Immunocytochemical localization of substance P in the spinal trigeminal nucleus of the rat: a light and electron microscopic study.

The neuropeptide substance P is a transmitter candidate for certain primary afferent fibers which terminate in the substantia gelatinosa. In this study the light and electron microscopic localization of substance P in the substantia gelatinosa of the spinal trigeminal nucleus of the rat has been studied using immunocytochemical procedures. Substance P immunoreactive fibers were observed mainly in lamina I and outer lamina II. Ultrastructural analysis revealed immunoreactivity in unmyelinated fibers and in axon terminals which contained agranular spherical vesicles and large dense-cored vesicles and which made predominantly simple asymmetric axodendritic synaptic contacts. Immunoreactive terminals only rarely formed the central terminal of synaptic glomeruli and in only one example was a stained terminal possibly postsynaptic to an unstained terminal. The majority of synapses were onto small dendrites in outer lamina II and in some cases these dendrites were themselves presynaptic to other dendrites. Immunoreactive terminals also synapsed with the soma and proximal dendrites of large neurons on the border of laminae I and II. The results show that there are at least two distinct targets for substance P immunoreactive terminals in the substantia gelatinosa, namely the large lamina I neurons and lamina II probable interneurons. Some of the former may be projection neurons while some of the latter may correspond to the inhibitory islet cells described by Gobel and colleagues in the cat. In addition the results indicate that few substance P immunoreactive terminals receive axoaxonic synapses and emphasize instead the role of postsynaptic interactions. In particular the results suggest several sites at which substance P might interact postsynaptically with the neuropeptide enkephalin.