The Mechanisms For Differences In Sensitivities To Muscle Relaxants Between Facial Muscles And Limb Skeletal Muscles And The Related Changes After Injuries

Objective:To test the hypothesis that differences in sensitivities to rocuronium exist between facial nerve-and somatic nerve-innervated muscles,and to observe the changes in pharmacodynamic responses to rocuronium after facial nerve injury.To investigate the different pharmacodynamic responses to rocuronium among these muscles are related to the size of neuromuscular junction,the size of muscle fiber,as well as the quantitative,qualitative,and locational changes in nicotinic acetylcholine receptor(nAChR)subunits.To explore that differences in nAChR expression among these muscles are related to the agrin/Lrp4/MuSK and neuregulinl/ErbB signaling pathways.Methods:Male Sprague-Dawley rats were divided randomly into normal and facial nerve injury groups.Right sided orbicularis oris and gastrocnemius were obtained from normal group.Different levels of facial nerve injury were induced by crush axotomy in the injury groups.The injured orbicularis oris were obtained on days 1,3,7,14,30,60,and 90.The pharmacodynamic response to rocuronium were conducted in vitro,and the 50%inhibitory concentration(IC50)values were obtained from dose-response curves.Expression of the nAChR ?1,?1,?,?,and ? subunits was assessed using immunofluorescence staining.Expression of agrin,Lrp4,MuSK,MMP3,NRG1 beta isoform,ErbB2,ErbB3,and ErbB4 were assessed using reverse transcription-polymerase chain reaction and western blot analysis in these musclesResults:The IC50 values for rocuronium were significantly larger in the normal orbicularis oris than those in the gastrocnemius.The densities of nAChR subunits in unit cross-sectional area were higher in the normal orbicularis oris than those in the gastrocnemius.The normal orbicularis oris had a smaller muscle fiber cross-sectional area(CSA)and a larger ratio of endplate surface area(ESA)to muscle fiber CSA(ESA/CSA).The mRNA and protein levels of NRG1 beta isoform,ErbB2,ErbB3,and ErbB4 were higher in normal orbicularis oris than in the gastrocnemius,while no differences were observed in the mRNA or protein levels of agrin,Lrp4,MuSK,or MMP3.Increased IC50 values and reduced twitch tension were observed after facial nerve injuries,and the IC50 values of the injured orbicularis oris were positively correlated with the extent of facial nerve injury.Expression of the nAChR ?1,?1,?,?,and ? subunits increased significantly on the postsynaptic membranes of endplates and extra-junctional muscle membranes after facial nerve injury.agrin mRNA and protein levels were decreased significantly after facial nerve injury on days 1,3,7,14,and 30,and were negatively correlated with the extent of facial nerve injury.The mRNA and protein levels of Lrp4,MuSK,MMP3,NRG1 beta isoform,ErbB2,ErbB3,and ErbB4 were increased significantly after facial nerve injury and were positively correlated with the extent of facial nerve injuryConclusions:Facial nerve-innervated muscle was less sensitive than somatic nerve-innervated muscle,and the mechanisms underlying this result may be related to the densities of nAChR subunits in unit cross-sectional area,muscle fiber CSA and the ESA/CSA ratio.The higher expression of nAChR subunit in normal orbicularis oris may be related to the NRG1/ErbB,but not to the agrin/Lrp4/MuSK signaling pathway.Facial nerve injury caused the resistance to neuromuscular blockers and reduced twitch tension,which was related to qualitative,quantitative,and locational changes in nAChR subunits.Facial nerve injury differentially activated the agrin/Lrp4/MUSK and NRG1/ErbB signaling pathways,which both contributed to upregulate junctional and extra-junctional localization of the nAChR subunits.