Sensory Innervation of the Dorsal Portion of the Lumbar Intervertebral Disc in Rats

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Abstract
The vertebral levels of dorsal root ganglia innervating the dorsal portion of the L5–L6 intervertebral disc were investigated in rats using a retrograde transport method. The pathways and functions of nerve fibers supplying the dorsal portion of the disc were determined by denervation and immunohistochemistry. The dorsal portion of the lumbar intervertebral disc has been reported to be innervated segmentally, but anesthetic block of the paravertebral sympathetic trunks and the L2 spinal nerve can relieve discogenic low back pain. In the current study, the sensory innervation of the dorsal portion of the L5–L6 intervertebral disc was investigated, because the disc anatomically corresponds to the L4–L5 disc in humans, and the dorsal portion of the human L4–L5 disc is frequently subject to injury that causes low back pain. A retrograde transport of Fluoro-Gold (F-G; Fluorochrome, Denver, CO) was used. Subjects included nontreated control (n = 32) and sympathectomized rats in which paravertebral sympathetic trunks were removed from L2 to L3 (n = 9). In a ventral approach, Fluoro-Gold crystals were placed on the dorsal portion of the L5–L6 disc, and labeled neurons in the bilateral dorsal root ganglia from T10 to L6 were counted. Fluoro-Gold crystals did not leak from the dorsal portion of the L5–L6 disc in 14 of the 32 nontreated rats and in 5 of the 9 sympathectomized rats. These rats were used for analysis. Fluro-Gold–labeled neurons were found in dorsal root ganglia from T13 to L6 in the 14 control rats but only from L2 to L6 in the 5 sympathectomized rats. The dorsal portion of the L5–L6 disc of rats was shown to be multisegmentally innervated by the T13 to L6 dorsal root ganglia. The sensory fibers from T13, L1, and L2 dorsal root ganglia were shown to innervate the dorsal portion of the L5–L6 disc through the paravertebral sympathetic trunks. In contrast, those from the L3–L6 dorsal root ganglia may innervate the dorsal portion of the L5–L6 disc through the sinuvertebral nerves.