Substance P and leucine-enkephalin changes after chordotomy and morphine treatment.

Spinal cords of rats, cats and monkeys were transected; the animals were perfused at varying times. Other rats were injected with morphine and perfused 10 days later. Immunocytochemistry shows substance P (SP) present in control animals primarily in the substantia gelatinosa (SG) of the dorsal horn of the spinal cord. Slight SP immunoreactivity is found in the ventral horn and near the central canal. Starting a few days after transection, there is a buildup of reaction product in the dorsal horn, in sections cut from below the lesion; staining above remains the same. With time, after chordotomy, SP immunoreactivity appears in fibers in lamina V, only in sections below the lesion. Leu-enkephalin (LE) is also found in the SG, however, it is also present in quantity in the ventral horn and central canal areas. Chordotomy has no effect on its distribution indicating LE is intrinsic in the cord and probably contained within interneurons. Morphine increases SP immunoreactivity in the SG, laminae I, IV and V, and in the ventral horn, suggesting morphine analgesia is due to inhibition of intraneuronal SP release in regions specifically associated with pain--SG and lamina V.