Localization of prostaglandin E2 synthases and E-prostanoid receptors in the spinal cord in a rat model of neuropathic pain

Abstract

Prostaglandin E2 (PGE2) is a lipid mediator which plays a role in the generation of inflammatory and neuropathic pain. In the peripheral nervous system, PGE2 sensitizes nociceptive afferent neurons through E-prostanoid (EP) receptors. In the central nervous system, PGE2 modulates pain sensitivity and contributes to the development of neuropathic pain. However, the distribution of PGE2 and EP receptors in the spinal cord remains unclear. In the present study, we examined the expression of PGE2 synthases (microsomal PGE synthase [mPGES]-1, mPGES-2, and cytosolic PGE synthase [cPGES]) and EP receptors (EP1-4) in a rat model of neuropathic pain.We identified that mPGES-1 mRNA was upregulated in spinal endothelial cells after nerve injury and exhibited co-localization with cyclooxygenase-2 (COX-2). We detected that mPGES-2 mRNA and cPGES mRNA were expressed in spinal neurons and noted that their expression level was not affected by nerve injury. With respect to EP receptors, EP2 mRNA and EP4 mRNA were expressed in spinal neurons in the dorsal horn. EP3 mRNA was expressed in motor neurons, whereas EP1 mRNA was not detected in the spinal cord. Intrathecal injection of tumor necrosis factor alpha (TNFα) upregulated mPGES-1 mRNA in blood vessels in the spinal cord. Intrathecal injection of a TNFα-neutralizing antibody partially inhibited the upregulation of mPGES-1 mRNA after nerve injury.These results indicate that PGE2 is synthesized by COX-2/mPGES-1 in spinal endothelial cells after nerve injury. These results suggest that in neuropathic pain condition, endothelial cell-derived PGE2 may act on EP2 and EP4 receptors on spinal neurons and modulate pain sensitivity.