Sexual differences in the distribution of substance P immunoreactive fibers in the ventral horn of the rat lumbar spinal cord

The distribution of substance P (SP) in the rat spinal cord was investigated by peroxidase-anti-peroxidase immunocytochemistry combined with retrograde horseradish peroxidase (HRP) labeling via the cremaster muscle. In the male rats, a dense network of SP immunoreactive (SP-IR) fibers and terminals was detected in the ventral column of the L1 and L2 segments (Vent L1-2) with a different density and extent from the other segmental levels. These fibers and terminals were accumulated within and around the nucleus centromedialis lumbaris (CM) of the L1 and L2 segments. However, in the female rats, SP-IR fibers and terminals were sparse in the Vent L1-2 without particular segmental differences. HRP-positive motoneurons were located in the CM and surrounded by SP-IR fibers and terminals. These results indicate that the Vent L1-2 of the rat spinal cord shows sexual dimorphism with respect to the regional distribution of SP-IR fibers and terminals, and that motoneurons that innervate the cremaster muscle are innervated by dense SP-IR fibers and terminals.