Dopaminergic Modulation Shapes Sensorimotor Processing in the Drosophila Mushroom Body

To survive in a complex and dynamic environment, animals must adapt their behavior based on their current needs and prior experiences. This flexibility is often mediated by neuromodulation within neural circuits that link sensory representations to alternative behavioral responses depending on contextual cues and learned associations. In Drosophila, the mushroom body is a prominent neural structure essential for olfactory learning. Dopaminergic neurons convey salient information about reward and punishment to the mushroom body in order to adjust synaptic connectivity between Kenyon cells, the neurons representing olfactory stimuli, and the mushroom body output neurons that ultimately influence behavior. However, we still lack a mechanistic understanding of how the dopaminergic neurons represent the moment-tomoment experience of a fly and drive changes in this sensory-to-motor transformation. Furthermore, very little is known about how the output neuron pathways lead to the execution of appropriate odor-related behaviors. We took advantage of the mushroom body’s modular circuit organization to investigate how the dopaminergic neuron population encodes different contextual cues. In vivo functional imaging of the dopaminergic neurons reveals that they represent both external reinforcement stimuli, like sugar rewards or punitive electric shock, as well as the fly’s motor state, through coordinated and partially antagonistic activity patterns across the population. This multiplexing of motor and reward signals by the dopaminergic neurons parallels the dual roles of dopaminergic inputs to the vertebrate basal ganglia, thus demonstrating a conserved link between these distantly related neural circuits. We proceed to demonstrate that this dopaminergic signal in the mushroom body modifies neurotransmission with synaptic specificity and temporal precision to coordinately regulate the propagation of sensory signals through the output neurons. To explore how these output pathways ultimately influence olfactory navigation we have developed a closed loop olfactory paradigm in which we can monitor and manipulate the mushroom body output neurons as a fly navigates in a virtual olfactory environment. We have begun to probe the mushroom body circuitry in the context of olfactory navigation. These preliminary investigations have led to the identification of putative pathways for linking mushroom body output with the circuits that implement odor-tracking behavior and the characterization of the complex sensorimotor representations in the dopaminergic network. Our work reveals that the Drosophila dopaminergic system modulates mushroom body output at both acute and enduring timescales to guide immediate behaviors and learned responses

In silico prediction of regulators of neuronal identity through phylogenetic footprinting

How individual neurons in a nervous system give rise to complex function, behavior and consciousness in higher animals has been studied for over a century, yet scientist have only begun to understand how brains work at the molecular level. This level of study is made possible through technological advances, especially transgenic analysis of the cells that make up nervous systems. To date, no other system has been used as extensively as the nematode Caenorhabditis elegans in this pursuit. With just 302 neurons in the adult hermaphrodite, extensive neuronal maps at the anatomical, functional, and molecular level have been built over the past 30 years. One way to understand how nervous systems develop and differentiate into diverse cell types such as sensory or motor neurons that make higher level behaviors possible, is to unravel the underlying gene regulatory programs that control development. Throughout my PhD I investigated neuron type identity regulators to understand how nervous system diversity is generated and maintained using several bioinformatic approaches. First, I developed a software program and community resource tool, TargetOrtho, useful for identifying novel regulatory targets of transcription factors such as the cell type selector proteins termed terminal selectors evidenced to control terminal cell identity of 74 of the 118 neuron types in C. elegans. Analysis of terminal selector candidate target genes led to the further discovery that predicted target genes with cis-regulatory binding sites are enriched for neuron type specific genes suggesting an overarching theme of direct regulation by terminal selectors to specify cell type. Using this knowledge, I make predictions for novel regulators of neuronal identity to further elucidate how the C. elegans nervous system diversifies into 118 neuron types

Effect of early odorant exposure on the structure and output of the glomerular module

Early sensory experience has the capacity to dramatically shape the final anatomy and function of a sensory circuit. Both sensory deprivation and enrichment have major impacts on the development and maintenance of sensory circuit structure. We take advantage of the stereotyped structure of the olfactory system to investigate how early odorant experience changes the structure and output of the mouse olfactory bulb. The olfactory system lacks the stimulus-based topography seen in the visual or auditory sensory systems, a consequence of the high dimensionality of odorant stimuli. However, it possesses a highly stereotyped organization, making it an ideal model system in which to study the processing of sensory stimuli in a systematic and specific manner. Axons from olfactory sensory neurons that express the same odorant receptor converge into glomeruli, spherical structures in the olfactory bulb (OB). Glomeruli and their post-synaptic targets, including principal projection neurons, the mitral and tufted cells, form the basis of the glomerular module, which is the basic odor coding unit of the OB. In this dissertation, we leverage this specific structure to study how early odorant experience changes the composition of a glomerular module and impacts the odor-evoked activity of mitral cells. In Chapter 2, we use an in vivo dye labeling technique to examine how prenatal and early postnatal odorant exposure impacts the number of primary projection neurons connected to activated glomeruli. We find that significantly more mitral and tufted cells become associated with activated glomerular modules, suggesting that sensory input plays a major role in modulating OB circuit refinement in early development. In Chapter 3, we investigate how odor-evoked mitral cell activity across the dorsal OB is impacted following the same exposure paradigm used in Chapter 2. Using 2-photon calcium imaging of mitral cell somata, we find that early odorant exposure increases the number, amplitude, and reliability of excitatory odor-evoked mitral cell responses, potentially due to sensory enrichment during a developmental critical period. Together, these findings demonstrate that early odorant experience dramatically impacts OB anatomy and output, which may have significant implications on odor representation in the OB and olfactory perception. These changes may also influence olfactory-guided behavior, such as odor discrimination and preference