Visualizing synapse formation in arborizing optic axons in vivo: dynamics and modulation by BDNF

Dynamic developmental changes in axon arbor morphology may directly reflect the formation, stabilization and elimination of synapses. We used dual-col or imaging to study, in the live, developing animal, the relationship betwe en axon arborization and synapse formation at the single cell level, and to examine the participation of brain-derived neurotrophic factor (BDNF) in s ynaptogenesis. Green fluorescent protein (GFP)-tagged synaptobrevin II serv ed as a marker to visualize synaptic sites in individual fluorescently labe led Xenopus optic axons. Time-lapse confocal microscopy revealed that altho ugh most synapses remain stable, synapses are also formed and eliminated as axons branch and increase their complexity. Most new branches originated a t GFP-labeled synaptic sites. Increasing BDNF levels significantly increase d both axon arborization and synapse number, with BDNF increasing synapse n umber per axon terminal. The ability to visualize central synapses in real time provides insights about the dynamic mechanisms underlying synaptogenes is, and reveals BDNF as a modulator of synaptogenesis in vivo.