2020 Theses Doctoral

Neuronal Topography in a Cortical Circuit for Innate Odor Valence

Costantini, Daniel

The mouse olfactory system detects odorants with 1000 olfactory receptors (ORs). Olfactory sensory neurons (OSNs) express only 1 OR. OSNs expressing a common OR converge
on a single glomerulus, a stereotyped location in the olfactory bulb (OB). Thus, odorants are represented by a spatial map of glomerular activation. OB odor representations are then processed by five central brain regions. One region, cortical amygdala (CoA), receives spatially patterned and stereotyped axonal input from the OB and is both necessary and sufficient for innate behavioral responses to odor. However, CoA receives input from all glomeruli and forms a representation of every odor. It is not known why all odors are represented in CoA or how some odor representations elicit behavior while others do not. One hypothesis is that only rare neurons in CoA, not activated by most odors, participate in innate signaling. Another hypothesis is that all neurons in CoA participate in innate signaling, but for many odors, opposing CoA outputs cancel out downstream. These hypotheses were addressed by single nuclei sequencing and in situ hybridization which identified and localized neuronal cell types within CoA. Cell types are topographically segregated in regions well positioned to differentially receive inputs from genetically defined subsets of glomeruli. Therefore, the connectivity between OB and CoA may instantiate a genetically wired circuit from OB to cortex for innate odor processing. A number of rare and common cell types were identified. Thus, CoA may process two types of innate signals: (1) specific innate signals, produced by few glomeruli and processed by rare CoA cell types; (2) broad innate signals, produced by many glomeruli and processed by common CoA cell types through the integration of probabilistic information about the value of odorants.