Substance P and Calcitonin Gene-related Peptide Immunoreactive Sensory DRG Neurons Innervating the Lumbar Facet Joints in Rats

The rat L5/6 facet joint is innervated from L1 to L5 dorsal root ganglions (DRGs) multisegmentally. Sensory fibers from L1 and L2 DRGs were reported to innervate nonsegmentally through the paravertebral sympathetic trunks, while those from L3 to L5 DRGs segmentally innervate the L5/6 facet joint. The presence of substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactive (ir) nerve fibers has been demonstrated in the lumber facet joints, but their ratios have not been determined. Fluoro-gold (F-G) labeled neurons innervating the L5/6 facet joint were distributed throughout the DRGs for levels L1 to L5. Of the F-G labeled neurons, the ratios of SP-ir L1, L2, L3, L4 and L5 DRG neurons were 13, 15, 29, 31 and 30%, respectively, and those of CGRP-ir neurons were 17, 24, 44, 56 and 50%, respectively. The ratios of SP and CGRP-ir neurons in L1 and L2 DRGs were significantly less than those in L3, L4 or L5 DRGs. In conclusion, the neurons of L3, L4 and L5 DRGs may have a more significant role in pain sensation of the facets than L1 and L2 DRG neurons.