Characteristics of sensory DRG neurons innervating the lumbar facet joints in rats

The rat L5/6 facet joint, from which low-back pain can originate, is multisegmentally innervated from the L1 to L5 dorsal root ganglions (DRGs). Sensory fibers from the L1 and L2 DRGs are reported to non-segmentally innervate the paravertebral sympathetic trunks, whilst those from the L3 to L5 DRGs segmentally innervate the L5/6 facet joint. In the current study, characteristics of sensory DRG neurons innervating the L5/6 facet joint were investigated in rats, using a retrograde neurotransport method, lectin affinity- and immuno-histochemistry. We used four markers: (1) calcitonin gene-related peptide (CGRP) as a marker of small peptide containing neurons, (2) the glycoprotein binding the isolectin from Griffonia simplicifolia (IB4) or (3 the purinergic P2X(3) receptor for small, non-peptide containing neurons, and (4) neurofilament 200 (NF200) for small and large myelinated fibers. IB4-binding and CGRP and P2X(3) receptor containing neurons are typically involved in pain sensation, whereas NF200 is associated with pain and proprioception. Neurons innervating the L5/6 facet joints, retrogradely-labeled with fluorogold (FG), were distributed throughout DRGs from L1 to L5. Of FG-labeled neurons, the ratios of NF200 immunoreactive (IR) neurons and CGRP-IR neurons were 37% and 35% respectively. The ratio of IB4-binding and P2X3 receptor-IR neurons was 10%, significantly less than the ratio of CGRP-IR neurons to FG-labeled neurons. The ratios of IB4-binding and P2X3 receptor-IR neurons were significantly higher, and that of CGRP-IR neurons was significantly less in L1 and L2 DRGs than those in L3, L4 or L5 DRGs. Under physiological conditions in rats, DRG neurons transmit several types of sensations, such as proprioception or nociception of the facet joint. Most neurons transmitting pain are CGRP-IR peptide-containing neurons. They may have a more significant role in pain sensation in the facets via peptidergic DRG neurons.