Labeling of central projections of primary afferents in adult rats: a comparison between biotinylated dextran amine, neurobiotin and Phaseolus vulgaris-leucoagglutinin.

The efficacy of anterograde labeling of the central projections of primary afferent fibers were compared between biotinylated dextran amine (BDA), neurobiotin (NB) and Phaseolus vulgaris-leucoagglutinin (PHA-L) after injections into the L5 or T13 dorsal root ganglia (DRGs) of adult rats. Excellent labeling was obtained with BDA, which visualized fibers with fine terminal boutons in the L5 and T13 spinal cord segments, Clarke's nucleus and the gracile nucleus. Rarely observed crossed projections to the gracile nucleus and L5 ventral horn of the contralateral side could also be distinguished. Even in the most successful experiments, however, BDA labeled only about one-third of the axons originating from the injected dorsal root ganglion. BDA was also efficient as transganglionic tracer after application to the transected sciatic nerve. NB produced no significant labeling of the L5 primary afferents, and was only moderately effective on the T13 level. PHA injections resulted in sparse terminal labeling of the T13 and L5 afferents. Thus, BDA is an effective tracer for long-range labeling of primary afferent projections in the spinal cord and brain stem. Since not all stem fibers become labeled, however, the method does not allow quantification of all axon branches and terminals arising from the injected DRGs.