Multiple axonal tracing: simultaneous detection of three tracers in the same section.

Multiple neuroanatomical tract-tracing methods are important tools for elucidating the connectivity between different populations of neurons. Evaluation of the question as to whether two specific fiber inputs converge on a particular, identified population of projection neurons requires the application of a triple-staining procedure that allows the unequivocal detection of three markers in a single section. The present report deals with a combination of tracing methods using anterogradely transported Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine in conjunction with retrogradely transported Fluoro-Gold. These tracers were simultaneously detected according to a three-color paradigm, which includes the use of three different peroxidase substrates (nickel-enhanced diaminobenzidine, diaminobenzidine, and Vector VIP), thus resulting in three distinct precipitates: black, brown, and purple. We illustrate this method by showing convergence of projections arising from neurons located in two separate basal ganglia-related nuclei onto identified thalamostriatal projection neurons.