Long‐term histological analysis of innervation and macrophage infiltration in a mouse model of intervertebral disc injury–induced low back pain

Abstract Low back pain (LBP) is a leading cause of global disability. Multiple anatomical, cellular, and molecular factors are implicated in LBP, including the degeneration of lumbar intervertebral discs (IVDs). We previously described a mouse model that displays behavioral symptoms of chronic LBP. Here, we investigated the development of pathological innervation and macrophage infiltration into injured IVDs following a puncture injury in mice over 12 months. 2‐month old CD1 female mice underwent a single puncture of the ventral L4/5 IVD using a 30G needle, and were sacrificed 4 days and 0.5‐, 3‐, 6‐ and 12‐months post‐injury. Severity of disc degeneration was assessed using colorimetric staining. IVD innervation was measured by PGP9.5‐immunoreactivity (‐ir) and calcitonin gene‐related peptide‐ir (CGRP‐ir). Macrophage accumulation into IVDs was detected by F4/80‐ir. Mechanical IVD injury resulted in severe degeneration and increased PGP9.5‐ir nerve fiber density starting at 4 days that persisted for up to 12 months and dorsal herniations began to occur at 3 months. CGRP‐ir was also upregulated in injured IVDs, with the largest increase at 12 months after injury. Infiltration of F4/80‐ir macrophages was observed in injured IVDs by day 4 both dorsally and ventrally, with the latter diminishing in the later stage. Persistent LBP is a complex disease with multiple underlying pathologies. By highlighting pathological changes in IVD innervation and inflammation, our study suggests that strategies targeting these mechanisms might be useful therapeutically.