Dale M. Wachowiak - US grants

DESCRIPTION: (From Investigator's Abstract) The olfactory bulb is the first synaptic relay in the olfactory pathway, and its function is critical to the detection and discrimination of odors. Loss of normal olfactory function has been associated with nutritional deficits, among other health concerns. In addition, the olfactory bulb is a promising model for understanding sensory processing and perception by the brain. The proposed research will investigate the patterns of neural activity responsible for encoding odor information in the brain. The specific aims of this research are to understand the importance of coherent, odor-evoked oscillations of olfactory bulb activity in odor discrimination, the neural basis for these oscillations, and the relationship between odor-evoked oscillations and a separate, spatial code for representing odor information at the level of the olfactory bulb. The study will use optical imaging with activity-dependent dyes in order to characterize the temporal and spatial patterns of activity in the intact, in vivo olfactory bulb of the turtle. Experiments are proposed to address the temporal and spatial representation of odors by specific populations of olfactory bulb neurons and to test the role of oscillations in mediating odor discrimination at the behavioral level. This work will help elucidate how the olfactory bulb represents odor information and what parameters of olfactory bulb activity are important for discriminating odors.

DESCRIPTION (provided by applicant): The goal of this project is to understand how odors are represented by populations of olfactory receptor neurons at the level of their input to the olfactory bulb, and to analyze how these representations are modified at the synapse between olfactory receptor neurons and o1factory bulb neurons. While responses of single olfactory receptor neurons to odors have been well characterized, the strategy by which odor information is encoded across the entire population of neurons is poorly understood. A critical step in this process is the convergence of receptor neurons onto glomeruli of the olfactory bulb, where a spatial map of odor sensitivity is generated and the synaptic processing of olfactory information begins. This study will focus on the coding and processing of information about odors at this first, critical level of the olfactory pathway by selectively imaging activity in olfactory receptor neuron terminals. In particular, this study will focus on how different odorants, and different concentrations of the same odorant, are represented in terms of spatial patterns of input to olfactory bulb glomeruli. This study will also characterize a newly-established neuronal pathway for presynaptic inhibition of olfactory receptor neuron input to the olfactory bulb, the functional significance of which has yet to be explored. Specifically, this study will: 1) investigate how odors are represented by receptor neuron input to the olfactory bulb, 2) functionally characterize pathways mediating presynaptic inhibition of receptor neuron input to the olfactory bulb, and 3) investigate the role of this presynaptic inhibition in shaping the representation of odors. Together, these experiments will constitute a significant step towards understanding how odor quality is encoded at a specific level of the olfactory pathway. Understanding the basic functions of the olfactory system can potentially lead to improved diagnosis and treatment of diseases of the nervous system.

DESCRIPTION (provided by applicant): A fundamental step in understanding sensation is understanding how neural circuits in the brain transform patterns of sensory neuron activity into robust and efficient representations of the external world. Sensation is an active process in which the detection and initial encoding of sensory information is dynamically modulated by sampling behavior and behavioral state. Understanding how central circuits process sensory information in the context of active sampling is critical for understanding the neural basis of sensation in the behaving animal. The goals of this project are to understand how neural circuits in the mammalian olfactory bulb transform sensory inputs in vivo and in the context of active odor sampling. We will ask how certain bulb networks - in particular those mediating interactions between glomerular modules (interglomerular circuits) and those mediating inhibition within a glomerulus (intraglomerular circuits) shape the patterns of olfactory bulb output that are transmitted to cortex as a neural code for odor information. We will also ask how active 'sniffing' of odors at high frequency changes the operation of these circuits. We will use an innovative toolbox of genetic, optical and electrophysiological approaches that we have optimized for the in vivo dissection of circuit function, applied primarily in the awake, head-fixed mouse. There are two broad Aims designed to generate an understanding of the how the olfactory bulb network transforms sensory inputs acquired by the behaving animal. The first Aim focuses on interglomerular circuits and will map how these circuits influence mitral cell output from olfactor bulb glomeruli and how they are organized with respect to the glomerular odor map. We will use optogenetic activation of sensory input to genetically-tagged glomeruli expressing different odorant receptors, combined with selective imaging of excitation and inhibition from mitral cells innervating these glomeruli. The second Aim focuses on intraglomerular inhibition and will ask what role this inhibition plays in shaping the input-output functions of mitral cells during natura odor sampling. We will quantitatively compare responses of periglomerular versus mitral cells to optogenetically- and odorant-evoked inputs using imaging in the awake mouse, and perform whole-cell recordings from mitral cells during selective optogenetic suppression of intra- (but not inter-) glomerular inhibition. The overall impact of this project will be to advance a mechanistic understanding of how central circuits transform sensory inputs into the neural patterns of activity that underlie perception and an understanding of how these circuits function during behavior.

The proposed experiments use the mouse olfactory system as a model to understand how neurons restore functional connections to their targets in the central nervous system after massive neuronal loss and subsequent regeneration. The mammalian olfactory system has the remarkable capacity for regeneration and regrowth of its primary sensory neurons (olfactory sensory neurons) after even a complete loss. However, the regrowth of sensory neuron axons to their appropriate targets in the brain - the glomeruli of the olfactory bulb - appears to be imperfect, with many axons contacting inappropriate glomeruli. The proposed experiments will, for the first time, ask how this mistargeting alters the glomerular maps of activity that normally represent odor information. A genetically-encoded indicator of olfactory sensory neuron activation will be used to compare odorant representations in normal and epithelial lesion-recovered mice. In parallel, histological analysis of the retargeting of a single, molecularly-identified receptor type will permit a detailed anatomical analysis of this phenomenon. Experiments are proposed which a) investigate the time-course of regeneration of glomerular activity maps and the precision with which they recover after lesion;b) test whether the precision of sensory neuron retargeting depends on odorant-evoked activation of the neurons;and c) investigate the relationship between the odorant response properties of olfactory sensory neurons and their choice of glomerular target. The experiments should generate the first general picture of the extent to which functional representations of odors are altered after lesion and recovery, and point to mechanisms by which connections between olfactory neurons and their targets are reformed. Errors in the reinnervation of glomeruli are likely mechanisms underlying olfactory dysfunction in humans recovering from olfactory loss due to trauma or infection. These experiments may provide improved cellular-level explanations for such clinical deficits and could suggest therapeutic strategies. More generally, reestablishing appropriate neural connectivity is a prerequisite for the full recovery of function of any sensory or motor system. For example, incorrect connections to the CMSafter recovery of peripheral nerve fibers can cause hyperalgesia and other neuropathies. This work could potentially yield insight into treatment strategies for these and other syndromes related to inappropriate neuronal connections.

DESCRIPTION (provided by applicant): Sensation is an active process involving dynamic interactions of the animal with its environment, which leads to dynamic patterns of neural activity in sensory neurons and in the central processing stages that ultimately underlie perception. The proposed project will investigate sensory system function in the awake animal, using the olfactory system to understand the relationship between the dynamics of stimulus sampling, the temporal structure of sensory inputs to the olfactory system, and how these two factors shape processing in the olfactory bulb - the first stage of information processing in the mammalian olfactory system. Temporally dynamic activity patterns are a prominent feature of many different levels of olfactory processing, and dynamic activity at both slow (100 - 500 msec) and fast (10 - 50 ms) timescales is central to many models of odor coding. Much of the dynamics of odorant-evoked activity - at all of these levels - is structured around the respiratory cycle, which controls the access of odorant to the sensory neurons themselves. However, it remains unclear how the temporal structure of sensory input shapes odor representations and information processing. In addition, in the behaving animal respiration itself is highly dynamic, with animals actively changing the frequency, amplitude, and shape of individual sniffs during odor sampling (i.e., sniffing). Very few studies to date have examined the consequences of these changes in sampling behavior - and in the sniff-driven sensory input - for higher-order olfactory processing. At the same time, recent studies have pointed to the importance of circuitry in the glomerular layer of the olfactory bulb in shaping bulb output via mitral cells. These newly-described circuits have yet to be incorporated into a biophysically-realistic, computational model of olfactory bulb function. This project will use a combination of experimental and computational approaches to address two central questions: 1. How does the glomerular circuitry transform realistic patterns of sensory input into patterns of mitral cell output? 2. What are the consequences of the olfactory bulb transformation for the representation of odorants by mitral cell populations? Intellectual Merit: The project is a joint experimental and computational effort which uses spatiotemporal patterns of sensory neuron activity recorded from awake, behaving rodents as inputs to computational models of the olfactory bulb network. The project will also use electrophysiological and behavioral measures to test model predictions. This approach is unique and significant in that it uses - for the first time - natural sensory inputs to a model of olfactory bulb function, and also because it incorporates - for the first time - a realistic model of the circuitry around the olfactory bulb glomerulus, many features of which have only recently been described. The experiments are designed to provide a picture of how odor information is transformed at the first stage of synaptic processing and how this transformation is actively shaped by the animal's own sampling behavior. Broader Impacts: The proposed work should lead to important insights into how sensation can be actively modulated by sensory acquisition behavior. A potentially important biomedical impact of this work is the development of improved prosthetic devices - for example, artificial limbs which use sensor-driven proprioceptive information to help control movement. Another potential impact is the improved design of sensor devices for detecting analytes in a complex environment. The general concept of using naturalistic sensory information as inputs to model circuits is an idea that could also lead to important breakthroughs in understanding sensory processing in other modalities. Generation of the first circuit model of the recently-described glomerular network in the olfactory bulb will be an important shared resource for others in the field who wish to characterize computations at the first few synapses in the olfactory system.

DESCRIPTION (provided by applicant): Sensory systems process incoming information via multiple, parallel pathways that break the complex representation of the external environment into distinct processing streams. These parallel pathways encode a subset of stimulus features and contribute to specific aspects of a unified sensory percept - for example, motion and form in vision. While well-described and recognized as fundamental to sensory processing in other modalities such as vision, parallel pathways in the olfactory system remain poorly understood. This project will characterize for the first time how outputs from the olfactory bulb - the first synaptic level of processing in the olfactory system - differ in their representation of odor information as a function of their projection target. The project uses recently-developed optical, genetic and molecular tools to enable recording activity from individual olfactory bulb output neurons tagged according to their axonal projection to specific regions of primary olfactory cortex. To accomplish this, genetically-encoded Ca2+ reporter proteins (GCaMP) or light-activated channels (ChR2) will be expressed in olfactory bulb projection neurons using a viral vector driving Cre-recombinase dependent expression of the transgene and a mouse line expressing Cre-recombinase selectively in mitral and tufted cells of the olfactory bulb. The experiments take advantage of the fact - newly established by our laboratory - that these neurons can be infected through their axons, allowing expression in only those neurons projecting to specific areas of olfactory cortex. Odor responses in such target-defined neurons will then be recorded with in vivo imaging or with electrophysiology in both anesthetized and awake mice. All aspects of this methodology have recently been developed and work robustly in our laboratory. Using this approach we will ask how neurons projecting to distinct olfactory cortical areas differ with respect to their odorant- evoked response properties and their degree of modulation in the awake, behaving animal. The experiments will address important questions regarding the nature of olfactory processing streams that emerge from the olfactory bulb: Do projections to different cortical targets carry distinct representations of sensory information - a occurs, for example, in the parallel representations of motion and form in the visual system? If so, what is the nature of these differences in the context of odor coding and odor perception? In addition, do the different classes of olfactory bulb output neurons - classically identified by ther dendritic morphology and somatic location - map to distinct anatomical pathways according to their cortical targets or to distinct functional coding streams according to their odor response properties? Together the proposed experiments should lead to a significant advance in understanding parallel olfactory processing streams in a functional and behavioral context.

PROJECT SUMMARY We seek to better understand how the brain processes olfactory information by focusing on how circuits of the olfactory bulb control two fundamental aspects of sensory processing: the relationship between sensory input and olfactory bulb output as a function of stimulus intensity, and tuning of response specificity by lateral inhibition. Models of how olfactory bulb circuits mediate these operations exist, but generating and testing rigorous predictions from them has been limited by a lack of understanding of how circuits are organized with respect to olfactory bulb glomeruli. Glomeruli represent individual odorant receptors and so constitute the fundamental unit of information processing at this stage. Our strategy is to overcome this gap by, first, better defining the functional map of odor 'space' across glomeruli of the dorsal olfactory bulb. To achieve this we will functionally define glomeruli in terms of the odorants to which they have maximal sensitivity as well as their relative sensitivities across a carefully-selected odorant panel, yielding the first `odor sensitivity' map of the dorsal olfactory bulb and allowing for the rapid and reliable identification of individual glomeruli across animals. We will use these data to uncover new insights into the organization of glomerular odor maps and to generate a public resource for further exploration by the neuroscience community. We will next use this knowledge to rigorously test alternative models for how specific circuits shape the input-output functions of the olfactory bulb by comparing intensity-response functions of sensory inputs and glomerular outputs and by selectively removing particular interneuron types from the olfactory bulb network. We will also test alternative models for how inhibitory connections between different glomeruli are organized and how they may be shaped by odor experience, using odorants that selectively activate different combinations of identified glomeruli. Our experimental strategy builds on innovative approaches that are key to achieving a new level of understanding of how odor representations are transformed by olfactory bulb circuits. First, we are able to repeatedly identify and target many glomeruli across the dorsal olfactory bulb using a small number of diagnostic odorants and to efficiently map responses to many odorants in a single experiment. Second, we are able to selectively image from sensory inputs to glomeruli, projection neuron outputs from glomeruli, or both simultaneously, allowing us to precisely characterize input-output transformations by olfactory circuits. Finally, we will develop an improved model of the olfactory bulb network that is highly constrained by experimental data and which should lead to new insights into how this network alters odor representations and enable new predictions that can be tested with further circuit manipulations or behavioral assays.

PROJECT SUMMARY A fundamental step in understanding sensation is understanding how neural circuits in the brain transform patterns of sensory neuron activity into robust and efficient representations of the external world. Sensation is an active process in which the detection and initial encoding of sensory information is dynamically modulated by sampling behavior. Understanding how central circuits process sensory information in the context of active sampling is critical for understanding the neural basis of sensation in the behaving animal. The goals of this project are to understand how neural circuits in the mammalian olfactory bulb transform sensory inputs in vivo and in the context of active odor sampling, or sniffing. First, we will examine circuit-level determinants of the diverse patterns of excitatory drive onto mitral and tufted cells, focusing on the respective roles of sensory inputs and glomerular circuits in generating this diversity. Second, we will define if and how stimulus features themselves ? in particular, odorant chemistry and intensity ? relate to the dynamics of excitatory drive onto MT cells, as well as how these dynamics are shaped by sniffing behavior. Finally, we will ask how dynamics and glomerular patterns of excitatory input impact the input-output transformation of the olfactory bulb. The proposed experiments implement several innovative approaches to dissecting circuit function in the intact olfactory bulb; these include using second-generation optical reporters of glutamate to directly image excitatory signaling onto mitral/tufted cells with high temporal resolution, dual-color imaging of glutamate and calcium to simultaneously monitor presynaptic inputs and readouts of mitral/tufted cell activity from the same glomerulus, and the use of identified glomeruli with well-characterized response spectra to design powerful tests of model predictions. The overall impact of the project will be to advance a mechanistic understanding of the relationship between the dynamics of odor sampling and the resulting dynamics of neural activity in the olfactory bulb. These findings will pave the way for ultimately understanding the role that neural dynamics and active sampling play in odor perception.