The Effect And Regulatory Mechanism Of BDNF From RACC Neurons On Neuropathic Pain-related Aversion

BackgroundNeuropathic pain,as one of the most common types of chronic pain,always behaves in a spontaneous manner(a continuous or paroxysmal pain that is not related to an external noxious stimulus).There has been considerable evidence suggesting that patients with chronic pain suffer from much more affective disturbances than pain itself,such as anxiety,fear and inferiority.Thus,increased attention should be paid to the clinical treatment of pain affection.The rostral anterior cingulate cortex(rACC)is an important brain area involved in the formation of emotional pain.NMDA receptor NR2B subunits in the rACC play a key role during the process of pain-related aversion.Nerve injury significantly increases the expression of NR2B,but the administration of NR2B antagonist into the rACC completely blocks the upregulation and relieves the pain affect.Brain-derived neurotrophic factor(BDNF)is commonly regarded to be an activity-dependent neuronal modulator in central nerve system that plays an important role in synaptic plasticity,learning-memory and chronic pain.lately,a new study found increased BDNF expression in the ACC of rats with inflammation,while local injection of a TrkB receptor antagonist mostly alleviated neuronal hyperexcitability,cold stimulation-induced hyperalgesia and prevented passive avoidance behavior.Moreover,it was reported that the BDNF/TrkB-mediated signaling pathway in the spinal cord promoted the development of central sensitization and hyperalgesia,and that this process was dependent upon the activation of dorsal horn NR2B receptors.However,whether and how BDNF in the rACC contributes to neuropathic pain-related aversion is still unclear.Thus,the present study was aim to explore the mechanisms of BDNF in pain-related aversion formation in the rat model of neuropathic pain.ObjectiveTo establish the rat model of partial sciatic nerve injury and investigate the function and mechanisms of BDNF on neuropathic pain-related aversion.Methods and Results1.Endogenous BDNF is involved in neuropathic pain-related aversionMethods Rats were randomly divided into groups.Operated group(spared nerve injury model,SNI),Sham group(sham operation),Naive(no treatment).The paw withdraw threshold(PWT)of all rats,was evaluated at 1 day before and also on 1,3,7,10 and 14 days after surgery.The SNI rats were randomly divided into CTX-B group(CTX-B+rACC)and CTX-B-P group(CTX-B+PFC).On day 7 after surgery,clonidine-induced conditioned place preference(CPP)was performed to evaluate pain affect.After the behavioral tests,rats were decapitated and the rACC was quickly removed for the determination of BDNF and TrkB expression by western blot and immunohistochemistry.Results Compared with control group,the PWT was significantly decreased in surgical rats,and CPP was only acquired in SNI rats.However,when TrkB receptor antagonist CTX-B was pre-injected into rACC(not PFC)before CPP test,the preference phenomenon disappeared.Moreover,the expression of BDNF and its high affinity receptor TrkB were significantly upregulated in the rACC of SNI rats.2.Exogenous BDNF contributes to pain affection via regulating NR2B receptorsMethods Rats were randomly divided into groups.Naive group(no treatment),BDNF group(BDNF+rACC),DMSO group(DMSO+rACC),CTX-B group(BDNF+CTX-B),Ifenprodil group(BDNF+Ifenprodil),BDNF-P group(BDNF+PFC).After surgery,conditioned place avoidance(CPA)test was conducted to evaluate pain-related aversion.After behavioral tests,rats were decapitated and the rACC was quickly removed for the determination of expression of TrkB and NR2B by western blot and immunofluorescence.Results Compared with DMSO group,administration of exogenous BNDF into the rACC(not PFC)of normal rats was sufficient to produce CPA.However,when the TrkB or NR2B receptor antagonist was pre-injected into rACC before test,the BDNF-CPA phenomenon was completely disappeared.Moreover,exogenous BDNF significantly increased the NR2B expression when compared with control group,but this upregulation was mostly alleviated by CTX-B.The immunofluorescence results showed that TrkB was co-localized with NR2B receptors in rACC neurons.ConclusionBDNF/TrkB signaling pathway in the rACC contributes to neuropathic pain-related aversion via activation of NR2B receptors.The results of this study are demonstrated in animal experiments,which confers a clinical significance that a new strategy targeting BDNF and relative receptors in the rACC might be useful for the prevention of pain-related emotional disturbance due to peripheral nerve injury.