Activation of dorsal horn cannabinoid CB2 receptor suppresses the expression of P2Y(12) and P2Y(13) receptors in neuropathic pain rats

BACKGROUND: More evidence suggests that dorsal spinal cord microglia is an important site contributing to CB2 receptor-mediated analgesia. The upregulation of P2Y(12) and P2Y(13) purinoceptors in spinal dorsal horn microglia is involved in the development of pain behavior caused by peripheral nerve injury. However, it is not known whether the expression of P2Y(12) and P2Y(13) receptors at spinal dorsal horn will be influenced after CB2 receptor activation in neuropathic pain rats. METHODS: Chronic constriction injury (CCI) and intrathecal ADPbetaS injection were performed in rats to induce neuropathic pain. The paw withdrawal latency (PWL) was used to evaluate thermal hyperalgesia in neuropathic rats. The expression of P2Y(12) and P2Y(13) receptors, p-p38MAPK, and NF-kappaBp65 was detected with RT-PCR and western blotting analysis. RESULTS: Treatment with AM1241 produces a pronounced inhibition of CCI-induced thermal hyperalgesia and significantly inhibited the increased expression of P2Y(12) and P2Y(13) receptors at the mRNA and protein levels, which open up the possibility that P2Y(12) and P2Y(13) receptor expression are downregulated by CB2 receptor agonist AM1241 in CCI rats. Western blot analysis demonstrated that AM1241 reduced the elevated expression of p-p38MAPK and NF-κBp65 in the dorsal spinal cord induced by CCI. After administration with either SB203580 (p38MAPK inhibitor) or PDTC (NF-kappaB inhibitor), the levels of P2Y(13) receptor expression in the dorsal spinal cord were lower than those in the CCI group. However, in CCI rats, the increased expression of P2Y(12) receptor was prevented by intrathecal administration of PDTC but not by SB203580. In addition, minocycline significantly decreased the increased expression of P2Y(12) and P2Y(13) receptors. The similar results can be observed in ADPbetaS-treated rats. Intrathecal injection of ADPbataS causes thermal hyperalgesia and increased expression of P2Y(12) and P2Y(13) receptors in the dorsal spinal cord of naive rats. Moreover, intrathecal injection of AM1241 alleviates pain response and reduces the elevated expression of P2Y(12) and P2Y(13) receptors, p-p38MAPK, and NF-κBp65 in the dorsal spinal cord of ADPbetaS-treated rats. Intrathecal injection of SB203580 significantly inhibited the ADPbetaS-induced P2Y(13) receptor expression, without affecting P2Y(12) receptor expression. However, treatment with either SB203580 or PDTC effectively inhibited P2Y(13) receptor expression compared to ADPbetaS-treated rats. CONCLUSIONS: In CCI- and ADPbetaS-treated rats, AM1241 pretreatment could efficiently activate CB2 receptor, while inhibiting p38MAPK and NF-kappaB activation in the dorsal spinal cord. CB2 receptor stimulation decreased P2Y(13) receptor expression via p38MAPK/NF-kappaB signaling. On the other hand, CB2 receptor activation decreased P2Y(12) receptor expression via p38MAPK-independent NF-kappaB signaling pathway.