Existence of brain-derived neurotrophic factor and vanilloid receptor subtype 1 immunoreactive sensory DRG neurons innervating L5/6 intervertebral discs in rats.

The rat L5/6 intervertebral disc is innervated by L1 to L6 dorsal root ganglia (DRGs). T13 to L2 DRGs innervate the L5/6 intervertebral disc through paravertebral sympathetic trunks, whereas L3 to L6 DRGs directly innervate through sinuvertebral nerves on the posterior longitudinal ligament. The presence of substance P (SP)-immunoreactive (ir) and calcitonin gene-related peptide (CGRP-ir) sensory nerve fibers on the lumbar intervertebral disc has been established. SP and CGRP are markers of sensory neurons mainly involved with pain perception. The existence of SP-ir and CGRP-ir DRG neurons innervating the L5/6 intervertebral disc has been also demonstrated. Brain-derived neurotrophic factor (BDNF), which exists mainly in the small DRG neurons, plays an important neuromodulatory role in inflammatory conditions. Vanilloid receptor subtype 1 (VR1) in the DRG neurons and spinal dorsal horn is a channel that appears to confer responsiveness to heat and chemical stimuli. The presence of BDNF-ir and the VR1-ir DRG neurons innervating the L5/6 intervertebral disc has not. In this study of DRG neurons innervating the L5/6 intervertebral disc, the proportions of BDNF-ir in L1, L2, L3, L4, and L5 DRG neurons were 14%, 12%, 12%, 12%, and 13% and the proportions of VR1-ir L1, L2, L3, L4, and L5 DRG neurons were 10%, 8%, 24%, 19%, and 23%, respectively. Under physiological conditions in rats these neurons may transmit inflammatory and burning pain of the L5/6 intervertebral disc.