Max L. Fletcher, Ph.D. - US grants

DESCRIPTION (provided by applicant): The long-term goal of this project is to understand how learning can influence olfactory odorant representations at the first level of processing within the olfactory bulb. While the mammalian olfactory system has been shown to have a remarkable capability for undergoing experience-dependent plasticity, how such odor memories are imprinted in the adult olfactory neural circuit remains unclear. Although this process most likely involves changes at multiple stages in the olfactory pathway, one interesting site for plasticity is the olfactory glomerular layer. Within this layer, the anatomical organization of receptor neuron input allows odorant information to be transformed into an odorant-specific topographical map of glomerular activity. This activity pattern can be visualized in vivo using a newly-developed transgenic mouse with a GFP-based calcium indicator (G-CaMP2) expressed exclusively in olfactory bulb output neurons immediately postsynaptic to receptor input. Unlike previous imaging methods, this mouse allows us to observed purely postsynaptic odor maps in the glomerular layer for the first time. Using this mouse model we can directly test the hypothesis that olfactory learning significantly alters olfactory bulb postsynaptic glomerular odorant representations for the trained odorant. This will be accomplished by comparing odorant-evoked glomerular activity patterns in the same animal before and after associative conditioning. Preliminary data suggests that conditioning with a given odorant significantly alters glomerular responses to that odorant following training. Based on this, we plan to extend our findings by testing the hypothesis that these changes will serve to reduce the representational overlap between the trained odorant and similar odorants. Together, these studies will have a significant impact on our understanding of the neural basis of odor coding and role plasticity plays in shaping neural responses to sensory stimuli. PUBLIC HEALTH RELEVANCE: The sense of smell plays an important role in our daily life. Olfaction dysfunction is often times an early indicator of several major neurological diseases in humans including Alzheimer's disease, Parkinson's disease, and schizophrenia. The general goal of this grant is to understand the neural basis of olfactory processing which could help in the diagnosis and treatment of these diseases.

? DESCRIPTION (provided by applicant): The overall goal of this project is to directly test how cholinergic input from the nucleus of the horizontal limb of the diagonal band of Broca (HDB) alters olfactory bulb (OB) glomerular circuits and how this modulation ultimately affects odor coding, perception, and learning. We will use of both wide-field and 2-photon in vivo and in vitro calcium imaging in mice expressing genetically encoded indicators of neuronal activity in defined OB cell types, to test the novel overarching hypothesis that synaptically-released acetylcholine bidirectionally modulates mitral/tufted cell odor responses as a function of the prevailing odor intensity. We further test the hypothesis that this modulation is due to opposing muscarinic and nicotinic receptor actions on inhibitory periglomerular cells that differentially regulate the strength of presynaptic inhibition of olfactory sensory neuron input to the glomeruli. Using imaging and well characterized olfactory-mediated behaviors, we will also investigate how this cholinergic modulation of the glomerular odor representation affects odor perception and learning. Specifically, we test the hypotheses that HDB-evoked acetylcholine release in the OB: (1) enhances olfactory sensitivity, (2) dishabituates odor responses adapted at the peripheral olfactory sensory neuron level and (3) is critical for associative olfactory learning. By investigating receptor type-specific cholinergic modulation in morphologically- and physiologically-distinct neuron types at both the population and single cell levels, the experiments of this proposal will for the first time elucidate how cholinergic modulation of neural odor responses in the OB impacts olfactory perception and learning behaviorally. The overall findings will dramatically advance our understanding of how acetylcholine modulates sensory representation, odor coding, perception and behavior.

? DESCRIPTION (provided by applicant): Ingestive decisions play a key role in a number of human conditions including obesity, diabetes, anorexia, hypertension, and coronary artery disease. One of the most important factors regulating these decisions is the sense of taste. The gustatory cortex in mammals has been shown to be involved in taste learning and behavior, and electrophysiological studies have shown that taste-activated neurons in this area tend to be broadly tuned (with regard to taste quality), multi-modal, and modulated by behavioral state. However, it is not clear how taste quality itself is encoded in the GC, and whether stimulus tuning in individual neurons varies as a function of cell type or functional connection. Two-photon (2P) imaging is a technique that allows for the simultaneous recording of taste-evoked responses in a large numbers of neurons in vivo. However, only a couple of 2P studies in the taste system have been published to date. We will combine 2P imaging with specific identification of cell types, accomplished by either Cre-dependent expression or retrograde labeling. Collectively, these experiments will test the hypothesis that taste sensitivities and tuning profile vary in GC neurons as a function of cortical depth, cell type (pyramidal cell vs. interneuron) or projection pattern. Aim 1 will investigate compare taste responses in Thy1-expressing pyramidal cells and Calretinin-expressing interneurons to the greater population of all labeled cells in GC. Aim 2 will investigate taste responses in GC neurons retrogradely labeled from either gustatory thalamus or basolateral amygdala.

Project Summary Ingestive decisions play a key role in a number of human conditions including obesity, diabetes, anorexia, hypertension, and coronary artery disease. One of the most important factors regulating these decisions is the sense of taste. The gustatory cortex (GC) in mammals has been shown to be involved in taste learning and behavior, and electrophysiological and imaging studies suggest taste quality information is encoded in the activity of both specific and broadly responsive taste-activated neurons. However, it is not clear how taste quality is spatially organized across this brain area. We will use two-photon (2P) imaging to systematically map taste quality across the breadth of GC, evaluating the hypothesis that it has an overlapping organization, with likely overrepresentation of particular tastes at the anterior and posterior extremes. We will also combine 2P imaging with specific identification of cell types, accomplished by either Cre-dependent expression or retrograde labeling. Finally we will evaluate taste responses in GC following taste learning. Collectively, these experiments will shed light on how tastes are organized in this key brain area at a cellular level, and how they are modified with behavior. Aim 1 will systematically map taste responses along the anterior-posterior axis, and also with respect to dorsal-ventral location and depth. Aim 2 will compare taste responses in Thy1- expressing pyramidal cells and GAD-expressing interneurons to the greater population of all labeled cells in GC. Taste responses will also be examined in cells that project to contralateral cortex, or to amygdala. Aim 3 will investigate taste responses in GC neurons following taste aversion learning and extinction.