BACKGROUND: The corticospinal tract is the core structure of cerebral control of extremity movement and plasticity, which are prerequisites for movement rehabilitation after brain injury. The measurement and assessment of plasticity changes within the corticospinal tract has become one of the key goals in this field.OBJECTIVE: To explore the effects of biotinylated dextran amine (BDA) as a neural tracer in the rat corticospinal tract and the possibilities of assessing plasticity within the corticospinal tract.DESIGN: An observational experiment.SETTING: Department of Acupuncture of Chinese Medical College, Chongqing Medical University, Department of Neurology, the Second Affiliated Hospital, Chongqing Medical University.MATERIALS: Eighteen male adult Sprague Dawley (SD) rats of clean grade, weighing 200-250 g, were provided by the experimental animal center of Chongqing Medical University. The animal procedures in this study were in accordance with the animal ethics standards. BDA was provided by Vector Laboratories Company (USA, catalogue Sp-1140; serial number R0721).METHODS: This experiment was performed in the Laboratory of Chongqing Medical University between September and December 2006. Adult SD rats were used in the experiment and 15% BDA was injected slowly with a mini-syringe through two round (3 mm diameter) holes into the left sensory and motor cortex. The center of one hole was located 3 mm anterior from the anterior fontanel and 1.5 mm left of the midline; the second hole was located 1.5 mm posterior from the anterior fontanel and 4 mm left of the midline. Three injections were made at each hole at three different levels: 1.4, 1.2, and 1 mm ventral from the surface of the flat skull. After 14 days, the brains and spinal cords were removed and frozen. Sections were cut on a cryostat and BDA transportation absorbed by axons was observed under a fluorescence microscope.MAIN OUTCOME MEASURES: Axonal absorption and transportation of BDA was observed under fluorescence microscope.RESULTS: Eighteen SD rats were enrolled in this experiment; 12 rats were included in the final analysis and six were eliminated, resulting in a dropout rate of 33% (6/18). BDA injected into the left cortex was absorbed in the axons, and fluorescence was observed throughout the pyramidal neurons and axons of the left cerebral cortex. At 14 days after rejection, BDA was detected in the midbrain and cervical enlargement along the CST, and axonal structures and Ranvier nodes were clearly observed with 200× magnification. CONCLUSION: BDA injected into the cerebral cortex effectively traces the corticospinal tract and is biologically stable over long distance transportation. In addition, the method of BDA tracing is fairly simple to perform.