Quentin Gaudry - US grants

The early olfactory system, one of the best-characterized sensory systems, is well understood for its connectivity and how it represents odors. However, how odor representation is affected by endogenous neuromodulation remains controversial and elusive. In recent years, neuromodulation in olfactory circuits has garnered great attention in regards to the targets of modulatory cells within these circuits. To date, the manner in which olfactory circuits control the activity of modulatory neurons, and thus modulator release, has been entirely unexplored. Thus, no models presently exist demonstrating the natural pattern of modulator release in olfactory structures, rendering it impossible to study neuromodulation is a natural manner. Furthermore many modulatory neurons, including those involved in olfaction, are also known to possess co- neurotransmitters. The role of such co-transmission remains elusive. This proposal will address these gaps by focusing on the Drosophila antennal lobe, which is analogous to the olfactory bulb, but comprising far fewer neurons than its mammalian counterpart. We will use patch clamp physiology and develop novel approaches to map out the wiring diagram between serotonergic neurons and neurons of the first olfactory relay. Importantly, we will emphasize the feedback connections made from olfactory neurons back onto modulatory cells. Such connections shape activity in modulatory cells and may regulate modulator concentration locally in olfactory circuits. Additionally, we are screening RNAi collections to isolate tools to knock down the expression of individual neurotransmitters in serotonergic neurons that possess multiple transmitters. These RNAi tools will first be combined with optical imaging to determine how each transmitter influences olfactory coding in the antennal lobe. Next, we will use these reagents to alter synaptic function in flies performing an olfactory discrimination task to link sensory coding and perception.

Project Summary. Sensory networks continuously fine-tune how they process information to meet ongoing physiological demands. The nervous system achieves this flexibility via the release of ?neuromodulators? which alter the biophysical and synaptic properties of individual neuron classes within a network. This adjusts the influence of each class to optimize network dynamics for the appropriate context. Neuromodulation is ubiquitous and many neurological disorders result from, or are associated with, dysfunctional neuromodulatory systems. Despite the importance of neuromodulation for healthy sensory processing, our ability to predict the consequences of neuromodulation is limited by the diversity of modulatory receptors expressed by different classes of neurons. Each receptor has different effects and each class of neuron supports different features of sensory coding, so the effects of neuromodulation can be complex. We propose to address this issue in a genetically tractable model with fewer neurons and modulatory receptors; the olfactory system of Drosophila. The objective of this application is to determine how serotonin (5-HT) receptor subtypes affect key neuronal classes and the consequences for olfactory processing and odor-guided behavior. The long-term goal of this research is to determine the mechanistic basis for neuromodulation of sensory network dynamics. In vertebrate and invertebrate olfactory systems, the effects of 5-HT on odor-evoked responses vary across different neuron classes. However, it is difficult to determine how 5-HT alters olfactory processing without knowing the consequences of activating the 5-HT receptors expressed by each class. We recently completed a comprehensive atlas of 5-HT receptor expression within the olfactory system, so we now propose to manipulate the expression of individual 5-HT receptors in specific classes of neurons to determine how 5-HT affects individual neuron classes, the consequences for odor coding across olfactory brain regions and odor- guided behavior. In addition, we will use one of the first whole brain, nanometer resolution EM connectomes to establish single cell resolution connectivity rules of 5-HT neurons with each olfactory neuron class examined in this proposal. In Specific Aim 1 we will determine the receptor basis for the effects of 5-HT on local inhibitory networks within the first olfactory neuropil (the antennal lobe or ?AL?). In Aim 2 we will determine the contribution of direct modulation of cholinergic AL output neurons to the overall effects of 5-HT on olfactory information sent to downstream processing stages. Finally, in Aim 3 we will determine how 5-HT receptors modulate GABAergic AL output neurons that promote olfactory attraction. These experiments will establish how the overall effects of 5-HT emerge from neuron class-specific expression of 5-HT receptors, thus addressing a critical gap in our knowledge of healthy sensory processing.

This proposal examines the contribution of receptor trafficking in inhibitory interneurons for establishing glomerulus-specific neuromodulation during olfactory. Current models of neuromodulation in olfaction emphasize the activation of specific classes of neurons that express a specific and dedicated class of neuromodulatory receptor, for example a single serotonin receptor. In such models, local interneurons in the first stages of olfactory processing may differentially innervate distinct olfactory glomeruli depending on the modulatory receptors that they express. Here, we will challenge this view and develop tools to demonstrate that odor specific modulation may occur not solely by targeting distinct cell classes, but also by the dendrite specific modulation of local interneurons that span multiple olfactory glomeruli. Specifically, we will develop tools to tag and track each serotonin receptor type in identified classes of local interneurons in the Drosophila antennal lobe, the first olfactory relay. Additionally, we will tag and label serotonin receptors in projection neurons that span multiple olfactory neuropil to determine how serotonin receptors are trafficked across olfactory neuropil. The Drosophila antennal lobe is an excellent model system for addressing these issues as its organization is highly similar to the mammalian olfactory bulb, and the smaller number of cells in the antennal lobe render it easier to study and characterize. Receptor tagging will be achieved be editing the fly genome to express modified serotonin receptors that are fused with the 11th fragment of the green fluorescent protein molecule, GFP. A genetic binary expression system can then be used to express the remaining portion of the split-GFP molecule to selectively label serotonin receptors in a specific cell class or individual neuron. We combine these tools with expansion microcopy to examine the co-localization of serotonin receptors with presynaptic and postsynaptic release sites within local interneuron and projection neurons populations. Finally, we will generate a version of these tools using a split mCherry fluorescent molecule approach so that the co-localization of serotonin receptor classes can be examined when local interneurons express multiple classes of these receptors. These experiments will lay the anatomical ground work for subsequent studies examining functional branch-specific neuromodulation of wide-field local interneurons in early olfactory processing.

Project Summary. Sensory networks continuously fine-tune how they process information to meet ongoing physiological demands. The nervous system achieves this flexibility via the release of ?neuromodulators? which alter the biophysical and synaptic properties of individual neuron classes within a network. This adjusts the influence of each class to optimize network dynamics for the appropriate context. Neuromodulation is ubiquitous and many neurological disorders result from, or are associated with, dysfunctional neuromodulatory systems. Despite the importance of neuromodulation for healthy sensory processing, our ability to predict the consequences of neuromodulation is limited by the diversity of modulatory receptors expressed by different classes of neurons. Each receptor has different effects and each class of neuron supports different features of sensory coding, so the effects of neuromodulation can be complex. We propose to address this issue in a genetically tractable model with fewer neurons and modulatory receptors; the olfactory system of Drosophila. The objective of this application is to determine how serotonin (5-HT) receptor subtypes affect key neuronal classes and the consequences for olfactory processing and odor-guided behavior. The long-term goal of this research is to determine the mechanistic basis for neuromodulation of sensory network dynamics. In vertebrate and invertebrate olfactory systems, the effects of 5-HT on odor-evoked responses vary across different neuron classes. However, it is difficult to determine how 5-HT alters olfactory processing without knowing the consequences of activating the 5-HT receptors expressed by each class. We recently completed a comprehensive atlas of 5-HT receptor expression within the olfactory system, so we now propose to manipulate the expression of individual 5-HT receptors in specific classes of neurons to determine how 5-HT affects individual neuron classes, the consequences for odor coding across olfactory brain regions and odor- guided behavior. In addition, we will use one of the first whole brain, nanometer resolution EM connectomes to establish single cell resolution connectivity rules of 5-HT neurons with each olfactory neuron class examined in this proposal. In Specific Aim 1 we will determine the receptor basis for the effects of 5-HT on local inhibitory networks within the first olfactory neuropil (the antennal lobe or ?AL?). In Aim 2 we will determine the contribution of direct modulation of cholinergic AL output neurons to the overall effects of 5-HT on olfactory information sent to downstream processing stages. Finally, in Aim 3 we will determine how 5-HT receptors modulate GABAergic AL output neurons that promote olfactory attraction. These experiments will establish how the overall effects of 5-HT emerge from neuron class-specific expression of 5-HT receptors, thus addressing a critical gap in our knowledge of healthy sensory processing.