Light- and electron-microscopic evidence of costoring of immunoreactive enkephalins and substance P in dorsal horn neurons of rat.

The topographical localization of substance P (SP) and methionine-enkephalin-octapeptide (Enk-8) was examined immunohistochemically in the surface layer of the dorsal horn of rat cervical spinal cord. Although a few neurons were immunoreactive for Enk-8 in the intact animals, after an intracisternal administration of colchicine, immunoreactive Enk-8 neurons were numerous, and half of them indicated immunoreactivity also for SP. Some immunoreactive SP neurons appeared to show no immunoreactivity for Enk-8. Immuno-reactive nerve fibers, on the other hand, were numerous, and many of them contained both peptides. Electron-microscopic examination of the nerve fibers in tissue prepared by a freeze-drying procedure and stained by a postembedding procedure, revealed the costoring of both peptides in the same cored vesicles. The physiological significance of this costoring is discussed.