The coexistence of serotonin- and substance P-like immunoreactivity in the spinal cord of the rat as shown by immunofluorescent double labeling.

This study surveyed the coexistence of serotonin-like immunoreactivity (5-HT-IR) with substance P-like immunoreactivity (SP-IR) in fibers and terminals within various portions of the spinal cord of the rat. A previously characterized technique of immunofluorescent double labeling was used to stain 5-HT-IR red and SP-IR green, and a search was then made for single fibers that fluoresced both colors. These were found to be most common in the ventral horn, wherein 99% of 5-HT-IR fibers were also immunoreactive for SP. Coexistence of these substances was very rare in the superficial dorsal horn: fewer than 3% of 5-HT-IR fibers were immunoreactive for SP. In the region surrounding the central canal and in the intermediolateral cell column (IML), over half of all 5-HT-IR fibers and terminals were doubly labeled. In the white matter, doubly labeled fibers were most common in the ventral funiculus and somewhat less common in the lateral funiculus. They were rare in the dorsal columns. It is concluded that the coexistence of 5-HT and SP in nerve fibers and terminals is associated with somatic and sympathetic autonomic motoneurons. The role of the coexistence of 5-HT and SP in somatosensation, including pain, is unclear.