Project Summary/Abstract This is a continuation of an existing program funded by an RO1 that seeks to understand fundamental questions about the nature of the olfactory stimulus and its discrimination by primary sensory receptors, in order to better grasp the nature of the signal being forwarded to the brain. We supplement the standard organic chemistry approach to odor classification with one that includes a biological perspective based in principles of medicinal chemistry pioneered in the pharmaceutical industry and drug development. This allows us to not only consider physical chemical characteristics of molecules but also how they interact with the many olfactory receptors. This work, like much of olfactory cellular studies, utilized monomolecular stimuli to probe receptor function. However the olfactory system is usually confronted with complex stimulus blends of between tens and hundreds of components. To better understand the complexity of mixture discrimination we have begun utilizing a novel microscopic technique, SCAPE – that allows us to visualize large numbers of individual neurons in an intact tissue with single cell temporal and spatial resolution. Preliminary data from that work showed a remarkable and unexpected degree of inhibition and enhancement of responses by one component of the mixture on other components of the mixture. This raises a fundamental issue in olfactory discrimination as there appears to be considerable interaction between stimuli at the primary receptor, raising crucial questions about how the brain perceives and manages signals that differ depending on the presence of particular odors in a blend. It appears that many, if not all, odors can act as either an agonist or antagonist at different receptors. Thus depending on whether or not an antagonist to odor X, for example, exists in the blend, the signal from odor X will appear different to the brain. This level of complexity is unusual in sensory systems where signal processing of this sort occurs in the brain not the primary receptors. Understanding higher brain processing olfactory signals will require a greater understanding of the initial events at primary receptors. The olfactory code is more than the additive contributions of the olfactory receptors.