Improving the Application of High Molecular Weight Biotinylated Dextran Amine for Thalamocortical Projection Tracing in the Rat.

High molecular weight biotinylated dextran amine (BDA) has been used as a highly sensitive neuroanatomical tracer for many decades. Since the quality of its labeling was affected by various factors, here, we provide a refined protocol for the application of high molecular weight BDA for studying optimal neural labeling in the central nervous system. After stereotactic injection of BDA into the ventral posteromedial nucleus (VPM) of the thalamus in the rat through a delicate glass pipette, BDA was stained with fluorescent streptavidin-Alexa (AF) 594 and counterstained with fluorescent Nissl stain AF500/525. On the background of green Nissl staining, the red BDA labeling, including neuronal cell bodies and axonal terminals, was more distinctly demonstrated in the somatosensory cortex. Furthermore, double fluorescent staining for BDA and the calcium-binding protein parvalbumin (PV) was carried out to observe the correlation of BDA labeling and PV-positive interneurons in the cortical target, providing the opportunity to study the local neural circuits and their chemical characteristics. Thus, this refined method is not only suitable for visualizing high quality neural labeling with the high molecular weight BDA through reciprocal neural pathways between the thalamus and cerebral cortex, but also will permit the simultaneous demonstration of other neural markers with fluorescent histochemistry or immunochemistry.