Immunocytochemical localization of pro-opiomelanocortin-derived peptides in the adult rat spinal cord.

A dispersed descending pro-opiomelanocortin (POMC) fiber system has been demonstrated by peroxidase-antiperoxidase (PAP) immunocytochemistry in the adult rat spinal cord. beta-endorphin, adrenocorticotrophic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH) and 16K immunoreactive fibers exist in the spinal cord from cervical down to sacral level. Descending fibers running parallel in the dorsolateral and lateral funiculus send collaterals ventromedially or medially to terminate in the gray matter surrounding the central canal, where nociceptive neurons have recently been located, in addition to those nociceptive cells in the dorsal horn. After spinal transection at lower thoracic level, POMC peptide immunoreactivities disappeared below the lesion. Moreover, no POMC cell bodies were found in the spinal cord. Therefore, the descending fibers are most likely of supraspinal origin.