RAPID REMODELING OF RETINAL ARBORS IN THE TECTUM WITH AND WITHOUT BLOCKADE OF SYNAPTIC TRANSMISSION

Dynamic rearrangements of axon terminal arbors may be critical for est ablishing appropriate connections in the developing nervous system. He re, the changes in complex retinal axon arbors in the tecta of live Xe nopus larvae were followed during the formation of the topographic ret inotectal projection. Three-dimensional reconstructions of terminal ar bors made with a confocal microscope at hourly intervals revealed rapi d remodeling of arbor extensions. Shorter branches were and retracted very rapidly, suggesting that they probe the environment for the optim al sites to form stable branches. About 27% of longer branches were pr esent throughout the entire observation period and may be sites of sta bilized synaptic contacts. Treatment of the animals to block postsynap tic activity resulted in increased rates of arbor rearrangements, whic h may coincide with decreased synapse stability. These studies reveal the dynamic behavior of nerve arbors and provide estimates for the lif etimes of retinotectal branches.