Calcitonin gene-related peptide immunoreactive sensory DRG neurons innervating the cervical facet joints in rats.

The rat C5/6 facet joint is innervated multisegmentally from the C3 to the T3 dorsal root ganglia. Sensory fibers from the C3, C4, and C7-T3 dorsal root ganglia were reported to be innervated nonsegmentally through the paravertebral sympathetic trunks, while those from the C5 and C6 dorsal root ganglia were reported to segmentally innervate the C5/6 facet joint. The presence of calcitonin gene-related peptide-immunoreactive nerve fibers has been proved in the facet joints, but their ratios have not been studied. Fluoro-gold-labeled neurons innervating the C5/6 facet joint were distributed throughout the dorsal root ganglia from the C3 to T3 levels. Of the fluoro-gold-labeled neurons, the ratios of calcitonin gene-related peptide-immunoreactive C3, C4, C45 C6, C7, C8, T1, T2, and T3 dorsal root ganglion neurons were 30%, 22%, 43%, 47%, 21%, 19%, 25%, 36%, and 40%, respectively. The ratios of calcitonin gene-related peptide-immunoreactive neurons in the C5 and C6 dorsal root ganglia were significantly higher than those in the C3, C4, C7, and C8 dorsal root ganglia. In the physiological condition in rats, the neurons of the C5 and C6 dorsal root ganglia may have a more significant role in pain sensation of the facets than other dorsal root ganglion neurons.