Genetic Increases in Olfactory Bulb BDNF Do Not Enhance Survival of Adult-Born Granule Cells

Adult-born neurons produced in the dentate gyrus subgranular zone (SGZ) develop as excitatory hippocampal granule cells (GCs), while those from the subventricular zone (SVZ) migrate to the olfactory bulb (OB), where most develop as GABAergic olfactory GCs. Both types of neurons express TrkB as they mature. Normally ~50% of new olfactory GCs survive, but survival declines if sensory drive is reduced. Increases in endogenous brain-derived neurotrophic factor (BDNF) in hippocampus, particularly with wheel running, enhance dentate GC survival. Whether survival of new olfactory GCs is impacted by augmenting BDNF in the OB, where they mature and integrate, is not known. Here, we determined if increasing OB BDNF expression enhances survival of new GCs, and if it counters their loss under conditions of reduced sensory activity. Neurogenesis was assessed under normal conditions, and following unilateral naris occlusion, in mice overexpressing BDNF in the granule cell layer (GCL). OB BDNF levels were significantly higher in transgenic mice compared to controls, and this was maintained following sensory deprivation. Bromodeoxyuridine (BrdU) cell birth dating showed that at 12-14 days post-BrdU, numbers of new GCs did not differ between genotypes, indicating normal recruitment to the OB. At later intervals, transgenic and control mice showed levels of GC loss in deprived and nondeprived animals that were indistinguishable, as was the incidence of apoptotic cells in the GCL. These results demonstrate that, in contrast to new dentate GCs, elevations in endogenous BDNF do not enhance survival of adult-born olfactory GCs.