P2X3-immunoreactive primary sensory neurons innervating lumbar intervertebral disc in rats

The P2X3 receptor is normally localized in a sub-population of small-diameter dorsal root ganglion (DRG) neurons, and is thought to be related to pain perception. The aim of this study in rats was to examine P2X3-immunoreactivity in DRG neurons innervating the lumbar disc and in DRG neurons innervating cutaneous tissues. Fluoro-Gold was applied to the L5–L6 disc, the plantar skin of the hind paw (L4–L5 dermatomes), and the back skin (L1–L2 dermatomes). It has been reported that the L5–L6 disc is innervated by T13–L5 DRG neurons. We performed immunostaining using antibodies against the P2X3 receptor of T13–L5 DRGs to examine the L5–L6 disc, L4 and L5 DRGs to examine plantar skin and L1 and L2 DRGs to examine back skin. The P2X3-immunoreactivity was detected in 22.0 and 22.8% of neurons, labeled by Fluoro-Gold applied to plantar and back skin, respectively. However, P2X3-immunoreactivity was detected in only 4.0% of the neurons projecting to the L5–L6 disc. The proportion of P2X3-immunoreactive neurons was significantly larger in the DRG neurons innervating the plantar or the back skin, than in the DRG neurons innervating the lumbar disc. These results suggest that the P2X3 receptors are abundant in DRG neurons innervating cutaneous tissues, but not in neurons innervating the lumbar disc. It is likely therefore that the P2X3 receptor is less related to the mechanism of discogenic pain, than to cutaneous tissue pain.