Vertebrates have evolved parallel olfactory pathways to support their survival and reproduction. In recent years it has become clear that vertebrates from early-diverged fishes to humans have retained the capacity to detect environmental steroids using their olfactory systems. In rodents, including mice, the accessory olfactory system is enriched in steroid- sensitive olfactory neurons, called vomeronasal sensory neurons. These neurons are sensitive to excreted steroids like estrogens, androgens, and glucocorticoids found in urine; our laboratory recently discovered that this system also demonstrates broad sensitivity to bile acids found in feces. Bile acids are an incredibly important class of molecules and are critical for dietary fat absorption, but we poorly understand how chemosensory systems use bile acid information to influence animal physiology and behavior. The goal of this research proposal is to add depth to our understanding of the mechanisms of bile acid detection by the accessory olfactory system and the spatial representation of bile acid information in the brain. Specifically, the proposed research will determine the sensitivity of mouse vomeronasal sensory neurons for fecal odorants and several relevant monomolecular bile acid ligands, which will add breadth and depth to our understanding of fecal odor and bile acid detection in the sensory periphery. Subsequently, the synaptic terminals of feces- and bile acid-sensitive neurons, which ramify in the glomerular layer of the accessory olfactory bulb, will be functionally mapped in 3-dimensions using live light sheet microscopy. Finally, the bile acid sensitivity of projection neurons, called mitral cells, that directly target the medial amygdala and bed nucleus of the stria terminalis will be evaluated, determining how bile acid information is sorted into these parallel output channels. Combined, the results will show how bile acids are encoded and prepared by neurons in the accessory olfactory system in ways that help shape animal behavior. These data will improve our understanding of brain function, and will provide a foundation for further study into chemosensory impacts on mammalian social and reproductive behaviors.
Most mammals, including rodents that harbor harmful microorganisms, rely on chemosensory cues to gain information about the emitter, for example sex, species, health and sexual maturity. The rodent accessory olfactory system is a compact chemosensory pathway that detects and interprets information from metabolic byproducts and hormones in excretions like urine and feces, but we poorly understand how this information is decoded by the brain in ways that drive social and reproductive behaviors. This proposal will investigate sensory processing of fecal chemosignals and monomolecular steroids across connected levels of the mouse accessory olfactory system; the resulting data will inform our understanding about the chemosensory origins of steroid detection by chemosensory neurons and the circuit mechanisms that turn these external cues into behaviorally relevant representations in the brain.
