• Characterization of an olfactory subsystem

      Cockerham, Renee; Munger, Steven D. (2008)
      Mammals encounter a large number of different odor molecules that provide information about their environment. As the primary sense for rodents, many behavioral responses are based on olfactory information. Elucidating the ways that animals make sense of chemical stimuli is a difficult task, a consequence of the diversity of stimulus types and because chemical stimuli communicate disparate information ranging from food quality to reproductive status. The olfactory system uses several subsystems to process chemical stimuli. These subsystems can be defined by the stimuli to which they respond, the receptors and other molecules they express, and the connections they make to the central nervous system. One such subsystem contains sensory neurons that express components of a cGMP-based sensory transduction cascade, including the receptor guanylyl cyclase, GC-D, the cyclic nucleotide-gated channel subunit CNGA3 and the phosphodiesterase PDE2. These neurons send their axons to specialized regions of the caudal main olfactory bulb, the necklace glomeruli. However, little was known about the chemosensitivity, function, or neural connections of this novel olfactory subsystem. This dissertation contains three sets of experiments that aim to illuminate the biological role of this olfactory subsystem. The first set of studies characterizes the anatomy and function of GC-D-expressing neurons. My data, along with that of my collaborators, demonstrate that GC-D-expressing neurons respond to the natriuretic peptides guanylin and uroguanylin, as well as to the natural stimulus, urine, through an excitatory, cGMP-dependent signaling cascade that requires both GC-D and CNGA3. In the second phase of this study, I examined the sensory input and interglomerular connectivity of the necklace glomeruli. Necklace glomeruli exhibit extensive interglomerular connections with other olfactory subsystems and receive heterogeneous sensory innervation. These findings indicate that the necklace glomeruli integrate natriuretic peptide signals in the context of other olfactory information. In the final section of this dissertation, I investigated the potential behaviors mediated by the GC-D neurons and the necklace glomeruli. Efficient pup suckling, maternal anogenital licking, or detection of the natriuretic peptides guanylin and uroguanylin were all ruled out as behaviors mediated by the GC-D neurons and the necklace glomeruli. Together, these studies highlight the unique nature of the GC-D-expressing sensory neurons and the necklace glomeruli as a novel chemosensory subsystem.
    • Mapping Functional Circuitry in the Main Olfactory Bulb Using Intrinsic Flavoprotein and NAD(P)H Fluorescence Imaging

      Uytingco, Cedric; Munger, Steven D.; Puche, Adam C. (2015)
      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.