Physiology and Circuitry of Bile Acid Detection Within the Accessory Olfactory System

The mouse accessory olfactory system (AOS) supports social and reproductive behavior through the sensation of environmental chemosignals. These chemosensory cues provide a vast amount of information that the olfactory systems must then encode and translate into behaviorally relevant outputs. The initial detection of olfactory stimuli in the accessory olfactory system (AOS) is mediated by the vomeronasal sensory neurons (VSNs) in the vomeronasal organ (VNO), which relay the signals to the accessory olfactory bulb (AOB). Impaired vomeronasal signaling results in deficits in social communication and other behaviors. Despite decades of research, our understanding of how odors are encoded and processed within the AOS is still severely limited. For example, the extent to which the AOS discriminates and encodes chemosensory information at the peripheral level of the VNO is currently unknown. Furthermore, how the tuning of VSNs is then translated into a behaviorally relevant representation in the brain is still unclear. Understanding how the AOS processes and encodes chemosensory information will advance our understanding of how external cues can generate internal chemosensory representations that are critical for survival. Sensory adaptation is a source of experience-dependent feedback that impacts responses to environmental cues. In the mammalian main olfactory system (MOS), adaptation influences sensory coding at its earliest processing stages. However, sensory adaptation in the accessory olfactory system (AOS) remains relatively controversial, leaving many aspects of the phenomenon unclear. Thus, I investigated sensory adaptation in vomeronasal sensory neurons (VSNs) using in situ Ca2+ imaging. I found evidence for sensory adaptation in response to the monomolecular ligands, cholic acid (CA) and deoxycholic acid (DCA). These Ca2+ imaging experiments also revealed the presence of a slower form of VSN adaptation that accumulated over dozens of stimulus presentations delivered over tens of minutes. These studies help establish the presence of VSN sensory adaptation and provide a foundation for future inquiries into the molecular and cellular mechanisms of this phenomenon and its impact on mammalian behavior. A growing number of excreted steroids have been shown to be potent AOS cues, including bile acids (BAs) found in feces. As is still the case with most AOS ligands, the specific receptors used by vomeronasal sensory neurons (VSNs) to detect BAs remain unknown. To identify VSN BA receptors, we first performed a deep analysis of VSN BA tuning using volumetric GCaMP6f/s Ca2+ imaging. These experiments revealed multiple distinct populations of BA-receptive VSNs with submicromolar sensitivities. I then developed a new physiology-forward approach for identifying AOS ligand-receptor interactions, which I term Fluorescence Live Imaging for Cell Capture and RNA sequencing, or FLICCR-seq. FLICCR-seq analysis revealed five specific V1R family receptors enriched in BA-sensitive VSNs. These studies introduce a powerful new approach for ligand-receptor matching and reveal biological mechanisms underlying mammalian BA chemosensation. Finally, I've spent the remainder of my thesis laying the groundwork for exploring BA-mediated behaviors and the circuitry of BA information as it travels through the AOS. Importantly, my work opens many avenues for future studies. Here, I show evidence that five specific V1R receptors are BA-sensitive. The identification of ligands that can modulate the activity of orphan VRs is paramount to understanding their function and in turn understanding chemosensation. We now have more tools in the toolbox to understanding and studying the activation of VRs, their signal transduction, and their function.