Inverse expression of olfactory cell adhesion molecule in a subset of olfactory axons and a subset of mitral/tufted cells in the developing rat main olfactory bulb

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Abstract
The projection of olfactory sensory neuron (OSN) axons from the olfactory epithelium (OE) to the olfactory bulb (OB) is highly organized but topographically complex. Evidence suggests that odorant receptor expression zones in the OE map to the OB about orthogonal axes. One candidate molecule for the formation of zone‐specific targeting of OSN axon synapses onto the OB is the olfactory cell adhesion molecule (OCAM). OCAM+ OSNs are restricted to three of the four zones in the OE and project their axons to the ventral OB where they form synapses with mitral/tufted (M/T) cells. To determine when this zonal connection is established, we have examined OCAM expression in rat olfactory system, during seminal periods of glomerular formation. OCAM+ axons sort out in the ventral olfactory nerve layer of the OB before glomerular formation. Surprisingly, OCAM was also expressed transiently by subsets of M/T cell dendrites located in the dorsal OB. The expression of OCAM by OSN axons and M/T dendrites was asymmetrical; in the dorsal OB, OCAM− OSN axons synapsed on OCAM+ M/T dendrites, whereas in the ventral OB, OCAM+ OSN axons synapsed on OCAM− M/T dendrites. The restricted spatial map of OCAM+ M/T cells appeared earlier in development than the zonal segregation of OCAM+ OSN axons. Thus, OCAM on M/T cell dendrites may act in a spatiotemporal window to specify regions of the developing rat OB, thereby establishing a foundation for mapping of the OE zonal organization onto the OB. J. Comp. Neurol. 458:389–403, 2003.