Lumbar Spinal Cord Astrocytic Hyperactivation is Associated with Thrombospondin‐4 Upregulation during Persistent Low Back Pain

Abstract

Low back pain (LBP) is a leading cause of global disability. Due to a lack of understanding of its underlying mechanisms, this condition is poorly managed and necessitates novel approaches to improve treatment options. Here we used a preclinical rat LBP model induced by nerve growth factor (NGF) injections into the lumbar paraspinal muscles to study the neurobiological changes in the lumbar spinal cord that contribute to LBP. NGF injections induce behavioral changes, including hypersensitivity to a noxious mechanical stimulus (i.e., mechanical hyperalgesia) in the low back (LBP). NGF‐induced LBP was associated with spinal cord dorsal horn pain neuron hyperexcitability, a pro‐nociceptive process that contributes to central sensitization, a state implicated in persistent pain. We previously reported that the maintenance of NGF‐induced LBP was associated with astrocytic hyperactivation (i.e., increased glial fibrillary acidic protein, GFAP, with somatic & process hypertrophy) in the superficial dorsal horn (SDH), a location for pain processing and modulation. Our findings suggested a role for neuroimmune activation, a glial‐cell dependent process that promotes inflammation and central sensitization. Although glial hyperactivation was previously implicated in LBP patients, the mechanisms by which spinal astrocytes contribute to LBP are unclear. In this study we examined the potential role of the astrocyte‐dependent glycoprotein, thrombospondin‐4 (TSP‐4). TSP‐4 is predominantly expressed and released by spinal cord astrocytes and promotes inflammation and synaptic plasticity. Although unreported in LBP, spinal astrocytic TSP‐4 upregulation was implicated in neuropathic pain, in part, by promoting excitatory synaptogenesis. We hypothesized that persistent LBP and spinal astrocytic hyperactivation at day 14 (D14) induced by NGF injections would be associated with spinal cord dorsal horn TSP‐4 upregulation. To test our hypothesis, male rats received two injections, 5 days apart, of NGF (0.8μM/50μl) or vehicle into the left paraspinal muscles (5th lumbar vertebral level). At D14, lumbar spinal cord dorsal horns (L1–6) were harvested and evaluated for TSP‐4 mRNA (DNA microarray; n=3/group) and the location and cellular expression of TSP‐4 protein (n=4/group) via immunofluorescence with confocal microscopy. Compared to vehicle, NGF injected rats had significant ipsilateral low back mechanical hyperalgesia that persisted through D14. NGF rats also had upregulation of ipsilateral dorsal horn TSP‐4 mRNA expression (1.5‐fold increase) compared to controls. TSP‐4 immunofluorescence was significantly increased (2 fold) in the ipsilateral SDH in NGF rats. High magnification confocal z‐stacks demonstrated TSP‐4 immunolabeling was located in GFAP positive astrocytes. These findings demonstrate that spinal astrocytic TSP‐4 upregulation occurs during the maintenance of persistent LBP. Further work is required to determine the mechanisms that promote spinal TSP‐4 upregulation and its role in LBP. Our findings could help to reveal the central mechanisms contributing to LBP and to develop novel LBP treatments.Support or Funding InformationSLU Center for Neuroscience SCFN‐SGP‐01This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.