Retrograde tracing with Fluoro-Gold: different methods of tracer detection at the ultrastructural level and neurodegenerative changes of back-filled neurons in long-term studies.

Among the available retrograde fluorescent tracers Fluoro-Gold (FG) is particularly advantageous because it (1) is not only detectable by fluorescence microscopy but also immunocytochemically, resulting in an almost complete staining of the dendritic arbor, (2) is visible in lysosome-like structures allowing for the identification of projection neurons at the ultrastructural level, and (3) remains in the labeled neurons for extended periods of time. Photoconversion and immunostaining for FG, respectively, result in a stable, electron-dense reaction product. Thus, the retrogradely labeled cells can be analyzed quantitatively in the light- and electron microscope for their structural characteristics and input synapses. Long-term studies of back-filled neurons provided evidence for neurotoxic effects of FG in these cells.