SYNAPTIC PLASTICITY AND COMPETITION DURING OPTIC REGENERATION IN GOLDFISH TECTUM

The regenerating optic projection in goldfish is a model system for studying how patterned synaptic connections are made. Ultrastructural studies are described which examine synapse formation and fiber outgrowth on tectum during optic regeneration. These were done in order to identify possible mechanisms by which the initially diffuse optic projection is spatially restricted between 30 and 60 days. An improved horseradish peroxidase labeling protocol was developed to label regenerating optic fibers on tectum and a column sampling method was used to examine the laminar distribution of optic afferents in the primary optic innervation layer of normal fish and fish regenerating for 14-240 days. Optic and non-optic fibers and synapses per column were then quantified. Two further experiments in fish regenerating during the elimination of impulse activity using tetrodotoxin (TTX) or after tectal ablation were then done to examine how the number of optic synapses is regulated.;These findings indicate optic synapses are rearranging during map refinement. This rearrangement appears to be the consequence of the elimination of optic fibers and not due to map refinement, because optic fibers were similarly eliminated under impulse blockade when map refinement has previously been shown to be inhibited with TTX. This evinces that activity determines the final outcome of synapse rearrangment. The synaptic counts in half tectum indicate that the number of postsynaptic sites available to optic fibers is constrained by tectum. Thus, regenerating optic fibers are apparently competing for a fixed number of these sites. The data are consistent with the hypothesis that these sites are made available when some optic fibers are eliminated.;Labeling showed that synaptogenesis was rapid and most optic fibers were fasciculated. Counts showed that normal numbers of optic synapses were made at 30, 60 and 240 days even though numbers of optic fibers were 9-10 times normal at 30 days and 4-5 times normal at 60 and 240 days. This was found to be unaffected by neural impulse blockade. The optic projection compressed onto half tectum also showed normal numbers of optic synapses per column indicating that about half the normal number of optic synapses is made on half tectum.