses under spike-timing-dependent plasticity (STDP), iii) dynamic synapses under node-success-driven plasticity (NSDP), and iv) dynamic synapses under both NSDP and STDP mechanisms. We observe that small-world structures emerge when critical dynamics are combined with STDP mechanisms in a particular type of topology. Moreover, we go beyond simple spike pairs of STDP, and implement spike triplets to assess their influence on the dynamics of the system. To the best of our knowledge this is the first study that implements this version of STDP in the context of critical dynamics in complex networks.

abstract: Sensory gating is a process by which the nervous system preferentially admits stimuli that are important for the organism while filtering out those that may be meaningless. An optimal sensory gate cannot be static or inflexible, but rather plastic and informed by past experiences. Learning enables sensory gates to recognize stimuli that are emotionally salient and potentially predictive of positive or negative outcomes essential to survival. Olfaction is the only sensory modality in mammals where sensory inputs bypass conventional thalamic gating before entering higher emotional or cognitive brain regions. Thus, olfactory bulb circuits may have a heavier burden of sensory gating compared to other primary sensory circuits. How do the primary synapses in an olfactory system "learn"' in order to optimally gate or filter sensory stimuli? I hypothesize that centrifugal neuromodulator serotonin serves as a signaling mechanism by which primary olfactory circuits can experience learning informed sensory gating. To test my hypothesis, I conditioned genetically-modified mice using reward or fear olfactory-cued learning paradigms and used pharmacological, electrophysiological, immunohistochemical, and optical imaging approaches to assay changes in serotonin signaling or functional changes in primary olfactory circuits. My results indicate serotonin is a key mediator in the acquisition of olfactory fear memories through the activation of its type 2A receptors in the olfactory bulb. Functionally within the first synaptic relay of olfactory glomeruli, serotonin type 2A receptor activation decreases excitatory glutamatergic drive of olfactory sensory neurons through both presynaptic and postsynaptic mechanisms. I propose that serotonergic signaling decreases excitatory drive, thereby disconnecting olfactory sensory neurons from odor responses once information is learned and its behavioral significance is consolidated. I found that learning induced chronic changes in the density of serotonin fibers and receptors, which persisted in glomeruli encoding the conditioning odor. Such persistent changes could represent a sensory gate stabilized by memory. I hypothesize this ensures that the glomerulus encoding meaningful odors are much more sensitive to future serotonin signaling as such arousal cues arrive from centrifugal pathways originating in the dorsal raphe nucleus. The results advocate that a simple associative memory trace can be formed at primary sensory synapses to facilitate optimal sensory gating in mammalian olfaction. / Dissertation/Thesis / Ph.D. Biology 2012