Innervation of the lumbar intervertebral disc by nerve growth factor-dependent neurons related to inflammatory pain

We used anatomic tracers and immunoreactivity in rats to define dorsal root ganglion neuron populations innervating the lumbar discs in physiologic and inflammatory states.

To investigate the percentages of calcitonin gene-related peptide-immunoreactive (CGRP-ir) and isolectin B4 (IB4)-binding neurons innervating lumbar discs.

Small neurons are classified into two types. One contains CGRP and expresses the nerve growth factor receptor. The other binds IB4 and expresses the glial cell line-derived neurotrophic factor receptor.

A neurotracer, Fluoro-Gold, was applied to the L5-L6 disc in rats. Five days later, 50-microL saline (control group: n = 8) or Complete Freund's adjuvant (inflammatory group: n = 8) was applied to the disc. Seven days after the second operation, T13-L5 dorsal root ganglions were processed for double staining of CGRP and IB4.

Of the Fluoro-Gold-labeled neurons, 50.1 +/- 4.6% (mean +/- SEM) were positive for CGRP and 0.7 +/- 0.6% positive for IB4 in the control group, while 65.6 +/- 4.7% were positive for CGRP and 1.0 +/- 1.0% positive for IB4 in the inflammatory group. The percentage of CGRP-ir neurons was significantly higher than that of IB4-binding neurons in both groups (P < 0.001, each). The percentage of CGRP-ir neurons in the inflammatory group was significantly higher than in the control group (P < 0.05).

We found that most small neurons innervating the disc were CGRP-ir. Furthermore, disc inflammation caused an increase in CGRP-ir neurons but not IB4-binding neurons, suggesting that CGRP-ir, nerve growth factor-dependent neurons are more responsible for discogenic pain.