Effects of permeability of midtectal barriers in goldfish on compression of the visuotectal projection rostrally and regenerative escape caudally

Optic synapse number but not density is constrained during regeneration onto surgically halved tectum in goldfish: HRP-EM evidence that optic fibers compete for fixed numbers of postsynaptic sites on the tectum

The number of optic synapses in the half tectum of goldfish was counted by using an improved HRP-labeling protocol and a columnar sampling method that spanned the entire optic innervation layer, S-SO-SFGS. It was previously found by using this procedure in intact tectum that the normal number of optic synapses was regenerated by 30 days and maintained thereafter even in the absence of impulse activity. This suggested that the number of synapses in this system was intrinsically fixed. In order to examine whether this limit was imposed by optic fibers or by target cells, optic synapses were counted in surgically halved tecta which received compressed optic projections consisting of regenerating optic fibers from the entire retina. We reasoned that if synapse number is a function of the number of afferents, then there should be twice the normal number of optic synapses per column; on the other hand, if their number is fixed by target, then their number per column should be normal. We found that the number of optic (labeled) synapses was normal in sample columns from fish at 70 days and 160 days after optic nerve crush. Thus, retinal ganglion cells, on average, formed half as many synapses on the half tectum compared to intact tectum, indicating the number of optic synapses was limited by the tectum. The number of nonoptic (unlabeled) synapses was also found to be normal. By contrast, the S-SO-SFGS was found to be 88-103% thicker compared to normal fish, apparently because of a 20-fold increase in the number of optic fibers. As a result, the density of synapses was about half normal in half tecta, and so, in contrast to synapse number, synaptic density is not constrained during regeneration. We infer from these data that optic fibers compete for limited numbers of postsynaptic sites during regeneration and suggest that this competition promotes neural map refinement and the various plasticities described for this projection.

Selective retinal reinnervation of a surgically created tectal island in goldfish. I. Light microscopic analysis

Through anatomical and physiological studies of the regenerating retinotectal projection of goldfish, we sought to determine whether the establishment of a topographic projection is attained through a refinement of an initially less precise pattern of innervation. A 1-mm-wide mediolateral strip of caudal tectum was removed so that a small island of tectal tissue was spared at the caudal pole, and the contralateral nerve was either crushed (TIX) or left intact (TI). The presence of regenerated axons in the ablated zone and the reinnervation of the caudal island were assessed with anterograde and retrograde labeling methods in the following postoperative intervals: early, 20-50 days; middle, 50-110 days; and late, more than 170 days. The anterograde radioautographic method revealed that the appropriate layers of the tectal island became reinnervated by optic axons during the early period. During the middle and late periods, one to several large, discrete bundles bridging the lesion zone along the surface of exposed subtectal structures were readily identified both by radioautography and by anterograde or retrograde labeling following application of horseradish peroxidase to the transected optic nerve or tectal island, respectively. In contrast, the anterograde horseradish peroxidase method did not reveal axon bundles extending caudal to the half-tectum in the absence of a tectal island. Among TIX cases, retrograde horseradish peroxidase labeling of the contralateral nasal retina was more widespread in the middle period than in the late period, a result we interpret as reflecting an improvement in topographical precision with time. The area of retinal labeling among TIX cases in the late period was similar to that following caudal tectal injection in cases with simple nerve crush, although it was still elevated above normal control values. Physiological maps indicated a focal representation of the nasal retina in the tectal island in both periods and did not reveal a transient extreme convergence of retinal input. These findings are discussed in relation to Sperry's chemoaffinity theory.

Selective retinal reinnervation of a surgically created tectal island in goldfish. II. Electron microscopic analysis

In the preceding study (Edwards et al., '85), we showed that regenerating optic axons reestablish a topographically restricted projection to a caudal tectal island created by surgical removal of a 1-mm-wide strip of caudal tectum in goldfish. In the present ultrastructural study, we evaluated the dependence of this axonal outgrowth on the presence of tectal target tissue caudal to the gap. Axon counts in the lesion zone were compared between cases with complete caudal tectal ablation and cases with ablation sparing a caudal tectal island (with and without optic nerve crush). During the postoperative interval of 20-50 days (early period), up to about 1,000 unmyelinated axons with features characteristic of optic axons were present in numerous small subpial bundles in both preparations. In the subsequent interval of 50-110 days (middle period), less than 200 axons were counted caudal to simple half-tecta, whereas 4,000-14,000 myelinated and unmyelinated axons were present in a few large bundles which crossed the lesion zone of tectal island cases. In this period, optic terminals could be demonstrated in the tectal island using the anterograde horseradish peroxidase method. At 170-300 days after surgery (late period), bridging bundles contained between 2,000 and 6,000 largely myelinated axons. We conclude that caudal tectal tissue is not necessary for the initial outgrowth of a small number of axons beyond a rostral half-tectum. The target is essential, however, for the maintenance of these axon fascicles and for the subsequent massive outgrowth of axons to the island. The contributions of glial guidance, diffuse exploratory outgrowth, and target-produced trophic factors to the formation of an initially exuberant projection to the island are discussed. A process of selective axon collateral withdrawal is proposed to account for the decrease in axon numbers within bridging bundles in the late period and for the late restriction in the retinal origin of the island projection indicated by results in the preceding study (Edwards et al., '85).