Despite the prevalence of mental illness in society, little is known about the basic neural circuitry regulating human behavior. Evidence suggests that mice and humans share the essential neuroanatomy leading to basic behaviors such as aggression and mating. However, unlike humans, these behaviors are thought to be regulated in mice primarily through secreted chemical cues known as pheromones. In order to develop valuable mouse models to study these conserved neural circuits, it is critical to first understand the chemical ligands that initiate activation of these pathways in the mouse. The objective of this application is to determine the molecular role of a particular class of pheromone binding proteins called Major Urinary Proteins (MUPs) in mediating pheromone signaling between mice. My central hypothesis is that MUPs are a critical component of the pheromone signal excreted by one animal and detected by another. This hypothesis is based on the following previously published observations as well as my own preliminary findings: 1) MUPs are expressed in male urine in different combinations between mice suggesting a role in information coding. 2) MUPs contain a central binding pocket which binds known pheromone ligands. 3) Behavioral studies suggest MUPs are involved in certain pheromone-mediated behaviors. 4) My preliminary studies show that MUP-ligand complexes directly activate VNO neurons in vitro. Based on these findings, it is my working hypothesis that MUPs interact directly as ligands or indirectly as ligand carriers with VNO neurons to effect pheromone signaling. I will test the extent to which MUP-ligand complexes, MUP ligands alone, and MUPs alone (without ligands) activate VNO sensory neurons by calcium imaging dissociated VNO neurons. In addition, based on the fact that mice express different combinations of MUPs in their urine, it is my working hypothesis that different MUPs will activate discrete populations of VNO neurons, providing a potential mechanism by which mice can discriminate between conspecifics. I plan to test this working hypothesis by purifying individual MUPs from urine and comparing their activation specificity by calcium imaging on populations of dissociated VNO neurons. By completing the proposed study, I will have determined the exact nature of the MUP protein and ligand interaction with sensory neurons. In addition, I will have determined the function of the heterogeneous regulation of MUP expression between mouse strains. Elucidating these mechanisms will provide an important step forward in understanding pheromone signaling at a molecular level, which will advance our understanding of the downstream molecular pathways conserved in humans that regulate basic behaviors. [unreadable] [unreadable] [unreadable]