Oral Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

INTRODUCTION: Chronic low back pain, primarily associated with intervertebral disc (IVD) degeneration, remains challenging for pharmacological and surgical interventions. A deeper understanding of discogenic pain pathophysiology is necessary to develop improved treatments. Spinal fusion is poorly indicated in many discogenic pain subjects revealing a need to identify additional pain sources beyond degenerated IVDs. IVDs are innervated with nociceptive fibers from dorsal-root-ganglia (DRGs) and spinal cord (SC) dorsal horn, implicating them as potential pain sources. A comprehensive investigation of IVD, DRG, and SC following IVD injury is needed to understand their complex interactions in discogenic pain progression. This study determined the kinetics of DRG and SC changes following lumbar annulus fibrosus (AF) puncture injury in a rat discogenic pain model. 

METHODS: Approved by IACUC, thirty-seven skeletally-mature Sprague-Dawley rats were assigned to Naïve or AF-injury groups. AF Injury involved anterior AF puncturing of L3-4, L4-5, and L5-6 IVDs followed by PBS injection [1]. Rats were euthanized at 3-days, 1-week, 2-weeks, and 8-weeks post-injury, and compared with Naïve rats without surgical interventions. Lumbar spines (L2-6), L2 DRGs and lumbar SCs were isolated and processed for histology Safranin-O/fast-green/hematoxylin for general morphology, and immunohistochemistry for spine inflammation (CD68), pain sensitization (substance P, SubP), neuroinflammation (Iba1), and neuronal remodeling (GFAP).

RESULTS: AF injury induced moderate IVD degeneration with acute and widespread spinal inflammation, particularly at injured AF region. DRG SubP and Iba1 were acute and temporary, peaking at 3-days post-injury and decreasing by 8-weeks (Fig 1). DRG GFAP also significantly increased at 3-days and remained elevated until 8-weeks (Fig 1), suggesting DRG remodeling. SC SubP and GFAP were significantly increased by 2-weeks and remained elevated until 8-weeks (Fig 2). SC Iba1 peaked at 2-weeks and restored to Naïve levels by 8-weeks (Fig 2). 

DISCUSSION: AF injury caused temporal and tissue-dependent changes in sensitization and inflammation. Elevated SubP in DRG and SC suggests peripheral and central sensitization might be associated with the pain-like behavioral phenotype previously demonstrated in similar rat models with IVD injuries [1,2,3]. AF injury-induced IVD degeneration caused rapid spinal macrophage activation (CD68) that progressed to DRG macrophages (Iba1) and gradually to SC microglia (Iba1), indicating inflammatory crosstalk between spine and neural tissues. AF injury also triggered GFAP elevation in DRGs and SC, indicating chronic satellite glial cell and astrocyte activation, suggesting chronic neuronal remodeling. Early macrophage activation and sustained glial cell activation in DRG and SC were similar to those associated with neuropathic pain [4-6], revealing neuropathic components in IVD degeneration likely arising from chronic inflammation or spinal instability. In conclusion, IVD injury involves pathology to peripheral and central nervous systems with distinct early and late responses in addition to IVD degeneration. Future therapeutic strategies for chronic discogenic pain may consider spinal tissue interactions and chronicity with early interventions managing acute inflammation and later strategies managing chronic DRG and SC sensitization and remodeling.

ACKNOWLEDGEMENT: Supported by NIH/HIAMS Grants R01AR078857 and P30AR079206.