The mammalian brain ensures adaptive behavior through its large capacity for cellular and circuit plasticity. The diverse scales of neural plasticity range from single synapse modification 
to network remodeling that accompanies ongoing neurogenesis 
. Plasticity mechanisms accommodate changing environmental stimuli that are continuously relayed to the brain via multiple sensory modalities. Among sensory systems, the olfactory system possesses a large capacity for circuit plasticity through continued generation of new neurons in adult life. Such continuous incorporation of new neurons implies persistent, large-scale remodeling of synaptic connections, the nature of which is not well known.
Within the olfactory system, the axons of olfactory sensory neurons (OSNs) expressing the same odorant receptor 
converge onto discrete glomeruli in the main olfactory bulb (MOB) 
. Organized around glomeruli, groups of mitral/tufted cells, as well as various interneurons, form connected networks that extend into all layers of the olfactory bulb 
. These networks likely represent unitary modules for odor information processing 
and may be functionally analogous to barrels in the somatosensory cortex or ocular dominance columns in the visual system.
The functional organization within and between MOB glomerular units has been the subject of intense investigation. Lateral interactions between glomeruli are mediated primarily by dendrodendritic synapses between mitral cells and granule cells 
, and the electrophysiological properties of these synapses have been well characterized 
. Although mostly studied as singly recorded neurons or synaptically coupled pairs, these experiments support the notion that populations of neurons associated with multiple glomeruli are highly interconnected. Among the most studied forms of intrabulbar circuitry, granule cells provide inhibitory feedback onto spatially distant glomeruli by forming synapses with the lateral dendrites of mitral cells 
. In addition, synaptic inputs from both local short axon cells (SACs) and distant cortical neurons provide direct regulation of granule-mitral cell synapses 
. Despite a central role in olfactory processing, the relative connectivity of individual granule cells to different cell types, the spatial organization of granule cell synaptic partners, and the regulation of granule cell connectivity by sensory stimulation remain unclear.
New GABAergic granule and periglomerular cells in the MOB are continually generated throughout adulthood 
. Whereas many adult-born neurons fail to establish and maintain dendrodendritic synapses and ultimately undergo apoptosis 
, granule cells born during early stages of postnatal development tend to be long-lived and form stable synaptic connections 
. We thus sought to define the patterns of cellular connectivity formed by postnatal-born granule cells in the MOB and determine how new granule cell microcircuits are influenced by sensory input. In the present study we employed monosynaptic circuit tracing using pseudotyped rabies virus together with a conditional red-fluorescence mouse reporter strain to label newborn olfactory bulb interneurons and their presynaptic partners in vivo 
. We show that postnatal-born granule cells make synaptic connections with cortical inputs and multiple olfactory bulb cell types. The pattern of monosynaptic connectivity shows a clustered organization that is characterized by extensive presynaptic inputs from anatomically distinct short axon cells. Moreover, increased sensory experience by odor enrichment enhances SAC connectivity onto postnatal-born granule neurons. These results define the presynaptic repertoire of novel inputs onto newborn granule cells, and support a model whereby clustered patterns of organization in the olfactory bulb extend from local short axon cells to cohorts of deep granule cells that span the laminae of the olfactory bulb. The identification of numerous short axon cells presynaptic to new granule cells reveals unanticipated cellular interactions that occur during granule cell (GC) synapse development. Experience-driven changes in SAC-GC connectivity provides a circuit basis for refining or remodeling synaptic inputs upon exposure to a complex sensory environment through ongoing neurogenesis.