Persistent neuronal labeling by retrograde fluorescent tracers: a comparison between Fast Blue, Fluoro-Gold and various dextran conjugates.

The permanence of retrograde neuronal labeling by the fluorescent tracers Fast Blue, Fluoro-Gold, Mini-Ruby, Fluoro-Ruby and Fluoro-Emerald was investigated in adult rat spinal motorneurons at 1, 4, 12 and 24 weeks after tracer application to a transected muscle nerve. After 1 week, the largest number of retrogradely labeled motoneurons was found with Mini-Ruby, Fluoro-Gold and Fluoro-Ruby, while Fluoro-Emerald yielded a smaller number of labeled cells. With increasing survival time, all of these tracers exhibited a marked decrease in the number of labeled neurons. Fast Blue also produced very efficient staining after 1 week and, in addition, the number of Fast Blue-labeled cells remained constant over the entire time period studied. Also in embryonic spinal cord tissue exposed to Fast Blue. the label persisted for at least 6 months after transplantation into adult spinal cord. Double-labeling experiments combining Fast Blue with Fluoro-Gold, Mini-Ruby, Fluoro-Ruby or Fluoro-Emerald showed that all these substances were non-toxic and that the time-related decrease in the number of neurons labeled by the latter tracers was due to degradation or leakage of the dyes. Thus, Fast Blue would be the tracer of choice for motoneuronal labeling in long-term experiments, whereas the usage of the other tracers should be restricted to experiments of limited duration.