Encoding and Processing of Accessory Olfactory System Odorants

The accessory olfactory system is a mouse olfactory subsystem dedicated to the processing of nonvolatile odors originating from the excretions of other animals. Accessory olfactory system activity drives behaviors critical for mouse survival such as mating, aggression, and predator avoidance. Odorants are detected by sensory neurons in a specialized structure called the vomeronasal organ, and these neurons project to the accessory olfactory bulb. The accessory olfactory bulb is the first site of information integration and processing within the accessory olfactory system, and this processing can be regulated by noradrenaline and other neuromodulators. Despite years of targeted research, there are only a few known ligands for this system limiting our ability to understand how odorants are processed. Part of my thesis has focused on the discovery of novel ligands for this system. I discovered that feces are a novel and potent source of accessory olfactory system odorants. Fecal-driven responses in the accessory olfactory bulb are unique from other known odorant sources and exhibit sex-selectivity, indicating that feces are a potentially rich source of information for the mouse. I determined that bile acids, which vary in identity and concentration by both sex and species, are one class of fecal chemosignals. Accessory olfactory bulb responses to bile acids show patterns of overlapping and specific activity, supportive of a bile acid combinatorial code. During my thesis I have also studied the role of noradrenaline on stimulus processing in the accessory olfactory bulb. I found that noradrenaline has different effects on spontaneous and stimulus-evoked activity. In a small subset of cells noradrenaline increases spontaneous activity, while the majority of cells show suppressed stimulus-evoked activity. This suppression is not consistent across stimulus responses. Some responses are immediately suppressed; some require multiple exposures to stimuli, while others are completely resistant to noradrenaline. Theses results indicate that noradrenaline can have heterogeneous and nuanced actions on accessory olfactory bulb activity.