Effects of target neuron loss on olfactory sensory neurons in adult mice

Olfactory sensory neurons (OSN) expressing the same odor receptor (OR) project their axons to topographically fixed glomeruli in the olfactory bulb (OB). This topographic map results from axon guidance mechanisms determined by ORs, glia and molecular guidance cues. The present study examined the organization of mature OSNs expressing the P2 OR in adult mice after ablation of bulb neurons with N-methyl-D-aspartate (NMDA). Rapid neuronal degeneration was followed by progressive laminar disorganization of the OB and glomerular shrinkage. P2 axon targeting and convergence was maintained within degenerating glomeruli for up to 2 weeks. After that time, fewer P2 axons were observed in the lesioned OB with fewer P2 neurons in the olfactory epithelium (OE). By 3 weeks, the mature OSN population was reduced and the immature population was increased. These results suggest that bulbar synaptic contacts do not maintain sensory axon convergence in the adult, but regulate neuronal survival in the OE.