Normal numbers of retinotectal synapses during the activity-sensitive period of optic regeneration in goldfish: HRP-EM evidence implicating synapse rearrangement and collateral elimination during map refinement

Abstract

Optic and nonoptic fibers and synapses were counted in the primary optic innervation layer (S-SO-SFGS) in anteromedial tectum in normal goldfish and in fish 30, 60, and 240 d after the optic nerve was crushed. A newly developed "cold-fill" HRP-labeling protocol was used to label optic afferents for electron microscopy, and counts were then made on EM photomontages of columns through the HRP-labeled S-SO-SFGS. Normal numbers of retinotectal synapses were present at 30 d regeneration, at a time when activity-dependent refinement of the optic projection is incomplete. Normal numbers were also found at 60 and 240 d, when refinement is largely completed. In contrast to this constancy in optic synapse numbers, there was nearly 10 times the normal number of optic fibers in the SFGS at 30 d, and these were reduced by 50% at 60 d, remaining over 4 times normal at 240 d. These findings imply extensive rearrangement of optic synapses during map refinement. They also indicate that synapse rearrangement is associated with the elimination of optic collaterals.

Imaging of individual normal and regenerating optic fibers in the brain of living adult goldfish

“…4), and there is reason to think that many of these incorrectly positioned axonal branches are eliminated with time (Becker and Cook, 1988). Electron microscope studies show that there are nearly 10 times the normal number of optic fiber profiles in the tectum at 30 days following optic nerve crush, and most of these are lost by 60 days (Hayes and Meyer, 1989). Whether there is a period of net retraction of entire fibers or fiber branches, or whether retraction of an individual fiber branch is associated with a concomitant advance of another branch of this same fiber, are issues that remain to be addressed using in vivo imaging technology.…”

Growth dynamics and morphology of regenerating optic fibers in tectum are altered by injury conditions: An in vivo imaging study in goldfish

“…During regeneration, fibers reform the normal number of synaptic connections by 1 month. This number remains constant thereafter even though fibers are changing positions to refine retinotopy (17,31). If half of tectum is removed to produce a compressed projection, the number of optic synapses per unit projection area of tectum is the same as in the normal tectum (14,32).…”

“…Good lamination and normal numbers of synapses are restored. And perhaps most importantly, fish recover excellent pattern and color vision (1,17,28,29,33,37).…”

Restoration of visual function following optic nerve regeneration in bluegill (Lepomis macrochirus) × pumpkinseed (Lepomis gibbosus) hybrid sunfish

“…A small 4.58 tilt that continued to persist at 15-16 weeks post-crush indicates that refinement may still be ongoing. Hayes and Meyer 1989! indicated that synaptic refinement in the goldfish could persist through 34 weeks post-transection.…”

“…Within two weeks of optic nerve transection, retinal axons re-invade and branch across the optic tectum Stuermer & Easter, 1984;Rankin & Cook, 1986;Hayes & Meyer, 1988;Schmidt et al, 1988!. Synaptogenesis has been documented within three weeks after optic nerve crush in the goldfish, with subsequent regeneration proceeding across the tectum in an anterior to posterior gradient~Schmidt & Edwards, 1983; Hayes & Meyer, 1989;Meyer & Kageyama, 1999!. There is often an overproduction of optic nerve axons early in regeneration~Schmidt et al, 1988!, and although the retinotopic map is roughly restored by week 5, refinement of the axons and synapses continues for several more weeks~Meyer, 1980;Meyer & Kageyama, 1999;Meyer et al, 1985!. Sunfish are primarily visual predators and feed throughout the day on a wide variety of prey~Mittelbach, 1984; Janssen & Corcoran, 1993!. Sunfish visual projections have been mapped Striedter & Northcutt, 1989;Northcutt & Butler, 1991!, and this family of fishes has served as a model for physiological and behavioral assessment of optic nerve regeneration~Schwassmann & Kruger, 1965;Northmore, 1981, 1991 studies used simple visual tests such as light detection~Northmore, 1989;Northmore & Masino, 1984! or contrast sensitivity~North-more & Celenza, 1992!…”