Cell surface carbohydrates reveal heterogeneity in olfactory receptor cell axons in the mouse

Cell surface carbohydrates, both in the olfactory system and elsewhere, have been proposed to play critical roles in axon guidance and targeting. Recent studies have used plant lectins to study the heterogeneous distribution of carbohydrates in the olfactory system. One lectin, Dolichos biflorus agglutinin (DBA), heterogeneously labels subsets of glomeruli. In the olfactory epithelium DBA labeled a subset of olfactory sensory neurons (OSNs) including their cilia, dendrites, and somata. OSN axons were also labeled and readily observed in the olfactory nerve and bulb. The patterns of glomerular innervation by DBA labeled (DBA(+)) axons were diverse; some glomeruli contained many labeled axons, while others contained few or no labeled axons. To characterize the heterogeneous innervation of glomeruli, we double labeled olfactory bulbs with DBA and an antibody to olfactory marker protein (OMP). OMP colocalized in most, but not all, DBA(+) axons. To determine if those axons that did not express OMP were immature, we double labeled olfactory bulbs with DBA and anti-GAP-43. GAP-43 rarely colocalized with DBA, suggesting that DBA(+) axons are not, as a population, immature. Triple labeling with all three markers revealed a small subset of DBA(+) axons which did not express either OMP or GAP-43. Electron microscopy established that DBA labels axons in the olfactory nerve and DBA-labeled axons form typical glomerular axodendritic synapses.