Olfactory neurons expressing closely linked and homologous odorant receptor genes tend to project their axons to neighboring glomeruli on the olfactory bulb

We have characterized two separate odorant receptor (OR) gene clusters to e xamine how olfactory neurons expressing closely linked and homologous OR ge nes project their axons to the olfactory bulb. Murine OR genes, MOR28, MOR1 0, and MOR83, share 75-95% similarities in the amino acid sequences and are tightly linked on chromosome 14. In situ hybridization has demonstrated th at the three genes are expressed in the same zone, at the most dorsolateral and ventromedial portions of the olfactory epithelium, and are rarely expr essed simultaneously in individual neurons. Furthermore, we have found that olfactory neurons expressing MOR28, MOR10, or MOR83 project their axons to very close but distinct subsets of glomeruli on the medial and lateral sid es of the olfactory bulb. Similar results have been obtained with another m urine OR gene cluster for A16 and MOR18 on chromosome 2, sharing 91% simila rity in the amino acid sequences. These results may indicate an intriguing possibility that olfactory neurons expressing homologous OR genes within a cluster tend to converge their axons to proximal but distinct subsets of gl omeruli. These lines of study will shed light on the molecular basis of top ographical projection of olfactory neurons to the olfactory bulb.