Leukotriene synthases and the receptors induced by peripheral nerve injury in the spinal cord contribute to the generation of neuropathic pain

Abstract

Leukotrienes (LTs) belong to a large family of lipid mediators, termed eicosanoids, which are derived from arachidonic acids and released from the cell membrane by phospholipases. LTs are involved in the pathogenesis of inflammatory diseases, such as asthma, rheumatoid arthritis, and peripheral inflammatory pain. In the present study, we examined whether LTs were implicated in pathomechanism of neuropathic pain following peripheral nerve injury. Using the spared nerve injury (SNI) model in rats, we investigated the expression of LT synthases (5-lipoxygenase; 5-LO, Five lipoxygenase activating protein; FLAP, LTA4 hydrolase; LTA4h and LTC4 synthase; LTC4s) and receptors (BLT1, 2 and CysLT1, 2) mRNAs in the rat spinal cord. Semi-quantitative RT-PCR revealed that 5-LO, FLAP, LTC4s, BLT1, and CysLT1 mRNAs increased following SNI, but not CysLT2 mRNAs. Using double labeling analysis of in situ hybridization with immunohistochemistry, we observed that 5-LO, FLAP, and CysLT1 mRNAs were expressed in spinal microglia. LTA4h and LTC4s mRNAs were expressed in both spinal neurons and microglia. BLT1 mRNA was expressed in spinal neurons. The p38 mitogen-activated protein kinase inhibitor, but not MEK inhibitor, reduced the increase in 5-LO in spinal microglia. Continuous intrathecal administration of the 5-LO inhibitor or BLT1 and CysLT1 receptor antagonists suppressed mechanical allodynia induced by SNI. Our findings suggest that the increase of LT synthesis in spinal microglia produced via p38 MAPK plays a role in the generation of neuropathic pain.

Leukotrienes in Nociceptive Pathway and Neuropathic/Inflammatory Pain

“…31) As predicted from the RT-PCR data, the expression level of BLT1 mRNA was low, even after spinal nerve injury, and therefore the increase in BLT1 mRNA in the spinal cord was hard to see in darkfield photographs. The brightfield photograph showed the aggregation of silver grains on cells with large nuclei that were lightly stained by hematoxylin in the gray matter after SNI.…”

“…31) Initially, we examined the mRNA of 5-LO in the spinal cord. In the spinal cord of naïve rats, we detected very few aggregations of silver grains, suggesting very low mRNA levels (Fig.…”

“…2A, B). 31) The administration of inhibitor started at 2d after SNI surgery using an osmotic pump. The nerve injury induced a decrease in the withdrawal threshold indicating mechanical hypersensitivity and the administration of 5-LO inhibitor (2.4 nmol/d) attenuated the mechanical hypersensitivity significantly compared to the vehicle-treated group.…”

“…In order to determine the effect of antagonists of LT receptors on established pain behaviors, the osmotic pump with antagonist of BLT1 or CysLT1 was implanted at the 6th day after SNI surgery and pain behaviors were measured from 7 to 14 d. 31) Neither antagonist attenuated the decrease of withdrawal threshold after SNI surgery.…”

“…[28][29][30] Here, the detailed expression patterns of enzymes for LTs synthesis and the receptors in the rat spinal cord are summarized and the detailed alteration of expression of these molecules after peripheral nerve injury and their roles in neuropathic pain are also reported. 31) …”