08; Figure 5Q), which is greater than the variation seen in synapses sampled from the neuron imaged at daily intervals, where R2 = 0.25 (Figure 4F). These data indicate that synaptic rearrangements associated with branch extension and stabilization occur at least over ABT-263 molecular weight a time course of hours. The analysis of synaptic contacts revealed a conversion from clustered immature synaptic contacts on extending dendrites

to fewer mature contacts onto stable dendrites. This is accompanied by decreased divergence, measured as the number of postsynaptic profiles contacted by individual presynaptic boutons. The results suggest that presynaptic boutons undergo structural reorganization, possibly corresponding to the dynamics of axon branches. We therefore conducted an analysis of synaptic circuit formation from the point of view of the axon. The labeled neuron imaged at daily intervals had an elaborate local axon arbor that exhibited dynamic branch extension, stabilization, and retraction (Figure 6A). We identified a total of 170 axodendritic and axosomatic synaptic contacts from 102 boutons made by 374.3 μm of reconstructed axon branches for an average synapse density of 0.45 synapses/μm of axon branch length. We examined the ultrastructural features of the presynaptic boutons with respect to the dynamics of the axon branches based on the in vivo

two-photon images (Figures 6A–6F). We analyzed 203.40 μm from 14 stable branches, 107.81 μm from

9 extended branches, and 63.09 μm from 10 retracted branches. Unlike dendrites, the synapse density of stable axon Sorafenib cell line branches was significantly higher than that of extended and retracted branches (stable: 0.62 ± 0.03 synapses/μm; extended: 0.27 ± 0.04 synapses/μm, p < 0.001; retracted: crotamiton 0.21 ± 0.05 synapses/μm, p < 0.001, post hoc Mann-Whitney test after Kruskal-Wallis test; Figure 6G). The synapses formed by stable axon branches were significantly more mature than synapses formed by extended and retracted axon branches (maturation index; stable: 41.91 ± 1.48, n = 130; extended: 26.02 ± 2.67, n = 26, p < 0.05; retracted: 30.65 ± 2.58, n = 14, p < 0.05, post hoc Kruskal-Wallis test; Figure 6H). When we analyzed the divergence of individual presynaptic boutons, we found that each axon bouton contacted between one to four partners. In contrast to dendrites, presynaptic boutons from stable axon branches form connections with more postsynaptic partners than boutons from extended or retracted axon branches (stable: 1.83 ± 0.09 connections/bouton, n = 71; extended: 1.37 ± 0.11 connections/bouton, n = 19, p < 0.005; retracted: 1.08 ± 0.18 connections/bouton, n = 9, p < 0.01, post hoc Kruskal-Wallis test; Figure 6I). Similar to dendritic filopodia, axonal filopodia were found at a higher density on extended axon branches (0.24 filopodia/μm) compared to stable axon branches (0.12 filopodia/μm, p < 0.