Substance P--positive structures in rat spinal cord--a longitudinal bundle ventral to the central canal.

Using the immunoperoxidase method, the distribution of substance P (SP)-positive structures was studied in different levels of the rat spinal cord. In general, the density of SP-positive sites proved to be higher in lumbar and sacral than in cervical and thoracic segments. In transverse sections, the density was highest in lamina I, and decreased in dorso-ventral direction. Following colchicine injection into the spinal cord, many SP-positive cell bodies could be visualized in the dorsal and lateral horn as well as in the lateral spinal nucleus. In front of the central canal, an SP-immunoreactive longitudinal bundle (FLCV) showing close relation to the ependyma, extends throughout the whole length of the spinal cord. This bundle, which also displays acetylcholinesterase activity, electronmicroscopically consists of thin myelinated and unmyelinated fibers, some of which containing numerous synaptic vesicles. SP-positive fibers penetrating between ependymal cells and abuting on the luminal surface of the central canal probably arise from the FLCV.