Advanced Oxidation Protein Products Caused Hyperalgesia In Rats And The Underlying Molecular Mechanisms

Hyperalgesia is a multifactorial situation caused by injury or dysfunction of the nervous system,this situation results in the increasing response to noxious stimuli.Because pain hypersensitivity can persist long after the healing of the initial damage,it causes major health problems around the world.The mechanism of hyperalgesia is very complex.Oxidative stress is when excessive amount of reactive oxygen species(ROS)is formed or when the antioxidant capacity is decreased,and such imbalance may be a key factor to hyperalgesia.Advanced oxidation protein products(AOPPs)are protein products containing dityrosine and cross-linking formed primarily as a consequence of oxidative stress.AOPPs have been considered as a novel marker of oxidant-mediated protein damage.Recently,AOPPs have been recognized as a marker of oxidative stress and as a mediator of inflammation involved in the pathophysiology of many diseases.However,the role of AOPPs in the mechanism of hyperalgesia remains unknown.Our study aims to investigate whether AOPPs have an effect on hyperalgesia and the possible underlying mechanisms.To identify the AOPPs involved,we induced hyperalgesia in SD rats by injecting complete Freund's adjuvant(CFA)in hindpaw.The level of plasma AOPPs in CFA-induced SD rats was 1.6-fold in comparison with what in normal SD rats.After intravenous injection of AOPPs-modified rat serum albumin(AOPPs-RSA)in SD rats,the paw mechanical thresholds,measured by the electronic von Frey system,significantly declined.Immunofluorescence staining and Western Blots indicated that AOPPs increased expressions of NADPH oxidase 1(Noxl),NADPH oxidase 4(Nox4),transient receptor potential vanilloid 1(TRPV1)and calcitonin gene-related peptide(CGRP)in the dorsal root ganglia(DRG)tissues.In-vitro studies were performed on primary DRG neurons which were obtained from both thoracic and lumbar DRG of rats.Results indicated that AOPPs triggered reactive oxygen species(ROS)production in DRG neurons,which were significantly abolished by ROS scavenger N-acetyl-L-cysteine(NAC)and small-interfering RNA(siRNA)silencing of Nox1 or Nox4.The expressions of Noxl,Nox4,TRPV1 and CGRP were significantly increased in AOPPs-induced DRG neurons.And relevant siRNA or inhibitors notably suppressed the expressions of these proteins and the calcium influxes in AOPPs-induced DRG neurons.In summary,our study provided for the first time that plasma AOPPs were increased in CFA-induced hyperalgesia rats.We identified a novel mechanism by which AOPPs sensitized TRPV1 to induce sustained mechanical hyperalgesia.Sensitization required activation of Nox1/Nox4 and was probably mediated by direct generating ROS to enhance channel gating.AOPPs-induced sensitization of TRPV1 caused intracellular Ca2+ increasing,enhanced CGRP release,and exacerbated pain hypersensitivity.