| BackgroundMorphine is the agonist of opioid receptors,the main drug for relieving acute and chronic pain in clinic,in addition,its analgesic effect is very strong.However,after repeated use,the analgesic effect will decrease significantly and result in morphine tolerance.Neuropathic Pain(NPP),caused by damage or disease of somatosensory nervous system,is very common in various types of pain.Opioids are an important drug for the treatment of neuropathic pain in clinic,but the adverse reactions such as drug dependence reduce the therapeutic effect greatly.The emergence of opioid tolerance makes it impossible that opioids play the greatest role in clinical use.In order to reduce the occurrence of this situation and improve the quality of life of patients after using opioids,it has great clinical significance to understand the mechanism of opioid tolerance and to restore its efficacy to the greatest extent.Therefore,people have been looking for effective ways to make opioids exert their maximum analgesic effect and reduce the incidence of adverse reactions.Dexmedetomidine(DEX),a highly selective alpha 2-adrenergic receptor(α2-AR)agonist,has sedative and anti-anxiety effects,and slight effects on respiration and circulation,which make it used widely in clinic.Some studies have found that dexmedetomidine can reduce the release of inflammatory factors in the body,and a large number of literatures have shown that inflammatory factors such as TNF-α,IL-1β can activate and maintain inflammatory and neuropathic pain,which is closely related to the occurrence and development of pain,and can be activated or activated by(Nuclear Factor kappa-B,NF-κB).TLR4/NF-κB pathway is an important regulatory signal of immune injury and pain transmission.So,can dexmedetomidine affect the formation of morphine tolerance by regulating TLR4/NF-κB signaling pathway during morphine tolerance? ObjectiveIn order to explore the mechanism of dexmedetomidine in morphine tolerance and the possible signaling pathway,we tested the behavior of chronic morphine tolerance in normal and neuropathic pain model rats after being used dexmedetomidine,and detected the expression of relating proteins in spinal cord(SC)which may participate in morphine tolerance and cause hyperalgesia.Method 1 The effects of different doses of dexmedetomidine subcutaneously on morphine tolerance in normal ratsSD male rats were randomly divided into 5 groups(n=6): Saline group,Mor+Saline group,12.5 ug/kg DEX+Mor group,25 ug/kg DEX+Mor group and 50 ug/kg DEX+Mor group.Tail-flick was measured at 1 day before morphine injection,1,3,5,7 days when morphine was injected,and the maximum analgesic efficiency(%MPAE)would be calculated.Paw withdrawal mechanical threshold(PWMT)and Paw withdrawal thermal latency(PWTL)were measured at 1 day before administration and the eighth day in administration.According to the above experimental results,the best dose group of DEX was selected.2 The effects of dexmedetomidine subcutaneously on morphine tolerance in neuropathic pain ratsSD male rats were randomly divided into 5 groups(n=6): Sham group,SNL+Saline group,SNL+Mor group,SNL+Mor+DEX group and SNL+DEX group.The dose of DEX injection was selected from the above-mentioned pain behavior test results.Neuropathic pain models were prepared by L5 spinal nerve ligation(SNL),moreover,tail-flick,PWMT and PWTL were measured at the first 2 days before administration and at 1,3,5,8,10 days in administration,at the same time % MPAE would be calculated.3 The effects of intrathecal injection of morphine on the expression of TLR4,phosphorylated NF-κB p65,TNF-α and IL-1β in rats spinal cordSD male rats were randomly divided into five groups(n=3): Control group,intrathecal injection of morphine for 1 day,3 days,5 days,7 days group.Western-blot technique was used to detect the expression levels of TLR4,phosphorylated NF-κB p65,TNF-α and IL-1β in L4-5 spinal cord of rats in each group at different time points.4 The effects of dexmedetomidine subcutaneously on expression of phosphorylated NF-κB p65,TNF-α and IL-1β in the morphine tolerant rats spinal cordSD male rats were randomly divided into four groups(n=3): Saline group,Mor+Saline group,DEX+Mor group and DEX group.The dose of DEX injection was selected from the pain behavior test results.After 6 days of medicine injection,the expression levels of phosphorylated NF-κB p65,TNF-α and IL-1β in L4-5 spinal cord were detected by Western-blot.5 The effects of dexmedetomidine subcutaneously on expression of phosphorylated NF-κB p65,TNF-α and IL-1β in the morphine tolerant rats spinal cord with neuropathic painSD male rats were randomly divided into 5 groups(n=3): Sham group,SNL+Saline group,SNL+Mor group,SNL+Mor+DEX group and SNL+DEX group.After 6 days of medicine injection,the expression levels of phosphorylated NF-κB p65,TNF-α and IL-1β in L4-5 spinal cord were detected by Western-blot.Result 1 25ug/kg DEX can delay the formation of morphine tolerance in rats with less adverse reactionsThere was no significant difference in PWMT and PWTL at 1 day preoperative of each group(P > 0.05).Compared with Mor+Saline group,12.5 ug/kg DEX+Mor group had no significant increase in %MPAE,PWMT and PWTL(P > 0.05),however 25 ug/kg DEX+Mor group and 50 ug/kg DEX+Mor group increased significantly(P < 0.05);The sedative effect of 50 ug/kg DEX+Mor group is too serious.So according to comprehensive analysis of sedation and analgesia,we choose 25 ug/kg DEX as the best dose.2 DEX can delay the formation of morphine tolerance in rats after nerve injuryThere was no significant difference in PWMT and PWTL between each group at the first 2 days before operation(P > 0.05),and also in Sham group at each time point and preoperative baseline(P > 0.05),but SNL + Saline group had decreased significantly;compared with SNL + Saline group,the %MPAE,PWMT and PWTL of SNL + Mor group were decreased(P < 0.05),the %MPAE decreased to the baseline from starting injection to the fifth day(P < 0.05).Compared with SNL+Mor group,the %MPAE,PWMT and PWTL in SNL+Mor+DEX group were significantly higher(P < 0.05),besides,single dexmedetomidine group was also higher than SNL group(P < 0.05).3 TLR4/NF-κB signaling pathway is involved in morphine toleranceCompared with Control group,the expression levels of TLR4 and p-p65 in spinal cord of rats increased significantly after intrathecal injection of morphine(P < 0.05),and the expression levels of inflammatory factors TNF-α and IL-1β also increased significantly(P < 0.05).4 DEX can mediate morphine tolerance through TLR4/NF-κB signaling pathwayCompared with Saline group,the expression levels of p-p65,TNF-α and IL-1β in spinal cord of rats in Mor group were significantly higher(P < 0.05),while the expression levels of p-p65,TNF-α and IL-1β in Mor + DEX group were lower than those in Mor group(P < 0.05).5 DEX can mediate morphine tolerance in rats with neuropathic pain via TLR4/NF-κB signaling pathwayCompared with Sham group,the expression levels of p-p65,TNF-α and IL-1β in SNL+Saline group increased,and increased more significantly after intrathecal injection of morphine(SNL+Mor group)(P<0.05).However,after injecting dexmedetomidine(SNL+Mor+DEX group)the expression levels were decreased significantly(P < 0.05).ConclusionsDexmedetomidine can delay the formation of morphine tolerance and enhance the analgesic effect of morphine,the mechanism may be related to inhibiting the activation of TLR4/NF-κB signaling pathway in spinal cord of rats and decreasing the expressions of inflammatory factors. |
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