Characterisation of tectal ephrin-A2 expression during optic nerve regeneration in goldfish: implications for restoration of topography.

EphA receptors and their ligands the ephrin-As, expressed as retinal and tectal gradients, are required for the development of retino-tectal topography [Neuron 25 (2000) 563] and its restoration during goldfish optic nerve regeneration [Mol. Cell. Neurosci. 25 (2004) 56]. We have reported previously that, during regeneration, a transient EphA3/A5 gradient is formed by differential expression across the entire retinal ganglion cell (RGC) population [Neurosci. Abs. 33 (2003) 358.2; Exp. Neurol. 183 (2003) 593]. In retino-recipient tectal layers, ephrin-A2 is normally expressed by only a sub-population of cells, but during regeneration, there is a graded increase with more expressing cells caudally than rostrally [Exp. Neurol. 166 (2000) 196]. Here, we examine the characteristics of tectal ephrin-A2 expression during regeneration. We report that the level of ephrin-A2 expression is comparable for all ephrin-A2-positive cells in normal animals and during regeneration. Using double-labelling immunohistochemistry for ephrin-A2 and specific cell markers (NeuN for neurons, GA5 for astrocytes, NN-1 for microglia/endothelial cells and 6D2 for oligodendrocytes), we demonstrate that ephrin-A2-expressing cells, as in normal animals, are exclusively neuronal. Moreover, double labelling with BrdU showed that ephrin-A2 is expressed in resident cells and not those generated during optic nerve regeneration [Brain Res. 854 (2000) 178, 153 (1978) 345].