Expression Of Vesicle-associated Genes In Dorsal Root Ganglion Neurons And Regulation After Peripheral Nerve Injury

The unmyelinated C-afferents of small dorsal root ganglia (DRG) neuronsconducting nociceptive signals terminate in lamina II of the spinal cord. Previousstudies showed that after peripheral nerve injury, the synaptic vesicles are notaccumulated at presynaptic zone of C-afferent terminals, whereas a relativelynormal presynaptic localization of synaptic vesicles is seen in thick-myelinatedafferents in laminae III and IV. We presume that the modification of thevesicle-associated molecules that are expressed in small DRG neurons is involvedin this pathological process. Using cDNA array, we identified that the expression ofeleven vesicle-associated molecules were strongly changed in rat DRG afterperipheral nerve injury. Using in situ hybridization and immunohistochemistry, weidentified their cellular distribution and found that among these eleven genes,synaptoporin was specially localized in small DRG neurons andsynaptoporin-positive fibers were projected to the superficial layer of dorsal spinalcord. The expression of synaptoporin was significantly upregulated in both mRNAlevel and protein level in rat DRG after nerve injury. It is well known thatsynaptoporin is a membrane protein of synaptic vesicles and can interact withVAMP2. Taken together, we suggest that the up-regulation of synaptoporin mayinvolve in the abnormal distribution of synaptic vesicles in C-fibers.Immunohistochemistry revealed the differential expression of synaptoporinand synaptophysin in primary sensory neurons. In the present study, we showedthat synaptoporin was expressed in subsets of small neurons that contain eithercalcitonin gene-related peptide or isolectin B4, and was distributed in their afferentterminals in laminae I-II of the spinal cord. Synaptophysin was expressed in 57% ofsynaptoporin-containing small DRG neurons and in large DRG neurons. In thespinal dorsal horn, synaptophysin-immunolabeling was weak in the afferent fibersin lamina I, outer lamina II and the dorsal part of inner lamina II, but strong in theafferent fibers in laminae III-IV. However, a subpopulation of isolectin B4-positivesmall DRG neurons expressed both synaptoporin and synaptophysin, and theirafferent fibers were mainly distributed in the ventral part of inner lamina II. Thus,synaptoporin is a major synaptic vesicle protein in Aδ-and C-fibers in bothphysiological and neuropathic pain states.In the present study, we demonstrate that in DRG neurons SNAP-25 is onlyexpressed in large population and is absent from IB4/CGRP-positive small ones,suggesting that SNAP25 is not involved in the exocytosis of most peptidergic Aδ-and C-fibers. About 68% of SNAP25-containing DRG neurons express TrkB,indicating that the vesicle fusion process SNAP25 involved in may be regulated byBDNF and NT4/5. The homolog protein SNAP-23 is expressed in most (if not all)DRG neurons and has strong expression since early development.Co-immunoprecipitation demonstrates the participation of SNAP-23 in theformation of SNARE complex. These data indicate that SNAP-23 substitutes forSNAP-25 in the exocytosis of small DRG neurons and may have a more popularfunction in peripheral nerve system.