Ropivacaine Promotes Axon Regeneration by Regulating Nav1.8-mediated Macrophage Signaling after Sciatic Nerve Injury in Rats

Abstract

Background: Continuous nerve block with ropivacaine is commonly performed after repair surgery for traumatic peripheral nerve injuries. After peripheral nerve injury, tetrodotoxin-resistant voltage-gated sodium channel Nav1.8 is upregulated and contributes to macrophage inflammation. This study investigated whether ropivacaine promotes peripheral nerve regeneration through Nav1.8-mediated macrophage signaling. Methods: A sciatic nerve transection-repair (SNT) model was established in adult Sprague-Dawley rats of both sexes. The rats received 0.2% ropivacaine or 10 μM Nav1.8-selective inhibitor A-803467 around the injured site or near the sacrum for 3 days. Nerve regeneration was evaluated using behavioral, electrophysiologic, and morphological examinations. Moreover, myelin debris removal, macrophage phenotype, Nav1.8 expression, and neuropeptide expression were assessed using immunostaining, enzyme-linked immunosorbent assay, and Western blotting. Results: Compared to the SNT-plus-vehicle group, the sensory, motor, and sensory-motor coordination functions of the two ropivacaine groups were significantly improved. Electrophysiologic (mean ± SD: recovery index of amplitude, vehicle 0.43 ± 0.17 vs. ropivacaine 0.83 ± 0.25, n = 11, P < 0.001) and histological analysis collectively indicated that ropivacaine significantly promoted axonal regrowth (percentage of neurofilament 200 [NF-200]-positive area: vehicle 19.88 ± 2.81 vs. ropivacaine 31.07 ± 2.62, n = 6, P < 0.001). The authors also found that, compared to the SNT-plus-vehicle group, the SNT-plus-ropivacaine group showed faster clearance of myelin debris, accompanied by significantly increased macrophage infiltration and transition from the M1 to M2 phenotype. Moreover, ropivacaine significantly attenuated Nav1.8 upregulation at 9 days after sciatic nerve transection (vehicle 4.12 ± 0.30-fold vs. ropivacaine 2.75 ± 0.36-fold, n = 5, P < 0.001), which coincided with the increased expression of chemokine ligand 2 and substance P. Similar changes were observed when using the selective Nav1.8 channel inhibitor A-803467. Conclusions: Continuous nerve block with ropivacaine promotes the structural and functional recovery of injured sciatic nerves, possibly by regulating Nav1.8-mediated macrophage signaling.