Tracing of corticospinal fibers by extracellular pressure-injection of biocytin into the motor cortex in rats.

The aim of this study was to determine if biocytin, a low molecular analog of biotin, could reliably label details of corticospinal fibers at the lumbar level in rats. Biocytin (5%) was injected extracellularly by pressure into the unilateral hindlimb area of the motor cortex. Frozen sections of the injection sites and the lumbar segments were incubated with avidin-horseradish peroxidase (HRP). Terminal labeling of corticospinal fibers at the lumbar segments could be observed within 2-3 days of the injection. These results indicate that anterograde tracing with biocytin can be applied to label axons in a long tract such as the corticospinal tract in rats. Biocytin stain provided a view of a higher level of detail than wheat germ agglutinin (WGA)-HRP.