A modification of biotinylated dextran amine histochemistry for labeling the developing mammalian brain.

Biotinylated dextran amine (BDA) has proven to be an excellent anterograde tracer in adult mammalian brains, having some advantages over other anterograde tracers such as Phaseolus vulgaris-leucoagglutinin (PHA-L) and biocytin. However, results are inferior when BDA is used in neonatal mammals. To improve the sensitivity and quality of BDA labeling in neonatal mammalian brains, the tetramethylbenzidine-sodium tungstate (TMB-ST) method for horseradish peroxidase (HRP) histochemistry was modified and used in BDA histochemistry. After BDA application to the visual cortex of neonatal rat and cat, contralateral and ipsilateral cortical and subcortical regions were examined for BDA-labeled exons and terminals. The modified BDA histochemistry produced corpus callosum (CC) axons in neonatal rat and cat that were heavily and continuously labeled. The distribution, trajectories, branching and termination of individual CC axons, and even possible axon-axon contracts, were clearly identified in exquisite detail, even at low magnification. The quality of BDA labeling in the ipsilateral lateral geniculate nucleus and superior colliculus was similar to that of the CC axonal labeling. These results indicate that the modified BDA histochemistry provides a very sensitive and reliable approach to revealing the detailed distribution and morphology of projecting axons and terminals in the developing mammalian nervous system.