Olfaction-the sense of smell-is the major means though which we sense our chemical surroundings, and as such, it has a significant impact on quality of life. One of the overarching goals in the field of olfactory neurobiology is to understad the neural pathways that connect an olfactory stimulus, its representation in the brain as a percept, and the circuits that drive downstream behaviors. Perhaps the greatest obstacle to understanding these neural pathways is the complexity and physical size of the nervous system. For example, mice, which have numerous olfactory-guided behaviors, have ~1000 different odorant receptors, 5-10 million olfactory sensory neurons, and 1800 distinct collections of axons and dendrites, called glomeruli, in the olfactory bulb, the brain area specialized in processing the initial steps of olfaction. In this proposal, we describe a strategy to study the neural circuiry underlying olfaction in a system that is at least 10 times simpler than mice, the zebrafish larvae. The key innovation of this strategy is our use of a recently identified, innate olfactory-guided behavior to study olfactory circuits. Although five-day-old zebrafish exhibit several innate behaviors, until recently there were no known robust olfactory-guided behaviors in zebrafish larvae. Here we demonstrate that zebrafish larvae exhibit a stereotyped, olfactory-guided fear behavior in response to components in an organic solvent extraction of zebrafish skin. Ultimately, using this behavior as a guide, we will be able to connect the sensation of specific molecular structures with the types of olfactory receptors they bind, the types of olfactory sensory neurons they activate, and the neural circuit they excite in order to generate behavior.
In Specific Aim 1, we describe experiments to identify which of the three olfactory sensory neuron types is stimulated by the behaviorally active odorants. These results will enable future experiments that allow us to narrow the search for the relevant receptors and signaling pathways underlying this olfactory-guided behavior.
In Specific Aim 2, we describe two separate but complementary strategies to identify the anatomy of the olfactory neural circuit at the first olfactory-processing step in the brain, the glomeruli of the olfactory bulb.
In Specific Aim 2. 1, w identify which glomeruli are a part of the neural circuit underlying the olfactory behavior by imaging their neural activity in response to sensory reception of purified, behaviorally active molecules.
In Specific Aim 2. 2, we identify the relevant glomeruli by ablating them and looking for a diminished behavioral response. The results of these experiments will increase our understanding of the neural circuits underlying a innate, olfactory-guided behavior, and will provide a solid foundation for an R01 application to support future studies on the elucidation of neural circuits mediating innate behaviors.
Olfaction, the sense of smell, has a significant impact on quality of life and is an important research endeavor. In this application, we use the zebrafish larva-a simple and highly tractable model system-to study the neural circuitry underlying an olfactory-guided fear behavior. Our strategy enables us to study the neural mechanisms that underlie not only olfaction, but also chronic fear and anxiety, conditions that present a significat burden on human mental health.