Abstract
Low back pain (LBP) is the primary cause of disability globally. There is a close relationship between Modic changes or endplate defects and LBP. Endplates undergo ossification and become highly porous during intervertebral disc (IVD) degeneration. In our study, we used a mouse model of vertebral endplate degeneration by lumbar spine instability (LSI) surgery. Safranin O and fast green staining and μCT scan showed that LSI surgery led to endplate ossification and porosity, but the endplates in the sham group were cartilaginous and homogenous. Immunofluorescent staining demonstrated the innervation of calcitonin gene-related peptide- (CGRP-) positive nerve fibers in the porous endplate of LSI mice. Behavior test experiments showed an increased spinal hypersensitivity in LSI mice. Moreover, we found an increased cyclooxygenase 2 (COX2) expression and an elevated prostaglandin E2 (PGE2) concentration in the porous endplate of LSI mice. Immunofluorescent staining showed the colocalization of E-prostanoid 4 (EP4)/transient receptor potential vanilloid 1 (TRPV1) and CGRP in the nerve endings in the endplate and in the dorsal root ganglion (DRG) neurons, and western blotting analysis demonstrated that EP4 and TRPV1 expression significantly increased in the LSI group. Our patch clamp study further showed that LSI surgery significantly enhanced the current density of the TRPV1 channel in small-size DRG neurons. A selective EP4 receptor antagonist, L161982, reduced the spinal hypersensitivity of LSI mice by blocking the PGE2/EP4 pathway. In addition, TRPV1 current and neuronal excitability in DRG neurons were also significantly decreased by L161982 treatment. In summary, the PGE2/EP4 pathway in the porous endplate could activate the TRPV1 channel in DRG neurons to cause spinal hypersensitivity in LSI mice. L161982, a selective EP4 receptor antagonist, could turn down the TRPV1 current and decrease the neuronal excitability of DRG neurons to reduce spinal pain.
Highlights
Low back pain (LBP) is the primary cause for disability globally [1], with a 1-month prevalence of 23.2% [2]
We found an elevated concentration of prostaglandin E2 (PGE2) in the porous endplate of lumbar spine instability (LSI) mice
Since PGE2 is the cyclooxygenase 2 (COX2) product in the inflammatory environment, we examined COX2 expression, prostaglandin E synthase (PGES) expression, and PGE2 concentration in L4-L5 endplates at 8 weeks in the two groups. Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) and immunostaining showed an increase in COX-2 expression at 8 weeks in the LSI group relative to the sham group (Figures 3(a)–3(c))
Summary
Low back pain (LBP) is the primary cause for disability globally [1], with a 1-month prevalence of 23.2% [2]. Since LBP is generally a persistent symptom, about 2/3 of the patients with LBP complained about their pain-related symptoms even after 12 months [3]. This persistent painful condition is associated with the development of multiple physical and psychosocial disabilities [4]. In patients with LBP, researchers have detected signal changes in the degenerative endplates by magnetic resonance imaging (MRI) [10, 11]. The close relationship between Modic changes or endplate defects and LBP has been verified in some previous studies [12, 13]
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