DiI labeling.
Spinal cords were dissected out with the DRGs attached, motor root was cut to prevent backfilling of motor neurons, and crystals of 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI; Molecular Probes, Eugene, Oregon, United States) were inserted in DRGs. Labeled spinal cords were incubated at 37 °C for 8 d and sectioned on a vibrotome at 100 μm. Sections were observed under the fluorescent microscope and photographed, and images were transferred to Adobe Photoshop, inverted, and adjusted for brightness and contrast. Photoconverted in 0.15% DAB (3,3′-diaminobenzidine; Sigma) in 0.1M Tris buffer (pH 8.2).