Persistent retrograde labeling of adult rat retinal ganglion cells with the carbocyanine dye diI

To study the retrograde labeling of intact and axotomized retinal ganglion cells (RGCs) over long periods of time, we applied the carbocyanine dye diI to the superior colliculi (SC) and dorsal lateral geniculate nuclei (dLGN) in adult albino rats and examined the retinas by fluorescence microscopy after different periods of survival. Retrogradely labeled RGCs, which were observed in the retinas as early as 3 days after application of the dye, gradually increased in density so that by 7 days more than 80% of the RGCs were labeled and by 30 days diI-labeled cell densities were similar to those observed after short applications of other tracers. Using short-term retrograde labeling with fast blue (FB) as an independent marker of RGCs, it was determined that these neurons remained labeled with diI for periods of up to 9 months without apparent leakage of the tracer to other retinal cells. In addition, diI labeling persisted in the somata of more than 80% of axotomized RGCs whose contact with the source of label had been interrupted for 3 months. Thus, we propose that retrogradely transported diI is a useful label for quantitative studies of neuronal populations, even after axotomy.