The olfactory bulb (OB) is the first relay center in the mammalian olfactory system and its major local inhibitory interneurons, granule cells (GCs), are continually produced throughout adulthood. Adult-born GCs develop many dendritic spines which form reciprocal synaptic contacts with projection neurons. While olfactory sensory experience crucially regulates the spine development of adult-born GCs, it is not well known with which projection neurons among many projection neurons adult-born GCs make synaptic contacts and whether the choice of target projection neurons for adult-born GCs' synaptic contacts is regulated by the activity of the target projection neurons.
To address this question, we have established a mouse system in which channelrhodopsin was expressed in a subpopulation of mitral cells (MCs), the major projection neurons in the OB, and they are activated by photostimulation. Expression of channelrhodopsin-mCherry fusion protein revealed that both channelrhodopsin-expressing and non-expressing MCs were present in a given OB region. Dendritic spines of adult-born GCs were visualized by infecting with GFP-expressing virus. We are examining whether adult-born GCs make more synaptic contacts onto photoactivated MCs, compared with channelrhodopsin-negative MCs.
During the analysis of contact formation of adult-born GCs' dendritic spines onto MCs at various ages of adult-born GCs, we found that individual adult-born GCs form spines contacting somata of a limited number of MCs (perisomatic-targeting spines) from their early developmental stage. At the subsequent stage, adult-born GCs form well-studied spines that contact lateral dendrites of MCs (dendritic-targeting spines). We are examining whether formation of each of these subcellular-targeting spines is controlled by the activity of target MCs by the above optogenetic strategy.