Distribution of synaptic vesicle proteins within single retinotectal axons of Xenopus tadpoles

In the developing retinotectal projection, retinal axon arbor structure changes rapidly within the target tectal neuropil at stages when the visual system functions to process visual information. In vivo imaging of single retinotectal axon arbors shows that up to 50% of the arbor branch length can be restructured within 8 h and short branchtips have average lifetimes of 10 min. To determine if presynaptic sites are restricted to the relatively stable part of the arbor or if they are also located on the more dynamic portions of the arbor, punctate staining of synaptic vesicle proteins (SVP) synapsin 1 and synaptophysin was mapped within individual retinal axons using double-label confocal immunocytochemistry. We report that SVP puncta were distributed throughout the retinotectal axon arbor. Notably, short branchtips, which are known to be extremely dynamic, contain the presynaptic machinery necessary for synaptic transmission. These data support a model in which activity-dependent mechanisms can influence presynaptic axon arbor morphology by modifying the rate of dynamic rearrangements of axonal branchtips.