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-t erm expansion of connections, but it is not known how fast such change s can occur or what triggers sprouting of terminals. We studied sprout ing of optic fibers into an area denervated by local microinjection of beta-bungarotoxin (beta-BTX), a specific presynaptic neurotoxin with phospholipase A(2) activity that destroys nerve terminals at the neuro muscular junction. After injection of 0.1 pmol of beta-BTX, the optic terminals fired spontaneously with decreasing amplitude and became sil ent 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 vi sual field. Horseradish peroxidase (HRP) staining of the entire optic nerve showed a denervated region at the injection site with beaded, de generating fibers at its edge. Between 3 and 9 days later, optic units were recorded within the injection zone whose receptive fields lay ju st outside the scotoma in the visual field, indicating that intact sur rounding terminals had sprouted into the area. These sprouts made func tional connections, as indicated by field potential recordings and cur rent source-density analysis. At this time, HRP staining also demonstr ated 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 regen erated to reclaim their tectal sites. The results show that the retino tectal projection of goldfish is very dynamic, since intact optic fibe rs can sprout into adjacent vacant postsynaptic territory within 2 to 3 days, much faster than previously reported. In a final experiment, w e 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 a ctivity does not block regeneration in this system. One possible mecha nism for this rapidly triggered sprouting is that arachidonic acid lib erated by beta-BTX acts as a sprouting factor to attract surrounding h ealthy fibers into the denervated region but requires activity at the terminals to be effective. (C) 1996 John Wiley & Sons, Inc.