Accumulating evidence indicates that peripheral nerve injury (PNI) activates spinal microglia that are necessary for neuropathic pain. Recent studies using bone marrow (BM) chimeric mice have reported that after PNI, circulating BM-derived cells infiltrate into the spinal cord and differentiate into microglia-like cells. This raises the possibility that the population of spinal microglia after PNI may be heterogeneous. However, the infiltration of BM cells in the spinal cord remains controversial because of experimental adverse effects of strong irradiation used for generating BM chimeric mice. In this study, we evaluated the PNI-induced spinal infiltration of BM-derived cells not only by irradiation-induced myeloablation with various conditioning regimens, but also by parabiosis and mice with genetically labelled microglia, models without irradiation and BM transplantation. Results obtained from these independent approaches provide compelling evidence indicating little contribution of circulating BM-derived cells to the population of spinal microglia after PNI.

P2X4 receptors (P2X4R) are a family of ATP-gated non-selective cation channels. We previously demonstrated that activation of P2X4R in spinal microglia is crucial for neuropathic pain, a highly debilitating chronic pain condition, suggesting that P2X4R is a potential therapeutic target for treating neuropathic pain. Thus, the identification of a compound that has a potent inhibitory effect on P2X4R is an important clinical challenge. In the present study, we screened a chemical library of clinically approved drugs and show for the first time that duloxetine, a serotonin and noradrenaline reuptake inhibitor, has an inhibitory effect on rodent and human P2X4R. In primary cultured microglial cells, duloxetine also inhibited P2X4R-, but not P2X7R-, mediated responses. Moreover, intrathecal administration of duloxetine in a model of neuropathic pain produced a reversal of nerve injury-induced mechanical allodynia, a cardinal symptom of neuropathic pain. In rats that were pretreated with a serotonin-depleting agent and a noradrenaline neurotoxin, the antiallodynic effect of duloxetine was reduced, but still remained. Based on these results, we suggest that, in addition to duloxetine’s primary inhibitory action on serotonin and noradrenaline transporters, an inhibitory effect on P2X4R may be involved at least in part in an antiallodynic effect of intrathecal duloxetine in a model of neuropathic pain.

Microglia, the resident immune cells of the central nervous system, are constitutively mobile cells that undergo rapid directional movement toward sites of tissue disruption. However, transcriptional regulatory mechanisms of microglial motility remain unknown. In the present study, we show that interferon regulatory factor-8 (IRF8) regulates microglial motility. We found that ATP and complement component, C5a, induced chemotaxis of IRF8 wild-type microglia. However, these responses were markedly suppressed in microglia lacking IRF8 (Irf8 −/− ). In a consistent manner, phosphorylation of Akt (which plays a crucial role in ATP-induced chemotaxis) was abolished in Irf8 −/− microglia. Real-time polymerase chain reaction analysis revealed that motility-related microglial genes such as P2Y 12 receptor were significantly suppressed in Irf8 −/− microglia. Furthermore, Irf8 −/− microglia exhibited a differential expression pattern of nucleotidedegrading enzymes compared with their wild-type counterparts. Overall, our findings suggest that IRF8 may regulate microglial motility via the control of microglial gene expression.