Activation and connectivity in the olfactory system of a basal vertebrate

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In many vertebrates, olfactory sensory input takes place in more than one location within the nasal cavity, and odour information is processed in a spatially precise manner in the olfactory bulb. However, the rationale behind the organization of the olfactory subsystems is not well understood. The objective of this research program is to investigate the odor sensitivity, biochemical profile and neural connectivity in subdivisions of the peripheral olfactory organ and the olfactory bulb in the lamprey, a basal vertebrate that diverged from the main vertebrate line of evolution over 500 million years ago. While teleost fish smell hundreds of odours, and mammals respond to thousands of odorous molecules, the lamprey has an olfactory repertoire of a few dozen odourants. Yet olfaction is a key sensory modality, as pheromones direct lampreys during spawning migration and reproductive behaviour. These characteristics enable the lamprey to be utilized as a reductionist model for investigating vertebrate olfaction. We have recently utilized electrophysiology of an in vitro lamprey nose-brain prep to characterize the odour activation map of the olfactory bulb. The lateral region responds to amino acids, yet the olfactory sensory neuron population that projects into this bulbar region is unknown. We will utilize neuronal tract tracing to find the peripheral location and the cell morphology of these olfactory sensory neurons; and we will investigate the signal transduction pathways of these olfactory sensory neurons through optical imaging and electrophysiology. Tract tracing will be used to investigate the dendritic arbors of lateral bulbar projection neurons, to examine the degree of convergence of the neurons that convey amino acid olfaction. While the lateral region of the olfactory bulb is stimulated by amino acids, there is a less clear picture of the spatial response map to pheromones. We will investigate if there are specific regions devoted to specific pheromones associated with migration and reproduction. Tract tracing will be used to identify the peripheral location of the pheromone sensing neurons, the central projections and the corresponding bulbar projection neurons. In derived vertebrates, olfactory sensory neuronal subtypes express specific conductance channels that are partnered with signal transduction pathways. The recent annotation of the sea lamprey genome opens up opportunities for investigating the expression of specific conductance channels expressed by the different populations of sea lamprey olfactory sensory neurons. This information is important for defining the lamprey olfactory subsystems in terms of the general vertebrate model. This research program probes for commonality and divergence in the evolution of the vertebrate nervous system. It is significant because it will contribute knowledge of the how and why of olfactory sensory input and the ensuing central connections. On a broader scale, this research will contribute to knowledge regarding the evolution of the brain and how the vertebrate brain handles information from diverse chemical stimuli. Translation of this research will directly benefit wildlife management of the invasive sea lamprey in the Great Lakes, and endangered populations in Europe and the North American west coast.