Topographic refinement of the goldfish retinotectal projection: sensitivity to stroboscopic light at different periods during optic nerve regeneration.

Abstract

When the severed optic nerve of a goldfish regenerates, the restored retinotectal projection is at first only grossly topographic. Refinement occurs later, by a mechanism that is thought to depend on correlation in the electrical activity of neighbouring retinal ganglion cells because it can be blocked by exposure to tetrodotoxin or diffuse stroboscopic (strobe) light. To study the sensitivity of retinotectal map refinement to strobe light at different periods during regeneration, four equivalent groups of goldfish with severed right optic nerves and ablated right lenses were interchanged, at 21 day intervals, between strobe (S) and diurnal (D) light to generate four different exposure sequences. After 84 days, a localized iontophoretic injection of WGA-HRP was made into each left tectum to label retinal ganglion cells with terminal arbors at the injection site, and the degree of clustering of the labelled cells was estimated statistically to assess map refinement. Retinae exposed to the sequences SDDS, SSDD or DSSD were broadly similar to each other and to those seen previously after exposure for similar total periods to diurnal light, constant light or strobe light with the lens in place. However, those kept in diurnal light for the first 42 days and in strobe light thereafter (DDSS) revealed significantly less refinement, equivalent to that seen previously after just 42-44 days in diurnal light. Thus diffuse strobe light itself neither sharpens nor unsharpens the regenerated map: its immediate effect seems only to be the indefinite postponement of whatever refinement would otherwise have occurred.(ABSTRACT TRUNCATED AT 250 WORDS)