Rapid activity-dependent sprouting of optic fibers into a local area denervated by application of beta-bungarotoxin in goldfish tectum.
The retinotectal projection is known to be capable of extensive long-term expansion of connections, but it is not known how fast such changes can occur or what triggers sprouting of terminals. We studied sprouting of optic fibers into an area denervated by local microinjection of beta-bungarotoxin (beta-BTX), a specific presynaptic neurotoxin with phospholipase A2 activity that destroys nerve terminals at the neuromuscular junction. After injection of 0.1 pmol of beta-BTX, the optic terminals fired spontaneously with decreasing amplitude and became silent within 1 to 2 h. Outside the injection zone, the retinotectal map was normal, so the silent zone was associated with a scotoma in the visual field. Horseradish peroxidase (HRP) staining of the entire optic nerve showed a denervated region at the injection site with beaded, degenerating fibers at its edge. Between 3 and 9 days later, optic units were recorded within the injection zone whose receptive fields lay just outside the scotoma in the visual field, indicating that intact surrounding terminals had sprouted into the area. These sprouts made functional connections, as indicated by field potential recordings and current source-density analysis. At this time, HRP staining also demonstrated retinal innervation within the injection zone. By 12 days, normal maps with no scotoma were recorded and HRP staining was normal at the injection site, indicating that the beta-BTX-damaged fibers had regenerated to reclaim their tectal sites. The results show that the retinotectal projection of goldfish is very dynamic, since intact optic fibers can sprout into adjacent vacant postsynaptic territory within 2 to 3 days, much faster than previously reported. In a final experiment, we showed that this sprouting is activity-dependent, since it could be prevented by blocking retinal activity with intraocular tetrodotoxin (TTX) during the first 2 days postinjection, even though TTX block of activity does not block regeneration in this system. One possible mechanism for this rapidly triggered sprouting is that arachidonic acid liberated by beta-BTX acts as a sprouting factor to attract surrounding healthy fibers into the denervated region but requires activity at the terminals to be effective.
Schmidt
J T
JT
Department of Biological Science, State University of New York at Albany 12222, USA.
Lemere
C A
CA
D000818:Animals; D002038:Bungarotoxins; D003714:Denervation; D006054:Goldfish; D006735:Horseradish Peroxidase; D009416:Nerve Regeneration / Q000187:drug effects / Q000502:physiology*; D009900:Optic Nerve / Q000502:physiology*; D003336:Tectum Mesencephali; D013779:Tetrodotoxin / Q000494:pharmacology
D002038:Bungarotoxins; D013779:Tetrodotoxin; D006735:Horseradish Peroxidase