Nathan N. Urban, Ph.D. - US grants

The long term goal of this project is to better understand the mechanisms of early olfactory coding in the mammalian brain, in particular to address questions about how odor-evoked activity from different populations of olfactory receptor neurons. Is integrated. Mitral and tufted (M/T) cells are the principle input and output neurons of the main olfactory bulb, a structure which is known to play an important role in the separation and identification of odors. M/T cells do not synapse on each other, but are connected by reciprocal dendroendritic synapses that they make with inhibitory granule cells. The granule cell-mediated maternal inhibition of mitral cells is thought to be an important mechanism for integrating patterns of odor-evoked activity. Long-ranger lateral inhibition requires the activation of distal regions of mitral cell dendrites, but little is known about the mechanisms underlying action potential propagation or transmitter release in these structures. The aims of this project are to measure directly the propagation of activity in mitral cell maternal dendrites and to understand the mechanisms responsible for the activity dependence of propagation that has been observed. The role of synaptic input in regulating action potential propagation also will be examined in order to determine how the pattern of lateral inhibition exerted by an active mitral cell might be modified by the ongoing activity of other mitral cells in the bulb. The results of these experiments will provide insights into the neural basis of sensory coding and into how deficits sensory coding might arise from damage to this system.

DESCRIPTION (provided by applicant): The overall goal of this project is to determine how the output of the main olfactory bulb regulated by is the local circuitry of this structure. In particular, this proposal focuses on studying lateral inhibition, a ubiquitous feature of neuronal circuits thought to play a central role in determining the specificity of stimulus-evoked responses and in regulating network dynamics in many brain regions, including the olfactory bulb. Gaining a better understanding of functional properties of neural circuitry such as lateral inhibition is of central importance to many areas of neuroscience and the olfactory bulb presents an interesting and tractable model system in which to study fundamental questions about the regulation of neural activity by circuitry. These experiments are designed to address the hypothesis that one function of inhibitory circuitry connecting principal neurons associated with different glomerular modules in the olfactory bulb is to mediate a functional lateral inhibition. Specifically we use paired recordings from mitral cells to study the properties of synaptic circuits making functional connections between mitral cells associated with different glomeruli. Using this approach we will test whether lateral inhibition between pairs of individual mitral cells occurs when mitral cell activity is similar in frequency to what has been reported from in vivo recordings made during odor stimulation. We also will investigate short- term plasticity of lateral inhibitory connections, and study the interaction of recurrent and lateral inhibition. In conjunction these results will provide important insights into the role played by inhibitory circuits in shaping sensory representations, especially in the olfactory bulb and in understanding how abnormalities of inhibition can lead to functional deficits in the olfactory bulb and other brain networks.

[unreadable] DESCRIPTION (provided by applicant): The long-term goal of this project is to understand how the properties of dendrites and circuits influence the integration of pheromone-evoked activity in the mammalian accessory olfactory system. Integration of synaptic inputs arriving at thousands of sites across the dendritic tree is a basic mechanism that influences the functional properties of all neuronal types. In particular types of neurons the mechanisms of synaptic integration are thought to be highly specialized to process certain types of inputs, for example to detect short time delays in the auditory system. The dendritic trees of accessory olfactory bulb (AOB) mitral cells have a complicated and highly specialized morphology. These cells seem to be designed to sample inputs arriving at multiple locations (glomeruli), and then to convey some information about this activity to the mitral cell soma from which it can be passed on to higher brain areas. We propose to use physiological and computational techniques to improve our understanding of synaptic integration in the cells and circuits of the accessory olfactory bulb. In particular we will examine the influence of active and passive properties of AOB mitral cell dendrites on the propagation of synaptic activity from the tufts to the somata of these cells. We also will examine how inputs to multiple tufts are integrated, and determine how activity at one tuft is influenced by signals arriving at other tufts. Overall the experiments proposed will provide increased understanding of many basic issues of synaptic and dendritic processing in neurons, and in particular they will provide valuable information on the integration of chemosensory information in the accessory olfactory system. [unreadable] [unreadable]

[unreadable] DESCRIPTION (provided by applicant): Neurons convey messages more effectively when firing together. Even modest increases in synchronization result in large changes in firing rate for downstream neurons. Synchronous activity, especially oscillatory synchrony, is observed in many brain networks and is thought to play an important role in representation of stimuli, propagation of activity and computation. Alterations of synchrony, especially in the gamma frequency range, have recently been implicated in the etiology in a number of cognitive disorders, most notably schizophrenia. However, linking the alterations in synchrony with the alterations in synaptic and cellular properties has proven difficult. Here we describe experiments designed to examine a novel mechanism for the generation of synchronous oscillations which we call stochastic synchrony. Recently, substantial interest has arisen in theoretical work describing synchronization of oscillating neurons by aperiodic, partially correlated, noisy inputs. We have shown experimentally that such inputs can generate oscillatory synchrony in uncoupled neurons (olfactory bulb mitral cells) and have proposed that this mechanism may account for the development of fast (gamma frequency) synchronous oscillations in the olfactory bulb. Here we propose further theoretical and experimental investigations of noise-induced synchronization in neurons more generally. Specifically, we propose to analyze the dependence of noise-induced synchronization on properties of the noisy inputs and on the dynamics of the oscillators. By combined experimental and theoretical investigation, we will determine which channel types are most critical for the development of synchrony by this mechanism. We also propose to study the interaction of noise-induced and connectivity-induced synchronization, as in many cases these two phenomena are likely to both be involved in generating patterns of synchronous activity across brain networks. By exploring this novel mechanism of gamma oscillations we hope to better understand how alteration of cellular and circuit-level properties can interfere with the development of normal gamma oscillations. Such work will have importance for understanding disorders such as schizophrenia, which are associated with altered gamma activity. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): The olfactory system is a model system for the study of many aspects of neuronal circuits and function. A central question that arises in many sensory systems, including the olfactory system is the extent to which patterns of spiking (as opposed to just the rate of spiking) are involved in the coding of sensory information. In recent years, considerable attention has been paid to this issue in the olfactory system. The work we describe in this proposal investigates cellular and circuit-level mechanisms that may make such temporal coding possible, focusing on the role of granule cell-mediated inhibition in regulating mitral cell firing. Despite years of work on mechanisms and function of spike time reliability and precision in many brain areas, little or no work has focused on the role of inhibition in generating accurate and reliable spike times. Clearly, temporal coding of spiking in excitatory neurons will require cooperation of interneurons and the nature of this cooperation is the focus of the experiments that we propose here. By imaging large populations of olfactory bulb granule cells in vitro we have shown that glomerular stimulation results in long latency, prolonged firing of granule cells, indicating that granule cell activity evolves over periods of seconds following a single stimulus. This long-lasting inhibition will be critical for the evolution of neuronal activity in the olfactory bulb. Understanding the mechanisms by which it is generated and regulated will be essential to understanding the transformations of odor-evoked activity in the bulb. Thus, we propose experiments to evaluate the hypothesis that long latency, granule cell mediated inhibition regulates the timing of mitral cell activity for hundreds of milliseconds following mitral cell activation. PUBLIC HEALTH RELEVANCE: Despite years of work, scientists still do not understand the nature of the signals sent from one neuron to another and what aspects of neural activity are part of the signal, vs. part of the noise. Experiments in this proposal investigate some features of the temporal patterns of neuronal activity in the olfactory bulb and seek to determine the cellular and circuit level mechanisms that generate these firing patterns. Such studies are critical for understanding which aspects of neuronal activity are involved in signaling from one neuron to the next.

The PIs and Co-PIs of grants supported through the NSF-NIH Collaborative Research in Computational Neuroscience (CRCNS) program meet annually. This will be the fifth meeting of CRCNS investigators. The meeting brings together a broad spectrum of computational neuroscience researchers supported by the program, and includes poster presentations, talks and plenary lectures. The meeting is scheduled for June 7-9, 2009 and will be held at Carnegie Mellon University and the University of Pittsburgh.

DESCRIPTION (provided by applicant): We are interested in how high frequency (30+ Hz) brains oscillations are generated using the mouse olfactory bulb as our model system. These oscillations are a prominent feature of neural activity in many brain areas including the olfactory system. The mechanisms that underlie these oscillations, as well as the functional role they play are poorly understood, but they have recently been implicated in many aspects of brain function and disease. In the previous funding period, we have described a novel mechanism, called stochastic synchrony, by which correlated fast fluctuating inputs can generate synchronous gamma-frequency (30-80Hz) oscillations in populations of neurons. While much of our previous work used olfactory bulb neurons as a model system for analysis of this novel mechanism of synchronization, this prior work ignored many details of olfactory bulb neurons and circuits in order to describe the general features of this phenomenon. In this application we propose to extend our previous work by considering how key features of olfactory bulb circuitry and physiology modulate stochastic synchrony. We are particularly interested in whether the observed heterogeneity of neural properties is a useful feature of neural networks or is a bug that results from the intrinsic imprecision of biological systems. Homogeneity should enhance synchrony but recent data suggests that heterogeneity across neurons of the same type may provide certain functional advantages. We are also interested in how the synaptic connectivity of the olfactory bulb may facilitate or disrupt synchrony. Exploring these mechanisms will improve our understanding of the function and disorders of synchrony, especially in the context of the vertebrate olfactory system

What make a neuron become active? This question, central to our understanding of brain processes, is both a biophysical question about underlying biological mechanisms, and a statistical question about the features of incoming stimuli to which a neuron responds. The overall goal of this project is to forge a link between the biological mechanisms of neuronal activity and the computational process by which neurons encode features of incoming stimuli. More specifically, this project seeks to understand the biological underpinnings of stimulus coding by neurons.

Dynamical models of neurons that incorporate detailed information about the ion channels that these cells express provide a detailed, biophysical account of neuronal activity. These kinds of models have been used widely and can incorporate and constrain an impressive amount of biological detail. Unfortunately they provide little insight into the meaning of neuronal activity or into the kinds of computations and transformations of stimuli that neurons are performing. On the other hand, models derived from statistical approaches are able to capture the often-noisy and complex relationships between neural activity and the stimuli that a neuron receives. These models provide insight into how specific features of incoming stimuli are extracted and combined by populations of neurons.

These approaches will be combined through collaboration of a team at Carnegie Mellon University (Urban and Kass) and one at Bar-Ilan University (Korngreen) with expertise in the application of statistical and biophysical models to single neuron data. The work will focus on two neuron types that have several important features in common. Olfactory bulb mitral cells and layer 5 neocortical pyramidal cells are two classes of large neurons that receive distinct sources of input inputs onto different divisions of their elaborate dendritic trees. To forge this connection between dynamic and statistical models, this project will develop detailed biophysical models using recently described methods and extend the framework of current statistical models to allow the interpretation of the functional consequences of ion channels and their localization on specific classes of inputs. Applying these improved methods, and examining the consequences of changing biophysical properties on the ability of neurons to robustly and effectively represent stimuli will generate a novel account of the linkage because biological mechanisms and single neuron computation. A companion project is being funded by the Israel Binational Science Foundation (BSF).

This project was developed at an NSF Ideas Lab on "Cracking the Olfactory Code" and is jointly funded by the Physics of Living Systems program in the Physics Division, the Mathematical Biology program in the Division of Mathematical Sciences, the Chemistry of Life Processes program in the Chemistry Division, and the Neural Systems Cluster in the Division of Integrative Organismal Systems. The project is a synergistic combination of laboratory experiments and computer modeling that will lead to better understanding of how animals use the sense of smell to navigate in the real world. Almost universally, from flies to mice to dogs, animals use odors to find critical resources, such as food, shelter, and mates. To date, no engineered device can replicate this function and understanding the code used by the brain will lead to many novel applications. Cracking codes, from neural codes to the Enigma code of WWII, is aided by a deep understanding of the content of messages that are being transmitted and how they will be used by their intended receivers. To crack the olfactory code, the team will focus on how odors move in landscapes, how animals extract spatial and temporal cues from odor landscapes, and how they use movement for enhancing these cues while progressing towards their targets. The proposed work encompasses physical measurement of odor plumes, behavioral measurement of animals' paths through olfactory environments, electrophysiological and optical measurement of neural activity during olfactory navigation, perturbations of the environment via virtual reality and of neuronal hardware via genetics, and multilevel mathematical modeling. The PIs will teach and work with undergraduate, graduate and postdoctoral students and especially recruit students from underrepresented groups in science. The project's results may lead to improved methods for the detection of explosives, new olfactory robots to replace trained animals, and new theoretically-grounded advances in robotic control. The project will inform the development of technologies that interfere with the ability of flying insects (including disease vectors and crop pests) to locate their odor target, thus opening a new door for developing 'green' technologies to solve problems that are of global economic and humanitarian importance.

This proposal is a synergistic combination of laboratory experiments and computational modeling that will probe how animals use olfaction to navigate in their environment. Specifically, this effort seeks to solve the difficult problem of olfactory navigation through the following aims: (i) Generate and quantify standardized, naturalistic odor environments that can be used to perform empirical and theoretical tests of navigation strategies; (ii) Determine phenomenological algorithms for odor-guided navigation through behavioral experiments in diverse animal species; (iii) Determine how odor cues for navigation are encoded and used in the nervous system by recording neuronal data and simulating putative neural circuits that implement these processes; (iv) Manipulate olfactory environments and neural circuitry, to evaluate model robustness. In contrast to previous attempts to understand olfactory navigation, the present strategy emphasizes mechanisms that are biologically feasible and explores the wide range of temporal and spatial scales in which animals successfully navigate. The project will generate datasets of immediate use and importance to scientists in theoretical biology and mathematics, engineering (fluid mechanics, electronic olfaction, and robotics) and biology (neuroscience, ecology and evolution).

The olfactory system can perform remarkable tasks in the identification and detection of odors. The circuits that perform these tasks are poorly understood. Recent work has demonstrated important differences in in vivo odor-evoked responses between two types of projection neurons in the olfactory bulb, the mitral and tufted cells. Evidence suggests that differences in the connectivity and local circuit properties of mitral and tufted cells likely play a key role in generating differential stimulus selectivity and response patterning that may be key for olfactory system function. Here we analyze circuit-level differences between olfactory bulb projection neuron types using electrophysiological and optogenetic approaches. We specifically propose the central hypothesis that tufted and mitral cell populations associated with different glomeruli represent parallel, weakly interacting and functionally complementary pathways of olfactory bulb output. We describe experiments and analysis to test specific elements of this hypothesis by examining the local circuit mechanisms that underlie the differences in odor- evoked responses between tufted and mitral cells. We are especially interested in differences related to the inhibitory circuitry of the olfactory bulb. We will test this central hypothesis by performing experiments designed to assess the strength, sources and plasticity of inhibitory connections made onto mitral and tufted cells. In Aim 1 we will determine how lateral inhibition differentially affects spike timing in mitral and tufted cells. We predict that tufted cells will be less strongly inhibited by activation of neighboring glomeruli than will mitral cells. In Aim 2 we will determine the different sources of mitral and tufted cell input. A critical question for understanding olfactory bulb circuitry is whether inhibition onto mitral and tufted cells arises from the same or different cell types. In Aim 3 we will examine differences in experience-dependent plasticity of synapses to mitral and tufted cells. New data that we have collected represents the first example of plasticity in these local circuits. We propose to examine the mechanisms and loci of this plasticity.

Project Summary/Abstract The olfactory system can perform remarkable tasks in the identification and detection of odors. The circuits that perform these tasks and how they are influenced by experience are poorly understood. Our recent work has demonstrated that early prenatal experience results in changes in odor-evoked responses, circuit connectivity and lateral inhibition. Here we propose to analyze experience-dependent circuit-level changes in olfactory bulb lateral inhibition using electrophysiological and optogenetic approaches. Specifically, we will perform experiments and analysis designed to address the following three specific Aims. Aim 1) To determine if odor conditioning alters the intrinsic properties of olfactory bulb neurons. In many parts of the brain, one consequence of increased excitatory input is reduced excitability of principle neurons. We will test whether the excitability of mitral and/or tufted cells as well as particular interneuron types differs in animals subject to long-term odor exposure. Aim 2) To determine how odor conditioning changes synaptic properties in olfactory bulb circuits. Our preliminary data indicate that exposure to an M72 ligand increases the strength of lateral inhibition onto tufted, but not mitral cells following activation of the M72 glomerulus. We will evaluate this plasticity by determining the circuit mechanisms of plastic change. Specifically we will determine which synapses or cells are responsible for these enhanced responses. Aim 3: To determine how odor conditioning affects odor-evoked responses. Our published and preliminary data show that long term exposure causes a broad increase in mitral cell odor evoked responses (measured by in vivo 2-P calcium imaging in anesthetized animals). In this Aim we propose to expand on this observation by examining odor-evoked responses in tufted cells and specifically in mitral and tufted cells identified as receiving input in glomeruli that respond to the conditioned odor.