Gina G. Turrigiano - US grants

9421233 Turrigiano The ability of animals to learn and remember arises from the nervous system. To learn and remember the properties of the neural circuits within the nervous system are modified with experience. These experience-dependent modifications include modifications in both the intrinsic properties of individual neurons and the properties of the synaptic connections between neurons. Models of learning have stressed almost exclusively the role of synaptic modifications in encoding experience, but recent work has shown that experience, in the form of activity in the presynaptic neurons, can profoundly alter the intrinsic firing properties of neurons. This finding identifies an additional powerful mechanisms by which activity can shape neural circuit output. The goal of this work is to determine the mechanisms by which activity regulates intrinsic neuronal firing properties and to ask how this regulation is coordinated with the regulation of synaptic properties to encode experience. This will lead us to a better understanding of how we learn and how we remember.

It is generally accepted that the abilities to learn and remember arise through activity-dependent modifications in the properties of the neural circuits underlying behavior. The outputs of neural circuits depend on an interplay between synaptic connections and the intrinsic firing properties of individual neurons, but despite the importance of these intrinsic neuronal properties for circuit function, studies on the mechanisms of learning have stressed almost exclusively the role of synaptic modifications. We have shown that isolated lobster stomatogastric ganglion neurons in culture undergo activity-dependent transitions between tonic firing and burst firing states in a calcium-dependent manner. These transitions are produced by regulation of the expression of ionic conductances. These studies indicate that the intrinsic firing properties of neurons, like synaptic strengths, can be continuously modified by ongoing activity, and that the firing properties of a neuron are in part a function of its recent history of activation. This proposal will follow up on these findings by using electrophysiological and calcium imaging techniques to address the mechanisms by which activity modifies intrinsic neuronal firing properties. The role of intracellular calcium, protein synthesis, and protein kinases in this process will be addressed. We will determine whether local activation of individual neurites can locally modify the magnitude or distribution of conductances, and whether this modification acts to oppose or to potentiate synaptic inputs. The long-term goal of this proposal is to understand how intrinsic neuronal properties and synaptic strengths are conjointly regulated to encode experience. In order to study the interactions between intrinsic and synaptic plasticity, we have begun to develop a cortical culture system in which the firing properties and synaptic properties of specific populations of neurons can be studied, and where spontaneous synaptic activity is high. The additional time for research afforded me by the award of a K02 grant will allow me concentrate my personal effort on developing this new culture system. I believe this system will provide essential information about the building blocks of learning and memory, and the role of experience in determining and modifying nervous system structure and function.

DESCRIPTION (Investigator's Abstract): During learning and development, the number and strength of synaptic inputs received by a neuron may change dramatically. Such changes are crucial for sculpting functional circuits and generating behavioral flexibility, but they raise a compelling problem for the nervous system: that is how do neurons and circuits maintain stability in their firing properties in the face of such dramatic synaptic configuration? In particular, how do neurons maintain their firing rate in the correct dynamic range despite large fluctuations in the total amount of synaptic excitation they receive? One possibility is that neuronal activity levels can regulate synaptic strengths to maintain firing rates within certain boundaries. In preliminary experiments we have tested this hypothesis. We found that activity can bidirectionally modify the amplitude of miniature excitatory synaptic currents (mEPSCs) between cultured cortical pyramidal neurons. These modifications act to maintain stability in firing rates; increased activity decreases excitatory synaptic strength, and vice versa. In addition, inhibition in these cultures in regulated by activity in the opposite direction from excitation. This activity dependent regulation of synaptic strengths could serve to maintain relatively constant firing rates over broad changes in the number and strength of synaptic inputs. In addition, because this regulation acts to oppose traditional long-term potentiation, it can prevent saturation of synaptic strength arising from the correlation-based synaptic modifications thought to underlie some forms of leaning and memory. The proposal has five specific aims: 1) Hemostatic regulation of firing rates and mEPSC amplitude in cortical pyramidal; 2) Do excitatory synaptic strength vary as a function of firing rate or receptor activation?;3) Homeostasis of inhibitory synaptic connections between cortical interneuron and pyramidal neurons; 4) Role of neurotrophins and calcium influx in synaptic homeostasis; and 5)synaptic homeostasis in vivo.

9726944 TURRIGIANO Much of the development of the mammalian central nervous system occurs after birth, and requires experience in order for functional circuits to form. A central problem in neurobiology has been to elucidate the rules by which experience, in the form of patterned neural activity, determines which synaptic connections are maintained and which are lost. An important hypothesis is that neurons compete with each other in an activity-dependent manner for the establishment of synaptic connections. This is thought to occur through competition between synapses for limiting quantities of a neurotrophic agent that is essential for maintenance of the synapse. Presynaptic neurons that are successful at exciting the postsynaptic neuron obtain more neurotrophic agent and therefore out-compete neurons that are not. Although this model has generated much interest, it has not yet been directly tested. Cortical circuits have both inhibitory and excitatory synaptic connections. Inhibitory connections, where electrical activity in a presynaptic neuron inhibits electrical activity in the postsynaptic neuron, is essential for the proper functioning of cortical circuits, but despite this importance most research to date has concentrated almost exclusively on elucidating the rules by which excitatory synaptic connections are formed. While the rules for formation of inhibitory connections have not been established, an understanding of these rules is crucial for understanding the genesis of cortical circuits. The goal of Dr. Turrigiano's research project is to ask whether the formation of inhibitory synaptic connections involves competitive mechanisms, and to ask whether this competition occurs through the activity-dependent uptake of a trophic agent. These experiments will help to complete our understanding of neural circuit development, and will test a prominent model for how activity wires up the mammalian central nervous system.

DESCRIPTION (provided by applicant): Recent experimental work has identified a novel form of synaptic plasticity (synaptic scaling) in which changes in the level of ongoing cortical activity scale the strength of all of a neuron's excitatory and inhibitory inputs up or down in the direction needed to stabilize network activity. The direction of change in synaptic strengths depends on the identities of both the presynaptic and the postsynaptic neurons, so that excitatory to excitatory (E to E), excitatory to inhibitory (E to I), and I to E connections are all regulated independently, and in the correct direction to stabilize firing rates. Here we propose a collaboration between the Turrigiano lab (experimental) and the Wang lab (computational) to study the computational consequences of these scaling rules for the function of highly recurrent cortical networks. The Wang lab has found that robust models of persistent activity states in prefrontal cortex (such as those thought to underlie working memory) require that synaptic strengths be "tuned" very finely, and synaptic scaling of excitatory inputs offers an effective and biologically plausible means of performing such synaptic fine-tuning. In this proposal we will work back and forth between experimental and theoretical studies to determine more generally whether the plasticity rules we have uncovered are suitable and sufficient to allow the fine-tuning and optimization of activity states in highly recurrent cortical networks. We will ask whether the scaling rules are implemented differently for different classes of neuron (pyramidal vs. interneurons) and how they interact with classic Hebbian-type learning rules to stabilize and optimize network activity. Computational studies of synaptic scaling are still in their infancy, and many assumptions about how this plasticity will function and interact with classical forms of plasticity such as LTP and LTD are entirely untested. By working back and forth between experiment and theory we hope to begin to illuminate this important subject, and expect these studies to open up a wealth of new ideas about how recurrent networks are set up and stabilized during deveiopment and learning. These studies will shed light on the mechanisms by which perturbations in network activity, such as those produced by chronic use of addictive substances, cause fundamental alterations in cortical circuitry and computational capabilities.

ABSTRACT NOT PROVIDED

DESCRIPTION: Sensory experience plays an important role in refining the connectivity of primary visual cortex, but the identity of the synaptic plasticity mechanisms that contribute to this refinement are still under debate. Most research has concentrated on the role of correlation-based plasticity mechanisms such as LTP and LTD, but these mechanisms are highly destabilizing and are unlikely to be sufficient to explain all of activity-dependent development. Using cultured cortical networks we identified a novel form of homeostatic synaptic plasticity that scales excitatory and inhibitory synaptic strengths up and down in the correct direction to stabilize the activity of cortical networks, and more recently we have demonstrated a similar phenomenon in vivo. Here we propose to examine the role of this homeostatic synaptic scaling in experience-dependent plasticity in vivo using a classic sensory deprivation paradigm, monocular deprivation (MD) and binocular deprivation (BD) using lid suture. MD and BD have been used extensively to study activity-dependent plasticity, but the effects of these manipulations on intracortical circuitry have never been probed in detail. We will approach this problem by manipulating activity in rodent visual cortex through MD and DR, then cutting slices of primary visual cortex and obtaining whole-cell recordings to measure quantal currents and paired synaptic transmission. We will ask whether the quantal currents for excitatory and inhibitory synapses are scaled in the opposite direction in response to altered visual input, whether this scaling displays critical periods as do other forms of activity-dependent plasticity, and whether the rules for synaptic scaling are specific for particular classes of excitatory and inhibitory inputs. These experiments will lay an important foundation for understanding the detailed changes in cortical circuitry that arise as a result of altered sensory experience, and will have important implications for the mechanisms of visual abnormalities such as amblyopia.

Circuits of neurons in the mammalian cortex are composed of many thousands of neurons, each of which may receive synaptic contacts from hundreds of other neurons. A central goal of neuroscience is to understand how such complex circuits are wired up correctly during development. It is generally believed that basic cortical circuitry is set up through genetic programming, but that during development this rough connectivity is refined in an activity-dependent manner. Synapses that are effective at driving activity in the postsynaptic neuron are retained, while synapses that are ineffective are lost. Despite the generality of this observation the molecular machinery that allows patterned neuronal activity to "reward" and enhance effective synapses and "punish" ineffective synapses remains unknown. In this project, Dr. Turrigiano's laboratory will test the role of calcium-calmodulin dependent protein kinase II (CaMKII) in such synaptic rewiring of cortical circuits. CaMKII is an attractive candidate to transduce patterned activity into changes in synaptic connectivity because it is activated when synapses effectively drive the postsynaptic neuron, and can have a number of important effects on synaptic transmission. In previous work the PI found that activation of CaMKII has two distinct effects on synaptic connectivity. In the presence of activated CaMKII, synapses that continue to effectively drive the postsynaptic neuron are structurally enhanced, while synapses that do not are eliminated. This suggests that activation of a single molecule can generate an activity-dependent remodeling of synaptic connectivity. In this proposal the PI will combine physiological, molecular, and time-lapse imaging to study the cellular and molecular mechanisms that allow CaMKII to act as either a "punishment" or a "reward" signal for synapse formation. This proposal will foster undergraduate education because Brandeis undergraduates will participate in these experiments in the form of senior honors research projects. In addition the PI has an excellent track record in training women in quantitative scientific methods.

DESCRIPTION (provided by applicant): We request funds for the purchase of a Leica Spectral Confocai-Multiphoton Microscope (Leica SP2 MP) for combined imaging and electrophysiological experimentation. There is no multiphoton microscope at Brandeis, and our single confocal microscope cannot be used for combining imaging and electrophysiology. Such an instrument would greatly facilitate the N.I.H. funded research of the user group. It will also benefit other life science researchers at Brandeis interested in live imaging of biological material. The research of the five investigators in the user group is now well poised to take advantage of combined confocal/multiphoton imaging and electrophysiology. Projects include: mechanisms of synapse formation, synaptic plasticity, neuromodulation, and the regulation of intrinsic excitability. Four of the five investigators have a track record in confocal and quantitative imaging, and a different subset of the four have extensive electrophysiological experience. All five investigators have reached the limit of what can be achieved without an integrated system for combining electrophysiology and live imaging. The Leica SP2 MP instrument would be housed in the Imaging Facility that currently houses our confocal. Because we have had a confocal facility for 10 years, mechanisms of operation and maintenance are already in place. The Imaging facility is managed by microscopist with 20 years of experience. He will commit 50% of his time to running and maintaining the SP2 MP, and will be responsible for training as well. Brandeis University and the Biology Department are committed to maintaining this facility. In addition to the user group, the confocal/multiphoton facility will be available to all life science researchers at Brandeis who have a need for this technology on a time-available basis.

DESCRIPTION (provided by applicant): The Gordon Research Conference (GRC) on Neural Circuits and Plasticity (formerly Neural Plasticity) has been held in alternate years since 1977. We are requesting partial support for the June 26-July 1, 2005 meeting, and the 2007 and 2009 meetings, at Salve Regina University in Newport, Rl. The GRCs were established to stimulate scientific interchange in an informal setting. Interchange is promoted by the informal nature of the conference and by the large amount of time committed to questions during the sessions. The GRC rule prohibiting publication or citation of the meetings and presentations promotes open discussion of the latest results and current ideas. This format has been particularly useful for the Conference on Neural Circuits and Plasticity, a highly interdisciplinary meeting in which the formation, function, and plasticity of neural circuits is explored at the molecular, cellular, systems, and computational levels. The 2005 Conference will have 9 scientific sessions, with a keynote talk by Dr. Bert Sakmann, a Nobel Laureate and one of the most renowned scientists studying circuit function and plasticity. The scientific sessions will cover a range of topics of current interest in the field, from synaptogenesis to sensory coding. The speakers and session chairs are all either world leaders in their fields or up-and-coming junior investigators. Afternoon poster sessions provide an opportunity for all interested participants to present and discuss their work, and each session will include 1 or 2 short talks selected from the submitted abstracts. This format worked very well in previous years to promote young investigators. Question and answer periods are generously scheduled, and social events will permit new participants to meet speakers and session chairs informally. Past participants have found these informal interactions to be a highlight of the meeting, and an important source of scientific advancement for junior people. We are requesting support to fund the attendance of underrepresented groups in the field of Neural Circuit plasticity, including women and ethnic minorities.

ABSTRACT Memory and other cognitive functions reside in part in the pattern and strength of synaptic connections between neurons. Understanding the molecular determinants of synaptic strength has been a long- standing goal of neuroscience, and advances in this field stand to influence our understanding of virtually every neurological disorder from Autism to Alzheimer's disease. Over the past decade biochemical and conventional molecular and genetic approaches have begun to piece together how interactions between neurotransmitter receptors and other synaptic proteins regulate and control synaptic strength and plasticity, but a major limitation is that there is little or no structural information about how proteins are arranged into signaling complexes at the synapse. Many signaling molecules can only interact with immediately adjacent proteins, and this localization may itself be regulated by experience. Understanding how functional signaling complexes are generated and how they in turn regulate synaptic strength thus requires that we probe the spatial arrangements of proteins within the postsynaptic density (PSD). Conventional approaches do not have sufficient resolution to allow the position of synaptic proteins to be mapped within these tiny (<1 [unreadable]m) synaptic structures. Here I propose to develop tools to map the spatial arrangements of individual synaptic proteins (such as glutamate receptors) within the PSD, and to determine how these spatial arrangements are influenced by synaptic plasticity, using super resolution light microscopy. By mapping the relative positions of many different proteins within the postsynaptic membrane and PSD we will be able to generate a 3 dimensional model of the protein lattices that comprise the postsynaptic side of the synapse. This method has the promise to put a vast array of biochemical and molecular data on protein-protein interactions into a structural context that is essential for its interpretation, and will add a powerful new tool to the analysis of synaptic function.

DESCRIPTION (provided by applicant): The proper functioning of the mature visual system depends upon experience-dependent refinement of cortical circuitry during early postnatal development. Visual deprivation during this early critical period (CP) can lead to loss of visual responsiveness to the deprived eye (amblyopia). Despite decades of research it is still unclear which cellular plasticity mechanism(s) contribute to this loss of visual responsiveness, and by extension are required for the normal development of visual function. For example, long-term depression (LTD) of cortical excitatory synapses has been suggested to be necessary and sufficient for loss of visual responsiveness following MD, while other studies have raised the possibility that potentiation of inhibition plays a critical role in this process. Recently the maturation of inhibition from fast-spiking GABAergic basket cells (FS cells) has been implicated in the regulation of CP onset, but why mature inhibition is necessary for CP plasticity is still unknown. During the past 2 funding periods we found that monocular deprivation (MD) during the rodent CP dramatically potentiates inhibitory transmission at FS synapses onto star pyramidal neurons (SP, the major excitatory cell type within L4 of rodent V1), by inducing a novel form of long-term potentiation of inhibition (LTPi) at this synapse. Further, LTPi is developmentally regulated, turns on coincidently with the opening of the CP, and manipulations that delay or advance the CP also delay or advance the ability of FS synapses to express LTPi. In this proposal we wish to test the central hypothesis that LTPi plays a critical role in the lossof visual responsiveness induced by visual deprivation, so that FS cells initiate the CP by gaining the ability to express LTPi. To this end we will exploit our mechanistic understanding of LTPi to block its induction in vivo, and determine the role LTPi plays in the cortical response to monocular deprivation (MD). Further, we will evaluate the relative importance of LTD and LTPi in MD-induced loss of visual responsiveness within L4, and ask whether these two forms of plasticity are induced in a synergistic manner during MD. We will combine in vivo and in vitro electrophysiology, viral-mediated gene transfer, and optogenetics to achieve these goals. These experiments will settle a long-standing debate about the mechanisms underlying amblyopia, and have the potential to radically alter our view of the cellular underpinnings of CP plasticity. )

DESCRIPTION (provided by applicant): One of the deepest and most central problems in neuroscience is how neurons and circuits manage to balance the need for plasticity with the requirements of neuronal and circuit stability. Pioneering work over the past two decades from the investigators on this program project has established that neurons and circuits solve this stability problem by engaging a set of homeostatic plasticity mechanisms that act to stabilize neuronal excitability around an activity set-point. The study of homeostatic plasticity has now blossomed into a large and diverse field, and a substantial body of literature describing the phenomenology of homeostatic regulation of neuronal excitability and synaptic strength now exists. However, despite much progress, key elements of the conceptual framework for understanding homeostatic regulation of neuronal activity remain completely obscure. In this proposal, we wish to address four outstanding questions in the field: 1) How are neuronal activity set-points built, and what are the molecular identities of the set-point components? 2) How are different set-points in different neurons established? 3) What are the downstream targets of the set-point machinery, and how are these organized to effectively coordinate different forms of homeostatic plasticity? And finally 4), how do network relationships influence set-points? This program project will bring together four of the top researchers in the field of homeostatic plasticity in a synergistic enterprise to determine how activity set-points are built. Our premise is that this process is so fundamental to all nervous systems that it is likely that th functional structures underlying it are evolutionarily conserved. Therefore, we now propose a collaborative and synergistic set of experiments that will leverage the advantages of different vertebrate and invertebrate preparations to uncover the molecular identity of the sensors and effectors that regulate homeostasis of activity set-points. The experiments described here will test novel hypotheses about how a truly fundamental problem in neuroscience is solved, and in the long term are likely to result in a set of new strategies for interventions in numerous neurological disorders that result from impaired homeostatic and compensatory mechanisms.

My lab pioneered the study of homeostatic plasticity in neocortical neurons and networks. Recently we found that neocortical neurons both in vitro and in vivo have a firing rate set point (FRSP) around which they regulate their average firing. This is accomplished through a set of homeostatic plasticity mechanisms, including synaptic scaling and homeostatic regulation of intrinsic excitability, that act to restore excitability when it is perturbed by changes in sensory drive or synaptic input. We have made significant inroads into understanding the molecular signaling pathways and machinery that accomplishes this homeostatic regulation of firing; in particular, we know that cell-autonomous changes in average neuronal firing that modulate somatic calcium influx lead to changes in signaling through the calcium-dependent kinase CaMKIV, and this in turn produces transcription-dependent homeostatic alterations in the surface expression of ion channels and glutamate receptors. Despite this progress, it remains largely mysterious how neocortical neurons (or any other cell type) build a stable firing-rate set-point out of calcium-dependent signaling pathways. Here we wish to test the central hypotiiesis that FRSP in neocortical neurons is the equilibrium point of opposing calcium-dependent signaling pathways that regulate excitability, and that this basic principle for how to build a stable FRSP wdll generalize to fundamentally different cell types. We have three major aims: i) test the prediction that FRSP can be modulated by altering the strength of specific calcium-dependent signaling pathways, such as CaMKIV; 2) Identify other elements that act in opposition to CaMKIV and together comprise the FRSP, and 3) determine which homeostatic effectors are downstream of CaMKIV to generate homeostatic compensation. This project wdll illuminate the mechanistic underpinnings of a fundamental aspect of neuronal physiology, and through collaborative work the Other Pis on this Program Project.

This core provides administrative support for the program project in the form of oversight of day to day interactions, data sharing, group meetings, and formulation of an external advisory board. The core PI is Gina Turrigiano, Professor of Biology, Brandeis University.

? DESCRIPTION (provided by applicant): Homeostatic synaptic scaling is an important form of plasticity thought to be essential for maintaining stable function in developing neural circuits. Synaptic scaling scales the strength of all of a neuron's excitatory synaptic strengths up or down in the correct direction to stabilize neuronal firing rates. These homeostatic adjustments in synaptic weights are accomplished in large part through changes in the synaptic accumulation of GluA2-containing AMPAR at synapses, and appear to operate on all excitatory synaptic inputs onto a given neuron in response to changes in the neuron's own firing. Despite great recent interest, the molecular and biophysical mechanisms that enable this homeostatic adjustment of AMPAR abundance during synaptic scaling are still poorly understood, and many of the assumptions underlying this model of synaptic scaling (such as its global nature) remain largely untested. In this proposal we aim to illuminate the mechanisms that lead to enhanced synaptic AMPAR abundance during scaling up, and to test the idea that this form of plasticity acts on all excitatory inputs to stabilize neuronal firing in vivo. This proposal is built around or recent observation that the AMPAR-binding protein GRIP1 is essential for the regulated increase in synaptic AMPAR abundance during scaling up, and that this process requires direct interactions between GRIP1 and GluA2. Here we proposed to determine how (at the biophysical level) this regulated interaction between GluA2-GRIP1 drives an increase in synaptic AMPAR abundance, by using a variety of cutting edge imaging approaches. We will test two alternative models: first, that GRIP1 traffics to synapses along with AMPAR and enhances synaptic capture of the receptor, and second, that GRIP1 enhances synaptic delivery of modified AMPAR that have an enhanced affinity for synaptic scaffolds. Further, we will use the tools we have generated through these in vitro studies to selectively disrupt AMPAR trafficking during synaptic scaling up in vivo, in order to probe the mechanism and function of synaptic scaling within intact neocortical circuits.

Project Summary Behavioral states such as sleep and wake profoundly affect the patterns of activity and neuromodulatory tone within neocortical circuits, but the function of these state changes on learning and experience-dependent plasticity remain controversial. It has been postulated that wake is when Hebbian mechanisms are engaged, while sleep serves to homeostatically ?renormalize? synaptic strengths/firing rates that were perturbed by experience-dependent changes in the waking state. We study the homeostatic plasticity mechanisms that stabilize firing rates and circuit function within primary visual cortex (V1), and can track this process in freely behaving rodents. Perturbing firing through monocular visual deprivation (MD) initially suppresses firing (1- 2d MD), but firing rates then rebound to control levels over a 2 d period despite continued MD. Further, we can perturb firing in the other direction using an MD followed by eye re-opening (ER) paradigm, and observed that firing rates are potentiated by ER and again slowly return to baseline values. This ?firing rate homeostasis? is accomplished in part through synaptic scaling up or down of excitatory synapses onto pyramidal neurons within V1. We can follow this process in freely behaving animals cycling between natural bouts of sleep and wake, to directly determine when the homeostatic restoration of firing occurs. In the last funding period we made the surprising discovery that upward and downward firing rate homeostasis are oppositely regulated by sleep and wake states: upward occurs gradually during each bout of active wake and is suppressed by sleep, while downward is enabled by sleep and suppressed by wake. Our work reveals that sleep and wake states are critically important for gating homeostatic plasticity, and act to segregate upward and downward homeostatic processes into distinct behavioral states. How this is accomplished mechanistically is entirely unknown, as is the function this segregation might serve. Here we propose to determine the features of waking/sleeping states that enable/suppress firing rate homeostasis, and to test whether the underlying synaptic plasticity mechanisms are themselves directly gated by sleep and wake. Finally, to gain insight into the behavioral/functional consequences of this gating, we propose to test how sleep and wake orchestrate Hebbian and homeostatic plasticity within V1 during a vision-dependent learning task. These experiments promise to illuminate fundamental features of visual cortical physiology, and to shed light on the function of sleep and wake states in coordinating synaptic plasticity during learning.

The overall goal of my NS-supported research program is to understand the mechanisms that stabilize the function of central nervous system (CNS) microcircuits during experience- dependent plasticity and learning. Over the past ~20 years of NS support we discovered and characterized several forms of homeostatic plasticity, including synaptic scaling and intrinsic homeostatic plasticity, that are postulated to sense perturbations in mean neuronal activity, then bidirectionally adjust synaptic and cellular properties to keep activity within a set point range. Our recent work has focused on a) identifying the cellular and molecular mechanisms of these homeostatic forms of plasticity in order to bolster our mechanistic and functional understanding, and to generate tools that allow us to selectively block homeostatic plasticity in vivo; and b) to determine what aspect of neuronal activity is under homeostatic control in intact CNS circuits in vivo. We recently showed that the mean firing rates of neocortical pyramidal neurons in freely behaving animals return back to an individual baseline following prolonged perturbations to sensory drive, strongly supporting the idea that neocortical neurons homeostatically regulate their mean firing around an individual 'firing rate set point'. Such a process is theoretically important for preventing circuit hypo- or hyperexcitability during experience-dependent development, as well as to short-circuit the positive feedback nature of Hebbian plasticity rules that can degrade memory fidelity. We now have (or are developing) the tools to disrupt homeostatic plasticity and firing rate set points in vivo, allowing us to assess the impact of this disruption on network function and memory storage. The major goals of my NS- supported research program going forward are: 1) to determine how activity set points are built, and how individual neurons can have set points that are orders of magnitude different from each other; 2) to understand how multiple homeostatic mechanisms cooperate with each other to stabilize network activity in the face of profound perturbations; and 3) to test the role of synaptic scaling and intrinsic homeostatic plasticity in memory encoding and generalization. These studies will have important implications for our understanding of neurological disorders that arise from aberrant circuit excitability (epilepsy, autism-spectrum disorders). They may also provide a new avenue into understanding disorders such as PTSD that are likely to arise from excessive generalization during aversive learning.

PROJECT SUMMARY Primary cilia are sensory organelles that are now known to be present on nearly every neuron type in the mammalian central nervous system. In the developing nervous system, cilia are essential for progenitor proliferation, neuronal migration, and the establishment of synaptic connectivity. However, although cilia are also present on mature neurons, their roles in the postnatal brain are poorly understood. Intriguingly, cilia concentrate neuropeptide and amine receptors, and cilia dysfunction has been linked with multiple neuropsychiatric diseases, suggesting that cilia-dependent neuromodulator signaling may be critical for the maintenance and plasticity of neural circuits. The overall goal of this exploratory R21 is to investigate the role of ciliary signaling in the acute modulation of excitatory synapse formation and function in the postnatal brain. In preliminary experiments, we have found that acute disruption of cilia in postnatal cortical neurons increases excitatory synapse number and strength. Consistent with this finding, spontaneous neuronal firing rates (driven by synaptic input) are also increased indicating a disruption of excitation/inhibition (E/I) balance. Since E/I imbalance contributes to multiple neuropsychiatric and metabolic disorders, our results raise the novel and exciting possibility that ciliary signaling is essential for generating and maintaining correct E/I balance in multiple postnatal circuits. We will take advantage of the complementary expertise of the co-PIs (Sengupta ? cilia biology, Turrigiano ? synaptic physiology) to: Aim 1. Establish a role for ciliary signaling in the regulation of E/I balance and circuit excitability. Aim 2. Explore the mechanisms by which ciliary signaling modulates synaptic properties. Results from this work have the potential to open up new avenues for understanding how correct E/I balance is dynamically maintained in the postnatal brain, and provide insights into how cilia dysfunction contributes to the regulation of mental health.

Project Summary This is a proposal to continue a long-standing postdoctoral training program at Brandeis University. This program integrates intensive quantitative and experimental training in a collaborative environment to produce neuroscientists who go on to successful careers applying these skills to solve the myriad problems posed by disorders of the nervous system. We will take a diverse and talented set of individuals with Ph.D.s in neuroscience, physics, math, computer science, engineering, and biological sciences and give them systematic and integrative training in the use of modern quantitative/analytical approaches bridging levels of analysis to understand nervous system function. We have designed a modular and collaborative training structure that allows trainees with disparate backgrounds to acquire the skills, training and flexibility they need to thrive as modern neuroscientists. Our program comprises a two-year sequence that systematically builds quantitative literacy, fluency with statistical methods and quantitative tools, training in rigor, experimental design and ethics, and grant-writing and communication skills. We intend to recruit 4 Postdoctoral Trainees/year into this 2-year program for a total of 8 slots, which maintains the previous level of support. We will magnify the impact of this training program by encouraging all Neuroscience postdocs, not only those appointed to the training grant, to participate. This will serve the purpose of building a collaborative postdoc community that collectively possesses and disseminates a broad set of quantitative skills, and where horizontal and vertical transmission of these skills will be an important feature of training. The richer this trainee and mentor community is, the more effective the training will be. To this end, we are committed to enhancing the diversity of our training program at all levels, and a set of concrete steps to accomplish this are proposed.