The Role Of Endocannabinoid System In The Analgesia Induced By Spinal Cord Stimulation

Neuropathic pain is induced by disease or direct injury to the somatosensory nervoussystem. This long-lasting chronic pain usually causes serious damage to the patient’sphysical and mental health. Neuropathic pain not only disturbes the patients’ work, life andsocial activities, but also brings enormous economic burden to the family and society.Patientsoftenstayin astressstateforalongtimebecauseofthepainthatcan’tbeeffectivelycontrolled. As a consequence, these patients more or less suffer from mental disorders suchas depression or anxiety. There are various kinds of treatment of chronic pain, such as drugtherapy, physical therapy, interventional therapy, surgery and so on, but most of them havenarrow indications and have limitations to their applications. Besides, the effect of all thetreatments mentioned above is not ideal to relieve the pain, and cannot improve the patient’squality of life obviously. So, it is necessary to find more effective therapeutics.As one of the methods of neuromodulation, spinal epidural electric stimulation (Spinalcord stimulation, SCS) has been shown superiority in alleviating neuropathic pain, includingreduced dose of analgesic drugs, increased activity of patients and improved life quality.However, not every patient can get benefit from this therapy, there are still considerablenumber of patients get limited pain relief after SCS. For one hand, this might be due to theinappropriate parameters setting of the stimulation. On the other hand, the limitedunderstanding about the mechanisms of SCS may contribute its failure in the treatments.Therefore, it is particularly critical for us to make clear the mechanisms of SCS inducedanalgesia, and eventually guide the clinical usage of SCS.Previous studies have found that the inhibitory neurotransmitters such as GABA and5–HT in the spinal cord participate in the analgesic action of SCS. In addition, endogenousmorphine system, adenosine and choline are also involved in this process. Microglia has animportant regulating role in the development of neuropathy pain, SCS may also achieveanalgesic effect through the regulation of microglia activity. The endocannabinoid system (eCBs) is another important endogenous analgesic pathways in the body, but the role ofeCBs in the analgesic mechanisms induced by SCS is still unknown. This experiment wasdesigned to explore the action of eCBs in SCS.Experiment1: Construction of animal models for SCSObjective: To make neuropathic pain and epidural SCS model in SD rats, and evaluateefficiency and stability of the model.Methods: Adult male SD rats (weight200-250g) were used. The basic value ofhindpaw mechanical withdraw threshold (PMWT) was measured after3d acclimatization.Then, the neuropathic pain model were made by ligation of the left L5spinal nerve (SNL).3d after surgery, the PMWT was tested again after eliminating rats with paralysis of lowerlimb.5d after SNL, rats with significant decrease of PMWT were selected to make spinalepidural SCS model. The homemade stimulating electrode was placed in the spinal epiduralspace at T10-T11levels. Five more days later, general situation of the rats was evaluated,and PMWT was measured after SCS electrodes implanted.Results: The PMWT of left hind paw of rat after SNL was significantly reduced (P <0.001) and had little changes after the implantation of epidural electrodes. The rats are allin normal condition after model making, indicating by normal weight growth, no lower limbparalysis and normal reflex responses to external stimuli.Experiment2: The effect of SCS on mechanical allodynia induced by SNLObjective: To find the appropriate parameters of stimulation, and replicate theanalgesic effect of SCS on SNL neuropathic pain model.Method: Male SD rats (200-250g) were selected for this experiment. The constructionof SNL and SCS model were the same as experiment1.5d after SCS model, the electricalstimulation was given at frequency of2,10, and35Hz respectively. A30min duration andthe stimulation intensity threshold causing leg movement (motion-threshold intensity) werechosen. The value of PMWT was tested during the continuing stimulation of epiduralelectrode to find the best frequency of stimulus that can alleviate the mechanical allodynia.Results: Under the continuous wave with motion-threshold intensity (150-200mV),the stimulation in10-35Hz significantly improved the insilateral PMWT of rats after SNL (P <0.001), while the stimulation of2Hz had little effect on PMWT (P>0.05).Experiment3: The effect of intraperitoneal injection of CB1receptorantagonist AM251on SCS induced analgesia.Objective: To observe whether the CB1receptor antagonist can reverse the analgesiceffect of SCS.Methods: Male SD rats (200-250g) were selected for this experiment. Theconstruction of SNL and SCS model were the same as Experiment1. After elimination offailure model, a continuous wave stimulation with20Hz frequency and motion-thresholdstrength (150-200mV) were used.30min after observation of the analgesic effect of SCS,CB1receptor antagonist AM251was applied through intraperitoneal injection and thevalue of PMWT was measured during second SCS after injection of AM251.Results: CB1receptor antagonist AM251through intraperitoneal injection reversedthe analgesic effects of SCS (P <0.001), indicated that the eCBs is involved in theanalgesia effect of SCS.Conclusion:The homemade stimulating electrode can be used to making SCS animal model inrats. The CB1receptor antagonist reverses the analgesic effects of SCS, indicated that theeCBs is involved in the analgesia effect of SCS.