A note on the use of biocytin in anterograde tracing studies in the central nervous system: application at both light and electron microscopic level.

A new neuroanatomical method based on the anterograde transport of biocytin has recently been reported. The validity of this method is examined closely on a number of well established pathways in the central nervous system, as well as the experimental parameters necessary for its effective use. iontophoretic application of this amino acid-biotin complex allows the placement of very discrete injections. At the injection sites neurones appear to be completely filled, whereas fibres of passage are left unlabelled. From the injection site axons can be traced at the light microscopic level to their terminal fields, where their pattern of termination and morphology can be clearly visualised. These anterogradely labelled fibres can be examined further by electron microscopy to identify synaptic specializations. The uptake and transport of biocytin, following iontophoretic injection, appears to be similar to that observed using techniques employing radiolabelled amino acids or Phaseolus vulgaris-leucoagglutinin (PHA-L), in that it is transported predominantly in the anterograde direction. However, biocytin offers some advantages over these techniques in that its detection is relatively easy at electron microscopic levels. Furthermore, biocytin appears to be transported rapidly making it possible to study short pathways under acute experimental conditions.