Recent techniques for tracing pathways in the central nervous system of developing and adult mammals

Over the last 20 years, the choice of neural tracers has increased manyfold , and includes newly introduced anterograde tracers that allow quantitation of single-axon morphologies, and retrograde tracers that can be combined w ith intracellular fills for the study of dendritic arbors of neurons which have a specific projection pattern. The combination of several different tr acers now permits the comparison of multiple connections in the same animal , both quantitatively and qualitatively. Moreover, the finding of new virus strains, which infect neural cells without killing them, provides a tool f or studying multisynaptic connections that participate in a circuit. In thi s paper, the labeling characteristics, mechanism of transport and advantage s/disadvantages of use are discussed for the following recently introduced neural tracers: carbocyanine dyes, fluorescent latex microspheres, fluoresc ent dextrans, biocytin, dextran amines, Phaseolus vulgaris leucoagglutinin, cholera toxin and viruses. We also suggest the choice of specific tracers, depending on the experimental animal, age and type of connection to be stu died, and discuss quantitative methodologies. (C) 2000 Elsevier Science Inc .