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

Abstract

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

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

Summary of background data: 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.

Methods: 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.

Results: 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).

Conclusions: 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.