Human spinal neurons: innervation by both substance P and enkephalin

The distribution of the peptides substance P and methionine-enkephalin in relation to neurons in the human thoracic and lumbar spinal cord has been studied with the unlabeled peroxidase anti-peroxidase method employing antibodies directed against substance P and methionine-enkephalin. The peptides have been localized in varicosities and terminal-like processes in close apposition to the soma and proximal dendrites of neurons in the marginal zone, the intermediolateral nucleus, intermediomedial nucleus, the reticular nucleus of lamina V and the ventral horn; some cells in all these regions are innervated by both peptides.The findings provide morphological evidence of these peptides in the human spinal cord. The demonstration of innervation by both peptides of single neurons in laminae I and V provides some morphological basis to models that postulate the interaction of substance P and enkephalin in pain modulation. The innervation of other neuron types indicates that these neuropeptides may be involved in other spinal functions as well.