Fibronectin/integrin system is involved in P2X4 receptor upregulation in the spinal cord and neuropathic pain after nerve injury

We have previously shown that activation of the ATP-gated ion channel subtype P2X₄ receptors (P2X₄Rs) in the spinal cord, the expression of which is upregulated in microglia after nerve injury, is necessary for producing neuropathic pain. The upregulation of P2X₄Rs in microglia is, therefore, a key process in neuropathic pain, but the mechanism remains unknown. Here, we find a fibronectin/integrin-dependent mechanism in the upregulation of P2X₄Rs. Microglia cultured on dishes coated with fibronectin, an extracellular matrix molecule, expressed a higher level of P2X₄R protein when compared with those cultured on control dishes. The increase was suppressed by echistatin, a peptide that selectively blocks β₁ and β₃-containing integrins, and with a function-blocking antibody of β₁ integrin. In in vivo studies, the upregulation of P2X₄Rs in the spinal cord after spinal nerve injury was significantly suppressed by intrathecal administration of echistatin. Tactile allodynia in response to nerve injury and intrathecal administration of ATP- and fibronectin-stimulated microglia was inhibited by echistatin. Furthermore, intrathecal administration of fibronectin in normal rats increased the level of P2X₄R protein in the spinal cord and produced tactile allodynia. Moreover, the fibronectin-induced allodynia was not observed in mice lacking P2X₄R. Taken together with the results of our previous study showing an increase in the spinal fibronectin level after nerve injury, the present results suggest that the fibronectin/integrin system participates in the upregulation of P2X₄R expression after nerve injury and subsequent neuropathic pain.