A comparison of the ultrastructure of substance P and enkephalin-immunoreactive elements in the nucleus of the dorsal lateral funiculus and laminae I and II of the rat spinal cord.

The ultrastructure of substance P (SP)- and enkephalin (ENK)-immunoreactive elements in the nucleus of the dorsal lateral funiculus (NDLF) and in laminae I and II of the spinal cord was examined in the rat using the peroxidate-antiperoxidase (PAP) technique. Electron-microscopic observations were made of a large number of immunolabelled terminals (n = 428; many followed in serial sections), axons, and immunoreactive cell bodies and dendrites which were occasionally encountered. Morphometric analysis was used to describe and compare the fine structural features of immunolabelled elements. Both SP- and ENK-immunoreactive terminals contained clear synaptic vesicles and dense-cored vesicles of similar size but the ENK-immunoreactive profiles contained significantly more dense-cored vesicles than SP-immunolabelled profiles. Both SP- and ENK-immunoreactive profiles in the dorsal laminae of the dorsal horn contacted mainly smaller dendritic elements. Only rarely were axo-axonic interactions noted. The NDLF contains widely scattered cell bodies dispersed within a neuropil which is rich in synaptic complexes and is interdigitated between fascicles of myelinated and unmyelinated axons. Numerous SP- and ENK-immunoreactive profiles were observed in the NDLF, many of which made asymmetric synaptic contacts with NDLF neurons. Although both the dorsal gray and NDLF contain large numbers of SP- and ENK-immunoreactive elements which are similar in morphology in both regions, the NDLF can be distinguished from laminae I and II by a number of criteria, including the nature of the neuropil, principle sources of SP innervation, and the termination patterns of ascending projections.