, 2001, Hua and Smith, 2004, Meyer and Smith, 2006, Niell et al., 2004, Ruthazer et al., 2003, Ruthazer et al., 2006 and Sanchez et al., 2006). Nevertheless, these studies were limited by analysis of either pre- or postsynaptic neurons and the relatively low spatial resolution of fluorescence light microscopy. Questions such as whether new axonal or dendritic branches initiate synaptic contacts, whether synaptic contacts are required to stabilize branches, and whether Galunisertib cost synaptic activity affects synaptic contacts on new branches had not been directly addressed.

Our results clearly demonstrate that newly extended dendritic branches not only form synapses, but surprisingly, they have a significantly higher

synapse density than stable branches. Extending dendritic branches have a higher Tenofovir mw density of filopodia than stable branches and 60% of filopodia on extending dendritic branches have synapses, consistent with previous observations that dendritic filopodia are sites of synaptogenesis (Fiala et al., 1998, Nikolakopoulou et al., 2010 and Toni et al., 2007). Note that the dendritic filopodia analyzed by EM in this study are too small to be observed by in vivo imaging. The increased synapse maturation on stable dendritic branches is consistent with the proposed role of synapses in stabilizing mafosfamide neuronal structures. Similarly in axons, the increase in synaptic maturation and decrease in divergence of contacts with stable dendrites support the synaptotrophic hypothesis. Therefore, our data provide further support for the synaptotrophic hypothesis by demonstrating that activity-dependent synapse maturation correlates with dendritic branch stabilization. It is interesting to note that the extensive synapse elimination we observe was not directly predicted by the synaptotrophic hypothesis and suggests that fewer stronger synapses are more effective at stabilizing developing axons and dendrites. In contrast to dendrites,

MSBs on stable axon branches are the principle sites of synaptogenesis and new synapses were rarely found on axon filopodia. MSBs have been observed throughout the CNS in both developing and adult tissue; however, the relation between MSBs and circuit development is unclear, partly because in vivo imaging with presynaptic markers cannot resolve synaptogenesis at MSBs (Meyer and Smith, 2006 and Ruthazer et al., 2006). Therefore, our ability to characterize inputs onto dynamic and stable dendritic branches has allowed us to demonstrate that dynamic dendrites preferentially contact MSBs compared to stable dendrites within the same arbor. New dendritic spines in adult brain preferentially synapse with MSBs (Knott et al., 2006 and Toni et al., 2007).