Spontaneous activity generated within the olfactory bulb establishes the discrete wiring of mitral cell dendrites

In the mouse olfactory bulb, sensory information detected by ~1,000 types of olfactory sensory neurons (OSNs) is represented by the glomerular map. The second-order neurons, mitral and tufted cells, connect a single primary dendrite to one glomerulus. This forms discrete connectivity between the ~1,000 types of input and output neurons. It has remained unknown how this discrete dendrite wiring is established during development. We found that genetically silencing neuronal activity in mitral cells, but not from OSNs, perturbs the dendrite pruning of mitral cells. In vivo calcium imaging of awake neonatal animals revealed two types of spontaneous neuronal activity in mitral/tufted cells, but not in OSNs. Pharmacological and knockout experiments revealed a role for glutamate and NMDARs. The genetic blockade of neurotransmission among mitral/tufted cells reduced spontaneous activity and perturbed dendrite wiring. Thus, spontaneous network activity generated within the olfactory bulb self-organizes the parallel discrete connections in the mouse olfactory system.