We have developed a genetic approach to visualize axons from olfactory
sensory neurons expressing a given odorant receptor, as they project
to the olfactory bulb. Neurons expressing a specific receptor project
to only two topographically fixed loci among the 1800 glomeruli in the
mouse olfactory bulb. Our data provide direct support for a model in
which a topographic map of receptor activation encodes odor quality in
the olfactory bulb. Receptor swap experiments suggest that the olfact
ory receptor plays an instructive role in the guidance process but can
not be the sole determinant in the establishment of this map. This gen
etic approach may be more broadly applied to visualize the development
and plasticity of projections in the mammalian nervous system.