Tract-tracing in developing systems and in postmortem human material using carbocyanine dyes

© 2006 Springer Science+Business Media, Inc. All rights reserved. Rapid progress in neurobiology and genetics demands knowledge of fundamental aspects of brain development including the connectivity patterns within developing and adult brains. The primary focus of this chapter is on neuroanatomical tract-tracing using carbocyanine dyes which have several advantages over traditional tracing methods. First utilized for in vitro studies, a major breakthrough in the late 1980s was the demonstration that carbocyanine dyes act as anterograde and retrograde tracers in fixed tissue, eliminating the need for diffusion of tracers in vivo. Moreover, carbocyanine dyes are more efficacious than classical tracing methodologies especially during early stages of development, and consequently have been used to reveal the spatiotemporal patterns of axonal development in different species. Furthermore, the unique properties of the carbocyanine dye tracing method have opened up new avenues for tracing connections in human postmortem specimens. This is a key step in determining the precise connectivity of neural circuits in the human brain, and subsequently to relate this knowledge to pathological cases. The success of carbocyanine dyes as tracers, both in vitro and in fixed material, is reflected in the flurry of publications throughout the 1990s and into the present. However, there are relatively few systematic studies that have tested parameters to optimize their use or to give practical advice to enhance their efficacy. This chapter aims to bring together some of our experiences with the carbocyanine dye tracing method drawn from our studies in mammalian, reptilian, and human and nonhuman primate specimens.