Regeneration of dopaminergic neurons in goldfish retina.

The conditions necessary to trigger regeneration of dopaminergic neurons were investigated in the goldfish retina. Intraocular injection of 6-hydroxydopamine (6-OHDA) was used to destroy dopaminergic neurons, and neuronal regeneration was monitored by injections of the thymidine analog bromodeoxyuridine (BUdR). Regenerated dopaminergic neurons, (identified by double-labeling with anti-tyrosine hydroxylase and anti-BUdR antibodies) were found within 3 weeks after 2 injections of 0.6 mg/ml 6-OHDA (estimated intraocular concentration), but not after injection of lower doses. All retinas with regenerated dopaminergic neurons also contained other types of regenerated neurons, including cones and ganglion cells, consistent with nuclear counts which revealed non-selective cell loss (34-36%) in both the outer and inner nuclear layers after exposure to the high dose, but not lower doses of 6-OHDA. Regenerated neurons were produced by clusters of dividing neuroepithelial cells probably derived from rod precursors in the outer nuclear layer. These results demonstrate that dopaminergic neurons will not regenerate after they are selectively ablated but only as part of a developmental process that involves generation of multiple cell types.