Kevin Franks - US grants

DESCRIPTION (provided by applicant): Olfactory information is encoded in a combinatorial fashion by olfactory bulb glomeruli, which individually represent distinct chemical features of different odorants. This information is transmitted to the piriform cortex, where it is presumed to form combinations of these discrete signals, and leads to an odor percept. The focus of this proposal is to understand how cells in piriform cortex integrate information from olfactory bulb. I will attempt to gain fundamental insights into how olfactory information is represented in the brain by asking specific questions about the integrative properties of piriform cortex neurons. The experiments proposed here will determine the number of glomerular inputs onto single cells in piriform cortex (Aim 1). I will determine how many glomeruli are required to activate a single piriform cortex neuron and how combinations of coactive inputs from different glomeruli are integrated in piriform cortex neurons (Aim 2). I will also determine how higher brain areas affect the integration properties of piriform cortex neurons. I will specifically ask whether inputs onto piriform cortex neurons from orbitofrontal cortex alter the impact of sensory inputs from the bulb (Aim 3). These experiments will require the use of new techniques that will be developed during the mentored phase and used extensively during both the mentored and independent phases of the award. These data will shed light on the anatomical and physiological strategies employed by central neural circuits to process sensory stimuli. Such information is essential to understanding both the normal and pathological states of the central nervous system. The mentored phase of this award will take place in the laboratories of Dr. Richard Axel and Dr. Steven Siegelbaum at Columbia University. Dr. Axel has extensively studied the molecular, cellular and circuit-level basis for olfaction. Dr. Siegelbaum has made key discoveries illustrating how regulation and expression of ion channels affect neural circuits and alter behavior. Both Dr. Axel and Dr. Siegelbaum have distinguished track records in mentoring fellows through the transition to independence. RELEVANCE (See instructions): The central olfactory system is one of the most tractable brain circuits, permitting fundamental insights into how the brain processes sensory information. A clear understanding of these processes is crucial for an understanding of brain function in health and disease. For example, problems with odor perception are early indicators for serious neurological diseases, including Parkinson's and Alzheimers'Disease.

The ability to identify an odor is crucial for avoiding predators, obtaining food or finding a mate. The concerted activity of distributed ensembles of neurons in piriform cortex (aPCx) is thought to lead directly to the formation of an odor percept however the coding strategies used to represent different features of an odor stimulus, like odor identity and odor intensity, remain poorly understood. aPCx consists of a heterogeneous population of neurons that likely serve distinct roles in shaping odor representations and/or conveying this information to downstream target areas. Little is known about how they actually do so as most investigations into cortical odor coding were agnostic to cell type or target projection. Moreover, most previous studies were performed in anesthetized animals, where both the cells' functions and consequent cortical representations may be dra- matically altered. The objective here is to understand how different features of an odor stimulus are repre- sented in diverse populations of aPCx neurons, and elucidate the underlying neural circuit mechanisms that shape these representations. The approach used will be to record odor-evoked spiking in large ensembles of functionally identified aPCx neurons in awake mice. The central hypothesis is that independent coding strate- gies are used to represent odor identity and odor intensity, and that different types of aPCx cells play distinct roles in shaping these representations. The rationale for these studies is that determining how odors are repre- sented in aPCx of an awake, behaving animal is crucial for understanding olfactory system function. To this end the following three aims are proposed: Aim 1: To determine how piriform cortex simultaneously rep- resents odor identity and odor intensity. Preliminary studies suggest that odor identity is encoded in distrib- uted ensembles of aPCx neurons, and these ensembles are largely concentration-invariant (i.e. a spatial iden- tity code); and odor intensity is encoded by the synchrony of the ensembles activity (i.e. a temporal intensity code). Aim 2: To determine odor responses in diverse subtypes of aPCx neurons, defined genetically, by laminar organization or by distinct projection targets. In vivo optogenetic tagging will be used to identify responses in defined subtypes of neurons. Odor responses in different subtypes of neurons are predicted to reflect their specific roles in shaping the ensemble or the feature of the odor stimulus most relevant for different target regions. Aim 3: To determine how recurrent circuitry shapes cortical odor representations. Output from principal neurons will be unilaterally blocked, permitting simultaneous bilateral recordings of control and ?feedforward? circuits. Preliminary data suggest recurrent circuits dramatically shape the size, timecourse and gain of odor responses in aPCx. This proposal is innovative because it deploys novel tools and methods to re- cord odor-evoked activity in large populations of neurons in awake animals and ascribe distinct response prop- erties to identified cortical circuit elements. This contribution is significant because it will provide deep insight into how odors are represented in piriform cortex through understanding underlying circuit mechanisms.

Project Summary The enormous diversity of neural cell types is a defining characteristic of the brain. Different neural circuits consist of a myriad of distinct cell types, each with specific intrinsic properties and patterns of synaptic connectivity, which transform neural input and convey this information to downstream targets. However, despite their fundamental importance in neural processing, our understanding of how individual cell types differentially contribute to neural circuit function and computation remains poor. Here, the investigators leverage a highly tractable neural circuit, the mouse olfactory (piriform, PCx) cortex, to determine how information about odor stimuli is encoded, transformed, and conveyed to its different downstream target areas. The objective of this proposal is to register diverse odor responses observed in PCx neurons onto identified neural cell types, defined by their morphology, intrinsic properties, and connectivity. This will be achieved via a collaborative, multidisciplinary, iterative computational-experimental approach, involving computational modeling, in vivo two­ photon imaging, in vitro electrophysiology, behavior, chemogenetics and decoding analyses. The investigators' working hypothesis is that different features of an odor - its identity, intensity, and valence - are selectively extracted and encoded by distinct subsets of PCx neurons by virtue of their different intrinsic and local circuit properties, and then selectively transmitted to different target areas. In the two aims proposed, the investigators will image activity evoked by different odorants at multiple concentrations in subpopulations of PCx neurons in awake, behaving mice. They will compare their imaging data with simulated odor-evoked activity in a computational model in which they incorporate the specific intrinsic properties and patterns of local synaptic connectivity of these subpopulations of PCx neurons. In Aim1, the investigators will image and model odor responses in two morphologically distinct subtypes of principal neurons, semilunar cells and superficial pyramidal cells. In Aim 2 they use a similar approach, but with subpopulations of PCx neurons defined by their specific projection targets. Mice will be performing a go/no go odor discrimination task during imaging, allowing characterization of responses to odors with different identities, concentrations or valence. This experimental-computational approach will determine the extent to which the distinct intrinsic properties and specific connectivity patterns of different cell-types accounts for differences in their odor responses. Crucially, mismatches between modeling and experimental results will reveal additional properties of these cells and circuitry that may determine their odor responses, which can and will be tested experimentally. Achieving the goals of this proposal will therefore provide a coherent framework for understanding how different features of an odor stimulus can be selectively extracted, encoded and conveyed to appropriate downstream targets.

Summary (Project 2: How olfactory information is transformed from bulb to cortex) Understanding how sensation is transformed into perception is a central challenge for neuroscience. Sensory information is detected by receptors that interact with the external world. This information is then routed, through multiple stages of processing, to cortical sensory areas, where perception first emerges. In this project we take advantage of the relatively ?shallow? organization of the rodent olfactory system to determine how odor infor- mation is directly transformed from olfactory bulb to piriform cortex; essentially, examining the transformation of sensory representations of the olfactory input in the bulb to cortical representations of the olfactory percept within a single stage of processing. To do this, we will combine the technical expertise of the Franks Lab (Duke Uni- versity) at recording and analyzing activity of large populations of piriform neurons, with the expertise of Rinberg Lab (New York University) at patterned optogenetic activation of olfactory bulb glomeruli. Combining our efforts provides us the unique opportunity to directly activate defined areas of olfactory bulb (i.e. Input) while recording from populations of neurons in piriform cortex (i.e. Output). This provides a unique opportunity to directly and systematically determine how olfactory information is transformed from an elemental sensory representation in bulb into a holistic perceptual representation in cortex. We will then use a variety of molecular genetic tools to selectively disrupt specific components of this circuit, which will reveal the distinct cellular operations that each component plays in implementing this transformation. This project will therefore provide deep mechanistic insight into how the brain transforms olfactory sensory information into cortical odor percepts. Thus, we are confident that we will reveal the logic of the transformation of olfactory information from bulb to cortex However, the logic and implementation of the bulb-to-piriform transformation is also likely to be instantiated at multiple other sites throughout the brain, and often at sites that are deeply embedded within the brain, such as the hippocampus, where access to both the Input and Output are more challenging, and so operations performed at these deeper areas are more difficult to interpret. We are therefore optimistic that lessons learned while probing our olfactory circuit will generate generalizable principles that will also apply to multiple other, similarly organized systems.