Expression and co-expression of VR1, CGRP, and IB4-binding glycoprotein in dorsal root ganglion neurons in rats: Differences between the disc afferents and the cutaneous afferents

Study Design. The expression of vanilloid receptor 1 (VR1), calcitonin gene-related peptide ( CGRP), and isolectin B4 ( IB4)-binding glycoprotein in dorsal root ganglion (DRG) neurons innervating the lumbar disc and the plantar skin was investigated. Objective To characterize the DRG neurons innervating lumbar discs and those innervating cutaneous tissue in rats. Summary and Background Data. Small nociceptive DRG neurons are divided into nerve growth factor (NGF) sensitive and glial cellline-derived neurotrophic factor (GDNF)-sensitive neurons. CGRP and IB4-binding glycoprotein are recognized as specific markers for NGF and GDNF-sensitive neurons, respectively. VR1 is localized in small DRG neurons. Methods. Using histochemical staining and retrograde tracing methods, the expression of VR1, CGRP, and IB4-binding glycoprotein in DRG neurons innervating the L5-L6 disc and the plantar skin was examined in rats. Results. DRG neurons innervating the disc showed positive staining as: 23.4% VR1, 54.4% CGRP, and 1.0% IB4-binding glycoprotein. The following distribution was found for DRG neurons innervating the skin: 35.1% VR1, 41.1% CGRP, and 19.5% IB4-binding glycoprotein. Percentages of neurons positive for VR1 and IB4-binding glycoprotein were significantly lower in DRG neurons innervating the disc than in DRG neurons innervating the skin (P < 0.05), while no significant difference was observed in the percentage of neurons positive for CGRP. Conclusions. VR1 is less abundant in lumbar disc than in cutaneous tissue. Our data suggest that nociceptive-information from the disc is transmitted mostly by NGF-sensitive neurons, while that from the cutaneous tissue is transmitted by both NGF-sensitive and GDNF-sensitive neurons.