Denervation affects the influence of CGRP on OPG/RANKL system in the fracture healing process: an experimental study in rats

Abstract

Objective To study the influence of sensory/motor nerve injury on fracture healing and the influence of calcitonin gene related peptide (CGRP) on osteoprotegerin (OPG)/receptor activator nuclear factor kappa B ligand (RANKL) system during the fracture healing process through the establishment of selective transection of sensory/motor nerve in rats with tibia fracture as an animal model and to elucidate the peripheral nerve injury regulation mechanism during fracture healing. Methods Sixty Wistar rats were divided into 3 groups randomly:motor nerve (ventral root) transection + tibia fracture group (anterior rhizotomy group,ART group) ; sensory nerve (dorsal root) transection + tibia fracture group (poeterior rhizotomy group,PRT group) ;simple tibia fracture group (sham operated group,SO group).The transverse tibia fracture was treated by intramedullary nailing.The rats were sacrificed at postoperative day 7,14,21 and 28 respectively to harvest the bony calluses 5 mm proximal and 5 mm distal to the fracture site.The expression of CGRP,OPG and RANKL was detected by immunohistochemistry and analyzed by Image-proplus 6.0 software.The comparison among the groups was analyzed using SPSS 16.0. Results Strong positive expression of CGRP was seen at each time point in SO group.At postoperative day 14 and 21,the differences of CGRP expression between SO group,ART group and PRT group were significant ( P ＜ 0.05).OPG expression was strongest at postoperative day 7 in SO group,while remaining high even with gradual decrease.The differences in OPG expression were significant ( P ＜ 0.05 ) at postoperative day 14 between PRT group and SO group and between PRT group and ART group,while that difference was seen between SO group and PRT group and between SO group and ART group at postoperative day 21.RANKL expression in the SO group peaked at postoperative day 21,and later gradually decreased.At postoperative day 14 the differences between PRT group and ART group and between PRT group and SO group were significant ( P ＜0.05),while at postoperative day 21 the differences between SO group and ART group and between SO group and PRT group were significant ( P ＜ 0.05). Conclusion During the fracture healing process,the influence of sensory nerve fibers was significantly higher than that of the motor nerve fibers.CGRP can regulate the OPG/RANKL expression ratio,thus affecting the healing process.Denervation (especially the sensory nerves) causes reduced CGRP regulation on OPG/RANKL expression,which is not favorable for fracture healing.Complete innervation is a necessary condition for the normal healing of fractures. Key words: Sensory nerve; Motor nerve; Calcitonin gene related peptide; Osteoprotegerin; Receptor activator nuclear factor kappa B ligand