Which cell types are targeted by neurogenesis in the olfactory bulb? To answer this question (among others), Bardi et al. used a optogenetic mouse model selective for adult-born olfactory cells to test various cell types for post-synaptic responsiveness following light stimulation.
So if the (randomly chosen) cells in the olfactory bulb slices respond to light, as measured via patch-clamping, that means that they must receive input from an adult-born cell. They found that 86% (65/76) of tested cells projecting to cortical regions had such functional connections, whereas only 21% (9/42) of tested local neurons did. Although the proportion is biased towards neurons with longer-range axons, it is clear that adult-born neurons have a diverse set of synaptic targets.
In one of their slices they labelled responsive cells with biocytin/streptavidin and visualized them with confocal microscopy. Different colors correspond to different postsynaptic cell types, as you can see below.
Since all of the adult-born olfactory neurons communicate with inhibitory GABAergic signals, their role would be more limited if they only targeted cells projecting to cortical regions. This study highlights another level of complexity, showing that these neurons could in certain cases act to inhibit inhibitory interneurons, thus adding to excitatory signals leaving the olfactory bulb from particular axons. Layers upon layers.
Bardy C, et al. 2010 How, when, and where new inhibitory neurons release neurotransmitters in the adult olfactory bulb. J Neuro, PubMed, doi: 10.1523/JNEUROSCI.4543-10.2010.