Central projections and connections of lumbar primary afferent fibers in adult rats: effectively revealed using Texas red-dextran amine tracing

Signals from lumbar primary afferent fibers are important for modulating locomotion of the hind-limbs. However, silver impregnation techniques, autoradiography, wheat germ agglutinin-horseradish peroxidase and cholera toxin B subunit-horseradish peroxidase cannot image the central projections and connections of the dorsal root in detail. Thus, we injected 3-kDa Texas red-dextran amine into the proximal trunks of L(4) dorsal roots in adult rats. Confocal microscopy results revealed that numerous labeled arborizations and varicosities extended to the dorsal horn from T(12)–S(4), to Clarke's column from T(10)–L(2), and to the ventral horn from L(1–5). The labeled varicosities at the L(4) cord level were very dense, particularly in laminae I–III, and the density decreased gradually in more rostral and caudal segments. In addition, they were predominately distributed in laminae I–IV, moderately in laminae V–VII and sparsely in laminae VIII–X. Furthermore, direct contacts of lumbar afferent fibers with propriospinal neurons were widespread in gray matter. In conclusion, the projection and connection patterns of L(4) afferents were illustrated in detail by Texas red-dextran amine-dorsal root tracing.