Abstract
Background: The importance of P2X purinoceptors, CB2 receptor and microRNA-124(miR-124) in spinal cord microglia to the development of neuropathic pain was demonstrated in numerous previous studies. The upregulation of P2X4 and P2X7 receptors in spinal dorsal horn microglia is involved in the development of pain behavior caused by peripheral nerve injury. However, it is not clear whether the expression of P2X4 and P2X7 receptors at dorsal spinal cord will be influenced by CB2 receptor or miR-124 in rats after chronic sciatic nerve injury.Methods: Chronic constriction injury (CCI) of the sciatic nerve was performed in rats to induce neuropathic pain. Tests of the mechanical withdrawal threshold (MWT) were carried out to assess the response of the paw to mechanical stimulus. The expression of miR-124, P2X4, P2X7 and CB2 receptor were detected with RT-PCR. The protein expression of P2X4, P2X7 and CB2 receptor, RhoA, ROCK1, ROCK2, p-p38MAPK and p-NF-kappaBp65 was detected with Western blotting analysis. Results: Intrathecal administration of CB2 receptor agonist AM1241 significantly attenuated CCI-induced mechanical allodynia and significantly inhibited the increased expression of P2X4 and P2X7 receptors at the mRNA and protein levels, which imply that P2X4 and P2X7 receptors expression are down-regulated by AM1241 in CCI rats. Western blot analysis showed that AM1241 suppressed the elevated expression of RhoA, ROCK1, ROCK2, p-p38MAPK and NF-κBp65 in the dorsal spinal cord induced by CCI. After administration with Y-27632 (ROCK inhibitor), SB203580 (P38MAPK inhibitor) or PDTC (NF-κB inhibitor), the levels of P2X4 and P2X7 receptors expression in the dorsal spinal cord were lower than those in CCI rats, which imply that the ROCK/P38MAPK pathway and NF-κB activation may contribute to the increased expression of P2X4 and P2X7 receptor. On the other hand, in CCI rats, AM1241 treatment evoked the increased expression of CB2 receptor and miRNA-124, which can be inhibited by intrathecal injection of CB2 receptor antagonist AM630, which indicate that the increased expression of miRNA-124 may be medicated by CB2 receptor activation. In addition, the increased expression of P2X4 and P2X7 receptors in the dorsal spinal cord of CCI rats were inhibited by miRNA-124 agomir. Furthermore, intrathecal injection of miRNA-124 agomir could efficiently inhibit the ROCK/P38MAPK pathway and NF-κB activation in CCI rats. Moreover, AM1241 treatment significantly inhibited the expression of P2X4 and P2X7 receptors, and this suppression is enhanced by pretreatment with miRNA-124 agomir. On the contrast, the inhibitory effect of AM1241 on the expression of P2X4 and P2X7 receptor can be reversed by pretreatment with miRNA-124 antagomir.Conclusions: In CCI rats, intrathecal injection of AM1241 could efficiently induce the increased expression of miRNA-124, while inhibiting the ROCK/P38MAPK pathway and NF-κB activation in dorsal spinal cord. CB2 receptor/miRNA-124 signaling induced the decreased P2X4 and P2X7 receptors expression via inhibit the ROCK/P38MAPK pathway and NF-κB activation.
Delta opioid receptor activation modulates affective pain and modality‐specific pain hypersensitivity associated with chronic neuropathic pain
