Evidence that biocytin is taken up by axons

Unexpected anterograde labeling is systematically observed in the pontine nuclei following iontophoretic injection of biocytin in the substantia nigra pars reticulata. Control experiments using WGA-HRP have led to deny the existence of a nigro-pontine pathway. The possibility that biocytin is taken up by fibers of passage has been tested. Deposits of biocytin in the corpus callosum result in a massive axonal labeling of this fibrous system. This study, in contrast to previous reports, stresses that biocytin is easily taken up and transported by axons. Hence, this tracer has to be used with careful controls when injected in structures crossed by fibrous tracts.