An LPA species (18:1 LPA) plays key roles in the self-amplification of spinal LPA production in the peripheral neuropathic pain model

Background: We previously reported that nerve injury-induced neuropathic pain is initiated by newly produced lysophosphatidic acid (LPA). Results: In this study, we developed a quantitative mass spectrometry for detecting LPA species by using Phos-tag. Following nerve injury, the levels of 18:1, 16:0 and 18:0 LPA in the spinal dorsal horn significantly increased at 3 h and declined at 6 h. Among them, 18:1 LPA level was the most abundant. In the same preparation, there were significant elevations in the activities of cytosolic phospholipase A(2) (cPLA(2)) and calcium-independent phospholipase A(2) (iPLA(2)), key enzymes for LPA synthesis, at 1 h, while there was no significant change in phospholipase A(1) activity. Pharmacological studies revealed that NMDA and neurokinin 1 receptors, cPLA(2), iPLA(2) and microglial activation, as well as LPA(1) and LPA(3) receptors were all involved in the nerve injury-induced LPA production, and underlying cPLA(2) and iPLA(2) activations. In the cells expressing LPA(1) or LPA(3) receptor, the receptor-mediated calcium mobilization was most potent with 18:1 LPA, compared with 16:0 or 18:0 LPA. Moreover, the intrathecal injection of 18:1 LPA, but not 16:0 or 18:0 LPA, caused a spinal LPA production and neuropathic pain-like behavior. Conclusion: These results suggest that 18:1 LPA is the predominant ligand responsible for LPA(1) and LPA(3) receptors-mediated amplification of LPA production through microglial activation.