Multiple Neuroanatomical Tract-Tracing: Approaches for Multiple Tract-Tracing

Experimental neuroanatomical tracing techniques are fundamental to the study of the structure of the central nervous system. In the last few decades, many new methods for axonal tracing and cell labeling have been introduced. Neuroanatomical tracing applied as an isolated method produces relatively straightforward answers, for instance, whether there is connectivity from compartment Y in nucleus A to layer X in area B. However, questions that deal with the intrinsic complexity of brain circuits require the application of multiple-tracing paradigms in which two or even three different tracers are combined in single histological sections. With such paradigms we can handle questions like “are the fibers arriving in layer X of area B in contact with neurons that project to compartment Z in nucleus C,” “do these projection neurons receive as well innervation from area W,” and “what is the neurochemical signature of these connectivity-identified neurons?”We illustrate this approach with examples from our studies on pallidonigral connectivity in association with nigrostriatal efferent neurons.