A novel substance P pathway linking the dorsal and ventral horn in the upper lumbar segments of the rat spinal cord.

We report here the presence of a novel substance P pathway existing at a segmental level in the upper lumbar region (L1/L2) of the rat spinal cord. Substance P-containing fibres were seen directly linking a number of discrete areas of the rat spinal cord, including dorsal and ventral regions. These fibers were found closely associated with a specific group of motoneurones, the mediolateral motor nucleus in the ventral horn. This motoneurone group appeared as a 'focusing' nucleus around which substance P-containing tracts appeared to congregate. An intrinsic spinal cord origin of substance P-containing neurones in these interconnecting pathways is suggested as deafferentation achieved either by section of appropriate dorsal roots, or pharmacologically by neonatal treatment with capsaicin, had no effect on the appearance of the nucleus in the ventral spinal cord or of the associated substance P-containing tracts. Likewise, mid-thoracic lesions of the spinal cord (to determine supraspinal contributions) including hemisection, dorsolateral funiculus lesions and lesions of the region around the central canal, induced no change. The significance of these interconnecting substance P-immunoreactive fibre tracts is not known. However, the intimate association of substance P-containing fibres with a specific group of motoneurons indicates that they may modulate motor function.