Morphological features of endomorphin-2-immunoreactive ultrastructures in the dorsal root ganglion and spinal dorsal horn of the rat.

Endomorphin-2 (EM2)-immunoreactive (ir) fibers and terminals in the superficial laminae (lamina I and II) of the spinal dorsal horn (SDH) primarily come from neurons in the ipsilateral dorsal root ganglion (DRG), which are important for nociceptive information transmission and modulation. However, the morphological features of EM2-ir neurons and fibers in the DRG and terminals in the SDH under ultrastructural levels have not been completely revealed. The present study observed the distributions of EM2-ir neurons, fibers and terminals in the DRG and SDH and detected their ultrastructural features using immunoelectron microscopy. EM2-ir neurons in the DRG are primarily small or medium in size and account for 17.2% of all neurons in the DRG. EM2-ir large dense-core granule vesicles (LDCVs) are dispersed in the cytoplasm and fibers. Most of the central processes of DRG neurons were thin myelinated and unmyelinated fibers and contained a few EM2-ir LDCVs. An intensive string of EM2-ir fibers with beads and terminals were observed in the superficial laminae of the SDH, other than EM2-ir neurons. EM2-ir products were also detected sparsely in the fibers and terminals. The average diameter of terminals was 94.41 ± 18.13 nm. EM2-ir terminals formed different types of synapses, most of which were asymmetrical (91%). EM2-ir LDCVs colocalized primarily with spherical small clear vesicles in asymmetrical synapses and flat vesicles in symmetrical synapses. The average length of postsynaptic dense zones (PSDs) measured in the asymmetrical synapses was 317.00 ± 31.67 nm. These results indicate that EM2-containing structures are distributed in the cytoplasm of DRG neurons, the central processes and terminals in the SDH and provide morphological evidence for the antinociceptive effects of EM2 in the SDH.