Visualizing axons in the Drosophila central nervous system using immunohistochemistry and immunofluorescence.

How axons in the developing nervous system navigate to their correct targets is a fundamental question in neuroscience. Studies of axon guidance in the embryonic central nervous system (CNS) of Drosophila melanogaster have proved instrumental to the identification of molecules and mechanisms that regulate wiring. The relative simplicity of the Drosophila embryonic CNS, the advantages of genetic approaches, and the ability to analyze the pathfinding decisions of well-defined axon groups or even individual axons have all contributed to our understanding of the fundamental mechanisms of axon guidance and target selection. In this protocol, we describe methods for using fluorescent antibody staining as well as horseradish peroxidase (HRP) immunohistochemistry to reveal axon-projection patterns and identify specific guidance molecules. In the presence of 3,3'-diaminobenzidine tetrahydrochloride (DAB), HRP forms a brown, stable precipitate that allows visualization of axon populations under white light. This can be quite advantageous when teaching basic embryonic anatomy and dissection techniques, because unlike fluorescent antibody staining, HRP can be visualized in a lighted room. HRP detection does not allow precise colocalization of signals, nor can multiple colors be visualized simultaneously. These objectives are easily achieved with fluorescent antibody staining.