Novel Cell Types and Circuits in the Mouse Main Olfactory Bulb

The mammalian olfactory system is strikingly shallow. While peripheral input in other sensory systems is sequentially processed by brainstem, midbrain, and thalamic nuclei before reaching primary sensory and associational cortices, olfactory input is processed by only a single region of the brain – the main olfactory bulb – before reaching higher cortical areas. A tremendous amount of neural processing is thus compressed within the main olfactory bulb, making this region of the brain uniquely well suited for investigating fundamental principles of neural processing. Currently, the identity and functional roles of multiple cell types and circuits within the main olfactory bulb remain almost entirely unknown, significantly limiting our understanding of olfaction. Herein, I describe a set of studies addressing this broad gap in knowledge. In Chapter 1, I introduce the known cellular and circuit components of the main olfactory bulb. In Chapter 2, I examine the complexity in biophysical cell-to-cell differences among mitral cells, a class of principal neurons in the main olfactory bulb, and quantify how this within-class diversity regulates mitral cell synchrony. In Chapter 3, I systematically explore synaptic and intrinsic biophysical properties to functionally establish mitral cells and tufted cells as two distinct classes of principal neurons in the main olfactory bulb. In Chapter 4, I reveal that disinhibitory circuitry mediated by a largely uncharacterized class of interneurons is widespread throughout the main olfactory bulb and critically involved in regulating the sensory-evoked activity of inhibitory granule cells. In Chapter 4 Appendix, I provide the first quantitative evidence for the morphological and functional subdivision of granule cells into two distinct classes that separately interact with mitral cells and tufted cells. In Chapter 5 and Chapter 5 Appendix, I molecularly identify a novel class of deep short-axon cells and show that this class of interneurons integrates centrifugal cholinergic input with broadly tuned sensory input and provides highly divergent synaptic output to dynamically regulate the balance of activity between mitral cells and tufted cells. Finally, in Chapters 6 and 7, I present general conclusions from these studies and provide a reappraisal of inhibitory circuitry within the main olfactory bulb.