Mapping Functional Circuitry in the Main Olfactory Bulb Using Intrinsic Flavoprotein and NAD(P)H Fluorescence Imaging

Abstract

The canonical olfactory sensory neurons (OSNs) and olfactory bulb glomeruli within the main olfactory system play critical roles in the recognition of odors. However, it is now recognized that within the main olfactory system there are several distinct subsystems, such as the GC-D-expressing (GC-D+) OSN/necklace glomeruli (GC- D/necklace) subsystem, that play specialized chemosensory roles. Little is known about how these olfactory subsystems process sensory information, although previous finding from our lab suggest that the GC-D/necklace subsystem uses a coding strategy distinct from the combinatorial coding employed by the canonical main olfactory system. For example, the necklace glomeruli exhibit extensive interglomerular connections and, in contrast to canonical glomeruli of the main olfactory bulb (MOB), receive heterogeneous sensory inputs. This arrangement suggests an integrative circuit that could associate food odors with the social cues that have been shown to stimulate GC-D+ OSNs. To better understand the functional circuitry associated with the necklace glomeruli, and by extension core principles of sensory processing within this olfactory subsystem, my dissertation establishes an approach, novel to the olfactory system, where stimulus-induced increases in intrinsic flavoprotein and NAD(P)H fluorescence signals can be used to map functional circuits in the main olfactory bulb that are associated with single, identified glomeruli. This approach allowed me to examine the spatiotemporal spread of stimulus-dependent intrinsic signals following stimulation of individual canonical glomeruli as well as the functional connectivity between neighboring glomerular circuits. Results of my studies suggest the presence of reciprocal connections between the interglomerular-interneuron and mitral-granule-mitral pathways under disinhibited conditions. Additionally, my examination of the functional circuitry associated with the necklace glomeruli suggests that the GC-D/necklace subsystem is functionally integrated with the canonical main olfactory system within the MOB. Together, these studies introduce and employ a novel and accessible tool to examine connectivity within the MOB circuitry to provide new insights into canonical and subsystem-specific odorant processing.