Olfactory discrimination largely persists in mice with defects in odorant receptor expression and axon guidance

Microarray analysis reveals that nearly one quarter of all odorant receptor genes are down regulated in β3GnT2？/？ mice compared to controls. Analysis of OR expression by quantitative PCR and in situ hybridization demonstrates that the number of neurons expressing some odorant receptors, such as mOR256-17, is increased by nearly 60% whereas for others such as mOR28 the number of neurons is decreased by more than 75% in β3GnT2？/？ olfactory epithelia. Analysis of axon trajectories confirms that many axons track to inappropriate targets in β3GnT2？/？ mice, and some glomeruli are populated by axons expressing more than one odorant receptor. Results show that mutant mice perform nearly as well as control mice in an odor discrimination task. In addition, in situ hybridization studies indicate that the expression of several activity dependent genes is unaffected in β3GnT2？/？ olfactory neurons.Results presented here show that many odorant receptors are under-expressed in β3GnT2？/？ mice and further demonstrate that additional axon subsets grow into inappropriate targets or minimally innervate glomeruli in the olfactory bulb. Odor evoked gene expression is unchanged and β3GnT2？/？ mice exhibit a relatively small deficit in their ability to discriminate divergent odors. Results suggest that despite the fact that β3GnT2？/？ mice have decreased AC3 activity, decreased expression of many ORs, and display many axon growth and guidance errors, odor-evoked activity in cilia of mutant olfactory neurons remains largely intact.Understanding the organization of connections in the mammalian olfactory system is a challenge because the rules used to generate a map of axon trajectories from the olfactory epithelium to the olfactory bulb (OB) appear to differ considerably from other sensory modalities, such as the visual and somatosensory systems [1-3]. It is now clear that regulation of adenylyl cyclase activity and generation of cAMP is one of the major contributors of guidance information in