Involvement Of P2X7Receptor Of RVM In Bone Cancer Pain: Mechanism OfDescending Facilitation

Patients with tumors live longer than last century by the improvement of therapy.However,in latestage of cancer,pain becomes one of the most important problem whichaffects both patients'emotion and quality of life. Also, it will lead patients and theirfamilies to stop active treatment. So there exists much need of the mechanism study ofcancer pain. Bone cancer pain is one of the most common types of chronic pain, which iscaused by tumors metastasize from distant sites such as lung, breast and prostate to bonein late phase. Bone cancer pain is so severe and hard to control. It has a magnificentsignificance to study the mechanism of the cause and development of bone cancer pain.Compared with neuropathic and inflammatory pain, cancer pain has its own complicatedmechanism and remains unclear, so regular therapy has less effect. The chronic paincauses plasticity change of central nervous system, and then causes a series of long termchanges. Recent studies show that the chronic pain will lead continuous positivefeedbackenlargement of brain—medulla—spinal cord descending facilitation, which leadto the maintenance and development of chronic pain. When there exists continuousnociceptive stimulation, RVM will have nervous plasticity and continuous facilitatorypain. Endogenous descending facilitatory system releases5-HT in the spinal dorsal horn asmain transmitter for its work. Bone cancer pain is also a state of chronic pain, however,it's unclear whether descending facilitatory system is activated and how it works.In the model of neuropathic and inflammatory pain, it is proved that the activation ofglia cause the activation of descending facilitatory system. Inhibition of the gila's functioncan suppress mechanical allodynia. P2X7receptor in peripheral and spinal cord involves inthe neuropathic and inflammatory pain. There is still no evidence that microglia and P2X7receptor of RVM participate in the maintenance of bone cancer pain. Through the papersand pre-experiments, we predict that P2X7receptor and microglia may have effect ondescending facilitatory system. Therefore, in a rat model induced by intra-tibia inoculationof Walker256mammary gland carcinoma cells, we investigated the possible mechanismof microglia and P2X7receptor of RVM in descending facilitatory system involve in thedevelopment of bone cancer pain. Immunohistochemistry, Western-blot, ELISA,RT-PCR and behavior test are mainly used.1. Establishment of a rat bone cancer pain model At the7th Day after injection of Walker256mammary gland carcinoma cells into tibiamedullary cavity, tests showed stable and significant mechanical allodynia. Mechanicalallodynia can also be observed in the contralateral limb. However,thermal hyperalgesia isnot so stable. X-ray showed the ruin of bone tissue at14th and21th day after the modelestablished. The astrocyte is significantly activated in the spinal dorsal horn. All above areparallel to the past reports.2. The descending facilitator effect of Glia and P2X7receptor of RVM in bone cancer painmodelAfter the establishment of bone cancer pain model in rat, Fos positive neurons inspinal dorsal horn, RVM, PAG, ACC had a significant higher range than the shamgroups at the7th and14th day. It revealed that descending facilitatory system has beenactivated by cancer pain. We investigated the expression of glia and P2X7receptor ofRVM in bone cancer pain model via Western-blot and immunohistochemistry. The resultshowed that microglia, astrocytes and P2X7receptor were robustly activated in RVM.The mechanical allodynia can be suppressed by the inhibition of microglia or P2X7receptor. We use RNAi to block the expression of P2X7receptor in RVM, which couldalso suppress the mechanical allodynia and the activation of microglia and astrocytes. Atthe same time, the expression of D'serine increased, thus inhibit the expression of5-HT ofRVM. This may be one of the mechanisms that activate descending facilitator.3. Involvement of5HT3receptor mediated descending facilitatory in bone cancer painIntrathecal injection of Ondansetron, an antagonistof5HT3receptor, can significantlysuppress the mechanical allodynia and the activation of related nuclei of descendingfacilitatory system. The expression of5-HT3receptor's mRNA didn't increase in spinaldorsal horn via RT-PCR. Meanwhile, the expression of5-HT increased in spinal dorsalhorn in the bone cancer pain model. Therefore, bone cancer-induced mechanical allodyniamay be caused by the increase of5-HT expression, not5-HT3expression in spinal cord.Taken together, following the bone cancer pain model of inoculation with Walker256cells, descending facilitatory ways were significantly activated. Activation of Gliaand P2X7receptor of RVM may take part in the development and maintenance of bonecancer pain. Our study indicated that glia and P2X7receptor of RVM might become a novel target for bone cancer pain treatment.