Enkephalin-containing neurons in substantia gelatinosa of spinal trigeminal complex: Ultrastructure and synaptic interaction with primary sensory afferents

The ultrastructural morphology and synaptic associations between enkephalin-containing neurons and sensory afferents was examined in the caudalis portion of rat trigeminal complex, the region corresponding to Rexed's layer II of the dorsal horn^12,23. Specific antiserum to Met⁵-enkephalin was localized by the peroxidase-antiperoxidase technique in normal adult rats and in rats having electrolytic lesions of the trigeminal ganglia at 2, 4 and 6 days prior to sacrifice. In both normal and lesioned animals, the enkephalin-like immunoreactivity (ELI) was diffusely distributed throughout the cytoplasm of neuronal perikarya, dendrites, myelinated and unmyelinated axons, and axon terminals. The axons terminals were characterized by the presence of both small (40-60 nm) clear, and large (60-100 nm), dense-core vesicles and by the formation of predominantly axodendritic synapses. Optimal visualization of degenerating terminals occured at 4 days after lesions of the trigeminal ganglia. Approximately one-third of the degenerating afferents were in either direct contact or were separated from an enkephalin-containing process by an intervening unlabeled dendrite. The occurrence of a well defined membrane density between a degenerating and enkephalin-labeled bouton was rare. Most commonly, the processes showing ELI were either unassociated or formed a triad arrangement with the degenerating afferents. Within the triad, the degenerating bouton formed a junction with an unlabeled dendrite which in turn was synaptically connected at the same level to an enkephalin-labeled process, usually a dendrite. We conclude that an intermediary neuron may be involved in interaction between primary sensory afferents and enkephalin-containing neurons in spinal trigeminal complex.