Changes in the retinotectal projection pattern during growth and optic nerve regeneration in the goldfish

In the goldfish, retinal ganglion cells (RGC) send their axons through the optic nerve and tract to the contralateral optic tectum where their terminal arborisations form a precise retinotopic map. The cut optic nerve regenerates to reestablish this map. Retina and tectum grow throughout life but with different modes of growth. Nevertheless, the map remains retinotopic. A caudal shift of RGC terminal arbors was proposed to allow for the different modes of growth: the hypothesis of shifting connections. The retinotectal projection of the normal goldfish was studied using the neuronal tracer horseradish peroxidase (HRP) and was shown to be consistent with this hypothesis. It had been proposed that, in regeneration, RGC can utilise the similar firing patterns of their near-neighbours to recognise them and to form terminal arborisations next to them on the tectum. An anatomical method to measure map precision was developed to test this hypothesis. This method, using the retrograde transport of wheatgerm agglutinin conjugated to HRP (WGA-HRP), is described. When WGA-HRP is used appropriately, its uptake can be restricted to terminal arborisations. Measurement of nearest- neighbour distances of RGC so labelled gives an estimate of map precision. In regeneration in standard laboratory conditions, RGC labelled after injection of WGA-HRP at a standard tectal site were initially scattered. As regeneration proceeded, a retinotopic cluster of labelled RGC was seen and scattered cells were eliminated. Fish with optic nerve cut and lens ablation on one side were maintained in tanks in diurnal, continuous or stroboscopic light. Stroboscopic light obliterates local similarities in firing patterns. Fish in stroboscopic light had less precise maps after optic nerve regeneration than controls. This result is consistent with the hypothesis that locally similar firing patterns are important in establishing precision in the regenerating retinotectal map