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Time‐dependent lumbar spinal cord astrocytic hyperactivation occurs in the deep dorsal horn during the maintenance of nerve growth factor‐induced low back pain

Low back pain (LBP) is a complex condition that is a substantial global socioeconomic problem. LBP is poorly managed, in part, due to a lack of efficacious treatments. Determining the underlying mechanisms contributing to LBP will help to discover new therapies to impact this global crisis. In this study, we used a preclinical model of LBP induced by nerve growth factor (NGF) injections into the lumbar paraspinal muscles. It was previously shown that NGF injections induce persistent low back mechanical hyperalgesia (hypersensitivity to a noxious mechanical stimulus) and nociceptive neuronal hyperexcitability in the lumbar spinal cord deep dorsal horn (DDH), which contributes to central sensitization. Central sensitization is key to the development of persistent pain. Neuroimmune activation (astrocytic & microglial hyperactivation & enhanced proinflammatory cytokines) also plays a critical role in central sensitization. We previously reported that astrocytic hyperactivation occurred in the ipsilateral lumbar spinal cord superficial dorsal horn, where pain is processed and modulated, during the maintenance of NGF‐induced persistent LBP. Recent reports examining the induction of persistent LBP suggest that glial hyperactivation also occurs in the DDH, where pain projection neurons are located. The objective of this study was to investigate astrocytic hyperactivation in the DDH during the maintenance of persistent LBP. We hypothesized that NGF injections would induce astrocytic hyperactivation in the lumbar spinal cord DDH during the maintenance of LBP. We tested our hypothesis using the NGF LBP model and immunofluorescence with confocal microscopy. We administered two injections (50μl) 5 days apart on Day 0 (D0) and Day 5 (D5) of NGF (0.8μM) or vehicle (PBS) into the left paraspinal muscles (5th lumbar vertebral level) in male rats. Low back mechanical hyperalgesia (LBP) was assessed before injections on D0 and D5, and at other time points (D2, D5+4 hours, D7, D10, & D14). Lumbar spinal cords (L1–L6; formalin fixed & transversely sectioned at 30μm) were evaluated for immunofluorescence labeling of the astrocyte marker, glial fibrillary acidic protein (GFAP), in the DDH bilaterally (Image J; inter‐ and intra‐rater reliability ICC= 0.77 & 0.83). NGF‐injected rats displayed significant reductions in ipsilateral low back noxious mechanical thresholds post‐injection on D5 through D14 compared to controls. At D14, NGF rats (n=4) demonstrated a significant increase in mean fluorescence intensity for GFAP, in the DDH ipsilateral to injections with characteristic appearances of hyperactivated astrocytes (i.e., hypertrophic cell bodies & processes) compared to vehicle (n=4). No significant changes were observed at D5 (4 hours post‐NGF injection) or D7. These findings suggest that time‐dependent astrocytic hyperactivation occurs in the DDH during the maintenance of LBP, further supporting the role that spinal cord neuroimmune activation plays in persistent LBP. This study provides a foundation that may help identify novel therapies to improve treatment efficacy and impact the problem of LBP.