Resolvin D2 Relieving Radicular Pain is Associated with Regulation of Inflammatory Mediators, Akt/GSK-3β Signal Pathway and GPR18

Neuroinflammation induced by protruded nucleus pulposus (NP) has been shown to play a significant role in facilitation of radicular pain. Resolvin D2 (RvD2), a novel member of resolvin family, exhibits potent anti-inflammatory, pro-resolving and antinociceptive effects. But the effect of RvD2 in radicular pain remains unknown. The radicular pain rat models were induced by application of NP to L5 dorsal root ganglion. Each animal received intrathecal injections of vehicle or RvD2 (10 ng µl−1 or 100 ng µl−1). Mechanical thresholds were determined by measuring the paw withdrawal threshold for 7 days. The expressions of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and transforming growth factor-β1 (TGF-β1) in ipsilateral lumbar segment of rat spinal dorsal horns were measured by using ELISA and real time-PCR. Western blot was used to measure the expressions of phosphorylated Akt (p-Akt) and phosphorylated glycogen synthase kinase 3 beta (p-GSK-3β). The expressions and distributions of RvD2 receptor, G-protein-coupled receptor 18 (GPR18), were also explored in the spinal cord of rats by using double-label immunofluorescence. RvD2 treatment caused significant reductions in the intensity of mechanical hypersensitivity and spinal expressions of TNF-α and IL-6. Meanwhile, RvD2 increased the expressions of TGF-β1 and regulated Akt/GSK-3β signaling. Furthermore, immunofluorescence showed that GPR18 colocalized with neurons and astrocytes in spinal cord. The results suggested that RvD2 might attenuate mechanical allodynia via regulating the expressions of inflammatory mediators and activation of Akt/GSK-3β signal pathway. RvD2 might offer a hopeful method for radicular pain therapy.