The main olfactory system
The mammalian olfactory system is defined by several spatially segregated subsystems with distinct molecular and functional characteristics, in contrast to the single sensory surface present in fish. The evolutionary steps of that transition is largely unknown. The amphibian olfactory system as evolutionary intermediate appears well-suited to investigate the molecular driving forces behind olfactory regionalization. Different olfactory subsystems exist within the main olfactory system. We found that in the olfactory system of larval Xenopus laevis two segregated odor processing streams coexist, sharply segregated in the main olfactory bulb and partially segregated in the olfactory epithelium. A lateral odor processing stream is formed by microvillous receptor neurons and is characterized by amino acid responses and Gαo/ Gαi as probable signal transducers, whereas a medial stream formed by ciliated receptor neurons is characterized by responses to alcohols, aldehydes and ketones, and Gαolf/ cAMP as probable signal transducers. To reveal candidates for the olfactory receptors underlying these two streams the spatial distribution of twelve genes from four olfactory receptor gene families has been determined. We found that some odorant receptors mimic the spatial distribution observed for thelateral or medial stream, other olfactory receptors appear noth to be associated with either stream. Odor responses to bile acids and amines and are also not lateralized, suggesting an incomplete segregation of these responses (for more information see Gliem et al., 2013). Preliminary data indicate that two additional subsystems exist within the main olfactory system of larval Xenopus laevis. We are currently characterizing these subsystems.